mid-cretaceous magmatic evolution and intrusion-related metallogeny...

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Mid-Cretaceous Magmatic Evolution and Intrusion-Related Metallogeny of the Tintina Gold Province,

Yukon and Alaska

Craig J.R. HartM.Sc.

University of British Columbia 1995

B.Sc. McMaster University 1986

This thesis is presented for the degree of

Doctor of Philosophy

Centre for Global MetallogenySchool of Earth and Geographical Sciences

The University of Western Australia

October 2004

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Supervisors

David I. Groves and Richard J. Goldfarb

Collaborators

John L. Mair, Lara L. Lewis, Moira Smith, Dan T. McCoy, Roger Hulstein, Paul Roberts, Tom Bundtzen Mike E. Villeneuve, Arne A. Bakke, David Selby, Chris Wijns

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All work presented in this thesis is original and that of the candidate, unless otherwise specified, acknowledged or referenced.

__________________________________

Craig J.R. Hart 18 October 2004

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“There is nothing so sobering as an outcrop.”

Francis Pettijohn (undated)

The apical portion of the Tuna Stock as exposed in eastern Yukon. This slightly pera-luminous pluton is a member of the mid-Cretaceous Tungsten Plutonic Suite.

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AbstractThe Tintina Gold Province (TGP) comprises numerous (>15) gold belts and districts

throughout interior Alaska and Yukon that are associated with Cretaceous plutonic rocks. With a gold endowment of ~70Moz, most districts are defined by their placer gold contributions, which comprise greater than 30 Moz, but four districts have experience significant increases in gold exploration with notable discoveries at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz). All significant TGP gold deposits are spatially and temporally related to reduced (ilmenite-series) and radiogenic Cretaceous intrusive rocks that intrude (meta-) sedimentary strata. The similar characteristics that these deposits share are the foundation for the development of a reduced intrusion-related gold deposit model. Associated gold deposits have a wide variety of geological and geochemical features and are categorized as intrusion-centered (includes intrusion-hosted, skarns and replacements), shear-related, and epizonal. The TGP is characterized by vast, remote under-explored areas, unglaciated regions with variable oxidation depths and discontinuous permafrost, which, in combination with a still-evolving geological model, create significant exploration challenges.

Twenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites and belts are defined across Alaska and Yukon on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features, when combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole-rock ferric:ferrous ratios define the distribution of magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction-generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly-younger, slightly-peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite-series belt in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension.

Intrusion-related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au and Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation associated with ilmenite-series plutons. However, there are some notable deviations from expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons which form the basis for the reduced intrusion-related gold deposit model.

Among the mid-Cretaceous TGP plutonic suites, the Tombstone, Mayo and Tungsten suites are the most metallogenically prolific. The Tombstone suite is alkalic, variably fractionated, slightly late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite

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dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone suite was derived from an enriched, previously-depleted lithospheric mantle, the Tungsten suite intrusions from continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo suite from similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source.

Each of these three suites has a distinctive metallogeny that is related to the source and redox characteristics of the magma. The Tombstone suite has a Au-Cu-Bi association that is characteristic of most oxidized and alkalic magmas, but also has associated, and enigmatic, U-Th-F mineralisation. The reduced Tungsten suite intrusions are characterized by world-class tungsten skarn deposits with less significant Cu, Zn, Sn, and Mo anomalies. The Mayo suite intrusions are characteristically gold-enriched, with associated As, Bi, Te, and W associations. All suites also have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral occurrences. Although processes such as fractionation, volatile enrichment, and phase separation are ultimately required to produce economic concentrations of ore elements from crystallizing magmas, the nature of the source materials and their redox state play an important role in determining which elements are effectively concentrated by magmatic processes.

Intrusion-related gold systems have an implied genetic relationship with a cooling pluton, but confident links are difficult to establish. U-Pb and Ar-Ar geochronology in the TGP has established main mineralizing events at circa 90 and 70 Ma, but precise temporal links between magmatism and mineralization have been difficult to confirm. An integrated geochronological approach to understand the magmatic-hydrothermal evolution of the Fort Knox, Clear Creek, Scheelite Dome, Mactung and Cantung intrusion-related deposits using SHRIMP zircon U-Pb, mica Ar-Ar, molybdenite Re-Os, and TIMS U-Pb data. TIMS U-Pb data, and SEM imagery of plutonic zircons indicate considerable inheritance and Pb-loss in the zircons. SHRIMP analyses avoid Pb-loss and inheritance and result in dates that are up to 4 m.y. older than the TIMS dates. Ar-Ar dates for magmatic biotite and hornblende indicate that most plutons cooled quickly, but Ar-Ar dates on hydrothermal micas from mineralization are 2-3 million years younger than the magmatic mica dates suggesting that Ar retention in hydrothermal micas may differ from that in magmatic micas. Re-Os molybdenite dates are in best agreement, and within the uncertainty of the SHRIMP dates for the host plutons. Direct comparisons of the absolute ages indicate magmatic-hydrothermal durations from 1.1 to 3.3 million years, which expand from 2.2 to 4.2 m.y. with the extremes of the uncertainties. These durations contrast thermal modeling which indicates rapid cooling to below ~250°C within 0.03 to 0.3 million years. Long-lived magmatic-hydrothermal systems may result from episodic replentishment of assembling magmatic plutons. Alternatively, the discrepancy may result from within-system uncertainties of geochronological decay constants and the compounding effects of decay constant errors across isotopic systems which conspire to increase the uncertainties of comparative dates.

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Contents

Abstract ......................................................................................................................... i

Contents ..................................................................................................................... iii

List of Appendices ...................................................................................................... vii

Acknowledgements ....................................................................................................viii

Chapter One

INTRODUCTION

Preamble .....................................................................................................................1

Characteristics of IRGS .............................................................................................2

Concerns of Classif cation .......................................................................................... 2

Thesis Aims and Objectives ......................................................................................... 3

Thesis Organisation .....................................................................................................3

Supportive and Associated Contributions .................................................................... 4

References ..................................................................................................................7

Chapter Two

GEOLOGY, EXPLORATION and DISCOVERY in the TINTINA GOLD PROVINCE, ALASKA and YUKON

Abstract ...................................................................................................................... 11

Introduction ................................................................................................................ 11

Historical Exploration Activity ..................................................................................... 12

The Early Years .......................................................................................................13

The Modern Era .......................................................................................................15

Contemporary Efforts ..............................................................................................15

Regional Geology of the TGP Belts and Associated Gold Mineralization .................. 18

Deposits of the Kuskokwim Mineral Belt, southwestern Alaska ..............................18

Fairbanks district, eastern Alaska ............................................................................ 20

Goodpaster district, eastern Alaska ......................................................................... 22

Tombstone gold belt, central Yukon ......................................................................... 24

Other Districts ..........................................................................................................25

Summary ...................................................................................................................28

Styles of Gold Mineralization ..................................................................................... 30

Intrusion-centered ....................................................................................................30

Sheeted Veins ....................................................................................................31

Skarns .................................................................................................................33

Replacements, Disseminations & Breccias ........................................................34

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Shear-related ores ...................................................................................................35

Epizonal Mineralization ............................................................................................37

Summary and Implications for Ore Genesis ..............................................................38

Exploration and Discovery Case Histories ................................................................. 41

Fort Knox .................................................................................................................41

Dublin Gulch ............................................................................................................42

Brewery Creek .........................................................................................................43

True North ................................................................................................................44

Donlin Creek ............................................................................................................45

Pogo ........................................................................................................................46

Exploration Methods & Targeting in the TGP..............................................................48

Pluton Distribution, Character and Lithology ...........................................................48

Elemental Associations ............................................................................................49

Placer-Related Targets ............................................................................................49

Structural Targeting .................................................................................................49

Geochemical Methods for TGP Exploration ............................................................50

Geophysical Methods for TGP Exploration .............................................................52

Drilling as an Exploration Tool ................................................................................. 53

Additional Exploration Considerations ....................................................................... 53

Conclusion .................................................................................................................56

References ................................................................................................................57

Chapter Three

The NORTHERN CORDILLERAN MID-CRETACEOUS PLUTONIC PROVINCE: ILMENITE/MAGNETITE-SERIES GRANITOIDS and INTRUSION-RELATED MINERALISATION

Abstract ......................................................................................................................67

Introduction ................................................................................................................67

Tectonic Framework ...................................................................................................70

Plutonic Sites .............................................................................................................71

Early and Mid-Cretaceous Plutonic Belts and Suites ................................................72

Tosina-St. Elias Belt .................................................................................................72

Nutzotin-Kluane Belt ................................................................................................75

St. Lawrence-Seward-Koyukuk Belt ........................................................................ 76

Tanacross-Dawson Range-Whitehorse Belt ............................................................79

Ruby-Kaiyuh-Nyac Belt ...........................................................................................79

Yukon-Tanana Uplands Plutons .............................................................................. 80

Anvil-Hyland-Cassiar Belt ........................................................................................ 81

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Southern Alaska Range ...........................................................................................82

Tok-Tetlin Belt ..........................................................................................................83

Tungsten Plutonic Suite ...........................................................................................83

Mayo Plutonic Suite .................................................................................................84

Fairbanks-Salcha Plutonic Suite .............................................................................. 84

Livengood - Tombstone Belt .................................................................................... 85

Redox State Characteristics ...................................................................................... 86

Aeromagnetic Character ..........................................................................................86

Magnetic Susceptibility ............................................................................................86

Ferric:Ferrous Ratios ...............................................................................................89

Space, Time & Metallogenic Distribution of Redox-Series Plutons ............................90

Distribution Patterns ................................................................................................90

Temporal Variations .................................................................................................93

Metallogeny .............................................................................................................95

Tectonic Setting of Redox-Series Plutons .................................................................. 98

Conclusions .............................................................................................................100

References ..............................................................................................................102

Chapter Four

SOURCE and REDOX CONTROLS on METALLOGENIC VARIATIONS in INTRUSION-RELATED ORE SYSTEMS, TOMBSTONE-TUNGSTEN BELT, YUKON

TERRITORY, CANADA

Abstract .................................................................................................................... 119

Introduction .............................................................................................................. 119

Tectonic Setting .......................................................................................................120

Characteristics of the Tombstone-Tungsten Belt .....................................................120

Plutonic Suites .........................................................................................................122

Nomenclature ........................................................................................................124

Tombstone Plutonic Suite ......................................................................................124

Mayo Plutonic Suite ...............................................................................................126

Tungsten Plutonic Suite .........................................................................................127

Intrusion-Related Metallogeny ................................................................................. 127

Geochemistry ...........................................................................................................129

Isotopic Data ............................................................................................................133

Redox State .............................................................................................................134

Zircon Character ......................................................................................................136

Discussion ...............................................................................................................136

Source Regions of Magmas .................................................................................. 136

Redox Characteristics of Source Materials ...........................................................139

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Relationships Between Redox State of Magmas and Metallogeny .......................139

Conclusions .............................................................................................................141

References ..............................................................................................................143

Chapter Five

INTEGRATED U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar GEOCHRONOLOGY of MINERALIZING PLUTONS in YUKON and ALASKA: DURATION of MAGMATIC-

HYDROTHERMAL SYSTEMS

Abstract ....................................................................................................................153

Introduction ..............................................................................................................154

Regional Geology - Tintina Gold Province ...............................................................154

Geology of the Plutons ............................................................................................156

Fort Knox ...............................................................................................................156

Clear Creek ...........................................................................................................156

Scheelite Dome .....................................................................................................158

Dublin Gulch ..........................................................................................................158

Mactung .................................................................................................................158

Cantung .................................................................................................................159

Geochronology ........................................................................................................159

Fort Knox ...............................................................................................................160

Clear Creek ...........................................................................................................163

Dublin Gulch ..........................................................................................................164

Scheelite Dome .....................................................................................................165

Mactung .................................................................................................................165

Cantung .................................................................................................................166

Summary ......................................................................................................................167

Discussion ....................................................................................................................169

Comparison of Data ................................................................................................169

Duration of the Magmatic-Hydrothermal Systems ..................................................171

Modeled Durations of Cooling Plutons ................................................................... 173

Durations of Cooling Plutons from Empirical Data .................................................174

Limitations of Data Integration ................................................................................ 174

The Road Ahead .....................................................................................................176

References ...................................................................................................................177

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Chapter Six

CONCLUSIONS

Conclusions .............................................................................................................183

Tintina Gold Province ............................................................................................183

Northern Cordillera Mid-Cretaceous Plutonic Province .........................................184

Source and Redox Controls on Tombstone -Tungsten Belt Intrusions .................. 185

Integrated Geochronology and Duration of TTB Systems .....................................185

Final Remarks ..........................................................................................................186

APPENDICES

1 Sample Locations .................................................................................................189

2 SHRIMP.data.........................................................................................................190

2 Ar-Ar data and plots .............................................................................................192

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Acknowledgements

This thesis is the result of encouragement and support from Richard Goldfarb and David Groves. Both have played so many roles to get me to this point that they cannot all be listed, but all are appreciated. Keep on challenging me! I am grateful for the opportunity to be part

of the Centre for Global Metallogeny and The University of Western Australia.

Sincere thanks are owed to Grant Abbott and the Yukon Geological Survey for their continued encouragement and support of this endeavour. As well, the encouragement of my YGS colleagues Maurice Colpron, Mike Burke, Don Murphy and Julie Hunt, and the smooth

administrative support of Rod Hill and Monique Raitchey are really appreciated. As well, Lara Lewis is thanked for field assistance, GIS-support, and for filling-in-the-gaps.

A special thanks to John Mair who has always challenged me with new information, insight and ideas, and made this thesis better as a result of them.

I am grateful for the collaborations and easy exchange of information afforded me by my Tintina Gold Province colleagues and collaborators over the years: Erin Marsh, David Selby,

Julian Stephens, Dan McCoy, Tim Baker, Jim Lang, John Thompson, Moira Smith, Jim Mortensen, Paul Jensen, Rainer Newberry, Al Doherty, Roger Hulstein, Gerry Carlson, Arne

Bakke, Tom Bundtzen, Cam Rombach and Mark Lindsey.

Thanks to the UWA SHRIMP crew, Neal McNaughton, Ian Fletcher, Matt Baggott, Matt Godfrey, Natalie Kositicin, April Pickard for SHRIMP support and insight.

Cheers to the CGM post-grad crew particularly office-mates John Pigois, Amit Eliyahu as well as Louis Bucci, Brock Salier, Chris Grainger, Steve Garwin, Erik van Noort for keeping

it sweeet. Janet Thicket provided assistance when needed, kept me out of trouble with administrators, and amazingly keeps the Centre running.

Reviews and comments of this thesis by Shunso Ishihara, Edward Spooner and Bernd Lehmann are sincerely appreciated.

Finally, thanks to my wife Kelly for her ongoing support, for shouldering a greater share of parenting responsibilities, and for enduring the tasks and challenges of being married to a field geologist. Also, heaps of thanks to Cooper and Anika for putting up with a part-time Dad over

much of the past three years, and for enduring numerous flights across the planet, ..... from -40°C to +40°C.

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Chapter 1 Introduction

Chapter One

IntroductionPreamble

A connection between granitic (sensu lato) rocks and gold ores has long been recognized (Agricola, 1556). Early models were developed early in the last century in response to observations made in the European mineral districts (deLaunay, 1900; Berg 1927) or those championed by American economic geologists (Lindgren, 1913; Spurr, 1923; Emmons, 1933). Since then, links between magmatically-derived hydrothermal fluids and gold deposits have been broadly speculated, intensively studied and hotly debated, but a geochemical or isotopic “silver bullet” that can unequivocally link the two remains elusive.

The development of field-based deposit models, such as the porphyry copper model of Lowell and Guilbert (1970), reinforced by theoretical, fluid inclusion, geochemical and isotopic data, has led to almost universal acceptance that porphyry copper±gold±molybdenum deposits are genetically related to a causative, normally hosting or proximal, intrusion. Similarly, associated high-sulfidation silver-gold epithermal deposits (e.g. Hedenquist and Lowenstern, 1994) and gold-bearing skarns (e.g. Meinert, 1992) are also considered to be the products of fluids derived from cooling intrusions. Other gold-bearing hydrothermal deposits that do not show such a close and consistent relationship to granitiod intrusions are more controversial. Orogenic gold deposits are a classic example in which there is a broad spatial and temporal connection to granitoid magmatism (Groves et al., 1998; Goldfarb et al., 2001), but direct genetic relationships to specific plutons or suites of intrusions are not supported by the bulk of evidence.

Recently, a new ore deposit class that encompasses a wide range of gold mineralization styles has been introduced. This classification, known as intrusion-related gold systems (IRGS), which lack proximal base-metal associations, emphasises a genetic relationship with intrusions having a low primary oxidation state (Thompson et al., 1999; Lang et al., 2000). As such, this deposit model contrasts markedly with the wide range of intrusion-related gold deposits (e.g. porphyry systems) that are characterized by associated economic copper mineralization and that are related to oxidized plutons (Sillitoe, 1991).

The IRGS model was largely developed from observations made on gold deposits and occurrences throughout central Alaska and Yukon, which collectively comprise the Tintina Gold Province (Hart et al., 2002). Development of the IRGS classification was based on the wide-array of gold deposit styles, occurrence types and granitoids that occur throughout the Tintina Gold Province (Newberry 1995; McCoy et al. 1997; Thompson et al., 1999; Lang et al., 2000; Thompson and Newberry, 2000), as well as some global examples (Sillitoe and Thompson, 1998). The IRGS classification has been used as a template to identify other regions in the world where like-deposits may exist.


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The model has been broadly adopted, is the topic of special volumes (Tucker and Smith, 2000; Lang and Baker, 2001) and has been used as a basis for exploration of gold targets throughout the world. Despite the relative infancy of IRGS, only having been characterised circa 2000, a search on the internet search engine Google (Oct. 2004) for “intrusion-related gold”, returns approximately 600 hits.

Characteristics of IRGS Based on observations from numerous authors as well as information presented herein, the

following characteristics of IRGS are as follows.

1) Tectonic setting is best characterized as the inboard portion of a continental margin, well cratonward from the continental margin arc.

2) Associated regional metallogeny is best characterized as a tungsten, or less commonly tin, district.

3) Associated plutonic rocks comprise small plutons composed of metaluminous to slightly peraluminous, intermediate to felsic granitoid compositions.

4) The plutons mostly belong to the ilmenite-series (Ishihara, 1977), suggesting a dominantly reduced primary oxidation state, although there are local variations in redox state.

5) There is a wide-range of styles of mineralization, typically centred around a causative pluton.

6) Mineralisation has low-total sulphide contents and have limited hydrothermal alteration interpreted to result from low fluid volumes.

7) Metal assemblages are gold-dominant with anomalous Bi, W, As, Te and/or Sb, and typically have non-economic base-metal concentrations.

8) Fluid systems are dominated by carbonic hydrothermal fluids.

Concerns of Classif cationMany of the features listed above for IRGS are non-diagnostic or can be subjectively

interpreted (e.g. levels of Bi or other elements), and granitoids are common in most epigenetic mineral districts. This presents a problem in unequivocal classification. Further, the classification has been broadened by some authors with additions and modifications (e.g. Rowins, 2000; Robert, 2001), or adapted to suit other diverse geological situations where several of the factors listed above are not met (Hall et al., 2001; McCuaig et al., 2001). The classification has even been applied-to deposits in geological settings lacking granitoids because of their possible and presumed occurrence at depth.

Deficiencies in classification are expected of immature and evolving deposit models, and these have been noted by many authors (Thompson and Newberry, 2000; Goldfarb et al., 2000; Lang and Baker, 2001; Groves et al. 2003). In order to establish more robust criteria for this class of deposits, these authors note the need for: 1) improved basic

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descriptions of the styles of mineralization, 2) better constraints on the timing and nature of events at all scales, 3) improved appreciation for the tectonic and magmatic settings, 4) research on associated magmatic-hydrothermal processes, and 5) increased understanding of structural controls on these systems.

It is within this framework of understanding and mis-understanding that this thesis is founded. Thesis aims presented below are mainly directed towards points 2, 3 and 4 as listed above.

Thesis Aims and ObjectivesThe principal aims of this thesis are to: 1) develop a better understanding of the gold

metallogeny of the Tintina Gold Province and the intrusion-related gold system model; 2) establish the mid-Cretaceous plutonic framework, particularly magmatic redox conditions, of the Tintina Gold Province; 3) establish controls on metallogenic variations within the Tombstone-Tungsten Belt of the Tintina Gold Province; and 4) establish a chronology for magmatic-hydrothermal systems within the Tombstone-Tungsten Belt.

Collectively, these aims are designed to: 1) provide a better foundation upon which to understand intrusion-related gold systems; 2) highlight the role of the mid-Cretaceous magmatic history and tectonics in controlling intrusion-related metallogeny; 3) determine the effect that magma source and redox plays in controlling intrusion-related metallogeny; 4) improve our understanding of the timeframes of magmatic-hydrothermal systems; and 5) develop an improved basis for exploration targeting.

Thesis Organisation In order to achieve the four principal aims, the research herein is presented as a series

of four papers which each form a chapter. Each paper deals with a specific component of the project that contributes towards achieving the aims and objectives of the research. The first paper establishes a foundation for understanding the Tintina Gold Province (TGP) and its associated gold deposits. The second paper establishes the nature of the mid-Cretaceous plutonic episode within the TGP. The third paper establishes the petrogenetic controls and associated metallogenic variations of the three most-prolific plutonic suites that comprise the Tombstone-Tungsten Belt (TTB). The fourth paper documents the geochronology of magmatic-hydrothermal systems across the TTB, and evaluates the durations of such systems.

At the time of submission (October 2004), the papers are at variable stages within the publication process. The first two papers are published, the third is “in press” and set to be published shortly, and the fourth is “in review”. Contributions from co-authors are defined at the beginning of each chapter. Cited references are presented at the end of each chapter.

Paper 1 Tintina Gold ProvinceThis paper resulted from a presentation on the Tintina Gold Province invited by the Society

of Economic Geologists, for their first stand alone meeting in Denver, USA, in May 2002. The associated paper, “Geology, Exploration and Discovery in the Tintina Gold Province, Alaska and Yukon” was published in the Society of Economic Geologists Special Publication 9, p. 241-274, in 2002.


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Paper 2 Northern Cordillera Mid-Cretaceous Plutonic ProvinceThis paper resulted from an invitation from Shunso Ishihara who was the editor of a

Special Issue of the journal of Resource Geology. This special issue was to commemorate Dr. Ishihara’s 70th birthday and his lifetime contribution to understanding intrusion-related ore systems. The paper, “The Northern Cordillera Mid-Cretaceous Plutonic Province: Ilmenite/Magnetite-Series Granitoids and Intrusion-Related Mineralisation” was published in Resource Geology, v. 54, no. 3, p. 253-280, in 2004.

Paper 3 Source and Redox of Tombstone -Tungsten Belt IntrusionsThis paper is based on to a presentation made at the Fifth Hutton Symposium on the Origin

of Granites and Related Rocks in Toyohashi, Japan in May 2003. The paper “Source and Redox Controls of Intrusion-Related Metallogeny, Tombstone-Tungsten Belt, Yukon, Canada” will be published shortly in the Fifth Hutton Symposium Volume to be published in the Transactions of the Royal Society of Edinburgh: Earth Sciences, and jointly distributed as a Special Volume by the Geological Society of America.

Paper 4 Comparative Geochronology and Duration of TTB DepositsThis paper is based on to a presentation made at the SEG 2004 Meeting in Perth, Australia,

under the theme “Cutting Edge Developments in Mineral Exploration”. The presented paper, “Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar Geochronology of Mineralising Plutons in Yukon and Alaska; Duration of Magmatic-Hydrothermal Systems” has subsequently been submitted to a special issue of Mineralium Deposita, and is currently in the review process.

Supporting and Associated ContributionsResearch directed towards developing improved understandings in intrusion-related gold

systems in Alaska, Yukon and China has been particularly fruitful as a result of collaborations with many colleagues. Many additional unabstracted oral and poster presentations have not been listed.

Papers 1. Goldfarb, R.J., Baker, T., Dube, B. Groves, D.I., Hart, C.J.R., Robert, F., in review.

World distribution, productivity, character, and genesis of gold deposits in metamorphic terranes. Society of Economic Geologists, 100th Anniversary Volume.

2. Mair, J.L., Hart, C.J.R., and Stephens, J.R., in review. Deformation history of the western Selwyn Basin, Yukon, Canada: Implications for orogen evolution and mid-Cretaceous magmatism. Geological Society of America.

3. Stephens, J.R., Mair, J.L., Oliver, N.H.S., Hart, C.J.R., Baker, T., 2004. Structural and mechanical controls on intrusion-related deposits of the Tombstone Gold Belt, Yukon, Canada, with comparisons to other vein-hosted ore-deposit types. Journal of Structural Geology, v. 26, p. 1025-1041.

4. Selby, D., Creaser, R.A., Heaman, L.M. and Hart, C.J.R., 2003. Re-Os and U-Pb geochronology of the Clear Creek, Dublin Guch and Mactung deposits, Tombstone Gold Belt, Yukon Canada: Absolute timing relationships between plutonism and mineralization. Canadian Journal of Earth Sciences, v. 40, p. 1839-1852.

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5. Groves, D.I., Goldfarb, R.J., Robert, F., and Hart, C.J.R., 2003. Gold deposits in metamorphic belts: Overview of current understanding, outstanding problems, future research, and exploration significance. Economic Geology, v. 98, p. 1-29.

6. Marsh, E.E., Goldfarb, R.J., Hart, C.J.R. and Johnson, C.A., 2003. Geology and geochemistry of the Clear Creek intrusion-related gold occurrences, Tintina Gold Province, Yukon, Canada. Canadian Journal of Earth Sciences, v. 40, p. 681-699.

7. Hart, C.J.R., Wang, Y., Goldfarb, R.J., Begg, G., Mao, J. and Dong, L., 2003. Axi and associated epithermal gold deposits in the western Tianshan, Xinjiang, PR China. In: Tectonic Evolution and Metallogeny of the Chinese Altay and Tianshan, Mao J, Goldfarb RJ, Seltmann R, Wang D, Xiao W, and Hart C.J.R. (eds). International Association on the Genesis of Ore Deposits, London, p. 209-226.

8. Goldfarb, R..J., Mao, J., Hart, C.J.R., Wang, D., Anderson, E., and Wang, Z., 2003. Tectonic and metallogenic evolution of the Altay Shan, Northern Xinjiang Uygur Autonomous Region, northwestern China. In: Tectonic Evolution and Metallogeny of the Chinese Altay and Tianshan, Mao J, Goldfarb RJ, Seltmann R, Wang D, Xiao W, and Hart C. International Association on the Genesis of Ore Deposits, London, p. 209-226.

9. Harris, M.J., Symons, D.T.A., Blackburn, W.H., Hart, C.J.R., and Villeneuve, M., 2003. Travels of the Cache Creek Terrane: a paleomagnetic, geobarometric and Ar/Ar study of the Jurassic Fourth of July Batholith, Canadian Cordillera. Tectonophysics, v. 262, p. 137-159.

10. Selby, D., Creaser, R.A, Hart, C.J.R., Rombach C., Thompson J.F.H., Smith M.T., Bakke A.A., and Goldfarb R.J., 2002. Application of Re-Os molybdenite dating for determining distinct episodes of mineralization: absolute timing of sulfide and gold mineralization at the Fort Knox and Pogo gold deposits, Alaska. Geology, v. 30, p. 791-794.

11. Hart, C.J.R., Goldfarb, R.J., Qiu, Y., Snee, L., Miller, L.D., and Miller, M.L., 2002. Gold deposits of the northern margin of the North China Craton: multiple late Paleozoic-Mesozoic mineralizing events. Mineralium Deposita, v. 37, p. 326-351.

Extended Abstracts/Short Course Notes1. Mair, J.L., Hart, C.J.R., Groves, D.I. and Goldfarb, R.J., 2003. The nature of Tombstone

Plutonic Suite rocks at Scheelite Dome, Tintina Gold Province: Evidence for an enriched mantle contribution. In: The Ishihara Symposium: Granites and Associated Metallogenesis, GEMOC Macquarie University, July 2003, p. 222-234.

2. Mair, J.L., and Hart, C.J.R., 2003. Reduced intrusion-related gold deposits: Examples from the Tintina Gold Province. In: Orogenic and Intrusion-related Gold Deposits, Short Course notes, Centre for Global Metallogeny, Perth, February 2003

3. Stephens, J.R., Mair, J.L., Hart, C.J.R., Oliver, N.H.S., and Baker, T., 2002. Structural controls on intrusion-related gold mineralization, central Yukon Territory, Canada: comparison to other deposit styles and potential for giant deposits. In: Giant Ore Deposits Workshop, Poster Abstracts, University of Tasmania, Hobart, Australia, p. 49-53.

4. Stephens, J.R., Mair, J.L., Hart C.J.R., Oliver N.H.S. and Baker, T. 2002. A structural exploration model for intrusion-related gold mineralization in the Tombstone Gold Belt, central Yukon. In: Regional Geologic Framework and Deposit Specific Exploration Models for Intrusion-Related Gold Mineralization, Yukon and Alaska: Notes from the third annual technical meeting, S. Ebert (ed) . Mineral Deposit Research Unit, University of British Columbia p. 92-108.

5. Stephens, J.R., Oliver N.H.S., Mair, J.L, Hart C.J.R., and Baker, T. 2002. A mechanical analysis of intrusion-related gold systems, central Yukon, Canada. In. Applied Structural Geology for Mineral Exploration and Mining. International Symposium Abstract Volume, Australian Institute of Geoscientists Bulletin 36.


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6. Hart, C.J.R., McCoy, D.T., Goldfarb, R.J., Smith, M., Roberts, P., Hulstein, R., Bakke, A.A., Bundtzen, T.K., 2002. Geology, exploration and discovery in the Tintina Gold Province, Alaska and Yukon. Global Exploration 2002: Integrated Methods for Discovery, E.E. Marsh, R.J. Goldfarb and W.C. Day (eds). Society of Economic Geologists, p. 25-26.

7. McCoy, D., Newberry, R., Hart, C.J.R., Bundtzen, T., Farnstrom, H., Werdon, M., Szumigala, D., 2002. Geology, Exploration, and Discovery in the Tintina Gold Province, Alaska and Yukon. AAPG Cordilleran Section Meeting, Anchorage

8. Mair, J.L., Hart, C.J.R., and Groves, D.I., 2002. Tectonic setting of intrusion-related gold mineralisation of the Tombstone Gold Belt, Yukon, Canada. In: Giant Ore Deposits Workshop, Poster Abstracts, University of Tasmania, Hobart, Australia, p. 31-35

9. Hart, C.J.R., Selby, D., and Creaser, R.A., 2001. Timing relationships between plutonism and gold mineralization in the Tintina Gold Belt (Yukon and Alaska) using Re-Os molybdenite dating. In: 2001: A Hydrothermal Odyssey, Conference Abstracts, P.J. Williams (ed), Economic Geology Research Unit Contribution 59, p. 72-73.

10. Lindsay M.J., Baker, T., Hart, C.J.R., and Oliver, N.H.S., 2001. The structural history and mineral paragenesis of the Brewery Creek gold deposit, Yukon Territory, Canada. In: 2001: A Hydrothermal Odyssey, Conference Abstracts, P.J. Williams (ed), Economic Geology Research Unit, Contribution 59, p. 118-119.

11. Stephens, J.R., Mair, J.L., Oliver, N.H.S., Baker, T., and Hart, C.J.R., 2001. Structural control on intrusion-related gold deposits in the central Yukon Territory, Tintina Gold Belt, Canada: Implications for exploration. In: 2001: A Hydrothermal Odyssey, Conference Abstracts, P.J. Williams (ed), Economic Geology Research Unit Contribution 59, p. 191-192.

Abstracts1. Lewis, L.L., Hart, C.J.R., and Garrett, R.G., 2004. Compatible behaviour of beryllium

in fractionating granitic magmas, Selwyn Magmatic Province. Abstracts with Program, Geological Association of Canada, SS10-P08.

2. Goldfarb, R.J., Groves, D.I., and Hart, C.J.R., 2004. Overview on gold deposits in metamorphic belts – orogenic and intrusion-related deposits. Ishihara Symposium Abstract Volume, Tokyo.

3. Hart, C.J.R., Villeneuve, M., Mair J.L., D. Selby, D., and Creaser, R., 2003. Integrated geochronology of intrusion-related gold systems: Examples from Yukon & Alaska. 13th Goldschmidt Conference, Kurashiki, Japan, Geochimica et Cosmochimica Acta, v. 67, p. A136.

4. Selby, D., Creaser, R.A., Heaman, L.M., and Hart, C.J.R., 2003. Re-Os and U-Pb geochronology of the Clear Creek, Dublin Gulch and Mactung deposits, Tombstone Gold Belt, Yukon, Canada: Absolute timing relationships between plutonism and mineralization. Abstracts with Program, Geological Association of Canada, Vancouver.

5. Hart, C.J.R., Mair, J.L., Groves, D.I. and Goldfarb, R.J., 2003. The influence of source variations on redox state and metallogeny of a cross-orogen plutonic province, Yukon, Canada. The Origin of Granits and Related Rocks, Fifth Hutton Symposium, Toyohashi Japan. Geological Survey of Japan, Interim-Report 29, p. 42.

6. Marsh, E., Goldfarb, R., Hart, C.J.R., and Farmer, G.L., 2001. The Clear Creek intrusion-related gold deposit, Tintina Gold Belt, Yukon, Canada. Abstracts with Program, Geological Society of America, Boston, Mass.

7. McCausland, P.J.A., Symons, D.T.A., Hart, C.J.R., and Blackburn, W.H., 2001. Preliminary paleomagnetism and geothermobarometry of the Granite Mountain Batholith, Yukon: Uplift remanence implies minimal tectonic motion for the Yukon-Tanana Terrane. Geological Association of Canada, Program with Abstracts, St. Johns.

7


8. Selby, D., Creaser, R.A., and Hart, C.J.R., 2001. Timing relationship between plutonism and gold mineralization: Re-Os molybdenite study of the reduced intrusion-related gold deposits of the Tombstone plutonic suite, Yukon and Alaska. GAC MAC Annual Meeting, St. Johns, Newfoundland, p. 134.

9. Hart, C.J.R., Baker, T., Lindsay, M.J., Oliver, N.H.S., Stephens, J.R., and Mair, J.L., 2000. Structural controls on the Tombstone Plutonic suite gold deposits, Tintina Gold Belt, Yukon. Geological Society of America, Cordilleran Section Abstracts with Programs, v. 32, p. A-18.

10. Farmer, G.L., Mueller, S., Marsh, E., Goldfarb, R.J., and Hart, C.J.R., 2000. Isotopic evidence on sources of Au-related mid-Cretaceous Tombstone Plutonic Suite granitic rocks, Clear Creek district, Yukon. Cordilleran Section Abstracts with Programs, Geological Society of America, v. 32 p. A-13.

ReferencesAgricola, Georgius, 1556: De Re Metallica: Froben Press, Basel; translated into English by

Hoover, Herbert Clark and Lou. Henry Hoover; Mining Magazine, London, 1912; (reprinted by Dover Publications, Inc., New York, 1950).

Berg, Georg, 1927. Zonal distribution of ore deposits in Central Europe; Economic Geology, v. 22, p. 113-132.

deLaunay, L., 1900. Les variations de felons métallifères en profondeur. Reviews Géneral de Science, v. 11, p. 575-580.

Emmons, W.H., 1926. Relations of metalliferous lode systems to igneous intrusions, Transactions of the American Institute of Mining, Metallurgy and Engineering, v. 74, p. 29-70.

Goldfarb, R.J., Groves, D.I., and Gardoll, S., 2001. Orogenic gold and geologic time: A global synthesis: Ore Geology Reviews, v. 18, p. 1–75.

Goldfarb, R.J., Hart, C.J.R., Miller, M., Miller, L., Farmer, G.L., and Groves, D.I., 2000. The Tintina Gold Belt: A global perspective. British Columbia and Yukon Chamber of Mines, Special Volume 2, p. 5-34.

Groves, D.I., Goldfab, R.J., Gebre-Mariam, M., Hagemann, S.G., and Robert, F., 1998. Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationshipis to other deposit types. Ore Geology Reviews, v. 13, p. 7-27.

Hedenquist, J.W. and Lowenstern, J.B., 1994. The role of magmas in the formation of hydrothermal ore deposits. Nature, . 370, p. 519-527.

Hall, G.A., Wall, V.J., and Massey, S., 2001. Archean pluton-related (thermal aureole) gold: The Kalgoorlie exploration model. In:2001: A Hydrothermal Odyssey, P.J. Williams (ed), Townsville, Economic Geology Research Unit Contribution 59, p. 66-67.

Ishihara, S., 1977. The magnetite-series and ilmenite-series granitic rocks. Mining Geology, v. 27, p. 293-305.

Lang, J.R., and Baker, T., 2001. Intrusion-related gold systems: the present level of understanding. Mineralium Deposita, v. 36, p. 477-489.


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Lang, J.R., Baker, T., Hart, C.J.R. and Mortensen, J.K., 2000 - An exploration model for intrusion-related gold systems. Society of Economic Geologists Newsletter 40, 1-15.

Lindgren, W., 1933. Mineral Deposits. McGraw Hill, New York and London, 930 p.

Lowell, J.D., and Guilbert, J.M., 1970. Lateral and vertical alteration-mineralization zoning in porphyry ore deposits. Economic Geology, v. 65, p. 373-408. .

McCuaig, T.C., Behn, M., Stein, H., Hagemann, S.G., McNaughton, N.J., Cassidy, K.F., Champion,

D. and Wyborn

L., 2001. The Boddington gold mine: A new style of

Archaean Au-Cu deposit. In 4th International Archean Symposium, Extended Abstracts. AGSO-Geoscience Australia Record 2001/37, p. 453-455.

Meinert, L.D., 1992. Skarns and skarn deposits. Geoscience Canada, v. 19, p. 145-162.

Newberry, R. J., McCoy, D.T., and Brew, D.A., 1995. Plutonic-hosted gold ores in Alaska: Igneous vs. metamorphic origins. Resource Geology Special Issue 18, p. 57-100.

Robert, F., 2001. Syenite-associated disseminated gold deposits in the Abitibi greenstone belt, Canada. Mineralium Deposita, v. 36, p. 503-516.

Rowins, S., 2000. Reduced porphyry copper-gold deposits; a new variation on an old theme. Geology, v. 28, p. 491-494.

Sillitoe, R.H., 1991. Intrusion-related gold deposits. In: Gold Metallogeny and Exploration, R.P. Foster (ed), Blackie, Glasgow, p. 165-209.

Sillitoe, R.H., and Thompson, J.F.H., 1998. Intrusion-related vein gold deposits: types, tectono-magmatic settings and difficulties of distinction from orogenic gold deposits. Resource Geology, v. 48, p. 237-250.

Spurr, J.E., 1923. The Ore Magmas: A Series of Essays on Ore Deposition. McGraw-Hill, New York, 915 p.

Thompson, J.F.H. and Newberry, R.J., 2000. Gold deposits related to reduced granitic intrusions. Reviews in Economic Geology, v. 13, p. 377-400.

Thompson, J.F.H., Sillitoe, R.H., Baker, T., Lang, J.R., and Mortensen, J.K., 1999. Intrusion-Related Gold Deposits Associated with Tungsten-Tin Provinces. Mineralium Deposita, v. 34, p. 323-334.

Tucker, T.L. and Smith, M.T. (chairs), 2000. The Tintina Gold Belt: Concepts, exploration and discoveries. British Columbia Chamber of Mines, Cordilleran Roundup, Special Volume 2, 225 p.

9

Chapter 2 Tintina Gold Province

Chapter Two

Geology, Exploration and Discovery in the Tintina Gold Province,

Alaska and Yukon

Craig J.R. Hart1,2, Dan T. McCoy3, Richard J. Goldfarb1,4, Moira Smith5, Paul Roberts5, Roger Hulstein6, Arne A. Bakke7, and Thomas K. Bundtzen8

1Centre for Global MetallogenySchool of Earth and Geographical SciencesUniversity of Western AustraliaCrawley, Western Australia, Australia, 60092Yukon Geological SurveyBox 2703 (K-10)Whitehorse, Yukon, Canada, Y1A 2C63 Placer Dome ExplorationP.O. Box 110249Fairbanks, Alaska, 995114 U.S. Geological SurveyBox 25046, MS 964, Denver Federal Center, Denver, Colorado, USA, 802255 Teck-Cominco Ltd.600-200 Burrard StreetVancouver, British Columbia, Canada, V6C 3L96 281 Alsek RoadWhitehorse, Yukon Territory, Canada, Y1A 4T17 Kinross Gold USABox 73726, Fairbanks, Alaska, USA, 997078 Pacific Rim Geological ConsultingBox 81906, Fairbanks, Alaska, USA, 99708


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Preface to Chapter TwoThis paper represents an invited presentation on the Tintina Gold Province by the Society of Economic Geologists for their first stand alone meeting in Denver, USA, in May 2002. This associated paper, “Geology, Exploration and Discovery in the Tintina Gold Province, Alaska and Yukon” was published in 2002 in Society of Economic Geologists Special Publication 9, p. 241-274. The spelling and format for citations and references follow those that are used in publications of the Society of Economic Geologists, a USA-based organisation.

Justif cation of Authorship: This manuscript is the culmination of several field seasons and numerous site visits throughout the Yukon and east-central Alaska. The manuscript, produced in the early stages of the Ph.D. project, is co-authored by many colleagues to bring together a breadth of exploration expertise about this vast region, in particular to include knowledge of developments in Alaska, as well as Yukon, where my experience is greatest. Dr. Dan McCoy, Dr. Richard Goldfarb, Mr. Arne Bakke and Dr. Thomas Bundtzen are all experts on Alaskan geology and mineralisation and have contributed to the exploration section on Alaska. Specifically, Dr. McCoy provided information about the Ester Dome region of the Fairbanks district, Dr. Goldfarb provided information about regional geochemical surveys and general Alaskan geology, Mr. Bakke was responsible for most of the exploration at Fort Knox and provided expertise about exploration efforts there, and Dr. Bundtzen provided information about the Kuskokwin region. Dr. Moira Smith and Paul Roberts are geologists who had worked at the Pogo deposit and they provided information regarding its discovery and geology. Mr. Roger Hulstein provided information regarding exploration techniques and applications. The candidate planned and produced the whole of the manuscript, initiated and developed the models presented herein, and compiled and constructed all of the figures and tables. The co-authors, particularly Dr. Goldfarb, provided editorial comments.

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AbstractThe Tintina Gold Province (TGP) comprises numerous (>15) individual gold belts and

districts throughout interior Alaska and Yukon with a gold endowment of ~68Moz. Most districts are defined by their placer gold contributions, which comprise greater than 30 Moz, but four districts have experience significant increases in gold exploration with notable new discoveries at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz). All significant TGP gold deposits are spatially and temporally related to reduced and radiogenic Cretaceous intrusive rocks that intrude (meta)- sedimentary strata. These like characteristics are the foundation for the development of an intrusion-related gold system model. Associated gold deposits have a wide variety of geological and geochemical features, but can be categorized as intrusion-centered (includes intrusion-hosted, skarns and replacements), shear-related and epizonal. Although intrusion-related gold systems have an implied genetic relationship with a granitoid, confident links are difficult to establish for many shear-related and epizonal deposits. U-Pb and Ar-Ar geochronology has established main mineralizing events at circa 90 and 70 Ma, but precise links temporal links between magmatism and mineralization have been difficult to confirm with Ar-Ar methods, but new Re-Os molybdenite dating confirms that Fort Knox is 91 Ma and that Pogo’s Liese veins are probably 104 Ma. Notable characteristics of the TGP pertinent to exploration, such as vast, remote under-explored areas, unglaciated regions with variable oxidation depths and discontinuous permafrost, in combination with an evolving geological model, create exploration challenges that can be overcome with the pragmatic application of selected exploration techniques and strategies. Regional stream geochemistry followed-up with soil geochemistry has been most effective although the application of the intrusion-related model to areas with known mineral occurrences has also been successful.

IntroductionThe “Tintina Gold Belt” is a vast, gold-rich region of interior Alaska and central Yukon that

has a 100-year-long history and encouraging potential. The concept of a “Tintina Gold Belt” was put forward in 1997 by junior exploration company personnel in an effort to highlight the region’s 68 million ounce (Moz) gold endowment (C. Freeman and G. Carlson, oral. commun., 2001). The Tintina Gold Belt’s underlying geology is complex, and the nature of the gold deposits so variable, that no specific geologic elements precisely define it (Smith, 2000). Although gold deposits allied with middle and Late Cretaceous intrusions are a key element, some workers include more marginal elements such as Tertiary epithermal deposits, Triassic Cu-Au deposits, and the Klondike placer deposits. These considerable variations in deposit styles and ages have resulted in a poor understanding of the relationships among the deposit types.

The Tintina Gold Belt is composed of numerous distinct gold belts and districts. We therefore follow the lead of Bundtzen et al. (2000), and define the region instead, as a gold

Geology, Exploration and Discovery in the Tintina Gold Province,

Alaska and Yukon


12

province (Fig. 1). Recent exploration efforts in the Tintina Gold Province (TGP) have focussed on a new class of intrusion-related gold deposits that are spatially, temporally and perhaps genetically associated with Cretaceous intrusions. These deposits account for approximately 35 Moz of in-ground resources and a considerable, but uncertain part of the placer production. The wide-ranging styles of mineralization allied with these intrusions have been incorporated within an intrusion-related metallogenic model (McCoy et al., 1997; Thompson et al., 1999; Hart et al., 2000; Lang et al., 2000; Thompson and Newberry, 2000; Lang and Baker, 2001). The main tenet of the model is that variations in deposit styles are accounted for, according to their position with respect to a central, presumably causative, pluton, and the depth of formation. This zoned model forms the basis for regional and property-scale exploration, but as some gold deposits lack defining features of the model, their genetic associations with a central pluton are ambiguous.

In this paper, the geology of the gold belts and districts that comprise the TGP are described, with particular emphasis on the nature of mid-Cretaceous intrusion-related gold mineralization in the Fairbanks district, Tombstone gold belt and Goodpaster district, and Late Cretaceous intrusion-related ores of the Kuskokwim region. The various styles of mineralization found within these districts are described, and the intrusion-related model used to account for their formation is evaluated in this context. Exploration and discovery case histories of significant deposits are presented, and followed by exploration strategies and methods that have proved effective in dealing with the unique exploration challenges of the region.

Historical Exploration ActivityConsiderable evolution in exploration trends and concepts, in response to changing metal

prices and advances in exploration and mining technology (Fig. 2) have characterized the 100 years of mining activity in the TGP. Most exploration in the TGP since ~1990 was focussed on mineralization that is spatially associated with mid- and Late Cretaceous plutonic rocks. In

Figure 1. Tintina Gold Province is composed of numerous gold belts and districts throughout central Alaska and Yukon. Most of the districts are mainly defined by placer gold production, but many regions, notably the Kuskokwim, Fairbanks and Tombstone regions have seen high levels of exploration and concomitant discovery throughout the 1990s. KK-Kuskokwim, RP-Ruby-Poorboy, FB-Fairbanks, CL-Circle, CH-Chulitna, KT-Kantishna, Bo-Bonniville, RS-Richardson, GP-Goodpaster, EG-Eagle, 40-Forty-mile, 60-Sixymile, KD-Klondike, DR-Dawson Range, TB-Tombstone, TG-Tungsten, HY-Hyland River.

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particular, exploration in the Fairbanks and Goodpaster districts of east-central Alaska (Fig. 3), the Tombstone gold belt of central Yukon (Fig. 4), and the Kuskokwim region of southwestern Alaska (Fig. 5) are emphasized.

The Early YearsRussian prospectors explored the southern Kuskokwim region of Alaska for gold as early

as the 1830s. Tens of thousands of would-be miners infiltrated the north during the Klondike gold rush of 1898, and a series of gold rushes in the region continued until 1916. American, Canadian and European prospectors and miners panned for gold and searched for lode sources in central Yukon beginning in 1885. Tens of thousands of would-be miners infiltrated west-central Yukon during the Klondike gold rush of 1898, and branched out to interior Alaska in pursuit of the next big rush. This occurred a few years later in the Fairbanks district (Fig. 3), where placer gold was discovered in 1901 and eventually led to a series of discoveries throughout the district such that it became Alaska’s principal gold producing region during

Figure 2. a) Relative lode gold exploration and mining activity, and placer gold production trends for the Tintina Gold Province. Note that the timescale is not linear. b) Total exploration expenditures in Alaska and Yukon, with Alaskan gold exploration expenditures. Data from Buntdzen et al., 1990, Burke, 2000, and other sources. C) Average monthly price of gold.


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the early 1900s. Gold production peaked in 1909 (Parker, 1929) and was followed by the introduction of dredges in the 1920, such that 12 were in operation by 1930 within the Fairbanks district. Fairbanks district placer production remained significant even after World War II, but most dredges were inactive by the end of the 1950s.

The first hardrock mines started on metasedimentary rock-hosted, gold-bearing quartz veins in 1910 and, by 1913, ten small mills were in operation (Brooks, 1915) in the Cleary Summit and Ester Dome areas of the Fairbanks district (Fig. 3). Unlike placer mining, lode mining declined significantly during WWI. By 1930, placer gold production (3.9 Moz) far outweighed hardrock production (100,000 oz) in the district. Lode activity increased during the Great Depression of the early 1930s, and with an adjustment of the gold price from $20 to ~$35/oz in 1934. As of 1960, total Fairbanks lode gold production was 239,247 oz (Cobb, 1973). Other small interior Alaska gold lodes produced intermittently from 1911 to 1960, but were economically insignificant.

In the Yukon, early prospecting in what is now the Tombstone gold belt was concentrated in the Mayo area, with placer discoveries between 1884 and 1901 on creeks draining mid-Cretaceous plutons at Dublin Gulch, Scheelite Dome and Clear Creek (Fig. 4). Initial lode staking in 1901 was followed up by underground exploration on numerous auriferous gold veins

Figure 3. Distribution of gold districts (ruled pattern) and significant gold deposits in east-central Alaska in relation to the distribution of Cretaceous granitoids (grey fill). Much of the region between the Tintina and Denali faults is underlain by Yukon-Tanana Terrane. Also shown is the Eocene Circle Hot-Springs pluton in the Circle district.

15


from 1912 to 1916. All early gold exploration focussed on high gold-grade sulfide-bearing quartz veins, although lodes of tungsten and silver were also explored. Total early lode gold production in the Yukon was minimal.

The Modern Era Modern exploration efforts in the 1960s and 1970s focussed on base metals, and gold

exploration languished. This was due, in part, to exploration success through the development of the porphyry copper model, the advent of helicopter-supported regional geochemical surveys, and the low price of gold as fixed by the gold standard. Granitoids throughout central Alaska were evaluated for their copper and molybdenum porphyry potential. In the Yukon, the focus was mainly on tungsten following the recognition of the Yukon’s high potential (Cathro, 1969) and the subsequent discovery of the Cantung skarn adjacent to a mid-Cretaceous pluton (Fig. 4) in 1971. Fuelled by high tungsten prices, continued exploration resulted in significant scheelite-rich skarn discoveries at Mactung, Bailey, Lened, Clea and Ray Gulch, all on the margins of mid-Cretaceous plutons. Follow-up of placer cassiterite occurrences in the Mayo area resulted in the discovery of a few sub-economic lode tin occurrences.

Contemporary EffortsContemporary exploration for intrusion-related gold deposits in the TGP followed the

boom in the price of gold in the early 1980s. In Yukon, large crystal-filled vugs hosting native

Figure 4. Distribution of gold deposits, belts and mid-Cretaceous plutons across central and southern Yukon Territory. Note that most of the mineralization is preferentially associated with plutons that form the leading edge of magmatism. Not all of the deposits shown are gold, some are tungsten (W) and silver (Pb). Dawson Range deposits are Late Cretaceous.


16

gold were discovered at Emerald Lake, a copper skarn was determined to be gold-rich at the Marn deposit, and exploration focussed on large sulfide-rich auriferous veins adjacent to the Antimony Mountain pluton (Fig. 4). Using sedimentary rock-hosted gold deposits in Nevada’s Roberts Mountains allochthon as an analogy, exploration for Carlin-type occurrences in analogous Selwyn Basin strata resulted in follow-ups of geochemical anomalies and properties with As, Sb and Hg signatures (e.g., Brick, Ida/Oro). Most of this field work was in variably calcareous country rocks within the thermal aureole of mid-Cretaceous plutons. In addition, gold exploration throughout all of Canada accelerated during the “flow-through” tax-incentive financing era of 1987 to 1990, during which time the Selwyn Basin-hosted Brewery Creek deposit was discovered in the TGP.

Significant changes in exploration targeting, resulted from the recognition of a new bulk-tonnage resource at the historic Fort Knox deposit near Fairbanks (Fig. 3), and the subsequent recognition that TGP plutons could host “porphyry-style” gold mineralization (Hollister, 1992). Exploration focussed towards finding additional large-tonnage, intrusion-related gold deposits in Alaska led to the discoveries in the early 1990s at Ryan Lode in the Fairbanks district, and Vinasale (DiMarchi, 1993) and Donlin Creek (Retherford and McAtee, 1994) in southwestern Alaska (Figs. 3 & 5). Exploration was spurred by technological advances that allowed profitable recovery of gram-level gold, either through conventional milling (Fort Knox) or heap leaching operations (Brewery Creek). Exploration staff involved with the Fort Knox discovery acquired and explored similar targets in Yukon (Scheelite Dome, Clear Creek; Fig. 4), and developed a resource at Dublin Gulch’s Eagle Zone in 1994.

Exploration and discovery of sedimentary rock-hosted gold ores at True North (Fig. 3), Brewery Creek and Scheelite Dome (Fig. 4), focussed additional exploration interest out of the granitoids and into the adjacent country rocks of the TGP. The discovery of the high-grade Liese veins at Pogo (Fig. 3) in 1996 further encouraged this shift in exploration focus, and perhaps more importantly, opened a large area of east-central Alaska and west-central Yukon not previously considered prospective. Exploration in the Tungsten/Hyland River belt of eastern Yukon during 1997-2000 focussed on thermal aureoles resulted in the discovery of the Sprogge and Fer occurrences (Fig. 4).

In Alaska, expenditures on precious metals exploration increased from a low of $US 6M in 1986, to $57M in 1990 and $42M in 1998. Total exploration expenditures for gold in Alaska and Yukon since 1990 are approximately $US 500M. Considerable exploration has been undertaken by Canadian mining companies with investment capital raised through share offerings on the stock market.

Of the region’s ~68 Moz Au endowment, approximately 31 Moz were recovered from placer production (Table 1). The current bedrock resource of approximately 35 Moz has mostly been discovered since 1990. Historical lode gold production through to 1995 accounts for only 1 Moz, but more than 2 Moz have since been produced, mainly from Fort Knox and to lesser extent, Brewery Creek. Current production and resource figures for the TGP districts and belts are given in Table 1.

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Table 1. Resource data and placer gold production for Tintina Gold Province occurrences and districts.

PRE-MINING RESOURCE PRODUCTIONDISTRICT, Deposit Mt gpt total

MozPlacer Moz

LodeMoz

Note/Reference

FAIRBANKS Fort Knox* 169.0 0.93 5.4 1.8 Bakke 2000, this paper

True North* 14.6 1.69 0.79 This paperRyan Lode* 3.6 3.04 0.347 0.03 This paper

Dolphin 27.7 0.7 0.67Gil* 9.2 1.37 0.43 This paper

Cleary Summit 1.4 34 1.59 0.301 Placer 8.1 Bundtzen 1996, ref ned ounces

District total 9.25 8.1 2.1

GOODPASTERPogo, reserve 9.0 18.86 5.8 This paper, Smith et al., 2000

Richardson placers 0.10 Bundtzen and Reger, 1977

KUSKOKWIMDonlin Creek, 122.1 2.91 12.3 M,I&I, 1.5gpt cutoff; Ebert et al., 2000,

minable reserve of 57 MtShotgun 32.8 0.93 1.1 Inferred, 0.55 gpt cutoff, Rombach

Nixon Fork 0.100 42 0.17 0.183 95-99 prod’n (C. Puchner, pers. comm., 2001), pre-65 (Cutler, 1994)

Vinasale 10.3 2.4 0.8 Bundtzen and Miller, 1997Other, lode 0.066 Bundtzen and Miller, 1997

Placer 3.16 Bundtzen and Miller, 1997District total 14.3 3.16 0.2

OTHER ALASKALiberty Bell 1.0 3.5 0.125 Nerco Press Release, 1.7 gpt cut-off

Golden Zone 12.1 3.25 1.2 .0016Illinois Creek 6.2 2.2 0.6 0.06 Flanigan, 1998

Honker 0.25 30 0.25 Flanigan, 1998Placer Circle 1.00 Bundtzen et al, 1984Placer Eagle 0.05 Cobb, 1973

Placer 40-mile 0.54 Cobb, 1973Placer Bonif eld &

Kantishna0.1 Cobb, 1973

Placer Rampart 0.20Placer Hot Springs 0.5

Placer Ruby-Poorman 0.45Sub-total 2.18 2.84 0.06

TOMBSTONE BELTMarn 0.3 8.6 0.10 Brown and Nesbitt 1987Horn 0.01 30 0.10 estimate

Brewery Creek* 17.2 1.44 0.85 0.27 Oxide, global resource of 40 MtDublin Gulch 99 1.19 4.1 Smit et al., 1996, minable

resource of 50.4Mt of 0.93gpt for Placer 0.38 estimate

District total 5.1 0.38 0.27

OTHER YUKONMoosehorn/Longline 0.1 36 0.05 0.004 estimate

Klondike 16 0.05 estimatePlacer 40-mile 0.3 estimatePlacer 60-mile 0.325 estimate

Dawson Range 0.05 0.06 estimateDistrict totals 16.73 0.114

GRAND TOTALS 36.63 31.30 2.82

Notes: Total resource plus placer production equals 67.94 million ounces (Moz). Tonnages are resources calculated using a variety of cut-offs and represent various levels of uncertainty that variably include measured, inferred, and indicated estimates. Unreferenced data from various unpublished sources, press releases and web sites. *indicates minable totals of producing mines. Some totals may appear incorrect due to rounding.


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Regional Geology of the TGP Belts and Associated Gold Mineralization

The TGP consists of several gold belts and gold-rich mineral districts that primarily underlie the region between the Tintina/Kaltag and Denali/Farewell fault systems in Alaska and Yukon, as well as the region northeast of the Tintina fault in Yukon, an area of ~500,000 km² (Fig. 1). The TGP is underlain by three main tectonic elements:1) Polymetamorphosed Neoproterozoic and Paleozoic metasedimentary and metaigneous assemblages of the pericratonic Yukon-Tanana terrane (YTT), which is divisible into a number of sub-terranes and underlies much of interior Alaska and west-central Yukon; 2) East-central Yukon, north of the Tintina fault, is underlain by basinal clastic rocks and limestone that were deposited as a continental margin assemblage known as the Selwyn Basin; 3) Southwestern Alaska’s amalgamated basement assemblages are overlain by great thicknesses of weakly deformed Cretaceous strata deposited in the Kuskokwim and related flysch basins.

Gold mineralization in most belts and districts is spatially and temporally related to mid or Late Cretaceous plutonic rocks that intruded across the regions’ diverse tectonic elements. The youngest, most inboard (cratonward) portions of the gold-associated mid-Cretaceous plutonic suites intruded YTT in interior east-central Alaska and Selwyn Basin strata in central Yukon. Late Cretaceous plutonic rocks that intruded the Kuskokwim basinal strata in southwestern Alaska have associated gold mineralization.

Post mid-Cretaceous dextral transcurrent motion along the Tintina-Kaltag fault system displaced southern YTT ~450 km to the northwest, likely in response to its collision with the northern and landward edge of the Wrangellia Terrane (Plafker and Berg, 1994). This resulted in the significant amount of offset of mid-Cretaceous, gold-related plutons between the Tombstone gold belt in Yukon, and the Fairbanks district in Alaska (Fig. 1). Similarly, the Denali–Farewell fault system, with a total of ~350 km of dextral displacement, cut Kuskokwim basin strata and offset many of the ca. 70 Ma plutons and gold deposits (e.g. Miller et al., 2002).

About fifteen distinct gold districts (or belts, as used for the Yukon) define the TGP in east-central Alaska and Yukon, with a westerly continuation of the province to the numerous districts that lie within the Kuskokwim region. Most of these districts are more notable for their placer production, as their hardrock sources are insignificant, or have not yet been discovered or exploited. District descriptions below emphasize those districts with significant lode exploration activity directed towards Cretaceous intrusion-related gold occurrences.

Deposits of the Kuskokwim Mineral Belt, southwestern AlaskaNumerous gold deposits associated with Late Cretaceous intrusive rocks comprise the 550-

km-long, northeast-trending Kuskokwim mineral belt in southwestern Alaska (Fig. 5). The belt includes a ~12 Moz Au resource at Donlin Creek, with less significant resources at the Shotgun, Vinasale and Nixon Fork deposits. Approximately 3.1 Moz of placer gold has been produced from the region, as well as 70,000 oz from lodes (Bundtzen and Miller, 1997).

The mineral belt is underlain by a thick succession of Upper Cretaceous clastic strata of the Kuskokwim Group, which was deposited as an overlap assemblage upon various accreted terranes (Fig. 5). Clastic sedimentation was initiated at about 95 Ma and continued until

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near the end of the Cretaceous (Bundtzen and Gilbert, 1993; Miller et al., 2000). The strata were deformed by broad regional folding throughout the Late Cretaceous. Dextral strike-slip motion along the Denali–Farewell and adjacent parallel fault systems, has further modified the architecture of the original basin(s) and associated basement terranes (Miller et al., 2002).

Magmatism within southwestern Alaska began at about 75 Ma and continued for approximately 20 m.y., likely in response to a period of NNE–SSW compression (Miller et al., 2002). Two main suites of Late Cretaceous to early Tertiary magmatic rocks are associated with gold mineralization (Bundtzen and Miller, 1997; Miller and Bundtzen, 1994). Alkalic, metaluminous, intermediate to mafic, volcanic-plutonic complexes, with equivalent, subaerial volcanic rocks, are associated with many deposits and prospects in the Kuskokwim region. These include the granite-hosted gold veins at Golden Horn and the gold-rich skarns at Nixon Fork. Felsic to intermediate, peraluminous porphyry dikes, sills, and stocks, dated mainly between 71 and 67 Ma, are spatially associated with some of the other gold deposits in the region, including Donlin Creek.

The alkalic volcanic-plutonic-hosted gold systems, many also enriched in copper, tungsten, and boron, have been described by Bundtzen and Miller (1997). They favor classification of

Figure 5. Generalized regional geology of the Kuskokwim mineral belt, modified from Bundtzen and Miller (1997).


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these gold deposits into an alkalic porphyry Cu-Au model. Golden Horn, the most important of these systems, occurs as auriferous stockworks within a monzonite body with chlorite and dolomite as the main alteration minerals. Muscovite-biotite-quartz and sericite-ankerite-quartz alteration assemblages were interpreted to pre-date the main period of gold and sulfide mineral deposition. Where the ~70 Ma granitoids are intrude Ordovician carbonates of the Farewell terrane, copper- and gold-rich calcic skarns developed (e.g. Nixon Fork, Newberry et al., 1997). Silver-tin-tourmaline veins, stockworks, and replacements in the Kuskokwim basin range in age from about 70-60 Ma and may be related to volcanic-plutonic-hosted gold lodes and/or to slightly younger magmatism (Bundtzen and Miller, 1997).

Most of the peraluminous-intrusion-associated, gold-bearing deposits occur as quartz-carbonate sulfide veinlet swarms and stockwork systems developed in small stocks, dikes, sills and adjacent sedimentary rocks. Arsenopyrite and pyrite are the dominant sulfide minerals, although stibnite and cinnabar are not uncommon. At some locations, gold and sulfide minerals, occur as disseminations and segregations in the igneous rocks (e.g., Vinasale, DiMarchi, 1993). At Donlin Creek and Vinasale, most of the gold is chemically bound within the arsenopyrite lattice (Dodd, 1998; McCoy, 2000). In addition, numerous small epithermal Hg- and Sb-rich veins and veinlets formed throughout the Kuskokwim region at about 70 Ma (Gray et al., 1997), mainly distal to the igneous rocks.

Limited isotopic dating suggests that gold occurrences in the Kuskokwim Group are relatively restricted in age. Dating of hydrothermal micas at Donlin Creek (Gray et al., 1997; Szumigala et al., 1999) Vinasale Mountain (DiMarchi, 1993), and Shotgun (Rombach and Newberry, 2001) consistently indicate deposition of gold at about 70 Ma, which is co-eval with the initial and most voluminous period of magmatism. Fluid inclusion data from the deposits suggest deposition of gold occurred at temperatures of approximately 200-250 ºC in an epizonal environment (perhaps ≤1 km), except at Shotgun which was a much hotter deposit (350-650 °C; Rombach, 2000).

Fairbanks district, eastern AlaskaThe most economically important gold deposits in the TGP are in the Fairbanks district of

interior Alaska (Fig. 6) and include the Fort Knox, Ryan Lode and True North deposits, all of which have contemporary production. The total gold resource of these and other deposits in the district exceeds 9 Moz, with historical placer production exceeding 8 Moz.

The gold deposits of the Fairbanks district are hosted within four YTT assemblages that vary between medium and high metamorphic grades - the Fairbanks Schist and Muskox assemblage are amphibolite facies, the Birch Hill is greenschist facies, and Chatanika terrane rocks are eclogite facies. Protoliths are dominantly clastic rocks with minor volcanic and carbonate rock components. Protolith ages are predominantly early to middle Paleozoic, except for the Fairbanks Schist which may be as old as Proterozoic. Sedimentary rocks of the Chatanika terrane reached peak eclogite facies at ~210 to 180 Ma, and were exhumed and thrust over YTT rocks during the Early Cretaceous (~135 Ma) (Douglas, 1997; McCoy, 2000). Dates on metamorphic minerals yield consistent cooling ages of 110 to 102 Ma (Wilson et al., 1985; McCoy et al., 1997; Douglas and Layer, 1999). Many of the rocks are altered to chlorite and secondary albite, a feature likely related to mid-Cretaceous plutonism (K. Clautice, written comm., 1997).

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Felsic to intermediate intrusions were emplaced throughout the district, and are best exposed at Gilmore, Pedro, and Ester domes (Fig. 6), where plutons and their hornfels zones form resistant topographic highs, drainages contain gold placers, and most of the economic lode gold mineralization is recognized. These late- to post-tectonic granites and granodiorites, emplaced at 2-5 km, are characterized as I-type, ilmenite-series, calc-alkaline plutons and considered to be arc-related in origin despite their elevated initial strontium ratios (Newberry et al., 1995; McCoy et al., 1997; Aleinikoff et al., 1999). The most reliable age dates indicate that the plutons were emplaced at about 91 Ma (McCoy et al., 1997; McCoy, 2000). An Eocene thermal event associated with subaerial volcanism is recognized by low-temperature hydrothermal alteration associated with Eocene fission track dates (Murphy and Bakke, 1993) and variably reset Ar-Ar dates throughout the district.

Gold mineralization in the district is hosted in moderately- to steeply-dipping, NW-striking (earlier) and NE-striking (later) faults that are thought to have had predominantly strike-slip movement (Robinson et al., 1990; LeLacheur, 1991; McCoy et al., 1997). Subsequent motion along the Tintina-Kaltag and Denali-Farewell fault systems resulted in new and reactivated NE-trending sinistral faults that cut much of central Alaska (LeLacheur, 1991). Oblique dip-slip movement along these faults, which continues today, has resulted in the current exposure of varying levels of the plutonic systems as evidenced by in both the geology and geobarometric data (Newberry and Burns, 1999; McCoy et al., 1997).

Gold mineralization was emplaced within the plutons (e.g., Fort Knox, Dolphin, part of the Ryan Lode), proximal to the plutons (within 100s to 1000’s of meters of outer contacts; e.g., most of the Ryan Lode, most skarns) and as far as 10 km from large igneous bodies (e.g., Hi-Yu, Gil). The nature of gold mineralization varies considerably within the district from discreet,

Figure 6. General geology and mineral deposits of the Fairbanks district. Placer workings are from Robinson et al., 1990. Faults are from Newberry et al. (1996).


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high-grade, shear-hosted, sulfide-rich veins (Ryan Lode), to fracture-hosted, sulfide-poor high-grade veins (Hi-Yu), to diffuse veins or disseminated ores (True North), to widespread, low-grade sheeted vein arrays (Fort Knox), to skarn-related ores (Gil). Mineralization other than gold-rich ores includes numerous, but sub-economic tungsten skarns, and small silver- and base-metal-rich replacements and veins (Metz, 1991). Alteration associated with gold mineralization varies from common early potassic and/or albitic, to more local late quartz-sericitic alteration.

Gold deposition in the district is approximately coeval with mid-Cretaceous plutonism. Most Ar-Ar dates on micas from the intrusions give ages between 92 and 86 Ma, with the spread of data resulting from cooling or resetting (McCoy et al., 1997; McCoy, 2000) as they are younger than an unpublished, widely quoted ~92 Ma U-Pb crystallization age of the granitoids (in Bakke, 1995). Ar-Ar dates on white mica from the Fairbanks district lodes are typically at the younger end of this spread and considered to be up to 4 m.y. younger than the pluton. Recent Re-Os dating of molybdenites from a gold-bearing sheeted quartz veins, of the type that compose the Fort Knox ore body, gave a 3 point isochron age of 92.4 Ma (Selby et al., 2001; Hart et al; 2001) indicating that mineralization was probably coeval with pluton crystallization.

Zoned metal assemblages typically vary from Au-Bi,Te±W, in and adjacent to, igneous rocks, to more As-and Sb-rich where the lodes cut metasedimentary rocks. Flanigan et al. (2000) show a systematic decrease in Bi:Au ratios and Au-Bi correlation coefficents for gold-bearing veins with distance from plutons in the Fairbanks district. McCoy et al. (1997) show that this relationship holds true even within a single deposit as the intrusion-hosted ore at the Ryan Lode deposit has a higher Bi:Au ratio and a better Bi-Au correlation than the schist-hosted veins. Some workers have made efforts to include all gold mineralization in the Fairbanks district in all-encompassing magmatic models (McCoy et al., 1997; McCoy, 2000), whereas others have invoked models in which intrusion and schist-hosted mineralization had a distinctly different genesis (Metz, 1991; Goldfarb et al., 1997; 2000).

Goodpaster district, eastern AlaskaThe Goodpaster district (Fig. 7), about 150 km east-southeast of Fairbanks, has seen

increased exploration activity following the 1996 discovery of the Liese ore body on the Pogo property which is the only significant resource in the district. The Blue Lead deposit, 40 km east of Pogo, has had minor gold production. Placer gold production from the region is limited to a few thousand ounces.

The Goodpaster district is mainly underlain by silliminite-grade Neoproterozoic to mid-Paleozoic paragneiss, and Mississippian(?) and Early Cretaceous (128 Ma, U-Pb) felsic orthogneiss (Smith et al., 1999) of the YTT. These rocks were uplifted from high-P and moderate-T conditions, relatively slowly throughout the Early Cretaceous (Hansen et al., 1991) in response to an extensive Early to middle Cretaceous extensional event (Pavlis et al., 1993; Rubin et al., 1995). The region’s structural history is complex and poorly understood, with at least two periods of metamorphism and ductile deformation, the latest extending from approximately 130-110 Ma. Rocks were imbricated along low-angle faults, and subsequently dismembered along a series of numerous NE-trending fault zones that parallel the Shaw Creek fault (Fig. 7). These faults may have controlled some of the region’s gold mineralization, and influenced the present shape and level of unroofing of mineralized zones.

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Intrusive rocks are voluminous, lithologically variable and largely mid-Cretaceous in age (Newberry, 2000). They include dikes, sills, small stocks and batholiths of granite, quartz monzonite, granodiorite, quartz diorite and diorite, and are largely calc-alkaline, I-type and moderately reduced. The largest single body is the Goodpaster batholith, dominated by an equigranular, medium-grained biotite granite interior and a relatively mafic border phase, with associated pegmatitic, dioritic and granitic dikes. Mid-Cretaceous intrusive rocks at Pogo are as old as ~107 Ma (U-Pb monazite), with U-Pb dates of approximately 94.5 Ma on a diorite body and ~93 Ma on the Goodpaster batholith also recorded (Smith et al., 1999).

Gold mineralization at Pogo occurs in a 2- to 3-km-wide, 15-km-long WNW-trending zone a few kilometers south of the southern margin of the Goodpaster batholith. This “Pogo Trend” includes thick, shallowly dipping, auriferous quartz veins, stockwork and sheeted vein zones, steeply-dipping, shear-hosted quartz veins and quartz-stibnite veins and breccias. The most significant mineralization is the Liese zone which comprises two or more tabular, gently-dipping, shear-hosted quartz veins (L1 and L2) averaging 7 m in thickness, has an estimated gold resource of 8.96 tonnes @ 18.86 g/t (Smith et al., 2000).

The Liese veins are partially hosted in a zone of 107 Ma granitic dikes, and alteration associated with the Liese zone is overprinted by alteration related to a diorite dyke dated at 94.5 Ma (U-Pb zircon). 40Ar/39Ar cooling ages on hydrothermal biotite and sericite typically return ages of ~91 Ma or younger, thus suggesting a protracted or multi-phase thermal history (Smith et al., 1999). Recent direct dating of molybdenite in the quartz veins by Re-Os methods (D. Selby, pers. commun.2001) suggests an age of mineralization of 104 Ma. Re-Os molybdenite ages of two other occurrences in the “Pogo Trend” are ~95 Ma.

Figure 7. Regional geological setting of the Goodpaster district showing relation of gold occurrences (stars) with batholiths of mid-Cretaceous (red fill) and of Late Cretaceous or Tertiary (pink fill) age.


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The Blue Lead and Grey Lead mines, ~40 km ESE of the Liese Zone at the eastern end of the “Pogo Trend”, had limited historical gold production from small, discontinuous, high-grade gold-quartz veins and stockworks along a gneiss and granodiorite contact. Auriferous veins have massive to vuggy white quartz with arsenopyrite, pyrite, stibnite, boulangerite and jamesonite, and have an association with Pb and Sb which is unlike the Liese zone mineralization.

Tombstone Gold Belt, central YukonGold deposits and occurrences in the Tombstone gold belt of Yukon include the Brewery

Creek deposit with about 270,000 oz of past production, the Dublin Gulch deposit with a geological resource of ~ 4 Moz, and various placer deposits with about 500,000 oz of gold production. The mineral occurrences are coincident with the broad arc-like trend of the 550-km-long Tombstone plutonic suite that spans central Yukon (Fig. 4). Plutons, and associated gold mineralization, are mainly in Neoproterozoic to Mesozoic continental margin strata of the Selwyn Basin. Middle Jurassic and younger northerly-vergent folds and thrusts deformed all units into the current ENE-trending fold belt geometry of the northern Selwyn fold belt (Gordey and Anderson, 1993). This includes the thrust-bounded assemblages marked by the Dawson, Tombstone, and Robert Service faults. A broad east-trending, shallow-dipping zone of highly-strained lower greenschist-grade psammitic rocks, known as the Tombstone strain zone, cuts the thrust faults (Murphy, 1997).

Several suites of mid-Cretaceous granitoids, known collectively as the Selwyn Plutonic Suite, were emplaced throughout the deformed Selwyn Basin strata (Anderson, 1983, 1987, 1988; Gordey and Anderson, 1993) and are divisible into the (from oldest to youngest) Anvil, Hyland River, Tungsten, south Lansing, and Tombstone suites (Mortensen et al., 2000). The gold-related Tombstone plutonic suite (TPS) consists of more than 110 stocks and plutons which are typically small (~5 km²) to medium (~100 km²). The TPS granitoids are generally medium -grained, to porphyritic, hornblende-biotite granodiorite and quartz monzonite, although they vary from quartz-poor alkalic rocks (tinguaite, syenite) to mafic-rich (pyroxenite, diorite) to quartz-rich, two-mica granitoids. Aplite and pegmatite occur locally. Geochemically, most of the TPS rocks are metaluminous, calc-alkaline, I-type with highly fractionated peraluminous muscovite- or tourmaline-bearing phases. Most TPS plutons have a low primary oxidation state, high initial Sr ratios (> 0.71; Armstrong, 1988) and heavy δ18O values (14-16 ‰; Marsh et al., 2001) that indicate a significant crustal component.

Gold occurrences are in the plutons, in the hornfels zones, and in structural zones or reactive lithologies as far as 20 km from major (exposed) plutons (Hart et al., 2000). Gold mineralization includes intrusion-hosted, sulfide-poor sheeted quartz veins (Dublin Gulch), sulfidized epizonal fracture networks and breccias in sills (Brewery Creek), sulfide replacements of variably calcareous strata (Scheelite Dome), well-developed, high-grade, sulfide-rich skarn (Marn, Horn), weakly-developed retrograde skarns (McQuesten, Mike Lake), and fault-hosted sulfide-rich Au-As-Sb veins (AJ, Hawthorne). Tungsten±Au skarns are common, and placer gold commonly occurs in creeks draining TPS plutons in unglaciated regions. Other significant deposit types include tungsten skarns, Sn-Ag greisens, and Ag-Pb veins.

The TPS intrusions are well-dated and consistently yield U-Pb ages of 92±2 Ma (Murphy, 1997). Although there are few published dates on the gold mineralization, the available

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analyses are mainly coincident with magma crystallization. At Clear Creek, 40Ar-39Ar dates of ~ 91 Ma have been reported on micas in gold-bearing veins (Marsh et al., 2001) and U-Pb dates at Emerald Lake overlap both 40Ar-39Ar cooling dates on the magmatic body and on gangue minerals in quartz veins at about 93 Ma (Coulson et al., in press).

Factor analysis of mineralized rock samples from the Clear Creek and Scheelite Dome each indicate that there are two geochemically distinct, but paragenetically-related, metal suites associated with gold. The dominant association is characterized by Au-Te-Bi ± W, As, which is also characteristic of the Fairbanks district (McCoy et al., 1997). The weaker association of Ag-Pb-Bi ±-Sb, Cu, Au, is more characteristic of Ag-Pb-Zn mineralization of the Keno Hill district located a few tens of kilometers to the east. Bismuth and Te correlate with gold in the more deeply-formed deposits, such as Clear Creek and Dublin Gulch (Marsh et al., 1999; Hitchens and Orssich 1995; Smit et al., 1996; Flanigan et al., 2000; Maloof et al., 2001), whereas Sb and Hg do so in the more shallowly-developed deposits such as Brewery Creek (Diment and Craig, 1999).

Other Districts The Ruby-Poorman district (Fig. 1), in west-central Alaska about 500 km west of

Fairbanks, has produced about 500,000 oz of placer gold. Geology is dominated by the Ruby batholith, a 115-110 Ma, coarse-grained, composite, peraluminous body with trace element and radiogenic isotope compositions suggestive of crustal melt derivation (Arth et al., 1989). South of the Kaltag fault and west of the Ruby batholith, several Au-Ag-As-Sn-Zn prospects and small deposits are associated with plutonic rocks of the same age and composition as the Ruby batholith. Deposits in the Illinois Creek area are the only lode deposits that have reported resource and production. These deposits include gold- bearing quartz veins, Zn-Pb-Ag-Au-Sn-rich replacement zones in carbonate rocks, and the highly-oxidized auriferous Illinois Creek shear zone that has produced ~56,000 oz Au and 220,000 oz Ag. All of the deposits have elevated concentrations of Bi and Te, but have significantly higher Ag/Au and Sn/Au ratios than do the other mid-Cretaceous intrusion-related deposits of interior Alaska (Flanigan, 1998).

Located between the Fairbanks and Goodpaster districts, the Richardson district (Fig. 3) consists of scattered gold placers with a total production of about 100,000 oz, and one past lode producer, the Democrat deposit, which produced ~2,000 oz Au (Bundtzen and Reger, 1977). The district is underlain by rocks of the YTT that are similar to those in the Goodpaster district. The Democrat deposit occurs in an argillically-altered and densely-veined rhyolite porphyry dike that is dated at ~90 Ma by 40Ar/39Ar on white mica (McCoy et al., 1997). Mineralization in the district appears to be at least partly controlled by the northwest-trending Richardson lineament (Bundtzen and Reger, 1977).

Better known for its VMS potential, the Bonnif eld district located 100 km south of Fairbanks (Fig. 3), is underlain by Devonian metamorphic rocks of the YTT, which include significant felsic metatuff as well as carbonaceous and variably calcareous phyllite. Metamorphism is greenschist facies. The district has historic placer production of about 75,000 oz, and only 7,000 oz of lode gold, all from the Liberty Bell gold mine , which is a replacement-style deposit adjacent to a mid-Cretaceous intrusion (~ 92 Ma K-Ar; J. Dilles, written comm., 1993). Mineralization consists of pyrite, arsenopyrite, pyrrhotite, native Bi and Bi±Pb±Sb


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sulfosalts, although the pluton itself also contains anomalous gold associated with an ankerite-tourmaline stockworks and fault-hosted veins (Suleyman, 1994).

The Kantishna district, about 170 km southwest of Fairbanks, has a total placer production of approximately 90,000 oz Au derived from creeks that drained small lode deposits. Approximately 8,000 oz of gold and 33,000 oz of silver were mined from lodes prior to 1980 (Bundtzen, 1981). Most lodes are within a 60 km-long, northeast-trending fault zone. The district is underlain by greenschist facies metapelite and amphibolite facies metavolcanic and metasedimentary rocks of the Spruce Creek sequence that correlate with the Paleozoic schists of the Fairbanks district (Newberry et al., 1996). Intrusive rocks in the district include hypabyssal plutonic rocks of intermediate composition, felsic quartz-feldspar porphyry of unknown age and affinity, and Eocene mafic dikes and plugs. Bundtzen (1981) identified three types of shear zone-hosted gold veins: 1) arsenopyrite- (scheelite)-Au quartz veins (i.e., Banjo, Spruce Creek); 2) galena-sphalerite-tetrahedrite-siderite veins (i.e., Quigley Ridge); and 3) massive stibnite-pyrite-quartz±Au veins (i.e., Slate Creek). Two gold-bearing arsenopyrite-scheelite-quartz veins in the schist yielded 40Ar/39Ar dates of ~90 Ma whereas hydrothermal mica associated with a galena-sphalerite-tetrahedrite-siderite vein was~68 Ma (McCoy, 2000).

The Chulitna district, located approximately halfway between Fairbanks and Anchorage in south-central Alaska (Fig. 1), is underlain by Permian to Jurassic volcanic and sedimentary rocks (Wrangellia terrane) and Jurassic-Cretaceous flysch of the southern Alaska Range. Gold deposits in the Chulitna district are primarily associated with hypabyssal, intermediate to felsic, 75-65 Ma reduced porphyritic plutons (Hawley and Clark, 1974; Swainbank et al., 1977). The Golden Zone is the district’s only significant deposit, and produced minor amounts of Ag, Au, Cu, and Pb. Recent drilling, however, has delineated a resource of approximately 600,000 oz Au, in a near-vertical quartz monzodiorite-hosted breccia pipe consisting of altered igneous rock fragments cemented by quartz-carbonate-sericite-sulfide. Potassium-Ar ages of ~70 Ma for primary biotite and secondary white mica indicate that Golden Zone mineralization was essentially contemporaneous with plutonism. Gold mineralization is directly associated with Bi-Te minerals, predominantly occurring as inclusions in arsenopyrite. The Au-As-Bi-Te metallogeny is similar to that seen in deposits throughout southwestern and east-central Alaska and the Yukon. However the presence of economically significant copper and latest Cretaceous dates suggest a closer association with deposits of the Kuskokwim region. (Gage and Newberry, 2001).

The Circle district, 90 km northeast of Fairbanks (Fig. 3), produced more than 1 Moz of placer gold since the original discovery in 1893 (Masterman, 1991). To date, the Circle district has had no reported lode production but recent exploration activity has discovered several occurrences, including Table Mountain, Joker and Portage Creek. The district is underlain by polymetamorphic schists of the YTT, that are intruded by two plutonic suites. The first is composed of intermediate to felsic calc-alkaline intrusions, similar to other gold-related plutons of the TGP. The largest of these, the Two-Bit pluton, forms the uplands above many of the highly productive placer creeks in the area and yields maximum 40Ar/39Ar ages of 89 Ma (McCoy et al., 1997). The predominant Au-As±Bi±Te±Zn occurrences, however are either within or peripheral to smaller plutons and dikes.

The second plutonic suite consists of highly fractionated, variably peraluminous, alkalic to

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very felsic syenomonzonite and granite plutons with accessory garnet and fluorite. This 58-50 Ma suite hosts sub-economic Sn-Ag±Bi±U mineral occurrences (Masterman, 1991; McCoy et al., 1997). The Eocene magmatic event, particularly the emplacement of the large Circle Hot Springs intrusion, has mainly reset many dates from earlier calc-alkaline plutons and associated occurrences resulting in complicated interpretations and an impression that the district is principally early Tertiary in age (McCoy et al., 1997). Despite Early Tertiary dates from the various gold occurrences, the geological attributes of the Table Mountain, Hope-Homestake, Joker and Portage Creek occurrences are more similar to those with a mid-Cretaceous plutonic association.

The Klondike, Forty-mile, Sixty-mile, Eagle districts, which occur along and near the Yukon-Alaska border, are principally placer gold producers, with subordinate lode production and no current reserves. Placer production from the Klondike district has been more than 13 million crude ounces of gold, but only about 50,000 oz of lode production is known. Forty-mile (combined Yukon and Alaska sides), Sixty-mile and Eagle placer production is about 800,000, 330,000 and 50,000 oz respectively. Each of these three districts is underlain by juxtaposed sub-terranes of the YTT and obducted ophiolite sequences of the Slide Mountain/Seventy-mile terrane. Although the Klondike region is cut by Late Cretaceous and Eocene dikes and small plugs of felsic magmatic rocks, Mesozoic plutonic rocks are otherwise lacking. The gold lodes, which have unequivocal characteristics of orogenic gold deposits, have few available dates, notably 134 and 140 Ma K-Ar dates on muscovite from one of the Klondike vein occurrences (Rushton et al., 1993).

The Forty-mile district contains numerous Late Triassic to Early Jurassic plutons and the nature of the gold mineralization is variable, possibly indicating several hydrothermal episodes (Newberry, 2000). The nature of gold mineralization in the Sixty-mile district is also variable, although some gold is probably related to Late Cretaceous intrusions (Glasmacher and Friedrich, 1992). The Eagle district has no significant lode resource, but has several small lode prospects and has had minor placer production. Several occurrences throughout the Eagle, Sixty-mile and Forty-mile districts were interpreted to suggest the presence of: 1) 184 Ma intrusion-related Au; 2) mid-Cretaceous intrusion-related Au and/or W; 3) Late Cretaceous Cu-Mo-Au, and 4) Eocene epithermal Au-Hg mineralization (Newberry et al., 1997).

The Dawson Range belt, in west-central Yukon (Fig. 4) hosts mainly porphyry copper and associated epithermal gold occurrences, and numerous small placer deposits. Episodic lode production has generated about 50,000 oz Au, whereas the placer production exceeds 200,000 oz. The Dawson Range is mainly underlain by YTT metamorphic rocks intruded by metaluminous I-type, mid-Cretaceous batholiths, and variably alkalic Late Cretaceous plugs. Both suites have a high primary oxidation state. Notable gold occurrences are at Mount Freegold, Mount Nansen, and the large, low-grade Casino Cu-Au-Mo porphyry. In westernmost Dawson Range near the Alaskan border, the Moosehorn Ranges area host numerous gold veins, such as the Longline deposit, and associated residual placers. Unlike most Dawson Range occurrences, the Longline veins occur in a structural array and have characteristics similar to veins associated with orogenic gold deposits such as banding textures in the veins, high Au:Ag ratios, low base-metal grades, and coarse gold. The veins contain arsenopyrite, but are not anomalous in bismuth.


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Like the Tombstone belt, the Tungsten and Hyland River belts of eastern Yukon overlap with the areas containing mid-Cretaceous plutonic rocks that intrude Paleozoic and older Selwyn Basin strata. The Tungsten suite is exposed across a 220-km-long region between the Mactung deposit to south of the Cantung deposit (Fig. 4). The plutons of this belt have lithological and geochemical similarities with the TPS, but are typically larger (to 250 km²), are more differentiated with a greater proportion of peraluminous two-mica granites, and are slightly older (e.g. ~97 to 95 Ma, Mortensen et al., 2000). Mineralization is dominated by the large Mactung, Cantung and Clea tungsten skarns with only a few small gold occurrences. The Hyland River belt of plutons (Tay River of Heffernan and Mortensen, 2000) include large intrusive masses that occupy ~ 40 % of the area (Fig. 4). The batholiths are dominated by magnetite-bearing, peraluminous, 98-96 Ma, two-mica monzongranites but the region also contains plutons of the older Anvil suite in the west, and younger Tungsten suite in the east (Heffernan and Mortensen, 2000). Associated gold occurrences include the Hyland, Fer, HY, Hit, Sun and Sprogge. All consist of structurally controlled zones of breccias, stockworks and veins with associated sulfide replacement bodies in adjacent calcareous strata. Pyrite and lesser arsenopyrite are the dominant sulfide minerals, but anomalous bismuth is reported at the Hyland deposit, which has a Main Zone oxide resource of 350,000 Moz Au. Mineralization is interpreted to lie above associated plutons as intrusive rocks are not exposed at surface.

SummaryThe numerous districts have a range of characteristics (Table 2), but most are dominated by

placer production as their hardrock sources are insignificant. Four districts have experienced significant increases in lode exploration, specifically in areas with Cretaceous intrusive rocks, with ensuing discoveries and resource definition - Fairbanks district (17 Moz), Kuskokwim mineral belt (17 Moz), Goodpaster district (6 Moz), and Tombstone gold belt (5 Moz). Metallogenically, the Fairbanks and Tombstone gold districts are also characterized by the numerous tungsten and lesser tin occurrences, whereas tin and mercury-antimony deposits characterize the Kuskokwim region. The Goodpaster district lacks a recognized metallogenic association with gold. Exploration for gold ores associated with Cretaceous intrusions is also notable in the Ruby (Illinois Creek), Kantishna (Quigley Ridge), Chulitna (Golden Zone), Bonnefield (Liberty Bell) and Circle (Table Mountain) districts in Alaska, and in the Dawson Range (Longline) and Sixty-Mile districts in the Yukon. Although most TGP districts are underlain by rocks of the YTT, the Kuskokwim region is an overlap sedimentary assemblage that is partly south of the Denali-Farewell fault system, the Chulitna district is underlain by rocks of the Wrangellia terrane to the south of the Denali fault system, and the Tombstone, Tungsten and Hyland River districts are on the ancient North American continental margin.

The tectonic setting(s) associated with emplacement of the Cretaceous plutonic suites, and formation of the gold lodes, are not well defined. Geochemical data on the intrusive rocks indicate variable settings, with tectonic discriminant plots (i.e., Pearce et al., 1984) indicating: 1) Plutons of the Fairbanks district are arc-related; 2) Tombstone gold belt plutons overlap the volcanic arc, within-plate and syn-collisional fields; and 3) the Late Cretaceous Kuskokwim intrusive rocks fall in the within-plate, syn-collisional and arc fields, although these plutons were notable effected by hydrothermal alteration (Newberry, 2000; Flanigan et al., 2000). The variability in geochemical data may partly reflect widespread alteration of many of the

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intrusions, but most likely indicates that the ca. 90-70 Ma calc-alkaline intrusions of interior Alaska and Yukon were emplaced into a variety of tectonic settings along a complexly deforming Cretaceous continental margin.

Fundamental features of gold deposits throughout most of the TGP are their associations with Cretaceous felsic intrusions that are reduced and radiogenic, and marine sedimentary or meta-sedimentary country rocks. Deposits allied with mid-Cretaceous intrusions are as old as 111 Ma (Illinois Creek deposit), or 104 Ma (Pogo deposit), but are dominantly 96 to 90 Ma (Fort Knox deposit, Tombstone gold belt). Late Cretaceous intrusive rocks and associated gold mineralization are mainly 71-65 Ma within the Kuskokwim mineral belt and Chulitna district. Most gold-associated plutons in the aforementioned districts and belts have low primary oxidation states, as indicated by low ferric/ferrous ratios (0.2 to 0.5), low magmatic susceptibilities, and ilmenite>magnetite (Newberry et al., 1995; McCoy et al., 1997; Lang and Baker, 2001). This feature appears to influence the magma’s ability to become enriched in gold during crystallization but the mechanism is poorly constrained (Thompson et al., 1999). Plutons of these districts also have elevated initial Sr and Nd ratios, radiogenic lead ratios and elevated δ18O values (Aleinikoff et al., 1987, 1999; Armstrong, 1988; Newberry et al., 1995; Newberry and Solie, 1994, 1995; Farmer et al., 2000; Marsh et al., 2001). These signatures indicate a component of crustal contamination that may play a role in their dominantly lithophile metallogeny (Thompson and Newberry, 2000). Most TGP associated Cretaceous plutons have intruded sedimentary or meta-sedimentary strata with variably reductive character and it is not known if the reduced signature of the magmas is primary or acquired feature.

Age District DepositsLate Cretaceous

71-67 Ma Kuskokwim Donlin CreekNixon ForkVinasaleShotgunGolden Horn

75-65 Ma Chulitna Golden Zone

Mid-Cretaceous 92-90 Ma Fairbanks Fort Knox

Ryan LodeGilCleary SummitEster DomeTrue North?

Richardson Democrat 90 Ma Kantishna Banjo

Slate Creek 92 Ma Bonnif eld Liberty Bell 89 Ma Circle Table Mountain

JokerPortage Creek

94-92 Ma Tombstone Brewery CreekDublin GulchScheelite DomeClear CreekMarn

96-92 Ma Tungsten/Hyland Hyland 107-104 Ma Goodpaster Pogo

Blue Lead ~111 Ma Ruby-Poorman Illinois Creek

Jurassic and older? 140 Ma and older? Klondike Sheba, Lone Star

Forty-MileSixty-MileEagle

Table 2. Districts and Deposits of the Tintina Gold Province divided by approximate ages.


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Styles of Gold MineralizationThe TGP is characterized by a wide range of mineralization styles that have spatial and

temporal relations with reduced and radiogenic Cretaceous felsic intrusions rocks. As a result of exploration and research efforts, mostly in the Fairbanks district and Tombstone gold belt, many of these styles of mineralization have been combined into an inclusive model for what have become known as intrusion-related gold systems (Thompson et al., 1999; Lang et al., 2000; Thompson and Newberry, 2000). Such deposits have the following features: 1) significant gold but uneconomic base metals; 2) characterized by lowfS2 ore assemblage; 3) association with reduced, ilmenite-bearing, I-type intrusions; 4) spatially-associated tungsten and tin occurrences; 5) a Bi-Te±W geochemical association for intrusion-hosted, and intrusion-proximal deposits, and an As-Sb±Hg signature for more distal country rock-hosted mineralization;

Despite their considerable variation in mineralization style, many intrusion-related deposits occur in a predictably zoned fashion with respect to a central, causative intrusion. Also commonly included within this model are epizonal styles of gold of mineralization, such as those that characterize the Donlin Creek deposit, and shear-related veins including Pogo’s Liese deposit. Descriptions of TGP mineralization are presented below in terms of these three main styles: 1) intrusion-centered; 2) shear-related; and 3) epizonal (Fig. 8).

Intrusion-centeredIntrusion-centered mineralization includes the range of deposit styles developed within

plutons and in the surrounding thermal aureoles, including sheeted vein arrays, skarns, and replacements, and breccias. They occur in outwardly zoned patterns with predictable variations in style, mineralogy and geochemical associations. As such, these various styles are the defining components for the intrusion-related gold system model, particularly defining characteristics of gold deposits that seemingly must be genetically associated with the central intrusion. These patterns have been fully recognized mainly with respect to the mid-Cretaceous intrusions of the TGP, although Late Cretaceous intrusions have associated skarn mineralization (i.e., Nixon Fork; see Cutler, 1994; Newberry et al., 1997). .Dublin Gulch (Fig. 9) and Fort Knox (Fig. 10) are excellent examples of intrusion-centered mineralizing systems as they have several styles of gold mineralization.

Figure 8. Schematic representation the three main styles of mineralization associated with Cretaceous intrusive rocks in the Tintina Gold Province. The genetic relationships that the various styles have with magmatism, as well as the nature of the relationships between the styles, are the focus of current research efforts.

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Sheeted VeinsSheeted vein arrays are characteristic of intrusion-related gold systems and are widely

developed at Fort Knox and Dublin Gulch, but occur to some extent in many of the mid-Cretaceous plutons of the Fairbanks district and Tombstone gold belt (see Bakke, 1995; Hitchins and Orssich, 1995; McCoy et al., 1997, Maloof et al., 2001). The steeply-dipping parallel veins are millimeters to centimeters thick, continuous for several tens of meters (Fig. 11), and typically form arrays in the apical parts of small plutons or in adjacent wall-rocks (Fig. 12).

Figure 9. Distribution of varying styles of mineralization, placer gold and placer scheelite at Dublin Gulch. Bismuthinite and scheelite has also been found in the placer concentrates. Data from Boyle (1965).

Figure 10. General geology of the Fort Knox pluton (from Bakke (1997). Sheeted veins occur throughout the pluton. The Stepovich W-Au skarn is southwest of the pluton, beyond the limits of the figure.


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The veins are dominated by massive, translucent to clear gray, and less often white quartz, with subordinate coarse-grained, white alkali feldspar and mica commonly on the margins. Total sulfide mineral content is low, varying from 0.1 to 3%, and includes pyrite>pyrrhotite>arsenopyrite with less common native bismuth, tetradymite, bismuthinite and molybdenite. Scheelite is locally common (to 1%). Many vein arrays have parallel and proximal pegmatite, aplite or lamprophyre dikes, and locally, pegmatite dikes show upward and along-strike transitions to sheeted quartz veins (Bakke, 1995). Vein density ranges from 1 to 20 per meter. Individual veins typically have grades of 3-50 gpt Au. Alteration varies from visually absent to zones 2-3 times wider than vein thickness and can include albitic or potassium feldspar selvages, carbonatization, and/or sericitization. Gold typically occurs as free-gold adjacent to or within bismuth minerals. Locally maldonite (Au2Bi) occurs, or more commonly, wormy intergrowths of Au and Bi indicate maldonite exsolution.

Variations of this style include interconnected bifurcating, white quartz veins that cut most of the sheeted vein ore at the Fort Knox deposit, but these are considerably less common at other deposits. The have a similar dominant orientation to the sheeted veins, range from hairline to several centimeters in width, and average about 15 per meter at Fort Knox (Bakke, 1995; McCoy et al., 1997). The veins also contain coarse-grained white mica, have sericitic selvages and more sulfide minerals and are characterized by quartz-sericite alteration of adjacent wall-rock. Their grades are erratic although locally very high. Bismuthinite and native gold grains often occur together whereas native bismuth and maldonite are absent (McCoy, 2000). Another variation is apparent at the Emerald Lake occurrence where parallel vein arrays link large miarolitic cavities (Duncan et al., 1997). None of the variants includes true multi-directional stockwork veins such as those that are typical of porphyry deposits.

Figure 11. Typical steep-dipping, intrusion-hosted, sheeted auriferous quartz vein array. This example is from Clear Creek area, Yukon. Marker pen is 14 cm.

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SkarnsGold and tungsten skarns occur near mid-Cretaceous plutons throughout the Fairbanks

district (Pedro, Spruce Hen, Stepovich) and Tombstone gold belt (Horn, Marn, Scheelite Dome), and with Late Cretaceous stocks in the Kuskokwim mineral belt (Nixon Fork). However, the TGP’s largest skarns are the scheelite-dominated Mactung, Cantung and Ray Gulch deposits in Yukon. Skarns specifically characterized as Au±W skarns (Au> 2g/t) occur adjacent to metaluminous calc-alkaline plutons that host sheeted vein ores. The Stepovich skarn, for example, is immediately south, and ~700 m above the Fort Knox intrusion (Fig. 6). Skarns are dominated by fine-grained aggregates of pyroxene, biotite and less commonly garnet. Ores occur as sulfide-rich assemblages characterized by pyrrhotite ± chalcopyrite ± bismuthinite ± scheelite that overprints a retrograde silicate assemblage (Allegro, 1987; Newberry et al., 1997). Some Au±W skarns contain significant arsenopyrite, such as the Table Mountain skarn (McCoy et al., 1997). The Ray Gulch tungsten skarn is adjacent to the Dublin Gulch pluton (Fig. 9), but lacks retrograde alteration and gold (Brown, 2001).

Gold±Cu skarns, such as the Horn, Marn (Brown and Nesbitt, 1987) and Mike Lake occurrences (Lilly, 1999) are often adjacent to alkalic plutons of the western Tombstone gold belt (Fig. 4). Skarn assemblages are dominated by coarse-grained hedenbergitic pyroxene, almandine garnet and wollastonite. The overprinting sulfide mineral assemblage is coarse-grained and dominated by massive pyrrhotite with ~2% chalcopyrite, 0.5% bismuthinite and free gold with fluorite, and locally scapolite. Gold grades are typically high (8-40 gpt Au). On the other side of the TGP, the Au±Cu skarn zones at Nixon Fork appear as vertical pipes that are zoned from garnet cores, through garnet-pyroxene zones, to wollastonite-idocrase-scapolite rims. They show a spatial association with granite porphyry dikes that cut the main Late Cretaceous quartz monzonite to quartz monzodiorite stock (Newberry et al., 1997).

Skarns with weakly developed, fine-grained calc-silicate assemblages are typically distal to plutonic contacts. The Gil deposit, for example, located ~ 9 km east of the Fort Knox deposit (Fig. 6), is dominated by variably retrograded pyroxene, amphibole, chlorite, calcite and epidote (Bakke et al., 2000). Gold mineralization hosts unevenly distributed amounts of pyrrhotite and

Figure 12. Generalized cross-section of the Dublin Gulch Eagle Zone. Distribution of sheeted veins are schematic but emphasize their position near the apex of the pluton adjacent to, and locally beyond, the wall-rock contracts. Modified from Smit et al., (1996) with drill-intersection from Hitchins and Orrsich (1997)


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arsenopyrite, lesser chalcopyrite and variable scheelite. Gold associated trace elements include Bi, Te±Mo±W±As (Bakke et al., 2000). However, much of the Gil skarn assemblage contains quartz veins similar to those in the Fort Knox, and they may carry the gold. The McQuesten and Aurex skarn occurrences in the Tombstone gold belt, consist of gold-enriched horizons in weakly developed skarn assemblages about 12 km from the nearest exposed pluton. Positive correlations of gold with bismuth and tungsten are evident, and arsenic anomalies are also common.

Replacements, Disseminations & Breccias Non-skarn-bearing mineralization styles within the hornfelsed aureole, either above or

adjacent to a mid-Cretaceous pluton, include variable replacements of and disseminations in calcareous and non-calcareous hornfels, and hydrothermal breccias. Polymetallic replacement bodies with elevated gold values have been recognized at the Cheechako and Nordale occurrences in the Fairbanks district. Disseminated to semi-massive sulfide replacements of calc-phyllite in the hornfels zone at Scheelite Dome are dominated by pyrrhotite and lesser auriferous arsenopyrite are widespread over several kilometers and cut by several generations of thin quartz-sulfide veins (Fig. 13; Mair et al., 2000; O’Dea et al., 2000). Replacement-style mineralization at the enigmatic Liberty Bell deposit in the Bonnifield district consists of arsenopyrite, pyrrhotite and bismuthinite with associated tourmaline-K-feldspar alteration near mid-Cretaceous granite dikes (Suleyman, 1994). The occurrence of rare garnet and epidote has encouraged this deposit to be classified as a skarn (Newberry et al., 1997). Decalcification and silica replacement of sandstone was reported in Brewery Creek’s North Slope zone (Diment and Craig, 1999).

Breccia-hosted mineralization is recognized in thermal aureoles in the Tombstone gold belt. Structural breccias occur in faults and at fault intersections at several locations, most notably at the Scheelite Dome (O’Dea et al., 2000) and Clear Creek (Stephens et al., 2000) occurrences. Magmatic-hydrothermal breccias are not common. The best such example may be the Bear Paw zone at Clear Creek. This is a clast-supported breccia dominated by country rock schist fragments with lesser granite fragments in a dark matrix of milled rock fragments

Figure 13. General geology of the Scheelite Dome area indicating the extensive gold mineralization outside of the pluton in, and beyond the thermal aureole (from Mair et al., 2000).

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and fine-grained tourmaline(?) and chlorite(?) that contains up to a few percent pyrrhotite, pyrite and chalcopyrite (Stephens and Weekes, 2000). A quartz matrix is locally present. Fractured, veined, brecciated and tourmaline-altered hornfels with quartz veining and sporadic gold values and anomalous Au, Bi, As and Sb outcrop at the Red Mountain and Black Hill occurrences near the town of Mayo (Murphy, 1997). In the Chulitna district, the Golden Zone deposit appears as a copper- and gold-rich breccia pipe, which has been suggested to be the top of a gold-rich porphyry system (Swainbank et al., 1977).

Shear-related oresThe most-common style of mineralization throughout the TGP ranges from large quartz

bodies in ductile shear zones, to brittle, gouge-filled sulfide-rich anastamosing shears, to disseminated ores along and adjacent to shears. Most of these ore-types are simply veins hosted in country rocks at some variable distances from, or locally within, Cretaceous intrusions. The most significant vein resources are in the Ester Dome and Cleary Summit regions in the Fairbanks district, and at the Pogo property in the Goodpaster district. The genesis of these ores is less clear than the above intrusion-centered mineralization systems as their origins may relate to a specific plutonic body or, in contrast, to a more regional process.

The approximately two dozen or so significant veins on Ester Dome are mainly within a 40 km² area and include the Ryan Lode, Grant, McQueen, Mohawk and Ready Bullion which are small past-producers, as well as the more recently discovered Silver Dollar and Rhyolite occurrences (Fig. 14). All but the southern Ryan Lode are hosted in schist. The veins vary from NNE- to ENE –trending, with the Ryan Lode and Ready Bullion in parts of more regional, NE-trending shear zones, whereas the other veins are mainly in secondary faults related to left-lateral shears (LeLacheur, 1991; Cameron, 2000). The shear zones generally exhibit brittle features with associated clay-rich fault-gouge, fault breccias or cataclasite. Associated sulfide

Figure 14. General geology and distribution of veins, placers and faults on Ester Dome. Veins from Robinson et al., 1990 and unpublished data. Faults from Newberry et al., 1996. 1-#2 Grant; 2 Stanford; 3-Alder; 4-Elmes; 5-Kevin’s Dream; 6-Dorothy; 7-Yellow Eagle; 8-Silver Dollar; 9-Lincoln; 10-Killarney; 11-Farmer; 12-McQueen; 13-St. Jude; 14-St. Paul; 15-Clipper; 16-Wandering Jew; 17- Mohawk; 19-McDonald; 20-Borovich; 21-Hudson; 23-Maloney; 24-Lookout. Dark yellow areas represent mined placer deposits as shown on Robinson et al. (1990).


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minerals include arsenopyrite and pyrite, with late stibnite. Some veins, such as at the Ryan Lode, have associated zones of diffuse fracture-controlled, low-grade mineralization.

Past-producing deposits in the Cleary Summit area include those developed on the Hi-Yu, Christina, Cleary Hill, McCarty and Chatham veins. These veins combine to form an 8-km-long array dominated by NW-trending and lesser NE-trending veins in schists of the YTT, which follow along the northern limb of the ENE-trending Cleary anticline (Fig. 15). The veins, mostly occurring as open-space fractures, are dominated by massive white and ribbon quartz, have high gold grades (~10 g/t) and have a variable sulfide mineral content. Gold-related phases include arsenopyrite, stibnite, jamesonite, and lesser tetrahedrite, boulangerite, pyrite, galena and rare bismuth-bearing minerals (Metz, 1991; Walsh and Rao; 1991; McCoy et al., 1997).

The Liese zone veins at Pogo occur as two or more, gently-dipping, parallel and tabular polyphase quartz bodies in gneiss that range from 1- to 20-m-thick (Smith et al., 1999). The earliest stage of mineralization is characterized by narrow shear veins that are largely ductile in nature, having formed during reverse motion along low-angle mylonite zones. Where the vein margins are not extensively sheared, extensional sheeted veins are present in the hanging-wall. Distinct zones of early quartz are granular with interstitial sericite or microcline (Moore, 2000). These are exploited by the much more voluminous main stage veins, which consist of white massive to banded quartz veins with pyrite-arsenopyrite bands that were emplaced during a change to brittle deformation and normal motion along the host structures (D. Rhys pers. comm., 2000). These veins are cut by narrow, steeply-dipping, sulfide-rich breccias. The Liese veins contain ~3% ore minerals, including pyrite, pyrrhotite, löllingite, arsenopyrite, chalcopyrite, bismuthinite, molybdenite, galena, sphalerite, various Ag-Pb-Bi±S minerals, maldonite, native bismuth, and native gold. Early biotite and later quartz-sericite stockwork and sericite-Fe-dolomite alteration are spatially associated with the Liese zone.

Shear-hosted veins at the Longline occurrence in western Dawson Range district in westernmost Yukon, occur within a structurally-controlled array hosted by mid-Cretaceous granodiorite (Ritcey et al., 2000). Veins are as thick as 1 m and dominated by massive and multiphase, coarse-grained white quartz. Many veins are locally vuggy, banded, have ribbon textures or rarely, sheared. Sulfide minerals, totaling less than a few percent, include

Figure 15. Distribution of veins in the Cleary Summit area. 1-Eagan; 2-Hi-Yu; 3-Basham; 4-Whitehorse; 5-Mizpah; 5-Too Much Gold; 7-McNeil; 8-Ohio-Mayflower; 9-Stringer; 10-Pennsylvanian; 11-Antimiony; 12-American Eagle; 13-Henry Ford; 14-McCarty; 15-Nordale; 16-Ebberts; 17-Pioneer; 18-Spirit; 19-Chatham; 20-Christina; 21-Quemboe; 22- Foster Hungerford; 23-Jupiter-Mars; 24-Blue Moon; 25-Rex; 26-Anna Mary; 27-BP; 28-Wyoming; 29-Cleary Hill; 30-Colorado; 31-Wackwitz. Modified from Robinson et al., 1990. Position of cross-cutting faults approximated from Newberry et al., 1996.

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arsenopyrite, galena, sphalerite, stibnite and boulangerite. Gold occurs as free grains to 2 mm or as infillings in sulfide minerals, mainly arsenopyrite.

In the Tombstone gold belt, arsenopyrite- and stibnite-rich veins, such as the AJ (Antimony Mountain) and Hawthorne (Scheelite Dome) veins, are country-rock hosted and within a few hundred meters of the intrusion, and are probably not unlike veins on Ester Dome in the Fairbanks district. However, silver-rich galena veins, such as the Peso and Rex near the Dublin Gulch deposit, are typically several, to several 10s of kilometers from the nearest exposed pluton. The silver veins of the Keno Hill district, which have produced > 200 Moz Ag, are 10 to 40 km from the nearest exposed pluton. Stibnite forms pods in fault zone-hosted quartz veins at the Scrafford, Frederich and Gilmer deposits in the Fairbanks district, and continuous gold-rich veins with documented antimony resources at the Stampede and Slate Creek deposits in the Kantishna district.

Quartz-filled shears in the Fort Knox intrusion form moderately-dipping zones of brecciated and granulated white quartz that range in thickness from 0.3 to 1.5m and are continuous for hundreds of meters (Fig. 10). The shears contain micron-sized gold associated with bismuthinite or bismite that add considerably to the total resource of the Fort Knox deposit. Intense phyllic alteration also overprints argillic alteration on vein margins and clay gouge, but may be related to post-gold hydrothermal alteration or supergene oxidation. The crushed nature of the quartz results from small (< 3m) displacements along the shears.

Epizonal Mineralization Some of the most important TGP deposits, with considerable gold resources, have features

compatible with emplacement at low temperatures and pressures. Notable examples of these high-level, perhaps best termed epizonal deposits include the True North (Harris and Gorton 1998; Bakke et al., 2000), Brewery Creek (Diment and Craig, 1999; Lindsay et al., 2000) and Donlin Creek (Ebert et al., 2000) deposits. All are characterized by structurally-controlled fracture networks of thin quartz-sulfide veinlets and matrix-filling breccias. The deposits are hosted in carbonaceous strata or dike/sill systems that have intruded the sedimentary rocks. The veins are locally drusy or composed of fine-grained quartz, and the sulfide mineral assemblage is dominated by arsenopyrite and pyrite with late stibnite. In rocks susceptible to alteration, it is most often characterized as weak phyllic, dominated by the formation of illite, weak silicification and disseminated pyrite. The deposits are also characterized by a similar metal assemblage of Au-As-Sb-Hg (Flanigan et al., 2000). In contrast to the more deeply-formed intrusion-centered and shear related ores, concentrations of Bi and W in the epizonal ores are typically below analytical detection limits.

Structurally, Brewery Creek (Fig. 16) and True North (Fig. 17) consist of several ore-bodies controlled by secondary structures (and their intersections) in the hanging-wall of a regionally significant thrust faults. Both Brewery Creek and Donlin Creek appear to have experienced a degree of extension following compression. Most Donlin Creek ores are structurally-controlled, as they are hosted in rigid intrusive rocks with contrasting rheology to the adjacent shales (L. Miller, oral commun., 2000). High-grade zones occur in more densely fractured rocks. Where unoxidized, gold is mostly in solid solution with sulfide minerals (even in extremely high-grade samples), either in arsenopyrite or in arsenian rims surrounding pyrite (Diment and Craig, 1999;


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Ebert et al., 2000; McCoy, 2000). Oxidation has liberated the gold in near surface zones at True North and Brewery Creek to depths of about 30m, leaving untested sulfide resources at depth.

Summary and implications for ore genesisEach of the three broad mineralizing styles has been included in the intrusion-related

gold system model. The differences between them reflect different conditions of formation (Thompson and Newberry, 2000). However, the nature of each style’s genetic links with magmatism are not always evident, particularly for the shear-related and epizonal examples. Links are typically characterized by numerous geological features, but where ambiguous,

Figure 16. General geology of the Brewery Creek area.

Figure 17. General geology of the True North area, Alaska. Modified from Bakke et al. (2000).

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geochronological or geochemical constraints are highlighted.

Intrusion-centered mineralizing systems typically have intrusion-hosted sheeted vein arrays as a central tenet. These systems have adjacent gold or tungsten skarns where suitable host-rocks are available. Within the thermal aureole, gold-bearing replacements, disseminations, more distal skarns and breccias occur. The variability of these occurrences reflect lateral variations in temperatures and changes in nature of the fluids away from a central pluton, with additional complexity supplied by variations in structural controls and reactive host-rock lithologies. These various styles of gold mineralization that are associated with mid-Cretaceous plutons in the TGP are the basis for the intrusion-related gold model (Fig. 18).

Sheeted vein arrays have clear geological links with magmatism, as identified by the transitional nature of the auriferous sheeted veins with pegmatite and aplite dikes (Bakke, 1995; McCoy et al., 1997). K-spar, mica, and tourmaline as gangue minerals, as well as sodic and potassic alteration likely indicate a magmatic fluid source. Temporal links between mineralization and magmatism as provided by Ar-Ar dates, indicated a gap of a few million years (McCoy et al., 1997) but new Re-Os dates on ore-stage molybdenite at Fort Knox indicate that ore formation may have been co-eval with granitoid crystallization (Hart et al., 2001). Skarn mineralization has a clear spatial and temporal association with the intrusive rocks, however as gold deposition in skarns typically results from a later, overprinting fluid (Meinert, 1998), the direct link between the mineralizing fluid and a particular magma is generally not-straightforward. The diverse styles of gold-bearing sulfide replacements, disseminations, breccias and distal skarns are developed within the thermal aureoles and as such have a consistent spatial relationship to magmatism. Limited data suggest that these replacement deposits at the most distal parts of the intrusion-centered model occurred at still relatively high temperatures (380-450°C; McCoy et al., 1997; Suleyman, 1994; J. Mair pers. comm., 2001) or in association with K-feldspar alteration. Importantly, intrusive host rocks are not the mineralizing magmas as they have cooled enough to be brittley fractured. More likely, mineralizing fluids were sourced from distal, or underlying plutons.

Shear-related gold veins occur at varying distances from a pluton or thermal aureoles, but are locally within the intrusions. As such, their genetic relationships with the nearest, or roughly coeval, pluton is difficult to ascertain (Sillitoe and Thompson, 1998). The gangue and sulfide mineralogy of shear-related veins purported to be intrusion-related are highly variable and differ significantly from intrusion-hosted sheeted vein arrays. In general, shear-hosted veins within the country rocks have higher percentages of sulfide minerals, lack bismuthoids and scheelite, as well as gangue K-feldspar and micas. This may relate to differences in wall-rock chemistry such that some elements (e.g. antimony, arsenic, mercury) are locally derived from the country rock sequences. Zoning from intrusion-proximal As to distal Sb to more distal Pb-Ag veins, although not well documented throughout the TGP, is consistent with expected metallogenic variations with decreasing fluid temperatures, particularly if these veins are considered part of the intrusion-centered model described above. Many vein sets however, such as those at Ester Dome or Cleary Summit, lack notable zoning. Liese veins have been interpreted to be a deep variety of reduced intrusion-related gold mineralization where the lack of zoning may result from the lack of significant thermal gradients between the causative intrusion and the site ore deposition.


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Pogo’s Liese veins are a notable exception to many of the aforementioned shear vein generalizations as it’s gangue and sulfide mineralogy are similar to sheeted veins. The L1 and L2 shear veins host bismuthoids and scheelite, and have mica and K-feldspar in the gangue. Elevated bismuth content or Bi:Au correlations of shear-related veins, has been suggested as an indicator of a magmatic fluid source (McCoy et al., 1997; Flanigan et al., 2000) however, alternative fluid sources may obtain such a signature through interaction with granitic rocks or through enriched country rocks (Goldfarb et al., 2000).

Confident and precise timing constraints of shear vein formation have been difficult to obtain as argon dates on fine-grained alteration minerals such as hydrothermal white mica give inconsistent results, due in part to partial resetting, and absolute ages of pluton crystallization are not well established by U-Pb methods. Two Ryan Lode Ar-Ar plateau ages of alteration white micas are discordant by one-and-a half million years at 87.6 and 89.1 Ma, and are variably younger than the ~90.4±0.6 Ma dates from the adjacent pluton (McCoy et al., 1997) for which U-Pb data is not available. Circa 91 Ma Ar-Ar dates on alteration minerals from Pogo’s Liese veins are approximately coeval with the nearby 92 Ma Goodpaster batholith (in Smith et al., 1999), but significantly younger than recently reported Re-Os molybdenum dates of ~104 Ma (Selby et al., in review). The uncertain reliability of the Ar-Ar dates for most shear-related veins makes it difficult to determine temporal relationships between mineralization and magmatism, and thereby assert a temporal association.

Many shear-related veins in the TGP (e.g. Liese zone, Longline, many at Cleary Summit) have features, such as banded sulfide minerals, ribbon quartz, tensile vein arrays, ductile shears, lack of zoning etc.. - that are characteristic of orogenic gold deposits (i.e., Groves et al., 1998). Additionally, many veins lack clear spatial or temporal associations, or zoning relations with a

Figure 18. A geological model of intrusion-hosted, proximal and distal styles of mineralization that develop around mid-Cretaceous plutons. Modified from Hart et al. (2000); Lang et al. (2000); and Lang and Baker (2001).

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central mineralizing pluton and are therefor difficult to accommodate within an intrusion-related model. However, since some of these veins also have features or spatial-temporal associations with magmatism, (e.g. Liese, Ryan Lode) a magmatic source cannot be entirely discounted. The Liese veins for example, may simply reflect a deeper level manifestation of the intrusion-related model (Thompson and Newberry, 2000).

The Brewery Creek, True North and Donlin Creek deposits have characteristics consistent with mineralization formed at low temperatures and pressures. These epizonal deposits are considered to represent higher-level manifestations of the intrusion-related gold model, in response to lower temperatures, and potentially greater wall-rock influence (Thompson and Newberry, 2001; Lang and Baker, 2001). Evidence linking epizonal mineralization to a magmatic source is incomplete. Potentially causative plutons for the True North deposits are more than 3 km away and igneous rocks within the deposit are limited to a few sparse, but altered, dikes. Although the Brewery Creek and Donlin Creek deposits are hosted mainly in sills and dikes, they are passive structural hosts to mineralization with potential source intrusions for the fluids yet to be identified. Brewery Creek ores are however, within a few kilometers of an intrusion-hosted sheeted vein complex (Classic zone) and may provide a link between the two styles, but this hasn’t been proven. Magmatism and mineralization, however, are essentially coeval at Donlin Creek (Buntdzen and Miller, 1997; Szumigala et al., 1999). Age dates for Brewery Creek and True North mineralization are not available.

Exploration and Discovery Case HistoriesExploration and discovery case histories are presented for the most significant intrusion-

related gold deposits of the TGP, to emphasize the effectiveness of various exploration techniques. They are presented according to similarities of deposit type in order to assist the reader in recognizing trends and themes between them.

Fort KnoxGold mineralization was known at the current Fort Knox deposit (Fig. 10) for almost 100

years before large-scale mining. This most recent discovery represents the development of a historic prospect into a bulk-tonnage deposit. Placer gold was discovered in 1901 at the mouth of Fish Creek, downstream from what is now the Fort Knox mine, but was considered too meager to be economically worked. Subsequent headwater prospecting for lode deposits in the and tributaries of Fish Creek resulted in the discovery of tungsten- and gold-bearing skarns. Gold-bearing quartz veins were discovered in 1913 over what is now the Fort Knox mine, and a small mill was assembled and three short shafts were sunk. U.S. Geological Survey (USGS) geologists then noted the occurrence of bismuthinite, and determined the presence of anomalous tellurium and a genetic connection with a body of porphyritic biotite granite (Prindle, 1913).

Modern work wasn’t undertaken until 1980, when the high gold price encouraged placer mining in the region, and bismuthinite nuggets containing abundant gold were discovered. Follow-up prospecting indicated that gold mineralization was not limited to a high-grade source, but was widespread. There are no published regional geochemical surveys from the area surrounding the deposit prior to initiation of a resource evaluation in 1987. Sporadic exploration efforts by various joint ventures continued until 1992, when Amax Gold, Inc.


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obtained the property. By the end of 1992, 427 holes had been drilled (317 RVC, 110 core) for a total meterage of nearly 80 km, with a 183 m average depth by the end of 1992. RC chips were panned and heavy mineral assemblages were recorded. Construction and pre-strip mining began in 1995 with production initiated in November 1996. A 1998 year-end calculation, incorporating the data for the past production, gave a reserve of 169 Mt averaging 0.93 g/t. Total gold production to the end of 2001 is approximately 1.8 Moz.

Soil sampling successfully delineated the ore-body in the early stages of exploration. The C-horizon soils, obtained using augers, provided the best geochemical response and defined a zone of >100 ppb Au that is approximately coincident with the ore-hosting pluton (Bakke, 1995). Hollister (1991) noted that bismuth in soils over the deposit was an obvious pathfinder, but that arsenic was not useful. This is due partly to the deposit’s low arsenopyrite concentration, and that the resulting low level arsenic anomalies are too diffuse to provide meaningful information. Correlations of Au with Bi, Te and frequently Mo and W were recognized but are too inconsistent to be dependable pathfinder elements (Bakke, 1995; McCoy et al., 1997). Bismuth and tellurium show identical distribution patterns to gold, when all three are abundant. Subsequent bulldozer trenching of anomalous zones confirmed the effectiveness of soil geochemistry and facilitated geological mapping. The Fort Knox orebody was not responsive to a ground magnetometer survey as a uniform low magnetic response exists over the intrusion, host-rocks and mineralized area. However certain units within the country-rock schists, particularly the amphibolite and some skarn horizons, show a strong magnetic response and the data are useful in reconnaissance exploration and mapping. Because of the success of the soil geochemistry, other geophysical techniques have not been widely applied.

Dublin GulchThe discovery of the Eagle Zone deposit at Dublin Gulch resulted from of the application

of the Fort Knox geological model to a region of analogous geology (Fig. 9). Similar to Fort Knox, the Dublin Gulch area consists of a small barely-exposed mid-Cretaceous pluton, associated historical lode gold prospects, a tungsten skarn, and placer gold deposits with considerable scheelite. In addition, native bismuth, galena and arsenopyrite were reported as heavy minerals in placer concentrates (Boyle, 1965). Episodic lode exploration since 1901 focussed on the thick (0.2 –1.5 m) sulfide mineral-bearing quartz veins. The Ray Gulch skarn deposit (5.4 Mt of 0.82 % WO3) attracted exploration interest beginning in 1978.

Seven stream silt samples collected by the Geological Survey of Canada (GSC, 1990) from a 5.5 x 1.5 km area that surrounds the Dublin Gulch pluton were variably anomalous in gold, arsenic, and tungsten (Fig. 19). Gold values were as high as 299 ppb, but are typically ~ 30 ppb with arsenic mostly between 300 and 1,300 ppm. Tungsten values are between 40 and 250 ppm, and are most likely derived from the scattered skarns. Half of the samples had anomalous antimony values from 6to 50 ppm. There were no analyses for bismuth or tellurium, but bismuth-bearing minerals were reported from placer concentrates, as was cassiterite.

Recognizing the similarities with Fort Knox, Ivanhoe Golfields Ltd., optioned the Dublin Gulch property in 1991. They targeted a region of anomalous surface soil samples (>50 ppb Au) that had been identified but not explored by previous exploration programs. Six excavator trenches over soil anomalies exposed quartz-arsenopyrite-rich quartz veins on both sides of an

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intrusive contact and weak gold mineralization within the intrusion. The region was recognized as a cupola, and a 1991 HQ drill hole in the intrusion intersected 105.2 m of 1.06 g/t Au (Hitchins and Orrsich, 1995). Subsequent core and RC drilling discovered the newly-defined Eagle Zone. Gold values in resulting core samples correlate well with bismuth, but not other metals (Hitchins and Orrsich, 1995). The Eagle Zone deposit has a flat aeromagnetic response.

Brewery CreekThe Brewery Creek deposit was a geochemical discovery. There were no known placer or

lode occurrences in this area, and plutonic rocks had not been previously mapped in the area. While exploring for Carlin-like deposits in the region in the fall of 1987, Noranda Exploration Co. Ltd., followed-up a weak mercury geochemical anomaly generated from the National Geochemical Reconnaissance stream silt geochemical survey data (Geological Survey of Canada, 1978). Using follow-up soil geochemistry, with subsequent trenching and reverse circulation drilling, the original discovery was made, and the positions of additional deposits in eight zones extending over a strike length of 12 km resulted from a similar methodology. Following changing ownership, development work and permitting, Viceroy Resource Corporation poured their first gold bar in November, 1996 and reached full production by May, 1997. Production from 1997 to 2001 was approximately 260,000 oz.

Samples from the GSC regional stream silt survey were analyzed for only a few metals in 1987, but indicated that the Brewery Creek area was regionally anomalous in mercury. Most of the ten closest samples to the deposit were characterized by values of >200 ppb Hg (Fig. 20). This led to a followed-up company silt sampling survey in the Brewery Creek region and new samples generated anomalous gold values. Analysis of the Geological Survey of Canada samples for additional metals (GSC, 1990) indicated that the single sample draining what would become the Brewery Creek deposit, contained 38 ppb Au, 27 ppm As, and 16ppm Sb. The gold value was the highest concentration within a 1,000 km2 area. In addition, many of the nearest stream sediments samples were also anomalous in antimony (>7 ppm). Regionally, the 27 ppm As value is not anomalous, but is elevated with respect to those within a 25 km radius.

Figure 19. Regional stream silt geochemical values for the Dublin Gulch area. Listed values are Au/As/W, all in ppm except gold in ppb. All sample are anomalous in both gold, arsenic and tungsten. Values on other locations are for gold only. Bd=below detection.


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Soil samples taken along a ridge above the anomalous silt sample, and returned values >2,000 ppb Au. Subsequent work outlined a 3.3-km-long, east-trending gold-in-soil anomaly, with 25-2,000 ppb Au and a uniform As:Au ratio of ~1000:1. Anomalous antimony and widely dispersed mercury anomalies were also uncovered. Trenching uncovered a south-dipping body of altered and brecciated intrusive rock with one chip sample averaging 3.1 g/t Au over 40 m. With limited outcrop on the property, soil geochemistry continued to be the primary exploration method for much of the life of the mine. Soil geochemistry was particularly effective as an exploration tool, because the region is mainly unglaciated and thus there are no overlying materials, except for local loess accumulations. Gold-in-soil anomalies were evaluated by excavator trenching, chip sampling and grid reverse-circulation drilling. Ground magnetic and induced polarization-resistivity surveys were undertaken with variable success, but the deeply weathered oxide ores are generally not responsive to most geophysical methods.

True NorthTrue North is an example of a historical discovery that has relied heavily on soil

geochemistry. Trenching has also been valuable, as few natural outcrops occur on the property, and the nature of the oxidized mineralization remained ambiguous until recently. Placer gold was first discovered on the south side of Pedro Dome in 1902 and placer mining of Dome and Eldorado Creeks continued through much of the first part of the century. Prospecting of hard-rock sources led to the mining of small amounts of gold and antimony from quartz veins at the Soo and Hindenburg deposits, as well as development of numerous small shafts on other prospects. Exploration of the northern part of what is now the True North deposit, in the late 1960s and 1970s, resulted in the discovery of five mineralized zones. Contemporary exploration was initiated in 1990 when a four-hole, 1000’ program by Amax Gold Inc. yielded encouraging

Figure 20. Regional geochemistry of the Brewery Creek area, undertaken prior to the discovery of the deposits, which are approximated in black. Values are Au/As/Sb/Hg, all in ppm except Au and Hg in ppb. Values from GSC, 1990. Sample responsible for discovery underlined. Larger dotes denote anomalous antimony. Although gold anomalies are scarce, the region has an elevated Sb values. The high Hg values reflect the anomalously high background values of the Earn Group. Patterned region denotes intrusive rocks. Region of Tintina Trench and Klondike River valley is underlain by thick alluvium (white). Dark shaded region is underlain by Earn Group black shales. All other areas are underlain by assorted Selwyn Basin clastic strata.

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results, property expansions and acquisitions. La Teko Resources Inc. acquired the property in 1993 and carried out exploration including adding 241 drill holes. In 1995, Newmont became a joint-venture partner, and aggressive integrated exploration resulted in the doubling of the resource to 42 t Au (Harris and Gorton, 1998).

The most successful exploration method consisted of grid-based, hand-held power auger soil sampling, which was used to acquire material from the bedrock interface (Harris and Gorton, 1998). Thicker, or frozen cover materials, required the use of ATV-mounted auger drills to obtain reliable samples. Samples were typically screened to–80-mesh, fire-assayed for gold, and analyzed for pathfinder elements by ICP. Soil geochemistry was followed by excavator or bulldozer trenching and chip sampling. Follow-up included short RC drilling. Chip samples were logged for lithology and integrated with high-resolution aeromagnetic data to construct bedrock geological maps. This resulted in the recognition of stratigraphic and structural controls on gold mineralization.

Aggressive drilling resulted in the discovery of several additional zones. For example, five RC holes drilled northwest of the Central Zone extended shallow, gently dipping, oxide mineralization ~120m beyond the previous limit. A ~1000-m-long, linear, but discontinuous, gold-arsenic soil anomaly within the Dome Creek Zone anomaly was explored with three RC-holes intersected partially oxidized mineralization, with gold values as much as ~5 g/t Au over 6 m.

Donlin CreekThe discovery of the Donlin Creek deposit resulted from follow-up work above historic

placer gold fields in southwestern Alaska. Placer gold was first discovered in the area circa 1909 and has been worked episodically until recent times. Lode mineralization was discovered in the 1940s, but exploration wasn’t initiated until 1974, when quartz vein samples yielded anomalous gold and were followed-up with bulldozer trenching. A rock and soil sampling program, undertaken by Calista Corp. from 1984 to 1987, was followed by bulldozer trenching, and channel, rock and soil sampling. RC and core drilling outlined several mineralized regions and identified a resource of ~230,000 oz Au. Greater than 100,000 m of core and RC drilling by Placer Dome, has resulted in the definition of a 12.3 Moz resource, although that figure will change following recent drilling by Novagold.

Despite the region’s poor exposures, prospecting of the rubble crop along ridge crests has been effective in identifying regions of altered intrusive rocks and mineralization (Ebert et al., 2000). Soil sampling, supplemented with auger sampling to obtain near-bedrock samples, is effective with values >250 ppb Au indicative of mineralization (Ebert et al., 2000). Aeromagnetic data show a subtle magnetic depression that defines the region of rhyodacite dikes, and associated alteration, which host the ores. The low magnetic contrasts between rock units (6 nT) was enhanced by removing the polar gradient to develop a residual signature, which results in better detail (Ebert et al., 2000). The IP chargeability yields broad anomalies, but aids in geological interpretations, whereas resistivity data are less interpretable. Coincident magnetic lows and gold-in-soil anomalies are the best near-surface targets.

Regional geochemical sampling prior to extensive trenching, was undertaken by the U.S.G.S. Minus-80-mesh stream sediment and panned, heavy-mineral concentrate samples were collected


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in small-order tributaries above their junctions with the placer mines on Donlin Creek, but only a few hundred meters downstream from the current Donlin Creek orebody (Gray et al., 1988). Analyses by semi-quantitative emission spectrography, with relatively high lower determination limits (e.g. 0.5 ppm Ag, 200 ppm As, 10 ppm Au, and 100 ppm Sb) yielded few favorable results. But, subsequent analysis of the same stream sediments by ICP by Motooka et al. (1988) gave values of 110 ppm As on American Creek, 100 ppm As on Queen Gulch, and 41 ppm As on Snow Gulch. These ICP data are effective at targeting Donlin Creek-style mineralization. There were no ICP analyses for Au, Te, and Hg, and no anomalies for Ag, Bi, Sb and W, which had high lower detection limits (0.3, 6.3, 10, and 9.1 ppm, respectively). Re-analysis using atomic absorption techniques (Hopkins et al., 1991) indicated that the two samples with highest arsenic by ICP, also contained anomalous gold at 11 and 35 ppb – this latter sample also contained 1.6 ppm Hg. However, tellurium was less than the 50 ppb lower limit in all eight sediment samples from the Donlin Creek area. In summary, stream sediments with >40 ppm As and >10 ppb Au, with less consistently >0.2 ppm Hg, may best target favorable regions in the Kuskokwim flysch sequence for Donlin Creek-like mineralization.

Surprisingly, heavy-mineral concentrate samples were unsuccessful indicators of potential targets at Donlin Creek. Two such panned samples contained 2,000 ppm Sn, which could reflect a high background in the dikes that host the Donlin Creek deposit, or some form of contamination. Given the dominance of gold-bearing arsenopyrite and stibnite in the deposit, the lack of anomalous gold, arsenic and antimony in the concentrate samples is surprising. The fine-grained nature of these minerals may have resulted in their being washed away during the panning process.

PogoDespite a telegraph line along the Goodpaster River valley since the early 1900’s, surface

exposures of large (2 m diameter) quartz boulders with visible gold lay undiscovered less than 1 km away. Results from a late 1970s U.S. Geological Survey regional geochemical survey of the Big Delta quadrangle also indicated the presence of the “concealed” Pogo deposit. About two dozen sites surrounding the subsequently discovered mineralization at the head of Pogo and Liese Creeks and on Shawnee Peak were sampled. Materials included minus-80-mesh and heavy-mineral-concentrates of stream sediment. For anomaly enhancement, the non-magnetic fraction of the heavy-mineral-concentrates were analyzed and, in addition to the raw stream sediments, oxalic acid-leached residues of the sediments were studied. The creek draining the northwestern side of the Liese veins (sample #668; O’Leary et al., 1978) is characterized by some of the most anomalous values from the entire regional survey with 0.1 ppm Au in minus-80-mesh sediments, 500 ppm As in the leachates, and 10,000 ppm W and 2,000 ppm As in panned concentrates. The arsenic and gold values are most significant, as numerous other concentrate samples from the quadrangle also contained as much as 1% W with no Au. The lack of anomalous bismuth in the concentrates is surprising considering its enrichment in the ores, but this may partly reflect the high lower-determination limit for the element using the emission spectrographic analytical techniques.

A geological resource of the two gently-dipping tabular L1 and L2 veins was estimated at > 4 Moz Au by late 1997. Mineralization at Pogo was originally discovered in 1981 by WGM, Inc. by follow-up prospecting of multi-element Au-W-As stream sediment anomalies

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that led to the discovery of high-grade quartz float near Pogo and Liese creeks. A decade later, exploration funding was secured through a Teck and Sumitomo Metal Mining Co. joint venture and subsequent soil sampling and prospecting, resulting in the identification of a several-km² Au-in-soil anomalies. Three drill holes were drilled in 1994, two of them hit what would later be recognized as the L1 vein (Fig. 21; Roberts et al., 2001). Originally interpreted as a series of steeply-dipping quartz veins, by late 1996 the intersections were re-interpreted as a shallowly-dipping body that was confirmed in 1997, along with the discovery of a second zone at greater depth. A geological resource for the two zones (L1 and L2) was estimated at >4 million oz of gold by late 1997, and an underground exploration program commenced after the 1999 season.

Subsequent property-scale exploration has relied on traditional exploration methods. Helicopter-assisted stream sediment sampling is the most effective “first pass” sampling technique (Roberts et al., 2001). Analysis of the fine sand fraction, collected from relatively high-energy environments, was more effective than a standard silt sample. Bulk-leach extractable gold (BLEG) and panned, heavy-mineral concentrate samples have also been effective. Stream sediment samples exhibiting multi-element anomalies, particularly As, W, and Bi, were followed up with prospecting and contour soil sampling. Soil sampling is effective, but as B-horizon soils are poorly developed, C-horizon samples are taken, sometimes using augers, particularly in regions with overlying loess (Roberts et al, 2001). Samples were analyzed for gold by fire assay, and a full suite of other elements, in particular, Bi, Te, As, Ag, but also Mo, Sb, Cu and Pb, which are locally useful pathfinder elements. Use of a low detection limit package that includes Bi and Te is considered critical. Geological mapping employed geophysical, air photo, field traverses, soil pit logging. Alteration is difficult to map in the field due to the weathered rocks, but sericite-dolomite alteration is recognized as the best indicator of proximity to mineralization. Diamond drill targets are selected using a number of indicators, including the presence of extensive, >100 ppb gold-in-soil anomalies, multi-element soil anomalies, and surface exposures of large, gold-bearing quartz boulders.

Figure 21. Surface projections of the Liese 1 and 2 zones with locations of drill hole collars, and current underground workings. Modified from Smith et al. (2000) and TeckCominco website. Stream sediment values of 35 and 200 ppb Au were determined for Liese and Pogo creeks in 1981. Discovery holes drilled in 1994 are plotted with their significant intersections. Data from Roberts et al. (2001).


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Exploration Methods & Targeting in the TGP

Pluton Distribution, Character and LithologyRegionally, gold occurrences that comprise the Tombstone gold belt are associated with the

most inboard (cratonward) limit of mid-Cretaceous plutons (Fig. 4). Plutons of the Fairbanks district, which are obviously prospective, likely represent the western continuation of the inboard plutons of the Tombstone belt. Regional delimiting criteria, however, cannot be applied to the vast plutonic masses of Cretaceous intrusions in east-central Alaska (including the Goodpaster district (Fig. 3) and Late cretaceous intrusions of southwestern Alaska), partly because mapping and age dating are not sufficiently detailed.

The Fort Knox and Dublin Gulch deposits are in small (~1 km²), barely unroofed plutons, where their cupolas are not entirely eroded away (Figs. 9 & 10). As such, the apical portions of partly unroofed, or even totally unroofed small plutons (up to a few km²) appear to be better targets for intrusion-related gold systems than larger plutons (e.g. Mustard, 2001). Elongate plutons, such as those at Fort Knox and Dublin Gulch, with associated sub-parallel dikes, may be indicative of structural control on both emplacement and focussing of hydrothermal fluids.

Lithological features and compositions of granitoids may indicate gold favorability. McCoy et al. (1997) identified porphyritic granodiorite and quartz monzonite plutons as being empirically more prospective. Prospective plutons may show signs of magmatic volatile production such as aplites and pegmatites, miarolitic cavities, tourmaline-bearing phases or veins, pneumatolitic textures and greisens, and unidirectional solidification textures. Similarly lithogeochemical characteristics could differentiate productive from barren systems, but existing data on mineralized systems indicate considerable variability. Fairbanks district plutons allied with gold mineralization (Newberry et al., 1990; McCoy et al., 1997) typically follow a calc-alkaline differentiation trend and have normative granodiorite and granite compositions. They are metaluminous to weakly peraluminous and plot dominantly as I-type. Plutons of the Tombstone gold belt have large variations in compositions with clinopyroxene, hornblende, biotite or biotite-muscovite as dominant mafic phases. Some plutonic phases are as mafic as pyroxenite and gabbro. However, gold-associated plutons in the western Tombstone gold belt are variably alkalic, as are some gold-associated plutons in the Kuskokwim district. Though highly altered, Late Cretaceous igneous rocks at Donlin Creek plot as granites and granodiorites have S-type characteristics, are peraluminous and have elevated radiogenic signatures (compiled data of Newberry, 2000). However, in all cases and in contrast to classic porphyry deposits, the mineralized plutons result from fluids generated elsewhere, and as such, any particular rock features or geochemical compositions of a specific granite suite may not be controlling factors in mineralization, but that gold enrichments may be controlled by other magmatic factors.

Using primary oxidation state and alkalinity indices, Leveille et al. (1988) suggest that reduced plutons are more favorable gold targets, and favorable plutons in eastern interior Alaska were identified (Burns et al., 1991). However, elsewhere plutons that are classified as part of the intrusion-related gold deposit group, of both Phanerozoic (e.g. Mustard, 2001) and Precambrian (e.g. Robert, 2001) age, are clearly oxidized. Thus, the oxidation state of the intrusion is a critical local controlling parameter on gold favorability within the TGP

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Elemental Associations Trace element abundances, and correlations with gold, reflect the temperature of formation

and the nature of the host rocks. Deposits with elevated Bi,Te±W,Mo,Sn concentrations formed at high temperatures and are characterized by feldspathic alteration, low sulfidation state (indicated by pyrrhotite-native bismuth-loellingite) mineralization as seen in intrusion-hosted early sheeted veins (Dublin Gulch), bifurcated vein networks (Fort Knox) and stockworks (Shotgun). At Pogo, geochemical data indicate strong correlations between Au, Bi and Te, and weaker correlations among Au, Ag, and As. The presence of As may reflect scavenging from sedimentary host rocks. Deposits with high As, Sb±Hg concentrations are hosted in carbonaceous sedimentary rocks and have features characteristic of lower temperature epizonal deposits (Brewery Creek, Donlin Creek).

Placer-Related TargetsLocalities upstream from placer gold occurrences are obvious targets. In the Tombstone gold

belt, the placer deposits have spatial relationships with intrusion-related gold systems at Dublin Gulch, Scheelite Dome and Clear Creek. However, in the Fairbanks area, despite >8 Moz of placer gold from the region, placers associated with the largest deposits (Fort Knox and True North) are minor, suggesting that the majority of gold may be coming from other deposit types (e.g. shear-related gold deposits), or from deposits that were completely eroded. Evaluating placer concentrates, or sampling heavy mineral concentrates for Bi-bearing minerals or scheelite may be beneficial, as was the case in placers draining both Fort Knox (Bakke et al., 1995) and Dublin Gulch (Fig 9; Boyle, 1965). However, placers containing scheelite, or cassiterite are typically too widespread to be useful deposit-scale pathfinders, and may be useful only as regional indicators.

Placers containing stibnite, cinnabar and arsenopyrite may be indicative of a proximal epizonal occurrence, as all are found in placers draining the Donlin Creek deposit (Ebert et al., 2000). The True North and Brewery Creek deposits did not generate placers as their gold is chemically bonded to sulfide minerals and even when liberated by oxidation, is too fine-grained. Gold from intrusion-hosted deposits like Fort Knox and Dublin Gulch is typically fine-grained, and of higher fineness than schist-hosted deposits in the same regions. Petrographic and trace element analysis of placer gold may aid in determining deposit style or proximity to source (Newberry, written comm., 1998; Knight et al., 1999). Placers draining intrusions, but lacking magnetite, may indicate a reduced source pluton. However, the lack of placer gold should not be a deterrent to lode exploration as Pogo and Fort Knox have only a limited placers, and much of the eastern Yukon has been glaciated and stripped of whatever placer gold occurrences may have existed.

Structural TargetingRegional structural targeting has not yet proven to be an effective exploration criterion, and

only limited success has been achieved at a property scale. In the Fairbanks area, the dominant regional faults are terrane bounding thrust faults, and a series of ENE-trending faults that locally contain shear-zone hosted mineralization (Fig. 6, 14; Newberry et al., 1996). Many of these structures are post-ore faults that juxtapose different structural levels. Discriminating between the syn-mineral and post-mineral faults is challenging, but has resulted in exploration successes


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on Ester Dome (Cameron, 2001). As a whole, NE-, NW-, N-, and E-trending faults are common throughout gold districts in the east-central Alaskan part of the TGP. However, much more detail is needed before aspects of timing, abundance and relative displacement can be assessed to adequately target ores (e.g. Newberry and Burns, 1999).

Low-angle faults and associated structures controlled the emplacement of the several lodes that comprise each of the Brewery Creek and True North deposits. Despite characterization as disseminated, both of these deposits have strong structural controls with linear ore zones (Figs. 16 & 17). As well, they lack any evidence of specific stratigraphic control, which may be an important distinction from many of the Carlin-type disseminated gold deposits in Nevada.

Property-scale structural controls are important in most TGP deposits. Moderately-dipping reverse fault arrays were important in focussing auriferous fluids at the Scheelite Dome deposit (Fig. 13; Mair et al., 2000; O’Dea et al., 2000). All intrusion-hosted sheeted vein arrays occur as tensile features resulting from far-field stresses. Throughout the Tombstone gold belt, many mineralized tensional zones trend easterly, indicating that north-south extension was dominant during gold deposition (Poulsen et al., 1997). The sheeted vein sets locally continue outside of the intrusion, where they are paralleled by pegmatite, aplite, and lamprophyre dikes, and by sulfide-bearing quartz veins (e.g. Fort Knox, Dublin Gulch). Such extensional zones at Clear Creek are anomalous in gold and continue for several kilometers along strike (Marsh et al., 1999). These extensional zones, which may have facilitated magma emplacement, may be related to early, north-trending strike-slip faults (Stephens et al., 2000).

Geochemical Methods for TGP ExplorationRegional geochemical surveys are extremely effective in outlining gold favorable terrain

for follow-up efforts, as noted in the Pogo and Brewery Creek discovery case-histories. The U.S.G.S. regional geochemical survey of the Big Delta 1o x 3o quadrangle used minus-80-mesh stream sediments, heavy-mineral-concentrate of stream sediments, and willow leaf and twig samples collected from low-order streams (Foster et al., 1979). Sample and data treatment are as described previously for the Pogo Case History. In addition to indicating the presence of the Liese veins at Pogo, other anomalies from the U.S.G.S. survey indicate the regional extent of potential mineralization within the Goodpaster River region. Anomalies in concentrate samples in the order of 200-300 ppm Bi characterized streams on the northern side of the Goodpaster River and a number of stream sediments from this area contained 0.05 ppm Au, suggesting continuation of gold mineralization ~5-10 km north of where it is currently known at Pogo. About 15-20 km southeast of Pogo, near the historic Blue Lead lode and placer occurrences, many oxide residue samples contained 500-1,500 ppm As, and a heavy-mineral-concentrate sample contained 300 ppm Bi, confirming the As-Au-Bi signatures as most favorable for exploration throughout the district.

Regional stream geochemical surveys in Yukon, undertaken by the GSC, included analyses of silt and water samples, with a ~13 km² sample density. Analytical techniques and elements analyzed vary between surveys – typically the minus 80 mesh fraction was analyzed. The data readily identify broadly anomalous regions that are mainly coincident with the distribution of the mid-Cretaceous granitoids, and locally identify known deposits and occurrences (Fig. 22).

Most unexposed bedrock the TGP is overlain by variably oxidized regolith since, despite the

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region’s far-northern location, much of central Yukon and Alaska were beyond the limits of most glacial events during the past 2.5 million years. This effects exploration sampling strategies and methodologies. Positive effects are that the surficial materials are mainly of local derivation and have not been dispersed by glacial transport. Additionally, soil and stream sediment samples are neither diluted nor contaminated with transported glacial materials. Negative factors include locally thick loess accumulations, and metal-leached B-horizon soils. These and other sampling challenges, such as the presence of discontinuous permafrost, have encouraged the use of handheld and ATV-mounted auger drills to collect C-horizon soils. Deep oxidation and periglacial solifluction may give diffuse geochemical anomalies with low anomaly contrast or irregular anomaly patterns. Most deposits give surface soil anomalies of 40 to 100 ppb Au, and C-horizon anomalies of 100 to 250 ppb Au, but soil samples >1 ppm Au are not uncommon over many mineralized zones. Confident sampling of oxidized rocks is difficult as gold may be concentrated in friable limonitic or goethitic coatings that are easily removed by rainfall or during trenching.

Soil sampling is perhaps the single most useful tool for exploration on a property scale, particularly in light of the lack of exposed rocks. Arsenic, Te, Bi, Sb and base-metals have proved to be useful pathfinder elements for gold at some properties (e.g. Pogo) using commercially available multi-element analytical packages with low detection limits for Bi and Te.

Figure 22. Regional stream silt geochemical data for arsenic from central Yukon as compiled from Geological Survey of Canada open files, showing the distribution of arsenic anomalies. The anomalous regions are mostly correlative with exposure and contact aureoles of mid-Cretaceous granitoids, although not all plutons have associated arsenic anomalies (e.g. Brewery Creek). Background values are < 50 ppm As, whereas most of the peaks have values >500 ppm As.


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Geophysical Methods for TGP ExplorationThe usefulness of geophysical methods in interpreting the nature of the underlying geology

in regions of poor outcrop cannot be overstated - in particular aeromagnetics in combination with resistivity methods has proven useful in the TGP (e.g., Scheelite Dome: O’Dea et al., 2000). The most successful application may be for the discovery of unexposed or unmapped plutons, where indicated by the magnetic anomalies generated from thermal aureoles that developed where these bodies intruded reducing sedimentary rocks (Fig. 23). For example, intrusive rocks at the Brewery Creek mine weren’t known prior its discovery, but their aureoles have a pronounced magnetic signature compared to the country rocks and are easily discerned from an airborne geophysical survey of the region (Hart et al., 2000). However, the usefulness of geophysical methods in delimiting the many types of gold ore bodies in the TGP is limited.

The magnetic expressions of the intrusions themselves are otherwise low and flat. They are difficult or impossible to differentiate from the siliciclastic metasedimentary rocks that are the hosts for most systems. This is particularly notable because intrusions with low primary oxidation states have been considered more favorable causative agents of gold mineralization (Leveille et al., 1988) and these plutons have low magnetic susceptibilities. However, more detailed ground surveys have shown that mineralized zones at Scheelite Dome are coincident with magnetic and resistivity lows (Hulstein et al., 1999) which are thought to represent structures. Post-acquisition processing can also enhance the data as shown at Donlin Creek (Ebert et al., 2000). Radiometric surveys have proven useful in locating unexposed plutons, more highly fractionated regions of exposed plutons, or regions with potassic alteration. High-K, biotite-rich alteration in thermal aureoles was discovered using gamma-rays at the Bear Paw occurrence (Clear Creek deposit) where it is coincident with a magnetic low (Stephens and

Figure 23. Total field aeromagnetics for the Clear Creek area west of Mayo. Most plutons have a low primary oxidation state (i.e. reduced) and co-commitment low magnetic response. Plutons intruding the carbonaceous Paleozoic sediments give rise to magnetic hornfels thought to result from pyrrhotite formation. Plutons intruding the more siliciclastic Proterozoic strata yield flat aeromagnetic responses with no response from the aureole. Several locations with high anomalies may indicate roof zones above unroofed or partly roofed plutons in carbonaceous strata.

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Weekes, 2001). Gravity lows may indicate the presence of larger plutonic bodies at depth, but the regional quality data is locally coarse (10 km grid), and not useful for discerning smaller features.

Drilling as an Exploration ToolAlthough core drilling is preferred during the initial exploration stages to maximize

geological information, the choice/decision between drilling core vs. reverse circulation (RC) is usually made on factors other than geological ones. These often include, time (RC is quicker and the exploration season is short), access (core rigs are helicopter portable and result in less environmental damage), total cost per foot (related to time, i.e. camp costs), and accessibility to water, which can be difficult in many unglaciated regions. However, there are reports of diamond drill holes in oxidized ground with almost 30 m of poor recovery.

Where large, quality samples are desired from a target with fractured and oxidized ground conditions, RVC will generally provide a better sample and ultimately, more reliable assay data - particularly above the water table. This was apparent at Dublin Gulch where twinning of RC and core (HQ) holes showed significant differences in oxidized rock but comparable results in unoxidized rock (Hitchins and Orssich, 1995). Additionally, as much of the gold may be in oxidized sulfide minerals as goethite or in gouge in fault zones, it may be washed away with sludges during core drilling. However, in wet holes, RC sample quality diminishes considerably with increasing water. Data from wet RC holes drilled for the Fort Knox ore reserve calculations were considered unreliable compared to twinned core holes and was disregarded. Previous problems with down-hole contamination using RC rigs have been largely eliminated with the new center face sampling bits and suitably-sized compressors. Routine drilling at Fort Knox uses large PQ diameter core with triple-tube core barrels to reduce down-hole contamination and ensure large sample sizes.

Additional Exploration ConsiderationsGrades and tonnages of the different deposit types of the TGP vary considerably (Fig. 24)

between the high-grade, low-tonnage skarns, to the bulk-tonnage intrusion-hosted deposits that have grades of ~1 g/t Au. Veins in metasedimentary rocks are also typically higher grade and have a range of tonnages. Epizonal deposits, typically with grades of ~1.4 to 3 g/t, are characterized by refractory ores. Numerous “replacement-style” deposits with ambiguous origins lie in the middle ground of both tonnage and grade. Intrusion-hosted, epizonal and vein deposits all have the potential to host resources of greater than 100 t Au.

Real and estimated capital and operating costs of existing and potential gold deposits in the TGP are impacted by wide variances in interrelated factors including available infrastructure, beneficiation costs, and political/environmental factors concerning land usage, access and potential permitting. The two larger gold operations in the TGP (Fort Knox and Brewery Creek) are supported by work forces that are housed in nearby towns. Development of both deposits required relatively short spur roads, and at Fort Knox a short power line. Development of more remote deposits (Donlin Creek, Vinasale, Dublin Gulch) would require roads and transmission lines that would be hundreds of kilometers long, or on-site power generation. On-site diesel generation is particularly expensive for projects whose ores have a high work index


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– specifically weakly altered or unoxidized granitic ores. The enormity of Fort Knox mine and mill complex within the Fairbanks area, likely requires that very large or very rich deposits need to be discovered there to initiate a new development. However, a considerable amount of gold from otherwise sub-economic ores will likely be mined, and processed in the Fort Knox mill.

Remote areas are difficult and expensive to access and as such, are less likely to be well-explored. Potential mines in remote regions, like the Kuskokwim region, would likely require the provision of supplies along seasonally open rivers, which would increase both operating cost and risk. Mining in remote and unspoiled parts of the northern Cordillera require environmental sensitivity. Conversely, development of deposits in the Fairbanks district may be impeded by their proximity to suburban development. Because environmental considerations are paramount, operations designed with zero discharge, such as Fort Knox and Brewery Creek, are likely to be permitted more quickly.

Mine valuation, on both capital and operating costs, is effected significantly by its beneficiation method. The nature of the ore and the amount of oxidation are two critical factors. The Fort Knox, Dublin Gulch, Pogo, Shotgun and the Golden Zone deposits have gold amenable to cyanide leach regardless of surface oxidation. Some deposits with refractory gold may be economically viable due to relatively deep oxidation (Brewery Creek, Ryan Lode, and True North). Additionally, oxidized rocks have a low work index such that at Brewery Creek, run-of-mine ores were taken to the leach pad without crushing. Oxidation depth is important, as the vertical extent the of Brewery Creek and True North oxide orebodies are limited and overlie untested sulfide resources. Deeply oxidized deposits only occur in areas that escaped recent glaciations such as interior Alaska and west-central Yukon. Glaciation through most of the elevated ranges of the Kuskokwim region has removed most deeply oxidized ores. The capital costs associated with constructing roasting or bio-leach facilities for such deposits in relatively remote regions will impact their development. Alternatively, cyanide extractive heap-leach operations of oxidized ores is viable, despite the extremes of a northern climate. Burying drip lines and exothermic reactions from within the heap are sufficient to keep the fluids from

Figure 24. Grade and tonnage plot for deposits of the Tintina Gold Province. Data from Table 1.

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freezing. Gold recovery is year-round, but significantly lower during the winter months (Fig. 25).

In general, support for exploration and development of gold resources in the TGP by the Alaskan and Yukon governments is positive. In Alaska, exploration expenditures can be written off against future taxes on gold production, state secured loans may assist and provide new infrastructure, and regulatory and taxation policy is more positive than in most other places in the USA. Land tenure and taxation policy is stable and non-prohibitive and native land tenure was resolved decades ago in a mutually positive manner. Conversely, the high value of United States currency, remoteness, and lack of “flow-through” financing make exploration very expensive. In Yukon, exploration benefits from government-funded incentive programs, exploration tax credits of 25%, federal “flow-through” tax-credits of 15% and a cheap Canadian dollar. However, uncertainty over the on-going native land-claims process and the establishment of protected areas have hindered aggressive exploration activity.

Obstacles impeding increased exploration expenditure within the TGP are those that have effected gold exploration elsewhere – notably the decreasing value of gold and decreased availability of venture capital for exploration. However, discoveries, such as the Pogo deposit, can catalyze and facilitate the financing of numerous grassroots exploration programs throughout the TGP. Compared to most of the world, or elsewhere in North America, the TGP has excellent mineral potential, security of tenure, excellent geoscience databases, and is still mainly under-explored. Furthermore, geological and exploration models are still in an evolutionary state and new deposit types are yet to found – most likely through creative and aggressive exploration approaches. Continued pragmatic exploration and ensuing discoveries will ensure that the Tintina Gold Province’s golden tradition continues to mature.

Figure 25. Monthly gold production figures from the Brewery Creek heap leach gold mine, Yukon. From company Annual Reports.


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ConclusionsThe TGP is defined by a wide range of gold deposit styles that are spatially and temporally

associated with reduced and radiogenic mid- and Late Cretaceous intrusions that are hosted in sedimentary or metasedimentary strata across Alaska and Yukon in the North American Cordillera. They account for an in-ground resource of approximately 37 Moz Au and an undefined portion of the region’s 30 Moz of placer gold production. Deposits in the TGP form the basis for the development of the intrusion-related gold system model. The wide range of deposit styles within the model can be catagorized as intrusion-centered (includes intrusion-hosted, skarn, thermal aureole-hosted replacements, breccias, distal skarns etc..), shear-related veins, and epizonal. The variations in deposit styles result from differences in emplacement depth and the nature of the host rocks. A genetic association with intrusive rocks is implied, but difficult to confidently establish for many shear-related and epizonal deposits. Even intrusion-hosted deposits are mineralized by granitoids that are generally not exposed making links to a specific magmatic phase difficult to document.

U-Pb and Ar-Ar geochronology has constrained the TGP’s important magmatic and mineralizing events at circa 90 and 70 Ma, precise deposit-scale temporal links between magmatism and mineralization using Ar-Ar have been difficult to confidently establish. Re-Os molybdenite dating has proven useful in yielding precise determinations and indicate a previously unconfirmed ~104 Ma mineralizing event at Pogo’s Liese veins.

Most TGP deposits were discovered through the application of traditional exploration methods – notably by following-up multi-element regional geochemical anomalies with soil sampling. Government-sponsored regional geochemical data effectively discern mineral-rich regions. Anomalous soil values are best obtained using augers and sampling the C-horizon. Trenching is required to obtain meaningful samples as much of the TGP is unglaciated and the rocks are deeply oxidized. Some TGP deposits were found through the application of the intrusion-related model to areas with known gold occurrences.

AcknowledgementsConversations about various aspects of the Tintina Gold Province with Grant Abbott, Tim

Baker, Mike Burke, Al Doherty, Brian Flanigan, Curt Freeman, Gerry Carlson, Jim Lang, Bill Lebarge, Mark Lindsay, John Mair, Erin Marsh, Marti Miller, Rainer Newberry, Cam Rombach, and Julian Stephens, are appreciated. Reviews of an earlier manuscript by Dick Nielson are Shane Ebert appreciated, and valuable comments and suggestions from John Thompson helped to construct the present one.

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Chapter 3 Mid-Cretaceous Plutonic Suites

Chapter Three

The Northern Cordilleran Mid-Cretaceous Plutonic Province:

Ilmenite/Magnetite-Series Granitoids and Intrusion-Related Mineralisation

Craig J.R. Hart1,2, Richard J. Goldfarb3, Lara L. Lewis2 and John L. Mair1

1 Centre for Global MetallogenySchool of Earth and Geographical SciencesUniversity of Western AustraliaCrawley, Western Australia, Australia 6009

2 Yukon Geological SurveyBox 2703 (K-10)Whitehorse, Yukon, Canada Y1A 2C6

3 United States Geological SurveyDenver Federal Centre Box 25046 (MS964)Denver, Colorado, USA 80225

Chapter 3 Mid-Cretaceous Plutonic Province Suites

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Preface to Chapter ThreeThis paper results from an invitation by Shunso Ishihara (Editor) to contribute to a Special Issue of Resource Geology as published by the Society of Resource Geology based in Japan. This Special Issue was initiated to commemorate Dr. Ishihara’s 70th birthday and his lifetime contribution to understanding intrusion-related ore systems. This paper “The Northern Cordillera Mid-Cretaceous Plutonic Province: Ilmenite/Magnetite-Series Granitoids and Intrusion-Related Mineralisation” was published in 2004 in Resource Geology, v. 54, no. 3, p. 253-280. The spelling and format for citations and references follow those that are used in Resource Geology.

Justif cation of authorship: This manuscript resulted from a large GIS compilation undertaken by the candidate as part of the Ph.D. study involving integration of varied datasets from Alaska and Yukon. Dr. Richard Goldfarb contributed expertise about Alaskan mineral deposits and provided editorial expertise. Ms. Lara Lewis compiled the magnetic susceptibility data that were acquired through the candidate’s field work, and provided GIS-support for some of the figures. Mr. John Mair provided insight about redox controls and petrogenesis of magmatic systems, and has been involved in ongoing regional igneous petrologic studies with the candidate. The candidate defined and researched the plutonic suites, compiled and interpreted the redox data, and made all of the interpretations with respect to metallogeny.

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The Northern Cordilleran mid-Cretaceous Plutonic Province: Ilmenite/magnetite-series

granitoids and intrusion-related mineralisation

AbstractTwenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites and belts are defined

across Alaska and Yukon, in the northern North American Cordillera, on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features are combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole-rock ferric:ferrous ratios to ascertain the distribution of magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction-generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly younger, slightly peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite-series belts in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension.

Intrusion-related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au and Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation associated with ilmenite-series plutons. However, there are some notable deviations from expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons: these form the basis for the newly recognized reduced intrusion-related gold deposit model.

IntroductionRegional metallogenic characteristics of intrusion-related mineral deposits are mainly

governed by the nature of their associated granitoids. Among the variables that affect intrusion-related metallogeny, the most important is probably the primary redox state of the magma. This recognition led Ishihara (1977, 1981) to develop a classification of magnetite- and ilmenite-series granitoids and to note that each series had specific metallogenic associations. Generally, magnetite-series granitoids, or magmas with high-oxidizing potential, form magnetite-bearing plutons that generate sulphide-rich Cu, Au, Mo, Pb, and (or) Zn deposits, whereas those with low-oxidizing potential are ilmenite-series and preferentially generate oxide deposits rich in tungsten and tin. This relationship between magmatic redox state and metallogeny is one of the most powerful first-order discriminators of intrusion-related metallogeny and a critical factor in regional mineral exploration targeting.

The oxidation state of a pluton is mostly a response to the nature of the source materials from which it was derived (Carmichael, 1991). Magnetite-series plutons are typically derived


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from the partial melting of mafic components in the mantle wedge that are hydrated and oxidized in response to dehydration of components in the subducting slab. The resultant magmas are most often emplaced as oxidized, arc-building, calc-alkaline plutons. Ilmenite-series magmas are mostly derived from or mixed with the partial melts of metasedimentary rock sequences that contain reducing carbonaceous strata (Ishihara 1981). These magmas typically result from melting of metasedimentary continental crust in a continental back-arc position, possibly in response to crustal thickening, or, less- commonly, from near-trench subduction-related melting of a flysch in an anomalously hot accretionary wedge. These differing tectonic settings responsible for the generation of magnetite- and ilmenite-series granitoids make it possible to ascertain, or even re-assemble, the tectonic settings in which plutonic belts may have formed.

Ilmenite- and magnetite-series plutonic belts typically lie parallel to the continental margins and their circum-Pacific distributions have been compiled by many workers (Takahashi et al., 1980; Ishihara 1984, 1998; Takagi and Tsukimura, 1997). However, the northeastern Pacific Ocean margin (the northern Cordillera of northwestern North America) is a region where distributions of these belts are particularly poorly known (Ishihara 1998), and a regional comprehensive description of such has been lacking from the literature. This region of North America, mainly underlain by the State of Alaska, USA, and the Yukon Territory, Canada (Fig. 1), hosts intrusions emplaced during multiple stages of magmatism. These include pre-accretionary oceanic arc plutons as old as Devonian, and syn- and post-accretionary continental margin Mesozoic-Cenozoic magmatic rocks. Together, these variably overlapping assemblages have imparted a complex distribution of plutons and batholiths throughout the region. In particular, overprinting Jurassic, Early Cretaceous, mid-Cretaceous, Late Cretaceous and early Tertiary magmatic events have combined to generate a complex pattern of plutons of differing origins, redox states, and, therefore, related metallogeny.

The most regionally extensive and metallogenically important magmatic episode in the northern Cordillera was that of the mid-Cretaceous, which resulted in the deposition of world-class intrusion-related tungsten, gold, silver, and copper deposits, and significant resources of uranium, molybdenum, and tin. The regional distribution of mid-Cretaceous plutonic suites and belts of differing oxidation states permits characterization of the nature of intrusion-related metallogeny, and forms the basis for developing an understanding of the tectonic settings in which various plutonic belts and deposit types form. This is particularly important because: 1) there are notable differences in the distribution of magnetite- and ilmenite-series plutonic belts between orogens along the western and eastern portion of the Pacific rim (Ishihara, 1998); and 2) a new class of gold deposits associated with reduced, ilmenite-series intrusions has recently

Figure 1 (following page). Generalized tectonic elements in Yukon and Alaska with significant faults, features and locations cited in text. Note the distribution of the mid-Cretaceous plutons which extend from near the Pacific Ocean to several hundred kilometers inland. Insular, Intermontane, and Koyukuk belts are mostly physiographic distinctions. Distances on faults refer to amount of displacement. Terranes are denoted by labels: SD-Seward Terrane; KO-Koyukuk Terrane; RB-Ruby Terrane; WR-Wrangellia; CG-Chugach Terrane; YTT-Yukon-Tanana Terrane; ST-Stikinia; CT-Cassiar Terrane. NWT-Northwest Territories. Faults: PRF-Pacific Range Fault; DT-Dawson Thrust; SCF-Shaw Creek Fault; VF-Volkmar Fault.

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been recognized (Lang et al., 2000; Thompson and Newberry, 2000). These differences highlight varying mechanisms of magma and metal generation across orogenic belts and may help to develop a better understanding of regional metallogenic variations and controls.

Herein, a framework of Early and mid-Cretaceous plutonic belts and suites for Alaska and Yukon is established. This is used as a framework upon which the redox state of plutonic belts and the intrusion-related metallogeny is constructed.

Tectonic FrameworkThe northern North American Cordillera is composed of variable amounts of exotic and

pericratonic crustal fragments, or terranes, which were accreted to the ancient continental margin during the Jurassic to early Tertiary (Gabrielse and Yorath, 1991; Monger et al., 1982; Plafker and Berg, 1994). Many of the accreted terranes are composed of primitive island arcs, oceanic crust, and flysch, whereas others represent fragments of variably metamorphosed continental margin assemblages of uncertain origin (Fig. 1). The ancient continental margin lacks exposed crystalline rocks because it is overlain by >15 km of deformed passive margin sedimentary strata that variably accumulated between the Mesoproterozoic and the Mesozoic. Notable within these otherwise carbonate-dominated platforms in Yukon and western Northwest Territories (NWT) is the Selwyn Basin, a late Neoproterozoic to Carboniferous clastic basin with important sequences of carbonaceous shales (Abbott et al., 1986). The Cassiar Terrane represents a block of the ancient continental platform that has been displaced by Mesozoic strike-slip movement along the Tintina Fault system (Fig. 1).

The Yukon-Tanana, Ruby, and Seward terranes, as well as several other smaller crustal fragments, represent polydeformed metasedimentary rock terranes that have continental, or pericratonic, affinities and mostly occupy a position between the ancient continental margin and outboard accreted terranes (Mortensen, 1992; Plafker and Berg, 1994). The accreted terranes are dominated by primitive Mesozoic island arcs, such as those that are characteristic of the Wrangellia, Stikinia, and Koyukuk terranes (Fig. 1), but these may also include older basement assemblages. Obducted oceanic rocks, such as those of the Cache Creek, Angayucham, Slide Mountain, and Seventy-Mile terranes, are mostly Paleozoic. They typically have a thin, elongate distribution, locally with eclogite and blueschist, which forms an interface at the leading edge of the accreted terranes with the ancient continental margin. Overlap basinal assemblages, mostly in the form of Jurassic to present-day flysch basins, occur along the relatively young accretionary margin of southern Alaska, as well as overlapping previously accreted terranes in the interior.

The nature and timing of many of the accretionary events remain controversial, but most evidence in Alaska and Yukon suggests a period of assembly involving “Intermontane” components in the Early Jurassic, and another, involving “Insular” components, in the Early to mid-Cretaceous (Monger et al., 1982; Monger 1989; Mihalynuk et al., 1994; Plafker and Berg, 1994) The farthest outboard components are still involved in accretion and margin-parallel translation, with continental to oceanic arcs still growing in far southwestern Alaska (Pavlis et al., 2004).

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Numerous suites of Jurassic, Cretaceous, and Tertiary plutonic and volcanic rocks were emplaced into this complex array of terranes and deformed continental margin strata (Armstrong, 1988; Miller, 1994). Most voluminous are Cretaceous rocks that formed first in response to rapid convergence between the North American and Pacific plates during the Cretaceous normal superchron (124-83 Ma) and secondly during the final subduction of the Kula plate that started at ca. 83 Ma (Engebretson et al.,1985). This latter rapid subduction, beneath what is now southern Alaska, caused rapid dextral oblique transpression in terranes that were previously coupled with the North American margin. Ensuing dextral strike-slip faulting along the Tintina-Kaltag (450 km) and Denali-Fairweather (350 km) fault systems (Fig. 1), combined with modern day deformation, have added further complexity to the terrane architecture (Plafker and Berg, 1994; Sisson et al., 2003 and papers therein).

For the purposes of this paper, it is convenient to use the terms “Insular” to refer to accreted island-arc terranes in the region south of the Denali-Fairweather fault system and “Intermontane” for the region of pericratonic and arc terranes between the Denali-Fairweather and Tintina-Kaltag fault systems . The North America margin refers to autochthonous North America and displaced equivalents.

Plutonic SuitesEarly and mid-Cretaceous magmatism was widespread throughout Alaska and Yukon,

with related plutons cutting across boundaries of tectonic elements and occurring from the near-trench environment along the current continental margin to 700 km inboard from the current plate margin (Fig. 2). Robust dating methods (U-Pb zircon) have aided significantly in defining the temporal division of the plutonic elements into distinct intervals. However, as many parts of the region lack these accurate analyses, particularly in Alaska, there are large tracts where less robust and relatively imprecise dates (K-Ar, Rb-Sr) are believed to be skewed towards erroneously young ages due to partial resetting. Nonetheless, several Cretaceous plutonic events within the region can be defined (Fig. 2). Early Cretaceous (145-135 Ma) plutonism was sparse and localized in the Insular terranes, and may define the final stages of Late Jurassic subduction-related arc magmatism that occurred on the seaward side of the evolving orogen. For the most part, there was an earliest Cretaceous Cordilleran-wide magmatic lull (Armstrong, 1988) that continued to ca. 115 Ma. Increased convergence rates between oceanic plates and North America resulted in a dramatic flare-up in magmatism at ca. 115-109 Ma that generated several plutonic suites across the breadth of the northern Cordilleran orogen. Magmatic activity continued through ca. 99 Ma and then migrated further inland until it terminated dramatically at 90 Ma. The 115 to 90 Ma magmatism was notably the most widespread, voluminous, diverse, and metallogenically important in both Alaska and Yukon. Latest Cretaceous magmatism, initiated at ca. 74 Ma, was widespread, but concentrated in southern Alaska, with less voluminous events in western and eastern Alaska, and central Yukon (Moll-Stalcup, 1994).

Early to mid-Cretaceous plutonism post-dates deformation of the host terranes, with only the earliest Cretaceous plutons that intrude the Insular terranes (Hudson, 1979), and the ca. 122-115 Ma plutons in the Yukon-Tanana Uplands of the Intermontane region (Day et al.,


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2003), showing evidence of internal deformation. These bodies also typically cut the youngest regional deformational structures and the major terrane-bounding faults. Most plutons were intruded after the accretion of the more landward of the Intermontane terranes to the continental margin, and syn- to post-accretion of the Insular terranes. As such, most of this plutonic epoch represents transitions from syn- to late- or post-orogenic intrusions.

Early and mid-Cretaceous plutons form several distinct belts and, where plutons within a belt share similar lithological characteristics and age ranges, they are characterized as plutonic suites (Fig. 2). More than 20 plutonic suites and belts are identified herein and are based, in part, on terminologies and distributions used by Anderson (1988), Woodsworth et al. (1991), Miller (1994), Mortensen et al. (1995, 2000), and Newberry (2000), but many names and distributions have been modified or added. As well, available digital databases of geology (USGS, 1997; Gordey and Makepeace, 2003), geochronology (Breitsprecher et al., 2003, USGS 1999, 2002), aeromagnetics (Saltus, 1997; Saltus and Simmons, 1997; Saltus et al., 1999; Lowe et al., 2003), and mineral deposits (USGS, 1998; Deklerk, 2003) were integrated on a GIS platform and used extensively in making the compilation and associated interpretations presented herein.

Early and Mid-Cretaceous Plutonic Belts and SuitesHerein, the Early and mid-Cretaceous plutons of Alaska and Yukon are divided into 25

separate plutonic belts and suites. Their distributions are shown in Figure 2 and a summary of their characteristics are given in Table 1. As many of these plutonic belts are along-strike with belts of similar geologic settings, lithologies or age ranges, they are grouped to form 13 larger plutonic belts which are described below, largely in order of decreasing age and increasingly landward position.

Tosina-St. Elias Belt (145-135 Ma)Earliest Cretaceous plutonism is represented by a >600-km-long belt that is cored by

plutons in the southern Wrangell Mountains in Alaska and the St. Elias plutonic suite in southwestern Yukon, but may also include plutons in the western Chugach Mountains in southern Alaska (Figs. 1 and 2) and the Chichagof plutonic suite in the southeastern Alaska panhandle (Hudson, 1979; Miller, 1994). These suites mainly intrude rocks of the Paleozoic to Triassic Wrangell and Alexander arc terranes, and to a lesser extent of the Chugach terrane, and are exposed mostly between the Denali and Border Ranges fault systems. Plutons vary from large elongate batholiths to small individual stocks. The plutonic suites are dominated by tonalitic series, variably foliated biotite-hornblende quartz diorite, tonalite, diorite, and granodiorite, with lesser trondhjemitic rocks, that form the Tonsina-Chichagof belt (Hudson 1983) and may include plutons ascribed to the Chitina arc (Plafker et al., 1989). Potassium-

Figure 2. Distribution and age ranges of mid-Cretaceous plutonic suites and belts across Alaska and Yukon. Age ranges given here may differ from those in the text or literature as the older ages are emphasized. Many suites, particularly those in Alaska, have a wide range of age dates that typically result from K-Ar determinations that are susceptible to partial resetting and give dates that are too young. YT Uplands-Yukon-Tanana Uplands, NWT-Northwest Territories

73



74

Table 1. Characteristics of Early to m

id-Cretaceous plutonic suites and belts in A

laska and Yukon with data pertinent to redox-series. Listings are in approxim

ate decreasing age. M

agnetite-series plutonic suites are shaded, those that are weakly m

agnetite-series are lightly shaded. Ilmenite-series rem

ain unshaded. Ages

in brackets are minim

um ranges likely to result from

variably reset determinations. M

agnetic susceptibility values are averages, ranges for some are given in

brackets. alk=alkaline; calc-alk=calc-alkaline; ilm=ilm

enite; mag=m

agnetite; na=data not available; trondhj-trondhjemitic; var=variable.

Suite/BeltAge (Ma)

AffnityFe

2 O3 /FeO

Mag Suscx10

-3 SIFe-Ti

MineralogyAerom

agCharacter

SeriesBest References

Tosina-St. Elias145-135

trondhj-tonaliticna

naprob magnetite

high, Chugach low

MagHudson (1979); Dodds and Campbell (1988); Pavlis et al. (1988)

Nutzotin-Kluane118-108

calc-alkaline>trondhj0.6-0.9

namagnetite,

titanitehigh

MagRichter et al. (1975a), Hudson (1979); Dodds and Campbell (1988)

Whitehorse

115-109calc-alkaline

0.5-4.012.9

magnetite, titanite

highMag

Morrison et al. (1979); Hart (1995, 1997, unpublished)

West Koyukuk

110-99alk>calc-alk>peralk

0.4-3.0na

titanite, magnetite

mixedMag, var.

Miller (1989)

St Lawrence Island108-100

alk>calc-alkna

namagnetite

naMag

Amato et al. (2002)Ruby

112-108 (99)aluminous>calc-alk

0.1-1.1na

ilmenite>titanitelow, mod-high

in northIlm

Miller (1989)

Kaiyuh112

aluminous>calc-alk0.3

nana

lowIlm?

Patton and Moll-Stalcup (2000); Patton et al. (1984)

Nyac111-108

aluminous?na

nana

lowIlm?

Yukon-Tanana Uplands

109-102 (93)aluminous

0.1-1.00.15 (0.02-0.38)

ilmenitelow

IlmDay et al. (2003); Dilworth (2003); W

erdon et al. (2004)

Anvil109-95

aluminous>calc-alkna

nana

low, mixedIlm

Pigage and Anderson (1985)Cassiar

109-99 (96)aluminous>calc-alk

0.3-1.40.77

titanite>magnetite>ilmenite

moderateW

eak Mag

Driver et al. (2000), Hart and Lewis (unpublished)

Hyland106-96

aluminous0.4-1.5

2.3na

moderateW

eak Mag

Hart and Lewis (unpublished)

Salcha106, 94

aluminous0.2-1.0, up to 6

0.08 (0.01-0.23)

ilmenitelow, mixed

IlmW

erdon et al. (2004)

Tok-Tetlin101-93

calc-alk0.06-0.3

nana

lowIlm

Richter et al. (1975a)Dawson Range

104-99 (93)calc-alkaline

1.5-5.55.1

magnetitemoderate-high

MagTempelman-Kluit and W

anless (1975); Selby et al. (1999); Hart (unpublished)

Southwest Alaska Range

100-88calc-alkaline

nana

probably magnetite

??Mag

Bouley et al. (1995)

East Seward100-96 (90)

alk-calc-alk?na

nana

highMag, var

Armstrong et al. (1986)

East Koyukuk98-83

calc-alkaline0.5-5.0

natitanite,

magnetitehigh

MagMiller (1989)

Tungsten98-94

aluminous0.1-0.3

0.16ilmenite

lowIlm

Gordey and Anderson (1993); Hart et al. (2005)

Mayo96-93

aluminous>calc-alk0.15-0.6

0.09titanite

lowIlm

Murphy (1997); Hart et al. (2005)Fairbanks

94-90aluminous>calc-alk

0.1-0.4, up to 20.33 Fort Knox

pluton onlytitanite

lowIlm

Blum (1983, 1985)

South Seward105, 93

aluminousna

nana

lowIlm?

Amato et al. (1994); Armstrong et al. (1986)Tanacross

92-90calc-alkaline

0.5-0.9na

moderateMag

Richter et al. (1976)Tombstone

92-90alkaline>peralk

0.2-1.01.8

magnetitehigh, mixed

MagAnderson (1987); Hart et al. (2005)

Livengood92-90

alkaline>peralkna

nana

highMag

Reifenstuhl et al. (1997a)

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Ar geochronology of uncertain reliability yields dates as young as 125 Ma (Chugach suite), but also a U-Pb date as old as 153 (Plafker et al., 1989): most dates are from 145 to 135 Ma. Tonalite-trondhjemite series plutons in the western Chugach Mountains, ranging from 135 to 125 Ma (Pavlis et al., 1988), may also belong to this belt. Plutons of the belt have high aeromagnetic responses, which together with their tonalitic calc-alkaline arc affinities (Plafker et al., 1989), designates them as magnetite series.

The region underlain by this plutonic suite has a high density of small intrusion-related mineral occurrences that are dominated by Cu-Au-Ag porphyry-style, skarn, and vein-style mineralisation. Among these occurrences are the Midas, London, and Cape porphyries, and the calcic iron skarns in the McCarthy area (Moffit and Mertie, 1923; MacKevett, 1976). The porphyry occurrences, which are associated with the Chitna Valley batholith, are interpreted to have formed in an offshore oceanic arc at ≥ 146-138 Ma and then were subsequently accreted to the continental margin (Young et al., 1997). Strontium initial ratios <0.7045 for the arc rocks (Arth, 1994) are consistent with a primitive oceanic origin.

Nutzotin-Kluane Belt (118-108 Ma)Bounded to the north by the Denali fault system, the plutons of the Nutzotin-Kluane belt

form a narrow, 800-km-long belt (Nutzotin-Chichagof belt of Hudson, 1979) that occurs throughout the northern Wrangell Mountains in Alaska (i.e., the Chisana arc of Barker, 1987) and the Kluane Ranges suite in southeastern Yukon (Fig. 2), and into the Alaskan panhandle, where they form the Chichigof belt of Richter et al. (1975a). They mainly intrude rocks of the Wrangellia and Alexander terranes, as well as adjacent Jura-Cretaceous flysch. The plutons are typically large epizonal plutonic complexes and stocks that are elongate and parallel with the regional fabric (Woodworth et al., 1991). They are dominated by hornblende-biotite granodiorite, with lesser quartz diorite and diorite, calc-alkaline, I-type intrusions (Richter et al., 1975a; Campbell and Dodds, 1983; Dodds and Campbell, 1988). Plutons have a wide range of K-Ar dates from 147 to 108 Ma and thus some included igneous bodies may be part of the older Tosina-St. Elias suite. Dodds and Campbell (1988) define the age range of Kluane Range suite intrusions to be 117-108 Ma. Aeromagnetic maps indicate that these rocks in Yukon have a highly magnetic character. To the south, in the southeastern Alaska panhandle, the Klukwan-Duke belt comprises 110-100 Ma mafic-ultramafic Ural-Alaska-type complexes that were deeply emplaced mainly within the Jura-Cretaceous flysch belt. This 500 km x 40 km belt consists of rocks that are concentrically zoned from dunite cores, outward through peridotite, to olivine and hornblende pyroxenite (Foley et al., 1997).

Numerous intrusion-related mineral deposits in the northern Wrangell Mountains part to this belt include the quartz diorite- and granodiorite-associated Baultoff, Carl Creek, Horsfeld, Bond Creek and Orange Hill porphyry copper and copper-molybdenum deposits (Young et al., 1997). The adjacent Triassic limestones host gold-bearing skarn deposits (Newberry et al., 1997). These deposits have ages of 114 to 105 Ma (Richter et al., 1975a, b; Richter, 1976). The main eight porphyry deposits have a combined resource of about 1,000 Mt averaging 0.20-0.35% Cu, as well as grades of about 0.02% Mo and minor silver and gold enrichments (Hollister et al., 1975). All are presently unmined, in large part due to their relatively remote


76

location. Calcic copper, iron, and/or gold skarns occur on both the northern and southern, more accessible flanks of the Wrangell Mountains (Richter et al., 1975a; MacKevett, 1976), with the Nabesna deposit being the economically most significant deposit. Almost 2 t Au have been recovered from pyrite lenses in fractured limestone associated with garnet-pyroxene skarn (Wayland, 1943). The paleotectonic setting for these intrusion-related deposits in the Wrangell Mountains is uncertain, but where the belt continues into the northern part of southeastern Alaska (i.e., Bruce Hills porphyry deposit), magmatism is interpreted to have taken place near the paleo-continental margin (Young et al., 1997). Potentially large iron resources (e.g., Klukwan, 3.2 billion tonnes of rock averaging 16.8% Fe) occur as magnetite in the outer hornblende pyroxenite zone of many of the Klukwan-Duke intrusions (Goldfarb, 1997), as well as possible economic occurrences of Ti and PGE. Potential intrusion-related mineral resources in the Kluane Range of this belt are unknown because they are sited in a national park.

St. Lawrence-Seward-Koyukuk Belt (113-99 Ma)Early Cretaceous plutons form an east-trending belt from St. Lawrence Island in the

Bering Sea, east to the southern/central Seward Peninsula and across the northern and eastern Koyukuk Basin (Fig. 2). If the Koyukuk-Seward-St. Lawrence belt is interpreted as a single belt, then it links several diverse lithotectonic terranes, and may also be viewed as the eastern extension of the Okhotsk-Chukotsk plutonic belt in eastern Russia (Miller, 1989). The St. Lawrence Island and East Seward plutons intrude the Proterozoic and Paleozoic pericratonic assemblages that comprise the Seward terrane, whereas the West and East Koyukuk plutons intrude the accreted Koyukuk volcanic arc terrane and the overlapping Cretaceous terrigenous sedimentary rocks. The belt often follows near the tectonic contact between the Koyukuk and Seward terranes. The St. Lawrence Island, East Seward, West and East Koyukuk plutons are all slightly alkaline in composition, with hornblende- and clinopyroxene-bearing monzonites and syenites, but some plutons are more typical calc-alkaline granodiorites (Miller, 1972, 1989; Patton and Csejtey, 1980; Patton and Box, 1989; Amoto et al., 2002). Accessory magnetite and titanite are reported for most plutons (Csejtey and Patton, 1974; Miller, 1989). The alkaline plutons typically yield dates of 113 to 99 Ma, with most between 109 to 105 Ma (Miller, 1989). The belt likely developed in response to north-dipping subduction.

In addition, a volumetrically small, but significant suite of highly alkaline, silica-undersaturated ultrapotassic intrusive bodies occur, mainly within the area of the West Koyukuk suite (Miller, 1989). West Koyukuk plutons apparently have mixed oxidation states, as indicated from both Fe2O3/FeO ratios (Fig. 3) and aeromagnetic signatures. Most of the plutons have relatively modest and flat aeromagnetic responses, but the large, ultrapotassic Selawik Hills pluton is defined by a notable aeromagnetic low and a few plutons (e.g., Shiniliaokok Creek pluton) have very high aeromagnetic signatures. The mixed signature may reflect the overlapping distributions and differing sources of the alkalic and ultrapotassic suites.

Two other, smaller, plutonic suites have also been identified within this belt (Miller, 1994). Plutons in East Seward, including the 96-91 Ma Darby pluton and other associated plutons to the north, are evolved silicic granites with moderate to high aeromagnetic responses, high Fe2O3/FeO ratios (Miller and Bunker, 1976) and moderately radiogenic strontium ratios (0.708-

77


0.711, Arth, 1987). The slightly younger East Koyukuk plutonic suite forms two 150-km-long belts (east- and southwest-trending) that are mostly defined by large, hornblende>biotite, calc-alkaline, granodioritic to granitic plutons in the eastern Koyukuk terrane (the east half of the Hogatza plutonic belt of Miller et al., 1966). These bodies yield K-Ar ages of 89-79 Ma, have high Fe2O3/FeO ratios, and low initial strontium ratios of 0.7038-0.7056 (Arth et al. 1989a; Miller 1994). Although intrusions of both the East Seward and East Koyukuk suites are mainly metaluminous, there are a few weakly peraluminous examples. Associated aplite dykes, tourmaline-rich zones, miarolitic cavities, coeval volcanic rocks, and broad hornfels aureoles characterize many intrusions. Abundant accessory magnetite and titanite have been reported (Miller 1989), and the plutons have high to moderate aeromagnetic signatures, and are oxidized, as defined by Fe2O3/FeO ratios from 1 to 3 (Fig. 3). Isotopic and trace element data indicate that there is no continental crustal component to these shallowly emplaced East Koyukuk bodies (Miller, 1989; Arth et al., 1989a).

In addition, a 200-km-long east-trending belt of younger, silicic plutons in southern Seward Peninsula, which forms the core of the Bendeleben and Kigluaik Mountains, yields 87-81 Ma K-Ar dates that represent cooling ages related to uplift of the granites and gneissic country rocks (Armstrong et al., 1986). The granites and high-grade metamorphic rocks developed in response to an Albian Barrovian metamorphic event between 105 and 92 Ma and the former are thus interpreted as anatectic plutons (Armstrong et al., 1986; Miller et al., 1992; Amato et al., 1994). The large Kachuik and East Bendeleben Mountains plutons have low and flat aeromagnetic signatures, but ages of 100-90 Ma that are similar to the rest of this suite.

The mainland region is characterized by small and relatively unstudied, intrusion-related U-Th, Be, REE-bearing pegmatite, and polymetallic Ag-Sn-Bi-Pb-F vein occurrences. There are also a few small tin showings in the southern part of St. Lawrence Island (e.g., Kangukhsam Mountain). Widespread, but weakly disseminated, molybdenite is known from the Sevuokuk quartz monzonite pluton (e.g., Poovookpuk Mountain and Booshu Camp) on westernmost St. Lawrence Island (Patton and Csejtey, 1980), but the most significant mineralisation is the Poovookpuk Mountain and West Cape Cu-Mo porphyry occurrences, which are associated with plutons in the north.

Gold mineralisation in the southern Seward Peninsula developed in response to a ca. 110 Ma metamorphic event that localized small orogenic lode-gold deposits in the greenschist facies rocks in the southern part to the peninsula (Goldfarb et al., 1997). Although gold vein formation is coeval with the onset of magmatism, the deposits are located 40-50 km south and west of the main magmatic bodies. These historically uneconomic lodes were eroded to form the main beach and river placers that yielded almost 200 t Au in the early 1900’s. Silver-rich, base-metal vein deposits in the higher grade metamorphic rocks of the Seward Peninsula (i.e., Independence, Omalik) show a closer spatial association, and are isotopically compatible, with mid-Cretaceous magmatism (Goldfarb, 1997), although some workers suggest that these deposits represent Paleozoic syngenetic mineralisation (e.g., Schmidt, 1997). The Darby granite porphyry in the southeastern part of the peninsula is highly enriched in uranium (e.g. Eagle Creek deposit) and was the obvious source for uraniferous alluvium that was remobilized into an Eocene roll-front type deposit (Thompson, 1997).


78

The most notable Koyukuk intrusion-related occurrences are the small Wheeler and Clear Creek U-Th-F prospects, which are associated with an ultrapotassic intrusion (Miller and Elliott 1977), and the Placer River molybdenite occurrence. Mineralisation associated with the other intrusions form the Hogatza and Hughes mineral districts, which include many very small polymetallic vein, scheelite vein, and gold placer showings. Placer thorianite of the Hogatza deposit is associated with the Zane Hills (101-84 Ma) biotite-hornblende quartz monzonite pluton (Staatz, 1981), and there is similar mineralisation downstream from the Indian Mountain (84 Ma) hornblende granodiorite pluton.

Figure 3. Whole rock ferric:ferrous ratios for selected mid-Cretaceous plutonic suites in Alaska and Yukon. Data from: Richter et al. (1975a); Luthy et al. (1981); Blum (1983); Miller (1989); Solie et al. (1990); Burns et al. (1991, 1993); Newberry et al. (1990a), Newberry and Solie (1995); Patton and Moll-Stalcup (2000); and Hart (unpublished). The dashed line represents the boundary between magnetite- and ilmenite-series granitoids.

79


Tanacross-Dawson Range-Whitehorse Belt (112-91 Ma)Extending from east-central Alaska through western Yukon, south to the British Columbia

border, the Tanacross-Dawson Range-Whitehorse plutonic belt (Fig. 2) consists of plutons in the Tanacross area, the Gardiner pluton of eastern Alaska (Richter et al., 1975a), and batholiths of the Dawson Range in eastern Yukon (Klotassin batholith of Tempelman-Kluit and Wanless, 1975, 1980). It also can likely be extended, by lithological similarity, to include the Whitehorse plutonic suite in southern Yukon (Morrison et al., 1979; Hart 1995, 1997). Most intrusions form large northwest-trending batholiths that intrude metamorphic rocks of the Yukon-Tanana terrane, except for the Whitehorse suite that comprises smaller epizonal plutons mainly intruding rocks of the Stikine terrane (Fig. 1). Coarse-grained, euhedral hornblende-biotite granodiorite is the dominant lithology, with locally more mafic diorite and gabbro, and locally fractionated bodies of quartz monzonite and biotite granite (MacIntyre and Coffee Creek suites in Yukon; Hart, 1997), and even more felsic components such as leucocratic quartz monzonite (Richter et al., 1975a). These rocks are metaluminous in composition and follow a calc-alkaline fractionation trend. They decrease in age to the west, with rocks of the Whitehorse suite dated from 112-109 Ma, Dawson Range from 106-99 Ma, and Tanacross from 99-91 Ma. Initial strontium ratios average 0.7045 and 0.706 for the Whitehorse and Dawson Range intrusions, respectively (Hart, 1995; Selby et al., 1999). All rocks have coarse primary magnetite and high aeromagnetic responses. The Dawson Range and Whitehorse suites have moderate to high magnetic susceptibilities and high Fe2O3/FeO ratios (Figs. 3 and 4). Thus, all plutons of this belt are classified as magnetite-series.

Intrusion-related mineralisation is mostly limited to small molybdenite and copper porphyry and skarn occurrences (e.g., Toni-Tiger), and small gold-bearing veins, porphyries and placers (e.g. Mt. Freegold/Antoniuk in the eastern Dawson Range). Significant mineralisation, however, is associated with rocks of the Whitehorse plutonic suite, which generated the numerous Cu-Au-Ag skarns of the Whitehorse Copper Belt that produced 7t Au, 23t Ag and 123,000 t of Cu prior to 1983 (Tenney, 1981).

Ruby-Kaiyuh-Nyac Belt (117-108 Ma)The Ruby-Kaiyuh-Nyac belt of interior Alaska is dominated by the northeast-trending,

400-km-long batholiths of the Ruby geanticline (Fig. 2). The plutons intrude Proterozoic and Paleozoic metamorphic rocks of the Ruby terrane and the Paleozoic and Mesozoic oceanic rocks of the Angayucham-Tozitna terranes. Batholiths are dominated by coarse-grained to porphyritic leucocratic biotite granite, with lesser granodiorite (mainly in the north), muscovite-biotite granite, syenite, and monzonite. Plutons are moderately to weakly peraluminous with >0.8% normative corundum (Miller, 1989), particularly in the larger plutons to the south, although neither cordierite, nor garnet, have been identified. Initial strontium and neodymium ratios are generally radiogenic, but decreasingly so to the north (Arth et al., 1989b). Assorted U-Pb, Ar-Ar, and K-Ar ages range from 112 to 108 Ma. The plutons have generally low ferric:ferrous ratios (~0.5 at 70% SiO2 ; Fig. 3), low magnetic susceptibilities, and generally low aeromagnetic responses, particularly in more southerly batholiths. Ilmenite and lesser titanite have been reported (Miller, 1989), and the batholiths are likely mainly ilmenite-series. Geochemical and isotopic characteristics are consistent with


80

their formation by melting of, or contamination with, Paleozoic continental crust (Miller, 1989; Arth et al., 1989b). Fewer data are available for the smaller plutons in the Kaiyuh (112 Ma) and Nyac (117-108 Ma) regions, but they are interpreted to form the more southerly extensions of the Ruby suite, as they are similar-aged plutons, with similar lithologies and geochemistry (Patton et al., 1984; Miller, 1989).

Intrusion-related mineral deposits in the Ruby-Kaiyuh-Nyac belt are sparse, with a few small tungsten skarns, tin greisens (i.e., Sithylemenkat), and assorted tin, niobium, and gold placers (i.e., Ruby, Melozitna and Tofty districts). The widespread occurrence of cassiterite in placers suggests that the small, but numerous, lode occurrences represent the remains of a larger tin province that was eroded during unroofing of the geanticline (Hudson and Reed, 1997). Minor copper-molybdenum-tungsten showings occur in the north of the belt, where some of the granites are more oxidized (Patton and Miller, 1970; Clautice, 1983; Clautice et al., 1993; Baker and Foley, 1986; Blum et al., 1987). The most significant mineralisation is a series of Ag-As-Au±Sn-Zn-Pb prospects and deposits immediately south of the Kaltag Fault, including the 113 Ma Illinois Creek deposit and the Honker and Perseverance silver-bearing vein occurrence (Flanigan, 1998).

Yukon-Tanana Uplands Plutons (109-102, 93 Ma)Voluminous Cretaceous intrusions in the central Yukon-Tanana Uplands, mainly the

area between the Shaw Creek and Volkmar faults, are characterized by large batholiths that intrude amphibolite-grade Paleozoic metasedimentary and metaigneous rocks of the Yukon-Tanana terrane (Fig. 2). Slightly to moderately peraluminous plutons vary from fine-grained to coarse-grained, porphyritic leucocratic granite and granodiorite, to less-common quartz monzonite (Smith et al., 1999; Dilworth, 2003; Day et al., 2003). Biotite dominates over sparse hornblende, and muscovite is rare (e.g., Newberry et al., 1990b). Most reported U-Pb dates for batholiths and plutonic rocks are from 109 to 102 Ma, but many dates from dykes and smaller and more metaluminous/mafic bodies are about 93 Ma, which is similar to a large number of Ar-Ar dates on igneous and hydrothermal micas (Smith et al., 1999; Dilworth et al., 2002; Selby et al., 2002; Day et al., 2003; Werdon et al., 2004). The nature of the plutonic rocks, large batholiths, and high regional metamorphic grade suggest that the intrusions were emplaced at mid-crustal levels. The batholiths all have low and flat aeromagnetic signatures and mostly have Fe2O3/FeO ratios <0.5 (Fig. 3). Limited work on the Goodpaster batholith indicates that ilmenite and titanite are dominant over magnetite, and it has low magnetic susceptibilities (0.1-0.2) and Fe2O3/FeO ratios (Dilworth, 2003; Werdon et al., 2004).

Mineralisation in the region is dominated by the high density of occurrences that form the 40-km-long, southeast-trending Pogo-Blue Lead gold belt located south of the Goodpaster batholith. Mineralisation includes the world-class Pogo deposit (5.6 Moz Au; Smith et al. 1999), but the genetic relationship between gold mineralisation and the intrusions is controversial (Goldfarb et al., 2000; Hart et al., 2002; Groves et al., 2003; Rhys et al., 2003). The 104 Ma age of the Pogo deposit (Selby et al., 2002) is the same as the age of the dominant plutonic event in the region, but is also only a few million years younger than the regional metamorphic event that affected the host rocks (Day et al., 2003). Some smaller

81


gold occurrences in the area have the same age as the Pogo deposit (i.e., Blue Lead, 105 Ma, Newberry et al., 1998), whereas, other gold prospects have been dated at ca. 94 Ma (i.e. Shawnee Peak, Prospect 4021; Selby et al. unpublished). The uplands region is geologically complex and poorly studied, and, except for a few small molybdenum- and tungsten-rich porphyry and skarn prospects (i.e. Rainy Mountain, Boulder Creek, Section 21, and Lucky 13 occurrences), also with the same bimodal age distribution (e.g., Newberry et al., 1998), the role of the granitoids in generating economically significant gold mineralisation is equivocal.

Anvil-Hyland-Cassiar Belt (110-96 Ma)The Anvil (110-98 Ma), Hyland River (106-96 Ma) and Cassiar (110-100 Ma) plutonic

suites (Fig. 2) comprise a >900-km-long belt (reconstructed across Tintina Fault) of large, like-aged plutons and batholiths with similar, peraluminous-leaning geochemical and lithological characteristics (Pigage and Anderson, 1985; Driver et al. 2000; Heffernan and Mortensen, 2000) (Fig. 2). All suites intrude Paleozoic and Neoproterozoic stratigraphy that comprises the ancient North American continental margin (Selwyn Basin) or displaced pieces of the margin (Cassiar Terrane). Lithologies are dominated by variably porphyritic, peraluminous to slightly peraluminous biotite granite and granodiorite, with local muscovite-bearing phases. The Anvil suite of plutons consists of three phases: peraluminous muscovite-biotite granites and two groups of metaluminous to peraluminous hornblende-biotite granodiorites to granites. The Cassiar batholith includes biotite>hornblende granodiorite and quartz monzodiorite in the south and muscovite-biotite granite in the north. As is typical in these plutons, there are a wide range of, dominantly radiogenic, isotopic ratios (i.e., initial strontium ratios of 0.706 to 0.740; εNd=-2.7 to -17, δ18O of 10 to 12 per mil are reported for the Cassiar batholith; Driver et al., 2000).

Magnetic susceptibilities for Hyland River rocks are wide-ranging but mostly moderate (Fig. 4), as are aeromagnetic responses which show that the Anvil suite rocks are low and less magnetic than Hyland or Cassiar suites. Ferric:ferrous ratios of Quiet Lake batholith rocks, considered part of the Cassiar suite, are between 0.5 and 1.0 (C. Hart, unpublished). Biotite of the Seagull suite, a highly fractionated sub-suite of the Cassiar suite (Liverton and Alderton, 1994), has compositions that indicate reduced fO2 between NNO and QFM (Liverton, 1999). Together, most of these data are consistent with an ilmenite-series classification suite, but Driver et al. (2000) note that titanite is common in most of the Cassiar batholith, and that magnetite is dominant over ilmenite except in the northern parts of the batholith. Heffernan and Mortensen (2000) suggest that magnetite is the common oxide mineral in Hyland intrusions. We interpret the Anvil suite to be ilmenite-series, whereas the Cassiar and Hyland suites although leucocratic and generally containing low amounts of Fe-Ti oxides, are weakly oxidized, and possibly weakly magnetite-series. The Seagull suite intrusions are reduced and ilmenite-series.

Associated mineralisation is characterized by numerous tungsten±molybdenum skarns and veins, with distal Ag-Pb-Zn veins. In particular, the Cassiar suite intrusions and adjacent calcareous strata define a particularly favorable environment for scheelite skarn formation, with dozens of small occurrences and a few small deposits (Risby, Obvious, Stormy). The


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most significant mineral occurrences are a series of tin skarns, including the JC tin skarn (1.25 Mt of 0.2 SnO2; Layne and Spooner, 1991) that is associated with the Seagull suite, and the Logtung porphyry tungsten deposit (162 Mt of 0.14% WO3) intrusion also within the Cassiar belt.

Southern Alaska Range (100-88 Ma)A poorly exposed cluster of mid-Cretaceous granodiorite, diorite and pyroxenite intrusions

occurs in the southern Alaska Range of southwestern Alaska, with the full areal extent of the plutons uncertain. These intrusions, nonetheless, are recently recognized as being associated with important mineralisation in that they host the very large Pebble porphyry copper-gold-molybdenum deposit (Bouley et al., 1995), as well as other similar occurrences such as the Neocola deposit. Recent company estimates suggest the Pebble deposit contains 16.5 billion

Figure 4. Magnetic susceptibility values for selected Yukon plutonic suites. The magnetic susceptibility boundary between magnetite and ilmenite series granitoids is approximately 3.0 (Ishihara, 1977).

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pounds of copper and 26.5 million ounces of gold within an inferred resource of 2.74 billion tones grading 0.27% Cu, 0.015% Mo, and 0.3 g/t Au, and hydrothermally altered rocks occur over an area of approximately 90 km2 (Northern Dynasty Minerals, Ltd., January 21, 2004). The intrusions are non-radiogenic and likely have a high mantle contribution (Young et al., 1997). Dating of the Pebble and Neocola deposits and host intrusions indicate ages are mostly between 100 and 88 Ma (Young et al., 1997; Schrader et al., 2001).

Tok-Tetlin Belt (101-93 Ma)This 600-km-long northwest-trending Tok-Tetlin belt consists of numerous elongate

plutons and small batholiths that occur north of the Denali fault system, from the northernmost Alaska Range to western Yukon (Fig. 2). Igneous rocks include the Tok-Tetlin and Cheslina intrusions in Alaska (Richter et al., 1975a), and some of the Nisling Range granodiorite plutons in Yukon (Tempelman-Kluit, 1976), which have intruded rocks of the southern Yukon-Tanana and Windy-McKinley terranes. These rocks are poorly studied, but consist of hornblende-biotite granodiorite, quartz monzonite, quartz monzodiorite, and pyroxenite. The belt may extend further north to the Bonnifield area where similarly reduced granitoids occur, or alternatively, plutons in this region may define a southerly extension of the Fairbanks-Salcha suite. Ages are mostly 101-93 Ma, but there are a few outliers at 107 and 88 Ma. Despite their high iron contents and apparent metaluminous compositions, the plutons generally have low aeromagnetic responses and low Fe2O3/FeO ratios (<0.3; Fig. 3) indicating a reduced character. As such, these rocks are interpreted to be ilmenite-series. Only a few small stibnite- and gold-bearing vein prospects have been recognized in association with this suite of plutons.

Tungsten Plutonic Suite (97-94 Ma)Forming a 300-km-long northwest-trending belt of small to moderate-sized plutons along

the eastern margin of the Selwyn basin, the Tungsten plutonic suite straddles the border of western NWT and eastern Yukon (Fig. 2). The 97-94 Ma plutons consist of leucocratic biotite granite, monzogranite, and quartz monzonite. Biotite is dominant, hornblende is present locally, and sparse muscovite is reported (Gordey and Anderson, 1993), but garnet is rare, and cordierite not reported. Intrusions are weakly to moderately peraluminous (Hart et al., 2005). Initial strontium ratios are high, typically >0.720. All plutons have low aeromagnetic responses, low whole rock Fe2O3/FeO ratios (0.1-0.3; Fig. 3), and low magnetic susceptibilities (<1; Fig. 4). Associated skarn assemblages are very reduced (Dick and Hodgson, 1982), skarn-forming fluids were methane-bearing (Bowman et al., 1985; Gerstner et al., 1989), and biotite compositions of the granitoids have very low Fe3+/Fe2+ ratios and, therefore, low fO2 (Keith et al., 1990). Intrusions of the Tungsten suite are characterized by accessory ilmenite, and thus represent ilmenite-series granitoids. World-class scheelite skarn deposits at Mactung and Cantung (57 Mt of 0.95% WO3 and 9 Mt of 1.4% WO3, respectively), and smaller deposits at Clea and Lened (Sinclair, 1986) are the most significant intrusion-related deposits. These deposits have associated sub-economic copper, and local zinc and tin enrichments. Small tin greisens, and distal lead-zinc and antimony vein occurrences are also associated with this suite of igneous rocks (Hart et al., 2005).


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Mayo Plutonic Suite (95-92 Ma)Approximately one hundred small (1-20 km2), 95-92 Ma plutons comprise the 400-km-

long Mayo plutonic suite (Hart et al., 2005) that spans across central Yukon (Fig. 2). The plutons intrude the northern margin of the Neoproterozoic to Paleozoic Selwyn basin. The plutons are dominated by porphyritic to coarse-grained leucocratic quartz monzonite, but also include granodiorite and biotite granite (Murphy, 1997). Biotite is dominant, although locally hornblende and clinopyroxene are present. Most plutons are metaluminous, with high SiO2 plutons verging to weakly peraluminous compositions. Lamprophyre dykes compose a small, but important component of most of the Mayo plutons. Initial strontium isotopic values of 0.710 to 0.730 indicate that the rocks are highly radiogenic. All plutons contain titanite as the dominant Fe-Ti phase, and rare magnetite and ilmenite. The plutons have low aeromagnetic responses (Hart et al., 2000), low Fe2O3/FeO ratios (0.2-0.5; Fig. 3), and low magnetic susceptibilities (0.09-0.4, Fig. 4).

Intrusions of the Mayo suite are associated with numerous gold occurrences, both intrusion- and contact aureole-hosted, including deposits at Dublin Gulch, Scheelite Dome, and Clear Creek (Murphy, 1997; Hart et al., 2000). Economically significant gold placer deposits have been worked for more than one hundred years in areas downstream of the intrusion-related gold lodes. The intrusions are also responsible for numerous tungsten skarn deposits (Ray Gulch, Kalzas) and a few small tin occurrences (Sinclair, 1986). In addition, the Ag-Pb-Zn vein deposits of the Keno Hill district, which produced >200 Moz Ag, are proximal to one of these intrusions, but the genetic relationship is unclear (Lynch et al., 1990).

Fairbanks-Salcha Plutonic Suite (94-90 Ma)Clusters of small plutons, forming a 150-km-long belt from the Fairbanks to the Richardson

areas (Fig. 2), have several associated intrusion-related mineral occurrences. The Fairbanks-Richardson plutons all intrude the dominantly greenschist grade, early Paleozoic and older Fairbanks Schist. Compared to intrusions of the Yukon-Tanana Uplands, the Fairbanks and Salcha plutonic suites represent sparse and isolated plutons rather than vast batholiths exposed south of the Shaw Creek fault. The Fairbanks-Salcha granitoids may be the western continuation of the Mayo plutonic suite that has been offset dextrally along the Tintina Fault.

Fairbanks area plutons are dominated by the Gilmore Dome pluton, the smaller Pedro Dome pluton, and the gold-rich Fort Knox (Vogt) stock (Newberry et al., 1996). These intrusions are characterized by weakly peraluminous porphyritic, biotite>hornblende granite and fine-grained granodiorite (Forbes, 1982; Blum, 1983, 1985; Newberry et al., 1990b; McCoy et al., 1997). Strongly peraluminous aplite and pegmatite phases are also present. Biotite is the dominant mafic mineral, with subordinate hornblende in some plutons, although hornblende is particularly common in the Fort Knox stock. Reliable dating from the region indicates a narrow range of 94-90 Ma for pluton ages, which is similar to ages of the Richardson district intrusions (McCoy et al., 1997). Initial strontium ratios are approximately 0.712 (Blum, 1985; McCoy et al., 1997). The plutons have low magnetic susceptibilities, flat aeromagenetic signatures that are indiscernible from the background schists, low Fe2O3/FeO ratios of 0.1-0.4 (Fig. 3), low fO2 values of -17 to -14 (McCoy et al., 1997), and lack magnetite

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as a primary accessory mineral, but contain ilmenite (McCoy et al., 1997) and appreciable titanite (Blum, 1985). Newberry et al. (1990b) define these as subduction-related, variably fractionated I-type plutons, which have been contaminated with perhaps 5-15 percent crustal material.

Plutons in the adjacent Richardson district consist of a few sparse ca. 90 Ma high-level biotite granodiorite and quartz-feldspar porphyry stocks and dykes (McCoy et al., 1997). Intrusive rocks in the Richardson district consist of high-level quartz-feldspar porphyry intrusions (McCoy et al., 1997), which are generally more oxidized than most of the Fairbanks plutons (Fig. 3). Few studies exist on the plutons in the Salcha River area, but the igneous rocks include granite, granodiorite, and-two mica granites that yield two populations of ages of ca. 106 and ca. 94 Ma (Werdon et al., 2004). They also have low aeromagnetic signatures, low magnetic susceptibilities, and contain ilmenite (Werdon et al., 2004), although they are slightly more oxidized than the Fairbanks intrusions (Fig. 3).

Mineralisation in the Fairbanks area is dominated by the low-grade, high-tonnage, producing Fort Knox gold deposit (5.5 Moz), which is hosted in a small and variably porphyritic granite stock (Bakke, 1995). Sheeted, low-sulfide quartz±feldspar veins, dated at 92.4 Ma by Re-Os (Selby et al., 2002), contain most of the low grade (~0.93 g/t Au) gold resource, and are reported in places to be transitional with mineralised feldspar-rich veins that also cut the various phases of the Fort Knox stock (Bakke, 1995; McCoy et al., 1997). Also in the Fairbanks area, the sheared margins of a small hornblende-biotite tonalite stock host ca. 90 Ma arsenopyrite-rich gold mineralisation at the Ryan Lode deposit. Other ca. 93 Ma plutons in the Fairbanks area are spatially associated with small, sub-economic tungsten skarns (i.e., Stepovich and Spruce Hen; Allegro, 1987; Newberry et al., 1997). Additionally, the region has yielded >8 Moz of placer gold (Bundtzen et al., 1996) and contains a large number of schist-hosted gold veins that have been genetically related to distal 94-90 Ma intrusions (McCoy et al., 1997), although this relationship has been questioned by some workers (i.e., Goldfarb et al., 1997; Hart et al. 2002). Similarly, the undated, presently-producing True North gold deposit, a thrust-hosted stockwork to disseminated-style deposit, is several kilometres from an intrusion and lacks intrusion-related characteristics as defined by Lang et al. (2000) and Hart et al. (2002). The few significant Au±Ag, As, Bi, Te, Sb, and Pb occurrences in the Richardson district are related to an 88 Ma biotite granodiorite and quartz feldspar porphyry stock (McCoy et al., 1997).

Livengood - Tombstone Belt (92-90 Ma)A narrow, 300-km-long east- to northeast-trending belt of quartz-alkalic plutons that intrude

rocks of east-central Alaska (Reifenstuhl et al., 1997a; quartz-alkalic plutons of Newberry, 2000, herein called the Livengood suite) is correlated across ~400 km of dextral displacement along the Tintina fault system to correlate with the Tombstone plutonic suite of west-central Yukon, as shown in Fig. 2 (Anderson, 1987; Hart et al., 2005). Each of the suites consists of approximately ten compositionally zoned bodies that are dominated by moderately alkalic phases, such as biotite-, hornblende-, and/or pyroxene-bearing monzonite and syenite, with lesser quartz-monzonite, monzodiorite, monzogabbro, and silica-undersaturated high-K phases,


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such as tinguaite (Anderson, 1987). All bodies have ages between 92 and 87 Ma (Reifenstuhl et al., 1997b; Anderson, 1987), and elevated initial strontium ratios of approximately 0.710 (Reifenstuhl et al., 1997a; Lang, 2001). Magnetite-bearing, quartz-poor marginal phases have been recognized in some plutons (Hart et al., 2005), locally with zonations to magnetite-barren quartz-rich interiors (Abercrombie, 1990; Reifenstuhl et al., 1997a). Most plutons have a moderate to low, but internally variable, aeromagnetic response, locally with concentric zoning and highly magnetic phases (Hart et al., 2000). Tombstone suite rocks have a wide range of Fe2O3/FeO (Fig. 3) and magnetic susceptibility values (Fig. 4), and include many igneous bodies that are oxidized, and they are, therefore, assigned to the magnetite-series.

Uranium-Th-REE mineralisation (Roy Creek in Alaska, Tombstone/Ting in Yukon) is most commonly associated with igneous rocks in both suites of the belt, but the Tombstone suite is also associated with the Brewery Creek gold deposit and the Marn and Horn Au-Cu-Bi skarns in Yukon (Brown and Nesbitt, 1987). In addition, the Wolverine and Sawtooth Mountain occurrences in Alaska and the similar Antimony Mountain occurrence in Yukon are each dominated by stibnite veins with anomalous precious metal concentrations.

Redox State Characteristics

Aeromagnetic CharacterTotal field aeromagnetic signatures are responsive to magnetite content, such that

rocks with larger volumes of magnetite generally yield elevated aeromagnetic responses (Grant, 1985). This is particularly the case for mid-Cretaceous rocks that cooled through the Currie temperature during the Cretaceous normal superchron, such that they are not affected by inverted dipole effects from cooling during magnetic reversals. As such, regional aeromagenetic surveys over the mid-Cretaceous magmatic rocks in Yukon and Alaska allow for first-order interpretations about the magnetic character and, therefore, the redox series of plutons. Aeromagnetic data (e.g., Saltus 1997; Saltus et al. 1999; Lowe et al. 2003) indicate that some suites, such as the Nutzotin, Cassiar, or Tungsten suites, show consistent responses, be they either high, moderate, or low, respectively. Others responses, such as for the East Seward suite, are variable (Fig. 5), potentially indicating complexities in either the redox state of the magmas of the suite, or problems in the assignment of particular plutons to a specific suite. This is particularly a problem in Alaska, where less detailed mapping and less precise age-dating make suite assignment of individual plutons less certain. Complex redox states may also be a characteristic intrinsic to peralkaline rocks, which are typically hotter and remain above the solidus longer, and thus have a greater capacity to incorporate reducing sedimentary rocks. Additionally, sub-solidus effects could cause the oxide mineralogy to change as a function of the fO2 of the magmatic fluids.

Magnetic SusceptibilityMagnetic susceptibility measurements are essentially a direct indicator of the average

magnetite volume in a rock. Determinations are herein presented for many of the plutons in the Yukon suites (Fig. 4), but few are available for Alaskan granitoids. Our measurements

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were made on several freshly broken, unoxidized, surfaces using a KT-9 handheld magnetic susceptibility meter, with a pin-spacer to account for analytical variations from uneven surfaces. All values presented here are averages of 6-10 determinations per rock, at x10-3 SI units. Ishihara et al. (2000) suggested that the value of 3.0 (equivalent to 100x10-6 emu/g in Ishihara, 1979) be used to separate magnetite- from ilmenite-series granitoids, at 70% SiO2.

Some plutonic suites have restricted ranges of magnetic susceptibility, whereas many have wide ranges of values. The calc-alkaline Dawson Range is the most magnetic, with a mean value of 5.1 that supports the presence of primary magnetite. However, among the wide range of values for the Dawson Range, there are many of <0.1 from highly fractionated, iron-poor rocks. Similarly, plutons from the alkalic Tombstone suite yield a wide range and a slightly elevated average value of 1.8, supporting the occurrence of primary magnetite in some samples (Hart et al., 2005). The Mayo and Tungsten plutonic suites are the least magnetic. All measurements, exclusive of those for a few pyrrhotite-bearing rocks from the latter suite, are <1.0, and average values are 0.09 and 0.16, respectively. The Hyland River and Cassiar suites have wide ranges, with average values of 2.3 and 0.77 respectively. These rocks, although dominantly peraluminous with highly radiogenic isotopic signatures (i.e., Driver et al., 2000), locally host titanite and minor magnetite and have slightly elevated aeromagnetic signatures and are thus classified as weakly magnetite-series.

Measurements of plutons directly associated with mineral deposits are more restricted in range (Fig. 6). Plutons at Dublin Gulch, Mactung, Cantung and Logtung are all associated with tungsten skarns and yield low values, mostly <0.5 representing ilmenite-series suites. The values for the intrusion at Mactung are particularly low, averaging only 0.03. The gold-enriched Fort Knox pluton has similar low values, as does the Dublin Gulch pluton, both

Figure 5. Plutons of the East Seward and eastern South Seward plutonic belts are shown with underlying total field aeromagnetics (white is high, black is low) to highlight their aeromagnetic character and variability. Plutons are named.


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are gold-dominant systems but have associated tungsten mineralisation. This low magnetic character and reduced state of plutons associated with gold deposits helped provide the basis for the reduced intrusion-related gold deposit model (Lang et al., 2000; Thompson and Newberry, 2000). The Whitehorse pluton has high magnetic susceptibility values (average 13) that support the presence of considerable primary magnetite and indicates that associated copper-gold skarn mineralisation is related to a highly-oxidized, magnetite-series calc-alkaline pluton.

Most of the reduced plutons and suites have a high proportion of determinations between 0.1 and 0.5. Although a value of 3.0 has been suggested as the division between magnetite and ilmenite-series granitoids (Ishihara, 2000), the reduced rock units measured in this study generally do not exceed 0.5, and this value may best provide an empirical division between magnetite and ilmenite-series granitoids for the rocks in the northern Cordilleran mid-Cretaceous plutonic province.

Figure 6. Magnetic susceptibility values for selected plutons associated with significant intrusion-related mineral deposits.

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Ferric:Ferrous RatiosAn approximation of the oxidation state of a magma can also be obtained from

determinations of whole-rock ferrous and ferric iron contents. Magnetite and ilmenite-series granitoid groups have been divided at an Fe2O3:FeO ratio of 0.5, at 70% SiO2 (Ishihara, 2000). Compiled Fe2O3:FeO whole-rock ratios, determined from major element geochemistry on mid-Cretaceous granitic rocks in Alaska and Yukon, range from as low as 0.06 to as high as 8.1 (Fig. 3). Many plutonic suites yield narrow ranges of ratios that increase with increasing silica. Suites with large data ranges and poor positive correlations with silica may partly result from incorrect suite assignment of plutons due to poor age control or insufficiently detailed mapping. The most reduced determinations were on plutons of the poorly understood Tok-Tetlin belt, where two of the ratios are 0.06, with others ≤ 0.3, but together showing decreasing redox conditions with increasing silica content. Plutons in the Dawson Range and East Koyukuk are among the most oxidized, with Fe2O3:FeO ratios consistently >1.0, and positive correlations with silica. The generally lower values for the Tanacross intrusions (0.4-1.0) suggest that they may not be part of the same suite with the Dawson Range plutons as suggested above.

Data from the Fairbanks area have a wide data range (0.1-2.0), with both increasing and decreasing ratios with increasing silica. Ratios from Salcha suite rocks are all higher than those from the Fairbanks suite for a given silica content. Rocks from the Yukon-Tanana Uplands also have a wide range of ratios (0.08-1.0), probably indicative of a several plutonic suites that have yet to be individually distinguished in this region.

Plutons from western Alaska are similarly varied. Rocks from the East Koyukuk suite have ratios clearly indicative of oxidized magmas (1-3), and ratios that increase with silica. West Koyukuk data are mostly from the ultrapotassic suite of intrusions and show wide oxidation-state variations (0.3-5), but are generally oxidized (>0.8) without any trends with varying silica contents. The Ruby intrusions mostly have ratios between 0.4 and 1.0, and show a broader trend of decreasing redox state with increasing silica. These values are higher than expected for these mainly crustal melt-derived plutons that lack magnetite (Miller, 1989; Arth, 1994). The lower Fe2O3:FeO ratios for the Kaiyuh intrusions may result from significant amounts of contamination in the magmas by carbonaceous sedimentary country rocks.

Central and eastern Yukon plutons show clear trends of increased redox ratios with fractionation, with the alkalic Tombstone suite (0.2-2) being more oxidized and mostly within the limit typical of magnetite-series rocks. The plutons from the Mayo (0.15-1) and Tungsten (0.1-0.3) suites show values clearly below the dividing limit of 0.5. Hart et al. (2005) indicate that only the Tungsten suite rocks contain ilmenite and that the Mayo suite is dominated by titanite. However, according to Ishihara (1977, 1981), both may be classified as ilmenite-series due to their overall low contents of iron-oxide minerals.


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Space, Time and Metallogenic Distribution of Magnetite/Ilmenite Series Plutons

Distribution PatternsThe mid-Cretaceous plutonic epoch in Alaska and Yukon is represented by several, mostly

northwest-trending, plutonic belts that form a magmatic province locally as wide as 700 km, although this may, in part, result from duplication by displacements along the Denali and Tintina fault systems, as discussed below. Individual belts are generally 50- to 200-km-wide and follow the regional structural fabric. Magnetite-series plutonic belts are preferentially hosted in more seaward tectonic settings, whereas ilmenite-series intrusions are hosted in more landward settings (Fig. 7), presumably farther from paleo-subduction zones. Magnetite-series granitoids are mostly emplaced into the primitive arc terranes, whereas ilmenite-series intrusion are dominantly intruded into the ancestral North American continental margin, or displaced or metamorphosed pericratonic equivalents. There are, however, notable deviations from these associations. The magnetite-series plutons of the St. Lawrence Island, East Seward, and Tanacross-Dawson Range belts are hosted in pericratonic terranes (Seward and Yukon-Tanana terranes) and the magnetite-series Livengood and Tombstone intrusions are in North American continental margin strata, as are the weakly magnetic but slightly peraluminous Hyland and Cassiar suites of plutons. Only the ilmenite-series intrusions of the Tok-Tetlin belt occur in a seaward position and, importantly, ilmenite-series rocks are not hosted in primitive accreted terranes. These deviations indicate that neither the basement terranes, nor the immediate host rocks, may be the controlling element in determining magmatic redox state.

Plutonic suite distribution patterns in the northern Cordillera are complicated by the large dextral strike-slip displacements which, when reconstructed, would place the Insular Belt rocks 350 km south of their current position with respect to the Intermontane Belt rocks due to movement along the Denali fault system (Fig. 8). Similarly, Intermontane Belt rocks would, in turn, be about 430 km south of their current position with respect to ancestral North American rocks due to movement along the Tintina fault system. These reconstructions would have the effect of removing the telescoping and duplication of the more outboard magnetite-series plutonic belts, such that the northern end of the Nutzotin belt may correlate with the Whitehorse plutons, and the northern Cassiar suite would correlate with the southern Hyland intrusions. As a result, the plutonic belts may have originally been thinner, perhaps as much as one-half of their current 700 km width (Fig. 8).

Another complicating feature specific to plutonic belts in westernmost Alaska, is that they appear to be related to a different subduction zone than that that generated plutonic belts in southern and central Alaska. However, it is likely that, at least the St. Lawrence, East Seward, and Koyukuk belts are a single belt that has been bent across an oroclinal bend as shown in Fig. 7 (North Alaskan orocline of Johnston, 2001). The belt may continue to the southwest, under-cover, as the region is characterized by along-strike deep magnetic highs (Saltus et al., 1999). The belt likely bends again, this time to the east across the Kulukbuk Hills orocline. This east-trending belt potentially joins with the plutons in the southern Alaska Range making it possible that they may all have originally been part of the same magmatic belt (Fig. 7).

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Circum-Pacific magnetite and ilmenite-series plutonic belts have varied distributions (Ishihara 1998). Much of the eastern Asian continental margin comprises coastal magnetite-series, and inboard and more regionally extensive ilmenite-series granitoids. There are notable variations and uncertainties in these patterns that result from tectonic complexities such as the formation of marginal basins (Sea of Japan) and oceanic arcs (Philippines), and the broad, but submerged, continental shelf. On the Japanese Islands, paired fore-arc ilmenite-series and back-arc magnetite-series belts typify the Late Cretaceous-Paleogene granitoids and this pattern is repeated by the overprinting Miocene-Quaternary magmatic arcs (Ishihara 1981). The belts in Japan tend to be narrow (80-100 km), but on the Asian mainland, they are as broad as several hundred kilometers.

The Pacific coast of the Americas is dominated by magnetite-series granitoids belts, locally with thin coastward ilmenite-series belts such as in Northern Chile (Ishihara et al., 1984) and the Sierra Nevada batholith (Bateman et al. 1991). However, the opposite pattern is apparent in the Peninsular Ranges where ilmenite-series are dominant and are inboard from coastal magnetite-series granitoids (Gastil et al., 1990). In Peru, a thick coastal magnetite-series

Figure 8. Reconstruction of dextral displacements along the Tintina and Denali fault systems to show approximate alignment of magnetite and ilmenite-series belts as they would have been in the mid-Cretaceous, prior to early Tertiary offset.

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plutonic belt dominates with more restricted, inboard ilmenite-series belt (Ishihara et al., 2000).

The patterns presented in this paper for the northwestern North American Cordillera emphasize a broad, coastal magnetite-series belt with a broad, landward ilmenite-series belt, and a thin, innermost magnetite-series belt. The pattern is generally similar to that in the Peninsular Ranges and the eastern Asian continental margin but is broader in distribution, more restricted in time, and reflects a differing tectonic setting.

Temporal VariationsThe Early to mid-Cretaceous magmatic epoch spans approximately 55 million years, from

145 to 90 Ma, and was represented by episodic plutonism across the northwestern Cordillera. This was initially dominated by the widespread emplacement of magnetite-series plutons, subsequently followed by widespread ilmenite-series plutons, and terminated with a final minor episode of magnetite-series plutons. The oldest magnetite-series components (145-125 Ma) may have been pre-accretionary and coeval with Late Jurassic oceanic arc-building events prior to their accretion to form part of the Insular terranes on the outermost parts of the orogen. Subsequent magmatism (118-99 Ma) was also magnetite-series, with metaluminous, arc-type, calc-alkaline plutonism occurring further inboard, through the eastern Insular terranes to Stikinia in the Intermontane belt, and to the far northwest through the West Koyukuk belt and across St. Lawrence Island. Ilmenite-series granitoids were first emplaced at about 113 Ma, likely in response to crustal thickening in response to terrane accretion. These granitoids include the intrusion of the large Ruby-Kayuh-Nyac system, and then the belts of peraluminous granitoids that characterize the Yukon-Tanana Uplands. The later-stage suites (109-96 Ma; Cassiar, Hyland) are slightly more oxidized than those in the other ilmenite-series belts and are classed here as weakly magnetite-series. These were followed by the emplacement of reduced, ilmenite-series suites (98 to 92 Ma; Tungsten, Mayo, Fairbanks, Salcha). The last plutonic event is defined by the 92-90 Ma alkalic, variably magnetite-series Livengood and Tombstone suites (Figs. 2 and 7).

There is an overall inland younging trend in the ages of the plutonic belts regardless of their oxidation state. But magnetite-series plutonic belts were active 5-10 million years prior to the initiation of significant ilmenite series plutonism at ca. 109 Ma. Otherwise, magnetite and ilmenite series plutonism was contemporaneous during the period from 109 to about 95 Ma. However, ilmenite-series plutonism begins to dominate late in this episode (97-92 Ma), and to have been followed by the small, late, alkalic magnetite-series event. In broad agreement, Ishihara (1981) notes that magnetite-series suites in Japan are older than ilmenite-series granitoids, in both the Cretaceous and Miocene belts.

Figure 9 (following page). Distribution of significant mineral deposits and occurrences associated with mid-Cretaceous plutons in Alaska and Yukon. Metallogenic patterns developed emphasize a Cu-Fe-Mo tenor associated with outboard magnetite-series granitoids. Inboard ilmenite series granitoids have significant associated tungsten mineralisation, as well as notable gold associations. Alkalic rocks also have associated U-Th mineralisation. The dashed line is the main contact between magnetite and ilmenite series granitoids.


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MetallogenyMagnetite and ilmenite-series granitoid belts are characterized by distinctly different

metallogenic associations (Ishihara 1977, 1981; Blevin and Chappell 1992; Blevin et al., 1996). These associations characterise the Alaska and Yukon plutonic suites (Fig. 9), but there are important variations from the more classical examples. Details of the mineral deposits are summarized in Table 2. Mineral occurrences displaying a Cu-Au-Fe tenor are strongly associated with the magnetite-series plutons in the Insular terranes and in the St. Lawrence-Seward-Koyukuk belt. Molybdenum prospects are also present in the latter belt of igneous rocks. Tungsten mineralisation best characterizes rocks of the ilmenite-series, but is also associated with the weakly magnetite-series plutons that together form a >1000-km-long belt through the Cassiar, Hyland, Tungsten, Mayo, and Fairbanks plutonic belts, after reconstruction of the Tintina fault system displacement (Fig. 8). Magnetite-series alkalic plutonic belts each contain intrusion-related U±Th occurrences, locally also with gold and copper enrichments, as is displayed by the Tombstone, Livengood and alkalic parts of the Koyukuk suites.

Gold mineralisation also has an association with the ilmenite-series plutonic belts, particularly those of the Mayo and Fairbanks suites (Fig. 9). This association is counter to most granite-series metallogenic schemes (i.e. Blevin et al., 1996), but forms the basis of the intrusion-related gold system model (Thompson et al., 1999; Lang et al., 2000; Thompson and Newberry, 2000) that emphasizes an association between gold and moderately reduced intrusions. This association had been originally recognized empirically by Leveille et al. (1988) for intrusions in central Alaska.

Despite the variability of igneous rock compositions and metallogenic associations in the mid-Cretaceous plutonic epoch, significant intrusion-related tin mineralisation is rare, except for the JC and similar deposits associated with those very reduced intrusions of the western Cassiar suite (Seagull suite). Polymetallic mineralisation, characterized by Sn-Ag but also typically including Cu-Pb-Zn-Bi-Au, is represented by a few greisens and vein deposits, such as Illinois Creek in Alaska and Logan in Yukon.

Silver -Pb-Zn-bearing, Sb-rich, and other varieties of polymetallic vein occurrences are difficult to confidently characterize as intrusion-related, but in many regions, including the world-class Keno Hill silver district, mineralisation is associated with ilmenite-series plutons. The polymetallic occurrences are likely coeval with intrusion-related gold and tungsten mineralisation, and appear to be deposited as part of a cooling paragenetic hydrothermal sequence distal to the plutons. Polymetallic ores are also related to magnetite-series plutonism in accreted terranes, such as surrounding porphyry copper deposits of the Nutzotin plutonic belt in the Wrangell Mountains. However, in some districts polymetallic mineralisation is of uncertain affinity, particularly those occurrences on the ancient continental margin where syngenetic origins for polymetallic ores are also possible (e.g., East Seward, West Koyukuk). Silver-rich Pb-Zn mantos are preferentially developed above unroofed plutons that intrude North American strata (e.g., Midway Ag), and some have associated gold mineralisation (e.g. Ketza River Au). The redox state of those plutons is uncertain because they are not exposed, but they are likely to be ilmenite or weakly magnetite-series plutons of the Cassiar suite.


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Table 2. Intrusion-related mineral deposits and significant occurrences associated with Early and mid-Cretaceous felsic intrusions in Alaska and Yukon. Data compiled from Newberry et al. (1997), Nokelberg et al. (1994); Young et al., (1997), Sinclair (1986), Hart et al. (2000), Deklerk (2003) and specific references cited in table. Bold=deposit, not-bold=prospect/showing. *=district. Fairbanks district includes Gil, Yellow Pup, Stepovich, Spruce Hen, Pedro, Cleary; East Interior district includes Mitchell, Deer Creek, Oscar, Champion, Iron Creek; Charley River district includes Wolverine, Cresent Ck., Twin Mtn. and Creek Mtn.; Lucky 13 district includes Bonanza, Beef Ridge, Valley Ridge; Upper Chena includes MR45, MR52, N. Fork Salcha.

Deposit(*=district)

Commodity,Style

Miner’n Age(Ma)

Associated Plutonic Suite

References

Insular BeltMt. Hayes/Rainey Creek

Fe (Cu, Au) skarn 150 Nutzotin Rose (1966), Newberry et al. (1997)

McCarthy Fe (Cu, Au) skarn 140 Tosina MacKevett (1976), Newberry et al. (1997)

London & Cape Cu porphyry 140 Tosina Nokleberg et al. (1987)Midas Cu-Au porphyry 140 Tosina Nokleberg et al. (1987)Zackly Cu-Au skarn 130 Nutzotin? Ford (1988)Rambler Au skarn 114 Nutzotin Weglarz (1991)Nabesna Au-Cu-Ag skarn 114 Nutzotin Weglarz (1991)Bond Creek Cu porphyry 114 Nutzotin Richter et al. (1975b)Horsfeld Cu porphyry 111 Nutzotin Richter et al. (1975b)Baultoff Cu porphyry 111 Nutzotin Richter et al. (1975b)Carl Creek Cu porphyry 111 Nutzotin Richter et al. (1975b)Poovookpuk Mtn Cu-Mo porphyry 108 St. Lawrence Bundtzen et al. (1994), Patton

and Csejtey (1971, 1980)West Cape Mo-Cu porphyry 108 St. Lawrence Patton and Csejtey (1971, 1980)Orange Hill Cu porphyry (minor

skarn)105 Nutzotin Richter et al. (1975b)

Neacola Cu-Mo porphyry 94 Southwest Alaska Range

Reed and Lanphere (1969, 1972)

Pebble Cu porphyry 90 Southwest Alaska Range

Bouley et al. (1995), Young et al. (1997)

Intermontane BeltWhitehorse Copper Belt

Cu skarn 111 Whitehorse Tenney (1981)

Antoniuk/Mt. Freegold

Au porphyry, veins 108 Dawson Range Deklerk (2003)

Blue Lead Au 105 YT Uplands McCoy et al. (1997)Illinois Creek Ag-Sn, Au veins 105 Kaiyuh Flanigan (1998)Paternie Cu-Mo-Au porphyry 103 McCoy et al. (1997)Forty Mile Au-Cu 102 McCoy et al. (1997)Table Mountain W Au skarn 90 Livengood/Fairbanks Newberry (1987); McCoy et al.

(1997)Liberty Bell Au-Ag-Bi-Cu skarn 92 Tok-Tetlin Yesilyurt (1994)Fairbanks* W skarn 93 Fairbanks Allegro (1987)East Interior* Cu skarn 90 YT Uplands Burleigh et al. (1994); Newberry

et al. (1997)Upper Chena* W skarn 92 Salcha Albanese (1984); Newberry et al.

(1997)Charley River* W skarn YT Uplands Foley & Barker (1981); Newberry

et al. (1997)Lucky 13* W skarn 92 YT Uplands Burleigh et al. (1994);

Newberry et al. (1997)McLeod Mo porphyry Kiayuh West (1954)Fort Knox Au sheeted veins 94 Fairbanks Bakke (1995)Ester Dome Au veins 90 Fairbanks McCoy et al. (1997)Democrat Au 90 Salcha McCoy et al. (1997)Roy Creek Felsic U Livengood Burton (1981)Elephant Au 90 Livengood McCoy et al. (1997)

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Deposit(*=district)

Commodity,Style

Miner’n Age(Ma)

Associated Plutonic Suite

References

Koyukuk BeltPlacer River Mo porphyry 103 Seward/KoyukukZane Hills Cu-Au porphyry ~100 East Koyukuk Miller (1976), Miller and Elliot

(1977)Clear Creek Felsic U ~100 East Koyukuk Miller (1976)Wheeler Creek Felsic U ~100 East Koyukuk Miller (1976)Billiken Fe skarn 100 East Seward Gamble and Till (1993)Eagle Creek Felsic Plutonic U 97 East Seward Miller and Bunker (1976)Independence Ag-Pb-Zn veins East Seward Hudson et al. (1977)Quartz Creek Pb-Zn-Ag veins West Koyukuk Bundtzen et al. (1994)Perseverance/Beaver Creek

Pb-Zn-Sb veins Koyukuk Nokleberg et al (1994)

Purcell Mtn. Mo-Cu porphyry KoyukukWindy Creek Mo porphyry East Seward Hudson et al. (1977)Kanuti W skarn 91? Ruby Clautice (1983)

North American MarginLogtung W porphyry 112 Cassiar? Abbott (1986); Noble et al.

(1984)Ketza River Au-Ag manto 108 Cassiar Fonseca (1998)Silver Hart Ag-Zn-Pb skarn 103 Cassiar Abbott (1983)Risby W-Cu skarn 103 Cassiar Deklerk (2003)Quartz Lake Ag-Zn-Pb manto - Hyland? Vaillancourt (1982)Hyland Au Au disseminated - HylandBailey W-Cu skarn - Hyland Deklerk (2003)Keno Hill Ag-Pb-Zn veins - Mayo Lynch et al. 1990Sa Dena Hes Pb-Zn-Ag-Sn skarn/

manto- Hyland Deklerk (2003)

Logan Pb-Zn-Ag ? ~102 Cassiar Deklerk (2003)JC Sn skarn ~101 Cassiar (Seagull) Layne and Spooner (1991)Mactung W-Cu skarn 95 Tungsten Dick and Hodgson (1982),

Atkinson & Baker (1986)Cantung W skarn 95 Tungsten Mathieson and Clark (1984);

Bowman et al. (1985)Ray Gulch W skarn 95 Mayo Lennan (1986)Dublin Gulch Au sheeted veins 94 Mayo Hitchins and Orssich (1995),

Maloof et al. (2001)Scheelite Dome Au veins, sheeted

disseminated94 Mayo Mair et al. (2000)

Clear Creek Au veins, sheeted 93 Mayo Marsh et al. (2003)Clea W skarn ~93 Tungsten Godwin et al. (1980)Lened W skarn ~93 Tungsten Glover and Burson (1986)Midway Ag-Pb-Zn manto Cassiar Bradford and Godwin (1988)Kalzas W vein/stockwork 91 Mayo Lynch (1989)Brewery Ck. Au disseminated 92 Tombstone Diment and Craig (1999);

Lindsey et al. (2000)Marn/Horn Au-Cu skarn 92 Tombstone Brown and Nesbitt (1987)Ting/Noting U felsic pluton 90 Tombstone Deklerk (2003)

Table 2. (cont.)


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The plutonic belts with little genetically-associated metallic mineralisation are those that form ilmenite-series batholiths such as in Ruby and Yukon-Tanana Uplands, and the magnetite-series batholiths of the Tanacross-Dawson Range. All of the plutons in these belts intruded amphibolite-grade metamorphic rocks, after peak deformation and probably at mid-crustal levels. The associated voluminous, ilmenite-series magmas, were low temperature, viscous, and mainly anhydrous, and are therefore, unlikely to have exsolved a metal-bearing magmatic hydrothermal fluid. The highest densities of intrusion-related mineral deposits are associated with the reduced and radiogenic, ilmenite-series Fairbanks and Mayo suites, and the oxidized and primitive, calc-alkaline, magnetite-series Nutzotin-Kluane belt. Magmas related to mineralisation in these belts intruded to form small plutons at shallow crustal levels; many remain partly or completely unroofed. Small-batch, higher-temperature hydrous magmas emplaced at high crustal levels, where pressure and temperature gradients are steep, appear to best dictate prospectivity. Given these conditions, magmatic processes responsible for volatile phase separation and metal-enrichment operate efficiently.

Tectonic Setting of Redox-Series GranitoidsThe regional and temporal variations in redox-series granitoids reflect their evolving

tectonic settings, source materials, and magmatic processes. It is widely recognized that oxidized magmas are mostly the calc-alkaline and metaluminous products generated by partial melts in the mantle wedge above a subducting slab. These melts can be modified by assimilation-fractional crystallization (AFC) processes during ascent and interactions with overlying, non-carbonaceous crust (Gill, 1981; Ishihira, 1981). Oxidized alkalic (potassic) magmas are derived through melting of metasomatized mantle, typically during post-collisional to extensional regimes in rifting environments (Bailey, 1983). Ilmenite-series magmas are typically generated in compressional settings through the melting of, or interaction of melts with, reduced, carbonaceous sedimentary rocks that are typical of continental margin sequences (Ishihira, 1981).

Most of the magnetite-series plutonic belts in Alaska and Yukon are typical calc-alkaline, metaluminous, I-type magmas that were originally generated in the mantle wedge or mafic lower crust in response to subduction-related hydration melt. The plutons are mostly uncontaminated because they intruded primitive arc terranes, except for the plutons of the Tanacross-Dawson Range belt that intruded Paleozoic and older metasedimentary rocks of the Yukon-Tanana terrane and have elevated initial strontium ratios (0.706). Despite the contamination, these radiogenic rocks are still oxidized and mostly strongly magnetic. The Cassiar and Hyland suites are locally weakly magnetic and categorised as weakly magnetite-series. They are slightly peraluminous, have highly radiogenic isotopic ratios, have characteristics akin to S-type granitoids, and were generated far inboard from the continental margin. These features indicate a substantial contribution from continental crust, which typically would result in a more reduced melt. However, large parts of the Neoproterozoic crust underlying this part of Yukon consist of oxidized, hematitic sandstone and siltstone (Hyland Group; Gordey and Anderson 1993), and there are limited amounts of carbonaceous materials. In addition, a mafic igneous component has been suggested as a partial source for these weakly oxidized magmas (Driver et al. 2000). Similarly, the variably magnetite-

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series Livengood and Tombstone suites intruded Selwyn Basin strata on the ancestral North American margin and were not reduced. The oxidized and alkalic melts were, however, likely derived through partial melting of ancient metasomatized mantle and had little interaction with the mid to upper crust (Hart et al., 2005).

Ilmenite-series magmas in Alaska and Yukon developed from melting of continental crust in response to crustal thickening associated with terrane collision, accretion, and obduction (Miller, 1989, 1994; Arth, 1994). This is most likely the case for the large batholiths that comprise the Ruby geanticline and Yukon-Tanana Uplands batholiths, as the country rocks in both regions are amphibolite-grade, with voluminous magmatism immediately following peak metamorphism. The once-continuous Fairbanks-Mayo-Tungsten ilmenite-series belts appear to have had a more complex origin that involved interactions of mantle-derived components with the crustal melts (Hart et al., 2005). Smaller belts, such as the South Seward and some in the Chugach region, appear to have been localized in shear zones during thrusting, and were synchronous with metamorphism or orogenic collapse (e.g. Amato et al., 1994). The most enigmatic ilmenite-series belt is the Tok-Tetlin belt that is very reduced, yet sited between nearly contemporaneous magnetite-series belts. These anomalous rocks could result from the melting of a large flysch component, which is interpreted to exist at depth between the Yukon-Tanana Terrane and Insular terranes (Stanley et al., 1990; Beaudoin et al., 1992) following early Cretaceous thickening. Isotopic evaluation of granitic rocks in central Alaska by Aleinikoff et al. (2000) failed to find flysch generated melts, but only one of their samples were from the Tok-Tetlin belt.

The tectonic setting and processes required to form reduced magmas are much more varied than those needed for oxidized magmas (Ishihara 1981). The melting of, or contributions from, a continental lithosphere containing carbonaceous materials is the most general example for generating reduced melts. However, other scenarios include in-situ reduction of oxidized magmas due to assimilation of wall-rocks (Ishihara and Sasaki, 1989), a depth-dependent mechanism whereby volatiles and, therefore, fO2, are lower for magmas generated below the zone of slab dehydration (Gastil et al., 1990), or that deeply-emplaced non-radiogenic and primitive magmas may locally source from reduced materials to become ilmenite-series (Bateman et al., 1991).

The temporal and spatial distributions of magnetite- and ilmenite-series granitoids in Alaska and Yukon can be explained by a model in which latest Jurassic to Early Cretaceous, arc-generated, I-type, magnetite-series granitoids (Tosina-St. Elias) are built upon offshore terranes prior their accretion to form the Insular Belt. Initial closure of the Gravina Ocean and collision of those terranes with the edge of the continental margin resulted in a short magmatic lull (135-120 Ma), which may have been coincident with a change in subduction polarity. Subduction of Pacific plates (probably the Farallon plate) beneath the new continental margin initiated in the late Early Cretaceous generated magnetite-series granitoids (Nutzotin-Kluane). Magmatism then migrated further landward (Whitehorse -Dawson Range-Tanacross) in response to final closing of the Gravina basin (Fig. 8) between the Insular and Intermontane tectonic elements. The collision of the Insular terranes resulted in compression and tectonic thickening behind the arc, which gave rise to metamorphism and partial melting of the lower crust beneath the


100

Yukon-Tanana and Ruby terranes. This initially generated syn-metamorphic orthogniesses at ~120-112 Ma. However, during a subsequent or continued period of uplift, extension, and orogenic collapse, decompression-assisted melting generated vast, locally foliated but mainly undeformed, ilmenite-series (Ruby, Yukon-Tanana Uplands, Anvil) and weakly magnetite-series (Hyland, Cassiar) batholiths from ~112 Ma until ~97 Ma. The inboard components of post-collapse, upper crustal associated extension provided the framework for emplacement of small upper crustal plutons of the Fairbanks, Mayo and Tungsten suites at 95 Ma. The final gasp of inboard extension resulted in magnetite-series alkalic plutons of the Livengood and Tombstone suites at 92-90 Ma, and the lamprophyre dykes associated with the Mayo suite.

Northern Cordilleran magmatism was mostly a response to subduction of the Pacific oceanic plates beneath the North American margin. Mid-Cretaceous plutonism ended abruptly at 90 Ma, probably in response to oceanic plate vector readjustments at ca. 95 Ma (Engebretson et al., 1995). The Kula plate migrated rapidly northward after detaching from the Farallon plate at 85 Ma (Engebretson et al., 1985). Significant coupling of crustal fragments along the continental margin with the Kula plate resulted in at least 500 km, and possibly as much as 2000 km (Irving and Wynne, 1991; Johnston et al., 1996), of net displacement along the Tintina fault system and allied structures. During the Paleocene, displacement jumped seaward to the Denali fault system, and resulted in an additional 350 km of displacement of Insular terrane elements. These rapid displacements resulted in the buckling of the Koyukuk and Intermontane tectonic elements in western Alaska resulting in the development of the the North Alaska and Kulukbuk oroclines (Johnston, 2001).

ConclusionsThe mid-Cretaceous plutonic episode (145-90 Ma) in the northern North American

Cordillera is the most widespread, voluminous, diverse and metallogenically significant plutonic event in the northern Cordillera, with a current width of nearly 700 km. The plutonic province was emplaced across an accretionary margin that includes primitive seaward terranes added onto an ancient continental margin. Geological, geochemical, isotopic, and geochronometric attributes of the mid-Cretaceous plutons are compiled to define 23 plutonic suites or belts that represent this magmatic province. The oxidation states of these plutonic suites and belts, as indicated by their aeromagnetic signatures, magnetic susceptibilities, and whole-rock ferric:ferrous ratios, define distinct and continuous magnetite and ilmenite-series belts that are today bent and offset by post-emplacement structural complexities.

Magnetite-series plutonic belts are preferentially hosted in the more seaward tectonic settings, whereas ilmenite-series intrusions are hosted in more landward settings, farther from paleo-subduction zones. Magnetite-series granitoids mainly intruded the primitive arc terranes, and ilmenite-series intrusions were dominantly emplaced into the ancestral North American continental margin or into displaced or metamorphosed pericratonic equivalents. However, there are important exceptions to these generalizations. One exception is the presence of magnetite-series plutons in continental rocks, but its correlative of ilmenite-series plutons in primitive terranes or coastal settings is rare.

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Overall temporal trends define magnetite-series belts that young inland, with younger ilmenite-series belts further cratonward. The oldest and most seaward plutonic episodes (145-125 Ma) are arc-type magnetite-series and may include pre-accretionary oceanic arc formation. Successive continental arc magmatism (118-99 Ma) is also magnetite-series, with metaluminous calc-alkaline plutonism occurring inland of the 145-125 Ma belts. Ilmenite-series plutonism, initiated at about 112 Ma in response to crustal thickening, was dominated by the formation of large, slightly peraluminous batholiths. Later plutonic suites (109-96 Ma) are similarly slightly peraluminous, but are more oxidized and designated as weakly magnetite-series belts. A final magmatic episode led to the emplacement of widely scattered ilmenite-series granitoids during a minor extensional event at 98 to 92 Ma. In the final phases of extension, magnetite-series alkalic plutonic suites and lamprophyres were emplaced at 92-90 Ma in the most inland locations.

Mineral deposits dominated by copper, gold, and iron are mainly associated with magnetite-series plutons, there are also some local examples of molybdenum occurrences associated with these intrusions. Magnetite-series alkalic plutonic belts are characterized by gold and copper enrichments, as well as by uranium and thorium occurrences. Tungsten mineralisation best characterizes the ilmenite-series plutons. Perhaps uniquely, intrusion-related gold mineralisation in the northern Cordillera is associated with ilmenite-series plutonic belts, most particularly in the Mayo and Fairbanks suites. Such an association is counter to most granite-series metallogenic schemes (i.e. Blevin et al., 1996), but forms the basis of the reduced intrusion-related gold system model of Thompson et al. (1999) and Lang et al. (2000). Base-metal (Pb-Zn±Ag) mineralisation directly associated with mid-Cretaceous plutons is not common, but small polymetallic occurrences are found distal to both magnetite and ilmenite-series mineralizing plutons. Significant associated Sn mineralisation is rare. The plutonic belts with the fewest mineral occurrences are those of both ilmenite-series and magnetite-series that are dominated by large batholith complexes. Plutonic suites with the highest density of associated mineral deposits are those of the highly-radiogenic ilmenite-series Fairbanks and Mayo suites and the magnetite-series Nutzotin-Kluane belt, which consist of mainly small plutons that were emplaced at upper crustal levels.

AcknowledgementsThe senior author (CJRH) would like to thank Shunso Ishihara for his encouragement and

patience in awaiting completion of this manuscript. In addition, the attendance of JLM to the Ishihara symposium and of CJRH to the Hutton symposium are acknowledged as pivotal events in making this manuscript viable. David Groves and an anonymous reviewer are thanked for their contributions for improving the manuscript. Furthermore, the support of the Centre for Global Metallogeny, Yukon Geological Survey and the US Geological Survey are appreciated.


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Chapter 4 Source and Redox Controls

Chapter 4

Source and Redox Controls on Metallogenic Variations in Intrusion-Related Ore Systems,

Tombstone-Tungsten Belt, Yukon Territory, Canada

Craig J.R. Hart1,2, John L. Mair1, Richard J. Goldfarb3 & David I. Groves1

1Centre for Global MetallogenySchool of Earth and Geographical SciencesUniversity of Western AustraliaNedlands, WA, 6009Australia

2Yukon Geological SurveyBox 2703 (K-10), Whitehorse, Yukon Y1A 2C6 Canada

3United States Geological SurveyBox 25046 (MS964)Denver, Colorado, 80225 USA


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Preface to Chapter FourThis paper reflects a presentation made at the Fifth Hutton Symposium on the Origin of Granites and Related Rocks in Toyohashi, Japan in May 2003. “Source and Redox Controls of Intrusion-Related Metallogeny, Tombstone-Tungsten Belt, Yukon, Canada” will be published shortly in the “Fifth Hutton Symposium Volume on the Origin of Granites and Related Rocks” in Transactions of the Royal Society of Edinburgh: Earth Sciences, and jointly distributed as a Special Paper by the Geological Society of America. The spelling and format for citations and references follow those that are used by the Royal Society of Edinbugh.

Justif cation of authorship: This manuscript results from field work and observations from across the Tombstone-Tungsten belt, with follow-up rock sampling, analyses and compilation of a significant amount of whole-rock, trace, rare-earth and redox geochemical data as part of the Ph.D. study. Mr. John Mair assisted with the petrology of rock samples and in presenting the geochemical diagrams, and provided insight into the interpretations. Dr. Richard Goldfarb was involved in the fieldwork and provided editorial assistance. Dr. David Groves suggested the study, facilitated the interpretations through discussions, and provided editorial assistance.

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Source and Redox Controls on Metallogenic Variations in Intrusion-related Ore Systems, Tombstone-Tungsten Belt,

Yukon Territory, Canada

Abstract

The Tombstone, Mayo and Tungsten plutonic suites of granitic intrusions, collectively termed the Tombstone-Tungsten Belt, form three geographically, mineralogically, geochemically and metallogenically distinct plutonic suites. The granites (sensu lato) intruded the ancient North American continental margin of the northern Canadian Cordillera as part of a single magmatic episode in the mid-Cretaceous (96-90 Ma). The Tombstone suite is alkalic, variably fractionated, slightly oxidized, contains magnetite and titanite, and has primary, but no xenocrystic, zircon. The Mayo suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone suite was derived from a enriched, previously-depleted lithospheric mantle, the Tungsten suite intrusions from continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo suite from similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source.

Each suite has a distinctive metallogeny that is related to the source and redox characteristics of the magma. The Tombstone suite has a Au-Cu-Bi association that is characteristic of most oxidized and alkalic magmas, but also has associated, and enigmatic, U-Th-F mineralisation. The reduced Tungsten suite intrusions are characterized by world-class tungsten skarn deposits with less significant Cu, Zn, Sn, and Mo anomalies. The Mayo suite intrusions are characteristically gold-enriched, with associated As, Bi, Te, and W associations. All suites also have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral occurrences. Although processes such as fractionation, volatile enrichment, and phase separation are ultimately required to produce economic concentrations of ore elements from crystallizing magmas, the nature of the source materials and their redox state play an important role in determining which elements are effectively concentrated by magmatic processes.

IntroductionRegional-scale metallogenic and ore-element variations of intrusion-related ore systems

have been ascribed to a variety of processes. The tectonic setting, magma composition, degree of fractional crystallization, and redox state all potentially play a role in determining the metallogeny of an intrusion-related mineral deposit district (e.g. Ishihara 1981; Titley 1982, 1991; Blevin and Chappel 1992; Blevin et al. 1996; Barton 1996; Lang and Titley 1998). Controversies have typically focused on the relative importance of either the source materials or magmatic-hydrothermal processes in enriching a system in a particular metal suite. Economic geologists commonly emphasize the role that magmatic-hydrothermal processes play in intrusion-related metal enrichments, whereas igneous petrologists emphasize the importance of the nature of the source materials.


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The Tombstone-Tungsten Belt (TTB) in Yukon Territory, Canada, forms a 750-km-long belt of dominantly felsic intrusions that were emplaced across the ancient western North American continental margin from 96 to 90 Ma. The TTB is the innermost and youngest of numerous plutonic belts that intruded the continental margin in the mid-Cretaceous. Although magmatism occurred over a short time interval and across a single tectonic element, pluton compositions vary considerably along the belt, as do the associated mineral occurrence types. Several deposits and numerous occurrences of gold, molybdenum, silver, tin, copper, and uranium, as well as world-class tungsten deposits, are associated with plutons of the TTB (Hart et al. 2000). This well-mineralized region represents the eastern part of the Tintina Gold Province (Hart et al. 2002), which notably hosts numerous intrusion-related gold systems across Yukon and interior Alaska (Thompson et al. 1999; Goldfarb et al. 2000; Lang et al. 2000). Based on pluton distribution, nature, and composition, the belt can be divided into three main groups of intrusions. From west to east, they comprise the Tombstone, Mayo and Tungsten plutonic suites, with each having its own distinctive metallogenic signature.

This paper presents data on the geological, mineralogical, and geochemical features that characterise each of the plutonic suites. In particular, petrology and major, minor, and trace element geochemical and isotope data are applied to characterise and compare the three suites. This is used to constrain the nature of the potential source materials that generated their magmas, and the processes that contributed to the diversity of lithologies. In addition, the metallogeny of each suite is evaluated in the context of these magmatic source variations, with particular emphasis on the role of oxidation state and fractionation.

Tectonic SettingThe northern North American Cordillera is underlain by numerous tectonic elements that

include outboard accreted island arc and oceanic terranes, accreted terranes with pericratonic affinities, and variably displaced or deformed portions of the ancient continental margin (Gabrielse et al. 1991). Whereas the mechanisms of terrane assembly and accretion remain contentious, most of the assembly and growth of the current continental margin occurred during ongoing convergent tectonism between Early Jurassic and mid-Cretaceous time (Gabrielse and Yorath 1991; Plafker and Berg 1994). Following the waning of collision and accretionary tectonics, extensive magmatism occurred across much of the newly assembled continental margin during mid-Cretaceous time (Armstrong 1988). Hundreds of batholiths and plutons of the mid-Cretaceous magmatic episode are distributed in broadly orogen-parallel belts and geographically restricted clusters. Plutons within each belt have similar characteristics and ages, and, as such, comprise plutonic suites (Mortensen et al. 2000; Hart et al. 2004a; Fig, 1). There is a large variation in the characteristics between the plutonic suites: some are typical arc-related, metaluminous and calc-alkaline I-types (e.g., Whitehorse suite, Hart 1997), others comprise peraluminous granitoids that were mainly derived from crustal melts (e.g., Cassiar suite, Driver et al. 2000), and others consist of variably alkalic, locally silica-undersaturated plutons (e.g., Tombstone suite, Anderson 1982).

Among the most interesting and economically prospective of the mid-Cretaceous plutonic suites are those that comprise the most inboard plutonic belt, the Tombstone, Mayo and Tungsten suites (Fig. 1). These plutons, which form the TTB, were emplaced into a thick

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package of variably calcareous, quartzose, and argillaceous Neoproterozoic, and locally carbonaceous Paleozoic strata, which were deposited within the Selwyn Basin along the ancient North American margin. Although carbonate platforms comprise much of the ancient North American margin, the Selwyn Basin developed in response to Neoproterozoic and Paleozoic rifting events that resulted in an attenuated crust, and the development of euxinic shale basins and associated Pb-Zn sedimentary exhalative deposits (e.g., Faro; Abbott et al. 1986). Selwyn Basin strata were structurally thickened and locally metamorphosed to lower greenschist facies as a result of outboard terrane accretion immediately prior to mid-Cretaceous plutonism (Murphy 1997). Numerous features associated with the plutons, such as east-trending dyke swarms and parallel sub-vertical extensional veins, suggest that the region underwent a period of post-collisional extension immediately following compression. As such, the TTB intrusions were emplaced during post-collision in an inboard part of the continental margin.

Characteristics of the Tombstone-Tungsten Belt More than 100 plutons, as well as numerous dykes and sills, of the TTB form the most

inboard plutonic belt of the mid-Cretaceous magmatic episode. The plutons intruded the northern Selwyn Basin, to the south of the long-lived, and likely basin-controlling Dawson thrust fault (Fig. 2; Abbott 1995). Within the TTB, the Tombstone, Mayo, and Tungsten plutonic suites are defined on the basis of their distributions and lithological similarities. The term “plutonic suites” is used, as it is common in the North American lexicon for a regional group of coeval and lithologically similar plutons. This approach differs somewhat from the

Figure 1 Regional tectonic elements of Yukon and the distribution of mid-Cretaceous plutonic suites, shown in various shaded tones. The Tombstone-Tungsten Belt (TTB) includes the distribution of the Tombstone, Mayo and Tungsten plutonic suites. Light shade is distribution of Selwyn Basin. Dotted lines are distribution limits of plutonic suites. Dark lines are fault, with teeth are thrust faults. Light lines are tectonic boundaries.


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use of “suites” in the Australian context, which is more dependent upon the magma source according to geochemical and isotopic parameters, and typically refers to several lithologies within a single or numerous plutons (White 1995; Chappell 1996).

Each of these three plutonic suites in the TTB forms a linear belt, with associated dykes paralleling their trends. The Tombstone suite is mostly located along a 120-km-long, southeast trend that parallels the Tintina Fault (Fig. 1). The plutons were discordantly emplaced into folded and faulted Neoproterozoic to Mesozoic sedimentary rocks in the western part of the Selwyn Basin (Fig. 2). Notably, the plutonic belt cuts the major Cretaceous structural features, such as the Robert Service and Tombstone thrust faults (Fig. 2), indicating a negligible role played by the faults in magma focussing. However, the trends of the Tombstone and Mayo suites are interpreted to reflect structural trends associated with Neoproterozoic rift events.

The Mayo suite of intrusions forms an ESE-trending, 370-km–long belt from the Tintina Fault to the eastern border of the Yukon Territory (Fig. 3). Country rocks are dominantly folded and thrust-faulted quartzose and argillaceous strata of the Neoproterozoic Hyland Group, with carbonaceous shale and chert of the Devono-Mississippian Earn Group in the east.

The Tungsten suite of intrusions forms an almost 200-km-long, SE-trending linear belt from Macmillan Pass to south of the townsite of Tungsten (Fig. 4). The plutons intrude carbonaceous and calcareous Paleozoic strata and lesser quartz-rich and pelitic Neoproterozoic strata of the easternmost Selwyn Basin.

In east-central Alaska, intrusions of equivalent age and lithology to the Mayo and Tombstone suites form the Fairbanks-Salcha and Livengood suites, respectively, which crop out in the world-class Fairbanks gold district south of the Tintina Fault (Hart et al. 2004). These intrusions are interpreted as the western parts of the Mayo and Tombstone suites that were offset by ~430 km of dextral transcurrent displacement on the Tintina Fault during the Late Cretaceous and Early Tertiary (Mortensen et al. 2000; Murphy and Mortensen 2003).

Plutons throughout the TTB have well-developed, resistant-weathering, contact-metamorphic aureoles that are about half-the-width of adjacent intrusions. None of the plutons have associated volcanic rocks. Equilibrium contact metamorphic assemblages and fluid inclusion data from syn-plutonic quartz-veins constrain pluton emplacement pressures to mostly 1.0 to 2.5 kbar (Baker and Lang, 2001; Marsh et al., 2003). Extensive geochronology (in Gordey and Anderson 1993; Murphy 1997; Mortensen et al. 2000; Coulson et al. 2002; Hart et al. 2004b; Mortensen unpublished) indicates that the TTB plutons were emplaced over a narrow time interval of 99 to 90 Ma with the Tombstone suite of intrusions being emplaced at the younger end of the range. Essentially contemporaneous 40Ar-39Ar dates from magmatic biotite and hornblende with U-Pb zircon dates (Coulson et al. 2002; Hart et al. 2004b) indicate that the region has not been affected by post-intrusion thermal events.

Plutonic Suites Intrusions of the TTB commonly occur as isolated plutons, particularly within the

Tombstone and Tungsten suites. However, the Mayo intrusions typically occur in clusters, commonly with numerous associated dykes, which collectively comprise intrusive complexes.

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Figure 4 Distribution of the Tungsten plutonic suite in eastern Yukon and westernmost Northwest Territories (NWT) and associated intrusion-related mineral occurrences. Outlined, unshaded plutons do not belong to the Tungsten plutonic suite, O’Grady Batholith is alkalic and a distal member of the Tombstone suite. Pluton names are italicized. Significant mineral occurrences are named. Mineral occurrences often have the same name as the pluton. Geological information from Gordey and Makepeace (2003). Mineral deposit information from Yukon Minfile (2003).

Figure 2 Distribution of Tombstone plutonic suite and associated intrusion-related mineral occurrences. Pluton names are italicized. Significant mineral occurrences are named. Geological information from Gordey and Makepeace (2003). Mineral deposit information from Yukon Minfile (2003).

Figure 3 Distribution of the Mayo plutonic suite across central Yukon and associated intrusion-related mineral occurrences. Clear Creek, Scheelite Dome and Dublin Gulch also have significant placer gold. Pluton names are italicized. Significant mineral occurrences are named. Mineral occurrences often have the same name as the pluton. Unfilled plutons belong to other suites. Geological information from Gordey and Makepeace (2003). Mineral deposit information from Yukon Minfile (2003).


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The Tombstone plutons are well-zoned, comprising several distinct lithological phases that form nested concentric zones or phases adjacent to each other. The Mayo intrusions most commonly consist of a main phase stock with gradational lithological variations potentially indicative of multiple intrusive phases. In addition, these plutons characteristically have a variety of associated dykes, including intermediate to mafic lamprophyres, porphyritic granitic dykes, and subordinate aplites and pegmatites. The Tungsten suite intrusions have comparatively minor, within-pluton, lithological phase variations and lack associated intermediate to mafic phases. However, plutons of the Tungsten suite commonly have associated pegmatites, aplites, and greisens. Intrusions of all suites are wallrock inclusion-poor, with only sparse occurrences of microdiorite enclaves. The Tungsten and Mayo intrusions are weakly porphyritic, with white megacrystic alkali feldspar. Characteristics of the plutonic suites are summarized in Table 1.

Nomenclature After first designating those plutons along the border between Yukon Territory and

Northwest Territories (NWT) as the Selwyn plutonic suite (Anderson 1983), Anderson (1988) and Woodsworth et al. (1991) subsequently included all mid-Cretaceous plutons in the Selwyn Basin within this group. The plutons were subdivided into two-mica, transitional and hornblende-bearing phases. Anderson (1988) further encouraged the additional use of more regional names, such as Anvil plutonic suite (Fig. 1).

The Tombstone plutonic suite has traditionally referred to those geographically restricted plutons in the westernmost part of the TTB that have alkalic affinities (Tempelman-Kluit 1970; Woodsworth et al. 1991), with the Tombstone Mountain batholith being the type locality. Tombstone plutonic suite was subsequently used by Mortensen et al. (1995) and Murphy (1997) to refer to all plutons that intruded along the northern margin of the Selwyn Basin between Dawson City and the NWT border (Fig. 1). Furthermore, Mortensen et al. (1995, 2000) renamed the Selwyn plutonic suite intrusions near the NWT border, the Tungsten plutonic suite, but included only those smaller plutons that were circa 94 Ma, which thus excluded the batholiths that Anderson (1988) had included within the Selwyn plutonic suite.

The term Tombstone plutonic suite is herein retained as originally applied to the dominantly alkalic intrusive rocks near the Tintina Fault, but in contrast to Murphy (1997) and Mortensen et al. (1995), does not include the east-trending belt of intrusions in central Yukon Territory. These plutons, between the Tombstone and Tungsten suites, are designated the Mayo plutonic suite. The Tungsten plutonic suite, as applied to the small peraluminous intrusions near the Yukon Territory-NWT border by Mortensen et al. (2000), is retained. Together, these three suites form the TTB. The Tombstone suite intrusions are characteristically alkalic and metaluminous; the Mayo intrusions are sub-alkalic and mostly metaluminous, and distinctive from the more felsic Tungsten suite intrusions that are dominantly peraluminous.

Tombstone Plutonic SuiteTombstone suite intrusions are characterized by their alkalic character, limited distribution,

and larger, well-zoned plutons (Anderson 1987, 1988; Gordey and Anderson 1993; Abercrombie 1990; Smit et al. 1985; Duncan et al. 1998). They include six main intrusions, as

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Table 1 Summary of characteristics of the plutonic suites

Tombstone Mayo Tungsten ReferencesAges 92-90 Ma 95-92 Ma 97-94 Ma 1, 2,9,10

Dominant lithologies

Alkali feldspar syenite to quartz syenite

Monzonite to granodiorite Granite to monzogranite

Pluton size Moderate Small Small

Plutons Zoned, maf c margins felsic cores

Simple, later maf c phases Simple, textural variations, some fractionated phases

Grain size Coarse, cumulate Medium to f ne grained, locally porphyritic

Medium grained, weakly porphyritic

Maf c phases Pyroxene (aegerine-augite)>hornblende>biotite

Biotite>hornblendeclinopyroxene common

Biotite>>muscovite 7

Dominant Fe-Ti indicator mineral

Magnetite>titanite Titanite Ilmenite>>titanite

Accessory minerals Epidote, allanite, melanite, apatite, f uorite, zircon

Allanite, apatite, zircon Garnet, apatite, monazite, tourmaline, allanite, zircon

4, 7

SiO2 range 50-70 % 55-75 % 66-76 % 3, Table 3

ASI Metaluminous, except where highly fractionated(0.65-1.1)

Metaluminous to weakly peraluminous(0.6-1.15)

Weakly peraluminous (1.0-1.2)

Alkalinity Alkaline to peralkaline subalkaline subalkaline

Fe2O3/FeO 0.2-1.1 0.15-0.45 0.1-0.3 Fig. 11

Average magnetic susceptibility

(x10-3SI)

1.79 0.11 0.16 Fig. 10

Cr (ppm) Most <20, some 20-80 Most 20-100, some 100-600

Most <20 3, 10

Inherited zircons None Some Considerable

Initial Sr ratio 0.710-0.720 0.7115-0.7140 0.717-0.737 3, 4, 7, 8

NdT-7 to -9 -8 to -13 -13 to -15 3, 5, 10

Oxygen isotopes 9-11 11-14 9-13 6, 7, 10

ZST 820°C 780°C 750°C Table 3

Associated mineralization

Au-Cu-BiU-Th-F

Au-Bi-Te, W, AsAg-Pb

W-(Cu-Zn-Mo)

Characterization Alkalic, slightly oxidized, metaluminous, radiogenic, syenite cumulates

Metaluminous, moderately reduced, radiogenic, biotite granodiorite

Weakly peraluminous, reduced, radiogenic, biotite granite

Notes: ASI-Aluminum saturation index; ZST – Zircon saturation temperature. References: 1-Anderson 1993 (in Gordey and Anderson 1993); 2-Murphy 1997; 3 – Lang 2000, 4-Abercrombie 1990, 5-Farmer et al. 2000, 6-Marsh et al. 2003, 7-Anderson 1988, 8-Gareau 1986; 9-Coulson et al. 2002; 10-Hart and Mair unpublished. Na-not available.


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well as swarms of sills, dykes, and small stocks along their trend (Fig. 2). Two outliers from the linear distribution are the isolated Emerald Lake pluton (Fig. 3) and O’Grady batholith (Fig. 4), which occur several hundred kilometres east of the other intrusions in the suite.

Tombstone plutons are typically circular to sub-circular, relatively large (10-80 km2 in plan), and typically composed of multiple concentric zones of lithologically distinct phases. The dominant rock types are coarse-grained alkali-feldspar syenite and quartz syenite. Many plutons have mafic marginal phases, such as pyroxenite and hornblende diorite, as well as central, more felsic, monzonitic or granitic phases that are nested in the syenite. Tombstone intrusions also locally contain distinctive alkalic, quartz-undersaturated phases such as tinguaite (feldspathoid syenite).

The dominant mafic minerals include augite, aegerine, hornblende, local biotite, and lesser melanitic garnet. Hornblende (arfvedsonite) typically replaces earlier formed augite. Fe-Ti accessory phases are magnetite and titanite; other accessory minerals include zircon, fluorite, apatite, and nepheline (Anderson 1988; Abercrombie 1990).

Mayo Plutonic SuiteMayo suite plutons are generally small in the west (1-5 km2), larger in the east (20-80

km2), but notable is the anomalously large Roop Lakes pluton (125 km2) (Fig. 3). This suite has a characteristic spatial and temporal associations with intrusion-related gold deposits. Mayo intrusions typically form isolated plutonic centres, or clusters of small felsic to mafic plutons. Intrusions are typically dominated by a main porphyritic quartz monzonite that characteristically includes less-evolved magmatic phases and locally forms weakly composite plutons (e.g., Dublin Gulch). Alternatively, some main phase plutons are cut by monzonite to diorite stocks and dykes (e.g., Scheelite Dome). Three small plutons in the Clear Creek area are dominated by a main-phase quartz monzonite, whereas more mafic lithologies dominate three others in the same area. A variety of porphyritic, aplitic, and pegmatitic felsic dykes, as well as calc-alkaline lamprophyre dykes, typically cut all plutons and their hornfelsed aureoles.

The early, mainly quartz monzonite plutons are weakly to moderately porphyritic, with white potassium feldspar phenocrysts that are locally >1cm in length. The phenocrysts occur in a fine- to medium-grained matrix of predominantly quartz and feldspar, with lesser biotite. Biotite is more abundant than hornblende, and clinopyroxene, although typically rare, is common in some plutons. Accessory minerals include titanite, allanite, apatite, and zircon. Intermediate intrusions are mineralogically similar to the earlier, more felsic main phase intrusions, but with a greater proportion of clinopyroxene, biotite, hornblende, and titanite. These more mafic phases locally contain unusual textures, with two generations of clinopyroxene. Early clinopyroxene is typically Mg-rich augite (Mg# >80) and is poorly preserved. Finer euhedral diopside grains (Mg # ~ 60) occur amongst quartz, and are interpreted to have formed late in the crystallisation sequence. Lamprophyre dykes include both sub-alkalic amphibole-rich spessartites and alkalic phlogopite/biotite-rich minettes (cf. Rock et al. 1991). Deuteric alteration is most extensive in intermediate to mafic intrusive rocks, with pyroxenes altered to amphibole and biotite. Titanite is ubiquitous and magnetite is locally present only in late granitic dykes, whereas ilmenite is rare, occurring only as fine inclusions within mafic phenocrysts in intermediate to felsic intrusions.

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Tungsten Plutonic SuiteIntrusions of the Tungsten suite are distinguished by their felsic, more homogeneous, and

consistently peraluminous character, as well as their strong association with tungsten skarn mineralisation (Anderson 1982, 1983, in Gordey and Anderson 1993). Plutons are typically small, generally less than 15 km2 in surface exposure.

Tungsten suite plutons are dominated by medium-grained quartz monzonite to monzogranite. Notably, these plutons lack both amphibole and clinopyroxene, which are common in quartz monzonite intrusions of the Mayo suite. Plutons typically feature gradational zoning to more felsic or peraluminous phases. Late-stage aplite or pegmatite dykes are locally common, as are tourmaline veinlets and rusty-weathering greisen-like alteration on joint planes in pluton cupolas.

Biotite, the dominant mafic mineral, ranges from brown to reddish, and commonly contains black pleochroic halos from radiation damage. Although biotite is typically the sole mica, primary muscovite is locally present in fractionated phases. Orthoclase forms both irregular coarse ophitic grains that incorporate earlier-formed biotite and plagioclase, as well as euhedral to subhedral simple-twinned laths. Ilmenite, the dominant oxide phase, typically occurs as fine grains in biotite. Subhedral plagioclase grains commonly feature oscillatory zoning, with sericitised grain cores. Accessory phases include allanite, zircon, apatite, and ilmenite, with trace amounts of garnet and tourmaline in more highly fractionated phases, aplites, and pegmatites. Magnetite is not present. Secondary ilmenite is strongly anhedral and associated with areas of chloritised biotite. Weak to moderate deuteric alteration is characterised by chloritisation of biotite, sericitisation, and carbonate alteration of plagioclase.

Intrusion-Related Metallogeny Each of the three plutonic suites has its own distinctive metallogenic association (Table 2),

although a wide range of factors, such as P-T of emplacement, nature of country rock, structure, and water-rock conditions, result in the diverse range of gold and/or tungsten-rich mineralisation styles as described by Hart et al. (2000). All suites, however, appear to have spatially associated Ag-Pb-Zn- and As-Sb-bearing veins that are peripheral to the plutons and are products of the final stages of intrusion-related hydrothermal activity.

The alkalic Tombstone plutons tend to be associated with mineral occurrences that are dominated by either a Au-Cu-Bi or U-Th-F association (Fig. 2). Enrichments of native gold (to tens of ppm) are hosted in pyrrhotite-rich skarns that contain percent-level copper and anomalous bismuth concentrations, such as characteristic of the Marn (Brown & Nesbitt 1987) and Horn deposits. An arsenic-rich epizonal gold deposit, Brewery Creek (0.3 Moz Au; Lindsay et al. 2000), is localized within, and adjacent to, monzonitic Tombstone suite sills and stocks (Fig. 2). Four of the Tombstone suite intrusions host U-Th-F mineralisation, with two of the occurrences being large-tonnage, low-grade deposits in disseminated and veinlet-hosted styles. Locally, highly-fractionated parts of the Tombstone intrusions contain Au-, Cu-, and Bi-bearing quartz veins and vug fillings, such as in the Emerald Lake pluton (Fig. 3), and Sn- and Ag-rich tourmaline greisens, such as in the Syenite Range pluton (Fig. 2). Mineralisation associated with the Tombstone suite is characterized by high halogen contents, as indicated by the presence of fluorite, scapolite, and, locally, sodalite.


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The Mayo intrusions are dominantly metaluminous, are most consistently associated with gold mineralisation, and have the highest abundance of associated mineral occurrences (Fig. 3). Approximately 1 Moz of placer gold have been recovered from creeks that drain areas underlain by the intrusions of the Mayo suite and surrounding hornfels. Gold occurrences include intrusion-hosted Au-Bi-Te-W quartz-alkali-feldspar sheeted vein arrays such as at Dublin Gulch (4 Moz Au; Hitchins & Orssich 1995; Maloof et al. 2001), Au±W skarns, and country rock-hosted Au-As quartz vein arrays such as those at Scheelite Dome (Mair et al. 2000). Solitary, shear- and fissure-hosted Au-As veins are also common features proximal to, or within, Mayo intrusions. Other types of mineralisation include tungsten skarns, Ag-Pb-Zn±Sb veins, and tin-bearing greisens. The area containing the Mayo suite of intrusions also hosts significant silver-rich quartz veins, with >200 Moz Ag recovered from veins of the Keno Hill district (Lynch et al. 1990). However, the genetic relationship of these veins to the intrusions is unclear. The Fort Knox gold deposit in Alaska (7 Moz Au) is hosted in a Mayo suite intrusion that has been offset by the Tintina Fault.

Table 2 Examples of mineralization associated with the plutonic suites

Deposit/Occurrence

Metals Mineralization Style

Ore Minerals Gangue minerals

Reference

Tombstone Plutonic SuiteMarn Au-Cu-Bi Skarn Pyrrhotite,

chalcopyrite, bismuthinite, electrum

Hedenbergite, almandine garnet

Brown & Nesbitt (1987)

Horn Au-Cu-Bi Skarn Pyrrhotite>pyrite, chalcopyrite, gold, bismuthinite,

Hedenbergite, garnet, f uorite

Brewery Creek-Moosehead Zone

Au-As-Sb Vein stockwork Pyrite, arsenopyrite Kaolinite Lindsay et al. (2000)

Ting/Noting U-Th-F Disseminated, veins Uraninite, molybdenite Fluorite Yukon Minf le (2003)

Mayo Plutonic SuiteDublin Gulch Au-Bi-W Sheeted veins Po, bismuthinite,

scheeliteQuartz, K-spar, sericite

Maloof et al. (2001)

Scheelite Dome Au-As Replacements, stockworks

Arsenopyrite, pyrite, pyrrhotite

Quartz, sericite, carbonate

Mair et al. (2000)

Clear Creek Au-Bi-W Sheeted veins Pyrite, bismuthinite, scheelite

Quartz, K-spar Marsh et al. (2003)

Ray Gulch W Skarn Scheelite Diopside Brown et al. (2002)

Keno Hill Ag-Pb Veins Galena Quartz, siderite Lynch et al. (1990)

Tungsten Plutonic SuiteMactung W-Cu Skarn Scheelite,

chalcopyrite, pyrrhotite

Diopsite, garnet, actinolite

Atkinson & Baker (1986)

Cantung W-Cu Skarn Scheelite, chalcopyrite, pyrrhotite

Diopside, epidote, garnet

Bowman et al. (1985)

Clea W-Cu Skarn Scheelite, chalcopyrite, pyrrhotite

Diopside, garnet, vesuvianite, biotite, f uorite

Yukon Minf le (2003)

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The peraluminous Tungsten suite intrusions are associated with the largest tungsten deposits in North America: the Cantung deposit (Bowman et al. 1985; Gerstner et al. 1989) hosts approximately 9 Mt of 1.4% WO3 and the Mactung deposit (Dick & Hodgson 1982; Atkinson & Baker 1986; Fig. 4) contains approximately 57 Mt of 0.95% WO3. Both deposits are developed in Lower Cambrian carbonates adjacent or above Tungsten suite intrusions. Some intrusions have highly fractionated phases that contain garnet, muscovite, tourmaline. Each deposit consists of several stratabound ore zones. The best ores (>1.5% WO3 and 0.2% Cu) are characterized by pyrrhotite-rich (>15%) pyroxene skarn with scheelite. The molybdenum content of the scheelite is low. Garnet-bearing skarns are much less common, contain less pyrrhotite and are lower grade. The skarns locally have sub-economic grades of zinc and molybdenite. Elsewhere, Tungsten suite intrusions generate small Sn, Mo and Au enrichments.

GeochemistryThe three plutonic suites are characterized by distinct major, minor, and trace element

geochemical signatures (Table 3). The Tombstone and Mayo intrusions have wide compositional variations, with SiO2 contents ranging from 50 to 75 wt %, whereas Tungsten intrusions have more restricted SiO2 contents from 65 to 75 wt % (Fig. 5A). The Tombstone suite rocks are distinguished from those of the Mayo suite by their higher alkalinity and lower MgO contents (and Mg#), particularly at ≤65% SiO2 (Fig. 5B). The Tombstone intrusions that contain <58% SiO2 are commonly nepheline normative, mainly due to their elevated alkali contents.

Aluminum saturation is also variable within the different suites (Fig. 5C). The Tombstone and Mayo suites display a wide range of aluminum saturation index (ASI) values, which is consistent with their broad range in SiO2 (cf. Chappell and White 2001). Generally, rocks with <65% SiO2 are metaluminous, with an ASI value <1, whereas those Mayo intrusions with SiO2 values >70% have ASI values slightly >1, as do those few highly fractionated Tombstone suite examples. Tungsten suite intrusions are entirely weakly peraluminous, with all values slightly >1.

The use of Rb, Sr, and Ba as fractionation indices is complicated by the relatively enriched nature of the intermediate to mafic intrusions of the Tombstone and Mayo suites (Fig. 6). Intrusions of the Tungsten suite, which have a restricted SiO2 range, feature a strong increase in rubidium content with increasing SiO2. Similarly, intrusions of the Tombstone suite show a clear increase in rubidium content with increased SiO2 content, although this occurs over a greater SiO2 range. In contrast, Mayo suite intrusions are characterized by only a slight increase in rubidium with increased SiO2. Strontium shows strong depletion with increasing SiO2 content in both the Tombstone and Tungsten suites, whereas the Mayo suite have no well-defined trends. Notably, the strontium contents of Mayo suite intrusions are considerably higher than those of the Tungsten intrusions at a given SiO2 content.

Interpretation of primitive mantle-normalised trace-element diagrams (Fig. 7) indicates that all groups are strongly enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE). In addition, all suites have distinct negative titanium and niobium anomalies (Fig. 7A). Most intrusions of the TTB also exhibit a negative phosphorous anomaly, but it is absent in mafic phases of the Mayo intrusions. Chondrite-normalised REE plots indicate


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Table 3 Representative geochemical data for Tombstone (Deadman pluton), Mayo (Dublin Gulch pluton) and Tungsten (Mactung pluton) plutonic suites.

Sample DEADMAN 1 DEADMAN 2 DEADMAN 3 DEADMAN 4DUBLINDIKE 1

DUBLIN QTZ-SYENITE

DUBLINPEGMATITE

DKEDUBLINDIKE 2

DUBLINMAIN PHASE

MACTUNGDIKE

MACTUNGTOURM PEG

MACTUNGMAIN

STOCK

MACTUNGDIKE IN STOCK

SiO2 57.8 64.7 58.6 68.3 59.6 69.5 61.0 59.8 67.2 75.7 72.6 68.1 72.3Al2O3 19.3 17.6 21.1 16.1 14.2 16.0 16.7 14.7 15.2 13.1 15.9 15.5 14.2CaO 3.76 2.34 0.85 1.53 5.49 2.63 3.94 6.41 3.46 1.07 0.65 3.18 1.96MgO 0.11 0.24 0.23 0.26 4.71 0.66 1.61 4.37 1.42 0.17 0.14 1.13 0.43Na2O 3.59 5.84 2.98 5.38 2.14 2.57 2.0 2.41 2.59 3.19 4.41 2.68 2.66K2O 8.6 5.43 12.1 4.9 5.44 4.37 9.81 4.28 4.5 4.31 5.32 4.45 5.17FeO (total) 2.85 1.92 2.53 1.8 5.12 1.9 1.64 5.58 3.07 1.01 0.34 3.53 1.89Fe2O3* 1.65 0.92 0.53 1.0 0.92 0.5 0.34 1.08 0.57 0.21 0.24 0.63 0.39FeO* 1.2 1.0 2.0 0.8 4.2 1.4 1.3 4.5 2.5 0.8 0.1 2.9 1.5MnO 0.13 0.05 0.06 0.04 0.1 0.02 0.04 0.1 0.04 0.05 0.05 0.07 0.04TiO2 0.304 0.198 0.303 0.175 0.551 0.343 0.592 0.673 0.459 0.054 0.02 0.487 0.176P2O5 0.03 0.02 0.02 0.01 0.16 0.08 0.16 0.17 0.13 0.0066 0.02 0.16 0.05Cr2O3 0.005 0.005 0.005 0.005 0.03 0.006 0.005 0.02 0.005 0.005 0.005 0.005 0.005LOI 0.65 0.6 0.4 0.65 1.05 1.05 1.8 1.1 0.85 1.4 0.45 0.95 0.5H2O+ 0.5 0.3 0.7 0.2 0.8 1.0 0.4 1.0 0.7 0.6 0.6 1.0 0.5Total 98.8 99.4 99.4 99.4 98.9 99.5 99.8 99.8 99.2 100.1 99.9 100.4 99.5

Mg# 0.04 0.11 0.08 0.13 0.48 0.26 0.50 0.44 0.32 0.14 0.29 0.24 0.19

F 1050 479 1520 10 271 478 10 1790 930 441 117 2350 561S % 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.03 0.005 0.005 0.005 0.005 0.005Cl 347 30 157 69 165 25 194 124 200 25 25 68 25CO2 % 0.06 0.3 0.01 0.4 0.06 0.005 0.39 0.32 0.23 0.01 0.06 0.08 0.005B 47 45 29 37 20 31 27 22 24 35 381 27 25

V 38 23 15 21 91 21 40 83 32 11 -5 47 -5Cr -20 -20 -20 -20 272 -20 37 178 30 -20 -20 57 -20Co 1 1 2 1 18 2 3 16 7 2 -1 5 1Ni 22 -20 -20 -20 60 -20 23 65 21 -20 -20 136 -20Cu -10 -10 -10 -10 -10 -10 -10 12 -10 -10 -10 -10 -10Zn 87 -30 108 -30 85 124 -30 89 59 33 -30 100 -30Ga 22 24 32 27 19 22 19 20 20 17 28 23 20Ge 1.2 1.2 1.5 1.3 1.4 1.3 2.8 1.5 1.2 1.3 5.0 2.1 2.2As 7 -5 -5 -5 5 10 -5 15 -5 -5 -5 -5 -5Rb 197 178 705 211 238 213 302 221 198 206 468 275 355Sr 3,380 1,110 599 637 467 476 640 438 465 262 70 365 89Y 11.4 17.2 30.4 15.5 22.9 4.6 32.6 23.0 18.1 18.9 58.0 20.9 45.9Zr 175 414 518 287 204 152 296 191 224 135 29 198 78Nb 12.8 22.9 40.3 24.0 15.5 13.9 22.6 14.0 14.2 12.1 26.4 16.4 18.5Mo -2 -2 2 -2 -2 -2 -2 -2 -2 -2 -2 -2 -2Ag -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5In -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1 0.1 -0.1Sn 2 2 3 2 3 4 8 4 1 5 7 12 11Sb 0.6 0.5 0.7 0.2 0.6 3.8 0.2 0.8 0.3 0.5 0.2 0.5 -0.2Cs 5.0 3.5 67.6 3.4 6.6 11.2 8.4 8.1 7.5 14.2 21.4 27.9 16.7Ba 8,590 2,760 857 1,660 1,620 2,930 3,780 1,370 1,380 603 56 993 119La 34.0 43.4 95.4 41.9 59.6 22.5 62.1 51.6 49.4 56.2 12.4 55.2 15.3Ce 69.7 75.5 164 71.9 99.4 39.4 111 94.7 88.4 100 28.7 103 31.1Pr 9.12 7.69 15.8 7.01 10.4 4.09 12.3 9.88 9.22 11.5 3.69 11.2 3.74Nd 36.8 25.3 49.9 22.1 36.4 14.3 44.9 34.2 32.0 40.7 15.3 38.8 14.6Sm 6.16 4.57 8.63 3.88 6.42 2.68 9.18 6.17 5.86 7.62 7.17 6.99 5.22Eu 1.43 1.12 1.69 0.894 1.34 1.01 1.61 1.25 1.28 1.38 0.175 1.46 0.433Gd 3.57 3.37 5.60 2.84 4.34 1.55 6.70 4.58 4.35 5.69 7.79 4.82 6.06Tb 0.42 0.57 1.02 0.48 0.78 0.22 1.13 0.76 0.68 0.74 1.86 0.75 1.39Dy 1.80 2.87 5.00 2.40 3.99 0.88 5.81 3.89 3.31 3.49 9.46 3.60 7.48Ho 0.29 0.54 0.96 0.48 0.76 0.13 1.11 0.76 0.62 0.63 1.72 0.69 1.52Er 0.85 1.53 2.64 1.30 2.01 0.32 2.94 2.07 1.59 1.79 4.64 1.88 4.09Tm 0.096 0.207 0.366 0.190 0.276 0.040 0.427 0.284 0.203 0.230 0.830 0.239 0.668Yb 0.78 1.38 2.24 1.14 1.71 0.25 2.75 1.83 1.27 1.59 5.55 1.57 3.81Lu 0.111 0.217 0.312 0.173 0.249 0.032 0.446 0.279 0.184 0.231 0.843 0.226 0.572Hf 4.3 9.8 10.8 7.9 5.6 4.5 8.4 5.3 6.2 4.3 4.4 5.5 4.1Ta 0.74 1.15 2.40 1.15 1.24 1.10 1.53 1.15 1.20 1.73 18.4 1.48 4.09W 0.6 0.6 3.2 0.8 0.5 1.7 377 2.1 2.2 0.6 2.8 0.8 0.9Tl 1.29 0.63 2.51 0.73 1.27 1.56 0.79 1.12 0.90 1.19 2.63 2.11 1.97Pb 55 13 73 13 22 30 8 20 16 26 51 38 44Bi 0.1 -0.1 -0.1 -0.1 -0.1 0.2 -0.1 -0.1 -0.1 -0.1 0.5 1.4 -0.1Th 10.6 29.7 68.5 27.4 31.6 13.2 29.1 25.7 25.3 25.8 13.2 27.4 21.2U 3.93 6.44 11.0 10.7 4.65 7.79 5.21 6.58 5.66 15.4 15.0 7.30 20.9

Zircon Saturation TemperaturesM 2.08 1.88 1.71 1.53 2.41 1.29 2.18 2.18 2.18 1.26 1.33 1.50 1.39Temp K 1018 1108 1144 1102 1008 1063 1055 1055 1055 1055 933 1070 1369Temp C 745 835 871 829 735 790 781 781 781 781 660 797 1096

Notes: Whole-rock data by XRF at XRAL-SGS, Canada. Trace element data by ICP-MS at Actlabs, Canada. FeO determined by titration. Cl and F by specific ion.*Fe2O3 is calculated from FeO total and FeO.

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Figure 5 Geochemical plot for the Tombstone, Mayo and Tungsten plutonic suites. All geochemical plots composed using selected data of C. Hart and J. Mair (unpublished), Lang (2000) and Abercrombie (1990), and that in Table 3. A – Alkali-silica plot. ASI=molar Al/Ca+Na+K. B) MgO-silica plot. C) ASI-silica plot.

Figure 6 Variation diagrams of Ba, Sr and Rb vs. silica for the Tombstone, Mayo, and Tungsten plutonic suites emphasizing varying source compositions and fractionation trends between the suites.


132

Figure 7 Multi-element variation diagrams for representative plutons of the Tombstone, Mayo and Tungsten plutonic suites. Data are normalized to primitive mantle values of Sun and McDonough (1989). A) LILE-data. B) REE. Data from Table 3, and associated unpublished data.

Figure 8 Plot of Cr (ppm) vs SiO2 for Tombstone, Mayo and Tungsten plutonic suite granitoids emphasizing the Cr-enrichment in Mayo intrusions. High values are interpreted to come, at least in part, from xenocrystic Cr-diopside. Overlapping data points with low Cr are at or below detection limit of 10 ppm.

133


strong enrichments of LREE, with negative slopes for intrusions of all groups (Fig 7B). The Tombstone and Mayo suites generally have no obvious europium anomalies, whereas more fractionated intrusions of the Tungsten suite have pronounced negative europium anomalies. On the trace-element discrimination diagrams of Pearce et al. (1984), each suite overlaps both the syn-collisional and volcanic-arc fields.

The variation in chromium concentration is pronounced (Fig. 8). Almost all Tungsten suite intrusions have concentrations below the analytical detection level (e.g., <20 ppm Cr). The Tombstone suite intrusions similarly contain low chromium levels, although more primitive intrusions contain as much as 100 ppm Cr. The Mayo suite intrusions have elevated, but variable chromium levels; felsic intrusions mostly contain <50 ppm, intermediate phases show 50-200 ppm, and mafic phases contain as much as 700 ppm Cr. Chromium content is concomitant with the presence of chromium diopside.

Isotopic DataIsotopic (Sr, Nd, and O) compositions can reflect the nature of magma source regions in the

middle to lower crust or mantle, and/or the extent of contamination from upper crustal sources (DePaolo et al. 1992). Isotopic data for plutons from the TTB are currently limited (Lang 2000), with most available data from intrusions in the Syenite Range (Abercrombie 1990), Clear Creek (Farmer et al. 2000; Marsh et al. 2003), Emerald Lake (Smit et al. 1985) and Scheelite Dome (J. Mair, submitted). Initial strontium isotope ratios for Tombstone suite rocks are 0.710 to 0.713 and εNd values range from -7.6 to -10.3 (Abercrombie 1990; Lang 2000). Our unpublished whole rock oxygen isotopic values range from 9 to 11 per mil. The Mayo intrusions have somewhat similar initial strontium isotope ratios from 0.712 to 0.714 (Kuran et al. 1982; Lang 2000) and εNd values of -8.3 to -12.5 (Farmer et al. 2000; Lang 2000). Whole-rock oxygen isotope values have been determined only for Clear Creek intrusions and range from 11.4 to 13.9 per mil (Marsh et al. 2003). Tungsten suite intrusions have very high initial strontium isotope ratios of 0.712 to 0.748 (Godwin et al. 1980; Gareau 1986; Lang 2000), and two rocks have εNd values of -13 to -15 (Lang 2000). Oxygen isotopes for Tungsten suite rocks include values of 9-13 per mil for the Cantung pluton (Bowman et al. 1985; Anderson 1988).

Both strontium and neodymium isotopic ratios are highly radiogenic in intrusions of all three plutonic suites. Notably, the more mafic and basic compositions, such as diorite, tinguaite and lamprophyres, also yield similar highly radiogenic values. However, Tombstone rocks may be slightly less radiogenic than Mayo suite rocks, and Tungsten suite rocks are significantly more radiogenic (Lang 2000). This suggests that the more easterly plutons of the Tungsten suite have larger contributions from the middle to upper crust. Limited analyses of Neoproterozoic sedimentary rocks in the Selwyn Basin indicate initial strontium isotope ratios (at ~95 Ma) of 0.720 to 0.760 (Kuran et al. 1982) and εNd values of ~-16 to -25 (Garzione et al. 1997; Creaser and Erdmer 1997). Consequently, the parent magmas to the plutons appear to be composed of large proportions of highly radiogenic crustal melts, but the plutons are less radiogenic than the country rocks (Fig. 9).


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Redox StateThe oxygen fugacity of a granitic magma can reflect the redox state of the source region,

as well as influence the processes of metal enrichment in the magma (Ishihara 1981; Candela 1989; Carmichael 1991; Blevin & Chappell 1992). Several features are indicative of the oxidation state of a pluton; these include its magnetic character, Fe-Ti mineralogy, and Fe2O3/FeO ratio. The Mayo and Tungsten suite intrusions have, for the most part, low and flat aeromagnetic signatures that are impossible to differentiate from the low background values of the sedimentary country-rock sequences (Hart et al. 2000). However, where these plutons intruded carbonaceous (i.e., reduced) sedimentary strata, they have contact metamorphic aureoles with elevated magnetic signatures, due to pyrrhotite development in hornfels. In contrast, Tombstone intrusions have an inherently zoned or heterogeneous magnetic character reflecting their varied lithologies, some of which do contain magnetite.

These aeromagnetic data are supported by magnetic susceptibility studies of the granitic rocks. Measurements using a KT-9 hand-held magnetic susceptibility meter on freshly broken rock surfaces indicate that the Tombstone suite rocks have a higher average magnetic susceptibility (1.79x10-3 SI) and a wider range (Fig. 10) than Mayo and Tungsten suite rocks (average 0.11 and 0.16x10-3 SI, respectively).

The dominant Fe-Ti-bearing accessory mineral can also reflect the oxidation state of a magma and be characterized as magnetite or ilmenite-series (Ishihara 1977, 1981). Most of the intrusive rocks in the region are titanite-bearing, but some Tombstone rocks contain significant magnetite. In the Mayo intrusions, titanite is dominant and ilmenite is rare. Magnetite is generally restricted to late granite dykes and occurs as scattered grains in some mafic dykes. Ilmenite is common in the Tungsten suite intrusions, whereas magnetite and titanite are absent.

Whole rock ferric:ferrous ratios of fresh granitic rocks provide an approximation of the redox potential for rocks lacking subsolidus oxidation (Burkhard 1991). Generally, the

Figure 9 Plot of εNd vs initial Sr ratio for Tombstone, Mayo and Tungsten plutonic suites using data compiled from Godwin et al. (1980), Kuran et al. (1982), Gareau (1986), Abercrombie (1990), Lang (2000), Farmer et al. (2000). Hyland Group field from data of Garzione et al. (1997), and Creaser & Erdmer (1997). Broad arrow indicates direction of increasing crustal component to magma.

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Figure 10 Histograms of magnetic susceptibilities for intrusive rocks of the Tombstone, Mayo and Tungsten plutonic suites. Measurements were made determining averages of 6-10 readings of various, freshly broken rock surfaces of a single sample using a handheld KT-9 Kappameter with a pin spacer to account for surface roughness. Most plutons are represented by at least 4-5 samples.

Figure 11 Changes in oxidation state (Fe2O3/FeO ratio) of Tombstone, Mayo and Tungsten intrusive rocks with fractionation, as represented by SiO2. Note the increased oxidation state with fractionation for all three suites. Scale is in log units to give greater resolution at low levels. The anomalously elevated values likely reflect partial secondary oxidation from weathering. The approximate ranges for oxidation states of porphyry copper deposits (PCD) and tin-tungsten deposits (SnW) are indicated by grey shading. The calculated approximate positions of selected oxygen buffers are shown (NNO=nickel-nickel oxide; QFM=quartz-fayalite-magnetite; Hem-Mag=hematite-magnetite) and were derived using the MELTS (Ghiorso and Sack 1994) calculator for multi-component silicate liquids by inputting absolute chemical compositions of all three suites, and constrained with T=850°C and P=2000 bars.


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intrusions have relatively low Fe2O3/FeO ratios, but a general trend of increased Fe2O3/FeO with increasing SiO2 content suggests increased fO2 with progressive magmatic differentiation for rocks of all three suites. The Tombstone intrusions have the highest Fe2O3/FeO ratios, whereas the Tungsten suite intrusions have the lowest (Fig. 11). In the Tombstone intrusions, the higher proportion of Fe2O3/FeO is expressed mineralogically as Fe3+-bearing silicates (andraditic garnet, hornblende), as well as local enrichments of magnetite. The least-felsic Tungsten suite intrusions plot below the QFM buffer (Fig. 11), and pyrrhotite is locally present in some highly-fractionated Tungsten suite aplitic phases. The Mayo intrusions are slightly reduced, with the greatest number of samples near the QFM buffer. Although many samples plot above the QFM buffer, most do not contain magnetite, which suggests that other parameters, in addition to oxygen fugacity, control the nature of the Fe-Ti minerals. Also notable is the dominance of titanite in the Mayo suite rocks that are below the QFM buffer, which is uncommon in most igneous rocks.

Zircon CharacterSeveral authors have suggested that the presence and nature, or lack of, inherited zircon are

useful guides for interpretations of the nature and thermal state of the source region for granitic melts (e.g., Watson & Harrison 1983; Chappell et al. 1987, 1998). Zircons in Tombstone suite intrusions are large (to mm length), typically anhedral, and show only weak element zonation (Fig. 12), with a notable absence of inherited cores (Fig. 12A). Zircons in rocks of the Mayo suite are dominantly euhedral and contain variable proportions, as much as 10%, of ancient inherited cores that are overprinted with thick, oscillatory-zoned, euhedral magmatic zircon rims (Fig. 12B). Tungsten suite intrusions contain zircons that are similar to those of the Mayo intrusions, but are smaller with thinner oscillatory-zoned rims, and contain a higher percentage of old inherited cores, many lacking overgrowths (Fig. 12C). Although the xenocrystic zircons may reflect the nature of the source material, their presence likely reflects low zircon saturation temperatures (Miller et al. 2003). Zircon saturation temperatures are modelled estimates of zircon solubility conditions in a melt that depends, in part, on melt composition and zirconium concentrations (Watson & Harrison 1983). It provides the best measurement of the temperature of a magma at its source (Miller et al. 2003). The Tombstone suite intrusions have zircon saturation temperatures of about 820°C, which is about 40o and 70o higher than average Mayo and Tungsten suite temperatures, respectively.

Discussion

Source Regions of MagmasConstraints on the source regions and redox states of each plutonic suite can be related to

the contrasting element characteristics of their associated mineralisation.

The Tombstone intrusions have a wide range of silica content that are indicative of a protracted fractional crystallization history. Variation diagrams of rubidium and strontium versus SiO2 also support significant fractionation (Fig. 6). The lack of negative europium anomalies in the REE diagrams is interpreted to reflect the suppression of early plagioclase growth due to the high alkalinity or internal water pressure of the parental magma. Cumulate

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textures suggest that density separation of mineral phases has strongly influenced the compositional variations. All rocks are alkalic, with ultrapotassic compositions for those that host accumulations of potassium feldspar. Silica undersaturated phases are feldspathoid-bearing. Non-cumulate mafic phases that contain approximately 50-55% SiO2 are shoshonitic in composition. The strong enrichment of LILE, LREE, and volatiles, and the high alkalinity and high neodymium and strontium isotopic values, suggest that a parental mafic melt was derived from a lithospheric mantle source that had been long-ago enriched by metasomatism prior to fusion and melt generation. However, the relatively low Mg#, and low chromium and nickel contents of these rock are best explained by early fractionation of olivine, pyroxene and spinels (Nelson et al., 1986).

The nature of the enriched components in intrusions of the Tombstone suite suggests that metasomatism was subduction related, because the mafic rocks, as well as the more evolved intrusions, feature distinct negative Nb-Ti anomalies - a characteristic commonly attributed to melt generation in a subduction-related setting (Pearce 1982; Tatsumi & Eggins 1995). More-evolved phases likely resulted from fractionation, potentially with a degree of crustal assimilation. This is supported by U, Th and Zr contents that all increase significantly as SiO2 increases from 50% to 60%. Rocks with >70% SiO2 represent highly fractionated, quartz-rich intrusions and, at >75% SiO2, are weakly peraluminous and locally contain tourmaline. Zircon saturation temperatures for the Deadman pluton range from 750-850°C (Table 3), and the lack of inherited zircon suggests that any assimilation of crustal material occurred at temperatures

Figure 12 Backscatter SEM images of zircons. A) Tombstone suite zircons are large, mainly anhedral to subhedral and have faint internal variations, B) Mayo suite zircons have large, finely oscillatory rims on early cores, some of which are xenocrystic, C) Tungsten suite zircons have thinner, oscillatory rims on a greater proportion of xenocrystic cores, but also include xenocrystic zircons lacking overgrowths.


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>850°C. In addition, many of the zircons for the Deadman pluton are anhedral, and are interpreted to have formed late in the crystallisation sequence, where they concentrated available uranium and thorium.

The Tungsten suite intrusions have numerous characteristics to indicate that they are mainly derived from an ancient crustal source. The abundance of inherited zircon, the variable, although highly radiogenic, strontium and neodymium isotope values, the positive δ18O values, the moderately to strongly reduced state of the rocks, a lack of associated intermediate and mafic phases, and the elevated ASI values are all consistent with a dominantly sedimentary-rock crustal source. However, the intrusions are only weakly peraluminous, are generally less peraluminous than typical S-type granites (cf. Chappell and White 2001), and are relatively rich in calcium and strontium for granitoids derived from metasedimentary rocks. These factors could be due to either a mafic crustal component, or a significant volume of meta-carbonate (calc-silicate) rocks amongst the quartzo-feldspathic and pelitic strata in the source region. Deep seismic profiles of the region (Snyder et al. 2002) show that Proterozoic platform sedimentary rocks are imaged at the Moho boundary. It is likely that these strata were involved in melting to re-establish the Moho after depression through deep crustal isotherms in response to upper crustal thickening in the Late Jurassic to Early Cretaceous. Zircon saturation temperatures calculated for the main phases of the Mactung pluton range from 770 – 800°C (Table 3). Inherited ancient zircon cores are abundant in the Mactung intrusion. The temperature likely corresponds to the point at which the inherited zircons were entrained, and does not necessarily rule out a more protracted petrogenesis.

Early, main-stage quartz monzonites to monzogranites of the Mayo suite are geochemically similar to Tungsten suite plutons (e.g., Fig. 5). They have positive δ18O values and evolved neodymium and strontium isotope characteristics. However, many early Mayo plutons contain at least a small proportion of hornblende, and some contain abundant clinopyroxene. This mineralogical difference, in combination with a lower proportion of inherited xenocrystic zircon, and an association with intermediate to mafic intrusions, indicates that either a mafic crustal component was included in the magma, or that early intrusions contained a component of both mantle- and crustally-derived material. In addition, Mayo suite intrusions contain elevated chromium contents, and are generally enriched in strontium relative to the Tungsten suite intrusions for a given silica content. Zircon saturation temperatures for felsic rocks of the Mayo suite range from 770 –790°C (Table 3), and the presence of inherited zircon suggests that they were incorporated during crustal melting close to, or slightly above, the temperatures estimated for zircon saturation. These zircon characteristics, along with the less pronounced fractionation trends in rubidium and strontium data, suggest that Mayo suite intrusions had considerably more interaction with crustal rocks than did the bodies of the Tombstone suite.

Mafic rocks of the Mayo suite have similarities to some Tombstone suite rocks in that they are alkalic in nature, volatile-rich, and formed relatively late in the plutonic history of the region. However, the mafic rocks of the Mayo suite typically have relatively higher Mg#’s and contain higher chromium concentrations. These are related to the common occurrence of xenocrystic chromium-diopside, which indicates that these mafic Mayo phases are unlikely to have undergone considerable fractionation, (Mair et al. in press). Intermediate lithological

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phases typically contain elevated chromium and, in some cases, have textures indicative of mixing mafic and felsic magmas. These phases likely result from the interaction between evolving felsic magmas in the crust and mantle-derived mafic magmas.

Isotope data for rocks of the Mayo plutonic suite are intermediate between those for the Tombstone and Tungsten suite plutons. The variations between the suites are interpreted to reflect the proportion of mantle-sourced material, with the Tombstone intrusions having the greatest component of material derived from a mantle source, followed by the Mayo suite. The radiogenic nature of mantle-derived phases is attributed to ancient metasomatism of the lithospheric mantle, which also enriched LILE, LREE, volatiles, and other incompatible phases (cf. Carlson & Irving 1994; Nelson et al. 1986). In addition, neodymium and strontium ratios may well have been shifted further to more-radiogenic values by increased crustal contributions. However, the values for mantle-derived rocks are considerably lower than ratios for both upper crustal strata of the Selwyn Basin and intrusions of the Tungsten suite.

Redox Characteristics of Source MaterialsIntrusions of the TTB range from magnetite-dominant relatively oxidized, to ilmenite-

dominant reduced bodies. These variations are interpreted to primarily reflect the varying proportion of magma contributed from different sources. The Selwyn Basin crustal rock package is dominated by marine siliciclastic rocks, some of which are basinal or off-shelf facies that contain graphite, and thus can be considered as strong reductants (Ishihara 1977, 1981). In contrast, mantle-derived phases are typically relatively oxidised, due to oxidising metasomatic agents that are introduced to the mantle wedge above subduction zones (cf. Carmichael et al. 1996). In mafic intrusions of the Mayo suite, the presence of sparse magnetite, rare barite inclusions in mafic phenocrysts, and high Mg# pyroxenes suggest that these rocks may have originally been oxidised. The Tombstone suite rocks are the most alkalic, show the least evidence of crustal interactions, and are the most oxidised. Conversely, the isotope data suggest a greater component of middle to upper crustal material in the Tungsten suite intrusions - these rocks will typically contain a higher proportion of reduced carbon and are therefore the most reduced.

Relationships Between Redox State of Magmas and MetallogenyThe associations between the oxidation state and the metallogenic expression of intrusion-

related hydrothermal systems have been well documented (e.g. Ishihara 1977, 1981; Blevin and Chappell 1992). Ishihara (1977, 1981) indicated that the compatibility and partitioning behaviour of metals is strongly influenced by the oxidation state of a magma, which can be estimated based on the dominance of either magnetite or ilmenite. The chalcophile Cu±Au association is generally associated with oxidized mantle-derived magmas generated in a subduction zone setting (Candela 1989). A high oxidation state is optimal for preventing early formation of sulphide globules in a magma, which can efficiently sequester chalcophile elements such as copper and gold (Burnham and Ohmoto 1980). Molybdenum enrichment is also favoured by oxidised conditions, but at a redox state lower than is typical of that associated with porphyry copper deposits (Ishihara 1981).

Numerous studies have indicated that the lithophile metal association of Mo, W and Sn is also strongly influenced by redox conditions (Ishihara 1981, Blevin and Chappell 1992; Candela


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1995). Redox conditions influence the valency of some elements, which, in turn, determines the compatibility of such elements in accessory minerals. For example, tin enrichment in fractionating melts is favoured under reduced conditions, where it occurs mostly as Sn2+, rather than as Sn4+. In more oxidised magmas, Sn4+ is favoured, but readily substitutes for Fe3+ and Ti4+ in mafic silicate minerals and in Fe-Ti phases (such as titanite), thus preventing tin enrichment (Ishihara 1978). Tungsten enrichment is similarly favoured by reduced conditions, although associated intrusions span a broader range of redox environments (Ishihara 1981). At higher fO2, tungsten has affinities with molybdenum and copper, being concentrated in minerals such as molydoscheelite (Sato 1980); under more reducing conditions, tungsten has affinities with tin (Ishihara 1981).

Other ore-associated elements that are variably enriched in the TTB systems are tellurium and bismuth. Bismuth is commonly strongly enriched in reduced gold and tungsten skarns, and by inference, the concentration of bismuth through magmatic processes appears to be favoured by an intermediate to reduced oxidation state (Meinert 1992). Conversely, tellurium is commonly enriched in epithermal mineralisation associated with oxidised alkalic intrusive complexes (Jensen and Barton 2000; Cooke and McPhail 2001), suggesting that its enrichment may be controlled by factors other than just redox effects. However, controls of bismuth and tellurium concentrations are equivocal, as they are anomalous in a variety of different deposit styles that developed under contrasting redox conditions (e.g. Ciobanu et al. 2003).

The precise role that redox conditions play in generating gold-enriched magmas remains enigmatic. Although gold is a siderophile element, it has strong chalcophile tendencies, and the formation of early sulphide globules in the melt, particularly copper- and iron-bearing sulphides, could effectively reduce the gold concentration in the residual melt (Cygan and Candela 1995). As a consequence, magmas too oxidized to precipitate sulphide minerals may be more effective at concentrating gold during fractionation, and indeed, gold-enriched magmatic-hydrothermal systems are recognized as being associated with highly oxidized magmas (Candela 1989). However, other workers have suggested that gold in oxidized melts can be sequestered by magnetite, as Tilling et al. (1973) report values >100 ppm Au in magnetite. Supporting this concept are intrusions in central Alaska with Fe2O3/FeO ratios below 0.6, which typically lack magnetite and plot below the NNO buffer, and appear to be more prospective for gold mineralisation (Leveille et al. 1988; Newberry & Solie 1995; McCoy et al. 1997). Recently however, sequestering of gold by magnetite has been shown experimentally to be only moderately efficient in felsic melts, even those that are gold-saturated (Cygan & Candela 1995; Simon et al. 2002, 2003). Despite these contradictory factors, gold is known to be highly incompatible in most silicate phases. Taken together, these factors indicate that redox state likely has a large influence on the partitioning of gold in the magmatic environment during fractionation

The intrusions and associated mineral occurrences of the TTB show metal correlations that are mostly consistent with the redox associations outlined above. The relatively more-oxidised Tombstone suite intrusions are associated with mineralisation dominated by Cu-Au (± Bi) and U-Th signatures. Although the Tombstone intrusions are considerably less-oxidised than those related to typical porphyry copper deposits, they are sufficiently oxidised to prevent

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early sulphide formation in the melt, into which copper and gold could partition (Cygan and Candela 1995). Uranium-Th mineralisation is interpreted to result from fluids generated during extended fractionation of undersaturated melts, which, at high temperatures, suppresses zircon crystallisation and uranium sequestering.

The Tungsten suite intrusions are reduced and associated with large tungsten deposits that also have minor enrichments of Cu, Mo, Zn, and Sn. Tungsten enrichment in the melts is favoured by the precipitation of ilmenite over titanite, as W4+ may substitute for Ti4+ in titanite due to their common valency (Ishihara 1978). The lack of extensive tin mineralisation may be due to insufficient fractionation of Tungsten suite intrusions, stripping of Sn2+ from the melt by biotite crystallization, or a lack of tin in the melt source area. North America, as a whole, is anomalously lacking in significant tin deposits (Ishihara 1981; Taylor 1979).

The Mayo intrusions are generally characterised by an intermediate oxidation state. They are only slightly more oxidized than intrusions of the Tungsten suite and slightly more reduced than those of the Tombstone suite. Consequently, they have a metal association that is broadly intermediate between that of the other suites, such that many Mayo intrusions are predominantly associated with gold deposits, but also includes subordinate tungsten mineralisation. The Mayo suite related gold deposits are characteristically associated with enrichments of Bi, Te, and As, and these ore systems are also generally copper-poor. In addition, tungsten mineralisation is widespread in the Mayo intrusions, but, unlike deposits associated with Tungsten suite intrusions, high tonnage resources are not recognized.

Conclusions Among the numerous potential controls on the regional metallogeny of intrusion-related ore

systems, the nature of the source rocks and the corresponding redox state of the magmas are among the most important. The TTB forms a single magmatic belt that was emplaced within the ancient North American continental margin in the mid-Cretaceous. Magmatism post-dates a period of terrane collision and crustal thickening. The nature of the plutons and their associated mineral deposits vary considerably across the TTB, becoming more reduced and radiogenic, and tungsten-rich/gold-poor to the southeast. On the basis of these variations, the belt is divisible into three plutonic suites, which, from west to east, are the Tombstone, Mayo, and Tungsten suites. The plutons are compositionally variable, but comprise dominantly granites to monzonites. There is a general trend from more metaluminous intrusions in the northwest (Tombstone suite) to peraluminous- intrusions in the southeast (Tungsten suite). All plutons have high initial strontium isotope ratios (0.710-0.730) and high whole-rock oxygen isotope values (+11 to +13 δ18O), and most plutons have low magnetic susceptibilities and low primary oxidation state (Fe2O3/FeO ratio ~ 0.2-0.3).

The Tombstone suite intrusions are alkalic and metaluminous, and include silica-saturated and undersaturated phases that result from fractionation and the effects of density separation of minerals to form cumulates. These relatively oxidized plutons have accessory magnetite and have associated Au-Cu-Bi and U-Th metallogeny.

Plutons of the Tungsten suite are peraluminous and have many features of S-type granitoids, such as locally abundant muscovite, garnet, and tourmaline. These plutons are reduced, as they


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lack magnetite and titanite, but contain minor ilmenite and locally pyrrhotite. The plutons have significant xenocrystic zircon. Plutons of the Tungsten suite are associated with world-class tungsten deposits, but lack association with significant gold occurrences.

Intrusions of the Mayo suite have some characteristics that are difficult to reconcile within the generalizations of “granite-series” models (e.g. Ishihara 1981; Blevin et al. 1996). They are characterized by titanite as the dominant iron- and titanium-bearing accessory mineral, whereas magnetite and ilmenite are typically too scarce to be diagnostic. As such, they are too reduced to be magnetite-series, but the titanite suggests the magmas are too oxidized to be considered ilmenite-series. They have characteristics that support neither an I-type, nor an S-type designation, in the sense of Chappel and White (1992). They are prodigious mineralizers despite being weakly-reduced, highly radiogenic, and locally peraluminous, low-temperature granites. The plutons, however, commonly contain amphibole and some contain clinopyroxene. Although felsic intrusions are volumetrically dominant, they are intimately associated with intermediate and mafic intrusions considered to represent hybrid phases developed through mixing with mafic magmas. The Mayo suite intrusions host significant gold mineralisation, with elevated Bi, As, W, and Te.

The distinctive lithological, geochemical, and isotopic parameters that characterise each plutonic suite are interpreted to reflect the varying proportions of different source materials. The main sources are sialic crust, and a mantle source that is strongly enriched in LILE, LREE, volatiles, and other incompatible phases, and locally contains depleted element enrichments. The influence of the crustal source gradually increases from northwest to southeast across central Yukon Territory, such that most granites in the southeast crystallized from melts that had a predominantly crustal source. The different source regions had a strong primary influence on the oxidation states of the parent magmas. Redox effects likely controlled the behaviour of metals during fractionation of the magmas, either promoting or inhibiting their residual concentration. Although the magma compositions, redox state, and nature of associated mineralization of TTB intrusions are relatively compatible with existing models of intrusion-related ore systems (e.g. Ishihara 1981; Blevin et al. 1996), it is remarkable for such variation in intrusion compositions and metallogeny to occur along a single contemporaneous magmatic belt.

Acknowledgements: Lara Lewis compiled the magnetic susceptibility data and assisted in the figure preparation. Continued support from the Yukon Geological Survey is appreciated by Craig Hart, and Rich Goldfarb recognizes the support of the U.S. Geological Survey’s Minerals Program. Previous Hutton Symposium volumes provided much of the basis for the senior author’s granite acumen, and works by Shunso Ishihara and Phil Blevin established foundations for integrating magmas and metallogenic processes. We thank K. Sato and an anonymous reviewer for their time and effort in reviewing this manuscript. The editorial assistance and patience of Vikki Ingpen is appreciated. This manuscript represents a portion of the senior author’s PhD dissertation at the Centre for Global Metallogeny at the University of Western Australia.

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Chapter 5 Integrated Geochronology and Duration

Chapter Five

Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar Geochronology of Mineralizing Plutons

in Yukon and Alaska: Duration of Magmatic-Hydrothermal Systems

C.J.R. Hart1, 2, M.E. Villeneuve3, J.L. Mair1, D. Selby4, and C. Wijns1

1Centre for Global MetallogenySchool of Earth and Geographical SciencesUniversity of Western AustraliaCrawley, WA, 6009, Australia

2Yukon Geological SurveyBox 2703 (K-10)Whitehorse, Yukon, Y1A 2C6, Canada

3Geological Survey of Canada601 Booth StreetOttawa, Ontario, K1A 0E8, Canada

4Earth and Atmospheric SciencesUniversity of AlbertaEdmonton, Alberta, T6G 2E3, Canada


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Preface to Chapter FiveThis paper is a publication based on a presentation made at the SEG2004 Meeting in Perth, Australia, under the theme “Cutting Edge Developments in Mineral Exploration”. The presented paper, “Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar Geochronology of Mineralising Plutons in Yukon and Alaska; Duration of Magmatic-Hydrothermal Systems” has subsequently been submitted to a special issue of Mineralium Deposita as an invited paper, and is currently in the review process. The spelling and format for citations and references follow those that are used in Mineralium Deposita.

Justif cation of authorship: This manuscript represents the culmination of several years of geochronological efforts to understand the evolution of the belt as based on new data generated as a result of this Ph.D. study. Dr. Mike Villeneuve provided Ar-Ar analyses of various minerals. Mr. John Mair has been involved with on-going collaborations as part of his Ph.D., and has supplied some additional Ar-Ar dates and SHRIMP analyses. Dr. David Selby is a long-time collaborator who has undertaken the Re-Os analyses in Dr. Robert Creaser’s isotope laboratory in response to a project established by the candidate during the Ph.D. study. Mr. Chris Wijns undertook the thermal modeling in response to variables suggested by the author. All sampling for SHRIMP and Ar analyses were undertaken by the candidate, except those at Scheelite Dome where were made in partnership with Mr. Mair. All compilations, tables, figures and interpretations are those of the candidate, or of the candidate in collaboration with one of the co-authors.

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Integrated U-Pb SHRIMP & TIMS, Re-Os & Ar-Ar Geochronology of Mineralizing Plutons

in Yukon and Alaska: Duration of Magmatic-Hydrothermal Systems

AbstractThe mid-Cretaceous plutons that underlie the Tombstone-Tungsten Belt of Yukona dn

Alaska have a high proportion of associated mineral deposits and occurrences. Establishing the age relationships of these complex ore-bearing magmatic-hydrothermal systems requires the integration of several geochronological methods such that robust mineral species representing differing stages of magma crystalisation and hydrothermal activity can be effectively and precisely dated.

SHRIMP zircon U-Pb, mica Ar-Ar, molybdenite Re-Os, and TIMS U-Pb data are compiled to establish the timing of magmatism and mineralisation (97-92 Ma), and the duration of magmatic-hydrothermal systems within intrusion-related gold and tungsten systems of the Tintina Gold Province, in Yukon and Alaska (1.1 to 4.2 m.y). Thermal modeling of these systems indicates that the plutons and their associated hydrothermal systems should cool to below the Ar-mica closure temperature (~250°C) very quickly (~30 k.y.) but episodic magma pulses can keep the systems viable for much longer periods (0.3 to 1.0 m.y). Establishing genetic relationships between mineralization and magmatism requires that the timing of these events be proven to within this narrow time-frame.

Existing TIMS U-Pb dating indicated that most plutons hosting mineralisation were emplaced circa 92±1 Ma. However, much of the zircon U-Pb data (and zircon SEM imagery) show evidence of ancient inheritance and Pb-loss, which cast doubt on the accuracy of the individual dates. SHRIMP dating of zircon was employed to avoid Pb-loss and inheritance. Resulting dates are up to 4 m.y. older than the TIMS dates which suggests that the zircons have notable, though inconspicuous Pb-loss, even from highly abraded fractions.

Ar-Ar dates on magmatic minerals indicate that most of these plutons cooled quickly as the dates are within one million years of the best U-Pb dates. Ar-Ar dates on hydrothermal biotite and muscovite from mineralisation within or adjacent to the plutons are generally within agreement, within the ±1 m.y. uncertainty of the SHRIMP dates, but are sometimes 2-3 million years younger than the magmatic mica dates. This suggests that Ar retention in hydrothermal micas may differ than that in magmatic micas. Re-Os molybdenite dates, both model dates and isochrons, are also in agreement, and within the uncertainty of the SHRIMP dates for the host plutons.

An integrated approach to documenting the duration and events within a magmatic-hydrothermal history seems valid, but many complicating factors conspire to make resolution of individual events within a one million year time-frame difficult, if not impossible. In addition to the within-system problems, uncertainties in the absolute values of the decay constants, and the compounding of decay constant errors across isotopic systems further increase the uncertainties. Until these problems are reduced, and calibrations between methods


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are improved, the slotting-in of events within a one million year window is best done as a process of relativity from within a single isotopic system. In systems lacking complex cooling histories, Ar-Ar is likely most effective at discerning events within timeframes of less than one million years,

IntroductionThe age of formation of the intrusion-related gold and tungsten systems across the

Tombstone-Tungsten belt (TTB) and within the Fairbanks District is an important component for understanding both regional and local chronologies. Establishing the timing of plutonism aids both our understanding of emplacement with respect to local structural and metamorphic events and the plutonic responses to far-field tectonic processes. As well, it may highlight systematic trends or variations along the belt that relate to causative factors affecting magma generation and emplacement. In addition, if these are truly intrusion-related deposits, the timing of mineralization should be essentially co-eval with magmatism, and any younger dates should relate to the duration of their associated hydrothermal systems. Discrepancies in such timing relations could relate to other factors, be they intrinsic to the systems or extrinsic responses to more regional events.

However, magmatic-hydrothermal systems are notoriously complex, typically with several magmatic phases and vein phases that may be mutually cross-cutting. These different phases may each have different minerals that are useful for dating. As a result, integrating numerous chronometers and different mineral species from different parts of the geological environment appears to be an effective method to understand the evolution of magmatic-hydrothermal systems.

Herein, new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon and Ar-Ar mica ages are integrated with existing conventional TIMS (thermal ionization mass spectrometry) U-Pb zircon and recently published Re-Os molybdenite age data. These data have been variably obtained from magmatic minerals from host plutons, gangue and ore minerals in mineralized rocks and alteration minerals in host rocks. This array of settings, materials and methods will allow us to establish temporal relationships between igneous intrusions and their associated hydrothermal mineralization. The data will better define the duration of magmatic-hydrothermal activity responsible for formation of intrusion-related gold and tungsten systems in the Tintina Gold Province, Yukon and Alaska. As well, the different isotopic methods will be evaluated and compared as chronometers in magmatic-hydrothermal systems.

Regional Geology - Tintina Gold ProvinceThe Tintina Gold Province is a vast region of Alaska and Yukon that comprises numerous

(>15) individual gold belts and districts with a total gold resource/production of more than 70 Moz (Hart et al. 2002). Much of this mineralization is spatially, and locally genetically associated with suites of mid-Cretaceous granitic plutons. Prolific among the plutonic suites are the Tombstone-Mayo and Tungsten suites of mid-Cretaceous plutons that form the 600-km-long Tombstone-Tungsten belt (Figure 1). This belt comprises numerous plutons with associated gold deposits in the Fairbanks area of east-central Alaska, USA, and in the Tintina

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Gold Belt in central Yukon, Canada. Together, these deposits form the basis of the intrusion-related gold model (Lang et al. 2000; Thompson and Newberry 2000). In addition, this belt includes world-class tungsten-rich skarn deposits that are also associated with the mid-Cretaceous plutons.

The plutons were emplaced within, but near, the northern and eastern margins of the Selwyn Basin, a Neoproterozoic to Mesozoic clastic basin that formed within the surrounding carbonate platforms (Fig. 1). Clear Creek, Scheelite Dome and Dublin Gulch plutons are hosted in pelites, psammites, quartzites and calc-silicate that comprise the of the Neoproterozoic Hyland Group (Murphy 1997; Stephens et al. 2000). The strata have locally undergone Cretaceous ductile deformation and greenschist grade metamorphism to form the Tombstone Strain Zone. These rocks cooled through ~250°C approximately 105 million years ago (Mair et al. in review a). The Fort Knox pluton formed in a similar setting but has been dextrally offset 430 km to the northwest along the Tintina Fault (Fig. 1), and is considered to be within the Yukon-Tanana terrane. Mactung and Cantung plutons intruded deformed, but essentially unmetamorphosed Neoproterozoic to early Paleozoic strata. A few Late Cretaceous McQuesten Plutonic Suite intrusions crop out 5-10 km from the Clear Creek and Scheelite Dome areas (Murphy 1997), but have had no obvious any thermal effects.

The gold-dominant plutons at Fort Knox, Clear Creek, Dublin Gulch and Scheelite Dome which are associated with gold deposits, are metaluminous to slightly peraluminous, moderately reduced ilmenite-series granitoids that form part of the Mayo Plutonic Suite (Hart et al. 2004, 2005). Similar plutons in the Fairbanks area, such as Fort Knox, are the fault-offset equivalents of the Mayo Suite. The Cantung and Mactung plutons are further east, more peraluminous, more reduced, ilmenite-series granitoids and are representatives of the Tungsten

Figure 1. Distribution of mid-Cretaceous granitoids (red) in Yukon and eastern Alaska which comprise part of the Tintina Gold Province. The youngest plutons form the Tombstone-Tungsten Belt (TTB-yellow shaded) and are associated with significant gold and tungsten mineralization and are located along margins of the Selwyn Basin (blue). The Fort Knox pluton and deposit are part of this belt, but have been displaced 430 km by the Tintina Fault.


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Plutonic Suite (Hart et al. 2004). All plutons consist of a main phase lithology, normally granite or quartz monzonite, but include other magmatic phases with subtle lithological and textural variations. All plutons are radiogenic, with initial Sr values >0.71. Most plutons have well-developed and resistant, rusty-weathering contact metamorphic aureoles up to 1-2 km wide of biotite hornfels with andalusite and biotite porphyroblasts and disseminated pyrite and pyrrhotite.

The plutons at Fort Knox, Clear Creek, Dublin Gulch and Scheelite Dome are dominated by several styles of gold mineralization, have placer gold in the creeks that drain them, and also have associated tungsten mineralization, mostly as small scheelite skarns. The Mactung and Cantung plutons are responsible for generating the largest, richest and most significant tungsten deposits in North America (Dick and Hodgson 1982; Matheson and Clark 1984). The plutons and their associated intrusion-related deposits were mostly formed between 8 and 4 km depth (McCoy et al. 1997; Baker and Lang 2001). All plutons, except for that at Fort Knox, lack evidence of significant post-emplacement thermal perturbations, although late-stage lamprophyres have been recorded at Clear Creek, Scheelite Dome and Cantung.

Geology of the Plutons

Fort KnoxThe Fort Knox pluton (or Vogt stock) hosts the Fort Knox gold deposit which is ~40

km northeast of Fairbanks in east-central Alaska (Bakke 1995; Bakke et al. 2000). The easterly elongate 1.1x0.6 km pluton intrudes previously-deformed, quartzose and pelitic metasedimentary rocks of the Fairbanks schist in the Yukon-Tanana terrane (Fig. 2a). The pluton consists of three textural phases that are mostly of sparsely K-feldspar porphyritic biotite>hornblende granites with late aplite and pegmatite dykes. Mineralization consists of low-sulphide sheeted and stockwork veins, and quartz shear veins with pyrite, bismuthinite, scheelite and lesser molybdenite as the main ore minerals. The Fort Knox deposit is hosted entirely within the intrusion and has approximately 5.5 Moz of contained gold at ~0.9 g/t (Bakke et al 2000). The pluton also has associated, but small, tungsten skarns in its periphery.

The Fort Knox pluton is clearly post-deformational. It lacks deformational fabric, cuts the regional fabric and overprints it with dykes and a hornfels assemblage. The metasedimentary host rocks yield white-mica Ar cooling ages that are variable but mostly between 120 and 105 Ma; biotite ages show more scatter (Douglas et al. 2002). The pluton has, however, been subjected to a younger thermal event as indicated by ca. 50 Ma zircon fission-track data (Murphy

and Bakke 1993).

Clear CreekThe Clear Creek area is characterized by six small plutons with diameters that range

from 0.2 to 3.5 km and, together with their associated hornfels, produce a resistant highland that forms the headwaters to Clear Creek (Fig. 2b). All plutons cut the fabric in the host metasedimentary rocks. Pluton compositions range from quartz monzonite to quartz diorite. The Pukelman stock is mainly porphyritic biotite>hornblende quartz monzonite, but pegmatite, aplite and lamprophyre dykes have also been recognized. The Clear Creek area hosts numerous occurrences and styles of gold mineralization as well as small tungsten skarns

157


A

B

C

D

E

F

Figure 2. General geological maps of the: a) Fort Knox; b) Clear Creek; c) Sheelite Dome; d) Dublin Gulch; e) Mactung; and f) Cantung areas.


158

(Stephens et al. 2000; Marsh et al. 2003). Intrusion-hosted mineralization is dominated by arrays of thin (0.1-2 cm) sheeted, variably auriferous quartz ± K-feldspar veins with 1-2% pyrite, arsenopyrite, and rare scheelite, molybdenite, chalcopyrite and bismuthinite, in its cupola. Alteration is weak, typically comprising thin selvages of vein proximal silicification. More than 129,000 oz of placer gold has been recovered from creeks draining the Clear Creek plutons (Allen et al. 1999).

Scheelite DomeThe Scheelite Dome area is dominated by the east-trending 4 x 2 km quartz-monzonite

intrusion which intrudes the ENE-trending, moderately SSE-dipping fabric that is developed in the Neoproterozoic Hyland Group (Fig. 2c). The Scheelite Dome stock, and adjacent metasedimentary rocks, are intruded by widespread east-trending dikes of monzonite, quartz monzodiorite, granodorite and lamprophyre, with subordinate aplite and pegmatite (Mair et al. in review b). The main phase quartz-monzonite contains clinopyroxene and biotite as the dominant mafic phases, with minor hornblende and accessory apatite, titanite, allanite and zircon. Lamprophyres include both spessartite and minette varieties. A well-developed contact metamorphic aureole, characterized by biotite, andalusite and potassium-feldspar in pelitic horizons, and wollastonite, diopside and plagioclase in calcareous horizons, extend for up to 500 m from the pluton margins. Associated mineralization is variable and includes a intrusion-hosted sheeted quartz veins, fissure and shear-type country-rock-hosted auriferous veins, tungsten and gold-tungsten skarns, silver-lead-zinc veins, and considerable placer gold (Mair et al. 2000).

Dublin GulchThe Dublin Gulch pluton is an elongate, northeast-trending 5.5x2 km body that intrudes

and cuts greenschist-grade quartzite and phyllite of the Neoproterozoic Hyland Group (Fig. 2d). The pluton is dominated by quartz monzonite with granite, granodiorite and lesser aplite. Mineralization is dominated by intrusion-hosted sheeted auriferous quartz vein arrays such as the Eagle and Olive zones, and the Ray Gulch (also known as Mar) scheelite skarn deposit (Lennan 1983; Hitchins and Orssich 1995; Maloof et al. 2001; Brown et al. 2002). The Olive zone consists of numerous, 0.5 to 10 cm thick quartz-dominant veins with up to a few percent ore minerals that are dominated by arsenopyrite, pyrrhotite and pyrite, with lesser bismuthinite, scheelite and molybdenite. The veins have 1-5 cm wide alteration selvages of quartz and muscovite, with lesser Fe-carbonate±chlorite. Molybdenite occurs rarely on vein margins and in alteration selvages. The Olive zone is similar in most respects to the Eagle zone but generally has thicker veins, coarser-grained sulphide minerals and more arsenopyrite.

MactungThe Mactung pluton (or Cirque Lake stock) is a 2.4x1.2 km pluton that has intruded

deformed, but unmetamorphosed clastic rocks of the Neoproterozoic Hyland Group, and carbonate strata of the Cambrian Rabbitkettle Formation (Fig. 2e). The pluton comprises three phases, megacrystic, equigranular and aplitic granitoids, with the equigranular phase commonly occurring as apophyses into the pelitic units (Anderson 1982). The main pluton is dominated by biotite quartz-monzonite, but includes granite, monzogranite and aplite

159


(Anderson 1982). Hornblende is absent, but the stock locally contains garnet, muscovite and tourmaline. Tourmaline occurs as coatings on fractures and in vein selvages (Anderson 1982; Atkinson and Baker 1986).

Skarn mineralization is dominated by pyroxene, actinolite ± garnet, biotite skarn overprinted by pyrrhotite, scheelite, chalcopyrite ± molybdenite (57 Mt at 0.95 % WO3; Atkinson and Baker, 1986). The skarn and hornfels are crosscut by quartz veins that contain scheelite and molybdenite with a greisen-type alteration (quartz-muscovite-tourmaline ± biotite).

CantungThe Cantung pluton occurs on the easternmost margin of Selwyn Basin in the westernmost

Northwest Territories. The 5x1 km, northwest –trending pluton is the exposed apex of the flat top of a pluton where it is downcut and exposed in the Flat River valley (Fig. 2f). The intrusion consists of weakly peraluminous, medium-grained biotite granite, with later aplitic dykes. Locally muscovite is evident, particularly proximal to the skarn mineralization where the pluton displays phyllic alteration. The cylinder-like Circular stock is lithologically similar, although coarser grained, and crops out 1.5 km to the north (Fig. 2f). The main ore zones, the Open Pit and E-Zone, host > 9 Mt of 1.4% WO3 hosted mainly as pyrrhotite-rich, pyroxene±biotite skarns (Mathieson and Clark 1984). Scheelite and lesser chalcopyrite are the main ore minerals. A late stage lamprophyre dyke cuts the skarn zones in the open pit.

GeochronologyInitial geochronology, using the K-Ar and Rb-Sr methods, began in the 1970s when

the region was widely explored for tungsten (as compiled in Sinclair 1986, and Gordey and Anderson 1993). These data indicated a general mid-Cretaceous age, but gave widely scattered results between 106 and 76 Ma. More recent exploration has further highlighted the importance of understanding these plutonic rocks. A significant TIMS U-Pb zircon campaign was undertaken in the region in the mid-1990s, in response to mapping efforts and the increased recognition of the gold potential of these intrusions (Murphy 1997; J. Mortensen unpub., in Lang 2001; Coulsen et al. 2002;). This work resulted in a large number of precise determinations that indicated that many of the plutons were emplaced over the short time interval at 92 ± 1.0 Ma (Mortensen et al. 2000). However, most of the U-Pb analyses were discordant due to the high proportion of xenocrystic zircon and potentially some degree of Pb-loss, thus complicating the zircon systematics and making interpretations equivocal. Argon-Ar methods were used to date various magmatic and mineralized rocks at Fort Knox and other mineral occurrences in the Fairbanks area (McCoy et al. 1997), and at the Emerald Lake pluton in eastern Yukon (Coulsen et al. 2002).

More recently, a campaign of Re-Os molybdenite dating was initiated to determine the age of mineralization at some Tintina Gold Province deposits (Selby et al. 2002, 2003). These authors provided additional, and still more precise, TIMS U-Pb data for the Dublin Gulch, Clear Creek and Mactung plutons, but discrepancies between U-Pb, Ar-Ar and Re-Os dates became apparent.


160

In an effort to improve on the accuracy of determinations on the timing of crystallization, SHRIMP U-Pb methods were applied and are presented below. This method, which utilizes a focused ion beam, allows targeting of portions of a zircon which lack both ancient inheritance and Pb-loss, to obtain a more accurate date of zircon crystallization. SHRIMP U-Pb analyses were undertaken on the Fort Knox, Clear Creek, Scheelite Dome, Dublin Gulch, Mactung and Cantung plutons (Appendix 2). The timing of magmatism and mineralization was also evaluated using Ar-Ar analyses on micas, which are well-developed in both magmatic and hydrothermal environments. Argon-Ar analyses from Scheelite Dome, Dublin Gulch, Mactung and Cantung are presented in Appendix 3.

Fort KnoxSeveral geochronological techniques have been applied at Fort Knox (Table 1, Fig. 3a).

Various plutons and various types of intrusion-related mineralization in east-central Alaska and the Fairbanks area have returned Ar-Ar dates of 91 to 83 Ma (McCoy et al. 1997). Several Ar-Ar white-mica dates from mineralized Fort Knox stockwork veins have returned similar ages from 88.1 to 86.8 Ma (as reported in McCoy et al. 1997). Argon-Ar analyses of magmatic phases yield dates of 88 Ma on biotite from the pluton and 87 Ma on white mica from a pegmatite (McCoy et al. 1997; D. McCoy and P.Layer unpub.). These dates are essentially concordant with the mineralization ages, and together are interpreted to represent an approximate 1 m.y. duration of magmatism and mineralization (McCoy et al. 1997). However, a U-Pb TIMS date of 92.5 ± 0.2 Ma was reported for the Fort Knox pluton (J. Mortensen unpub., reported in Bakke 1995, and Selby et al. 2002) and suggested a four to six million year difference between magmatism and mineralization.

In order to provide a more robust age for mineralization, Selby et al. (2002) applied the Re-Os method to three samples of molybdenite associated with gold mineralization. Six determinations yield a restricted range of dates from 93.0 to 91.8 Ma, and a mean of 92.6 ± 0.9 Ma which is the same as the reported U-Pb TIMS date for the plutons, defining approximately contemporaneous magmatism and mineralization. SHRIMP U-Pb analyses of zircons from the Fort Knox pluton yield a slightly older date of 93.5 ± 1.0 Ma. This older SHRIMP date likely results from avoidance of zones of Pb-loss which may cause a conventional U-Pb to yield a slightly younger result. However, the TIMS, SHRIMP and Re-Os dates all overlap at the two-sigma error level.

The Ar-Ar dates are up to six million years younger than the U-Pb dates. However, these younger dates are not indicative of the timing of mineralization as there are several indications that they are the result of partially resetting. Some Ar-Ar spectra from the Fairbanks area have early steps that indicate a period of ca. 53 Ma Ar-loss and others have stepwise increasing spectra that indicate resetting (D. McCoy pers. comm.; McCoy et al. 1997). Younger thermal activity at the Fort Knox pluton and environs is indicated by fission track ages of about 50 ± 5 Ma on zircon, indicating that temperatures exceeded -190°C (240 ± 50°C), and 48-36 Ma from apatite, suggesting rapid cooling to <120°C (Murphy and Bakke 1993). Eocene subaerial volcanic rocks (56-53 Ma) are recognized in the Fairbanks area, and currently crop out about 12 km downstream from Fort Knox (Roe and Stone 1993), and were likely responsible for partially resetting the Ar-Ar systematics of the magmatic and ore-related micas.

161


A

B

C


162

Figure 3. Graphical representations of compiled dates from the: a) Fort Knox; b) Clear Creek; c) Sheelite Dome; d) Dublin Gulch; e) Mactung; and f) Cantung areas. Ages shown as U-Pb in zircon and titanite are TIMS ages. Abbreviations: musc=muscovite, bio=biotite,

moly=molybdenite, peg=pegmatite.

D

E

F

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Table 1. Compilation of geochronological data from Fort Knox. Abbreviations; alt’n=alteration, musc=muscovite, qtz-quartz.

Sample Geology Method/Material Date (Ma) Reference

Several samples

Country rock schist Ar-Ar muscovite 120-105 Douglas et al. 2002

Mort FK Pluton U-Pb TIMS zircon 92.5±0.2 Mortensen unpub., in Bakke 1995; Selby et al. 2003

Hart FK1 Pluton U-Pb SHRIMP zircon

93.5±1.0 This paper

FK1-1 Intrusion-hosted qtz vein+musc alt’n

Re-Os molybdenite 93.0 ± 0.4 Selby et al. 2002

FK1-2 Intrusion-hosted qtz vein+musc alt’n

Re-Os molybdenite 92.2 ± 0.5 Selby et al. 2002

FK2-1 Fracture coating Re-Os molybdenite 92.9 ± 0.4 Selby et al. 2002FK2-2 Fracture coating Re-Os molybdenite 92.6 ± 0.4 Selby et al. 2002FK2-3 Fracture coating Re-Os molybdenite 91.8 ± 0.3 Selby et al. 2002FK3 Intrusion-hosted qtz

vein+musc alt’nRe-Os molybdenite 92.9 ± 0.4 Selby et al. 2002

Mean of 6 Re-Os molybdenite 92.6 ± 0.9 Selby et al. 2002Isochron of 6 Re-Os molybdenite 92.4 ± 1.2 Selby et al. 2002

93RNFN3 Stockwork vein Ar-Ar white mica 88.1 ± 0.4 McCoy et al. 199793RNFN4 Stockwork vein Ar-Ar white mica 87.9 ± 0.4 McCoy et al. 199793RNFN6B Stockwork vein next

to chalcedonyAr-Ar white mica 86.8 ± 0.4 McCoy et al. 1997

Pluton Ar-Ar biotite 88.9 ± 0.5 McCoy et al. unpublished

93RNFN5 Pegmatite Ar-Ar white mica 87.4 ± 0.3 McCoy et al. 1997Hornfels Ar-Ar biotite 88-93 steps McCoy et al.

unpublishedPegmatite Ar-Ar amphibole 95 Ma saddle-

shaped prof leMcCoy et al. unpublished

Clear CreekGeochronological results from Clear Creek are compiled in Table 2 and displayed in Figure

3b. Of the six plutonic stocks at Clear Creek, two are dated by U-Pb TIMS methods. The Pukelman stock has a U-Pb zircon date of 91.4 ± 0.8 Ma and a U-Pb titanite age of 91.5 ± 0.6 Ma, and the larger Rhosgobel stock yields a U-Pb zircon date of 91.4 ± 0.3 Ma (Murphy 1997). Subsequent U-Pb TIMS zircon dates of the Pukelman pluton by Selby et al. (2003) yield dates of 91.4 ± 1.9 and 92.3 ± 0.9 Ma. Most of the U-Pb determinations are discordant, such that the absolute zircon dates were determined by lower intercept regressions with concordia, but these type of dates typically have large inherent uncertainties (Ludwig 2001).

Initial Re-Os isotopic determinations on molybdenite within the granitoids indicate mineralization ages of ca. 98 Ma which are clearly in error as the dates are older than the host granitoids (Selby et al. 2003). Subsequently, two molybdenite samples associated with gold mineralization in the Pukelman pluton yielded mean Re–Os dates of 93.6 ± 0.3 and 92.5 ± 0.3 Ma, and a similar mean of 93.4 ± 0.4 Ma was determined for molybdenite from the nearby Saddle zone (Selby et al. 2003).

These imprecise U-Pb dates, and conflicting Re-Os dates encouraged SHRIMP U-Pb zircon determinations to be undertaken. The result was a date of 93.6 ± 1.8 Ma which is


164

approximately 1 m.y. older than the U-Pb TIMS zircon dates, and remarkably similar to the Pukelman Re-Os dates. An Ar-Ar date determined on biotite from hornfels adjacent to the Pukelman stock gave a date of 91.7±0.4, but two middle steps in the plateau are 92.6 ± 0.6 Ma (Marsh et al. 2003), similar to the Re-Os dates. An Ar-Ar date of muscovite from a quartz vein in the Rhosgobel zone gives a date of 90.0±0.3 Ma. This date may be partly reset 65 Ma Vancouver Creek stock which is 6-7 km to the south.

Table 2. Compilation of geochronological data from Clear Creek, with emphasis on the Pukelman pluton. Abbreviations: qtz=quartz; moly=molydenite; py=pyrite; musc=muscovite


92DH-151 Pluton U-Pb zircon 91.4 ± 0.8 Murphy 199792DH-151 Pluton U-Pb titanite 91.5 ± 0.6 Murphy 1997CC-1 Pluton U-Pb zircon 91.4 ± 1.9 Selby et al. 2003DS7-01 Pluton U-Pb zircon 92.3 ± 0.9 Selby et al. 2003PUK1 Pluton SHRIMP U-Pb zircon 93.6 ± 1.8 This paper

DS7-01-1 Qtz-moly vein Re-Os molybdenite 93.6±0.3 Selby et al. 2003DS7-01-2 Qtz-moly vein Re-Os molybdenite 93.6±0.3 Selby et al. 2003DS7-01-3 Qtz-moly vein Re-Os molybdenite 93.7±0.3 Selby et al. 2003DS7-01 mean 93.6 ± 0.3CC-3-1 Qtz-moly vein Re-Os molybdenite 92.6±0.3 Selby et al. 2003CC-3-2 Qtz-moly vein Re-Os molybdenite 92.4±0.3 Selby et al. 2003CC3 mean 92.5 ± 0.3CC-2-1 Saddle zone Re-Os molybdenite 93.4±0.4 Selby et al. 2003CC-2-2 Saddle zone Re-Os molybdenite 93.5±0.3 Selby et al. 2003CC2 mean 93.4 ± 0.4

98CCPU2 Hornfels Ar-Ar biotite 91.7±0.4 Marsh et al. 200398CCPU2 Ar-Ar biotite, 2 steps 92.6 ±0.6 Marsh et al. 200389CCRG13 Qtz-py-musc vein Ar-Ar muscovite 90.0±0.3 Marsh et al. 2003

Dublin GulchDublin Gulch results are compiled in Table 3 and displayed in Figure 3c. The Dublin Gulch

pluton has TIMS U-Pb zircon and titanite ages of 93.5+5.8/-0.5 Ma and 92.8±0.5, respectively (Murphy 1997). The U-Pb zircon data indicate significant inheritance in most zircon fractions, such that the date is determined from a single, nearly concordant fraction whose upper error is determined from its 207/206 age which results in an imprecise upper error limit. The absolute date however, is only slightly older that its titanite age. A more precise TIMS U-Pb zircon age of 94.0±0.3 Ma was largely based on two overlapping concordant fractions (Selby et al. 2003). Further support for this date is provided by a SHRIMP U-Pb zircon date of 94.2±1.0 Ma from this study.

Mineralization was dated using Re-Os in molybdenite analyses which yields a range of ages from 95.2 and 93.1 Ma. The age of 93.2±0.3 Ma is considered the best analysis due to its reproducibility and larger sample size (Selby et al. 2003). Coarse-grained biotite from auriferous quartz veins associated with the same molybdenite that was dated by Re-Os, gives an Ar-Ar date of 96.5±1.3 Ma. The analysis, however, is too old and an inverse isochron plot indicates that it hosts a component of excess Ar.

165


Table 3. Compilation of geochronological data from Dublin Gulch


94SD-240 Pluton, Eagle zone U-Pb zircon 93.5+5.8/-0.5 Murphy 199794SD-240 Pluton, Eagle zone U-Pb titanite 92.8±0.5 Murphy 1997DS16-01 Pluton, Olive zone U-Pb zircon 94.0±0.3 Selby et al. 2003

00CH DG1C Pluton, Olive zone U-Pb SHRIMP zircon 94.2±1.0 This paper00CH DG1C Pluton, Olive zone Ar-Ar biotite 96.5±1.3 This paper

DG1-1 Olive zone Re-Os molybdenite 94.3±0.3 Selby et al. 2003DG1-2 Olive zone Re-Os molybdenite 95.2±0.4 Selby et al. 2003mean 94.8DS2-01-1 Olive zone Re-Os molybdenite 93.1±0.3 Selby et al. 2003DS2-01-2 Olive zone Re-Os molybdenite 93.2±0.3 Selby et al. 2003mean 93.2

Scheelite DomeGeochronological data for Scheelite Dome are compiled in Table 4 and displayed in Figure

3d. The Scheelite Dome pluton yields a TIMS U-Pb zircon date of 92.1±0.5 Ma and a titanite age of 91.2±0.9 Ma (Murphy 1997) which are considerably more precise than previous K-Ar ages of between 90 and 99 Ma (Kuran et al. 1982). SHRIMP U-Pb zircon dates of 94.4 ± 1.6 and 94.6 ± 1.0 Ma are slightly older. Mair et al. (in review b) present several Ar-Ar dates for the Scheelite Dome area, including 92.7 ± 0.2 Ma on magmatic biotite, and 92.9 ±0.2 Ma on hydrothermal biotite from an auriferous quartz vein. A date of 91.2 ± 0.6 Ma is determined for biotite from an auriferous, granite-hosted quartz vein (Appendix 3).

Table 4. Compilation of geochronological data from Scheelite Dome


94DM-240 Pluton U-Pb zircon 92.1±0.5 Murphy 199794DM-240 Pluton U-Pb titanite 91.2±0.9 Murphy 1997SD Main Pluton U-Pb SHRIMP zircon 94.4 ± 1.6 Mair 2004SDMonz Pluton U-Pb SHRIMP zircon 94.6 ± 1.0 Mair 2004

Magmatic Ar-Ar biotite 92.7±0.2 Mair 2004Skarn Ar-Ar biotite 93.8±1.3 Mair 2004Hydrothermal Ar-Ar biotite 92.9±0.2 Mair 2004

00CH SD-1 Qtz-moly-scheelite vein

Ar-Ar biotite 91.2±0.6 This paper

GS-101 Pluton K-Ar biotite 90.4±5.8 Kuran et al. 1982SYA79-84 Pluton K-Ar biotite 99.3±3.8 Stevens et al. 1982

MactungGeochronological data for Mactung are compiled in Table 5 and presented in Figure 3e.

Five K-Ar isotopic dates for the Mactung pluton on biotite and muscovite are in the range 90-89 Ma (Wanless et al. 1974; Hunt and Roddick 1987). A TIMS U-Pb zircon date of 92.1 ± 0.4 Ma was recently obtained by Selby et al. (2003) and indicates only slight discordance from, but much higher precision than, the K-Ar dates. However, six Re-Os determinations on three molybdenite samples give a narrow range of older, but precise dates of 96.9 to 97.6 Ma, suggesting a five million year discordance with the U-Pb date. Selby et al. (2003) interpreted


166

this as evidence of earlier mineralation event that was coincidently intruded by the younger Mactung pluton, supporting a similar suggestion by Atkinson and Baker (1986) on the basis of alteration zones that were discordant to the position of the Mactung pluton.

SHRIMP U-Pb analyses of Mactung zircons return a date of 96.4 ± 1.0 Ma which is much more in line with the Re-Os in molybdenite dates and do not require an earlier mineralizing event. Three Ar-Ar dates were obtained on differing types of mineralization. Fresh, coarse-grained biotite from biotite-rich pyrrhotite skarn gives a date of 96.7 ± 0.6 Ma. In addition, coarse-grained muscovite from quartz-tourmaline-muscovite-molybdenite veins yield a flat plateau of 95.2 ± 0.9 Ma for veins cutting the pluton and 95.5 ± 0.6 Ma for veins cutting the hornfelsed country rocks. The Ar-Ar dates are also older than the TIMS zircon date and support a single magmatic and metallogenic event. Since the SHRIMP, Re-Os, and oldest Ar-Ar mica dates overlap at the two sigma uncertainty level, the SHRIMP determination may be slightly too young.

Table 5. Compilation of geochronological data from Mactung.


DS10-1 Pluton U-Pb zircon 92.1±0.4 Selby et al. 2003CH MT-1 Pluton U-Pb SHRIMP 96.4±1.0 This paper

MT-02-1 Quartz greisen vein Re-Os molybdenite 97.2±0.5 Selby et al 2003MT-02-2 Quartz greisen vein Re-Os molybdenite 96.9±0.5 Selby et al 2003DS8-01-1 Quartz greisen vein Re-Os molybdenite 97.1±0.4 Selby et al 2003DS8-01-2 Quartz greisen vein Re-Os molybdenite 97.1±0.4 Selby et al 2003DS9-01 Quartz greisen vein Re-Os molybdenite 97.2±0.4 Selby et al 2003DS12-01 Quartz greisen vein Re-Os molybdenite 97.6±0.4 Selby et al 2003Mean of 6 Re-Os molybdenite 97.2±0.2 Selby et al 2003Isochron of 6 Re-Os molybdenite 97.5±0.5 Selby et al 2003

00CH MT03 Skarn Ar-Ar biotite 96.7±0.6 This paper00CH 15 Greisen vein in

plutonAr-Ar muscovite 95.2 ± 0.9 This paper

00CHMT14A Greisen vein in hornfels

Ar-Ar muscovite 95.5 ± 0.6 This paper

FJ68-320-2 Pluton K-Ar biotite 89±4 Wanless et al. 1974ANMT-81-8-1 Pluton K-Ar biotite 89±2 Hunt and Roddick 1987ANMT-81-8-1 Pluton K-Ar biotite 90±2 Hunt and Roddick 1987ANMT82-331-1 Aplite dyke K-Ar muscovite 90±1 Hunt and Roddick 1987

CantungGeochronological data for the Cantung area are compiled in Table 6 and presented in

Figure 3f. Early K-Ar biotite dates for the pluton at Cantung give an age date of 93.6±2.6 Ma

(Archibald et al. 1978). A SHRIMP date on the same quartz monzogranite yields an older date of 96.6±1.2 Ma. Three Ar-Ar mica dates on skarn mineralization and associated veins range from 95.8 to 93.9 Ma, defining a narrower range than the mica K-Ar dates of 97.3 to 92.2 Ma. There are no available TIMS U-Pb data nor Re-Os analyses.

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Table 6. Compilation of geochronological data from Cantung.Sample Geology Method/Material Date (Ma) Reference

02CH CT-1 Cantung pluton SHRIMP U-Pb zircon

96.6±1.2 This paper

AHC-11 Circular stock, pluton K-Ar biotite 93.6±2.6 Archibald et al. 1978

02CH CT1-6 Biotite skarn Ar-Ar biotite 95.8±0.6 This paper02CH CT1-5 Qtz-bio-moly vein Ar-Ar biotite 94.4±0.6 This paper02CH CT2-21 Muscovite in quartz-

pyrrhotite veinAr-Ar muscovite 93.9±0.6 This paper

U43-19E Biotite skarn K-Ar biotite 94.6±2.5 Archibald et al. 1978ZKA-1 Greisen pluton K-Ar muscovite 92.2±2.4 Archibald et al. 1978U43-20A Tremolite skarn K-Ar biotite 97.3±2.7 Archibald et al. 1978

SummaryNewly acquired SHRIMP U-Pb zircon and Ar-Ar mica data from Tintina Gold Province

plutons and mineral deposits are integrated with previously published TIMS U-Pb, Ar-Ar and Re-Os geochronological determinations (Murphy 1997; McCoy et al. 1997; Selby et al. 2002, 2003) and presented in Table 7. All presented dates are given with their two-sigma errors, but some published data do not have indicated precision.

Table 7. Compilation of ages for six plutons and associated magmatic-hydrothermal mineralization in Yukon and Alaska. Dates are compiled from data sources referenced in the Tables 1-6.

Location(west to

east)

U-Pb zircon TIMS

U-Pb zircon

SHRIMP

Re-Os molybdenite

Ar-Ar magmatic

biotite

Ar-Ar hydrothermal

muscovite

Ar-Ar hydrothermal

biotiteFort Knox 92.5 ± 0.2 93.5 ± 1.0 92.4±1.2 87.4 ± 0.4

muscovite 88.1-86.8 ± 0.4 n/a

Clear Creek 92.3 ± 0.291.4 ± 0.8

93.6 ± 1.8 92.5-93.4 ± 0.5

91.7 ± 0.4 90.0 ± 0.3 n/a

Scheelite Dome

91.2 ± 0.9 94.6 ± 1.494.4 ± 1.0

n/a 92.7± 0.2 n/a 92.9± .2

Dublin Gulch

94.0 ± 0.392.8 ± 0.5

94.2 ± 1.0 95.2-93.1 ± 0.4

n/a n/a 96.5 ± 1.3

Mactung 92.1 ± 0.4 96.4 ± 1.0 97.5±0.5 96.7 ± 0.6 skarn

95.2 ± 0.9 95.5 ± 0.6

Cantung n/a 96.6 ± 1.2 n/a 95.8 ± 0.6 skarn

93.9 ± 0.6 94.4 ± 0.6

The accuracy of any individual geochronological method is difficult to judge in isolation. It is only from integration of age dates from other methods or minerals, where the systematics are different or better understood, that accuracy can be interpreted. For example, nearly co-eval SHRIMP and Re-Os dates and slightly younger Ar dates provide a pattern that is expected and predictable, and thus increases the reliability of the absolute dates. Contemporaneous Re-Os and Ar-Ar dates on hydrothermal minerals that are older than zircon U-Pb dates indicate a lack of accuracy of some U-Pb dates. This is particularly apparent at Mactung.

The near concordance of Re-Os molybdenite and the best U-Pb zircon dates suggest either a very short interval between magma crystallization and hydrothermal mineralization, or that


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intercalibration of the isotope systematics may be skewed. Both Re-Os in molybdenite and Ar-Ar in vein mica dates record the hydrothermal event, but the Re-Os dates are consistently older. The younger Ar dates could result from the cooling of the hydrothermal system as opposed to the timing of crystallization, or may indicate that there are intercalibration problems between the two systems.

Comparison of TIMS zircon dates on similar rocks from different laboratories indicates variations up to 1 m.y. although these dates are commonly expressed with uncertainties less than this range. Rhenium-Os dates on molybdenite are reproducible within various aliquots from the same sample, but variations of up to 1 m.y. are apparent between different samples from the same location (e.g. Clear Creek). A detailed evaluation of erratic Re-Os data has revealed the importance of sample homogenization which is best achieved with larger sample sizes (Fig. 4). These points and others emphasize that the precision expressed in the age dates is simple analytical precision, but is far outweighed by other, mainly geological, uncertainties.

Figure 4. Plot of sample size vs. age for molybdenite samples from Clear Creek showing the increase in reproducibility for sample sizes greater than 20 mg, using data of Selby et al. (2003). Similar results have been recorded by Creaser and Selby (2002) and mechanisms accounting for this variability are presented by Selby and Creaser (2004). Data points shown are listed in Table 2.

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Discussion

Comparison of data Before comparing the geochronological data, it is necessary to evaluate the TIMS U-

Pb analyses. The TIMS U-Pb zircon dates on plutons associated with mineralization were determined using conventional methods, whereby zircon populations consisting of tens to hundreds of grains with similar size and magnetic characteristics were analysed. These dates indicate that most of the plutons were emplaced at ca. 92±1.0 Ma (Murphy 1997; Lang et al. 2000; Mortensen et al. 2000; Coulson et al. 2002). However, most of the zircon U-Pb fractions give discordant and complex results. Part of this complexity is due to the inclusion of some ancient zircons which mostly occur as cores with magmatic overgrowths within the analysed population (Fig. 5a). This interpretation is supported by scanning electron microscope backscatter-emission (BSE) imagery of similar zircons in which there are obvious xenocrystic zircon cores represent a significant part of the zircon population, from 1-20% depending on the sample. The resulting analysis of these samples is discordant yielding an older 207Pb/206Pb age (Fig. 6). This requires the application of regression techniques to determine an age on the basis of a lower intercept with concordia. This method results in large inherent errors, particularly for young (i.e. Mesozoic) zircons (Ludwig 2001).

In addition, many of the analysed zircon populations had high uranium contents, and BSE examination of similar zircons from the same plutons indicates that many zircons have internal damage to their structure (Fig. 5b). These metamict regions result from radiation damage and are sites that are susceptible to loss of radiogenic product lead. Analysis of a fraction containing Pb-loss will result in dates that are too young. However, since the concordia curve is nearly parallel to the Pb-loss curve, the zircons may still appear to be concordant (Fig. 6). It is likely that most of the analysed fractions had a mixed component of both inherited zircon and Pb-loss. Most of the zircon populations that were analysed were subjected to abrasion to reduce Pb-loss from outer zones, but removal of such zones will only concentrate the effect of internal Pb-loss. It will also increase the effect of xenocrystic cores on the determined age. Improvements to the TIMS dates can be made by more judicious selection of fewer zircons, for example, demonstrated at Dublin Gulch (Selby et al. 2003).

Given that many of the TIMS zircon dates are predicated on models sensitive to assumptions regarding Pb-loss and xenocrystic components, such as lower regressions, there are reasonable grounds to question the accuracy of some of the dates. In order to avoid regions of a crystal that may have a contribution of each of these components, it is necessary to use the high spatial resolution of SHRIMP analysis, with supporting BSE imaging. The six SHRIMP U-Pb zircon dates presented here yield ages between 93.6±1.8 and 96.6±1.2 Ma. The absolute dates are all equivalent to, or up to 4.3 m.y. older than, their corresponding TIMS date, confirming that the TIMS ages are modified by from some degree of Pb-loss.

The Re-Os molybdenite results are available from four locations (Selby et al. 2002, 2003) and they are typically within the uncertainty of the SHRIMP U-Pb zircon dates, with few analyses deviating more than one million years. Argon-Ar dates on magmatic biotite indicate that most of these plutons (except Fort Knox) cooled relatively quickly, as the dates are just slightly younger than the SHRIMP U-Pb dates. This suggests that argon closure temperatures


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for the biotite (about 250°C) were reached within 1-2 m.y. of zircon crystallization. Biotite Ar-Ar dates from skarns show similarly good concordance with magmatic biotite and the best U-Pb zircon ages. Hydrothermal biotite and muscovite Ar-Ar dates from vein mineralization, within or adjacent to the plutons, are 1.0-3.4 m.y. younger than the SHRIMP U-Pb zircon dates, and are consistently younger than the magmatic mica dates. These patterns are integrated and displayed in Figure 7. Argon-Ar dates from micas at Fort Knox deviate considerably from these trends, with dates consistently 4-6 m.y. younger than the U-Pb zircon and Re-Os molybdenite dates, in response to partial resetting in response to a ca. 50 Ma thermal event specific to the Fairbanks area.

Figure 5. SEM backscatter-emission images of zircons showing: a) magmatic overgrowths on ancient inherited zircon cores; and b) regions of internal damage which are likely loci of Pb-loss. These examples are all from Scheelite Dome. The scale bars are all 100 microns.

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By virtue of its ability to avoid regions of inheritance and Pb-loss, the SHRIMP U-Pb in zircon dates are considered more accurate than the conventional TIMS U-Pb dates, although at the expense of analytical precision. This increased accuracy is supported by the closer coincidence of the SHRIMP dates with the Re-Os dates, and Ar-Ar dates on magmatic zircons, most notably as displayed at Mactung and summarized in Figure 7. The younger Ar-Ar dates on hydrothermal micas suggest: a) that the region experienced slow cooling; b) that mineralization was associated with a younger magmatic pulse; or c) that Ar retention in hydrothermal micas may be less robust than in magmatic micas. Slow cooling may be ruled out as the muscovite ages, which have a higher closure temperature to Ar diffusion, would yield older dates than the biotite. However, at Mactung, Cantung and Clear Creek, the muscovite dates are younger than the biotite dates, potentially indicating deviations in the long-held dogma of the higher Ar closure-temperature of muscovite.

Duration of the Magmatic-Hydrothermal SystemsThe direct comparison of dates representing magmatic and hydrothermal events from

the same deposits provides an opportunity to determine the evolution and duration of the magmatic-hydrothermal systems. This is possible since these systems, except Fort Knox, have not been affected by post-emplacement thermal effects, and thus the data can be interpreted to reflect the cooling history that represents the magmatic to hydrothermal transition. For Fort Knox is interpreted without the Ar data. For each system, direct comparisons are made of the best absolute ages representing magmatic and hydrothermal events to determine the absolute

Figure 6. Concordia plot showing hypothetical data points from TTB plutons whereby analysed zircon fractions have inheritance and Pb-loss. The result is an array of possible lower-intercept regression ages and an array of mixed nearly-concordant fractions, which are difficult to interpret accuratly.


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durations. Additionally, the data are compared at the extremes of the two-sigma uncertainty level to determine the potential durations. Anomalous data points have been excluded, which means that some of the TIMS data and the young Ar-Ar analyses that do not overlap other analyses have been omitted.

The TIMS U-Pb zircon and Re-Os molybdenite dates from Fort Knox indicate a contemporaneous magmatic-hydrothermal event at ~92.5 Ma. If weight is given to the SHRIMP U-Pb zircon date, an apparent absolute duration of 1.1 m.y., or 3.3 m.y including the two sigma errors of all three isotopic systems, is indicated. At Clear Creek, the absolute apparent duration is 1.9 m.y., or 4.0 m.y. at the two sigma error limit. At Scheelite Dome, by excluding the TIMS zircon analyses, the gap between 94.6 and 92.7 indicates an absolute duration of 1.1 m.y. and a potential duration of 2.2 m.y at the two-sigma error limits. At Dublin Gulch, excluding the Ar-Ar mica analysis which has excess Ar and the oldest Re-Os molybdenite determination which is from too-small a sample, the absolute range of dates is between 94.2 and 93.1 Ma for an absolute duration of 1.1 m.y, but a potential duration of 2.5 m.y. At Mactung, excluding the TIMS zircon analysis, the apparent duration as indicated by the absolute dates between the oldest U-Pb zircon determination at 96.4 and the youngest hydrothermal phase at 95.2 Ma, is 1.2 m.y. However, this is complicated by older Re-Os molydenite determinations at ~97.3 Ma, suggesting that the system could have a duration of as long as 3.7 m.y. including the broadest extremes of the two sigma errors. The Cantung magmatic-hydrothermal system evolved from 96.6 to 93.9 Ma for an apparent duration of 3.3 m.y, or 4.2 m.y. including the two sigma error limits.

The range of absolute ages from zircon crystallization to hydrothermal micas of the six sites suggests that duration of the magmatic-hydrothermal systems were 1.1 to 3.3 m.y., increasing to 2.2 to 4.2 m.y if the extremes of the uncertainties are included. Argon-argon in mica and U-Pb zircon data for magmatic phases from another TTB pluton, the Emerald Lake pluton, suggest that magmatism lasted less than one million years (Coulson et al. 2002).

Figure 7. Generalized summary of relationships between age determinations from a typical TTB pluton using different methods and materials.

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Table 8. Compilation of TTB data to assess the durations of magmatic-hydrothermal systems.

LocationOldest

magmatic date

Youngest hydrothermal

date

Absolute duration

Duration within

two-sigmaNote

Fort Knox 93.5±1.0 92.4±1.2 1.1 3.3 Ar data reset and excluded

Clear Creek 93.6±1.8 91.7±0.4 1.9 4.0 Anomalously young vein excluded

Scheelite Dome 94.6±0.9 92.7±0.2 1.1 2.2 Anomalously young vein excluded

Dublin Gulch 94.2±1.0 93.1±0.4 1.1 2.5 Excess Ar and small sample size Re-Os samples excluded

Mactung 96.4±1.0* 95.2±0.9 1.2 3.7Cantung 96.6±1.2 93.9±0.6 3.3 4.2

Ranges 93.5-96.6 1.1-3.3 2.2-4.2

* Oldest magmatic date could be up to 1 my older which would increase the absolute duration but not the

duration based on two-sigma errors.

Modeled Durations of Cooling PlutonsThe reliability of the estimates of duration of magmatic-hydrothermal events indicated

above can be evaluated by several methods. One method is by comparison with thermal models for similar geological settings. Several types of data indicate that small (up to 10 km3) magmatic bodies emplaced in the upper crust tend to cool to background temperatures in periods far less than 106 years (Cathles 1981). Much of the published thermal modeling is based on porphyry copper deposits that were emplaced as small high-level plutons. These data indicate rapid cooling to only slightly above background temperatures in as few as 10 k.y. to 30 k.y. (Elder 1977; Cathles 1977). The systems can be maintained for longer by increasing the size of the plutonic pluton (i.e. to 60 k.y.; Norton 1982) such that a body twice the size will take four times as long to cool (Cathles 1981). Most models simulate emplacement of magmas, as 1-2 km diameter cylinder-shaped plutons, into the cool, upper few kilometres of crust, with high permeability and fluid fluxes resulting in significant effects from convective cooling. Conductive cooling is, in some cases, up to two orders of magnitude slower. Cathles (1981) suggested that any magmatic body emplaced at 800°C regardless of size, cannot maintain a thermal anomaly greater than 200°C for more than 4 million years.

The typical TTB systems are unlike porphyry systems as they were emplaced at depths of 8-5 km and lack evidence of external fluid interactions. As a result, a conductive cooling model is erected of a typical TGP pluton as a cylindrical body, 2 km in diameter, emplaced immediately at 800°C into isotropic host rocks with an ambient temperature of 200°C (equivalent to a depth of 6 km with a 35°C/km thermal gradient). Results are shown in Figure 8a. Using only conductive cooling, the margin of the pluton cools to about 250°C in about 65 k.y. This short interval persists despite biasing every variable towards system longevity. Modeling of a much larger TTB pluton, using slightly different variables, gave similar results (Coulson et al. 2002). These time intervals are not significantly different from the convective cooling models, since conductive cooling is dominant until the system is below 400°C when convective cooling can operate. Therefore, even conductive cooling models cannot adequately account for magmatic-hydrothermal system durations in the range of 106 years.


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In the models, although the most effective method of maintaining the system in the hot hydrothermal realm (>300°C) longer is to have higher background temperatures, regional mineral assemblages do not indicate elevated temperatures. Another method to sustain magmatic-hydrothermal systems is the episodic injection of fresh pulses of hot magma. The model in Figure 8b demonstrates that a magmatic system can be maintained at >350°C for 125 k.y. and >250°C for almost 250 k.y. by three additional pulses of magma of equal volume to the initial mass, at intervals of 15 k.y. The timing between pulses is key, as short intervals will allow cooling of the system to be too rapid whereas too-long intervals require energy to be used to reheat an already cooled system and behave as many solitary systems. Sustaining hydrothermal systems is best-maintained by cumulative thermal episodes separated by intervals short enough to prevent rapid cooling and may be capable of keeping systems alive for a million years or so. Some locations, such as Clear Creek which consists of six individual stocks may have such a history. Other TTB systems that are represented by a single pluton,

Figure 8. a) Thermal models of a pluton represented by an infinite length, 2 km diameter cylinder of magma emplaced at 800°C into an environment of 200°C. These conditions best represent those of the Tombstone-Tungsten Belt. The pluton Note different scales and colour indicators of distance on the two figures. b) Same variables previous model, with three additional magma pulses of equal volume to the original at 15 ka intervals.

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particularly those of the Mayo Plutonic Suite such as Scheelite Dome and Dublin Gulch, are actually mixtures of at least two batches of magma, one from the mantle (Mair et al. 2005; Hart et al. 2005). Such systems may also be episodically recharged and maintain magmatic heat.

Durations of Cooling Plutons from Empirical DataSeveral empirical indicators of the duration of magmatic-hydrothermal systems have been

documented. Contemporary geothermal systems are recognized as having short lifespans of 10 k.y. to 200 k.y. (White and Heropolos 1986). However, some system, such as The Geyers in California, have been active for at least 1.1 m.y., and despite numerical modeling that suggests it should have cooled 0.5 m.y. ago, it is still >300°C (Norton and Hulen 2001). Exposed plutonic rocks in active arc environments are locally less than one million years old, particularly in porphyry copper deposit environments.

Detailed geochronological research on ancient magmatic-hydrothermal systems indicates variable lifespans. For example, quartz-sericite alteration at Chuquicamata may have been 2-3 m.y. later than potassic alteration (Reynolds et al. 1998), a 1.8±0.4 m.y duration is indicated at Rosario in the Collahuasi District (Masterman et al. 2004), and a 3.2 m.y lifespan is likely at La Escondita (Richards 1999). The Bingham porphyry was also constructed over at least 1.5 m.y. of magmatic and hydrothermal activity (Parry et al. 2001). Contrasting with these relatively long-lived events is the short magmatic to hydrothermal episode at Porgera which was possibly as short as 0.1 m.y, and 0.4 m.y. at most (Ronacher et al. 2002). Volcanism and mineralization at Round Mountain occurred in a period possibly as short as 0.05 m.y., but certainly within 0.5 m.y. (Henry et al. 1997), and hydrothermal activity associated with porphyry and epithermal mineralization at Luzon lasted only 0.3 m.y. but is bracketed within 1.3 m.y. of volcanic activity (Arribas et al. 1995). Similarly, at Batu Hijau, 2.5 m.y. of episodic magmatic activity was followed by a short, 80 k.y. hydrothermal event (Garwin 2002). So although hydrothermal activity associated with hydrothermal mineralization may have a limited duration, it commonly occurs within a longer period of protracted magmatism.

Limitations of Data IntegrationAn integrated approach to documenting magmatic-hydrothermal evolution and duration is

valid, but many complicating factors conspire to make resolution of individual events within a one or two million year time-frame difficult, if not impossible. Most obvious are variations in the nature of the minerals that are dated, such as cryptic inheritance or Pb-loss in zircons. Different micas may have variable capacities for argon retention, which may be dependent on simple factors such as volatile content (e.g. water) and grain size. Similarly, factors involved with the decoupling of rhenium and osmium in molybdenite are not fully understood and sample selection may significantly affect age determinations (Creaser and Selby 2002; Selby et al. 2003; Selby and Creaser 2004). Additionally, features specific to the Yukon and Alaska intrusion-related systems may not be fully understood, such as geothermal gradients and the number and timing of successive magmatic pulses.

There are, in addition, within-system problems that include uncertainties in the absolute values of the decay constants and the compounding effects of comparing decay constant errors


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across isotopic systems, which further conspire to increase the uncertainties. Until these problems are reduced, and calibrations between methods are improved, the elucidation of events within a 1 m.y. window is probably best done from within a single isotopic system. The U-Pb TIMS dates are precise enough, but zircons with composite histories decrease the ability to establish accuracy with confidence, and SHRIMP is incapable of achieving <1 m.y. levels of precision.

The Road AheadThe U-Pb method has only been applied to hydrothermal dating in rare cases, mainly due to

the lack of uranium-bearing minerals suitable for analysis. However, recent developments in U-Pb phosphate dating are making inroads in this direction (Vielreicher et al. 2003). Improved understanding of Re-Os systematics (e.g. Selby and Creaser 2004) and applications (Morelli et al. 2003) are important research avenues. Recent improvements in double-spiking of Re-Os analyses have increased the precision of this method, yet such techniques are only applicable in special situations (e.g., very young or in molybdenite with very low rhenium contents). Further, most magmatic-hydrothermal systems lack multiple generations of magmatic and hydrothermal molybdenite providing fewer opportunities for “event” determinations. In upper crustal systems lacking complex cooling histories, Ar-Ar may be one of the most applicable methods because datable minerals are typically available in both the magmatic and hydrothermal regime. Precision of absolute ages are limited by the “standard-based” nature of the method, but carefully chosen analytical protocols can result in relative ages determined with precisions less than ±0.1% (i.e. <0.1 m.y. at 100 Ma), potentially making the method capable of differentiating events that are 0.3 m.y. apart.

AcknowledgementsThanks to all those who have contributed to the geochronology of these systems, particularly to Jim Mortensen for his contributions in this region. The Yukon Geological Survey is thanked for continuing support of this research. Thanks also to Dan McCoy and Paul Layer for allowing inclusion of some unpublished data. We appreciated the contribution of Rob Creaser for access and developments in his Re-Os lab. Ar-Ar dates were performed at the Geological Survey of Canada in Ottawa. SHRIMP data were acquired on the SHRIMP II facility,

which is operated by a consortium consisting of Curtin University of Technology, the Geological Survey

of Western Australia and the University of Western Australia with the support of the Australian Research

Council. Zircon imaging was done at the Centre for Microscopy and Microanalysis at The University of Western Australia. This manuscript benefits from a thorough review by David Groves.

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Chapter 6 Conclusions

Chapter Six

ConclusionsEvaluation of intrusion-related mineralisation and the associated plutonic framework of

the Tintina Gold Province, with emphasis on the Tombstone-Tungsten Belt, has resulted in a number of conclusions. These are briefly presented and interpreted with respect to the goals and aims as indicated in Chapter One.

Tintina Gold ProvinceThe Tintina Gold Province (TGP) is a composite of numerous placer gold and lode gold

belts and districts that occur throughout central Alaska and Yukon. All factors combine to indicate a gold endowment of about 70 Moz, although this estimate is variable and dependent on changing resource estimates for the large Donlin Creek deposit. Approximately half of the endowment and most of the production is from placer gold, emphasizing the immaturity of the region. However, exploration activity for lode resources increased dramatically in the early 1990s in response to discoveries at Fort Knox, Brewery Creek, Pogo and Donlin Creek.

The belts and mineral districts that make up the TGP contain gold deposits that formed as a result of several different mechanisms and are not necessarily dominated by intrusion-related mineralisation. Much of the gold formed in response to geological events that ranged from Late Triassic-Early Jurassic to as young as early Tertiary. Plutons responsible for generating intrusion-related gold systems are all Cretaceous, probably mostly mid-Cretaceous, and are mostly in the Tombstone-Tungsten belts (TTB) and the Fairbanks district, regions previously considered more prospective for tungsten mineralisation.

Previous authors sought to include as much of the TGP gold mineralisation within a single, complex intrusion-related model, but the deposits are best understood by separating them according to their dominant geological setting. As such, gold and gold-related deposits and occurrences form three groups representing differing styles of mineralisation – namely shear hosted, epizonal and intrusion-centred deposits.

Recognizing the complexities within these regionally diverse mineral occurrences, a more refined, intrusion-centred ore-deposit model can be defined and developed according to observations mostly made in the TTB and Fairbanks district. This model emphasises spatial and temporal proximity to a central mineralising pluton and highlights outwardly-zoned metallogenic associations. Within this model, there is a large variation in the styles of gold mineralisation, largely in response to differing host rock types, structural variables, and the position and distance with respect to the source pluton. However, this intrusion-centred model results in a predictable geological and geochemical model that can be effectively used in exploration targeting.


184

Exploration methodologies targeting the apical regions of these moderately reduced, mid-Cretaceous intrusions are considered to be most successful. Regional aeromagnetic and geochemical surveys are efficient mechanisms for recognizing these intrusions and metalliferous haloes, but direct identification of deposits within these regimes remain more elusive.

Different mineralisation styles correlate to differing grades of the deposits. Intrusion-hosted deposits, such as Fort Knox, approximate 1 gpt Au but can be very large (200 t Au). Epizonal deposits may have refractory ores, such as at Donlin Creek, but may also be oxidized and more economic, such as at Brewery Creek. Shear-hosted vein deposits have grades equivalent to orogenic deposits (10-20 gpt) but their genetic association with a central mineralising intrusion is less certain. The largest deposit of this type in the TGP, Pogo, is particularly controversial in terms of its genesis.

Northern Cordillera Mid-Cretaceous Plutonic ProvinceTwenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites are defined within the

complex geological collage that underlies Alaska and Yukon, on the basis of their lithological, geochemical, isotopic and geochronometic characteristics. The oldest suites are in the most outboard of the accretionary terranes and there is a general younging trend cratonward, such that the youngest suites intrude the ancient continental margin. Plutons associated with intrusion-related gold mineralisation in the TTB and Fairbanks area have a narrow mid-Cretaceous age range of 96-92 Ma.

Redox characteristics are such that the older and more outboard suites are clearly magnetite-series, whereas those intruding the ancient continental margin, or its displaced or metamorphosed equivalents, are much more reduced. Of these, most are ilmenite-series, but some suites are slightly too oxidised, and/or magnetic to be ilmenite-series and are classified as transitional to weak magnetite-series.

The time-space-redox distribution of the plutonic suites allows reconstruction of the mid-Cretaceous plutonic episode. It was originally subduction dominant, with oxidised, metaluminous and isotopically primitive plutons. Vast regions of reduced, slightly peraluminous and radiogenic batholiths were generated during subsequent protracted collision and crustal thickening. Waning stages of plutonism gave rise to sparse and smaller, slightly reduced plutons in more cratonward positions. The final magmatic episode involved the emplacement of the most inboard, variably oxidised and alkaline plutonic suites during weak extension.

Associated mineralisation largely follows classical redox associations, with a Cu-Au-Fe metallogenic tenor with the magnetite-series plutons, and W-(Sn) mineralisation with the ilmenite-series plutons. However, there are notable differences, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons in interior settings and form the basis for the reduced intrusion-related gold deposit class.

185


Source and Redox of Tombstone -Tungsten Belt IntrusionsWithin the mid-Cretaceous TGP plutonic suites, the youngest and most cratonward are

amongst the most metallogenically prolific, hosting and/or generating numerous gold, tungsten and silver deposits. This 750-km-long belt of plutons is divisible into the Tombstone, Mayo and Tungsten plutonic suites.

The Tombstone suite is alkalic, variably fractionated, slightly oxidized, contains magnetite and titanite, and has primary, but no xenocrystic, zircon. The Mayo suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone suite was derived from an enriched, previously-depleted lithospheric mantle, the Tungsten suite intrusions from continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo suite from a similar sedimentary crustal source, but mixed with a distinct mafic component from an enriched mantle source.

Each suite has a distinctive metallogeny that is related to the source and redox characteristics of its magmas. The Tombstone suite has a Au-Cu-Bi association that is characteristic of the most oxidized and alkalic magmas, but also has associated and enigmatic U-Th-F mineralisation. The reduced Tungsten suite intrusions are characterised by world-class tungsten skarn deposits with less significant Cu, Zn, Sn, and Mo occurrences. The Mayo suite intrusions are characteristically gold-enriched, with associated As, Bi, Te, and W associations. All suites have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral occurrences.

The distinctive lithological, geochemical and isotopic parameters that characterise each suite result from varying proportions of magma from different sources, with lesser mantle influence in more easterly plutons. These variations in source components influence the oxidation state of the magma which, in turn, influences the nature of their associated metallogeny. Magma composition, redox state and metallogeny are relatively consistent with existing intrusion-related ore-deposit models. However, intrusion-related gold deposits are only one component of the wide variation in granitoid composition and associated metallogeny across this single, broadly coeval magmatic belt.

Integrated Geochronology and Duration of TTB Systems New SHRIMP U-Pb and Ar-Ar data are combined with existing TIMS U-Pb dates and

recently published Re-Os dates for five TTB magmatic-hydrothermal systems including Fort Knox (93.5-92.4 Ma), Clear Creek (93.6-91.7 Ma), Scheelite Dome (94.6-92.7 Ma), Dublin Gulch (94.2-93.1 Ma), Mactung (96.4-95.2 Ma) and Cantung (96.6-93.9 Ma). The data indicate ages that are up to four million years older than those first determined by TIMS U-Pb of zircon analyses, suggesting that the latter may be inaccurate, likely in response to a combination of a xenocrystic zircon component and Pb-loss from radiation-damaged zircons.


186

Comparison of the dates from the different geochronological methods on different minerals within a single system provide consistent and predictable results. The best zircon U-Pb dates of the granitoids reflect magma crystallization and are slightly older than the Re-Os molybdenite dates that reflect hydrothermal mineralisation. These Re-Os dates are however, older than the Ar-Ar dates on hydrothermal mica phases, suggesting that the magmatic-hydrothermal system remained above the closure temperature of the micas (~250°C) for some time (0.2-2 m.y.) after their crystallization.

Evaluation of most-robust geochronological data indicates that these magmatic-hydrothermal systems were active for as little as 1.1 m.y. or as long as 4 m.y. These durations are similar to those defined geochronologically for other intrusion-related systems, but are considerably longer than those indicated by thermal modeling. Episodic magmatism may be the best mechanism to maintain thermal energy within these systems for longer periods and explain this apparent discrepancy.

Final RemarksComplexities in the intrusion-related gold system (IRGS) model, as presented by some

authors, result from too broad a perspective. Previous authors attempted to place a wide array of gold deposits sited throughout the Tintina Gold Province into a single model, whereas only those uncontested intrusion-related deposits of the Tombstone-Tungsten belt and the Fairbanks district should be used to define a robust IRGS model (see Chapter 2). Only when the critical features of the model are defined, should other deposits be considered for inclusion in the model. A key to recognizing intrusion-related mineralization is the presence of a central mineralising pluton, within and around which there are predictable spatial patterns of different styles of mineral occurrences. The roles that such mineralizing plutons play in relation to epizonal and shear-hosted gold mineralisation remain contentious, and require further study for resolution.

There is considerable variation in the nature of magmatism and the nature of economic significance of mineralisation associated with the mid-Cretaceous plutonic episode. The most prospective mineralising plutons are those that result from emplacement of small batch magmas and are partly unroofed, as opposed to large magma-volume batholiths that were emplaced at deeper levels. Gold is associated with both primitive highly-oxidised plutons and radiogenic relatively-reduced plutons, but the oxidised plutons have subordinate gold associated with copper mineralisation, whereas reduced intrusions have a gold-only paragenesis. This emphasizes the importance of the nature of the melt source and its redox state to the metal associations of the magmatic-hydrothermal systems.

The most prospective plutonic suites for intrusion-related gold systems are those that were emplaced furthest inboard of the subduction zone and proximal to the ancient cratonic margin. The Tombstone-Tungsten Belt, is in this position, was the final product of a protracted 30 m.y.-long magmatic event, and was emplaced during weak extension. Significant along-strike variations in the belt are the result of differing proportions of differing source components. Extension in the central and western part of the belt promoted the involvement of deeper,

187


more-enriched mantle melts to mix with, and hybridise crustal melts which dominate in the eastern part of the belt. This mantle contribution essentially converted reduced Tungsten Suite tungsten-associated granites into moderately-reduced Mayo Suite gold-associated granites in the central part of the belt. That the mantle-derived component was gold-enriched is indicated by the gold-enrichments in the most westerly, mostly mantle-derived Tombstone Suite. The magmatic and metallogenic associations in the Tintina Gold Province should be tested in similar tectonic settings and plutonic belts considered to be prospective for similar mineral deposits, worldwide.


188

189

Appendix

Appendix iGeochonology Sample Locations

00CH MT-03 Mactung mine NTS 1050/8Sample taken from mine dumpZone 9 441815E 7017680N

Biotite-rich region in W-skarn indicative of potassic alteration. Biotite is coarse-grained and intergrown with pyrrhotite.

00CH MT-14A Mactung mine area, ~1.1 km north of adit NTS 105O/8Zone 9 447316E 7293548N

Muscovite in quartz-tourmaline-muscovite-molybdenite veins cutting hornfels. Re-Os sample as well.

00CH MT-15 Mactung mine area, ~ 300m east of adit NTS 105O/8Zone 9 428103E 7017812N

Muscovite in greisen on quartz-tourmaline coated joints in Mactung pluton.

00CH DG-1C Dublin Gulch deposit NTS 106D/4East side of upper Olive GulchZone 8 462000E 7100850N

Coarse-grained biotite in quartz-molydenite vein in granite, same as Re-Os sample.

00CH SD-1 Scheelite Dome NTS 115P/16Sheeted zone on north side of gulchZone 8 437350E 7073650N

Coarse-grained muscovite in quartz-molybdenite vein in granite, same as Re-Os sample.

02CH CT1-5 Cantung tungsten mine NTS 105H/16UndergroundZone 9 539333E 6870553N

Coarse-grained biotite from quartz biotite ±molybdenite veins cutting granite and aplite. Age of f uid exsolution.

02CH CT 1-6 Cantung tungsten mine NTS 105H/16Underground Zone 9 5393333E 6870553N

Biotite from a coarse-grained biotite skarn cross cut by pyrrhotite and chalcopyrite veinlets. Age of skarn mineralization.

02CH CT2-20 Cantung area, NTS 105H/16Circular stock above Cantung open pitZone 9 538388E 6871405N

Coarse-grained muscovite from muscovite-pyrrhotite greisens on fracture surfaces on the Circular stock. Age of mineralization, minimum age for granite.

02CH CT 2-21 Cantung open pit, NTS 105H/16Zone 9 539231E 6870208N

Coarse-grained greenish sericite developed in 1 cm diameter circular fragment in quartz-pyrrhotite vein cutting aplitic dyke at Cantung open pit. Age of hydrous mineralization.

Appendix

190

Appendix 2SHRIMP Data

Pmlq/0/Qe,/05R 4c/-3 "

/-4`lo o/-3M_,/05R

. σboolo

CH+.*. /-1 66 -+2- .+.36/ -+-.11/5 +---.21 6/+0 ı .+-CH+.0*. 1-. .5 -+-2 -+1.-- -+-.1124 +---.03 6/+2 ı -+6CH+./*. 034 2/ -+.2 -+4101 -+-.12/. +---.1- 6/+6 ı -+6CH+..*. .214 ./2 -+-5 -+-5/4 -+-.121/ +---./2 60+. ı -+5CH+.5*. .212 .-1 -+-4 -+.2/- -+-.1333 +---./3 60+6 ı -+5CH+.4*. ..26 52 -+-5 -+/.4/ -+-.1343 +---./4 60+6 ı -+5CH+/*. 5/6 ./6 -+.3 -+1631 -+-.1423 +---.00 61+1 ı -+5

CH+.3*. .425 165 -+/6 -+/-16 -+-.15-0 +---./4 61+4 ı -+5CH+0*. 46- /5 -+-1 -+004. -+-.154/ +---.0/ 62+/ ı -+5

Pmlq/0/Qe,/05R 4c/-3 "

/-4`loo/-3M_,/05R

. σboolo

MH+2*. ..66 /6/ -+/2 -+-5/1 -+-.2-0- +---016 63+/ ı /+/MH+.-*. .-6- /.- -+/- *-+-./1 -+-.2-0- +---023 63+/ ı /+0MH+.-*/ .053 05. -+/5 *-+.-31 -+-.1652 +---014 62+6 ı /+/MH+6*/ .615 /1/ -+.0 -+--04 -+-.142. +---01- 61+1 ı /+/

MH+..*. .251 ..3 -+-5 -+.464 -+-.12.- +---010 6/+6 ı /+/MH+.0*. 135 66 -+// -+/--0 -+-.120- +---010 60+- ı /+/MH+.1*. ..44 /1. -+/. -+./13 -+-.11.3 +---002 6/+0 ı /+.MH+.2*. ./55 .24 -+.0 -+.30. -+-.1363 +---01- 61+- ı /+/MH.4*. .-16 42 -+-4 -+//34 -+-.1001 +---000 6.+4 ı /+.MH.4*0 65- .04 -+.1 -+.1./ -+-.11.5 +---003 6/+0 ı /+.MH.4*1 .0/6 /-5 -+.3 .+22./ -+-.2//- +---026 64+1 ı /+0MH+.5*. .46/ 20- -+0. -+-320 -+-.133/ +---005 60+5 ı /+.MH+.6*. //24 .02 -+-3 -+./5- -+-.1423 +---01- 61+1 ı /+/

Pmlq/0/Qe,/05R 4c/-3 "

/-4`loo/-3M_,/05R

. σboolo

AD+.*. 110 6/ -+/. 0+4654 -+-.146- +---0-5 61+3 ı /+-AD+/*. 043 52 -+/0 3+.212 -+-.15-- +---021 61+4 ı /+0AD+0*. 124 ./- -+/4 4+5161 -+-.131/ +---04/ 60+4 ı /+1AD+1*. 016 6/ -+/4 2+43-- -+-.11/3 +---0.2 6/+0 ı /+-AD+2*. /12 20 -+// 4+106. -+-.1340 +---034 60+6 ı /+0AD+3*. 11. ../ -+/3 1+5.5- -+-.1521 +---0/2 62+. ı /+.AD+4*. 06. 53 -+/0 1+623/ -+-.13.0 +---0/3 60+2 ı /+.AD+5*. 042 ..- -+0- 1+31.2 -+-.1243 +---0.6 60+0 ı /+-AD+6*. 2.. 64 -+/- 1+6-/6 -+-.1542 +---0.. 62+/ ı /+-

AD+.-*. 640 .6- -+/- .+4160 -+-.14/4 +---/40 61+/ ı .+4AD+..*. 153 .06 -+/6 0+111/ -+-.2-43 +---/64 63+2 ı .+6AD+./*. 6/5 /-. -+// .+22-- -+-.1540 +---/43 62+/ ı .+5

/-4`lo o/-3M_,/05R

/-4`loo/-3M_,/05R

σ

/-4`loo/-3M_,/05R

σ

191

Appendix

AD+.0*. 52/ .42 -+/. .+/.3. -+-.16-4 +---/50 62+1 ı .+5

AD+.2*. ./4/ //3 -+.5 .+5152 -+-.1303 +---/36 60+4 ı .+4AD+.3*. ..22 /03 -+/. /+/0-/ -+-.1246 +---/36 60+0 ı .+4AD+.4*. 10. .-5 -+/3 1+0//3 -+-.1413 +---0-. 61+1 ı .+6AD+.5*. .-.4 .14 -+.2 /+.22. -+-.1364 +---/4/ 61+. ı .+4

Pmlq/0/Qe,/05R 4c/-3 "

/-4`loo/-3M_,/05R

. σboolo

JQ+6+1*. .0.0 36- -+21 *-+--24 -+-.131- -+---/25 60+4 ı .+3JQ+6+.*. ./0. 0/1 -+/4 *-+-103 -+-.145/ -+---/3. 61+3 ı .+4JQ+6+/*. ..64 6. -+-5 -+.146 -+-.16-3 -+---/30 62+1 ı .+4JQ+2+.*. .5-6 03- -+/. *-+-33- -+-.1635 -+---/3. 62+5 ı .+4JQ+3+0*. ./62 20 -+-1 *-+0-/4 -+-.2-1. -+---/36 63+/ ı .+4JQ+5+/*. .23- 22 -+-1 -+.2/- -+-.2-16 -+---/40 63+0 ı .+4JQ+6+4*. ./33 32 -+-2 -+-402 -+-.2-56 -+---/34 63+2 ı .+4JQ+3+.*. 554 25/ -+35 -+000. -+-.2/65 -+---/44 64+6 ı .+5JQ+0+/*/ .413 04/ -+// *-+/220 -+-.2101 -+---/36 65+4 ı .+4JQ+/+0*. .506 /31 -+.2 -+-/-4 -+-.2131 -+---/36 65+6 ı .+4JQ+4+.*/ .15 1. -+/5 -+3640 -+-.1325 +---/22 60+5 ı .+3JQ+4+1*. 546 .-- -+./ -+4/-. -+-.20.2 +---.41 65+- ı .+.

Pmlq/0/Qe,/05R 4c/-3 "

/-4`loo/-3M_,/05R

. σboolo

@Q+.*. ./4 12 -+04 .+6... -+-.2-/0 +---0/0 63+. ı /+-@Q+/*. .24 .-0 -+35 .+4-61 -+-.20/4 +---0.6 65+. ı /+-@Q+0*. ...3 003 -+0. -+-62/ -+-.2//1 +---/45 64+1 ı .+5@Q+1*. 626 /63 -+0/ *-+--46 -+-.2035 +---/50 65+0 ı .+5@Q+3*. 302 //6 -+04 -+/4-5 -+-.225/ +---/55 66+4 ı .+5@Q+4*. ..-1 04. -+02 *-+-34/ -+-.2.35 +---/42 64+- ı .+4

@Q+.-*. 0-4 .15 -+2- -+.52- -+-.16.. +---/55 62+1 ı .+5@Q+./*. 5/3 /02 -+/6 -+126/ -+-.1324 +---/41 60+5 ı .+4@Q+.1*. 2-/ 031 -+42 -+.446 -+-.2-20 +---/5. 63+0 ı .+5@Q+.2*. .-6 42 -+4. .+6-6. -+-.2.04 +---000 63+5 ı /+.@Q+.4*. 613 162 -+21 -+0-44 -+-.16/1 +---/4/ 62+2 ı .+4

/-4`loo/-3M_,/05R

σ

/-4`loo/-3M_,/05R

σ

Appendix

192

Selected samples were processed for 40Ar/39Ar analysis of bioitite, muscovite or sericite by standard mineral separation techniques, including hand-picking of clearest, least altered grains in the size range 0.5 to 1 mm. Individual mineral separates were loaded into aluminum foil packets along with a single grain of Fish Canyon Tuff Sanidine (FCT-SAN) to act as flux monitor (apparent age = 28.03 ± 0.28 Ma; Renne et al., 1998). The sample packets were arranged radially inside an aluminum can. The samples were then irradiated for 12 hours at the research reactor of McMaster University in a fast neutron flux of approximately 3x1016 neutrons/cm2.

Laser 40Ar/39Ar step-heating analysis was carried out at the Geological Survey of Canada laboratories in Ottawa, Ontario. Upon return from the reactor, samples were split into one or two aliquots of one to four grains each, and loaded into individual 1.5 mm-diameter holes in a copper planchet. The planchet was then placed in the extraction line and the system evacuated. Heating of individual sample aliquots in steps of increasing temperature was achieved using a Merchantek MIR10 10W CO2 laser equipped with a 2 mm x 2 mm flat-field lens. The released Ar gas was cleaned over getters for ten minutes, and then analyzed isotopically using the secondary electron multiplier system of a VG3600 gas source mass spectrometer; details of data collection protocols can be found in Villeneuve and MacIntyre (1997) and Villeneuve et al. (2000). Error analysis on individual steps follows numerical error analysis routines outlined in Scaillet (2000); error analysis on grouped data follows algebraic methods of Roddick (1988).

Corrected argon isotopic data are listed below, and presented as spectra of gas release or on inverse-isochron plots (Roddick et al. 1980). Each gas-release spectrum plotted contains stepheating data from one or two aliquots. Such plots provide a visual image of reproducibility of heating profiles, evidence for Ar-loss in the low temperature steps, and the error and apparent age of each step.

Neutron flux gradients throughout the sample canister were evaluated by analyzing the sanidine flux monitors included with each sample packet and interpolating a linear fit against calculated J-factor and sample position. The error on individual J-factor values is conservatively estimated at ±0.6% (2σ). Because the error associated with the J-factor is systematic and not related to individual analyses, correction for this uncertainty is not applied until calculation of dates from isotopic correlation diagrams (Roddick, 1988). If there is no evidence for excess 40Ar the regressions are assumed to pass through the 40Ar/36Ar value for atmospheric air (295.5) and are plotted on gas release spectra. Blank were measured prior and after each aliquot and levels vary between 40Ar=2.5-3.6x10-7 nm, 39Ar=4.2-13.3x10-9 nm, 38Ar=0.4-1.7x10-9 nm, 37Ar=0.4-1.7x10-9 nm, 36Ar=0.7-1.3x10-9 nm, all at ±20% uncertainty. Nucleogenic interference corrections are (40Ar/39Ar)K=0.025±.005, (38Ar/39Ar)K=0.011±0.010, (40Ar/37Ar)Ca=0.002±0.002, (39Ar/37Ar)Ca=0.00068±0.00004, (38Ar/37Ar)Ca=0.00003±0.00003, (36Ar/37Ar)Ca=0.00028±0.00016. All errors are quoted at the 2σ level of uncertainty.

Appendix 3Argon-Argon data

193

Appendix

References

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Roddick, J.C., 1988. The assessment of errors in 40Ar/39Ar dating. In: Radiogenic Age and Isotopic Studies, Report 2. Geological Survey of Canada, Paper 88-2, pp. 7–16.

Roddick, J.C., Cliff, R.A. and Rex, D.C., 1980. The evolution of excess argon in alpine biotites — a 40Ar/39Ar analysis. Earth and Planetary Science Letters, 48: 185–208.

Scaillet, S., 2000. Numerical error analysis in 40Ar/39Ar dating, Chemical Geology v.162, 269–298.

Villeneuve, M.E. and MacIntyre, D.G, 1997. Laser 40Ar/39Ar ages of the Babine porphyries and Newman Volcanics, Fulton Lake map area, west-central British Columbia. In: Radiogenic Age and Isotopic Studies, Report 10. Geological Survey of Canada, Current Research 1997-F, pp.131–139.

Villeneuve, M.E., Sandeman, H.A. and Davis, W.J., 2000. A method for the intercalibration of U-Th-Pb and 40Ar/39Ar ages in the Phanerozoic. Geochimica et Cosmochimica Acta, v. 64, p. 4017-4030.

Appendix

194

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195

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Appendix

196


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1&σ

197

Appendix

Appendix

198

mid-cretaceous magmatic evolution and intrusion-related metallogeny...

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