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Volcanology and geochemistry of the Cambrian Mount Read Volcanics in the Basin .Lake area, .
western Tasmania
by
Andrew T Jones
B.App.Sci. (RMIT) Hons. (Tas)
A thesis submitted in fulfilment of the requirements for the degree of
Master of Science
Centre for Ore Deposit and Exploration Studies, School of Earth Sciences
University of Tasmania
May 1999
1
Authority of Access
This thesis is not to be made available for loan or copying for one year from the date this statement
was signed. Following that time the thesis may be made available for loan and limited copying in
accordance with the Copyright Act 1968.
Signed: Andrew Jones Date: '2~/S/t~i1~~
ill
Declaration
This thesis contains no material which has been accepted for the award of any other degree or
diploma in any University, and to the best of my knowledge and belief, contains no copy or
paraphrase of material previously publlshed or written by another person, except where due
reference is made in the text of this thesis.
Ars~ 2./ls/'11Signed: Andrew Jones Date: I ,
ii
Abstract
The Basin Lake area is located in western Tasmania approximately 12 km north of Queenstown. A
stratigraphic section through the late Middle Cambrian Mount Read V91canics and overlying Late
Cambrian to Early Ordovician Owen Conglomerate is exposed in outcrop and diamond drillcore.
In this area, the Mount Read Volcanics consist of the Yolande River Sequence, Anthony Road
Andesite and Tyndall Group.
The Great Lyell Fault and South Henty Fault were active normal faults during the Cambrian and
bounded the extensional basin into which the lithologic associations of the study area were
deposited. NW to NNW striking fault sets present in the area may represent Cambrian transfer
faults. Silica-sericite-pyrite alteration has been focussed along these transfer faults and facies
changes occur across them. Devonian orogenesis has reactivated faults across the area.
The Yolande River Sequence forms the basal unit within the area and comprises volcaniclastic
turbidites and non-volcanic black mudstone deposited in a below-wave-base, probably deep water
setting. Components in the volcaniclastic units suggest a dacitic provenance and include abundant
pumice and volcanic crystals (feldspar, quartz) that were most likely supplied by explosive
eruptions at subaerial or shallow marine, extrabasinal or basin-margin vents.
Intrusive into and overlying the Yolande River Sequence is the Anthony Road Andesite, an
intrabasinal submarine dome, lava and intrusive complex largely comprising coherent and
autoclastic facies of basaltic andesite, hornblende andesite and rhyolite. Interleaved with the
coherent and autoclastic facies are black mudstone, limestone and ironstone. Limestone and
ironstone occur towards the stratigraphic top of the Anthony Road Andesite and at the base of the
overlying Tyndall Group.
The hornblende andesite and rhyolite associations of the Anthony Road Andesite have high-K
calc-alkaline affinities whereas the stratigraphically higher basaItic andesite association is
shoshonitic, with high P20S and light REE-enrichment. A similar transition from calc-alkaline to
shoshonitic compositions has been noted in the Que-Hellyer Volcanics and the Lynch Creek
basalts (Crawford et aI., 1992). The rhyolite association of the Anthony Road Andesite has no
recognised geochemical analogues in the Mount Read Volcanics.
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Ironstone contains single strand filamentous textures similar to bacterial mats found at
hydrothermal vent sites on the modem seafloor. Modem analogues favour deposition from low
temperature « 100°C) iron- and silica-rich, hydrothermal fluids on or within metres of the
seafloor. Base and precious metal-rich barite veins in Tyndall Creek and hematite-carbonate-barite
alteration with associated·silver mineralisation at Howards Anomaly occur at a stratigraphically
similar position to the ironstone facies. It is inferred that the limestone facies is laterally equivalent
to this stratigraphic position.
The Tyndall Group is the youngest lithostratigraphic unit of the Mount Read Volcanics and records
the final phases of volcanism and denudation of the source volcanic terrain. The group is
dominated by thick, crystal-rich volcaniclastic sandstone, breccia and conglomerate with lesser
limestone and rhyolite deposited in a marine below-wave-base setting by mass-flows. The
volcaniclastic mass-flow units in the Mount Julia Member and Zig Zag Hill Formation are thick,
coarse, and lacking finer interbeds which may suggest a shallower and more proximal depositional
setting than for the Yolande River Sequence. The occurrence of welded ignimbrite and the
dominantly dacitic to rhyolitic provenance of the Tyndall Group suggest the presence of felsic
caldera volcanoes in the source area.
The Owen Conglomerate conformably overlies the Tyndall Group in the Basin Lake area.
Sedimentary facies include conglomerate, sandstone and mudstone mainly derived from
Precambrian metamorphic basement. The conglomerates are well sorted and the clast population is
commonly well rounded indicating significant reworking in a high energy environment. Facies
variations indicate a change in conditions from below-wave-base to above-wave-base, with
deposition interpreted to have occurred in subaerial to submarine alluvial fans and fan deltas.
Modem analogues for the palaeogeography of the Basin Lake area can be found in deep marine
settings offshore from active subaerial volcanic terrains, such as the Bay of Plenty offshore from
the Taupo Volcanic Zone, New Zealand. The high-K calc-alkaline hornblende andesite and
rhyolite associations of the Anthony Road Andesite have geochemical analogues in modem arc
settings. The closest modem analogues of the shoshonitic basaltic andesite association occur in
post-collisional arc settings in Papua New Guinea, Turkey and tbe Himalayas.
v
Acknowledgements
I would like to acknowledge my supervisor Jocelyn McPhie for the assistance and advice which
she has extended to me and for her proof reading of the text of this thesis. I thank my unofficial
supervisors, Garry Davidson, Joe Stolz;Paul Kitto and Bruce Gerr;unell, for their expertise,
advice and proof reading of parts of the text. An APRA (Industry) scholarship funded this
research. Aberfoyle and Acacia are thanked for their fmancial support of the project.
The help, financial assistance and friendship extended to me by the former staff of Aberfoyle
Resources Ltd has been greatly appreciated. Robina Sharpe and Rob Lewis are acknowledged for
introducing me to the geology of the study area and for many helpful discussions. Garry Cooper
and Scott Williams are thanked for their help shifting drillcore and all the other various things they
have done while Richard de Bomford is thanked for his friendship throughout this study and for
his assistance with all things logistical. Dave Wallace, Chris Young, Hugh Skey, Steve
Richardson and John Hiscock are also thanked for their assistance.
Assistance with analytical work was extended to me from several quarters. In particular I would
like to acknowledge Phil Robinson, Simon Stevens, Naomi Deards, Katie McGoldrick, Nilar
Hlaing and Erin McGoldrick from the Geology Department, Mike Power, Christine Cook and Wis
Jablonski from the Central Science Laboratory and Mike Jacobsen and Richie Wooley from
Tasmania Development and Resources. Jeanette Harris, Peter Cornish, June Pongratz and
Christine Higgins are thanked for their assistance with innumerable problems.
Many staff members and postgraduate students of both CODES and the Geology Department have
assisted me with my studies. In particular, I would like to thank Matthew White, Alicia Verbeeten,
Ron Berry, Tony Crawford, David Cooke, Andrew McNeill, Marcel Kamperman, Steve Hunns,
Nathan Duhig, Dros Herlec, Singoyi Blackwell, Bill Wyman, Mark Doyle, Mohammad Adabi,
Zohreh Amini, Cathryn Gifkins, Anthea Hill, David Rawlings, Peter McGoldrick, Andrew
Tunks, Jamie Rodgers, Dave Selley, Bruce Anderson, Mark Duffett, Mike Roache, Steve Boden,
Rohan Wolfe and Stuart Smith. Thanks to Bruce and Liz Anderson and Cathryn Gifkins for
giving me somewhere to live right until the end.
A special acknowledgement for their consistent support and help with proof reading must go to my
parents, Anthony and Faye Jones. Joe McKibben and Rohan Williams are thanked for moral
support over the last three years. My final thankyou is to Bianca Manzi for her help with proof
reading and for believing.
vi
Table of Contents
Title Page Declaration Authority of Access Abstract Acknowledgements Table of Contents List of Figures List of Tables List of Plates
Chapter 1 Introduction 1.1 Aims 1.2 Location and access 1.3 Exploration and mining history 1.4 Previous work 1.5 Thesis organisation
Chapter 2 Regional Geology 2.1 Introduction 2.2 The Mount Read Volcanics
2.2.1 Central Volcanic Complex 2.2.2 Eastern quartz-phyric sequence 2.2.3 Basaltic-andesitic units 2.2.4 Western volcano-sedimentary sequences 2.2.5 Tyndall Group
2.3 Owen Conglomerate (Denison Group) 2.4 Cambrian economic mineralisation 2.5 Location of the study area in the Mount Read Volcanics
Chapter 3 Structural Geology 3.1 Introduction 3.2 Structural analysis of the Basin Lake area
3.2.1 Domain 1 Bedding, columnar jointing and flow banding Folds and cleavage
3.2.2 Domain 2 Bedding Folds and cleavage
3.2.3 Comparison of Domains 1 and 2 3.3 Faults
3.3. 1 South Henty Fault 3.3.2 Great Lyell Fault 3.3.3 NW to NNW faults 3.3.4 NE faults 3.3.5 . Discussion of faults
3.4 Summary and conclusions
Pa2e No.
i ii ill IV vi vii xi xiii xiv
1 1 2 4 7 9
10 10 10 11 14 14 15 16 18 18 19
20 20 21 21 24 24 26 26 26 30 30 30 33 34 34 36 36
vu
Chapter 4 Facies characteristics of lithostratigraphic units in the Basin Lake area 38
4.1 Introduction 38 4.2 Yolande River Sequence 38
4.2.1 Black mudstone facies 41 4.2.2 Volcanic mudstone facies 41 4.2.3 Coarse feldspar-pumice sandstone 41 4.2.4 Purniceous lithic-rich breccia 41 4.2.5 Sedimentation processes,-depositional setting and provenance
:;,:.of the Yolande River Sequence 42 4.3 Tyndall Group 43
4.3.1 Comstock Formation 43 LimestoneJacies 44 Volcaniclastic sandstone facies 44 Black mudstone facies 45 Crystal-rich volcaniclastic sandstone facies 45 Polyrnictic volcanic breccia facies 47
4.3.2 Zig Zag Hill Formation 47 Graded volcaniclastic sandstone/siltstone facies 47 Polyrnictic conglomerate facies 48 Coherent rhyolite and monomictic rhyolite breccia facies 48
4.3.3 Sedimentation processes, depositional setting and provenance of the Tyndall Group 50
4.4 Owen Conglomerate (Denison Group) 54 4.4.1 Jukes Conglomerate 54 4.4.2 Lower Conglomerate Unit 56 4.4.3 Newton Creek Sandstone 56 4.4.4 Middle Owen Conglomerate 57 4.4.5 Sedimentation processes, depositional setting and provenance
of the Owen Conglomerate 57 4.5 Post-volcanic dykes in the Mount Julia Member and Newton Creek Sandstone 58
4.5.1 Basalt dykes 58 4.5.2 Rhyolite dyke 59
4.6 Stratigraphic relationships 60 4.6.1 Stratigraphic relationships in the Basin Lake area 60 4.6.2 Comparison of stratigraphic relationships in the Basin Lake area
with the southern Mount Read Volcanics 60 4.7 Summary of depositional setting and provenance of the study area 61
Chapter 5 Lithofacies, facies associations and facies architecture of the Anthony Road Andesite 63
5.1 Introduction 63 5.2 Basaltic andesite association 64
5.2.1 Pyroxene-feldspar-phyric basaltic andesite 64 5.2.2 Facies architecture of the basaltic andesite association. 66
5.3 Hornblende andesite association 69 5.3. 1 Massive facies 69 5.3.2 Andesitic breccia facies 70 5.3.3 Matrix-supported andesitic breccia 73 5.3.4 Andesitic-dacitic volcaniclastic sandstone 73 5.3.5 ,Facies architecture of the hornblende andesite association 73
5.4 Rhyolite association 74 5.4.1 . Quartz-feldspar-phyric rhyolite facies 74 5.4.2 Monomictic breccia facies 75 5.4.3 Rhyolitic volcaniclastic breccialsandstone 75 5.4.4 Facies architecture of the rhyolite association 77
viii
5.5 Sedimentary facies association (non-volcanic) 77 5.5.1 Black mudstone facies 77 5.5.2 Limestone facies 78 5.5.3 Ironstone facies 78
5.6 Facies architecture of the Anthony Road Andesite 80 5.6.1 Depositional environment 80 5.6.2 Eruption and emplacement processes 80 5.6.3 Proximity to source vents 81 5.6.4 Character and evolution of the volcanic centre 81
Chapter 6 Volcanic Geochemistry of the Basin Lake area 83 6.1 Introduction 83 6.2 Mineral chemistry. 83
6.2.1 Analytical methods 83 6.2.2 Clinopyroxene 85 6.2.3 Arnphibole 87 6.2.4 FeTi oxide (magnetite) 88 6.2.5 Apatite 91 6.2.6 Cr-spinel 91 6.2.7 Discussion 92
6.3 Wholerock geochemical classification 95 6.3.1 Analytical methods 95 6.3.2 Element mobility 95 6.3.3 Geochemical characteristics of the Anthony Road Andesite 96
Basaltic andesite association 96 Hornblende andesite association 100 Rhyolite association 101
6.3.4 Geochemical characteristics ofrhyolite in the Tyndall Group 103 6.3.5 Geochemical characteristics of post-volcanic dykes: Mount Julia
Member and Newton Creek Sandstone 105 Basalt dykes 105 Rhyolite dyke 105
6.4 Discussion 106 6.4.1 Geochemical characteristics of the Basin Lake area 106 6.4.2 Comparison of the Basin Lake area with volcanic units elsewhere
in the Mount Read Volcanics 108 Anthony Road Andesite 108 Tyndall Group 113 Tholeiitic basaltic dykes 113
6.4.3 Modern volcanic successions comparable to the associations of the Anthony Road Andesite 115 The hornblende andesite association 115 The basaltic andesite association 116
6.5 Conclusions 117
Chapter 7 Alteration in the Anthony Road Andesite 119 7.1 Introduction 119 7.2 Alteration styles: distribution, mineralogy and textures 119
7.2.1 Groundmass and phenocryst alteration 120 7.2.2 Veins 121
7.3 Alteration geochemistry 123 7.3.1 Chlorite chemistry 123 7.3.2 .Wholerock geochemistry 126
Element mobilities 126 Mass change/ mass balance using immobile elements 126
7.3.3 Magnetite destructive alteration 131
IX
7 A Discussion 133 704.1 Timing of alteration 134 704.2 Comparison with alteration elsewhere in the Mount Read Volcanics 135
7.5 Conclusions 136
Chapter 8 The Lake Newton Ironstone 137 8.1 Introduction 137
8.1.1 Terminology 139 8.2 Field relationships and associated alteration 139 8.3 Ironstone mineralogy and texture 140
8.3. 1 Microbial fossils 142 8A Ironstone Geochemistry 142
8 A. 1 Analytical methods 142 8.4.2 Major and selected trace element geochemistry 145 8.4.3 Rare earth element geochemistry 145 80404 Geochemical characteristics of the Lake Newton Ironstone 145
8.5 Discussion 151 8.5.1 Origin ofthe Lake Newton Ironstone 151 8.5.2 Model for ironstone formation 151 8.5.3 Geochemical comparison with modem seafloor and ancient
VHMS-related ironstones 153 Modem seafloor ironstones 153 Ancient VHMS-related ironstones 153
8.6 Conclusions 154
Chapter 9 Synthesis 157 9. 1 Fades architecture of the Basin Lake area 157 9.2 Geological history and facies architecture of the study area 159 9.3 Mafic-intermediate successions in the Mount Read Volcanics 164 9A Importance of the Lake Newton Ironstone 168 9.5 Analogous modem volcanic settings 168
9.5.1 Palaeogeographic analogues 168 9.5.2 Geochemically similar volcanic settings 169
Bibliography Appendices
Appendix A AppendixB Appendix C AppendixD AppendixE Appendix F AppendixG Appendix H
172 191
Sample Catalogue Structural Data Carbon-Oxygen Isotopes Electron Microprobe Data Wholerock Geochemical Data X-Ray Diffraction Results Diamond Drillhole Locations and Graphic Logs of Drillholes Maps of the Study Area
x
List of Figures
Figure No. Page No.
1. 1 Location of the study area in western Tasmania. 3 1. 2 Location of economically prospeCtive areas in the study region. , 5
2. 1 Simplified geological map of western Tasmania. 12 2 . 2 Geological map of the central Mount Read Volcanics. 13 2.3 Diagrammatic cross section showing relationships between the major
lithostratigraphic units south and east of the Henty Fault. 17
3.1 Structural elements in the region. 22 3.2 Structural domains in the Basin Lake area. 23 3.3 Domain 1: stereoplot of poles to bedding. 25 3.4 Domain 1: lineations to columnar joints. 25 3.5 Domain 1: poles to flow banding in Tyndall Group. 25 3.6 Domain 1: poles to S2. 25 3.7 Domain 2: stereoplot of poles to bedding. 27 3.8 Domain 2: poles to Si, S2 and S3. 27 3.9 Domain 2: poles to So and Si, and the Fi fold axis. 29 3.10 Domain 2: poles to Si and S2, and the F2 fold axis. 29 3.11 Domain 2: poles to S2 and S3, and the F3 fold axis. 31 3.12 Sense of displacement criteria. 31 3.13 Generation 1 and 2 quartz-chlorite fibre veins in Newton Creek Sandstone. 33 3.14 Rose diagram of fault strikes in study area. 35 3.15 Poles to faults from Domains 1 and 2. 35
4.1 Geology of the Basin Lake area. 39 4 . 2 Graphic summary logs of drillholes TYN2 and Leech Hill. 40 4 . 3 Comparison of bedform characteristics. 46 4.4 Stratigraphic scheme used for the Tyndall Group. 47 4.5 Idealised stratigraphic column for the Tyndall Group in the Basin Lake area. 49 4.6 Idealised stratigraphic column for the Owen Conglomerate in the Basin Lake area. 55
5.1 Graphic logs of drillholes BL4, BL5 and BLD89-3, Basin Lake area. 67 5 .2 Facies relationships in the basaltic andesite association. 68 5.3 Facies relationships in the hornblende andesite association. 71 5.4 Facies relationships in the rhyolite association. 76 5.5 Cartoon of the interpreted facies architecture of the Anthony Road Andesite. 79
6.1 Wo-En-Fsprism showing mineral chemistry of clinopyroxene phases. 86 6.2 Mineral chemistry of amphibole phases, hornblende andesite association. 89 6 . 3 Multicomponent spinel prism displaying the mineral chemistry of magnetite
phases, hornblende andesite association. 90
xi
Figure No. Page No.
6.4 Mineral chemistry of chromite phases, depicted on a multicomponent spinel prism, basaltic andesite association. 90
6.5 Plot of 100Cr# versus 100Mg# for Cr-spinel phases, Mount Read Volcanics. 94 6.6 Plots of Na20, K20, Sr, Rb, Ba, Ti02, Nb and Y against Zr, hornblende
andesite association. 97 6.7 Plot of Si02 versus FeO*/MgO for the basaltic andesite, hornblende andesite
and rhyolite associations, Anthony Road Andesite. 98 6.8 Plot of Zrffi versus NbIY for volcanic units in the study area. 99 6.9 REE plots for the Anthony Road Andesite, Tyndall Group, post-volcanic dykes. 102 6.10 Plots of Na20 and K20 versus Si02 for rhyolites from the Tyndall Group. 104 6.11 Plot ofFeO*/MgO versus Si02 for Tyndall Group rhyolites and basaltic
dykes in the Newton Creek Sandstone and Mount Julia Member. 104 6.12 Plots of P20sffi02 (a.) and Ti/Zr (b.) versus Si02 for the Anthony Road
Andesite with the regional suites of Crawford et al. (1992). 109 6.13 REE plot comparing the Basin Lake area with the Mount Read Volcanics. 111 6.14 Plot of Ti/Zr versus Si02 for rhyolites from the Tyndall Group in the Basin
Lake area and regionally. 114
7.1 Mineral chemistry of chlorites. 125 7.2 Plot of AI203 vs Ti02 showing fractionation trend, and precursor composition. 129 7.3 Plot ofZrIY vs TilZr for altered and unaltered hornblende andesites. 129 7.4 Plot of mobile elements against immobile element ratio for magnetic
and non-magnetic hornblende andesite. a. Sr versus ZrlNb, b. CaO versus ZrlNb, c. MgO versus ZrINb. 132
8.1 Three known outcrops of ironstone: Lake Newton, Rowards Anomaly and Tyndall Ck. 138
8.2 REE patterns for diffusely bedded ironstone, Lake Newton Ironstone. 146 8.3 Ti02, MgO and Na20 versus AI203 for Lake Newton Ironstone. 147 8.4 Fe-(Ni+Co+Cu) lO-Mn diagram for diffusely bedded ironstone. 148 8.5 Fe-AI-Mn plot for diffusely bedded ironstone. 148 8.6 Onuma diagram of partition coefficients for mineral phases and ironstone REE. 150 8.7 Schematic illustration of Lake Newton Ironstone formation. 152 8.8 Major element comparison between ironstones. 155 8.9 Variation in ore equivalent horizons associated with Australian VHMS deposits. 156 8.10 REE comparison of the Lake Newton Ironstone and Thalanga ironstones. 156
9.1 Cartoon of the facies architecture of the study area. 158 9.2 Depositional setting and inferred sources for the Yolande Riv~r Sequence. 160 9.3 Interpreted emplacement setting for the Anthony Road Andesite. 161 9.4 Depositional setting and inferred sources for the Tyndall Group. 163 9.5 Depositional setting and inferred sources for the Owen Conglomerate. 166 9.6 a. Relationships of majorJithostratigraphic units, and b. Distribution of
geochemical suites. 167 9.7 The Taupo Volcanic Zone, North Island of New Zealand. 170
X11
List of Tables
Table No. Page No.
2 •1 Published geochronologic and biostratigraphic ages from the Mount Read Volcanics and the Owen Conglomerate. . 16
't:
5. 1 Average modal compositions of lavas and shallow intrusions, Anthony Road Andesite. 65
6. 1 Representative microprobe analyses of mineral phases, Anthony Road Andesite. 84 6. 2 Summary of geochemical characteristics from the Basin Lake area. 107 6.3 Five geochemical suites in the Mount Read Volcanics and their affinities. 108 6 •4 REE ratios for the Anthony Road Andesite and for suites IT and ill regionally
in the Mount Read Volcanics. 112
7.1 Selected microprobe analyses of chlorite, hornblende andesite association. 124 7.2 Chemical analyses of four samples from the hornblende andesite
association with four varying alteration assemblages. 128 7.3 Calculated precursor compositions, reconstituted compositions and mass changes
of major oxides for the four altered samples, hornblende andesite association. 130 7.4 Compositions of altered and relatively unaltered hornblende andesite. 133
8.1 Major and trace element analyses of the Lake Newton Ironstone. 143 8.2 Rare earth element data for the Lake Newton Ironstone and unaltered/altered
andesite and rhyolite, Anthony Road Andesite. 144 8.3 Mineral-melt partition coefficient data. 150
xiii
List of Plates
Plate No. Page No.
1. 1 Intense silica-sericite-pyrite alteration zone at pyrite corner, Anthony Road. 8 1.2 Base and precious metal-rich barite veins in Tyndall Creek, Antpony Road. 8 1.3 Base metal-rich massive sulphide clast in Newton Dam Spillway. 8
3.1 F2 folds in the Newton Creek Sandstone (Domain 2), Henty Canal. 32 3.2 Columnar jointing developed in an intrusion of hornblende andesite at
the base of the Newton Dam Spillway. 32 3.3 Anticline developed in Middle Dwen Conglomerate on the Tyndall Range. 32 3.4 Two generations of quartz-chlorite fibre veins on the Great Lyell Fault. 32
4. 1 Volcanic mudstone, Yolande River Sequence. 52 4.2 Pumiceous lithic-rich breccia with mudstone intraclasts, Yolande River Sequence. 52 4.3 Limestone in the Lynchford Member, Tyndall Group. 52 4.4 Crystal-rich volcaniclastic sandstone, Mount Julia Member, Tyndall Group. 52 4.5 Polymictic volcanic breccia, Mount Julia Member. 52 4 •6 Polymictic conglomerate and graded volcaniclastic sandstone/siltstone,
Zig Zag Hill Formation. 52 4. 7 Flow banded rhyolite, Tyndall Group. 53 4.8 Polymictic conglomerate of the Jukes Conglomerate, Dwen Conglomerate. 53 4.9 Pebble conglomerate of the Lower Conglomerate Unit, Dwen Conglomerate. 53 4.10 Bedded mudstone and sandstone, Newton Creek Sandstone, Henty Canal. 53 4.11 Pebble-cobble conglomerate of the Middle Dwen Conglomerate. 53 4.12 Photomicrograph of basaltic dyke, Henty Canal. 53
5.1 Photomicrograph of pyroxene-feldspar-phyric basaltic andesite. 72 5.2 Photomicrograph of the massive facies, hornblende andesite association. 72 5.3 Andesitic breccia facies (hyaloclastite), hornblende andesite association. 72 5.4 Andesitic breccia facies (mud matrix), hornblende andesite association. 72 5.5 Photomicrograph of quartz-feldspar-phyric rhyolite, rhyolite association. 72
8 •1 Ironstone as the matrix of a polymictic volcanic breccia ("Jasper Point"). 141 8.2 Photomicrograph of diffusely bedded ironstone, Lake Newton Ironstone. 141 8 . 3 Photomicrograph of filamentous texture in the Lake Newton Ironstone. 141
XlV
Chapter 1
Introduction
t·
This project has focussed upon the volcanic setting and facies relationships with in part of
the Cambrian Mount Read Volcanics, western Tasmania (Figure 1.1). The study area has
been restricted to two exploration tenements near Basin Lake (Figure 1.1): Lake Margaret
(E.L. 5/85 held at the initiation of this project under a joint venture agreement between
Aberfoyle Resources Ltd. and CRA Exploration Pty. Ltd.) and Basin Lake (E.L. 103/87
held at the initiation of this project under a joint venture agreement between Aberfoyle
Resources Ltd. and Acacia Resources Ltd.). The area provides an opportunity to study the
facies characteristics and relationships among three major lithostratigraphic units of the
Mount Read Volcanics: the Western volcano-sedimentary sequences (Yolande River
Sequence), the Anthony Road Andesite and the Tyndall Group. The overlying Owen
Conglomerate is also well exposed in the area.
The study region is considered to be prospective for volcanic-hosted massive sulphide
(VHMS)-type mineralisation, and locally the volcanics are intensely hydrothermally altered.
Previous studies (e.g., Creagh and Hungerford, 1990; Crawford et aI., 1992) have
recognised geochemical similarities between units within the study area and the volcanics
which host massive sulphide mineralisation at Hellyer and Que River, in the northern Mount
Read Volcanics. In addition, the association of mafic-intermediate volcanics with VHMS ore
deposits at Mount Lyell, Comstock, Hellyer and Que River is well documented (Corbett,
1990), making the Anthony Road Andesite of particular interest.
1.1 Aims
The primary objectives of this project are to:
1. Document the character and geometry of volcanic and sedimentary facies
associations within the study area.
2. Differentiate the major lithofacies present within the Anthony Road Andesite
using textural, petrographic and geochemical criteria, and establish a facies
architecture.
1
3. Compare and contrast the geochemical characteristics and alteration styles of the
Anthony Road Andesite with other andesite-dominated units in the Mount Read
Volcanics.
4. Clarify the stratigraphic position of the study region within the Mount Read
Volcanics.
The above aims have been achieved by means of field mapping, drillcore logging,
petrographic studies, geochemical studies (e.g., X-ray fluorescence, X-ray diffraction,
inductively coupled plasma mass spectrometry and electron microprobe) and isotopic work.
1.2 Location and access
The study area is located 12 km north of Queenstown and 14 km south of Rosebery in
western Tasmania (Figure 1.1). Road access from Queenstown is via the Zeehan Highway
and thence the Anthony Road (a road distance of approximately 22 km, Queenstown to study
area). From Rosebery, access is via the Murchison Highway and then the Anthony Road
(Figure 1.1). Unsealed roads branching from the Anthony Road provide additional access
within the study area. The study region constitutes an area of 16 km2, covered locally by
both button grass plains and thick temperate rainforest. In the northern part of the study area,
outcrop is relatively good. Elsewhere, Quaternary glacial and alluvial deposits up to 10 m
thick and dense vegetation cover much of the bedrock geology and the field work focussed
on road cuttings and drillcore. Topographic relief is low (elevation range 450 to 550 m
A.S.L.) although the Tyndall Range (Mount Tyndall elevation 1179 m A.S.L.) flanks the
eastern margin of the area. The principal development in the area is the Hydro Electric
Commission's Newton Dam, located in the northwest corner (Figure 1.1).
2
..., ~ + Moulll Black \D
§o
§o
~ ~
1Sterling Saddle North
MO\llIl + 5370000mN
Murchison
+ Williamsrord
x Hercules
ile Spur
+ Mount Read
Moxon Saddle
+ Gooseneck Hill
Red Hills
+ Mount Selin8
White Spur Dam
+ Newton Peak
Lake Plimsoll
~
o ~ ,...,
5360000mN
'L'lke Rolleslon
~
Q tn
+ Mount Tyndall
,,)
,6.<f. ,*'" ,
Lake HIUltley
+ Mounl Geikie ~Rosebery
• Queenstown
/{a1l1illon MOfiline Basin Lake
+ Boulder l-Iill Lake
Margarel
Figure 1.1 Location of the study area in western Tasmania. The Lake Margaret (E.L. 5/85) and Basin Lake (E.L. 103/87) exploration licences which comprise the study area are highlighted.
1.3 Exploration and mining history
The Basin Lake district has a long history of exploration activity, although mining activity in
the immediate area has been limited. The Tyndall Mine (Figure 1.2) exploits fault-related
vein-style Devonian mineralisation consisting of sphalerite, galena, chalcopyrite and
tetrahedrite in quartz veins, and was worked towards the end of last century (Twelvetrees,
1900; Purvis et al., 1983; Fitzgetald, 1987; Solomon et aI., 1988). The closest economic
deposit is the Cambrian VHMS-style, high grade, low tonnage, Henty Mine (2 km north of
the study area; Figure 1.2), which is currently being mined (Halley and Roberts, 1997).
Several features of the district have hampered exploration activity: patchy bedrock outcrop,
variable magnetic properties of the Anthony Road Andesite and Tyndall Group, and the
presence of numerous pyritic black shale units which generate misleading anomalous
responses using electrical geophysical methods (Creagh and Hungerford, 1990; Sharpe,
1993b, 1994). Nevertheless, the study area has been explored continuously from the mid
1960's to the present day, involving field mapping, costeaning, geochemical surveys (e.g.,
rock chip, soil, stream sediment) and geophysical surveys (e.g., heliborne and ground
magnetics, gravity, IP methods, various EM methods; Wells, 1973; Meares et al., 1980;
Sheppard, 1986; Creagh and Hungerford, 1990; Richardson, 1993; Sharpe, 1994). Various
exploration programs have culminated in target testing via diamond drilling; in excess of 30
diamond drill holes exist in the study region.
The study area includes four prospects which are of economic interest: Howards Anomaly,
Basin Lake, Tyndall Creek and Newton Dam Spillway (Figure 1.2; Sharpe, 1993b, 1994).
Intense alteration zones (e.g., sericite-silica-pyrite alteration at Leech Hill and "pyrite
corner") have also been recognised (Plate 1.1; Richardson, 1993; Sharpe, 1994). The four
prospects (discussed separately below) each involve mineralisation of a distinct style.
4
North
Pyrite Corner
Red Hills
+ Gooseneck Hill
x Henly Mine
\ Mount Julia +'
Lake Julia
+ Newton Peak
ewton all Creek mineralisation
'. \'.
+ Mount Tyndall
~
() 5355000mN
'<111 Q-ee '.'<,,.~
~ ~"
:J J:<.:-...
E-..
Basin Lake I. ,Pyrite zorfe Mount Geikie
HamillOn Aiol..ame Basin Lake Lake (,;.) Margaret g<5
5350000mN8 o
Figure 1.2 Map showing the IOC<1lion of economically prospective areas in the study region.
Howards Anomaly is a zone of intense to sporadic hematite-carbonate-chlorite-barite
alteration with associated silver mineralisation (up to 410 g/t Ag) and negligible base metal
mineralisation (Figure 1.2; Meares et aI., 1980; Purvis et al., 1983). This zone occurs near
the contact between the Anthony Road Andesite and the Tyndall Group and runs parallel to
this contact (striking NNW) for over 1 km (Meares et aI., 1980). Silver occurs as fine
grained freibergite, tetrahedrite, pyrargyrite and native silver, with the highest silver grades
associated with the most intense hematite alteration (Meares et al.! 1980; Purvis et al., 1983).
Pb isotope dating of the mineralisation at Howards Anomaly indicated a Cambrian age
(Purvis et aI., 1983; Sharpe, 1993b). It has previously been suggested that silver
mineralisation is related to seafloor hydrothermal venting (Purvis et al., 1983). Exploration
of Howards Anomaly was abandoned due to the evidently sporadic distribution of the
mineralisation.
The Basin Lake pyrite zone trends NNW and consists of massive to disseminated pyrite with
minor silver (best assay 62 g/t Ag) and low base metal grades (Figure 1.2; Fitzgerald, 1987;
Creagh and Hungerford, 1989; Richardson, 1993). The prospect occurs within strongly
sericite-altered volcanics at a similar stratigraphic position to Howards Anomaly (Le., near
the contact between the Anthony Road Andesite and the Tyndall Group; Creagh and
Hungerford, 1989; Sharpe, 1994). Massive pyrite extends for more than 10 m in drillcore
whereas disseminated pyrite is even more extensive (e.g., in diamond drill holes BL3, BL4,
BL5, BLD89-3; Appendix G).
Highly sheared silica-sericite-pyrite altered volcaniclastic rocks, cross-cut by barite-base and
precious metal-rich veins, are exposed in the bed of Tyndall Creek (east of the Anthony
Road; Plate 1.2; Figure 1.2; Sharpe, 1993b). Boulders containing mineralised veins have
also been discovered downstream (Sharpe, 1993). The prospect lies along the Tyndall Creek
Fault and the mineralisation may have been remobilised from a nearby source during
deformation (Sharpe, 1993). The ore mineralogy of the veins includes sphalerite, galena,
pyrite, chalcopyrite, chalcocite, tetrahedrite-tennantite and electrum (Kitto, 1993; Sharpe,
1993b). Pb isotopes indicate a Cambrian age for the Tyndall Creek mineralisation, though
the timing of remobilisation is uncertain (Sharpe, 1993b). Assay of the mineralised veins
(9.8% Zn, 3.3% Pb, 0.15% Cu, 66 g/t Ag, 4.5 g/t Au, 29% "Ba) shows it to be of a
particularly barite-, gold-, silver- and zinc-rich style (Sharpe, 1993b).
Clasts of base metal-rich massive sulphide occur within a unit of mass-flow deposited
polymictic volcaniclastic conglomerate exposed in the Newton Dam Spillway (Plate 1.3;
Figure 1:2; Gibson, 1991). Massive sulphide clasts contain sphalerite, galena, chalcopyrite,
pyrite and silver sulphosalts (pyrargyrite-proustite series; Gibson, 1991). The volcaniclastic
conglomerate contains cobbles and boulders of basalt and dacite (dacite dominant) and more
6
than ten massive sulphide clasts (maximum diameter 90 cm). The ore mineralogy, Pb
isotopes and S isotopes suggest derivation of the massive sulphide clasts from a Cambrian,
seafloor, polymetallic massive sulphide mound (Gibson, 1991; Sharpe, 1993b). The mound
may have been present in the source region of the mass flows, or else the clasts were
collected en route (Gibson, 1991; McPhie et aI., 1993). The outcrop in Newton Dam
Spillway is an ore-equivalent horizon, indicating development of po1ymetallic massive
sulphide mineralisation at this stratigraphic level (Large, 1992). ~,'.
1.4 Previous work
The Basin Lake district has been the focus of both exploration and academic research. Its
significance within a regional stratigraphic context has been discussed by Corbett (1990,
1992), McPhie and Gemmell (1992) and Pemberton and Corbett (1992). Crawford et al.
(1992) included samples from the study region in their review of the geochemistry of the
Mount Read Volcanics. Corbett (1986) and Corbett and Jackson (1987) provided regional
geological maps of the district at 1:25 000 scale. Two honours theses (Hutton, 1989 and
Gibson, 1991) are the most detailed geological studies undertaken within the region. Hutton
(1989) completed a section through the Western volcano-sedimentary sequences, Central
Volcanic Complex, Anthony Road Andesite, Tyndall Group and Dwen Conglomerate along
the Anthony Road, whereas Gibson (1991) studied the stratigraphy and mineralisation of the
region with particular emphasis on the Newton Dam Spillway.
Recent unpublished company reports for the Basin Lake licence (E.L. 103/87; e.g., Creagh
and Hungerford, 1989, 1990; Richardson, 1992, 1993; Sharpe, 1994) and Lake Margaret
licence (E.L. 5/85; e.g., Sharpe, 1993b) provide concise discussions of local geology,
structure and geochemistry. Company reports, overall, give insight into localities which have
remained of economic interest over a prolonged period (e.g., Howards Anomaly) and
present the results of past drilling, geochemical and geophysical work (e.g., Stevens-Hoare,
1975; Meares et al., 1980, 1981, 1982; Roberts and Cartwright, 1984; Sheppard, 1986;
Fitzgerald, 1987; Creagh and Hungerford, 1989, 1990; Sharpe, 1993b).
7
Plate 1.1 Intense silica-sericite-pyrite alteration in the Yolande River Sequence and the Anthony Road Andesite outcropping beside the Anthony Road at pyrite corner. Intense alteration outcrops beside the Anthony Road for over 100 m. (GR 379400E, 5354700N)
1/..
Plate 1.2 Base and precious metal-rich barite veins (v) in polymictic volcanic breccia of the Lynchford Member outcropping in Tyndall Creek, Anthony Road. Clasts of ironstone (i) also occur in this polymictic unit. (GR 380950E, 5357150N)
Plate 1.3 Clasts of base metal-rich massive sulphide (m) in polymictic volcaniclastic conglomerate outcropping in the Newton Dam Spillway. The number of base metal clasts outcropping
changes periodically as the spillway is eroded. (GR 380000E, 5358300N)
1.5 Thesis organisation
This thesis consists of nine chapters. The first chapter is a general introduction to the project
and states the project aims, whereas the second chapter is an overview of the regional
geology of the Mount Read Volcanics in western Tasmania. The structural geology of the
area of study is reported in chapter three and forms an important prelude to discussions of
the stratigraphy in chapter 4 and chapter 5. The geology of the Anthony Road Andesite is ..~.
described separately in chapter 5. Chapter 6 documents the geochemistry of selected
lithologies described earlier in chapters 4 and 5 and discusses regional comparisons based on
geochemical characteristics. Chapter 7 focuses on the mineralogy, styles and geochemistry
of alteration in the Anthony Road Andesite. The Lake Newton Ironstone, on the shores of
Lake Newton, has not been previously studied and its geological setting, petrography and
geochemistry are reported in chapter 8. A synthesis and model for the geological history is
presented in chapter 9.
9�
~
~
CJel ••
= = C
l =~ ....
~ ~
~
~
~
~
~ .... =
N
CJel= t..<
Chapter 2
Regional Geology
t 2.1 Introduction
Geologically, western Tasmania can be subdivided into four main components: the
Precambrian Tyennan Region, the Precambrian Rocky Cape Region, the Cambrian Mount
Read Volcanics and the Palaeozoic Dundas Trough (Figure 2.1; Corbett and Turner, 1989;
Corbett, 1992). The Tyennan and Rocky Cape Regions are blocks of continental crust, now
variably deformed and metamorphosed (Corbett and Lees, 1987; Turner, 1989). The Mount
Read Volcanics occur along the western margin of the Tyennan Region and form an arcuate
belt across western Tasmania (Figure 2.1; Corbett, 1992). The basallithostratigraphic unit in
the Mount Read Volcanics, the Sticht Range Beds, unconformably overlies Precambrian
rocks of the Tyennan Region (Corbett, 1992). The Dundas Trough contains Early Cambrian
sedimentary and volcanic rocks (Success Creek Group and Crimson Creek Formation),
allochthonous ultramafic-mafic complexes (low-Ti tholeiites and boninites) and the volcano
sedimentary sequences of the Dundas Group which interfinger with the Mount Read
Volcanics (Brown, 1989; Brown and Jenner, 1989; Corbett, 1992; Crawford and Berry,
1992). Late Cambrian to Early Ordovician siliciclastic conglomerates and sandstones (Owen
Conglomerate) overlie older lithostratigraphic units across western Tasmania (Figure 2.2;
Banks and Baillie, 1989; Corbett, 1992).
2.2 The Mount Read Volcanics
The late Middle Cambrian Mount Read Volcanics comprise calc-alkaline to tholeiitic
volcanics with lesser intercalated sedimentary units (Figure 2.1 and Figure 2.2; Corbett,
1992; Crawford et al., 1992). The volcanics have been deformed, IIletamorphosed (prehnite
pumpellyite or lower greenschist facies), and locally intensely hydrothermally altered
(Corbett, 1992; Crawford et al., 1992). The Mount Read Volcanics form the eastern margin
of the Dundas Trough (Figure 2.1; Corbett, 1992). Cambrian granitic rocks intrude the belt
at Mount Darwin, Murchison and Elliott Bay (Figure 2.1; Leaman and Richardson, 1989;
Corbett, 1992). There are few geochronologic and biostratigraphic constraints though
Perkins and Walshe (1993) suggest emplacement of the bulk of the Mount Read Volcanics
occurred during a narrow interval in the late Middle Cambrian, around 502.6±3.5 Ma.
10
However, some parts of the belt appear to be slightly younger (Adams et al., 1985; Jago and
Brown, 1989). Age data for major lithostratigraphic units of the Mount Read Volcanics, and
from the overlying Owen Conglomerate, are given in Table 2.1.
The Henty Fault Zone divides the Mount Read Volcanics into northern and southern sections
with stratigraphic correlations across the fault poorly constrained (Berry, 1989;Corbett,
1992). The Mount Read Volcanics have been subdivided by Corbett (1992) into seven
lithostratigraphic units, five of which are discussed below. The inferred stratigraphic and
facies relationships of the major associations are summarised in Figure 2.3. A regional
geochemical study of lavas and intrusions in the Mount Read Volcanics by Crawford et al.
(1992) recognised five geochemical suites: suites I, 11 and III with calc-alkaline to
shoshonitic affinities, and suites IV and V with tholeiitic affinities.
2.2.1 Central Volcanic Complex
The Central Volcanic Complex is dominated by feldspar-phyric rhyolitic and dacitic lavas,
intrusions and pumiceous volcaniclastic units (Corbett, 1992; McPhie and Gemmell, 1992).
Andesites are interbedded throughout the complex across both the northern and southern
Mount Read Volcanics (north and south of the Henty Fault Zone; Corbett and McNeill,
1988). The Central Volcanic Complex has an interdigitating relationship to the west with the
Western volcano-sedimentary sequences, and to the east with the Eastern quartz-phyric
sequence (Figure 2.3; Corbett, 1992). Recent studies (e.g., Corbett, 1992; McPhie and
AlIen, 1992) suggest that the Central Volcanic Complex was emplaced within a submarine
setting. Geochemical studies by Crawford et al. (1992) indicate medium- to high-K calc
alkaline affinities (suite I) for the majority of coherent units in the Central Volcanic Complex.
At Mount Lyell, massive and disseminated Cu-rich VHMS ore bodies are hosted within the
upper sections of the Central Volcanic Complex and thick pumice-rich units, also of the
Central Volcanic Complex, host the Rosebery and Hercules polymetallic VHMS deposits
(Figure 2.2; Corbett, 1992; McPhie and AlIen, 1992).
11
146" 147" 148"
o 50 100 !CM I ' --'
~.
,41
42"
1+::"+1 Devonion ~ granitoids
t=-::-~ Mathinna beds
f::: '';;'1 Middle to I~te Cambrian '''\':.:'' yolcano-sedunentary & ,
sedimentary sequences EllJott and correlates Bay
@ Cambrian granite
I~~::I~I Mt Read Volcanics
[Z] Ultramafic-mafic complexes
~':\ I Early trough deposits - Crimson Crk \ ,- Fm, Success Crk Grp and correlates
1:::--1 Precambrian
Figure 2.1 Simplified geological map of western Tasmania showing the Precambrian Rocky Cape and Tyennan Regions, the Palaeozoic Dundas Trough and the Cambrian Mount Read Volcanics (from Corbett and Turner, 1989),
12
'I~I.II.( (-..:~
~ ~~!/~c~'!!..f/
, r./ T
I \~"\-j) ~ 0-0 \ .< fJ
. 1 + I ~ ..... ~~+ r + + +~ +\
+ + ...'- + / /+ + ... + + + + \
(I + + + + + + + + + + + + + +
( ~ + + + + + + + )1 + + +09 + + + .±. ..... "- :;/+ + + + + +(-r-=- "-- "-- "-- "-
"-- "-- "-- "-~ + + + + + I "-90+ + + + + j ,,"""-- PCm """-- "-- "-
(I + + + + + "-- "-- "-- "-- "-
t"'-,,+ + + / "-- "-- "-- "-- "-"-- "-- "-- "-- "-- "-\ + +
"-- "-- "-- "-- "-(I( 1\+ + 0 00 , • + "-- - "-- .. "- '" I( (
/ I""TI Pos/-Devonion cover-Terliary (T).80 ( ( L...!.:.:!....J Permian-Jurassic (P-J)
( ( ( I: 09 : IDevonian granite( ( ( ( (
B La/tI Cambrian. Ordovician and
( (P£u( ( 0-0 Siluro-Devonian, sedimen/ory sequenctls
70 l ( ( CAIolBRIAN Western volcano-sedimen/ary1·::.c~;::1 sequences-Dundas Group. M/ Charltlr . . . . . . .. Group, Yolande River Stlquenctl
~ Tyndall Group volcanics
~I~ _ Andesi/es and basalis
Z a~ Eas/ern quarlz-phyric sequtlnctlo > o i:5 I~ XXxIGrani/e .t re/o/ed porphyry (x)'" ... :::El I':':::':::.":':":':) Ctln/ral Volcanic Complex-dominanl/y
::::':':;;.~~:::::::: ftlldspar-phyric volcanics
~ Slich! Range Beds - siliciclas/icl....!....-.!J sands/one .t conglomera/tI
_ Mafic (tho/eii/ic) 4c ullramafic rocks 40
~ Crimson Cree/( Forma/ion - maficl:::::::::::::: grtlywac/(e. muds/ontl. basall
I"Tl:::ITll Success Cree/( Group - sands/ontl,u..r:i1..U mudsfone. dolomite
PRECAt.lBRIAN 30
~ Rtlla/ively unme/amorphosed ~ quarlzi/tls .t sla/es
t ~ 3Me/oquorlzi/es. schis/s
~ MIM "-
1t' Prospect"-"-
"- ~ Btldding with facing
~
'9d'l o 5 10 15 20km0« ! ! ! ! I '9~ Scale
Figure 2.2 Geological map of the central Mount Read Volcanics showing the distribution of the major lithostratigraphic units (from Corbett, 1992). The large arrow gives a broad indication of the location of the study area.
13
2.2.2 Eastern quartz-phyric sequence
The Eastern quartz-phyric sequence predominantly consists of quartz-feldspar-phyric
rhyolitic and dacitic lavas and intrusions, volcaniclastic (and minor Precambrian basement
derived) mudstone to conglomerate, along with quartz-feldspar±biotite porphyritic intrusions
(Corbett, 1992; Pemberton and Corbett, 1992). Contact relationships are variable, and in
some places, the unit appears to be laterally equivalent to Central Volcanic Complex whereas ,;:.-.
in other places, volcaniclastic conglomerates of the Eastern quartz-phyric sequence pass
conformably into Tyndall Group (Figure 2.3; Pemberton and Corbett, 1992). Porphyries of
the sequence intrude· the Precambrian Tyennan Region along the eastern margin of the Mount
Read Volcanics (Corbett, 1992). The Cambrian Murchison Granite intrudes the sequence at
Mount Murchison (Figure 2.3; Corbett, 1992; McNeill and Corbett, 1992). Lavas and
intrusions of the Eastern quartz-phyric sequence have medium- to high-K calc-alkaline
characteristics (suite I; Crawford et al., 1992). Pemberton and Corbett (1992) suggested that
the Eastern quartz-phyric sequence represents a fades variation of Central Volcanic Complex
feldspar-phyric lavas.
2.2.3 Basaltic - andesitic units
Basaltic-andesitic units in the Mount Read Volcanics occur at contacts between, or
interdigitating with the Central Volcanic Complex, Western volcano-sedimentary sequences
and Tyndall Group (e.g., Anthony Road, Que-Hellyer, Sterling River, Miners Ridge-Lynch
Creek, Crown Hill, Comstock; Figure 2.2 and Figure 2.3; Corbett and McNeill, 1988;
Corbett, 1992; Pemberton and Corbett, 1992). The petrography of these units varies north
and south of the Henty Fault Zone, with plagioclase-pyroxene-phyric units present to the
north (and in the Henty Fault Wedge), and plagioclase-hornblende-phyric and plagioclase
pyroxene-phyric units dominant south of the Henty Fault, in the upper Central Volcanic
Complex and lower Western volcano-sedimentary sequences (Corbett, 1990, 1992;
Crawford et al., 1992). An association between basaltic-andesitic volcanics and several
VHMS deposits (e.g., Mount Lyell, Hellyer, Comstock, Que River) is well documented in
the Mount Read Volcanics (Corbett, 1990, 1992; Pemberton and Corbett, 1992).
Corbett (1992) and Pemberton and Corbett (1992) suggested that basaltic-andesitic
volcanism reflected a period of active extension and ritting which was focussed along major
faults. Siratigraphic studies provide partial support for correlation of the andesitic-basaltic
units within the Mount Read Volcanics (e.g., andesites at Mount Lyell/Comstock - Anthony
Road - Que-Hellyer; Corbett, 1990). Basaltic-andesitic units in the Mount Read Volcanics
14
are geochemically variable, with medium- to high-K calc-alkaline, shoshonitic and tholeiitic
affinities (Crawford et al., 1992).
2.2.4 Western volcano-sedimentary sequences
The western and northern parts of the Mount Read Volcanics are dominantly composed of.. 'f:
sedimentary successions, termed the Western volcano-sedimentary sequences (Figure 2.2;
Corbett, 1992). The Western volcano-sedimentary sequences comprise interbedded
volcaniclastic turbidites, micaceous siltstone, tuffaceous mudstone and black mudstone of
mixed volcanic and Precambrian basement provenance (Corbett, 1992; McPhie and
Gemmell, 1992; Pemberton and Corbett, 1992). Facies characteristics and fossils suggest
that the Western volcano-sedimentary sequences were deposited in a submarine, below
wave-base setting (McPhie and AlIen, 1992). Felsic lavas and intrusions are interspersed
throughout the sequences, with a major accumulation of mafic-felsic lavas, intrusions and
breccias (the Que-Hellyer Volcanics) occurring at the northern end of the Mount Read
Volcanics (Figure 2.2; Corbett, 1992).
The Western volcano-sedimentary sequences have been subdivided into the Dundas Group
(north of the Henty Fault as far as Pinnacles), Yolande River Sequence (south of the Henty
Fault) and the Mount Charter Group (Hellyer area; Corbett, 1992). The Yolande River
Sequence interfingers with and is overlain by the Central Volcanic Complex whereas the
Dundas Group overlies the Central Volcanic Complex (Figure 2.3; Corbett, 1992;
Pemberton and Corbett, 1992). In the northern Mount Read Volcanics, the Mount Charter
Group has an interfingering to overlying relationship with the Central Volcanic Complex
(Corbett, 1992). Trilobite fossils indicate an early Middle Cambrian to early Late Cambrian
age whereas radiometric dating of lavas from within the Que-Hellyer Volcanics gave an age
of 500 to 503.2±3.8 Ma (Table 2.1; Jago, 1986; Jago and Brown, 1989; Corbett, 1992;
Perkins and Walshe, 1993). The Mount Charter Group hosts the Hellyer and Que River
polymetallic massive sulphide ore deposits in the northern Mount Read Volcanics (Figure
2.3; Corbett and Komyshan, 1989; Corbett, 1992).
15
Table 2.1. Published geochronologic and biostratigraphic ages for lithostratigraphic units in the Mount Read Volcanics and the overlying Owen Conglomerate.
Lithology Dating Method Age
Owen Conglomerate Late Cambrian
5Newton Creek Sandstone Biostratigraphic '(post-Idamean pre-Paytonian)
Tyndall Group
6"Comstock Tuff' .(Lynchford) V-Pb 494.4 ± 3.8 Ma
6Volcaniclastic sandstone V-Pb 502.5 ± 3.3 Ma
(Anthony Road) late Middle Cambrian 2Comstock Limestone Biostratigraphic
(Boomerangian or Late Menevian)
6Anthony Road Andesite V-Pb; 40Arf39Ar 502.2 ± 3.5 Ma; 501.5 ± 5.7 Ma
6,IMurchison Granite 40Arf39Ar; K-Ar 501.0 ±5.7 Ma; 524 ± 15 Ma
4Darwin Granite V-Pb 510 (+64, -21) Ma
Mount Charter Group
6Vpper Rhyolitic Sequence V-Pb 503.2 ± 3.8 Ma late Middle Cambrian 3Que River Shale Biostratigraphic
(Undillan or middle to late Menevian)
6Que River Footwall Dacite V-Pb About 500 Ma
Central Volcanic Complex
6Mount Black Dacite V-Pb 494.9 ± 4.3 Ma
Data from IMcDougall and Leggo (1965), 2Jago et al. (1972), 3Jago (1977), 4Adams et al. (1985), 5Jago and Brown (1989) and 6perkins and Walshe (1993).
2.2.5 Tyndall Group
The Tyndall Group is the youngest lithostratigraphic unit in the Mount Read Volcanics.
Lower parts of the Tyndall Group are characterised by mass-flow deposited quartz-feldspar
crystal-rich volcaniclastic sandstone and breccia, and upper parts by volcaniclastic
conglomerate (Corbett et al., 1974; Corbett, 1992; McPhie and Gemmell, 1992; Pemberton
and Corbett, 1992; White and McPhie, 1996). Minor volumes of rhyolite, welded
ignimbrite, limestone and black mudstone are also present (Corbett, 1992). White and
McPhie (1996) have subdivided the Tyndall Group into the lower Comstock Formation,
comprismg the Lynchford Member and the Mount Julia Member, and the upper Zig Zag Hill
Formation. At Comstock (the type locality for the Tyndall Group), near Queenstown,
agnostid trilobites, echinoderm plates, hyolithids, small and inarticulate gastropods are
preserved within a basal limestone unit (Lynchford Member) and are suggestive of a Middle
16
to Late Cambrian age for deposition (Table 2.1; Jago et al., 1972; Corbett et aI., 1974).
Geochronology has further constrained the age of deposition for the Tyndall Group. A
volcaniclastic sandstone from the Anthony Road has been dated at S02.S±3.3 Ma whereas a
sample from the "Comstock Tuff" (Comstock Formation of White and McPhie, 1996) at
Lynchford yielded a considerably younger age of around 494±3.8 Ma (Table 2.1; Perkins
and Walshe, 1993). The age dating suggests that part of the Tyndall Group is younger than
the main belt of Mount Read Voleanics. On the Darwin Plateau, south of Queenstown, the , Tyndall Group non-conformably overlies the Cambrian Darwin Granite, and also indicates
that the Tyndall Group is younger than the rest of the Mount Read Volcanics (lones, 1993).
The Comstock Formation contains a distinctive pink (albite-silica) and green (chlorite)
banded quartz-feldsparf--IIlafic crystal-rich sandstone unit ("Comstock Tuff' or equivalents)
which outcrops at Comstock and Lynchford and along the Anthony and Cradle Mountain
Link Roads (Corbett et al., 1974; Pemberton and Corbett, 1992; White and McPhie, 1996).
The Mount Cripps Subgroup of the northern Mount Read Voleanics (Que-Hellyer area) is a
correlate of the Tyndall Group (Corbett and Komyshan, 1989; Pemberton and Corbett,
1992). Voleaniclastic conglomerate of the Tyndall Group and Eastern quartz-phyric
sequence are very similar and can only be distinguished where contact relationships with
Cambrian granites or the Central Volcanic Complex are exposed. Thus, conglomerates that
contain granite or porphyry clasts and which are overlain by the Owen Conglomerate have
been defined as Tyndall Group (Corbett, 1992; Pemberton and Corbett, 1992). The Tyndall
Group unconformably overlies the Central Volcanic Complex, Darwin Granite, Eastern
quartz-phyric sequence and Western volcano-sedimentary sequences and is in places
unconformably overlain by the Cambro-Ordovician Owen Conglomerate (Figure 2.3;
Corbett, 1992; Jones, 1993). The Henty Gold deposit is hosted by a sequence of rhyolite
lavas and volcaniclastic sandstones of the Tyndall Group (Figure 2.2; Halley and Roberts,
1997).
MURCHISON~~~ ~i~'~·i·i·iji·j·i'l·i·ii·i· -.' . . · · ~. ? GRANITE. . . . . . . .. ....
Figure 2.3 Diagrammatic cross section depicting the relationships among the major lithostratigraphic units south and east of the Henty Fault (from Corbett, 1992). The patterns and symbols are the same as in Figure 2.2. (MRB=Miners Ridge basalt; CT=Comstock Tuff; VC= volcaniclastic conglomerate; LCB=Lynch Ck. basalts; YRS=Yolande River Sequence; EQP=Eastern quartz-phyric sequence; QFL=Quartz-phyric lava)
17
2.3 Owen Conglomerate (Denison Group)
The Owen Conglomerate overlies the Mount Read Volcanics and is composed of siliciclastic
detritus (Figure 2.1; Corbett and Solomon, 1989). Units consisting of similar Precambrian
detritus are intercalated (in minor volumes) with the Mount Read Volcanics (e.g., in the
Eastern quartz-phyric sequence, Western volcano-sedimentary sequences, Tyndall Group;
Corbett, 1992). The Cambro-Ordovician Jukesian Orogeny was responsible for regional {-~
uplift of the Precambrian source, causing inundation of depositional environments by large
volumes of siliciclastic detritus and fonning a succession up to 1.5 km thick (Banks and
Baillie, 1989; Corbett and Turner, 1989; Corbett, 1992). Siliceous siltstone, sandstone and
conglomerate of the Owen Conglomerate characteristically display a distinctive purple-red
hematitic colouring, and conformably and unconformably overlie Precambrian and Cambrian
rocks throughout western Tasmania (Figure 2.3; Corbett and Turner, 1989).
Sedimentological and biostratigraphic studies suggest deposition of the Owen Conglomerate
during the Late Cambrian - Early Ordovician in a marine setting (Banks and Baillie, 1989).
Proximal turbidites in the lower Owen Conglomerate (e.g., Newton Creek Sandstone) were
deposited in a below-wave-base environment, whereas the Middle and Upper Owen
Conglomerate contain a fossil fauna, mud cracks and facies characteristics suggesting
deposition in a shallow marine to intertidal environment (Banks and Baillie, 1989; McNeill
and Corbett, 1992).
2.4 Cambrian economic mineralisation
Ore deposits of both Cambrian and Devonian ages are present in the Mount Read Volcanics.
This study is concerned with Cambrian volcanism and related processes; thus only
mineralisation that has been synchronous or closely related to Cambrian volcanism is
summarised here. Williams et al. (1989) and Large (1992b) provide reviews of Devonian
mineralisation in western Tasmania.
The major Cambrian ore deposits in the Mount Read Volcanics include Zn-Pb-Cu type (e.g.,
Hellyer, Que River, Rosebery, Hercules), Cu type (i.e., Mount Lyell) and Au-rich (i.e.,
. Henty) volcanic-hosted massive sulphide deposits (Large, 1992). Tasmanian Zn-Pb-Cu type
massive sulphide deposits are high grade (approximately 20% Pb+Zn+Cu), generally large
tonnage and significantly enriched in gold and silver (Large, 1992, 1992b). These ore
deposits. are similar in metal zonation, alteration mineralogy, alteration chemistry and S
isotopes (Large, 1992).
18
The 17 million metric tonne (Mt) Hellyer ore deposit is a mound-style massive sulphide
deposit with a zoned alteration pipe developed in the footwall beneath the deposit (Gemmell
and Large, 1992; Large, 1992). Unaltered volcanics away from the deposit grade through a
stringer envelope zone (sericite-quartZ±carbonate), sericite zone (sericite), chlorite zone
(chlorite-pyrite±sericite±quartz) to a siliceous core directly beneath the ore body (Gemmell
and Large, 1992). Footwall alteration has affected an area of 1500 m x 350 m around the
deposit and extends to a depth of at least 550 m (Gemmell and L3f.ge, 1992).
Rosebery is a 20 Mt sheet-style massive sulphide deposit with a semi-conformable quartz
sericite±pyrite-chlotite alteration zone extending for more than 2 km along strike from the
mineralisation (Green et al., 1981; Large, 1992). In contrast to the Hellyer deposit, alteration
at the Rosebery ore horizon is laterally continuous away from the deposit and alteration
zonation in the footwall is not developed (Large, 1992).
The massive sulphide deposits of the Mount Lyell field contain over 100 Mt at greater than 1
% Cu (Large, 1992). Cu mineralisation occurs dominantly in chalcopyrite-pyrite stockworks
with widespread disseminated mineralisation (Large, 1992). Intense sericite-silica
pyrite±chlorite alteration extends for 6 km x 1 km across the Mount Lyell field (Corbett,
1992).
Stratigraphic studies suggest that the Hellyer, Que River and Mount Lyell field massive
sulphide deposits occur at a similar stratigraphic level and are associated with basaltic
andesitic volcanics in the footwall (Corbett, 1992). The Hercules and Rosebery deposits are
hosted in a felsic pumiceous volcaniclastic succession that may be stratigraphically
equivalent to or higher than the Hellyer and Que River deposits (Corbett, 1992, McPhie and
AlIen, 1992). The Henty deposit is hosted within rhyolitic lavas and volcaniclastics of the
Tyndall Group, and is younger and stratigraphically higher than the other deposits in the
Mount Read Volcanics (Corbett, 1992; Halley and Roberts, 1997).
2.5 Location of the study area in the Mount Read Volcanics
The Basin Lake area is located in the central part of the Cambrian Mount Read Volcanics.
The major lithostratigraphic units in the area are the Yolande River Sequence, Anthony Road
Andesite, Tyndall Group and Owen Conglomerate. The Great Lyell and South Henty Faults
bound the study area to the east and west respectively. The Henty Mine is located 2 km north
whereas "the Mount Lyell field is located 10 km south of the Basin Lake area.
19
Chapter 3
Structural Geology
~-.
3. 1 Introduction
Two episodes of regional deformation, the Cambrian Delamerian Orogeny and the Middle
Devonian Tabberabberan Orogeny, have led to the development of variable fold trends,
faulting and in places, strong cleavage within the Mount Read Volcanics (Corbett, 1992;
Berry, 1994). The effects of both orogenies can be recognised in the structure of the Basin
Lake area.
Deformation related to the Delamerian Orogeny (510 - 490 Ma) has been divided into three
parts (Banks and Baillie, 1989; Williams et al., 1989; Corbett and Turner, 1989; Berry,
1994): i) middle Cambrian E-W extension and subsidence, accompanied by sedimentation
and volcanism (the Mount Read Volcanics); ii) N-S compression which produced E-W
trending folds; and iii) reactivation of earlier extensional faults as reverse faults (e.g., Henty
Fault) and development of upright open N-trending folds.
The Middle Devonian Tabberabberan Orogeny has a complex history in western Tasmania
(Seymour, 1980). Deformation was in places controlled by pre-existing Cambrian folds and
faults leading to tightening of N-trending folds and development of associated NNW
striking Devonian cleavage (Williams, 1979; Berry, 1994). Subsequent N- to NNE-directed
compression developed WNW-trending folds and thrusts with associated brittle wrench
faulting (sinistral movement on the NNE-striking Henty Fault; Corbett and Brown, 1976;
Berry, 1994).
The Tabberabberan Orogeny also involved prehnite-pumpellyite to lower greenschist facies
regional metamorphism of the Mount Read Volcanics (Corbett, 1992; Crawford et al., 1992;
Offler and Whitford, 1992) and emplacement of post-tectonic granites and associated contact
metamorphism (Williams et al., 1989).
Major faults that transect the Mount Read Volcanics (e.g., Henty Fault, South Henty Fault,
Great Lyell Fault, Rosebery Fault, Mount Charter Fault; Figure 3.1) are recognised as
Cambrian Delamerian structures which have been reactivated during later deformation,
20
including the Devonian Tabberabberan Orogeny (Seymour, 1980; Corbett and Lees, 1987;
Berry, 1989; Corbett, 1992; Halley and Roberts, 1997).
Interpretation of structure in ancient deformed volcanic sticcessions can be complicated by
the absence of bedding, especially in lavas and some volcaniclastic facies, the presence of
significant primary dip, and the marked competency contrasts and irregular contact
relationships between coherent andvolcaniclastic facies. In the study, these problems apply
mainly to the Anthony Road Andesite because it is domina~ed by lavas and shallow
intrusions. In general, the other lithostratigraphic units are well bedded and have provided
the bulk of the structural data.
3 . 2 Structural analysis of the Basin Lake area
In the Basin Lake area, bedding is well developed in volcaniclastic and sedimentary facies,
dominantly N to NW striking and youngs to the east. Three cleavages are observed with the
S2 cleavage being the most intense. Folds are widespread though best exposed in the
Newton Creek Sandstone in the northern study area (Plate 3.1). The Great Lyell and South
Henty faults bound the study area to the east and west respectively, and are interpreted to
have been active since the Cambrian (Figure 3.1; Corbett and Lees, 1987; Berry, 1989;
Halley and Roberts, 1997). NW to NNW and NE trending minor faults are also present.
Previous structural studies in the region have been undertaken by Hutton (1989), Gibson
(1991), Sliwa (1995) and Terry (1995).
The Great Lyell Fault separates two structural domains that differ in the orientation of
bedding, degree of folding and the intensity of cleavage. Domain 1 lies to the south and west
of the Great Lyell Fault and Domain 2 to the north and east of the Great Lyell Fault (Figure
3.2). Structural data used for interpretation in this study are listed in Appendix B.
3.2.1 Domain 1
Domain 1 occupies the bulk of the study area (Figure 3.2) and includes the Yolande River
Sequence, Anthony Road Andesite, Tyndall Group and the Jukes Conglomerate member of
the Owen Conglomerate. The principal lithofacies in these formations in Domain 1 are
shallow intrusions, lavas and volcaniclastic units with minor limestone and black mudstone.
The Newton Dam Spillway lies within this domain and has been studied in detail by Gibson
(1991).
21
-------
145·15
N
t
41°45
o lOkm I I I
Study area
x
42000
l"", ,- 61)
(J
+~
~
\ -
.....--r2·'
Figure 3.1 Structural elements in the region surrounding the study area. Modified after Berry (1989) from Corbett and Lees (1987). Dark lines represent faults with major synclines and anticlines also shown.
22
EL5,~5
~
Lake Margarel ~ I
~
North
S360000mN I km
S3~N
o Owen Conglomerate
Mount Read Volcanics
TyndaJl Group - volcanic! astics
o TyndaJl Group - rhyolite S3S6OOOmN
Anthony Road Andesite - undiITerenliated
o Anthony Road Andesite - rhyolite association
oYolande River Sequence
D Central Volcanic Complex
/ Fault
S3S4000mN
o » .....l
] S3S2000mN
Figure 3.2 Structural domains in the Basin Lake study area. The two domains are separated by the Great Lyell Fault
Bedding, columnar jointing and flow banding
Bedding (So) is well developed in volcaniclastic and black mudstone units throughout the
Yolande River Sequence and consistently dips and yourigs towards the east. Grading of
andesitic volcaniclastic units in drillcore indicates that the Anthony Road Andesite is also
east-younging.
'" The Tyndall Group is the youngest lithostratigraphic unit in the Mount Read Volcanics
(Corbett, 1992). The group is dominated by coarse sandstone, breccia, conglomerate,
siltstone and rhyolite. Bedding in the Tyndall Group predominantly strikes N to NW and
youngs and dips steeply to the east (50 0 to 85 0 Figure 3.3) but becomes overturned close to ;
the Great Lyell Fault.
Columnar jointing is present in a hornblende andesite intrusion of the Anthony Road
Andesite at the base of the Newton Dam Spillway (Plate 3.2). The polygonal columns are up
to 3 m long and 20 cm wide. The mean plunge of the column axes is 360 towards 298 0
,
implying that the orientation of the isothermal surface was close to a strike/ dip of 028 0 540 E
(Figure 3.4).
Flow banding is well developed within rhyolite lavas in the Zig Zag Hill Formation of the
Tyndall Group, both to the west and east of the Anthony Road. The planar flow banding
mainly strikes NW-SE with steep NE or SW dips (Figure 3.5) and is closely similar in
orientation to So across Domain 1 (Figure 3.3).
Folds and cleavage
Two cleavages, SI and S2, have been recognised in Domain 1. SI is uncommon and was
observed only in black mudstone of the Yolande River Sequence where it has been
crenulated by the later S2. The SI cleavage occurs as a slaty cleavage and is dominantly N to
NW striking. The S2 cleavage is widespread across Domain 1 but is best developed in the
black mudstone of the Yolande River Sequence and fine grained volcaniclastic facies of the
Tyndall Group. The S2 cleavage is a penetrative slaty cleavage in the finer grained
sedimentary and volcaniclastic facies and locally present as a spaced anastomosing cleavage
in the Anthony Road Andesite and coarse volcaniclastic sandstone of the Tyndall Group.
The S2 cleavage dominantly strikes N to NW and dips steeply (generally> 550 both E and
W; Figure 3.6). The high angle of S2 to So and dominant easterly dip of So throughout the
Tyndall Group suggest that Domain 1 occupies the eastern limb of a large wavelength (> 1
km) N?-trending upright to inclined, S21D2 anticline.
24
• ••• • •• • • •
Equal Area
n == 104
-=-...! ••••,,,-!!!!-a,..- •• •• + .~
.~ ..,-.. • • e i'"
.... .•"... - ••
Figure 3.3 Stereographic projection of poles to bedding in Domain 1 with the average great circle shown. The Fl fold axis is shown as a filled square.
Equal Area
n == 25
Figure 3.5 Stereographic projection of poles to flowbanding (closed circles) developed in rhyolite of the Zig Zag Hill Formation in Domain 1. The average great circle is shown.
Equal Area
n==4
Figure 3.4 Stereographic projection of column axes in columnar jointed (closed circles) hornblende andesite of the Anthony Road Andesite. The mean lineation (filled
square) plunges 360 towards 2980 • The great circle corresponds to the isothermal surface indicated by the column axes.
Equal Area
n == 71
Figure 3.6 Stereographic projection of poles to S2
(closed circles) in Domain 1.
25
F1 folds within Domain I are upright, open and of large wavelength (> I km) except close to
the Great Lyell Fault where reverse movement h~s resulted in overturning of So and
associated mesoscopic upright folding (F2; < 50 m wavelength). The F 1 (first generation)
fold axis plunges at 5° towards 345° (Figure 3.3). The rarity of SI in Domain 1 and the
widespread occurrence of S2 suggest that either the S2 forming event was stronger or that
the S2 cleavage has overprinted the SI cleavage and that related F2 folds have overprinted
earlier Fl folds (Figure 3.6). The F2fold axes plunge at between 30° to 60° towards the S to
SW.
3.2.2 Domain 2
The Owen Conglomerate is exposed within Domain 2, north and east of the Great Lyell
Fault (Figure 3.2) and comprises boulder-pebble conglomerate, coarse-medium sandstone
and black mudstone.
Bedding
Throughout Domain 2, bedding (SO) is best developed in thinly bedded to laminated black
mudstone. Bedding is less obvious in the siliciclastic conglomerate because beds are
commonly very thick and massive or graded. So strikes NW to NNW (283° to 350°) and
dips moderately to steeply (30° to 86°) both to the west and east (Figure 3.7). On the eastern
side of the study area the Middle Owen Conglomerate youngs to the west.
Folds and cleavage
Three cleavages, SI, S2 and S3, have been observed in black mudstone and quartz-mica
sandstone of the Newton Creek Sandstone in Domain 2.
SI is present at only three localities where it occurs as a slaty cleavage crenulated by another
cleavage, S2. SI strikes N to NW (005° to 305°) and dips steeply at between 70° and 84° to
both the ea~t and west (Figure 3.9). The intersection lineation L~ plunges at 60° towards
156°.
26
Equal Area • poles to So
n =138 Ii F 1 fold axis
Figure 3.7 Stereographic projection showing poles to So (closed circles), the best fit great circle and
the F1 fold axis for Domain 2.
• poles to SI + poles to S2
SI n=7 * poles to S3
S2 n = 37
S3 n=3
Figure 3.8 Stereographic projection of poles to SI (closed circles), S2 (crosses) and S3 (stars)
for Domain 2 of the study area.
27
The S2 cleavage is widely developed throughout Domain 2. It is a penetrative slaty cleavage
defined by grain alignment in black mudstone and a spaced cleavage in sandstone. S2 is
normal to bedding (convergent fan) in the hinges of small wavelength (1 to 50 m) SSE
trending folds (F2; Plate 3.1). The S2 cleavage strikes N to NW (010° to 300°) and dips
moderately to steeply (51 ° to 84°; Figure 3.8). The L1 intersection lineation plunges at 73°
towards 216°. The L~ intersection lineation plunges shallowly (16° to 33°) towards the SSE
(161 ° to 168°).
A third cleavage (S3) has been observed only at two localities where it occurs as a spaced
anastomosing cleavage crenulating S2. The S3 cleavage strikes NE (045° to 055°) and dips
moderately (55° to 70°) to the west (Figure 3.8). The L~ intersection lineation plunges at
62° towards 309°.
A large wavelength (> 1 km) F I anticline is well exposed in the Tyndall Range to the east
(Plate 3.3). The FI fold axis plunges at 22° towards 159° (Figure 3.8). The Lb intersection
lineation plunges at 59° towards 156° and approximates the F I fold trend (Figure 3.8). The
FI folds have been overprinted by the F2 folds which also have a SSE trend.
F2 folds are well exposed in the Newton Creek Sandstone (Plate 3.1). The F2 folds are
tight, asymmetric and similar in style with wavelengths commonly < 5 m, but rare examples
are up to 50 m. F2 fold vergence is to the east. The F2 fold axes measured in the field plunge
shallowly either towards the SSE or NNW, forming two distinct populations (Figure 3.9).
The L1 intersection lineation plunges shallowly (12° to 24°) towards the SSE (159° to 172°;
Figure 3.10).
The L~ intersection lineations closely match the F2 fold axis given by the poles to SI
cleavage and with one population of the measured F2 fold axes (Figure 3.9). The F2 folds
trend SSE in a similar orientation to the F I folds. The occurrence of two populations of F2
fold axes measured in the field, one trending SSE and one trending NNW, suggests
refolding of the F2 folds. The F2 folds observed are parasitic folds on the limbs of larger
wavelength F I folds.
No F3 folds were observed in the field although from the L~ intersection lineation, a plunge
of 50° to 60° towards WNW (280° to 309°) can be estimated for the F3 fold axis (Figure
3.11).
28
•• •
• • •• •
Equal Area • poles to So
So n=138 + poles to SI SI n=7 IiiI FI fold axis
+ : .."a, Lb intersection lineation .~
dI-;.r&f· .. \ _~. -~+i"l'
+. •• • It •••••••-;••+
•
+ •
•••• • a
.}:
•
• III
Figure 3.9 Stereographic projection of poles to So (closed circles), poles to S I (crosses), the F I fold axis
defined by the poles to So (solid square) and the Lb intersection lineation (small square) for
Domain 2.
Equal Area • poles to SI + poles to S2 SI n=7 a F2 fold axes measured S2 n= 37 * Lnintersection lineation IiI F2 fold axis calculated
Figure 3.10 Stereographic projection of poles to SI (closed circles) and S2 (crosses), the F2 fold axis
(closed square)defined by poles to SI' measured F2 fold axes (small squares) and theLn
intersection lineation (stars) for Domain 2.
29
3.2.3 Comparison of Domains 1 and 2
In both domains, the SI and S2 cleavages are similar in morphology and orientation. The S2
cleavage is the dominant cleavage in both domains. A spaced anastomosing cleavage, S3, is
present in Domain 2 but was not observed in Domain 1. Folds are rarely observed in
Domain 1 due to poor outcrop though F1 and F2 folds are well exposed in Domain 2. The SI
cleavage and F 1 folds may be related to deformation during ,the Cambrian Delamerian
Orogeny whereas the D2 deformation, which developed the S2 cleavage and F2 folds, is
more intense than Dl and thought to be related to Middle Devonian deformation based on the
similar orientation and style of folds to Delamerian and Tabberabberan structures elsewhere
in western Tasmania (Seymour, 1980; Berry, 1994).
3.3 Faults
The South Henty Fault and the Great Lyell Fault are major structures in the Mount Read
Volcanics. These faults, along with smaller faults, have affected stratigraphic relationships
across the study area. The strike of faults (Figure 3.2) has been established from
interpretation of air photos and helimagnetics, from logging of diamond drill core and from
outcrop mapping. The sense of displacement has been determined using three movement
indicators (Petit, 1987): i) quartZ±chlorite fibre veins and striations, ii) drag of cleavage into
the fault zone and ill) grooves in the fault surface (Figure 3.12).
3.3.1 South Henty Fault
The South Henty Fault and the North Henty Fault divide from the main NNE-oriented Henty
Fault Zone near Gooseneck Hill (Figure 3.1; McNeill and Corbett, 1992). The NNE-striking
South Henty Fault is present 1 km to the west of the study area and can be seen clearly on
helimagnetic images and air photos of the region (Figure 3.1). Corbett and Lees (1987)
suggest that the South Henty Fault was already active during the Cambrian, and could have
formed the western boundary of the basin in which the lithologic associations of the study
area accumulated. In a detailed study of the west-dipping Henty Fault Zone in the Rosebery
Tullah region, Berry (1989) recognised five phases of movement: i) a pre-Devonian east
directed thrusting and folding event, ii) high angle reverse faulting which post-dates
Devonian folding, iii) Devonian sinistral wrench faulting, iv) sinistral wrench faulting
(possibly in the Mesozoic) and v) normal faulting in the Tertiary. There is thus no preserved
record of the normal displacement proposed by Corbett and Lees (1987).
30
• poles to S2 Equal Area + p~les to S3
S2 n= 37 a L2 intersection lineation S3 n= 3 mF3 fold axis
1·
Figure 3.11 Stereographic projection of the poles to S2 (closed circles) and S3 (crosses), the F3 fold axis
(closed square) defined by the poles to S2 and theL~ intersection lineations (small squares)
for Domain 2.
•.1 et T.' }":'I
(iii)
Figure 3:12 Three sense of displacement criteria used during this study. (i) New minerals crystallised in the lee of a ledge on the fault plane. (ii) Drag of cleavage where the cleavage intersects the fault plane at a high angle to the fault striations. (iii) Grooves deepening to the right due to asperities on the opposing face. Diagrams have top block displaced to the right. Modified after Berry (1989).
Plate 3.1 F2 folds in the Newton Creek Sandstone (Domain 2), Henty Canal. Cutting is approximately 1.5m high. (GR 381350E, 5359480N)
Plate 3.2 Columnar jointing developed in an intrusion of hornblende andesite at the base of the Newton Dam Spillway. Yellow line follows the trend of the columns. (GR 379900E, 5358250N) ~
Plate 3.3 Anticline (FI) developed in Middle Owen Conglomerate on the Tyndall Range. Looking to the south.
Plate 3.4 Two generations of quartz-cWorite fibre veins on the Great Lyell Fault, Anthony Road. The second generation (2) clearly overprints the first generation (1). (GR 381400E, 5359300N)
32
3.3.2 Great Lyell Fault
The Great Lyell Fault is present along the eastern side of the study area, changing strike
from N to NNW in the Newton Creek area (Figure 3.1). The fault outcrops along Howards
Road but it is covered by glacial deposits elsewhere in the study area. The position of the
fault in drill holeNC2 (Appendix G) in conjunction with Aberfoyle's helimagnetic data,
suggest the fault is steeply (600 to 700
) west-dipping (Sharpe, 1994). The Great Lyell Fault
juxtaposes the Tyndall Group to the west against Middle Owell' Conglomerate to the east,
indicating several hundred metres of reverse movement.
Quartz and chlorite fibre veins occur on bedding planes in the Newton Creek Sandstone near
the junction of Howards Road and the Anthony Road (Plate 3.4) where the Great Lyell Fault
strikes NNW. Grooves on bedding planes and tension gash veins are also present. Two
separate generations of quartz and chlorite fibre veins are recognisable. At one locality the
lineations of generation 1 plunge at 45 0 towards 245 0 whereas the lineations of generation 2
plunge at 49 0 towards 165 0 (Appendix B; Figure 3.13; Plate 3.4). The first and second
generations .of fibre veins give a west-block-up displacement sense and indicate an initial
dextral-reverse movement followed by a sinistral reverse movement. The second generation
of fibre veins overprints and thus post-dates the first generation.
North
Equal Area
Bedding plane strike 110°
dipping 55° SW Generation 1 striation
.(plunge 45° towards 245°) Generation 2 striation
(plunge 49° towards 165°)
Figure 3.13 Stereographic projection of bedding in the Newton Creek Sandstone member of the Owen Conglomerate with plunges of generation I and 2 quartz-chlorite fibre veins from field locality 231. Data are listed in Appendix B. Fibre veins have a reverse sinistral and reverse dextral sense of movement respectively with the Great Lyell Fault striking NNW in this area.
33
To the north of the study area, the Great Lyell Fault intersects the Henty Fault Zone near
Gooseneck Hill and also continues to the south as far as Queenstown (Figure 3.1; Corbett
and McNeill, 1988; McNeill and Corbett, 1992). The Great Lyell Fault is thought to have
been a basin margin structure in the Cambrian (Corbett and Lees, 1987), because there are
major lithological changes in the Mount Read Volcanics across it. The Haulage
Unconformity between the Middle Owen Conglomerate and the Upper Owen Conglomerate
at Queenstown involved west block-up reverse movement on the Great Lyell Fault during
the Ordovician (Corbett and Solomon, 1989; Corbett and Turn~r, 1989). Uplift which has
put the Tyndall Group over the Owen Conglomerate is related to high angle thrusting on the
Henty Fault during Middle Devonian orogenesis.
3.3.3 NW to NNW faults
The strikes of minor faults in the study area are dominantly NW to NNW (Figure 3.14).
These faults commonly offset the Great Lyell Fault (Figure 3.2). Two major NW striking
faults are the Pyrite Corner Fault and the Tyndall Creek Fault, both of which have associated
intense alteration. The NW to NNW striking faults dip at moderate to steep angles (45° to
85°) to both the west and east, and have been active as reverse (n=29) and normal structures
(n=7) with both east-block-up and west-block-up displacements (Figure 3.15). Faults with
reverse displacement dominate and appear to pre-date the less common normal faults.
Helimagnetic and air photo interpretation suggest these structures controlled the distribution
of some stratigraphic units in the study area, especially the Tyndall Group in the north.
The presence of Cambrian alteration and interpreted stratigraphic changes across these faults
suggest that they were active during the Cambrian. The features observed on these structures
which indicate normal and reverse movement have been caused during reactivation, probably
following west-block-up reverse movement on the Great Lyell Fault.
3.3.4 NE faults
NE striking faults are not as common as the NW to NNW faults but have played an
important role in controlling the distribution of units in the study area (Figure 3.2, 3.14).
The reverse NW to NNW faults appear to be offset by and therefore pre-date the NE faults.
The NE faults are largely continuous and have not been offset. These faults are moderately
to steeply (50° to 84°) west-dipping and all appear to have been reverse structures, as
indicated by fibre veins (Figure 3.15).
34
•• ••
•• •
~
Figure 3.14 Rose diagram displaying the strikes of the faults across the study area.
Equal Area
n= 34 •
•• •• •
• • +
• •
•
Figure 3.15 Stereographic projection of poles to faults (closed circles) from Domain 1 and Domain 2.
35
3 .3 .5 Discussion of faults
It has proved difficult to distinguish Cambrian from Devonian faults. In the study region it is
interpreted that both the Great Lyell Fault and South Henty Fault were active during the
Cambrian. The Great Lyell Fault juxtaposes the Tyndall Group with the Middle Owen
Conglomerate whereas the South Henty Fault juxtaposes the Central Volcanic Complex with
the Yolande River Sequence, Anthony Road Andesite and Tyndall Group.
The NW to NNW striking fault set is associated with silica-sericite-pyrite alteration (Pyrite
Corner Fault), base metal mineralisation (Tyndall Creek Fault; Chapter 7) and facies changes
(Chapter 4) that suggest these faults were active during sedimentation/volcanism. They have
been reactivated following west-block-up reverse movement on the Great Lyell Fault during
Devonian orogenesis.
The NW to NNW and NE fault sets do not appear to have been as important in controlling
the distribution of stratigraphy in Domain 2 as in Domain 1.
3.4 Summary and conclusions
Three deformation events (DI-D3) have been responsible for development of folds, cleavage
and faults observed in the study area. Based on comparison with structural studies elsewhere
in western Tasmania, these deformation events have tentatively been correlated with
Cambrian (Dl) and Middle Devonian (D2,D3) orogenesis.
Across Domains 1 and 2, the SI slaty cleavage provides evidence for an early deformation
event (Dl) that developed large wavelength (> 1 km) upright open folds. The Dl event may
be related to the third phase of the Cambrian Delamerian Orogeny which produced upright,
open, N-trending folds across western Tasmania (Berry, 1994). Structures associated with
the D2 and D3 events (discussed below) correlate well with structures associated with Middle
Devonian deformation in western Tasmania.
The S2 cleavages in both domains are similar in morphology and N to NW orientation. The
S2 cleavage is largely axial planar to short wavelength « 50 m), SSE or NNW plunging F2
folds. The S2 cleavage and F2 folds are the dominant structures in the study area. The D2
deformation may correspond to the first phase of Middle Devonian orogenesis in western
Tasmania which caused the tightening of N-trending Cambrian folds and the widespread
development of a NNW cleavage (Berry, 1994).
36
D3 in the study area is recorded by a NE striking, anastomosing S3 cleavage. The D3
deformation may correspond with the second phase of Middle Devonian orogenesis in
western Tasmania which was associated with N- to NNE-directed compression (Berry,
1994).
Faults are common in the study area and include two regional faults, the Great Lyell and the
South Henty faults. Quartz-chlorite fibre veins within the studx. area indicate at least two
reverse movement episodes along the Great Lyell Fault. In addition, two fault sets, NW to
NNW striking and NE striking, occur in the study area. The NW to NNW faults are
dominant and record evidence of both reverse and normal movement. Alteration and
mineralisation occur along this fault set and these faults are thought to have been active
during sedimentation/volcanism. The NE faults are moderately to steeply west-dipping and
reverse. The NE faults offset and therefore post-date the NW to NNW faults.
37�
Chapter 4
Facies characteristics of lithostratigraphic units in the Basin Lake area
Chapter 4�
Facies characteristics of lithostratigraphic units in the� Basin Lake area�
4.1 Introduction
In the study area, the late Middle Cambrian Mount Read Volcanics comprise three
lithostratigraphic units: the Yolande River Sequence (part of the Western volcano
sedimentary sequences), the Anthony Road Andesite and the Tyndall Group (Figure 4.1).
The fourth lithostratigraphic unit in the region is the Late Cambrian to Early Ordovician
Owen Conglomerate which overlies the Mount Read Volcanics (Figure 4.1). Facies
characteristics and relationships for these lithostratigraphic units, except for the Anthony
Road Andesite (Chapter 5), are described and interpreted in this chapter.
4.2 Yolande River Sequence
The Yolande River Sequence comprises volcaniclastic and non-volcanic sedimentary facies
with minor rhyolitic intrusions. The Yolande River Sequence outcrops on the southern edge
of the study area though outcrop is continuous along the length of the Anthony Road to the
south, a distance in excess of 8 km (Figure 4.1; Corbett and McNeill, 1988). Hutton (1989)
recognised a true thickness of approximately 2 km in the region and subdivided the sequence
into lower and upper facies associations. A stratigraphic thickness of approximately 300 m
of the upper facies association is exposed in the study area along the Anthony Road. Two
drill holes (TYN2 and Leech Hill; Appendix G) also provide sections through the Yolande
River Sequence (Figure 4.2). In the study area, the Yolande River Sequence dips and
youngs to the northeast. An intrusion of columnar jointed rhyolite outcrops on the Anthony
Road, 3 km south of the study area.
Four mainfacies have been recognised, one is non-volcanic (black mudstone) and the rest
are volcaniclastic (volcanic mudstone, coarse feldspar-pumice sandstone, pumiceous lithic
rich breccia). These facies are described and interpreted below.
38�
!� i:i
S360000mN
5358000mN
5356000mN
5354000mN
5352000mN
.....-�=�r: .et::
'5== o rI.)
ELS/8S Lake Margaret
~
Norlh
lkm
o Owen Conglomerate
Mount Read Volcanics
Tyndall Group - volcaniclastics
o Tyndall Group ....0:
- rhyolite ..Q,j Anthony Road Andesite o - undifferentiated
o Anthony Road Andesite - rhyolite association
DYolande River Sequence
o Central Volcanic Complex
/ Fault
Figure 4.1 Distribution of major lithostratigraphic units in the Basin Lake area.
Diamond drillhole TYN2 Diamond drillhole Leech Hill Grainsize (mm) Grainsize (mm)
~ ~ ~ \0 N Depth (m) Depth (m) ;::; - N 00 \0 S ~ ~ 00 ~
0, I I I I I o
50 -,' - ",' ) 50
100 100
150 150
200 200
250
300
Yolande River Sequence
250
300
!\
1\
1\
~ - -- -- -
C3..' . .. .. .
Black mudstone
Coarse feldspar-pumice sandstone
350
400
!\ !\
L;] Pumiceous lithic-rich breccia
Anthony Road Andesite
~
Hornblende andesite association
450
/\ 1\
1\ /\
500
Figure 4.2 Summary graphic logs of diamond drillholes TYN 2 and Leech Hill. Both drillholes pass through the Yolande River Sequence. For detailed logs and drillhole locations refer to Appendix G.
4.2.1 Black mudstone fades
The black mudstone facies is exposed at several localities and intersected in drill holes Leech
Hill and TYN2 (Appendix G) but amounts to only 5 to 10 % of the Yolande River Sequence
in the study area. Black mudstone is laminated or massive and commonly strongly pyritic,
containing thin « 1cm) pyrite-rich bands and separate pyrite crystals. The black mudstone
facies is commonly intercalated with volcaniclastic turbidite'-'units and also occurs as
intraclasts (up to 70 cm across) in the turbidites. Intervals of black mudstone are commonly
less than 5 m thick.
4.2.2 Volcanic mudstone fades
The volcanic mudstone facies is distinguished from the black mudstone facies in outcrop by
being light grey to grey-green (Plate 4.1). Intervals of this facies are commonly very thick
(maximum 30 m), and typically massive, though laminated to thinly bedded units occur.
Slump folds, with wavelengths of 15 to 20 cm, and flame structures are locally present in
laminated to thinly bedded units.
In thin section, the volcaniclastic mudstone facies contains fragments of feldspar, quartz and
muscovite, and glass shards (McPhie et al., 1993) now altered to cWorite.
4.2.3 Coarse feldspar-pumice sandstone
Coarse feldspar-pumice sandstone is common in the Yolande River Sequence in the study
area. Relic pumice, feldspar crystal fragments, minor quartz and clasts of black mudstone in
a fine volcanic, originally glassy now altered, matrix comprise this facies with single units
commonly 10 to 15 m thick. In outcrop, this facies is commonly gradationally underlain by
the pumiceous lithic-rich breccia facies and grades into overlying volcanic mudstone facies.
In drill hole TYN2 (Appendix G), the pumiceous lithic-rich breccia facies is absent and a
repetitive sequence of coarse feldspar-pumice sandstone grading into volcanic mudstone,
with sandstone-mudstone 1 to 5 m thick, occurs (Figure 4.2).
4.2.4 Pumiceous lithic-rich brecda
The pumiceous lithic-rich breccia facies is clast- to matrix-supported and polymictic, and
dominantly composed of intraclasts of laminated black mudstone up to 70 cm in diameter
41�
(Plate 4.2). Hutton (1989) has also recorded clasts of andesite and dacite within this facies.
This facies is less than 5 m thick and is not intersected in drill holes in the area. The matrix
comprises ragged relic pumice (now altered to sericite and chlorite), feldspar and quartz
crystals and black mudstone. Poor sorting and the absence of grading and bedding are
features of this facies. Pumiceous lithic-rich breccia commonly grades upward into coarse
feldspar-pumice sandstone, forming the basal part of volcaniclastic turbidites in the Yolande
River Sequence in the study region.
4.2.5 Sedimentation processes, depositional setting and provenance of the
Yolande River Sequence
In the study area, the Yolande River Sequence is dominated by laminated black mudstone
and thick volcaniclastic turbidites (Figure 4.2). Laminated black mudstone consists of fine
detritus settled dominantly from suspension and thus represents the ambient sedimentation
regime in the basin. Where black mudstone occurs between volcaniclastic turbidites, it may
represent a significant time break between turbidity current events.
The three volcanic facies of the Yolande River Sequence represent single sedimentation units
which are nonnally-graded from pumiceous lithic-rich breccia bases through coarse feldspar
pumice sandstone into volcanic mudstone tops (Figure 4.3). These sedimentation units
resemble high density volcaniclastic turbidites (Lowe, 1982) in being thick (15-40 m),
having a high concentration of pumice and crystals, and displaying normal grading above
erosive bases (Figure 4.3). Volcaniclastic turbidites interbedded with black mudstone and the
total lack of tractional bedfonns suggest that the Yolande River Sequence was deposited in a
below-wave-base, relatively deep water setting. The repetitive cycling from coarse feldspar
pumice sandstone to volcanic mudstone in drill hole TYN2 (Appendix G) may indicate that
the turbidites in the Yolande River Sequence are either distal in character or that the
topography of the basin floor has controlled the turbidites to some degree (Walker, 1984;
Cas and Wright, 1987).
Medium to coarse grained volcaniclastic sedimentation units tens of metres thick are also
present in the Lower Devonian Merrions Tuff, New South Wales (Figure 4.3; Cas, 1983).
The high volumes of juvenile volcanic fragments in these units have been provided by
pyroclastic flow deposits which entered the sea (Cas, 1983).
The Yolande River Sequence volcaniclastic turbidites are composed predominantly of pumice
and volcanic crystals (feldspar» quartz). The sandstone facies contain between 65 and 71
wt% Si02 (Appendix E). The abundance of feldspar and the composition indicate a dacitic
42�
provenance for the Yolande River Sequence volcaniclastic turbidites. The presence of
abundant pumice and crystal fragments suggests that the volcanic component has been
sourced from explosive eruptions at vents at the basin margins or at shallow water
intrabasinal vents.
4.3 Tyndall Group
The Tyndall Group is the youngest lithostratigraphic unit of the Mount Read Volcanics and is
dominantly composed of volcanic facies with lesser non-volcanic sedimentary facies present
at the base and at the top of the group. The Tyndall Group outcrops discontinuously along
the Mount Read Volcanics and varies in stratigraphic thickness from 50-200 m in the Mount
Lyell area to approximately 1300 m in the Cradle Mountain Link Road area (White, 1996).
In the study area, the Tyndall Group has a stratigraphic thickness of approximately 750 m.
This study follows on from previous detailed studies of the Tyndall Group throughout the
Mount Read Volcanics by White (1996) and White and McPhie (1996).
The Tyndall Group is exposed along the Anthony Road and across the northern parts of the
study area (Figure 4.1) and occurs in drill holes TYN3, NC1, NC2, NC3, HA3, HA4 and
HA6 in the north and along the eastern side of the study area (Appendix G). White and
McPhie (1996) have divided the group into two formations, the lower Comstock Formation
(containing an upper and lower member) and the upper Zig Zag Hill Formation (Figure 4.4),
both of which are present in the study area. An idealised stratigraphic column for the Tyndall
Group is given as Figure 4.5.
In the study area, the Tyndall Group dips and youngs towards the northeast, except close to
the Great Lyell Fault where bedding is overturned.
4.3.1 Comstock Formation
The Comstock Formation contains two members: the basal Lynchford Member and the upper
Mount Julia Member (Figure 4.4; White, 1996). The Lynchford Member is poorly exposed
in the study area althoughit is intersected in several diamond drill holes. The Lynchford
Member contains three dominant facies: limestone, volcaniclastic sandstone and black
mudstone (Figure 4.5). The Mount Julia Member outcrops extensively along and east of the
Anthony Road. Good sections of the member also occur in diamond drillcore. Two facies
are distinguished in the Mount Julia Member in the study area: crystal-rich volcaniclastic
sandstone and polymictic volcanic breccia (Figure 4.5). The Mount Julia Member is
43�
distinguished from the Lynchford Member by being feldspar- and quartz-rich, by the
presence of abundant angular rhyolite and'dacite clasts and the overall dacitic to rhyolitic
composition. In situ welded ignimbrite occurs within this member at Zig Zag Hill (White,
1996), near Queenstown, and clasts of welded ignimbrite have been identified in the Mount
Julia Member in the study area.
Lynchford Member
Limestone· fades
Massive to bedded limestone occurs at the base of and within the Lynchford Member (Plate
4.3). The limestone is not exposed at the surface but has been intersected in diamond drill
holes near Howards Anomaly and along the eastern edge of the study area (e.g., NC3, HA4,
HA6; Appendix G).
Limestone units vary widely in thickness and abundance. Drill hole TYN5 contains ten
separate units and drill holes TYN3, TYN4, HA3, HA4, HA6, NC2, NC3 and BL2 all
contain three or more limestone units (Appendix G). Diamond drill hole NC3 intersected
three limestone units: one 53 m thick, one 110 m thick and the third unit 26 m thick
(Appendix G). Massive limestone is most commonly white whereas beds (0.5 to 3 cm thick)
in the bedded limestone commonly vary between white, red and purple (Plate 4.3), due to
variable levels of FeO*l. The limestone is largely sparry calcite with the smallest grains
around 20 /-Lm (0.02 mm) in diameter. Bedding is commonly well developed and defined by
finer (-0.02 mm) and coarser (-1.2 mm) grainsize. Detrital quartz and zircon occur locally
throughout the limestone but account for less than 5 % by volume. No fossils are clearly
visible though circular forms (to 2 cm in diameter) in several localities could represent the
remnants of fossils. The carbon-oxygen isotopic characteristics of the limestone are
discussed in Appendix C.
Volcaniclastic sandstone fades
Volcaniclastic sandstone of the Lynchford Member outcrops at two places along the Anthony
Road: 1. overlying the Anthony Road Andesite, and 2. in a faulted repetition around Tyndall
Creek. Volcaniclastic sandstone is also common throughout the Howards Anomaly area and
occurs irtterbedded with limestone in drillcore. The volcaniclastic sandstone facies largely
IFeO* =Fe203 + FeO
44
comprises euhedral to broken feldspar ± amphibole ± clinopyroxene ± magnetite crystals
with lesser quartz, together with minor blocky to subrounded ~eldspar-phyric dacite,
hornblende-phyric andesite, mudstone and ironstone clasts. Volcaniclastic sandstone is well
sorted and generally massive though in places shows well developed bedding, with single
beds 2 to 10 cm thick. Intervals of volcaniclastic sandstone to 50 m thick outcrop along the
Anthony Road.
'f"
Volcaniclastic sandstone in the Lynchford Member contains a lower volume of crystals and
less quartz than the crystal-rich volcaniclastic sandstone in the Mount Julia Member.
Black mudstone facies
Black to grey, commonly laminated mudstone occurs in the Lynchford Member along the
Anthony Road and in drill holes in the area (e.g., HA3, TYN1, HA5; Appendix G). One
unit approximately 80 m thick outcrops along the road south of Tyndall Creek and is
repeated but much thinner (2 m) north of the creek, due to faulting. The mudstones are
pyritic with well developed laminations and very thin «10 cm) sandstone interbeds.
Mesoscopic folds of tectonic origin occur within the mudstones.
Mount Julia Member
Crystal-rich volcaniclastic sandstone facies
Very thick, massive beds of crystal-rich volcaniclastic sandstone dominate the Mount Julia
Member in the study area. The sandstone comprises euhedral to angular feldspar and quartz
crystals with lesser hornblende, clinopyroxene and titanomagnetite crystals in matrix that
consists of fine albite and chlorite possibly replacing fine glassy particles. Angular clasts (to
20 cm) of rhyolite, dacite, ignimbrite and granite are dispersed throughout. Ragged chloritic
clasts, sometimes quartz±feldspar phyric, which might formerly have been glassy also
occur. Very thick (>50 m) intervals of sandstone show little evidence of internal grading but
pass upward into bedded (1 to 5 cm beds; Plate 4.4) sandstone/siltstone and downwards into
polymictic volcanic breccia.
The crystal-rich volcaniclastic sandstone facies commonly has a distinctive pink (albite-silica)
and green (chlorite) banding that is approximately bedding parallel (Plate 4.4). This pink
green banded facies has been named Comstock Tuff in previous regional work (Corbett et
al., 1974).
45
-------------S3
...------.'"
~~---
< .0 '.o:o,tO Clo' 0 .' • ,
S2 00'iO:. c.·"'O . ' -� 0' Q ~ '0 -.. '0 0
SI
R3 ~. \l�
• ~~'i•�
. ~\. R2J'o \ ..,.., 'V. -
Volcaniclastic turbidite, High-density
Yolande River Sequence turbidite
- - volcanic mudstone� . '. '. volcaniclastic sandstone� D: D volcaniclastic breccia
'0
I>
'\>
t7. (
. ,
Volcaniclastic megaturbidite, Merrions Tuff, NSW
, . ,
. . .�, .�
Q. . . ,
'Q•••
1:).
Volcaniclastic mass-flow deposit,� Mount Julia Member,�
TyndaU Group�
~ pumice clast 4J? mudstone intraclast
~ volcanic clast
.' )
.' '. ·.·.·.9. .., ... , .. ','~
w Wo •••• '° 0 ° 0�
° 0 ° •••••••�0
\>'
'0 • Cl ~, .
';7/).'1>'
C> ' p, ~. 0' P
• t> t> 't> r>1J lJ\;).~{:) ) .DO\.> {;>'\J
Subaqueous volcaniclastic mass-flow deposit
Figure 4.3� Comparison of bedform characteristics and grain size variations of a volcaniclastic turbidite and volcaniclastic mass-flow deposit from the Basin Lake area with a high-density turbidite (Lowe, 1982), volcaniclastic megaturbidite (Cas, 1979) and subaqueous volcaniclastic mass-flow deposit (Fiske and Matsuda, 1964). The high-density turbidite is the ideal sequence deposited (Lowe, 1982). Individual volcaniclastic megaturbidite units in the Merrions Tuff are up to 100 m thick (Cas, 1979). The example of a subaqueous volcaniclastic mass-flow deposit is from the Miocene Tokiwa Formation, Japan, and consists of massive and thin bedded lapilli-rich sandstone with massive units up to 40 m thick (Fiske and Matsuda, 1964), No scale implied,
Zig Zag Hill Formation
~ = 0
-~
-~ Mount Julia Member ~
"'Cl Comstock� = Formation�~
E-t Lynchford Member
Figure 4.4� Stratigraphic scheme used for the Tyndall Group throughout this study. From White and McPhie (1996).
Polymictic volcanic breccia facies
Polymictic pebble to cobble volcanic breccia comprises angular to subangular clasts of
porphyritic rhyolite and dacite, chloritic clasts and intraclasts of mudstone in a feldspar
quartz crystal-rich sandstone matrix (Plate 4.5). Lithic clasts commonly have a pink (albite
rich) halo in the adjacent sandstone matrix. Intervals of the polymictic breccia are up to 5 m
thick and commonly have a reversely graded lower part and a normally graded upper part,
and grade upward into the crystal-rich volcaniclastic sandstone facies (Figure 4.3).
Polymictic volcanic breccia is commonly c1ast-supported at the base becoming matrix
supported in the upper part.
4.3.2 Zig Zag Hill Formation
The Zig Zag Hill Formation is the upper formation of the Tyndall Group (Figure 4.4 and
4.5; White and McPhie, 1996). This formation is extensive across the study area, being
exposed along and both east and west of the Anthony Road. The Zig Zag Hill Formation is
dominated by polymictic conglomerate and includes a high proportion of Precambrian
derived detritus. The Zig Zag Hill Formation contains three facies: graded volcaniclastic
sandstone/siltstone, polymictic conglomerate, and coherent rhyolite and monomictic rhyolite
breccia (Figure 4.5).
Graded volcaniclastic sandstone/siltstone facies
This facies occurs in the northern part of the study area and comprises sandstone and
siltstone dominated by feldspar and quartz crystals with lesser titanomagnetite and altered
47�
ferromagnesian phases. Sedimentation units are commonly normally-graded from coarse and
medium, diffusely bedded sandstone to thinly bedded siltstone. Thin beds rich in heavy
minerals such as titanomagnetite are common in the siltstone. The thickness of sedimentation
units varies between 2 and 10 m. Flame structures, basal scour surfaces and low angle
cross-lamination are observed in the siltstone.
Pink (albite-silica) and green (chlorite) banding occurs in this faci~s, similar to that present in
the crystal-rich volcaniclastic sandstone facies of the Mount Julia Member.
Polymictic conglomerate facies
Clast- and matrix-supported polymictic conglomerate (Plate 4.6) with a feldspar-quartz
crystal sandstone matrix occurs in 2 to 15 m thick intervals that grade upward into graded
volcaniclastic sandstone/siltstone facies. Conglomerates are dominated by rounded to
subrounded clasts of siliceous rhyolite, dacite, chert and intraclasts of mudstone. In the top
of the Zig Zag Hill Formation, clasts of Precambrian quartzite, chert and quartz become
increasingly abundant.
Coherent rhyolite and monomictic rhyolite breccia facies
Coherent rhyolite and associated monomictic rhyolite breccia outcrops either side of the
Anthony Road in the north of the study area. Coherent rhyolite consists of euhedral to
resorbed quartz (:54 mm) and euhedral feldspar (:53.5 mm) phenocrysts in a now
microcrystalline quartzo-feldspathic, probably originally glassy groundmass. Chlorite
replaces a probable ferromagnesian phase and minor titanomagnetite is present. Outcrops of
coherent rhyolite along the Anthony Road are commonly 10 to 15 m thick, whereas outcrops
west of the Anthony Road are commonly greater than 50 m thick. The body of coherent
rhyolite west of the Anthony Road extends at least 2 km to the northwest whereas the body
east of the Anthony Road extends for over 500 m (Figure 4.1). Outcrops of coherent rhyolite
are strongly siliceous, pink to orange and flow banded, with flow bands ranging in width
from <1 cm to 5 cm (Plate 4.7).
Monomictic rhyolite breccia composed of angular clasts (to 15 cm) of flow banded to
massive rhyolite, with the same phenocryst population as the coherent rhyolite, outcrop
across the northern study area. Breccias are clast-supported and have a matrix of euhedral to
broken volcanic quartz and feldspar crystals. Monomictic rhyolite breccia (l to 5 m thick)
overlies the coherent rhyolite.
48
I=: 0..... 1d § 0
\ r
-\
,/"
\
I
--/
... t> [>~O vC> .,/ - VI>- t;7D-
~ \ I QV
/ .,/ ,/" '" '\ "l
Coherent rhyolite and monomictic breccia
~ ~ ~..... ~
~ Normally-graded mass-flow units of polymictic conglomerate and N volcaniclastic sandstone/siltstone
~ bJ)..... N
~ -------------------0 ~ I-l
'-' "S ~ Normally-graded mass-flow units of polymictic volcanic breccia
..j:::. \0
~ ~ < ~
I=: 0..... 1d §
cd..... ~
~
~
and crystal-rich volcaniclastic sandstone
0 -------------------~ ~
E-l ~ U 0 +-' CIJ
El 0 U
I-l
"S ~ "'0 I-l
~ - - - - 1 .... J
-~-~-~-~~. •• • I • • ' • .., . . . .... Laminated pyritic black mudstone
u I=: :>.
....:l 1 \ 1
r I r 11
Massive to bedded limestone interbedded with volcaniclastic sandstone
Figure 4.5 Idealised stratigraphic column for the Tyndall Group in the Basin Lake area.
4.3.3 Sedimentation processes, depositional setting and provenance of the
Tyndall Group
The Tyndall Group is dominated by volcanic lithofacies; limestone and black mudstone in the
Lynchford Member are volumetrically minor components of the group.
The limestone may represent a time break between deposition of t9-e Anthony Road Andesite,
stratigraphically below, and the volcaniclastic facies of the Tyndall Group. Identifiable
fossils have not been observed in limestone in the study area. Laminated black mudstone in
the upper Lynchford Member represents the ambient sedimentation regime in the basin. The
black mudstone in the upper Lynchford Member may represent a significant hiatus prior to
deposition of the Mount Julia Member.
Polymictic volcanic breccia and crystal-rich volcaniclastic sandstone in the Mount Julia
Member form single, normally-graded sedimentation units up to 40 m thick. These units are
characterised by a clast-supported lower region, with minor reverse grading at the base, a
massive crystal-rich volcaniclastic sandstone middle and a bedded crystal-rich volcaniclastic
sandstone top (Figure 4.3). The characteristics of the sedimentation units in the Mount Julia
Member indicate deposition from large volume, high concentration, volcaniclastic mass
flows. Clasts in the polymictic volcanic breccia are angular to blocky whereas volcanic
crystals are largely euhedral, suggesting these deposits are essentially syn-eruptive and have
undergone little reworking before final deposition.
In contrast to the Mount Julia Member, clasts in mass-flow units of the Zig Zag Hill
Formation have been reworked prior to resedimentation. Transport and deposition were
post-eruptive.
The thick normally-graded volcaniclastic mass-flow units in the Tyndall Group share
features with high density turbidites. Polymictic volcanic breccia in the Mount Julia Member
commonly has a thin reversely graded lower portion (cf. R2 layer of Lowe, 1982; Figure
4.3) with an overlying normally-graded breccia layer (cf. R3 layer of Lowe, 1982). White
(1996) suggested that gradual aggradation from a sustained high-density turbidity current
could explain the thick sedimentation units observed throughout the Tyndall Group. The
thickness of sedimentation units in the Mount Julia Member and Zig Zag Hill Formation
indicate continuing supply of large volumes of volcanic detritus into the depositional basin.
The contrast between the Mount Julia Member and Zig Zag Hill Formation in the Basin Lake
area and mass-flow-dominated units elsewhere in the Mount Read Volcanics, that are finer
50�
and interbedded with mudstone (e.g., Yolande River Sequence), support a shallower setting
for the Tyndall Group, although still below-wave-base.
The lower Lynchford Member has a more dacitic/andesitic provenance (based on the low
abundance of quartz crystals) whereas the Mount Julia Member and Zig Zag Hill Formation
have a rhyolitic/dacitic provenance (based on the dominance of quartz and feldspar crystals).
The occurrence of welded ignimbrite clasts, and in situ welded ignimbrite elsewhere in the
group (White and McPhie, 1996), combined with the crystal-rich nature of the facies suggest
that components in these facies were derived from subaerial or shallow marine explosive
eruptions. The large volume of pyroclasts and the presence of welded ignimbrite favours the
presence of felsic caldera volcanoes in the source region.
51�
Plate 4.1 Volcanic mudstone in the Yolande River Sequence, Anthony Road. This facies forms the top oflarge volume volcanic1astic turbidites common throughout the Yolande River Sequence. (GR 379070E, 5354425N)
Plate 4.2 Pumiceous lithic~Iich breccia with mudstone intrac1asts (mi), Yolande River Sequence along the Anthony Road. (GR 379000E, 5354335N) "
Plate 4.3 Bedded limestone in the Lynchford Member, Tyndall Group. From drill hole HA4 381m. (GR 380907E, 5357523N)
Plate 4.4 Crystal-rich volcanic1astic sandstone, Mount Julia Member, Tyndall Group along the Anthony Road. Albite and chlorite banding is a common feature in the Tyndall Group. (GR 381080E, 5357800N)
Plate 4.5 Polymictic volcanic breccia, Mount Julia Member, Tyndall Group along the Anthony Road. Angular volcanic c1asts (vc) occur in a crystal-rich sandstone (s) matrix. (GR 381065E, 5357630N)
Plate 4.6 Mass flow unit with polymictic conglomerate base (c) and graded volcanic1astic sandstone/siltstone top (s), Zig Zag Hill Formation, Tyndall Group along the Anthony Road. From drill hole NCl 172.5m. (GR 381212E, 5357318N)
52�
Plate 4.7 Flow banded rhyolite, Tyndall Group. East of Anthony Road. (GR 381270E, 5358875N)
Plate 4.8 Polymictic conglomerate of the Jukes Conglomerate, Dwen Conglomerate, Newton Creek. Contains well rounded metamorphic c1asts (m) and angular volcanic c1asts (v). (GR 381270E, 5359190N) "
Plate 4.9 Pebble conglomerate and quartz sandstone of the Lower Conglomerate Unit, Dwen Conglomerate. North of Howards Road. (GR 38141OE, 5359250N)
Plate 4.10 Bedded mudstone and sandstone, Newton Creek Sandstone in the Henty Canal. North of Howards Road. Cutting is approximately 3m high. (GR 3813lOE, 5359475N)
Plate. 4.11 Pebble-cobble conglomerate of the Middle Dwen Conglomerate, northern Anthony Road-Lake Plimsoll area. Clasts are well rounded and of metamorphic provenance.
Plate 4.12 Photomicrograph of basaltic dyke cross-cutting the Newton Creek Sandstone in the Henty Canal north of Howards Road. Clinopyroxene and plagioc1ase laths are dominant. Thin section AJ88, cross polarised light, x5 magnification. (GR 381511E, 5359494N)
53�
4.4 Owen Conglomerate (Denison Group)
The Late Cambrian to Early Ordovician Owen Conglomerate is exposed in the northern
region and along the eastern border of the study area (Figure 4.1). The Owen Conglomerate
is approximately 750 m thick in the northern study area and is dominantly composed of
Precambrian siliciclastic detritus derived from uplift of the Tyennan Region to the east
(Corbett and Lees, 1987; Banks artd Baillie, 1989). In the study area, the lower four of the ,
five subdivisions in the Owen Conglomerate, recognised by McNeill and Corbett (1992), are
present. Figure 4.6 is a generalised graphic log through the Owen Conglomerate across the
study region. The Newton Creek Sandstone member has been the subject of a detailed
honours thesis study by Terry (1995). The four stratigraphic units of the Owen
Conglomerate recognised in the study area are described below.
4.4.1 Jukes Conglomerate
Along Howards Road, the Jukes Conglomerate has a stratigraphic thickness of less than 50
m and is younging to the east. Conglomerate and coarse quartz-feldspar crystal-rich
sandstone predominate throughout the unit. Conglomerate beds are overall poorly sorted and
grade upward into sandstone. Distinction of polymictic conglomerates of the Zig Zag Hill
Formation (upper Tyndall Group) from basal Jukes Conglomerate is on the basis of the
volume of Precambrian-derived clasts. In this study, units with ~25 % Precambrian-derived
clasts have been assigned to the Jukes Conglomerate. Pebble-cobble conglomerate and
breccia, coarse volcaniclastic sandstone and siltstone of the Jukes Conglomerate occur along
Howards Road and to the east of the Anthony Road - Howards Road intersection.
The Jukes Conglomerate is distinctly purple and green due to the presence of hematite and
chlorite/sericite respectively. Clast- to matrix-supported polymictic conglomerates contain
clasts of orange quartz-feldspar-phyric rhyolite (to 40 cm), pink quartz sandstone (to 6 cm),
hematitic chert (to 6.5 cm) and cleaved quartzite (to 14 cm) in a coarse volcanic quartz and
feldspar crystal-rich matrix (Plate 4.8). Rhyolite clasts are commonly angular whereas
Precambrian-derived clasts are well rounded. The abundance of different clast types varies
widely, with rhyolite dominant at the base and the other sedimentary clasts (of Precambrian
derivation) becoming more abundant and eventually dominant, up sequence.
54�
--------------------
- - - ---------
--------
--------
o 000 0 0 0 °0° J00 Q 0 0 0.00 00
Middle Dwen Conglomerate ...:.', '. " ',',',', ',',', ','1 000°00 Qo 0 <0 o~oO °0 °0°0000'°0°°00°I--_(e_x..;;,p_o_se_d_m_o_s_tl,;"y_e_as_t_o_fth_e_st_u_dy;....,ar_e...;a)_-1-:"-":='-.!:::(.O 0 0 0 0 __---<"
Newton Creek Sandstone
(approximate thickness 600 m)
Lower Conglomerate Unit (approximate thickness 30 - 40 m)
Jukes Conglomerate (approximate thickness 50 m)
oeo1oO"t:::> oc00° 0° ........0.0 o<:l()}�
,. ::.::" ::) 0., On Cl <:>0 00.0 °0 0-0 "0 °0 ~J
<) 0 8 .,<:>" °00 DOOO°<l°OCo""J ---- .. -_.~'. . . . . . . . . . . . . .
.... -- _ ..... _~
.. ' .' .' .0 .' .' .' : :,' o • , • • • I I.
--------- - - - - -~ . . . .. ' .' .. . . . . • I • • • I • I ' 4
-..-_-- -""- ... -
Cl 0 D 0 0 0 '00ct, <0001 _~ .0 .. 0() p E) <010 00 <0/
{ '. " " " '. '. ' , " " ',( Q 00 0 "" 0 ~~ o ~ 0 0 <:> 0 vo 0 .[) .0 0 to 0 0 <0 0 0 0
)' 0 0 to ~.0$ 0 0 0 0
" ' " .. T
Figure 4.6 Idealised stratigraphic column for the Owen Conglomerate in the Basin Lake area,
Interbedded quartz sandstone and pebble-cobble conglomerate of metamorphic provenance
"'
Micaceous sandstone interbedded with pebble conglomerate of metamorphic provenance
Normally-graded units of micaceous sandstone and black mudstone
Interbedded quartz sandstone an<i.pebble-cobble conglomerate of metamorphic provenance
Interbedded crystal-rich sandstone and polymictic conglomerate-breccia of mixed volcanic and metamorphic provenance
4.4.2 Lower Conglomerate Unit
Grey-white cobble-pebble conglomerate and pink quartz sandstone constitute the Lower
Conglomerate Unit to the east of the Howards Road - Anthony Road intersection. The
Lower Conglomerate Unit is distinguished from the Newton Creek Sandstone in being
dominantly conglomeratic (cobble-pebble) and lacking mudstone beds, and from the Jukes
Conglomerate by the absence of rhyolite clasts. The Lower Conglpmerate Unit is between 30
to 40 m thick in the study area.
Conglomerates contain well-rounded clasts of Precambrian-derived cleaved quartzite (to 10
cm) and vein quartz (to 18 cm) with a coarse metamorphic quartz sand matrix (Plate 4.9).
Units are matrix-poor with clasts very closely packed. Thin discontinuous pink quartz
sandstone beds (1 to 5 cm) commonly form the top of normally graded cobble-pebble
conglomerate to sandstone units. The average clast size is between 3 to 8 cm. Some units are
reversely graded at the base, with a pebble-rich layer (approximately 5 cm thick) overlain by
a cobble to pebble-rich layer (clasts 3 to 8 cm) which grades into pebble conglomerate (clasts
1 to 2 cm) followed by pink quartz sandstone. The conglomerate part of such units is
commonly between 1.5 to 2 m thick whereas the sandstone top varies from 10 to 50 cm in
thickness.
4.4.3 Newton Creek Sandstone
The Newton Creek Sandstone (500 - 700 m thick; Terry, 1995) comprises pebble
conglomerate, medium grained micaceous sandstone and black mudstone. Good exposure of
well-bedded and well-sorted Newton Creek Sandstone occur in the Henty Canal (Plate
4.10). Jago and Brown (1989) recognised agnostid trilobites, from the post-Idamean to pre
Paytonian stage of the Late Cambrian, in shales in Newton Creek east of the Anthony Road.
In the lower Newton Creek Sandstone, micaceous sandstone, with weak internal
stratification, grading to thinly bedded laminated black mudstone is common. Pebbly
conglomerate is also common. Terry (1995) has distinguished seven separate facies in the
Newton Creek Sandstone; conglomerate, channels, thick bedded sandstone, thin bedded
sandstone, laminated siltstone and sandstone, siltstone/shale and slump sheet. Sedimentation
units comprising thickly bedded sandstone overlain by thinly bedded sandstone which
grades into mudstone are common throughout the lower Newton Creek Sandstone and
correspond to divisions A, Band E ofthe Bouma turbidite sequence (Bouma, 1962; Walker,
1984).
56�
Higher in the Newton Creek Sandstone, beds of pebble-cobble conglomerate (15 cm to >1 m
thick) with rounded quartzite clasts and coarse mica-quartz sand matrix grade into micaceous
sandstone «30 cm thick). Some conglomerate beds have a reversely-graded base and
normally-graded top. Flame structures and scours are common at the basal contacts of the
pebble-cobble conglomerate beds. In several localities, the conglomerates occupy channels,
2 to 5 m wide, which cut through the underlying stratigraphy. The cross-bedded laminated
siltstone-sandstone facies of Terry (1995) becomes more convnon in the upper Newton
Creek Sandstone. An overall upwards coarsening is observed throughout the Newton Creek
Sandstone (Terry, 1995).
4.4.4 Middle Owen Conglomerate
Purple to white, pebble to boulder, polymictic conglomerate interbedded with lenses of pink
coarse quartz sandstone comprise the Middle Owen Conglomerate on the Tyndall Range
(northeastern study area; Plate 4.11) east and north ofthe Great Lyell Fault.
Cobble conglomerate dominates the Middle Owen Conglomerate with lesser sandstone.
Conglomerate beds (to 20 m thick) are extremely clast-rich (up to 90%) and clast-supported.
Clasts of cleaved quartzite (average 10 to 15 cm) and white vein quartz (to 10 cm)
predominate with lesser chert and sandstone. Clasts are commonly well-rounded.
Conglomerates are poorly bedded and weakly graded to massive. Discontinuous, diffusely
bedded, pink, metamorphic quartz sandstone lenses a few centimetres to a few metres thick
occur interbedded with the conglomerates. Sandstone comprises less than 10% by volume of
the Middle Owen Conglomerate in this region. Some beds of massive conglomerate grade
into quartz sandstone. On the eastern edge of the study area (western side of the Tyndall
Range) units of the Middle Owen Conglomerate strike N-S, and dip and young to the west.
4.4.5 Sedimentation processes, depositional setting and provenance of the
Owen Conglomerate
Thin « 1 m) turbidites occur in the lower Newton Creek Sandstone and grade from massive
sandstone to bedded sand$tone into fine mudstone tops. Commonly the Bouma sequence
(Bouma, 1962) is not well-developed: many single units of fine sandstone and mudstone are
massive. Cross-bedding in siltstone becomes more common in the upper parts of the
Newtori Creek Sandstone. Poorly- to non-graded conglomerate is widespread in the upper
Newton Creek Sandstone and in the Middle Owen Conglomerate.
57
An overall upwards coarsening occurs throughout the Newton Creek Sandstone and Middle
Dwen Conglomerate. The influence of tractional processes, reflected in the occurrence of
cross-bedding, also increases upsequence in the Newton Creek Sandstone. Facies of the
Dwen Conglomerate have been interpreted to record deposition in an alluvial or submarine
fan setting with an upward shallowing from below-wave-base (lower Newton Creek
Sandstone) to above-wave-base (upper Newton Creek Sandstone, Middle Dwen
Conglomerate) conditions (Banks and Baillie, 1989). The well sqrted and well rounded clast
population indicate significant reworking in a high energy environment prior to final
deposition. The abundance of pebble to cobble conglomerate throughout the Dwen
Conglomerate suggests that topographic relief was sharp and that volumes of detritus were
high (Rust and Koster, 1984).
The siliciclastic detritus that dominates most of the Dwen Conglomerate was sourced from
the Precambrian Tyennan Region to the east (Banks and Baillie, 1989; Corbett and Solomon,
1989). The Jukes Conglomerate is an exception, containing both Precambrian metamorphic
and Cambrian volcanic clasts. The Jukes Conglomerate records the transition from the
volcanic lithofacies of the Mount Read Volcanics to lithofacies dominated by Precambrian
metamorphic detritus of the Dwen Conglomerate.
4.5� Post-volcanic dykes in the Mount Julia Member and Newton Creek
Sandstone
Basalt dykes cross-cut the Mount Julia Member in drill holes NC1 and NC2 (Appendix G).
Basalt dykes and a rhyolite dyke cross-cut the Newton Creek Sandstone in the Henty Canal,
north of Howards Road. Absolute ages are not available but geological relationships restrict
emplacement of both dyke types to a period following lithification of the enclosing sediments
but prior to the main Devonian deformation event (Chapter 3). The geochemistry of these
dykes is discussed in Chapter 6.
4.5.1� Basalt dykes
Basalt dykes in the Mount Julia Member vary in width from 0.4 m to 2.2 m. Top and bottom
contacts are sharp with chilled margins less than 10 cm wide. The dykes are vesicular with <
5 vol% vesicles now filled by either white calcite or dark green chlorite. The dykes have
magnetic susceptibilities of -2079 x 10-5 SI, reflecting the presence of fine magnetite. A
weak cleavage is observed in the dykes. Tabular plagioclase phenocrysts to 1 mm are
abundant (25-35 %) and minor clinopyroxene (-5 %) is also present. Alteration of the dykes
58
•
is quite intense with carbonate±epidote replacing plagioclase, chlorite replacing augite and
carbonate-chlorite±pyrite throughout the groundmass.
Two basaltic dykes, one 30 cm wide and one 1 m wide, are exposed cross-cutting the
Newton Creek Sandstone. A chilled margin is evident in the larger of the two dykes (Terry,
1995). The dykes have a sub-ophitic doleritic texture with abundant plagioclase laths (to
-1.5 mm; 50 %) partially enclosed by high-Ti augite (Plate 4.12). Plagioclase is fresh
whereas augite is commonly, though not always, altered to carbonate and chlorite. Trace
amounts of olivine replaced by chlorite and FeTi oxides altered to pyrite are also present.
Carbonate and chlorite alteration is quite strong throughout the dykes.
4.5.2 Rhyolite dyke
A 1 to 2 m x 20 m long quartz-feldspar-phyric rhyolite dyke cross-cutting bedding in
micaceous sandstone and black mudstone of the Newton Creek Sandstone is exposed in the
wall of the Henty Canal. The dyke has sharp contacts which have been strongly sheared.
Flow banding is developed in a 30 to 50 cm zone at both contacts with banding running
parallel to the contacts. The centre of the dyke is massive and unbanded.
The dyke has phenocrysts of embayed quartz (to -1.5 mm; <5 %) and euhedral feldspar (1
mm; 10-15 %). Feldspar is strongly altered to sericite and epidote and only ghosts of the
original phenocrysts remain. The groundmass was originally glassy with relic perlite
fractures, and has devitrified to a quartzo-feldspathic intergrowth; sericite and lesser pyrite
alteration is strong in most of the dyke.
Sedimentary rocks enclosing the dyke display no evidence of interaction or disruption caused
by emplacement, and were probably already lithified at the ti;IDe of emplacement. The
Newton Creek Sandstone and rhyolite dyke have been folded and cleaved. A reverse fault
cuts the dyke though offset is less than 1 m.
59�
4.6 Stratigraphic relationships
4.6.1 Stratigraphic relationships in the Basin Lake area
The contact between the Yolande River Sequence and the Anthony Road Andesite is marked
by strong cleavage development and intense silica-sericite-pyrite alteration at the "pyrite
corner" fault zone on the Anthony Road. Units of coarse feldsp~-pumice sandstone of the
Yolande River Sequence are interbedded with matrix-supported andesitic breccia of the
Anthony Road Andesite in drill hole BLD89-1 whereas in the Leech Hill drill hole (Appendix
G), the Yolande River Sequence is intruded by hornblende andesite dykes of the Anthony
Road Andesite (refer Chapter 5). It is thus apparent that the Anthony Road Andesite
intrudes, interfingers with, and also overlies the Yolande River Sequence.
In drillcore, andesitic-dacitic volcaniclastic sandstone in the upper Anthony Road Andesite
interfingers with volcaniclastic sandstone of the Lynchford Member of the Tyndall Group
(e.g., BLD89-3, HA3, HA5, TYN1; Appendix G). On the Anthony Road, clasts of
hornblende andesite occur in volcaniclastic sandstones of the Lynchford Member that directly
overlie massive hornblende andesite. Elsewhere in outcrop, this contact is commonly
strongly faulted.
Along the eastern side of the study area, the Great Lyell Fault separates the Tyndall Group to
the west from the Owen Conglomerate to the east. However, on Howards Road near the
western edge of the study area, volcaniclastic conglomerate and sandstone of the Tyndall
Group pass conformably into the Jukes Conglomerate at the base of the Owen
Conglomerate. Although there is a distinct change from volcanic to metamorphic clast types
between the Tyndall Group and the Owen Conglomerate, the change appears to take place
gradually through the Jukes Conglomerate.
4.6.2 Comparison of stratigraphic relationships in the Basin Lake area with
the southern Mount Read Volcanics
Stratigraphic relationships between the major lithostratigraphic units vary across the Mount
Read Volcanics and differ in places from the relationships observed in the Basin Lake area.
The Antl}ony Road Andesite intrudes, interfingers with, and also overlies the Yolande River
Sequence in the Basin Lake area. Broadly similar andesites and basalts (Lynch Creek
basalts) at Lynchford also intrude the upper Yolande River Sequence (Gibson, 1991;
60�
Corbett, 1992). These locations are 20 km apart and andesites and basalts are not continuous
in between.
In the study area, volcaniclastic sandstone of the Anthony Road Andesite is interbedded with
limestone and volcaniclastic sandstone of the Lynchford Member of the Tyndall Group. At
Comstock, the Tyndall Group interfingers with underlying andesitic rocks of the Crown Hill
andesite whereas in the Lynchfordarea, the Tyndall Group overlaps andesitic rocks (Lynch
Creek basalts) and close by sits directly on the Yolande River Sequence (Corbett, 1992).
Elsewhere, an erosional unconformity occurs between the Tyndall Group and the underlying
Mount Read Volcanics (e.g., South Darwin; Jones, 1993). The Tyndall Group may
conformably overlie the Anthony Road Andesite in the Basin Lake area due to deposition in a
relatively deep, central part of the basin, not affected by uplift or granite intrusion.
The Tyndall Group passes conformably into the Owen Conglomerate in the Basin Lake area
whereas elsewhere, an erosional unconformity occurs at the contact between these two
lithostratigraphic units (e.g., South Darwin, Lynchford, Comstock; Corbett, 1992; Jones,
1993). At Henty, 2 km to the north of the Basin Lake area, the Newton Creek Sandstone is
interbedded with volcanic conglomerates of the Tyndall Group (Halley and Roberts, 1997).
4.7 Summary of depositional setting and provenance of the study area
The Yolande River Sequence, Tyndall Group and Owen Conglomerate are dominantly
composed of sedimentary and volcaniclastic lithofacies; andesitic lavas and syn-volcanic
intrusions are abundant in the Anthony Road Andesite (Chapter 5) and rhyolite lava occurs in
the Zig Zag Hill Formation. The facies characteristics of the sedimentary and volcaniclastic
lithofacies provide constraints on the setting of deposition and the provenance.
The black mudstone in the Yolande River Sequence is non-volcanic and represents the
ambient hemipelagic sedimentation. Mass-flow units in both the Yolande River Sequence
and Tyndall Group are mainly composed of volcanic particles. The association of
volcaniclastic turbidites and black mudstone in the Yolande River Sequence suggest
deposition in a below-wave-base relatively deep water setting. The large volumes of
feldspar-phyric pumice and crystal fragments in the volcaniclastic turbidites were probably
derived from large-volume dacitic explosive eruptions, either at the basin margins or at
shallow water intrabasinal vents.
Thick sequences of mass-flow units without finer interbeds are present in the Mount Julia
Member, suggesting continual supply of large volumes of volcanic sediment into the basin.
61�
Corbett, 1992). These locations are 20 km apart and andesites and basalts are not continuous
in between.
In the study area, volcaniclastic sandstone of the Anthony Road Andesite is interbedded with
limestone and volcaniclastic sandstone of the Lynchford Member of the Tyndall Group. At
Cornstock, the Tyndall Group interfingers with underlying andesitic rocks of the Crown Hill
andesite whereas in the Lynchfordarea, the Tyndall Group overlaps andesitic rocks (Lynch
Creek basalts) and close by sits directly on the Yolande River Sequence (Corbett, 1992).
Elsewhere, an erosional unconforrnity occurs between the Tyndall Group and the underlying
Mount Read Volcanics (e.g., South Darwin; Jones, 1993). The Tyndall Group may
conformably overlie the Anthony Road Andesite in the Basin Lake area due to deposition in a
relatively deep, central part of the basin, not affected by uplift or granite intrusion.
The Tyndall Group passes conformably into the Dwen Conglomerate in the Basin Lake area
whereas elsewhere, an erosional unconformity occurs at the contact between these two
lithostratigraphic units (e.g., South Darwin, Lynchford, Comstock; Corbett, 1992; Jones,
1993). At Henty, 2 km to the north of the Basin Lake area, the Newton Creek Sandstone is
interbedded with volcanic conglomerates of the Tyndall Group (Halley and Roberts, 1997).
4. 7 Summary of depositional setting and provenance of the study area
The Yolande River Sequence, Tyndall Group and Dwen Conglomerate are dominantly
composed of sedimentary and volcaniclastic lithofacies; andesitic lavas and syn-volcanic
intrusions are abundant in the Anthony Road Andesite (Chapter 5) and rhyolite lava occurs in
the Zig Zag Hill Formation. The facies characteristics of the sedimentary and volcaniclastic
lithofacies provide constraints on the setting of deposition and the provenance.
The black mudstone in the Yolande River Sequence is non-volcanic and represents the
ambient hemipelagic sedimentation. Mass-flow units in both the Yolande River Sequence
and Tyndall Group are mainly composed of volcanic particles. The association of
volcaniclastic turbidites and black mudstone in the Yolande River Sequence suggest
deposition in a below-wave-base relatively deep water setting. The large volumes of
feldspar-phyric pumice and crystal fragments in the volcaniclastic turbidites were probably
derived from large-volume dacitic explosive eruptions, either at the basin margins or at
shallow water intrabasinal vents.
Thick sequences of mass-flow units without finer interbeds are present in the Mount Julia
Member, suggesting continual supply of large volumes of volcanic sediment into the basin.
61�
Mass-flows are crystal-rich and contain blocky to angular volcanic clasts which suggest
these units are syn-eruptive and their components have undergone little reworking before
final deposition. The Zig Zag Hill Formation, in contrast to the Comstock Formation,
contains mass-flow units with rounded volcanic clasts, suggesting significant clast.
reworking prior to final deposition.
The Tyndall Group contains well graded volcaniclastic mass-floW units and lacks tractional
bedforms, suggesting that deposition has occurred in a submarine setting in below-wave
base conditions. The coarser mass-flow units and the absence of black mudstone in the
Mount Julia Member and Zig Zag Hill Formation may indicate deposition of the Tyndall
Group in shallower water conditions and in a more proximal environment than the Yolande
River Sequence.
The abundance of pyroclasts, large-volume units and occurrence of welded ignimbrite clasts
suggest the Tyndall Group has been sourced from subaerial or shallow marine explosive
eruptions of rhyolitic and dacitic composition.
The Owen Conglomerate records an upward shallowing from below-wave-base to above
wave-base conditions with deposition in an alluvial to submarine fan environment (Banks
and Baillie, 1989; Terry, 1995). The Owen Conglomerate is distinct in comprising
sedimentary facies of Precambrian metamorphic provenance and recording significant
reworking in a high energy environment prior to final deposition. The influx of metamorphic
detritus in the Owen Conglomerate has been inferred to reflect uplift of the Precambrian
Tyennan Region during the Cambro-Ordovician Jukesian Orogeny (Banks and Baillie, 1989;
Corbett and Turner, 1989).
62�
Chapter 5
Lithofacies, facies associations and facies architecture of the Anthony Road Andesite
Chapter 5�
Lithofacies, facies associations and facies architecture of the� Anthony Road Andesite�
f·
5.1 Introduction
The Anthony Road Andesite comprises coherent and volcaniclastic facies ranging in
composition from basaltic andesite through to rhyolite. The term Anthony Road Andesite is
used throughout this study, not as a descriptive term, but as an informal name for the
lithostratigraphic unit which includes those rocks overlying the Yolande River Sequence and
underlying the Tyndall Group (Figure 4.1). The Anthony Road Andesite is the second most
voluminous (behind the Que-Hellyer Volcanics; Corbett, 1992) dominantly intermediate
composition volcanic unit in the Mount Read Volcanics. The Anthony Road Andesite does
not have simple internal stratigraphy because facies change in nature and thickness over short
lateral and stratigraphic intervals.
The Anthony Road Andesite is here subdivided into four facies associations, three of which
are volcanic and one being non-volcanic. The three volcanic facies associations, primarily
defined on the basis of texture, mineralogy and composition, are: 1. basaltic andesite; 2.
hornblende andesite, and 3. rhyolite. The volcanic facies associations are interleaved with a
non-volcanic sedimentary facies association comprising black mudstone, limestone and
ironstone.
This chapter discusses the characteristics of each of the four facies associations of the
Anthony Road Andesite, and provides an interpretation of the facies architecture. Aspects of
the geochemistry of the basaltic andesite, hornblende andesite and rhyolite associations are
reported in Chapter 6.
Throughout this chapter the volcanological terminology used is that employed by Cas and
Wright (1987) and McPhie et al. (1993) unless otherwise referenced. Five terms used
commonly in this chapter are defined below:
Volcanic - includes lavas, and lithified and unconsolidated volcaniclastic aggregates
(McPhie et al., 1993).
63
Coherent - solidified lava or magma (McPhie et al., 1993).
Volcaniclastic - composed predominantly of volcanic particles (Fisher, 1961).
Hyaloclastite - clastic aggregate formed by quench fragmentation of lavas and intrusions
(Cas and Wright, 1987; McPhie et al., 1993).
Peperite - rock generated by mixing of lava or magma with unconsolidated wet sediment
(Busby-Spera and White, 1987).
5.2 Basaltic andesite association
This association has only been observed in drillcore where it occurs in the upper Anthony
Road Andesite, close to the contact with the overlying Tyndall Group. The dominant
lithology is pyroxene-feldspar-phyric basaltic andesite. The intensity of alteration varies
significantly: in the southern part of the Basin Lake E.L., the association is relatively fresh
but to the north on the Lake Margaret E.L. (drill holes NC1, NC2), hematite-carbonate
alteration is ubiquitous (Appendix G). Petrography, mineral chemistry and wholerock
geochemistry concentrated on comparatively fresh samples from the southern Basin Lake
area.
In addition, stratigraphic thickness, volumes of lavas and shallow intrusions and the degree
of vesicularity differ north and south. The thickness of the association is difficult to
accurately ascertain in drill holes on the Lake Margaret E.L. because of the intensity of
alteration. In drill hole NC1, the thickness exceeds 200 m downhole (approx. 150 m true
thickness). Thicknesses are less than 10 m in drill holes on the Basin Lake E.L. Large
volumes of moderately vesicular to amygdaloidal breccia are present on the Lake Margaret
E.L. whereas the southern Basin Lake area is dominated by non- to poorly vesicular lavas
and shallow intrusions.
5.2.1 Pyroxene-feldspar-phyric basaltic andesite
Weakly Ghlorite-carbonate altered pyroxene-feldspar-phyric basaltic andesite have been
intersected in drill holes BL4, BL5 and BLD89-3 in the southern Basin Lake area (Appendix
G). Four units of basaltic andesite occur in drill hole BL4, five units in drill hole BL5 and
three units in drill hole BLD89-3 (Figure 5.1).
64�
..
The basaltic andesite is strongly augite-plagioclase-phyric (Plate 5.1). Augite occurs as
euhedral phenocrysts (0.9 mm) and microphenocrysts (0.15 mm) in the groundmass.
Plagioclase phenocrysts (2.5 mm) show albite and carlsbad twinning. Some plagioclase
phenocrysts have clinopyroxene rims. Feldspar also occurs as tabular microlites in the
groundmass. Minor crystal phases include apatite, magnetite and Cr-spine!. Cr-spinel is
present as honey brown microphenocrysts (0.2 mm) in the groun'dmass, and as inclusions in
augite phenocrysts. Augite is the dominant phenocryst phase « 24 modal %) whereas
plagioclase comprises up to 14 modal % (Table 5.1). Augite occasionally occurs in
glomeroporphyritic aggregates of up to 20 crystals. Euhedral phenocrysts of apatite occur
throughout but do not exceed 1 modal %.
Other uncommon components are strongly embayed quartz grains, most probably
xenocrysts, and gabbroic xenoliths consisting of intergrown augite and feldspar. The
groundmass of the basaltic andesites constitutes between 63 and 69 modal %. The basaltic
andesites are poorly vesicular, less than 6 modal % vesicles, with ovoid vesicles filled by
carbonate, quartz and chlorite or a combination. Coherent basaltic andesites are
hypocrystalline though the original glassy component is now recrystallised to a fine grained
quartzo-feldspathic intergrowth. Feldspar microlites in the groundmass of several samples
define a trachytic texture. The long axes of the smaller augite phenocrysts and the ovoid
vesicles are also aligned subparallel to the microlites of feldspar.
Alteration in the coherent basaltic andesites in the southern Basin Lake area is moderate, with
fresh augite but plagioclase phenocrysts commonly being replaced by sericite and
occasionally by chlorite and carbonate.
Table 5.1. Average modal compositions of lava and shallow intrusions from the basaltic andesite, hornblende andesite and rhyolite associations of the Anthony Road Andesite. Established by point counting.
Basaltic andesite Hornblende andesite Rhyolite Mineral n= 3; modal % n= 5; modal % n= 3; modal %
Quartz - 2 8 Plagioclase 13 22 17 Amphibole - 13
Biotite - - 1 Apati~ < 1 <1
Clinopyroxene 21 1.5 FeTiOxide (± Pyrite) < 1 1.2 6.5
Groundmass 61 60 67 Vesicles 3
65�
5.2.2 Facies architecture of the basaltic andesite association
The basaltic andesite is commonly massive though some units have brecciated margins.
Where brecciated, clasts are angular and the clast population is strictly monomictic. Clasts in
the brecciated margins commonly have a jigsaw-fit texture (in situ breccia) and grade into
coherent basaltic andesite towards the central parts of the lenses. Unbrecciated, finer grained
chilled margins 10 and 25 cm wide are present in some of the bas'altic andesite units. In drill
hole BU, the four units of basaltic andesite have black mudstone both above and below. The
contacts between basaltic andesite and black mudstone are commonly interdigitating and
irregular, indicating that the basaltic andesite has intruded the mudstone. In places close to
the contact, laminae in the mudstone have been disrupted or destroyed suggesting that the
mud was unconsolidated when the basaltic andesite was emplaced (cf. Kokelaar, 1982).
Fluidal clasts of basaltic andesite also occur within the mudstone. These disrupted contact
zones and the mixtures of basaltic andesite and mudstone are examples of fluidal peperite (cf.
Busby-Spera and White, 1987; McPhie et al., 1993). The occurrence of fluidal peperite at
both top and bottom contacts, indicates that the basaltic andesite intruded wet unconsolidated
mud and that intrusion was essentially syn-depositional (Kokelaar, 1982; Busby-Spera and
White, 1987).
An interpretation of the processes responsible for the facies relationships in the basaltic
andesite association, based primarily on drill hole BL4, is shown in Figure 5.2.
66�
• •
DDHBL5 DDHBLD89-3 Grainsize (mm) Grainsize (mm) -\0 C'l~ ~ .....Depth (m) :::; ~ <:'t 00 ~ Depth (m) :::; - <:'t 00 \0"'"
0 0
~ 1 ~lA
" ,,. \\50 --I ,50
~
~
100 11100~ 1 ~
150 -I , , '\ 150 -I \\
~
200 200
250 250
\\ 300300 1 If -/4 G Basaltic andesite
association
E;] 350~· 350~,\ Rhyolite association _ F4v\t
Coherent hornblende Purniceous sandstone, ~
~[!] andesite ~ Yolande River Sequence
400
~~~I Brecciated hornblende D Black mudstone andesite
• ~ ... 1 Andesitic sandstone Pebbly conglomerate/sandstone [J.. [:lJ Figure 5.1. Summary graphic logs of the Anthony Road Andesite in diamond drill holes BL5 and BLD89-3 from
the southern Basin Lake area. Detailed drill logs are given in Appendix G.
DDHBL4
Grainsize (mm) \0 C'l
Depth (m) S ~ N 00 ~
oSubmarine basaltic andesite intruding -
50 -""_--'---....
-
--
-Disrupted mud substrate
-- -~----
,.
---".--- --
100
0\ 00 Basaltic andesite association 150
~ D- -- - -
Pyroxene-feldspar-phyric basaltic andesite
Mudstone 200
Matrix-supported andesitic breccia
Andesitic breccia facies
Massive facies
Hornblende andesite association
[2J IESESI I~·:~:·I ~ ~
Feldspar - quartz sandstone/siltstone
250
Figure 5.2. Cartoon showing emplacement of the basaltic andesite association and a representative graphic log through 300
the association (BL4). Not to scale.
5.3 Hornblende andesite association
The hornblende andesite facies association is the most voluminous of the three volcanic
facies associations and comprises in the order of 70 vol% of the Anthony Road Andesite. It
is easily distinguished by being strongly porphyritic, with ubiquitous black hornblende
crystals up to 12 mm in diameter. The association consists of four facies: one coherent and ~--
three volcaniclastic. This association occurs over a 7 km strike length yet geochemical studies
indicate very little compositional variation (Chapter 6). There is a range from andesite
through to dacite (Si02 56 to 66 wt%). Feldspar and magnetite also occur as phenocryst
phases. The presence of magnetite, up to 1 to 2 modal %, accounts for the strongly magnetic
character of relatively fresh samples of this association. Parts of the association are affected
by alteration and magnetite has been destroyed; these parts are non-magnetic (Chapter 7).
5.3.1 Massive facies
The massive facies of the hornblende andesite association is widely exposed in the study
region and is common in drill holes in the Howards Anomaly (HA2, HA3, HA4) and
southern Basin Lake regions (BL5, BLD89-1, BLD89-3; Appendix G). Andesites and
dacites are evenly porphyritic containing phenocrysts of feldspar, hornblende,
clinopyroxene, magnetite, apatite and quartz in a microcrystalline groundmass (Plate 5.2).
Single units range from 5 m (BL5, HA3) to more than 50 m (TYN5, BLD89-2, BL1;
Appendix G), often with little apparent variation in phenocryst abundance and type.
Plagioclase feldspar (17 to 27 modal %) and hornblende (6 to 16 modal%) are the most
abundant phenocryst phases with clinopyroxene « 2.5 modal%; < 1 mm), magnetite « 3
modal%), apatite « 2 modal%) and quartz « 3.5 modal%; 1.6 mm) present in only minor
volumes (Table 5.1). Microphenocrysts of zircon are also present. Groundmass comprises
between 50 and 69 modal%.
Euhedral to resorbed plagioclase phenocrysts (3.2 mm) display albite twinning. Plagioclase
also occurs as tabular microlites in the groundmass. Plagioclase phenocrysts are commonly
albitised and locally replaced by disseminated chlorite, sericite, pumpellyite and epidote.
Coarse hornblende phenocrysts (0.3 mm to 12 mm) display brown to khaki green
pleochroism and are partly replaced by chlorite, epidote and calcite. Hornblende phenocrysts
commonly contain inclusions of plagioclase (to 0.7 mm), apatite and magnetite indicating the
synchronous crystallisation of hornblende and these phases.
69�
Magnetite and apatite phenocrysts are commonly closely associated, euhedral apatite (to 0.3
mm) often being enclosed within a magnetite phenocryst. The local abundance of apatite is
reflected in P20S contents of up to 0.42 wt% (Chapter 6). In altered samples, magnetite is
replaced by leucoxene and chlorite.
The groundmass in the massive fa.cies varies from holocrystalline to holohyaline, although
the originally glassy component has divitrified to a fine grained quartzo-feldspathic
intergrowth.
Outcrops of this facies are typically massive. Columnar jointing is present at one locality to
the southwest of the Newton Dam Spillway (Plate 3.2). Xenoliths within andesite occur at
the base of the Newton Dam Spillway and in the southern Basin Lake area. Contact
relationships are variable. In drill hole Leech Hill, the massive facies (2 to 8 m wide) has
intrusive contacts with pumiceous sandstones of the Yolande River Sequence whereas in drill
hole BLl the massive facies is overlain by andesitic breccia and sandstone indicating it
extruded (Figure 5.3; Appendix G).
5 . 3 . 2 Andesitic breccia facies
The andesitic breccia fades is widespread in drillcore (BLl, BL2, BL3, BL4, HA2;
Appendix G) and occurs in outcrop along the Anthony Road (Plate 5.3). The breccia is
strictly monomictic being composed of hornblende andesite clasts, the same as in the massive
facies. The clasts are blocky, angular and pebble to cobble grade with single breccia units
ranging in thickness from 2 to 60 m. The breccia is clast-supported, containing less than
50% matrix composed of fme andesitic fragments and euhedral to broken crystals of feldspar
and hornblende. At one locality, the matrix is composed of grey mudstone (Plate 5.4). Clasts
in the andesitic breccia commonly show jigsaw-fit texture (Plate 5.4).
Drillcore sections show that massive andesite commonly grades into andesitic breccia which
in turn grades into matrix-supported andesitic breccia and andesitic sandstone (Figure 5.3).
No evidence of bedding or grading has been observed in the andesitic breccia.
70�
DDHBLl DDHHA2
Grainsize (mm) Grainsize (mm)
~ ~ ~ ~ Depth (m) ~-.4NOO ~ Depth (m) ~.-4NOO ~
O....,....-........""""r-"""-----'----'--- o ....,...._...........................-+-........._
50 50
100 100
f)
o ID 1\ 150 1\ 1\
150
Hornblende andesite association !\
200 200 .........--_--:.....1
Massive facies Andesitic breccia Matrix-supported facies andesitic breccia
Figure 5.3. Reconstruction of facies relationships in the hornblende andesite association of the Anthony Road Andesite with representative graphic logs (BL1, HA2). Not to scale. Down hole thicknesses shown on graphic logs.
Plate 5.1� Photomicrograph of pyroxene-feldspar-phyric basaltic andesite, basaltic andesite association, Anthony Road Andesite. Cross polarised light. Thin section BL4 188.7m. Magnification x5. Clinopyroxene and plagioclase are dominant. (GR 380750E, 5353825N)
iF
Plate 5.2� Photomicrograph of the massive facies, hornblende andesite association, Anthony Road Andesite. Cross polarised light. Thin section 32-3/94. Magnification x1.25. Phenocrysts of hornblende, plagioclase and quartz are observed. (GR 379290E, 5352820N)
Plate 5.3 Andesitic breccia facies (hyaloclastite), hornblende andesite association beside the Anthony Road. (GR 380200E, 5355275N)
Plate 5.4 Andesitic breccia facies (mud matrix), hornblende andesite association along the Anthony Road. m=mud matrix; h=hornblende andesite clast. (GR 380300E, 5355310N)
Plate 5.5 Photomicrograph of quartz-feldspar-phyric rhyolite, rhyolite association, Anthony Road Andesite. Cross polarised light. Thin section 71-4/94. Magnification x 1.25. Flow banding is preserved in the groundmass while coarse quartz is ubiquitous. (GR 380395E, 5358420N)
72�
5.3.3 Matrix-supported andesitic breccia
Pebble-cobble grade, matrix-supported, dominantly monomictic breccia composed of angular
clasts of hornblende andesite occurs throughout drillcore (e.g., BLl, BL5, TYNl, HA5;
Appendix G). The matrix largely comprises feldspar, hornblende and clinopyroxene crystal
fragments along with finer volcanic lithic and formerly glassy particles. Units are up to 20 m
thick, though commonly less than 5 m, with clasts ranging up t640 cm across. Intervals of
breccia are poorly sorted and occasionally grade upward into andesitic-dacitic sandstone with
a decrease in the clast abundance. In drillcore, a gradational transition from andesitic breccia
facies through to matrix-supported breccias is common (e.g., drill hole BLl; Figure 5.3).
5.3.4 Andesitic-dacitic volcaniclastic sandstone
Sandstone intercalated with the matrix-supported andesitic breccia facies is well sorted and
composed of andesitic pebbles, and broken feldspar and hornblende crystal fragments. Beds
up to 5 m thick are typically normally graded to massive and some intervals are thinly bedded
to laminated (e.g., TYNl, HA5, HA7, BL3; Appendix G). In drillcore, andesitic-dacitic
sandstone interfingers with volcaniclastic sandstone of the Lynchford Member of the Tyndall
Group (e.g., BLD89-3, HA3, HA5, TYNl; Appendix G).
5.3.5 Facies architecture of the hornblende andesite association
In drillcore, the massive facies is commonly associated with the andesitic breccia, matrix
supported andesitic breccia and andesitic-dacitic volcaniclastic sandstone facies. In drill hole
Leech Hill, the massive facies has intrusive top and bottom contact relations and is the only
facies of the hornblende andesite association present, suggesting intrusion as dykes or sills
(Appendix G).
The andesitic breccia facies represents in situ hyaloclastite, with rare blocky peperite. The
observed jigsaw-fit texture of clasts implies that fragmentation has taken place in situ with
little subsequent clast movement (McPhie et al., 1993). Blocky peperite with a jigsaw-fit
texture of hornblende andesite clasts in a mudstone matrix indicates that unconsolidated wet
mudstone was present on the basin floor at the time of andesite emplacement. The mudstone
matrix is, baked and jointed, possibly the result of induration during andesite emplacement.
The presence in drillcore of the massive facies, overlain by the andesitic breccia facies which
grades into matrix-supported andesitic breccias indicates emplacement as lavas or domes.
73�
The presence of blocky peperite suggests that parts of the lavas and domes were partly
intrusive into the unconsolidated mud substrate.
A cartoon showing the relationships among the four fades of the hornblende andesite
association, based on drill hole logging, is shown in Figure 5.3.
i:
5.4 Rhyolite association
The rhyolite association occurs at two localities 5 km apart: 1. on the northern and southern
shores of Lake Newton and in drill holes HA7, HAS and NC4 in this area (Appendix G); 2.
in the southern Basin Lake area in drill holes BL1, BL5 and BLD89-3 (Appendix G). A
rhyolite facies, a monomictic breccia facies and a breccia/sandstone facies comprise the
association and in drillcore, the three are intimately related. This association ranges from
andesite to rhyolite (Si02 57 to 76 wt%) and is characterised by the presence of quartz (up to
10 mm). The lower Si02levels in some samples are related to alteration.
5.4.1 Quartz-feldspar-phyric rhyolite facies
Quartz-feldspar rhyolite is strongly porphyritic with ubiquitous, coarse quartz phenocrysts.
Plagioclase feldspar phenocrysts are also present though commonly altered to sericite and
chlorite. The rhyolite is typically flow banded but in some cases massive, and occurs in
intervals up to 200 m in true thickness (HAS; Appendix G). Some intervals of aphyric
rhyolite (to 30 m thick) are also present (drill holes BL1, HA7 and BLD89-3; Appendix G).
Plagioclase phenocrysts (:=:; 2.2 mm) comprise 7 to 23 modal% of the rhyolites and quartz
comprises 4 to 11 modal% (Table 5.1; Plate 5.5). Quartz phenocrysts in the range 2 to 4 mm
are more abundant than the coarser phenocrysts. Quartz phenocrysts are strongly embayed,
forming intricate skeletal shapes, and intensely fractured with a jigsaw-fit texture between
fragments. Up to 38 distinct fragments occur in the fractured quartz phenocrysts. Magnetite
and possible relic biotite (now largely chlorite) make up minor proportions of the rhyolites.
The groundmass is composed of fine, feldspathic and quartzo-feldspathic intergrowths
which was possibly originally volcanic glass. Recrystallisation in places has been controlled
by the relic perlitic fractures. The groundmass feldspar is albite; chlorite and fine pyrite, less
than 0.5 mm, are also widespread. Flowbanding preserved in the groundmass is now
overprinted by chlorite alteration but is well preserved close to phenocrysts. The variation in
74�
groundmass seems to be controlled by flowbanding with flowbands containing microlites
next to flowbands where microlites are absent.
In drill holes BL5 and HA7 (Appendix G), contact relations between the rhyolite and the
hornblende andesite association are sharp. In contrast, the rhyolite appears to pass almost
gradationally into the monomicticbreccia facies of the rhyolite association in drill holes BL5,
HA7 and HAS (Appendix G). In drill holes BLD89-3 and BLl (Appendix G), aphyric
rhyolite occurs above and below coarsely porphyritic rhyolite.
5.4.2 Monomictic breccia facies
Monomictic quartz-feldspar rhyolite breccia is common in this association, and intersected in
drill holes HA7, HAS and BL5 (Appendix G). In some places, clasts show a jigsaw-fit
texture that passes into monomictic breccia with no jigsaw-fit texture (e.g., HA7; Figure
5.4). Clasts to 20 cm across are angular to blocky, commonly flowbanded, and texturally
identical to the quartz-feldspar-phyric rhyolite facies. Coarse quartz phenocrysts are
ubiquitous in the clasts. Intervals of monomictic breccia are up to 10 m thick though 2 to 5 m
thick intervals are more common. The monomictic breccia facies shows close spatial
association with the rhyolitic volcaniclastic breccia/sandstone facies.
5.4.3 Rhyolitic volcaniclastic breccia/sandstone
Volcaniclastic matrix-supported breccia and sandstone, characterised by coarse (up to 10
mm) embayed quartz crystals, occur in drill holes HA7, HAS, BL5 and NC4 (Appendix G).
This facies occurs between units of quartz-feldspar-phyric rhyolite, and overlying the
monomictic breccia facies in drill hole NC4. Clasts of porphyritic rhyolite are up to 15 cm,
angular, blocky, and monomictic with a matrix of quartz and feldspar crystal fragments.
Volcaniclastic breccia/sandstone units are massive and non-graded.
75�
DDHHA7 DDHHA8
·Rhyolitic volcaniclastic breccia/sandstone
Monomictic breccia facies
Grainsize (mm) \0
S S N 00 ~Depth (m)
O...,...-----'-----'-----L----L----'-
\\
Grainsize (mm) \0
S S N 00 ~Depth (m) O...,...------'-----L.......----'---''---
-+
50
-....l 0\
..,..,...
-
-....
-- ~
-
~
-
\\ + .-:::: \\ ~
Partly extrusive rhyolitic cryptodome
---.
---
~ -
............ --
"'\..'- -='(J'-
'\-
'"""""' '--t
O~Q./
-- --. ---... -
100
150
200
100
150
2"00
I;
t~
~
\\t
T
n
+ ~
+
I~+ "'\ I Quartz-feldsparphyric rhyolite
Q~~OC5~ ~
Monomictic breccia facies
I~·:~:I Rhyolitic volcaniclastic breccia/sandstone
0- -- - -Mudstone 250 250
/1
+ \\
Figure 5.4. Reconstruction of facies relationships in the rhyolite association of the Anthony Road Andesite and two representative graphic logs (HA7, HAS). Not to scale.
5.4.4 Facies architecture of the rhyolite association
All facies of the rhyolite association contain distinctive coarse embayed quartz
crystals/phenocrysts and are thus considered to be genetically related. In drill hole NC4,
quartz-feldspar-phyric rhyolite is overlain by the monomictic breccia facies which in turn is
overlain by breccia and sandstone of the rhyolitic volcaniclastic breccialsandstone facies.
Monomictic breccia also overlies rhyolite in drill holes HA7 anlBL5 (Appendix G). In drill
holes BLD89-3 and BLl, coarsely porphyritic rhyolite has aphyric rhyolite, which may
represent a chilled margin, above and below (Appendix G). Thus, the rhyolitic volcaniclastic
breccialsandstone facies and monomictic breccia facies were most likely derived from the
quartz-feldspar rhyolite.
The combined facies relationships suggest that the rhyolite was intruded as domes (at least
300m x 200m x 200m), some of which were partly extrusive (Figure 5.4), and onlapped by
aprons ofhyaloclastite (monomictic breccia facies). The absence of intervening sedimentary
facies suggests not only a spatial but also temporal association between the emergent dome,
the monomict breccias and rhyolitic sandstones (Figure 5.4).
Interpreted relationships for the rhyolite association are shown in Figure 5.4.
5.5 Sedimentary facies association (non-volcanic)
Three non-volcanic sedimentary facies are present: 1. black mudstone; 2. limestone and 3.
ironstone. Although the volume of the sedimentary facies association is small « 5 %), the
association is widespread and important in constraining the depositional environment of the
Anthony Road Andesite.
5.5.1 Black mudstone fades
Black to grey mudstone is intercalated with the volcanic facies associations of the Anthony
Road Andesite (drill holes BLl, BL2, BL4, BL5, HA7, TYNl; Appendix G) throughout the
study area, especially in the lower to middle parts.
Mudst0l1:e in the Anthony Road Andesite varies from being massive and structureless to
thinly laminated. Mudstone intervals are commonly in the order of lO's of centimetres to a
few metres in thickness though much thicker intervals occur (up to 20 m in drill hole BL4;
Appendix G). Very thin beds of pyrite « 2 cm thick) and scattered euhedral and framboidal
77�
pyrite grains are common and suggest reducing conditions during deposition. The black
mudstone units generally have sharp contacts with interbedded much coarser volcaniclastic
facies.
5.5.2 Limestone fades !i:
Bedded to massive limestone is most common adjacent to and forming the matrix of the
matrix-supported andesitic breccia facies of the hornblende andesite association (drill holes
TYN3, TYN4, TYN5, BLl and BL5; Appendix G) in the middle and towards the top of the
Anthony Road Andesite. Limestone is similar in texture to the limestone which occurs in the
Lynchford Member of the Tyndall Group (Chapter 4). Limestone intervals are 2 to 5 m thick
though intervals up to 15 m thick occur (TYN5; Appendix G). Massive limestone is largely
comprised of calcite (white) whereas bedded limestone contains crystal fragments and clastic
detritus and is commonly hematitic to chloritic (pink to green). Beds are commonly planar
and range up to 10 cm in thickness.
5.5.3 Ironstone fades
Deep red to purple, quartz and hematite-rich ironstone, referred to as the Lake Newton
Ironstone (Chapter 8), outcrops on the shore of Lake Newton. Outcrop is limited by
vegetation and by fluctuating water levels in Lake Newton. In this area, ironstone was
observed in two circumstances: 1. as matrix in the matrix-supported andesitic breccia facies
of the hornblende andesite association and 2. as intervals of diffusely laminated ironstone (up
to 50 cm thick). Ironstone has not been intersected in drill holes and based on outcrop around
Lake Newton occurs over a stratigraphic interval of 15 to 20 m. Ironstone in the matrix of
polymictic volcanic breccia comprises the majority of the stratigraphic interval.
The ironstone occurs close to the stratigraphic top of the Anthony Road Andesite, at a similar
position to the limestone facies, and is overlain by the Tyndall Group. The ironstone facies is
discussed in greater detail in Chapter 8.
78�
SEDIMENTARY� FACIES�
ASSOCIATION�
RHYOLITE� ASSOCIATION�
HORNBLENDE� ANDESITE�
ASSOCIATION�
BASALTIC� ANDESITE�
ASSOCIATION�
limestone
ironstone
black mudstone I - - - I monomict breccia
';·.-·l,l:'-.~.. 't~ .• '~l~ • I 'I .: I •• '''; l ' , I ~ _.
'( ~ I \. I' !)' ~ ~ , ~, \ 1,
l'j";'I~' -, ~ ~:l'j'l'j ~ ., • _ 0
.' 1',(\ ,I 1,- 11 ".',
andesitic breccia facies
pyroxene-feldspar-phyric basaltic andesite
Figure 5.5 Cartoon of the interpreted facies architecture of the Anthony Road Andesite in the Basin Lake area.
5.6 Facies architecture of the Anthony Road Andesite
An understanding of the facies associations in the Anthony Road Andesite allows the facies
architecture to be reconstructed (Figure 5.5). The depositional environment, eruption and
emplacement processes, proximity to source vents and character of the volcanic centre are
discussed within this section on the facies architecture. f
5.6.1 Depositional environment
The occurrence of turbidites and black mudstones in the Yolande River Sequence (Chapter
4), underlying the Anthony Road Andesite, and of well graded mass-flow units and the
absence of evidence for tractional processes in the Tyndall Group, above the Anthony Road
Andesite, suggest that the depositional environment in the Basin Lake area was submarine
and below wave base.
Background sedimentation in the Anthony Road Andesite is represented by black mudstone
units which are intercalated with the volcanic associations. The mudstones are typical of
hemipelagic sediment settled from suspension. At the Henty Mine, to the north, limestone at
a similar stratigraphic level to that in the Basin Lake area contains a fossil assemblage
indicative of a shallow-water depositional environment (Halley and Roberts, 1997). The
absence of fossils in limestone in the study area may indicate deeper water conditions.
Within the volcanic associations of the Anthony Road Andesite, the main indicator of a
subaqueous setting is the presence of peperite and widespread hyaloclastite, and the lack of
tractional bedform features in volcaniclastic facies supports a below-wave-base setting.
5.6.2 Eruption and emplacement processes
The Anthony Road Andesite comprises exclusively effusive facies (e.g. lavas, domes) and
shallow syn-volcanic intrusions (e.g. sills, cryptodomes?, dykes). Because the Anthony
Road Andesite was erupted in a wholly submarine setting, the lavas and domes comprise
associations of coherent, in situ hyaloclastite and residemented hyaloclastite facies. Syn
volcanic intrusions can be recognised by peperitic margins. Evidence for extrusion comes
from th,e transitional contacts between massive andesite, in situ hyaloclastite and
resedimented hyaloclastite, and from the numerous andesite-derived volcaniclastic units
throughout the Anthony Road Andesite. Evidence for intrusions is based on observed
intrusive contacts and the occurrence of blocky peperite.
80
The basaltic andesite association consists of thin (10 m) lavas and shallow syn-volcanic sills
with fluidal peperitic margins.
Intrusive contacts to thin hornblende andesite units, with Yolande River Sequence above and
below, indicate the emplacement of andesite dykes. A close spatial association exists
between massive hornblende andesite, with intrusive and pepeiitic contacts, and derived
volcaniclastic deposits (e.g., in situ hyaloclastite, breccia and sandstone).
Domes, partly emergent domes and related autoclastic and volcaniclastic facies of the rhyolite
association occur at two separate localities in the study area. The presence of rhyolite-derived
volcaniclastic deposits transitionally overlying massive and brecciated rhyolite indicates
partial extrusion synchronous with sedimentation (Figure 5.5). The margins to the rhyolite
domes and intrusions are commonly quench fragmented as a result of interaction with
seawater and/or wet sediments. Domes with chilled margins also occur and appear to be
wholly intrusive with sharp top and bottom contacts in drillcore.
5.6.3 Proximity to source vents
All three volcanic facies associations of the Anthony Road Andesite are dominated by lavas,
domes, shallow syn-volcanic intrusions and related autoclastic facies that represent the
proximal products of intrabasinal, intermittently active centres. The spatial association of
autoclastic and coherent facies is a common feature of proximal environments of both
submarine and subaerial andesitic, dacitic and rhyolitic volcanoes (Hackett and Houghton,
1989; Cas, 1992; Kano et aI., 1993). Intrusions and lavas occur across a six to seven
kilometre strike length suggesting that a single conduit has not been responsible for all
volcanic activity. The dominance of shallow intrusions, lavas and domes of three distinct
compositions indicates that the Anthony Road Andesite represents the exposed remnant of a
Cambrian submarine, largely intermediate, polygenetic volcanic centre.
5.6.4 Character and evolution of the volcanic centre
The Anthony Road Andesite comprises thick successions of submarine lavas, intrusions and
domes, along with associated volcaniclastic deposits, in places upwards of 1 km thick. The
internal stratigraphy is not simple, with rapid lateral and vertical facies changes. Single units
range from 2 to 3 m (e.g., black mudstone) to 150 m (e.g., rhyolite dome) thick. The
dominance of lavas and shallow intrusions, in situ hyaloclastite and redeposited hyaloclastite
81�
interbedded with mudstone, and the lack of pyroclastic deposits, are consistent with the
centre being a intermediate-felsic dome, lava and intrusive complex deposited within a
below-wave-base submarine environment (Figure 9.3; Cas and Wright, 1987; Cas, 1992).
The distribution of proximal facies throughout the Anthony Road Andesite and along a 7 km
strike length indicates the presence of numerous active volcanic centres, suggesting that
constructional topography was widespread on the seafloor. 1:,
Facies of the hornblende andesite association have been emplaced throughout the
stratigraphic history of the Anthony Road Andesite. The rhyolite and basaltic andesite
associations have only been recognised in the Lake Newton and southern Basin Lake areas,
while facies of the hornblende andesite association occur along the 7 km strike of the
Anthony Road Andesite. The hornblende andesite association comprises in the order of 60 to
70 vol% of the Anthony Road Andesite, the basaltic andesite association 5 %, the rhyolite
association 10% with non-volcanic facies comprising the remainder.
The basaltic andesite association has only been recognised towards the top of the Anthony
Road Andesite (Figure 5.5). Emplacement of the basaltic andesite association appears to
have occurred over a narrow time interval. Lavas and volcaniclastic facies of the hornblende
andesite association continued to be emplaced after the basaltic andesite association.
The rhyolite association appears to be coeval with the upper/younger parts of the hornblende
andesite association and to postdate the basaltic andesite association which it overlies in drill
hole BL5 (Appendix G).
The sedimentary facies association occurs intercalated with the volcanic facies associations
throughout the Anthony Road Andesite. Although laminated to massive black mudstone
predominates throughout, units of limestone occur in the middle and upper part of the
Anthony Road Andesite and also in the basal Lynchford Member of the overlying Tyndall
Group. Ironstone has only been observed near the top of the Anthony Road Andesite.
82�
-< 0 joooo
oO
~ ~ _. = ~ ~
~
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= ~ :3_. rI:J.
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f""l'o
. 10'
1 = ~
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~
Chapter 6
Volcanic Geochemistry of the Basin Lake area
6.1 Introduction
This chapter presents the mineral chemistry of phenocrysts and crystals, and wholerock
geochemistry of shallow intrusions and lavas within the Basin Lake area. Data comes from
volcanic units of the Anthony Road Andesite, Tyndall Group and dykes in the Owen
Conglomerate. The textural and outcrop characteristics of the facies have been discussed in
chapters 4 and 5. Comparisons with the geochemistry of the Mount Read Volcanics
regionally, and with modem volcanic successions are made. Microprobe analyses referred to
in this chapter are tabulated in Appendix D and wholerock analyses are listed in Appendix E.
6.2 Mineral chemistry
The chemistry of phenocrysts from the basaltic andesite and hornblende andesite associations
of the Anthony Road Andesite and of crystals from crystal-rich volcaniclastic sandstone
facies in the Mount Julia Member of the Tyndall Group have been determined to enable
accurate chemical classification (Table 6.1). Microprobe analyses of amphibole from crystal
rich volcaniclastic sandstone in the Mount Julia Member from the same area were made
available by Matthew White (writ. comm., 1995). Though the groundmasses of lavas and
shallow intrusions in the Anthony Road Andesite are commonly altered, the phenocrysts are
often fresh, making them ideal for mineral chemistry studies.
6.2.1 Analytical methods
Mineral phases were analysed at the Central Science Laboratory, University of Tasmania on
a Cameca SX50 electron microprobe. The use of internal standards ensured analytical
precision. Microprobe analyses have been selectively stoichiometrically recalculated to fit
with advised cationic formulas (Leake, 1978; Morimoto et al., 1988; Deer et al., 1992). The
procedure of recalculation for_ each mineral phase is described below. Microprobe analyses
used for comparison in this chapter have also been recalculated so that cationic formula units
are consistent throughout.
83
Table 6.1. Representative mineral chemistry (single analyses) of the basaltic andesite and hornblende andesite associations of the Anthony Road Andesite.
Basaltic andesite association Hornblende andesite association
Mineral Clinopyroxene Magnetite Apatite Cr-Spinel Clinopyroxene Amphibole Magnetite Apatite Analysis R7CPX R3MAG+TI R9 APTI RI SP2 Rll CPX2 RI AMP3 . R2MAGl R8APT2
wt% Si02 51.46 0.25 0.10 0.10 51.67 48.83 • 0.32 Ti02 0.20 7.36 • 0.24 0.17 1.48 5.26 • A1203 1.14 1.76 • 9.69 1.86 7.28 1.14 • Cr203 • 0.18 • 54.35 0.41 • 0.31 • Fe203 0.32 51.40 • 5.04 0.02 • 55.07 • FeO 10.94 37.34 0.18 24.10 5.58 15.65 35.14 0.26 MnO 0.35 0.49 • 0.44 0.19 0.33 0.11 • MgO 13.80 0.23 • 6.08 17.09 13.33 0.04 0.05 CaO 21.22 • 55.67 • 21.32 10.98 • 53.14 Na20 0.21 • • • 0.13 1.29 • • K20 • • • • • 0.58 • • NiO 0.04 • 0.04 0.01 • • •· PZOS 0.33 • 41.49 • 0.32 • 43.32· S 0.04 • • • 0.11 • • • Cl 0.03 • 0.08 • 0.02 0.23 • 0.34 F • • 4.12 • • 0.16 •
,~\
4.40 ZnO • 0.01 • • • • • • VZOS • • • • • • 0.77 • BaO • • om • • • • 0.02 SrO • • 0.09 • • • • 0.08
6.2.2 Clinopyroxene
Electron microprobe analyses of clinopyroxenes have been stoichiometrically recalculated on
the basis of six oxygens and four cations using the recalculation program Mintab (Rock and
Carroll, 1990; Deer et al., 1992). P20S has been removed as apatite during recalculation.
Pyroxene nomenclature used is that endorsed by the subcommittee on pyroxenes (Morimoto
et al., 1988).
Anthony Road Andesite
Clinopyroxene occurs in both the basaltic andesite and the hornblende andesite associations
of the Anthony Road Andesite. Clinopyroxene phenocrysts analysed from pyroxene
feldspar-phyric basaltic andesite belongs to the Ca-Mg-Fe pyroxene group and vary
compositionally between augite and diopside. Analyses plot close to the boundary between
the diopside and augite compositional fields on a Ca2Si206 (Wo) - Mg2Si206 (En)
Fe2Sh06 (Fs) ternary diagram (Figure 6.1). The chemical variation in clinopyroxene
compositions is W042-4sEn38-49Fs7-17. Clinopyroxenes have Ti02 from 0.12 to 0.27 wt%
and NiO contents < 0.04 wt%. The highest Cr203 content is 0.79 wt%. CaO varies between
21 to 22 wt% whereas Ah03 is <1 to 2.5 wt%. 100Mg# (where Mg# = [Mg2+/{Mg2+ +
Fe2+}] varies from 69 to 87.
Clinopyroxenes analysed from lavas and shallow intrusions of the hornblende andesite
association (massive facies) plot within the Ca-Mg-Fe pyroxene group and are augites with
one diopside (Figure 6.1). The compositional range defined by these clinopyroxenes is
W041-4sEn43-s0Fs6-13 with 100Mg# varying from 77 to 89. CaO contents are between 20 to
22 wt% and Al203 is <1 to 3.5 wt%.
The two clinopyroxene populations are distinct (Figure 6.1) although three clinopyroxene
analyses from the basaltic andesite association fall in the field of the hornblende andesite
association. Augites from the basaltic andesite association are enriched in iron relative to
those from the hornblende andesite association. This difference in iron content is also noted
in whole rock geochemical analyses of the two facies associations.
85
diopside
hedenbergite
hedenbergite 20 20
00 0\
(f)
(see enlargement)
augite
pigeonite
5 ~---------~----------~5 clinoenstatite clinoferrosilite
50
Figure 6.1. Wo-En-Fs prism showing chemistry of clinopyroxene from the basaltic andesite (squares) and hornblende andesite associations (circles) of the Anthony Road Andesite. The fields for the compositional ranges of the Ca-Mg-Fe clinopyroxenes are from Morimoto et al. (1988).
Tyndall Group
Two clinopyroxene crystals analysed from crystal-rich volcaniclastic sandstones of the
Mount Julia Member belong to the Ca-Mg-Fe pyroxene group and plot within the augite field
on a Ca2Sh06 (Wo) - Mg2Sh06 (En) - Fe2Sh06 (Fs) ternary diagram (Morimoto et aI.,
1988). The variation in composition is W043-44Bn39Fs16. Clinopyroxenes are characterised
by around 51 % Si02, 10 % FeO* and low Ti02 (0.25 to 0.30 wt%). CaO varies between
21.4 and 21.6 wt% whereas Ah03 is <1.5 wt%. 100Mg# varies from 69.78 and 70.74.
6.2.3 Amphibole
Amphibole is characteristic of the hornblende andesite facies association of the Anthony
Road Andesite. Amphibole crystals also occur in crystal-rich volcaniclastic sandstones of the
Mount Julia Member, Tyndall Group.
All amphibole analyses have been recalculated stoichiometrically on the basis of 13 cations
(where 13 = Si + Ti + Al + Fe + Mn + Mg) excluding Ca, Na and K, based on the
recommendations of a pilot study of amphibole recalculation methods by Cosca et al. (1991).
Amphibole nomenclature employed here is based on the final report of the subcommittee on
the amphibole group (Leake, 1978).
Anthony Road Andesite
Amphiboles from the hornblende andesite association belong to the calcic amphibole group
having (Ca + Na)B ~ 1.34 and NaB < 0.67, where (Ca + Na)B is the sum of Ca and Na ions
allocated to the B site in the stoichiometric formula (Leake, 1978). The amphiboles have (Na
+ K)A < 0.50 and Ti < 0.50 and are classified as magnesio-hornblendes on the calcic
amphibole classification diagram of Leake (1978; Figure 6.2). These magnesio-hornblendes
vary in Mg# from 0.67 to 0.78 and have CaB> 1.34, as is common in naturally occurring
amphiboles (Leake, 1978). The Si02 content varies between 45 and 50 wt% whereas K20
varies between 0.45 and 0.80 wt%.
Tyndall Group
Amphiboles in sandstones of the Mount Julia Member regionally (M.J.White, written comm.
1995) fit the calcic amphibole group (Figure 6.2). The amphiboles have (Na + K)A < 0.50
87
and Ti < 0.50 and are classified as actinolite, actinolitic hornblende, magnesio-hornblende,
ferro-tschermakitic hornblende and tschermakite on the calcic amphibole classification
diagram of Leake (1978; Figure 6.2). Most data fall within the magnesio-hornblende field
and it is possible that the outliers which plot as actinolite, actinolitic hornblende, ferro
tschermakitic hornblende and tschermakite represent analyses of altered amphibole
phenocrysts. The analyses vary in Mg# from 0.49 to 0.75 and have CaB> 1.34 as is
common in naturally occurring amphiboles (Leake, 1978). The Si02 content varies between
40.70 and 52.99 wt% whereas K20 varies between 0.05 and 0.72 wt%.
6.2.4 FeTi oxide (magnetite)
FeTi oxide phases in the hornblende andesite and basaltic andesite associations of the
Anthony Road Andesite are titaniferous magnetite (henceforth referred to as magnetite).
Microprobe analyses have been recalculated stoichiometrically on the basis of 32 oxygens
and 24 cations, following the recommendations of Deer et al. (1992), using the Macintosh
application Mintab (Rock and Carroll, 1990).
The magnetite phenocryst in the basaltic andesite association falls in the field defined by
magnetites in the hornblende andesite association. Recalculated Fe203 and FeO contents in
magnetites vary from 47 to 57 wt% and 32 to 37 wt% respectively. The ratio of Fe203IFeO
ranges from 1.26 to 1.74. The Fe203IFeO ratio is very high in the Anthohy Road Andesite,
possibly reflecting its oxidised nature as is common of hornblende-phyric andesites (Gill,
1981). All the analysed magnetites are characterised by relatively low ulvospinel (Fe2Ti04)
contents, ranging from USPll-23. The magnetites all plot close to magnetite on the
multicomponent spinel prism (Deer et al., 1992), reflecting their low Cr203, Al203 and MgO
contents (Figure 6.3).
Magnetites display a range in minor element abundances: Ti02 3.78 to 7.74 wt%, V20S 0.46
to 1.14 wt%, Al203 0.02 to 2.74 wt%"and Cr203 0.18 to 0.35 wt% (1232 to 2395 ppm Cr).
Magnetite cores contain greater V205, Al203 and Ti02 than magnetite rims. In one analysed
magnetite phenocryst analysed in the hornblende andesite association, the observed
difference from core to rim is 1.1 -> 0.75 (V20S wt%), 7.67 -> 6.00 (Ti02 wt%) and 0.61
-> 0.19 (A1203 wt%).
88
• • • •
TYNDALL GROUP
o Amphibole from the Anthony Road
.. Amphibole from the Cradle Mt Link Rd
1:1 Amphibole from Lynchford
ANTHONY ROAD ANDESITE
• Amphibole from the Anthony Road Andesite
I I T I I Tremolitic
Tremolite hornblende -0.9
c • .~.C
0 • ~ A~
C Coc9od'•So •00 0
scherrnakiti P +' Actinolite Actinolitic Magnesio-hornblende Tscherrnakite C'l ornblendehornblende ~ 0 +0.5 0
~
::::::'
~
=<erro-FerroFerro-actinolite actinolitic Ferro-hornblende scherrnakiti ~ Ferro-tscherrnakite
hornblende lornblende
o I I I I
8 7.75 7.5 7.25 7 6.75 6.5 6.25 6 5.75
Si
Figure 6.2. Mineral chemistry of amphibole from the hornblende andesite association of the Anthony Road Andesite (n=58) and the Mount Julia Member of the TyndalI Group regionalIy. Data from the TyndalI Group is from White (written comm., 1995). The fields and nomenclature for the calcic amphiboles are from Leake (1978).
89
Magnesioferrite (n=12)
Chromite
Magnesiochromite
Figure 6.3. Multicomponent spinel prism displaying the mineral chemistry of magnetite (n=17) from the hornblende andesite association of the Anthony Road Andesite. After Deer et al. (1992).
Magnetite or Ulvospinel Fe304orFe2Ti04
Magnesioferrite MgFe204orMg2Ti04
Spinel MgAl204
Magnesichromite MgCr204
Figure 6.4. Mineral chemistry of chromite from the basaltic andesite association of the Anthony Road Andesite (N=9) on a multicomponent spinel prism (modified after Deer et al., 1992).
90
6.2.5 Apatite
Apatite is the dominant phosphorous-bearing mineral in the Anthony Road Andesite and its
occurrence is reflected in high wholerock P20S contents, 0.57 to 0.69 wt% in the basaltic
andesite association and 0.16 to 0.42 wt% in the hornblende andesite association.
Microphenocrysts of apatite occur in the groundmass and as inclusions in magnetite and
amphibole phenocrysts. Microprobe analyses of apatite have been calculated on the basis of
26 (0, OH, F, Cl). Studies on igneous melts indicate that up to 95 % of the phosphorous
content will crystallise as apatite with the remainder crystallising in garnets (or other silicate
phases) and in phosphate phases such as monazite and xenotime (Rankama and Sahama,
1950).
Apatites from the basaltic andesite and hornblende andesite associations are fluor-apatites
with 4.1 to 5.0 wt% F, and Cl contents < 0.34 wt%. The generalised formula for fluor
apatite is Cas(P04bF (Deer et al., 1992). The occurrence of fluor-apatite is not unexpected
as it is the most common form of apatite in igneous rocks (Deer et aI., 1992). The apatite
analysed from the basaltic andesite association contains slightly less P20S (41.45 wt%) and
F (4.1 wt%), and slightly more CaO (55.7 wt%) than apatites analysed from the hornblende
andesite association which contain - 43 wt% P20S, 4.3 to 5.0 wt% F and - 53 wt% CaO.
6.2.6 er-spinel
Cr-spinel is an accessory phase, present as microphenocrysts within the groundmass and
less commonly as inclusions within clinopyroxene phenocrysts, in pyroxene-feldspar-phyric
basaltic andesite of the basaltic andesite association of the Anthony Road Andesite.
Microprobe analyses of Cr-spinels have been recalculated on the basis of 32 oxygens and 24
cations, following the recommendations of Deer et al. (1992), using the recalculation
application Mintab (Rock and Carroll, 1990).
Cr-spinel compositions vary from chromite (Fe2+Cr204) through to magnesiochromite
(MgCr204) as shown on a multicomponent spinel prism (Figure 6.4), where chromite has
Fe2+ > Mg (Deer et al., 1992). The Cr-spinels show a variation in the 100Mg# from 31 to 51
and a variation in the 100Crt (given by 100Cr/[Cr+AID from 70 to 79. The crt displays a
broad n~gative correlation with Mg#. With decreasing Cr contents (54.68 to 43.49 wt%
Cr203) the Cr-spinels display an overall increase in Fe3+ (4.96 to 9.44 wt% Fe203), Ti
(0.21 to 0.32 wt% Ti02) and AI (9.74 to 12.23 wt% AI203) contents. Replacement of Cr by
AI and Fe3+ is a common feature in natural Cr-spinels (Deer et al., 1992).
91
6.2.7 Discussion
Amphiboles in sandstone of the Mount Julia Member of the Tyndall Group differ in
composition from amphiboles in the hornblende andesite association of the Anthony Road
Andesite, and have not been sourced from that unit (Chapter 5). '
Phenocryst chemistry indicates that the hornblende andesite association has either medium-K
or high-K calc-alkaline affinities (cf. Gill, 1981). The K20 content of amphiboles increases
from medium-K through higk-K calc-alkaline rocks to shoshonitic rock types (Gill, 1981).
K20 in the magnesio-hornblendes from the hornblende andesite association (0045 to 0.80
wt%) is greater than K20 content of amphiboles in medium-K calc-alkaline volcanics (e.g.,
0040 wt% K20 at Colima, Mexico; Luhr and Carmichael, 1980) and overlaps with K20
contents of amphiboles in some high-K calc-alkaline volcanics (e.g., 0.7 wt% K20, Rio
Grande, USA; Zimmerman and Kudo, 1979; Stromboli, Italy; Crawford et al., 1992), but
lower than K20 in other high-K calc-alkaline volcanics (e.g., 1.1 wt%, Banda Arc,
Indonesia). The hornblende andesite association shows overlap in K20 content with
amphiboles in shoshonites (e.g., 0040 to 1.54 wt% K20, northern Karakorum, China; 0.66
to 0.88 wt% K20, Kamchatka; Jezek and Hutchison, 1978; Kepezhinskas, 1989; Pognante,
1990). Mfinities of the hornblende andesite association are discussed further below.
Magnetite is a common phenocryst phase in hornblende-phyric andesites (Ewart, 1976). The
absence of ilmenite as a phenocryst phase in the basaltic andesite and hornblende andesite
associations of the Anthony Road Andesite could be due to low wholerock Ti02 contents
(0.39 to 0.72 wt%; cf. Ewart, 1976). Relatively high Ti02 content is a feature ofmagnetites
in andesitic volcanic rocks in general (Duncan and Taylor, 1969; Gill, 1981).
The high Fe203/FeO ratios in the hornblende andesite association (1.26 to 1.74) possibly
reflects its oxidised nature. For comparison, Fe203/FeO ratios of magnetites in calc-alkaline
volcanics from Rabaul, Papua New Guinea, range from 1.17 to 1.43 whereas ratios of
magnetites in tholeiitic volcanics from Tonga and Niua Fo'ou Island, S.W. Pacific, range
from 0.29 to 1048 (Heming and Carmichael, 1973; Ewart, 1976).
Compositions of Cr-spinels from pyroxene-feldspar-phyric basaltic andesite lavas and
shallow intrusions in the Anthony Road Andesite overlap with compositions of Cr-spinels
from the Hellyer Core Basalt (Jack, 1989), the Lynch Creek basalt (Dower, 1991) and with
two of the analyses on detrital Cr-spinels from the Miners Ridge Sandstone (Dower, 1991;
Figure 6.5). The pyroxene-feldspar-phyric basaltic andesite from the Anthony Road
92
Andesite, the Hellyer Core Basalt (Jack, 1989) and the Lynch Creek basalt (Dower, 1991)
also have similar whole-rock and rare earth element compositions. The similarity in 100Cr#
from Cr-spinels in these three units reinforces the overall geochemical similarity. Cr-spinels
from the tholeiitic Miners Ridge Basalt display a wide range in 100Cr# (47 to 98) and in
100Mg# (2 to 74) and plot away from all the other Cr-spinels (Figure 6.5; Dower, 1991).
Cr-spinels from the basaltic andesite association have lower Cr# than Cr-spinels from
Cambrian Tasmanian boninites which always have a Cr# > 0.80 (Figure 6.5; Brown and
Jenner, 1989).
Cr-spinels are possible discriminants of tectonic environments as their cation ratios change
with varying physico-chemical conditions (Irvine, 1967; Arai, 1992). The Ti02 content, Cr#
and Fe3#, given by Fe3+/(Cr+AI+Fe3+), can be used to distinguishbasalts from island arc,
intraplate and ocean-floor (MORB) settings (Arai, 1992). The Ti02 content in Cr-spinels
from the basaltic andesite association range from 0.21 to 0.41 wt% whereas the Fe3# ranges
from 0.60 to 0.12. On the discrimination diagrams of Arai (1992), Cr-spinels from the
basaltic andesite association of the Anthony Road Andesite plot within the fields for island
arc basalts. The large observed range in 100Mg# in the spineI data may be a result of
subsolidus re-equilibration caused by metamorphism (Brown and Jenner, 1989). The
100Mg# is thus not considered suitable for comparative purposes.
93
• Basaltic Andesite Association (n=9)
C Hellyer Core Basalt (n=6)
& Lynch Creek Basalt (n=1)
0 Miners Ridge Sandstone (n=3)100 - it+ Miners Ridge Basalt (n=l1)
,. "*"
90 - Cambrian Tasmanian Boninites
0
80 - &
0
1 •
• C
••
70 - dIi CC
• 0 •8 I::; u 8.... +
60 - + +
+ 50
+ ++
40 . I I I I • I I I I • I
lOO 90 80 70 60 50 40 30 20 10 o
100Mg/(Mg+Fe2+)
Figure 6.5. Plot of 100Cr# versus 100Mg# for Cr-spinels from across the Mount Read Volcanics. Data for the basaltic andesite association are from this study. Other sources are Hellyer Core Basalt (Jack, 1989), Lynch Creek Basalt (Dower, 1991), Miners Ridge Sandstone (Dower, 1991) and Miners Ridge Basalt (Dower, 1991). The field for Cambrian Tasmanian Boninites is from Brown and Jenner (1989).
94
6.3 Wholerock geochemical classification
The composition of the Anthony Road Andesite, Tyndall Group and post-volcanic dykes are
discussed separately below. Classification is based mainly on elements and ratios of
elements which display immobile behaviour, namely P20Sffi02, TilZr, NbIY and the REE.
Si02 and FeO*/MgO have been used as indices of fractionation. Chondritic normalising
factors used for the REE are those of Boynton (1984). All ge6chemical analyses quoted
throughout this chapter have been recalculated volatile free. The procedure used for
recalculation was 100 divided by the analysis total less the LOI to give the recalculation
factor (Crawford, 1994). Each oxide was then multiplied by the recalculation factor to give
the volatile free analysis.
6.3.1 Analytical methods
The majority of samples were analysed for major, trace and REE by XRF spectrometry on a
fully automated Philips 1410 spectrometer in the School of Earth Sciences at the University
of Tasmania. Major element abundances were determined on fusion discs following the
methods of Norrish and Hutton (1969). Trace element abundances were determined on
pressed powder pellets following the methods of Norrish and Chappell (1977) whereas six
samples were analysed for the REE at the University of Tasmania using the ion-exchange X
ray fluorescence spectrometry (XRF) procedure of Robinson et al. (1986). Nineteen samples
were analysed by Analabs for the REE using inductively-coupled plasma mass spectrometry
(ICP-MS). The analytical methods are detailed in Appendix E.
6.3.2 Element mobility
Rocks analysed for this study have all been affected to some degree by low-grade regional
metamorphism and/or hydrothermal alteration, making it necessary to base classification and
geochemical discrimination principally upon those elements which have remained immobile
during alteration. Element mobility within a unit has been tested by plotting elements on
bivariate diagrams. Coherent behaviour between elements, indicated by a linear array of
points projecting towards the origin, indicates relative element immobility, whereas a scatter
of points indicates the possibility of element mobility (Whitford et al., 1989; Stolz, 1992).
The immobility of a particular element is strongly affected by the intensity of alteration. The
elements generally found to be immobile during moderate intensity hydrothermal alteration
include P and the high field strength elements Ti, Zr, Nb and Y (Whitford et aI., 1989;
95
Crawford et al., 1992; Gemmell and Large, 1992). Gemmell and Large (1992) showed that
under conditions of intense alteration in the siliceous core of the alteration pipe at the Hellyer
massive sulphide deposit, all elements, including Ti, P, Zr, Nb and Y, were mobilised.
During low-intensity alteration, REE are generally but not invariably immobile (Humphris et
aI., 1978; Whitford et aI., 1988). In this study, only relatively unaltered samples have been
analysed for REE in order to avoid problems arising from REE mobility during alteration.
Bivariate diagrams for the Anthony Road Andesite indicate that some elements were mobile.
In particular, data for Na20, K20, Sr, Rb and Ba are scattered when plotted against Zr
(Figure 6.6). In contrast, Ti02, Nb and Y plotted against Zr define relatively linear trends
projecting towards the origin (Figure 6.6) and were probably immobile during alteration (cf.
Whitford et al., 1989; Stolz, 1992). Whitford et al. (1989) and Crawford et al. (1992) found
that Ti, Zr, Y, Nb, P and the REE in the Mount Read Volcanics have been the least mobile
and are the best elements for use in the classification and discrimination of rock types.
6.3.3 Geochemical characteristics of the Anthony Road Andesite
Basaltic andesite association
This association ranges from basalt to basaltic andesite with Si02 from 51 to 55 wt% at MgO
of 6.9 to 8 wt%. The basaltic andesite is strongly augite-plagioclase-phyric with lesser
apatite, magnetite and Cr-spinel phenocrysts present. The basaltic andesites have low Ti02
(0.50 to 0.59 wt%) but high P20S (0.57 to 0.69 wt%) and the transition metals Ni (132 to
150 ppm) and Cr (538 to 643 ppm). A plot of FeO*/MgO versus Si02 suggests
classification of the basaltic andesite association within the calc-alkaline magma series
(Figure 6.7; cf. Miyashiro,1974).
Basaltic andesites fall within the dacite-rhyodacite field on the ZrlTi vs Nb/Y plot of
Winchester and Floyd (Figure 6.8; 1977). This is attributable to the comparatively high Zr
contents of the association compared with orogenic basaltic andesites upon which the fields
were defined (Winchester and Floyd, 1977; Gill, 1981).
96
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o 50 100 150 200 250 o 50 100 150 200 250
Zr Zr
Figure 6.6. Bivariate'plots of K20, Rb, Sr, Ba, Na20, Ti02, Nb and Y against Zr for the hornblende andesite association of the Anthony Road Andesite.
97
The basaltic andesites display smooth positive REE patterns with no distinct anomalies
(Figure 6.9). The light REE, middle REE and heavy REE are all fractionated as shown by
steep negative slopes from La - Srn [(LalSm)N from 4 to 4.4], Srn - Dy [(SmlDY)N from 4.5
to 5.5] and Dy - Yb [(DyNb)N from 1.7 to 2]. The basaltic andesite association shows
extreme light REE-enrichment with up to 361 times chondritic La and (LaIYb)N of 33.4 and
44.6.
The basaltic andesites fit the geochemical criteria defined by Morrison (1980) and MUller et
al. (1992) as characteristic of the shoshonitic rock association, having both olivine and
hypersthene in the norm, low Ti02 « 1.3 wt%), high Al203 (> 14 wt%), high P205, high
concentrations of the large ion lithophile elements (Rb, Sr, Ba and presumably K20) and
showing light REE-enrichment. K20'levels now range from 0.79 to 1.95 wt%.
Anthony Road Andesite
o Basaltic andesite association 86
+ Hornblende andesite association
[J Rhyolite association .~~
78 $f";? [J C~
[J
[J70 [J [J
C'I + o + + i:i3 ++:+.+
+
62 +++
+ + .... +"'+++ Q
+ :1+ ... , +
tP 54
46 o 2 3 4 5 6
FeO*/MgO
Figure 6.7. Plot of Si02 versus FeO*/MgO for the three volcanic associations of the Anthony Road Andesite. Boundary between the tholeiitic and calc-alkaline fields is from Miyashiro (1974). FeO* =total iron.
98
Basin Lake area
o Basaltic andesite association (ARA)
• Hornblende andesite association (ARA)
o Rhyolite association (ARA)
EB Basaltic dyke (Newton Ck Sst, Mt Julia Mem
x Rhyolite dyke (Newton Creek Sandstone)
• Rhyolite lava (TyndalI Group)
Comendite Phonolite
Rhyolite
Trachyte
0.1 Rhyodacite
ZrfTi
Trachyandesite
Andesite
0.01 Nephelenite Andesite Basalt
0.1 1 10 NbIY
Figure 6.8." Plot of ZrfTi versus NblY for volcanic associations of the Anthony Road Andesite, rhyolite of the Tyndall Group and post-volcanic dykes. Fields for varying rock types are from Winchester and Floyd (1977).
99
Hornblende andesite association
The hornblende andesite association ranges from low-Si andesite to dacite with Si02 varying
from 56 to 66 wt%. A feldspar-hornblende-pyroxene phenocryst assemblage is dominant in
the low-Si andesites and a feldspar-hornblende±pyroxene±quartz assemblage is typical of
the dacites. There is significant variation in some major and trace element concentrations, in
particular Ti02 (0.39 to 0.72 wt%), MgO (1.81 to 6.18 wt%), Al203 (13.82 to 20.60 wt%),
P20S (0.16 to 0.42 wt%), Cr (5 to 218 ppm), V (80 to 305 ppm) and Ni (9 to 51 ppm).
Bivariate plots of major and trace elements for different samples commonly define linear
trends indicating that members of this association are co-magmatic.
On bivariate plots against FeO*/MgO, Ti02, FeO*, Ni, Cr, V and Ti/Zr decrease
monotonically with increasing fractionation, indicating the compatibility of these elements.
The decreasing linear trends displayed by Ti02, FeO* and Ti/Zr from andesites with 4 to 5
wt% MgO to dacites with 2 to 3 wt% MgO are indicative of the calc-alkaline affinities of this
association (cf. Miyashiro and Shido, 1975). Most analyses plot within the calc-alkaline field
on the FeO*/MgO versus Si02 diagram of Miyashiro (1974; Figure 6.7).
Magnetite with up to 7 wt% Ti02 (section 6.2.4) is an abundant accessory phase in andesite
lavas of the Anthony Road Andesite. Magnetite occurs in low-Si andesites (56 to 58 wt%
Si02) implying early crystallisation and that the magma was relatively oxidised (cf.
Miyashiro, 1974).
The mobility of the large ion lithophile elements precludes use of the K20 versus Si02
diagram to further subdivide this association within the calc-alkaline magma series
(Peccerillo and Taylor, 1976). The presence of hornblende phenocrysts suggests that
medium-K to high-K affinities are most likely, as hornblende is common only in medium-K
and high-K calc-alkaline andesites (Gill, 1981). Comparison of the K20 of amphiboles from
the hornblende andesite association with amphiboles from modern volcanic successions
favours high-K calc-alkaline affmities for the association (section 6.2.7).
The occurrence of apatite enclosed within magnetite phenocrysts (Chapter 5) indicates that
apatite was an early crystallising phase in the association. Fractionation of apatite is also clear
on a bivariate plot of P20S versus Si02 where P20S decreases with increasing Si02. The
close spatial association of magnetite and apatite phenocrysts is reflected in the similar
patterns,shown by FeO* and P20S (Gill, 1981).
The hornblende andesite association shows enrichment of the light and middle REE and flat
heavy REE patterns (Figure 6.9). Andesites and dacites of this association are light REE
100
enriched with La abundances up to 301 times chondritic La and (LaIYb)N from 14.8 to 37.2.
In contrast to the fractionated light REE [(La/Sm)N from 4.2 to 6.4] and middle REE
[(SmlDY)N from 3 to 5.1], the heavy REE are not significantly fractionated with (DylYb)N
generally close to unity (from 1.1 to 1.5).
Rhyolite association
All samples in the rhyolite association have abundant quartz and plagioclase phenocrysts.
However, on the basis of silica content only, the samples range from andesite through dacite
to rhyolite (57 to 76 wt% Si02). Silica, chlorite, K-feldspar and albite alteration are
commonly strong in this association, and most likely responsible for the observed variation.
The samples plot within the rhyodacite-dacite and trachyandesite fields on an immobile
element plot of Zrffi versus NblY (Figure 6.8; Winchester and Floyd, 1977). Major and
trace elements of the rhyolite association also vary widely, with 0.27 to 0.80 wt% Ti02,
13.55 to 22.46 wt% A1203, 0.51 to 2.88 wt% MgO, 0.14 to 0.25 wt% P20S, 4 to 21 ppm
Ni and < 2 to 66 ppm Cr.
This association has distinctly calc-alkaline trends with FeO*, MgO and Ti02 showing
monotonic decreases with increasing Si02. In addition, on a plot of Si02 versus FeO*/MgO
all samples (except for one altered sample) plot within the calc-alkaline field (Figure 6.7;
Miyashiro, 1974).
The rhyolite association shows steep light and middle (La to Dy) REE enrichment, quite flat
trends across the heavy REE (Figure 6.9) and no distinct anomalies. Fractionation of the
light REE and middle REE is shown by (La/Sm)N from 6.1 to 6.4 and (SmlDY)N from 3.4
to 5.7 respectively. The three samples analysed show very similar trends except with regard
to heavy REE abundances which differ for all three analyses (Figure 6.9). The heavy REE
display a flat unfractionated pattern reflected by (DylYb)N from 1 to 1.2. This association
shows light REE-enrichment with La between 237 and 218 times chondritic and (LaIYb)N
from 21.5 to 39.6.
The rhyolite association overlaps in P20S and Ti02 with the hornblende andesite association
and also has similar high levels of light REE-enrichment, suggesting that both associations
share high-K calc-alkaline affinities.
101
1000 1000 Basaltic Andesite Association, Anthony Road Andesite Rhyolite lavas, Tyndall Group
500
100 o BL5 184.25
• B1A199.55 o 41-5/94ARd
100 • 11/11/94
5010
10 I i i i I I I I I i i I I , I i
La Ce Pr Nd Srn Eu Gd Th Dy Ho Er Tm Yb Lu La Ce Pr Nd SmEu Gd Th Dy Ho Er Tm Yb Lu
1000 ---:j 100
Hornblende Andesite Association, Anthony Road Andesite j Basaltic dykes, Newton Ck Sst & Mt Jul Mem
o 16-11/94 50100 o AJ88• 32-3/94
• NC2379.85~ o HA3229.90
10 "'" 20
10 I I i i i I i I j i I I I i I I
La Ce Pr Nd Srn Eu Gd Th Dy Ho Er Tm Yb Lu La Ca Pr Nd Sm Eu Gd lb Dy Ho Er Tm Vb Lu
1000 1000 Rhyolite Association, Anthony Road Andesite Rhyolite dyke, Newton Creek Sandstone
500
100 o BLl 357.30
• BLD89-3 98.3
o 71-4/94 100 o OWENCOHERENT
5010
10 I I I I i I i I I I I I I I i ,
La Ce Pr Nd Srn Eu Gd Th Dy Ho Er Tm Yb Lu La Ca Pr Nd Sm Eu Gd lb Dy Ho Er Tm Vb Lu
Figure 6.9 Chondrite-normalised rare earth element plots for the basaltic andesite, hornblende andesite and rhyolite associations of the Anthony Road Andesite, rhyolite lavas from the Tyndall Group, basaltic dykes from the Newton Creek Sandstone and Mount Julia Member and rhyolite dyke from the Newton Creek Sandstone in the Basin Lake area. Chondrite normalisation values are from Boynton (1984).
102
6.3.4 Geochemical characteristics of rhyolite in the Tyndall Group
In the Zig Zag Hill Formation of the Tyndall Group, rhyolite is exposed over a strike length
of three kilometres (section 4.3.2). The rhyolites range from 75 to 80 wt% Si02. The scatter
of data on bivariate plots of Na20 and K20 versus Si02, indicates mobilisation of the alkali
elements (Na and K) which can be attributed to alkali exchange reactions between the glassy
groundmasses of these rhyolites and seawater (Figure 6.10; cC'Stolz, 1992). On a plot of
Zrffi versus NbIY, all samples plot within the rhyolite field on the sub-alkaline side of the
diagram (Figure 6.8; Winchester and Floyd, 1977). The rhyolite samples are all chemically
evolved with high S'i02 and FeO* from 0.20 to 2.12 wt%, Ti02 from 0.13 to 0.20 wt% and
MgO from 0.09 to 0.40 wt%. The rhyolites have low Cr « 2 ppm), Ni « 3 ppm) and V (2
to 7 ppm).
Further classification of these rhyolites within the sub-alkaline magma series is difficult due
to their evolved nature, the small geochemical range defined by the sample set and the
mobility of K20 and Na20. Five of seven samples plot within the calc-alkaline field on the
FeO*/MgO versus Si02 diagram of Miyashiro (Figure 6.11; 1974). The samples also fall
within the calc-alkaline fields on a plot of K20 versus Si02, but the K20 levels are unlikely
to be primary. Si02 is unsuitable as an index of fractionation because the rhyolites span only
a 5 wt% range. Minor local variations and the redistribution of Si02 during alteration could
also result in significant changes in such a tightly grouped sample set. Other fractionation
indices (e.g., FeO*/MgO) are similarly unreliable. Nevertheless, the trend of decreasing
MgO with decreasing FeO* is characteristically calc-alkaline in nature (Wilson, 1989).
Moreover, FeO* and MgO values of the Tyndall Group rhyolites compare closely with those
of calc-alkaline rhyolites from younger volcanic terrains but are noticeably less than those of
tholeiitic rhyolites (Ewart, 1979).
The Tyndall Group rhyolites show enrichment in the light REE relative to the heavy REE
with smooth negative slopes from La to Nd and (LaIYb)N of 8.5 and 11.7 (Figure 6.9). A
distinct negative Eu anomaly is present in both samples (EuJEu* ~ 0.45 and 0.59, where Eu*
is the interpolated value of Eu between chondrite-normalised Srn and Gd). The negative Eu
anomalies indicate the presence of residual plagioclase in the source. The heavy REE display
no enrichment with a flat pattern on the chondrite-normalised diagram [(DylYb)N of 1.1 and
1.2]. The light REE-enriched pattern for the Tyndall Group rhyolites is typical of calc
alkaline rocks (Gill, 1981).
103
:r',,I ' , , , I ' , , , I ' : '~ ~.' • , '~ 1: l~' ~.. I •••• I •••• I
-1:1 1:1
~:[
o ' I ! , J ,
60 65
Figure 6.10
80
70
S LU3 60
50
40 o
Figure 6.11
1:1 1:1 1:1j ~6t []
Z4 1:1 1:1
1:1[1
., -:f _,
, , , , , , , , , I I , , , I , I I , I I .... I .... I
70 75 80 85 60 65 70 75 80 Si02 Si02
Bivariate plots of K20 and Na20 against Si02 for rhyolite lavas from the Tyndall Group in the
Basin Lake area.
• Rhyolite lava (Tyndall Group)
IB Basaltic dykes (Mt Julia Member & Newton Ck Sst)
• • •
.-s:;.~
~~
,~ • Cl ~u .'~
C ~v
«;.,,~o
IB
2 3 4 5 6 7
FeO*/MgO
Plot of Si02 (wt%) versus FeO*/MgO for the Tyndall Group rhyolites and basaltic dykes intruding
the Mount Julia Member and the Newton Creek Sandstone in the Basin Lake area. The boundary between the tholeiitic and calc-alkaline field is from Miyashiro (1974).
104
85
8
6.3.5 Geochemical characteristics of post-volcanic dykes: Mount Julia
Member and Newton Creek Sandstone
Two groups of dykes occur in the study area: 1. feldspar-pyroxene-olivine-phyric vesicular
basaltic dykes and 2. a flow banded rhyolite dyke (Chapter 4). The basaltic dykes have been
logged in diamond drill holes NCl and NC2 (Appendix G) in the northern part of the study
area and are exposed in the Hydro Electric Commission's Henty Canal in the northern study
area. The flow banded quartz-feldspar-phyric rhyolite dyke is exposed in the Hydro Electric
Commission's Henty Canal in the northern study area.
Basalt dykes
All basaltic dyke samples are altered, containing a chlorite-sericite-carbonate assemblage
(Chapter 4) which accounts for the high LOI values for all samples (6 to 8 wt%). Despite the
effects of alteration, the Si02, Ti02, P20S, Ni, Nb, Y, V and Zr contents still reflect the
primary geochemistry.
The basaltic dykes have 45 to 49 wt% Si02, 7 to 7.7 wt% MgO and plot in the
andesitelbasalt field on the sub-alkaline side of a Zrffi versus NbfY diagram (Figure 6.8;
Winchester and FIoyd, 1977). The basaltic dykes have high Ti02 (1.63 to 1.88 wt%) and
low P20S (0.25 to 0.28 wt%). All samples plot in the tholeiitic field on the FeO*/MgO
versus Si02 diagram of Miyashiro (1974; Figure 6.11). Tholeiitic affinities for these dykes
are supported by plots of FeO* and Ti02 against FeO*/MgO and slight enrichment with
differentiation is observed. Analyses of basaltic dykes from the northern study area are
remarkably similar in Ti02, MgO, Ti/Zr, P20sffi02, Ti/V, Ni and NbfY suggesting that they
may be comagmatic.
The basaltic dykes display a minor negative slope from La to Yb indicating slight enrichment
of the light REE relative to the heavy REE (Figure 6.9). The slightly light REE-enriched
character of the dykes is given by the ratio of (LafYb)N which varies from 3.2 to 3.6. No
distinct anomalies are present.
Rhyolite dyke
The flow banded quartz-feldspar-phyric dyke is rhyolitic with 78 wt% Si02 at 1 wt% MgO.
This classification is supported on a Zrffi versus NbfY diagram (Figure 6.8; Winchester and
FIoyd, 1977). The dyke has FeO* of 4.08 wt%, Ti02 of 0.17 wt% and P20S of 0.04 wt%.
105
The major element analysis records 3.09 wt% K20 but only 0.01 wt% Na20. The low Na20
level indicates depletion by alteration processes.
The rhyolite dyke shows light REE-enrichment with a steep negative trend from La to Nd
and a (La/Yb)N ratio of 11.3 (Figure 6.9). A marke~ negative Eu anomaly is present with
EulEu* - DAD whereas the pattern is almost flat across the heavy REE [(DylYb)N of 1.2].
6.4 Discussion
6.4.1 Geochemical characteristics of the Basin Lake area
Table 6.2 summarises the geochemical characteristics of volcanic units within the Basin Lake
area. The hornblende andesite and rhyolite associations of the Anthony Road Andesite share
high-K calc-alkaline affinities. The basaltic andesite association has high-K calc-alkaline to
shoshonitic affinities and is the least evolved of the associations of the Anthony Road
Andesite. The rhyolite association is the most evolved of the three associations. REE patterns
for all the Anthony Road Andesite samples show fractionation of light REE and middle REE.
The major difference between the patterns for the three associations is the fractionated heavy
REE pattern of the basaltic andesite association marked by a (DylYb)N of 1.7. The Anthony
Road Andesite records the transition with time from high-K calc-alkaline to shoshonitic
compositions.
Rhyolites of the Tyndall Group are highly evolved and calc-a1kaline with light REE
enrichment though not as marked as that in the Anthony Road Andesite. All Tyndall Group
rhyolites show a marked negative Eu anomaly.
High Ti02 (> 1.6 wt%) tholeiitic basaltic dykes are geochemically distinct from the calc
alkaline to shoshonitic lavas and intrusions of the Anthony Road Andesite and Tyndall
Group. The basaltic dykes show the least light REE-enrichment of any rocks in the study
area.
106
Table 6.2 Geochemical characteristics of volcanic associations and major volcanicunits in the Basin Lake area.
Anthony Road Andesite Tyndall Group Basaltic Dykes
Basaltic Andesite association
Hornblende Andesite association
Rhyolite association
Rhyolite lavas Mount Julia Member & Newton Ck. Sandstone
Si02 (wt%) Ti02 (wt%) TilZr P20S I Ti02 (La/Yb)N
51 - 55 0.50 - 0.59 18.5 - 19.0 1.08 - 1.17
33.42-44.63
55 - 66 0.39 - 0.72
13.38-23.30 0.26-0.73
14.82-18.93
61 -75 0.27 - 0.80 12.5 - 27.6 0.25 - 0.59
21.53 - 39.63
>75 0.13 - 0.20 4.95 - 5.72 0.10 - 0.20
8.54 - 11.69
45 - 49 1.63 - 1.88 55.1-61.7 0.15-0.16 3.22 - 3.58
6.4.2 Comparison of the Basin Lake area with volcanic units elsewhere in
the Mount Read Volcanics
Lithogeochemistry has become an important tool for identifying magma types represented by
volcanic units throughout the Mount Read Volcanics. Crawford et al. (1992) defined five
distinct geochemical suites based on major and trace element geochemistry (Table 6.3). The
five suites can best be distinguished on plots of P20sffi02 and TilZr versus Si02.
Anthony Road Andesite
In addition to the Anthony Road Andesite, volcanic successions comprising basalts and
andesites occur in the Lynch Creek - Miners Ridge, Crown Hill, Sterling River, and Mt
Charter areas (Que-Hellyer Volcanics; Figure 2.2; Corbett, 1992). The hornblende andesite
and basaltic andesite associations of the Anthony Road Andesite are discussed in a regional
context below. The rhyolite association is unlike all other rhyolites elsewhere in the Mount
Read Volcanics.
The hornblende andesite association belongs to suite IT (Figures 6.12a,b; Crawford et al.,
1992) which also includes the Crown Hill andesite, Comstock andesite, Queenstown
Reservoir andesites and some units of the Lynch Creek basalts (Crawford et al., 1992). All
these units occur south of the Henty Fault Zone. On plots of TilZr, TiIV and P20Sffi02
against Si02 suite IT samples are tightly grouped (Figure 6. 12a,b).
Table 6.3 Affinities of the five geochemical suites identified in the Mount Read Volcanics by Crawford et al. (1992).
Suite Geochemical Affinities
I medium- to high-K calc-alkaline
IT high-K calc-alkaline
ill high-K calc alkaline to shoshonitic
N tholeiitic
V rift tholeiites
108
1.4, , , , i , i , , , i , i i , , , , i , i , i , i , , , , i , i i , , ,
• Hornblende andesite association 1.2
6 Basaltic andesite association 11 Rhyo1ite association + Rhyolite lava (Tyndal1 Group)
0.8
c: 0.6 11
E::: El ..... El lrl o 0.4 ~ I
.2:.~0.2
0 50 55 60 65 70 75
Si02 (wt%)
Figure 6.12a Plot of P205ffi02 vs Si02 for the volcanic associations of the Anthony Road Andesite and
rhyolites of the Tyndall Group. Fields for suite I, 11 and III for the Mount Read Volcanics are from Crawford et al. (1992).
50' , i i , , , , , , , i , , , , , , , , , i i , , , i i ' i' , " , ,
• Hornblende andesite association 6 Basaltic andesiteassociation
40 11 Rhyolite association + Rhyolite lava (Tyndall Group)
30
N:-;::, E-t
20
10
, , I I ! I , , I! I, , I , I I , I , , I! " I, I I , I , , , "
o 50 55 60 65 70 75 Si02 (wt%)
Figure 6.12b Plot ofTiJZr vs Si02 for the volcanic associations of the Anthony Road Andesite and rhyo1ites
of the Tyndall Group. Fields for suites I, 11 and III are from Crawford et al. (1992).
109
Suite IT rocks exhibit fractionated light and middle REE and flat, unfractionated heavy REE
(Figure 6.13; Crawford et aI., 1992). The major observed difference within suite IT is the
presence of negative Eu anomalies in both samples from the Queenstown Reservoir andesites
(Crawford et al., 1992). The Queenstown Reservoir andesites show the lowest light REE
enrichment with (La/Yb)N from 9.9 to 11.6 whereas the hornblende andesite association
shows the greatest enrichment with (La/Yb)N from 14.8 to 37.2 (Table 6.4). The lower light
REE-enrichment and greater Ti02 content of the Queenstown Reservoir andesites suggests
that they are unlikely to be comagmatic with the rest of suite IT (Crawford et al., 1992).
The basaltic andesite association of the Anthony Road Andesite falls within or close to the
suite ID field (Figure 6. 12a,b; Crawford et aI., 1992). The basaltic andesite association has
the lowest Ti02 « 1 wt%) and greatest P20S (> 0.50 wt%) and light REE-enrichment of
suite ID samples.
Basalts and basaltic andesites in the Mount Read Volcanics assigned to the high-K calc
alkaline to shoshonitic suite ID include the Hellyer Basalt, the Hollway Andesite, andesite in
the Pinnacles area, intrusions in the Howards Plains area and some units of the Lynch Creek
basalt (Jack, 1989; Coutts, 1990; Dower, 1991; Crawford et al., 1992; McKibben, 1993).
Ratios of TilZr, P20s/Ti02, ZrlY, Nb/Y and TiN are tightly constrained for suite Ill,
suggesting similar magmatic affinities regionally.
REE patterns for the most enriched samples of suite lIT are quite consistent regionallY and
have strongly light REE-enriched patterns (Figure 6.13). The most enriched samples also
show fractionated heavy REE patterns with (Dy/Yb)N of 1.5 to 2 (Table 6.4). The less
enriched samples with lower P20SITi02 and (La/Yb)N < 13 display negative Eu anomalies
and unfractionated heavy REE patterns with (Dy/Yb)N of around 1.1 (Crawford et aI.,
1992). The more enriched samples have similar REE patterns though (La/Yb)N varies
indicating the variable degrees of light REE-enrichment. Two samples of Lynch Creek basalt
[(La/Yb)N of 34.1 to 34.5], a sample of the Hollway Andesite [(La/Yb)N of 37.2] and a
sample of the basaltic andesite association [(La/Yb)N of 44.6] display the highest levels of
light REE-enrichment (Coutts, 1990; Crawford et al., 1992). The REE patterns of the more
evolved samples of suite ID could indicate similar source characteristics for these rocks.
110
1000 o Basaltic andesite association
• Lynch Creek basalts (482080)
o Hollway andesite (74086)
• Howards Plains basalt (Y408)
100 & Hellyer basalt (MR437)
10
Suite ill Mount Read Volcanics
La Ce Pr Nd Srn Eu Gd Tb Dy Ho Er Trn Yb Lu
1000 o Hornblende andesite association --- • Crown Hill andesite (Z632)
o Queenstown Reservoir andesite (39232)
100
10
Suite IT Mount Read Volcanics
La Ce Pr Nd Srn Eu Gd Tb Dy Ho Er Trn Yb Lu
1000
500
100
50
10
100
50
20
10
Rhyolite lavas, Tynda11 Group, Mount Read Volcanics
o Basin Lake area
• Basin Lake area
o Jukes (179/2)
• East Darwin (17911)
La Ce Pr Nd Srn Eu Gd Tb Dy Ho Er Trn Yb Lu
Basaltic dykes: Basin Lake and Henty Dyke Swarm
o Basin Lake area (Newton Ck Sst)
• Basin Lake area (Mt Jul Mrnb)
o Henty Dyke Swarm (SR8)
• Henty Dyke Swarm (stp234)
JL---.-----,--.-----,--.-----r-'I-,--,--.----.-1-.----.--.----.--La Ce Pr Nd Srn Eu Gd Tb Dy Ho Er Trn Yb Lu
Figure 6.13 Chondrite-normalised rare earth element plots for suite 11 and III of the Mount Read Volcanics, rhyolite lavas in the Tyndall Group across the Mount Read Volcanics and basaltic dykes from the study area and the Henty Dyke Swarm. Chondrite normalisation values are from Boynton (1984). Regional data from Crawford et al. (1992).
Table 6.4 Rare earth element ratios for the volcanic associations of the Anthony Road Andesite in the Basin Lake area and for suites IT and III regionally in the Mount Read Volcanics.
Lithology Suite II
Hornblende andesite association1 Hornblende andesite association1 Hornblende andesite association1 Rhyolite association 1 Crown Hill andesite2
Crown Hill andesite2
Queenstown Reservoir Andesites2
Queenstown Reservoir Andesites2 ........ ........ N
Suite III
Basaltic andesite association1 Hellyer basalt2
Hellyer basalt2
Hollway Andesite3
Howards Plains basalts2
Lynch Creek basalts2
Pinnacles andesite4
Sample number
ARd40-3/94 Ard48-3/94 ARd63-3/94 Q'FdAss71-4/94 CHillZ632 CHillM189 Q'tRes39232 Q'tRes38233
BL4199.55 334162 MR437 HollAnd Y408 482080 71-512
(LalSm)N
4.29 4.62 5.19 6.38 4.52 6.17
4 3.1
3.1-6.17
4.41 3.9
4.33 4.77 3.76 4.3
4.56
3.76-4.77
(DylYb)N
1.21 1.23 1.11 0.99 1.32 1.07 1.19 1.07
0.99-1.32
1.74 1.54
1.59 1.79 1.71 1.83 1.47
1.47-1.83
(SmlDY)N
2.92 3.42 3.29 3.42 3.39 3.57 2.48 3.04
2.48-5.66
1.97 .
3.6 3.07 4.38 4.13 4.49 2.79
1.97-4.49
(LalYb)N
14.82 18.93 18.44 21.53 19.75 22.91 11.56 9.85
33.42 21.08 20.59 37.23 25.89 34.46
'~."
18.69
Data sources. 1 this study, 2 Crawford et al. (1992), 3 Coutts (1990), 4 McKibben (1993)
Tyndall Group
Quartz-feldspar rhyolites occur in the Tyndall Group on Anthony Road, Gooseneck Hill,
Mount Jukes, East Darwin Plateau, Howards Road arid Cradle Mountain Link Road
(Crawford et al., 1992; Jones, 1993; White and McPhie, 1996). Crawford et al. (1992)
placed all dacite and rhyolite lavas of the Tyndall Group within the transitional, medium to
high-K calc-alkaline suite I (Figure 6. 12a,b). ,
Rhyolite samples from the Anthony Road - Gooseneck Hill area have higher SiOz (75 to 80
wt%), lower TiOz '(0.13 to 0.23 wt%) and lower FeO* « 2.12 wt%) compared with the
Jukes - Darwin samples which have SiOz (71 to 73 wt%), TiOz (0.30 to 0.45 wt%) and
FeO* (1.80 to 3.40 wt%; Doyle, 1990; Crawford et aI., 1992; McNeill and Corbett, 1992;
Jones, 1993). Some samples from the Jukes - Darwin area have relatively high LOI and KzO
and are evidently more altered than rhyolite samples from the Anthony Road - Gooseneck
Hill area. Other differences between the two areas are not attributable to alteration and reflect
the degree of fractionation. Samples from the Jukes - Darwin area have slightly higher Ti/Zr
for the same SiOz than the Anthony Road - Gooseneck Hill rhyolites, with lower degrees of
fractionation (Figure 6.14).
Tyndall Group rhyolites from East Darwin, Jukes area and the study region all show light
REE-enrichment [(LalYb)N being 10.6, 11.8 and 8.5 to 11.7 respectively; Figure 6.13;
Crawford et al., 1992]. In addition, all samples have relatively flat heavy REE patterns with
(DyIYb)N from 1.1 to 1.2 and a distinct negative Eu anomaly.
Tholeiitic basaltic dykes
Tholeiitic basaltic dykes in the Mount Read Volcanics include: 1. the Henty Dyke Swarm
which intrudes the northern Central Volcanic Complex and 2. dykes in the study area which
intrude the Mount Julia Member and Newton Creek Sandstone (Corbett and Solomon, 1989;
Crawford et aI., 1992; McNeill and Corbett, 1992; Terry, 1995). The Henty Dyke Swarm
intrudes the Central Volcanic Complex but not the Tyndall Group (Poltock, 1992; Crawford
et al., 1992), and is interpreted to be significantly older than the dykes of the study area.
Petrographically the basaltic dykes in the study area are similar to the Henty Dyke Swarm,
containing phenocrysts of plagioclase, high-Ti augite and olivine (Crawford et al., 1992).
However, the Henty Dyke Swarm dykes have lower FeO*, TiOz, PzOS, Zr, TiIV and higher
SiOz and Ti/Zr (Crawford et al., 1992). The dykes from the study area have lower SiOz than
the Henty Dyke Swarm.
113
Basaltic dykes from the study area show only slight light REE- and middle REE-enrichment
(Figure 6.9). Although one sample (STP234) of the Henty Dyke Swarm has an almost
identical REE pattern (Figure 6.13; Crawford et aI., 1992), two others (R8 and R195) have
almost flat patterns and exhibit a negative Eu and Ce anomaly respectively (Figure 6.13;
Crawford et aI., 1992). The degree of light REE-enrichment for the study area dykes
[(LaIYb)N from 3.2 to 3.6] is greater than the degree of light REE-enrichment observed in
the Henty Dyke Swarm [(LaIYb)N from 1.4 to 2.6; Crawford et al., 1992].
Differences in major, trace and REE contents and geological constraints on the timing of
emplacement indicate that a direct genetic relationship does not exist between the basaltic
dykes of the study area and the Henty Dyke Swarm.
Tyndall Group
• Basin Lake area
& Goosneck Hill
• MtDarwin
• Jukes
15 ,I I I I
•
•L 10
N ~ I &
• & .. •• 5 l- • • •
o I I I I
60 65 70 75 80 85
Si02
Figure 6.14 Plot of TilZr versus Si02 for rhyolite lavas from the Tyndall Group in the Basin Lake area and the regionally. Regional data sourced from Crawford et al. (1992) and Jones (1993).
114
6.4.3 Modern volcanic successions comparable to the associations of the
Anthony Road Andesite
The calc-alkaline suites of the Mount Read Volcanics (suites I and IT) have been compared
with modern orogenic calc-alkaline volcanics erupted in mature island arc (e.g., New
Zealand) and active continental margin settings (e.g., Andes; Crawford et al., 1992). In such
settings, medium- to high-K calc-alkaline volcanics spanning the basalt-andesite-rhyolite
spectrum are common (Gill, 1981; Middlemost, 1985). The suite with shoshonitic affinities,
suite III (Crawford et aI., 1992), is geochemically distinct, with the closest modern
analogues being post-collisional volcanics in regions of arc-continent collision (e.g., Papua
New Guinea Highlands, Himalayas, Alps; Crawford et al., 1992).
The hornblende andesite association
The hornblende andesite association of the Anthony Road Andesite is comparable with
modern day medium- and high-K calc-alkaline volcanics erupted in mature island arc and
continental margin settings, sharing the low Ti02, Zr and Y which are typical of arc
volcanism.1n particular, Y and Zr overlap with medium-K calc-alkaline volcanics from Java
and Bali, Sunda Arc (Whitford et al., 1979) and high-K calc-alkaline volcanics from Alicudi
and Stromboli, Aeolian Arc (Ellam et aI., 1988; Peccerillo and Wu, 1992). The hornblende
andesite association has P20S and (La/Yb)N ratios comparable with those of high-K calc
alkaline basaltic andesites and andesites, such as those of Stromboli, Aeolian Arc (Ellam et
aI., 1988), the Nevados de Payachata Volcanic Region, Andes, Chile (Davidson et al.,
1990), Java, Sunda Arc (Whitford et aI., 1979) and Eastern Srednogorie, Bulgaria (Manetti
et al., 1979). Samples of this association display the fractionated light REE and middle REE
with relatively flat heavy REE patterns that are common in calc-alkaline andesites from both
island arc and continental margin settings (Manetti et al., 1979).
The high-K calc-alkaline character of the hornblende andesite association is also evident from
P20S and (La/Yb)N ratios which are consistently higher than those in low-K calc-alkaline
andesites (e.g., Calbuco Volcano, Chile; L6pez-Escobar et al., 1995) and in medium-K calc
alkaline andesites (e.g., Salina, Southern Italy; Keller, 1974; Ellam et aI., 1988; Java and
Bali, Indonesia; Whitford et al., 1979; the Kastamonu area, Northern Turkey; Peccerillo and
Taylor, 1976). Although P20S in the hornblende andesite association overlaps with that of
some medium-K calc-alkaline andesites (e.g., Ararat Volcano and the Kars Plateau
Volcanics, Turkey), (La/Yb)N ratios are significantly lower in the Turkish andesites (Pearce
et al., 1990; Notsu et al., 1995).
115
The basaltic andesite association
The basaltic andesite association of the Anthony Road Andesite has lower Ti02 but is
enriched in P20S, Zr and the transition metals, Cr and Ni, relative to high-K calc-alkaline
basaltic andesites [e.g., Java, Sunda Arc (Ti02 0.91 wt%, P20S 0.33 wt%, Zr 152 ppm, Cr
8 ppm and Ni 8 ppm); Whitford et al., 1979; Alicudi, Aeolian ArC (Ti02 0.68 to 0.77 wt%,
P20S 0.23 to 0.30 wt%, Zr 102 to 125 ppm, Cr 154 to 370 ppm and Ni 27 to 77 ppm);
Peccerillo and Wu, 1992l.
Basaltic andesites with shoshonitic affinities characteristically have higher Ti02 contents than
the basaltic andesite association of the Anthony Road Andesite but some shoshonites overlap
in P20S, Zr, Cr and Ni. Shoshonite from Mount Hagen in the Papua New Guinea highlands
has Ti02 (1.06 wt%), P20S (0.51 wt%) and Zr (160 ppm) whereas shoshonite from Cape
Nelson, Papua New Guinea has Ti02 (0.84 wt%), P20S (0.49 wt%), Zr (220 ppm), Cr
(300 ppm) and Ni (54 ppm; Jakes and Smith, 1970; Mackenzie and Chappell, 1972),
comparing favourably in P20S with the basaltic andesite association.
Shoshonitic basaltic andesite of post-collisional origin from the Mus depression in Eastern
Anatolia, Turkey, overlaps in P20S (0.44 to 1.48 wt%) but has still lower Cr (6 to 41 ppm)
and Ni (6 to 21 ppm) with higher Ti02 (1.45 to 3.33 wt%) and Zr (397 to 631 ppm) than the
basaltic andesite association (Pearce et aI., 1990). Shoshonites which most closely
approximate major and trace element contents in the basaltic andesite association of the
Anthony Road Andesite are the post-collisional dykes in the Northwestern Alps, Italy, (Ti02
0.93 wt%, P20S 0.70 wt%, Zr 236 ppm, Cr 590 ppm, Ni 333 ppm; Venturelli et al., 1984)
and post-collisional dykes from northern Karakorum, China, (Ti02 0.75 to 2.13 wt%, P20S
0.72 to 1.7 wt%, Zr 226 to 692 ppm, Cr 105 to 580 ppm, Ni 36 to 403 ppm; Pognante,
1990).
Fractionated heavy REE patterns with (Dy/Yb)N, or (Tb/Yb)N [depending upon which REE
were analysed forl, significantly greater than unity are also recorded in shoshonites of the
Central Andes ([Tb/YblN from 1.55 to 1.66; Dostal et al., 1977), Vulcano, Aeolian Arc
([Tb/YblN of 1.60; Ellam et al., 1988), Eastern Srednogorie, Bulgaria ([Tb/YblN from 1.24
to 1.66; Manetti et aI., 1979), Lesbos, Greece ([Tb/YblN of 2.98; Pe-Piper, 1980) and
Nemrut, Turkey ([Tb/YblN of 1.49; Notsu et al., 1995). Garnet is the only common mineral
which strongly discriminates between the heavy REE and the observed fractionated heavy
REE patterns in shoshonites could be due to partial melting of garnet peridotite with residual
garnet present in the source region (Manetti et al., 1979; Pe-Piper, 1980).
116
Shoshonites occur in within-plate, intra-oceanic island arc (immature and mature),
continental margin arc and post-collisional settings (Morrison, 1980; MUller et al., 1992).
MUller et al. (1992) have developed criteria for discriminating among potassic volcanic rocks
. erupted in these four tectonic settings based on the premise that the geochemistry of the lavas
reflects the source characteristics and the magmatic processes occurring during emplacement.
The basaltic andesite association of the Anthony Road Andesite plbts outside the within-plate
field on graphs of Ti02 versus Ah03, and Y versus Zr, and within the continental arc/ post
collisional arc field on a Zr/A1203 versus Ti02/A1203 graph (MUller et al., 1992). Further
subdivision on a graph of CeIP20S versus ZrlTi and a ternary diagram of Zrx3 - Nbx50
CeIP20S indicates that all analyses plot within the continental arc potassic field (MUller et al.,
1992). Samples analysed from the Lynch Creek basalts and the Hellyer Basalt by Crawford
et al. (1992) also plot within the field for continental arc potassic volcanic rocks.
6.5 Conclusions
The study area includes rocks of diverse compositions. Although intermediate to felsic calc
alkaline volcanic units dominate, shoshonites and tholeiitic basaltic dykes also occur.
The Anthony Road Andesite consists of three geochemically distinct volcanic associations:
the basaltic andesite association with shoshonitic affinities, the hornblende andesite
association with high-K calc-alkaline affinities and the rhyolite association also with high-K
calc-alkaline affinities. The three associations can easily be distinguished from one another
using Si02, Cr, Ni and the Ti/Zr ratio.
The basaltic andesite association resembles the Hellyer Basalt, the Hollway Andesite, the
Howards Plains andesites and units of the Lynch Creek basalts, and belongs in suite III of
the Mount Read Volcanics as defined by Crawford et aI. (1992). Modern geochemical
analogues of the basaltic andesite association occur as post-collisional shoshonitic dykes in
the Himalayas and Northwestern Alps (Venturelli et aI., 1984; Pognante, 1990). The
hornblende andesite association is comparable with suite 11 of the Mount Read Volcanics,
displaying geochemical overlap with the Crown Hill Andesite, the Queenstown Reservoir
andesites and some andesites in the Lynch Creek basalts (Crawford et aI., 1992). Close
geochemical analogues for the hornblende andesite association are found in high-K calc
alkaline andesites and dacites from Stromboli in the Aeolian Arc (Ellam et al., 1988) and in
the Chilean Andes (Davidson et al., 1990). The rhyolite association of the Anthony Road
Andesite has similar P20SITi02 and REE concentrations to suite IT andesites and dacites.
117
However, rhyolites of comparable chemistry are unknown elsewhere in the Mount Read
Volcanics.
. Rhyolites in the Zig Zag Hill Formation ~f the Tyndall Group are the most evolved rocks in
the study area with> 75 wt% Si02 at < 0.5 wt% MgO. They resemble rhyolites in the
Tyndall Group at Gooseneck Hill and in the Jukes - Darwin area, showing comparable
trends on Ti/Zr versus Si02 diagrams and overlapping (LalYb)N. Similar evolved calc
alkaline rhyolites are known from island arc and continental margin settings (Ewart, 1979).
The feldspar-pyroxene-olivine-phyric basaltic dykes which intrude the Mount Julia Member
and the Newton Creek Sandstone post-date deposition of the Newton Creek Sandstone but
pre-date Mid-Devonian deformation. These tholeiitic basaltic dykes are geochemically
distinct from the high-K calc-alkaline to shoshonitic Anthony Road Andesite, having higher
Ti02 and Ti/Zr and showing only very slight light REE enrichment. These dykes are
significantly younger than the currently inferred age of the tholeiitic Henty Dyke Swarm.
From the geochemical compositions of volcanic units in the Anthony Road Andesite, a
subduction-related or collision-related tectonic setting can be inferred.
118
> f""!' - ~ ""1 ~
f""
!' .,..
0 =
.,.. =
f""!' =-
Cl
~
= ~>
~
f""
!'
=-~
= f""!'
10'1
0 ...
.,J
=
~
~
0 ~
Q. > =
Q.
~
rJ"J. .,..
f""!'~
Chapter 7
Alteration in the Anthony Road Andesite
7. 1 Introduction
Four events have affected the composition and mineralogy of the Cambrian Mount Read
Volcanics (McPhie et aI., 1993): 1. Cambrian syn-volcanic alteration; 2. regional
metamorphism accompanying major deformation; 3. localised syn-tectonic alteration, and 4.
contact metamorphism and hydrothermal alteration related to Devonian granitoid intrusion.
The effects of all/most of these events can be recognised in the Anthony Road Andesite, and
have been studied for the hornblende andesite association. The petrographic characteristics
of various alteration styles, mineral chemistry of metamorphic chlorite and a quantitative
assessment of the chemical effects of alteration in the hornblende andesite association of the
Anthony Road Andesite are presented below.
7.2 Alteration styles: distribution, mineralogy and textures
Alteration of the hornblende andesite association varies from selective replacement of
groundmass and/or phenocrysts to pervasive alteration with destruction of all primary
textures. Three dominant alteration assemblages have been recognised: 1. silica - sericite ± chlorite ± pyrite; 2. sericite - chlorite, and 3. albite - chlorite ± epidote ± sericite ± calcite ± hematite. A wide variety of other alteration assemblages has been observed, including locally
important leucoxene, goethite and pumpellyite.
Silica - sericite ± chlorite ± pyrite alteration occurs locally near the contact between the
Yolande River Sequence and the Anthony Road Andesite in the southern Basin Lake area: at
"pyrite corner" (Plate 1.1) and to the east around Leech Hill (intersected in drill hole Leech
Hill; Appendix G). The intensity of alteration varies from moderate in drill hole Leech Hill
where hornblende phenocrysts are replaced by chlorite and the groundmass is selectively
replaced by silica - sericite - pyrite to pervasive at "pyrite corner" where the silica - sericite
pyrite assemblage has totally replaced and destroyed all primary rock textures, and the rock
is now white to grey. Silica in this assemblage is of two types: 1. white quartz (0.3-4 mm in
diameter) which occurs disseminated throughout the rock, and 2. grey quartz which occurs
119
in monominerallic patches to 5 mm across. Pyrite occurs as cubes to 2 mm though more
commonly as finer grained aggregates. The alteration at "pyrite corner" extends for
approximately 300 m along the Anthony Road and is the most intense alteration observed in
. the Basin Lake area. The Devonian regional cleavage overprints alteration at "pyrite corner."
Sericite - chlorite alteration occurs throughout the hornblende andesite association in
combination with the albite - chlorite ± epidote ± sericite ± calcite ± hematite assemblage,
and as a distinct alteration type in drill holes BLD89-1 and BL4 (Appendix G). In these drill
holes, sericite - chlorite alteration is selective with plagioclase phenocrysts in andesites
containing disseminated sericite whereas the groundmass is replaced by patchy chlorite
sericite.
The most common alteration in the Anthony Road Andesite is the albite - chlorite ± epidote ± sericite ± carbonate ± hematite assemblage: This assemblage is widespread in outcrop and
throughout drill holes TYN1, TYN3, TYN4, TYN5, BL1, BL3, HA2, HA6 and HA7
(Appendix G). This alteration is typically of low intensity with phenocrysts preserved
though plagioclase is commonly pseudomorphed by epidote and albite or replaced by
disseminated sericite and chlorite. Albite - epidote - calcite occurs throughout the hornblende
andesites with albite - epidote in the groundmass commonly rimming zones of clear to white
calcite. Zones (5 to 50 cm wide) of epidote - sericite replacement of groundmass with total
phenocryst preservation occur along the Anthony Road. Albite - hematite - carbonate is a
common assemblage towards the stratigraphic top of the Anthony Road Andesite and
phenocrysts are replaced by albite and hematite (drill holes TYN4, TYN5, HA6, HA7;
Appendix F & G). Hematite and carbonate have not been observed in the lower Anthony
Road Andesite.
7.2.1 Groundmass and phenocryst alteration
The assemblage resulting from alteration of phenocrysts appears to be strongly controlled by
the phenocrysts primary composition. Though phenocrysts are commonly preserved, any
original volcanic glass component in the groundmass has been completely altered.
Plagioclase phenocrysts are ubiquitously albitised, displaying a pale pink dusting under
plane polarised light and being low in anorthite (An<1o) as determined by electron
microprobe (Appendix D). Fine grained sericite is commonly disseminated throughout
plagioclase, imparting a speckled appearance. Replacement of plagioclase by chlorite and/or
epidote can vary from domainal to complete. Uncommon pumpellyite also replaces
plagioclase phenocrysts in the Anthony Road Andesite.
120
Green to brown hornblende phenocrysts are most commonly partly or totally replaced by
pale green chlorite. Calcite and disseminated fine grained epidote are also common alteration
products of hornblende. Replacement of hornblende by euhedral pyrite is rare. The
hornblende phenocrysts contain primary inclusions of tabular plagioclase, magnetite and
apatite. The plagioclase inclusions are always albitised and where the hornblende is strongly
altered, the magnetite inclusions are usually replaced by chlorite and leucoxene.
Magnetite phenocrysts are common throughout the hornblende andesite association,
controlling the magnetic susceptibility of the volcanics. The degree of alteration of magnetite
closely mirrors that of plagioclase and hornblende phenocrysts: all three are either fresh or
altered to the same extent. Where altered, magnetite is replaced by chlorite or
leucoxene/goethite. Some altered phenocrysts contain cores of olive green chlorite with
margins of brown leucoxene.
The groundmass in most intrusions and lavas of the hornblende andesite association is now
composed of a fine grained quartzo-feldspathic intergrowth, commonly spotted with pale
green chlorite and lesser epidote, and sericite and calcite in the less altered samples. In
samples where the primary phenocryst phases have been altered, the groundmass is
ubiquitously altered and consists of chlorite with domains of quartz, calcite and epidote also
locally present. In some samples, euhedral pyrite is disseminated throughout the
groundmass, and fibrous green actinolite has been observed.
7.2.2 Veins
Veins and veinlets of varying compositions are widespread throughout the Anthony Road
Andesite. Age relationships among the veins are difficult to determine, as veins with
consistent mineralogy are not common. Relative timing can be determined from overprinting
relationships and vein mineralogy in a number of the vein sets.
Single veins are the most common vein type though en echelon and vein networks are also
observed. The majority of veins are of the order of 1 to 3 cm wide although some examples
are up to 15cm in width. Vein mineralogies include epidote, carbonate, quartz ±carbonate ± epidote ± chlorite, albite - epidote - chlorite, quartz - epidote - albite, quartz-pyrite, pyrite,
pyrite-epidote and hematite. Vein carbonate is dominantly calcite with dolomite and ankerite
forming only minor components (Appendix F).
121
Observed relationships among vein types were observed in drillcore. At 385 m in drill hole
NC2, a carbonate vein is cross-cut by a basaltic dyke (Appendix G). This relationship
indicates that the carbonate vein is pre-dyke emplacement. The basaltic dyke is altered and
has a cleavage most likely formed during the Devonian deformation event (Chapter 4).
Epidote occurs in networks of < 1 cm wide (commonly 3 to 5 mm) veinlets. In drill hole
TYN5, an early set of epidote veinlets has been cross-cut by carbonate veins that have
subsequently been cross-cut by another set of later epidote veinlet~ (Appendix G). Quartz in
veins varies between white and clear forms. Single quartz veins contain clear quartz and
similar clear quartz veins at Que River, northern Mount Read Volcanics, have been
interpreted as syn-tectonic in origin (Offler and Whitford, 1992). In drill hole HA2 (depth
123.5 m; Appendix G), an albite - epidote - chlorite vein is cross-cut by later white carbonate
veins whereas at 56.7 m, pyrite veinlets < 5 mm wide overprint white carbonate veins
(Appendix G). At 89.3 m (HA 2), epidote veins up to 4 cm wide cut andesitic breccias
which have been albite-altered prior to veining.
The mineralogy of the veins, especially the presence of epidote, albite and carbonate,
suggests they are either pre- or post-regional deformation veins and unlikely to be related to
Cambrian hydrothermal activity. Folded veins and/or veins offset by faults have not been
observed.
122
7•3 Alteration geochemistry
Chlorite chemistry and wholerock compositions have been determined in order to document
the geochemistry of alteration in the hornblende andesite association of the Anthony Road
Andesite. Mass balance calculations have been undertaken on samples from selected
alteration styles in the study area. Chlorite was the only alteration phase analysed by electron
microprobe as the results of previous studies were available for comparison.
7•3.1 Chlorite chemistry
Analyses of chlorite alteration were undertaken using a Cameca SX50 electron microprobe in
the Central Science Laboratory at the University of Tasmania. Analyses from the hornblende
andesite association in the study area are compared with analyses reported by Eastoe et al.
(1987) from the Mount Read Volcanics and with analyses reported by Offler and Whitford
(1992) from the Que-Hellyer area, northern Mount Read Vol~anics.
Probe analyses of chlorite have been recalculated using the computer application Mintab on
the basis of 28 oxygens (Rock and Carroll, 1990; Deer et al., 1992). Representative chlorite
analyses are listed in Table 7.1 with a full list given in Appendix D. In petrographic
examination, the chlorites analysed have a preferred orientation and are interpreted to be of
regional metamorphic origin. Chlorites are observed either replacing portions of the
groundmass or plagioclase phenocrysts (accompanying epidote in TS 624182*). The
chlorites are either ferro-clinochlores or magnesio-chamosites using the classification of
Bayliss (1975; Figure 7.1). The major compositional differences occur in the levels ofMgO
(12.3 to 18.6 wt%) and FeO (20.7 to 28.1 wt%) and are reflected in the Mg/(Mg+Fe) ratio
which ranges from 0.46 to 0.60 and the Fe/(Fe+Mg) ratio which ranges from 0.40 to 0.54.
Offler and Whitford (1992) found that chlorites of regional metamorphic origin overlapped
compositionally with chlorites of hydrothermal origin around the Que River deposit,
northern Mount Read Volcanics. The chlorites analysed from the study area overlap in
composition with the chlorite compositions given in Offler and Whitford (1992). A wide
range in Mg/(Mg+Fe) in chlorites across the Mount Read Volcanics is documented in Eastoe
et al. (1987). Only the immediate footwall to the Hercules ore deposit is distinctive having
Mg/(Mg+Fe) ratios (0.58-0.68) that are the highest in the belt.
* TS 624182 is from the Aberfoyle collection, Bumie
123
Table 7.1 Selected microprobe analyses of chlorite from the hornblende andesite association of the Anthony Road Andesite. A full list of chlorite analyses is given in Appendix D.
Magnesio-chamosite Ferro-clinochlore
(wt%) 75838 R2 CHL1 75838 R19 CHLl 624182 R7 CHL1 624182 R17 CHL2
Si02 25.86 25.58 27.85 28.24
Ti02 0.01 0.04 0.16 ( 0.04
Al203 17.07 18.55 16.61 16.48
Cr203 0.01 0.09 0.02
FeO* 27.09 27.45 21.80 21.63
MnO 0.5 0.46 0.45 0.57
MgO 14.95 14.27 18.6 18.36
CaO 0.08 0.16 0.09 0.07
Total 85.56 86.61 85.58 85.38
Number of ions calculated on the basis of 28 oxygens
Si 5.685 5.555 5.917 6.004
Aliv 2.315 2.445 2.083 1.996
AJVi 2.108 2.303 2.078 2.133
Ti 0.001 0.006 0.026 0.006
Cr 0.003 0.016 0.003
Fe2+ 4.98 4.987 3.873 3.846
Mg 4.898 4.622 5.893 5.821
Mn 0.093 0.085 0.08 0.102
Ca 0.018 0.038 0.021 0.016
Mg/(Mg+Fe) 0.496 0.481 0.603 0.602
Fe/(Fe+Mg) 0.504 0.519 0.397 0.398
124
3
2.8 Ferro-clinochlore
2.6
2.4
.:::: 2.2 < •2
1.8 • 1.6
1.4 •
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6
Alvi
Figure 7.1a Chlorite compositions from the hornblende andesite association of the Anthony Road Andesite. Diagram is after Bayliss (1975).
3
2.8
2.6
Magnesio-chamosite
,1 Ferro-clinochlore -
-2,4 -
> .~
2.2
2 t ~t -•• -
-
1.8 • -
1.6 ~ -
1,4 I 1 1 •
1 1 I
0.2 0.3 0,4 0.5 0.6 0.7 0.8 0.9
MglMg+Fe
Figure 7.1b Chlorite compositions from the hornblende andesite association of the Anthony Road Andesite. Diagram is after Bayliss (1975).
125
7.3.2 Wholerock geochemistry
Wholerock major and trace element geochemical analyses have been undertaken on variably
altered samples of the hornblende andesite association from outcrop and drillcore across the
study area. The analytical methods have been documented previously in section 6.3.1 and all
geochemical analyses are listed in Appendix E.
Element mobilities
The relatively linear trends which project towards the origin on bivariate plots (section 6.3.2)
indicate the immobility of the high field strength elements Ti, Zr, Nb, and Y and of Al203 in
the hornblende andesite association (Finlow-Bates and Stumpfl, 1981; Grant, 1986; Barrett
and MacLean, 1994). In contrast, FeO*, MgO, MnO, CaO, Na20, K20, BaO, Sr and Rb
show a scatter of points when plotted against Zr. Some scatter on the bivariate plots can be
accounted for by analytical error and by natural variation in composition but can also indicate
mass gains and losses due to mobility of these components during alteration (Liaghat and
MacLean, 1992; Barrett and MacLean, 1994).
Mass change/mass balance using immobile elements
The geochemical variations of the hornblende andesite association (Chapter 6) are interpreted
to reflect fractionation. This geochemical relationship allows use of the immobile elements as
monitors of mass transfer and calculation of the mass of components lost or gained during
alteration (MacLean, 1990).
The technique uses bivariate plots of immobile versus immobile elements for least altered
samples to outline the igneous fractionation trends for the rock series (MacLean and
Kranidiotis, 1987; MacLean, 1990; Leitch and Lentz, 1994). Incompatible immobile
elements are used preferentially, though in the volcanics of the study area the immobile
elements are observed to have been compatible (decrease in abundance with increasing
fractionation) as is observed in many calc-alkaline rock suites (Wilson, 1989; Barrett and
MacLean, 1994).
A fractionation trend was established from least altered samples using a plot of the
compatible immobile oxides Al203 versus Ti02 (Figure 7.2). Data from altered volcanics
plot away from the fractionation line but along separate alteration trends which project
through the origin (Figure 7.2; Liaghat and MacLean, 1992). The unaltered sample
126
(precursor) A1203 and Ti02 contents are given by the point at which the alteration line
intersects the fractionation line (Figure 7.2; Liaghat and MacLean, 1992). Fractionation
trends are established for each element or oxide on bivariate plots of the element versus
Ti02. The level of each element in the precursor is given by the intersection of precursor
Ti02 with the fractionation trend defined by the element - Ti02 bivariate plot (MacLean,
1990; Liaghat and MacLean, 1992).
A reconstituted composition (RC), samples mass after alteration, is calculated by
normalising the LOI (loss on ignition) of the corrected sample to the Ti02 level in its
precursor (Liaghat and MacLean, 1992). The reconstituted composition is given by (Barrett
and MacLean, 1994):
RC(wt%) = sample component (e.g., Na20) x (Ti02 level in precursor)/(Ti02 level in
analysed sample)
The procedure above is carried out for each component in the rock. The mass change due to
alteration can then be calculated for each component of the rock using (MacLean, 1990;
Barrett and MacLean, 1994):
Mass change due to alteration =RC - precursor
The technique described above has been used to assess mass changes due to alteration in the
hornblende andesite association. The four samples for which the mass change has been
calculated have undergone varying intensities and styles of alteration which include silica
sericite - pyrite (assemblage 1), sericite - albite - epidote (assemblage 3), albite - silica
sericite (assemblage 3) and epidote - sericite - chlorite (assemblage 2). The original sample
compositions are listed in Table 7.2. The precursor and reconstituted compositions and mass
change due to alteration are given in Table 7.3.
The most altered sample is from the Leech Hill drill hole which is interpreted to have
undergone moderate hydrothermal silica - sericite - pyrite alteration (Appendix G).
Calculations indicate that the Leech Hill sample has experienced mass addition of Si02
(+10.40 wt%) and FeO* (+4.43 wt%) and mass loss of Na20 (-5.64 wt%; Table 7.3). The
gain of SiOi and FeO* correlate with the influx of silica and pyrite respectively and are to be
anticipated during this style of alteration. Albite - silica - sericite alteration is visible in
sample 64-3/94. The effects of silica alteration are mirrored in the mass gain in Si02 (+10.88
wt%; Table 7.3). Sample 60-3/94 is epidote - sericite - chlorite altered hornblende andesite.
Mass change calculations indicate alteration has caused a net mass gain in CaO (+5.72 wt%),
which may correspond to the presence of visible epidote in the sample (Table 7.3). Albite
127
alteration is observed in sample 64-3/94 although after calculation a mass loss of 3.54 wt%
Na20 is recorded. This apparent contradicition may be due to inaccuracies in the
establishment of the fractionation trends.
The calculated mass changes correspond well with the observed alteration assemblage for the
moderately silica - sericite - pyrite altered sample (Leech Hill 500 m) and the albite - silica
sericite altered sample (64-3/94; Table 7.3). Problems with mass change calculations can
arise from the definition of fractionation trends on bivariate plots because all samples are
altered to some degree, leading to incorrect calculations of the precursor compositions.
Diagenetic and hydrothermal alteration commonly occur at high water/rock ratios while
regional metamorphism occurs at low water/rock ratios (MacLean, 1990). In samples which
have undergone regional metamorphism, there should be insignificant mass changes such as
those in the sericite - albite - epidote and epidote - sericite - chlorite altered samples (TYN1,
60-3/94).
Table 7.2 Chemical analyses of four samples from the hornblende andesite association with four varying alteration assemblages. Si=silica, Se=sericite, Py=pyrite, Ab=albite, Ep=epidote, CI=chlorite.
Hornblende andesite association
Alteration Si-Se-Py Se-Ab-Ep Ab-Si-Se Ep-Se-CI
Assemblage Type 1 Type 3 Type 3 Type 2
Sample No. Leech Hill 500m1 TYN1221.8m 64-3/94 60-3/94
Si02 61.77 61.18 63.32 60.44
Ti02 0.45 0.49 0.43 0.45
Al203 14.34 15.61 14.26 14.80
FeO* 10.25 7.26 7.34 7.49
MnO 0.32 0.12 0.11 0.11
MgO 4.91 3.90 3.59 4.39
CaO 4.51 3.40 5.31 7.63
Na20 0.89 5.37 2.32 2.99
K20 2.37 2.46 2.98 1.40
P205 0.20 0.20 0.21 0.22
LOI 13.44 1.81 3.64 2.02
Al203ITi02 31.87 31.86 33.16 32.89 Major element oxides in wt% *All samples LOI corrected to 100 IThe Leech Hill sample is from the Leech Hill drill hole interval 500-501 m (Corbett,1985)
128
• Unaltered hornblende andesite ... Silica-sericite-pyrite altered (Leech Hill) • Epidote-sericite-chlorite altered (60-3/94) • Sericite-albite-epidote altered (TIN 1)
20 rI + Albite-silica-sericite altered (64-3/94)
Fractionation trend 16 • •
12 ~ .... ~
~
0 C'l-<t: 8
\.<$'1{, /' ·o~
~... ~I{,'"
Precursor
4 /
o o 0.1 0.2 0.3 0.4 0.5 0.6
Ti02 wt% Figure 7.2 Alteration diagram used to calculate mass change in altered samples of the hornblende
andesite association. Ti02 and Al203 are immobile and allow a fractionation trend to be established. The point where the alteration line (which passes through the origin and the albite-silica-sericite altered sample) intersects the fractionation trend gives the precursor levels of Ti02 and A1203' Method after Barrett and MacLean (1994).
0.8
0.7 • 0.6
0.5 •
C'l
0 0.4..... ~
0.3
0.2
0.1
0 0 40 80 120 160 200
Zr
Figure 7.3 Plot of Ti02 versus Zr for samples of magnetic and non-magnetic hornblende andesite
of the Anthony Road Andesite. Data courtesy of Aberfoyle Resources Limited.
129
Table 7.3 Calculated precursor compositions, reconstituted compositions (nonnalised to Ti02 levels in the precursor) and mass changes of major oxides for the four altered samples of the hornblende andesite association.
Hornblende andesite association Alteration Si-Se-Py Sample No. Leech Hill 500m1 Precursor compositions Si02 59.61 Ti02 0.51 Al203 16.16 FeO* 7.19 MnO 0.08 MgO 4.02 CaO 2.66 Na20 6.65 K20 2.90 P20S 0.23
TOTAL 100 Reconstituted compositions Si02 70.01 Ti02 0.51 Al203 16.25 FeO* 11.62 MnO 0.36 MgO 5.56 CaO 5.11 Na20 1.01 K20 2.69 P20S 0.23
TOTAL 113.35 Mass changes Si02 10.40 Ti02 0 Al203 0.09 FeO* 4.43 MnO 0.28 MgO 1.54 CaO 2.45 Na20 - 5.64 K20 - 0.21 P20S 0
TOTAL 13.34 Major element oxides in wt%
Se-Ab-Ep Ab-Si-Se Ep-Se-Cl TYN1221.8m 64-3/94 60-3/94
59.61 61.2_8 61.28 0.51 0.49 0.49 16.16 16.24 16.24 7.19 6.58 6.58 0.08 0.11 0.11 4.02 3.64 3.64 2.66 2.59 2.59 6.65 6.18 6.18 2.90 2.67 2.67 0.23 0.22 0.22
100 100 100
63.68 72.16 65.81 0.51 0.49 0.49 16.25 16.25 16.12 7.56 8.36 8.16 0.13 0.13 0.12 4.06 4.09 4.78 3.54 6.05 8.31 5.59 2.64 3.26 2.47 3.40 1.52 0.21 0.24 0.24
104.00 113.81 108.93
4.07 10.88 4.53 0 0 0
0.09 0.01 - 0.12 0.37 1.78 1.58 0.05 0.02 0.01 0.04 0.45 1.14 0.88 3.46 5.72
- 1.06 -3.54 - 2.92 - 0.43 0.73 - 1.15 - 0.02 0.02 0.02
3.99 13.81 8.81
1The Leech Hill sample is from the Leech Hill drill hole interval 500-501 m (Corbett,1985)
130
7.3 .3 Magnetite destructive alteration
The magnetic properties of the hornblende andesite association are not unifonn across the
study area. Magnetic susceptibility ranges from high (> 1640 x 10-5 SI) to very low « 1 x
10-5 SI). The high magnetic susceptibility values correspond to approximately 0.2 volume%
of magnetite in the rock.
'"_:
Titaniferous magnetite is a common phenocryst phase in the hornblende andesite association.
Differing proportions of magnetite phenocrysts are partially responsible for the variations in
magnetic susceptibility readings. In more altered andesite samples, particularly samples with
sericite - silica ± pyrite alteration, titaniferous magnetite is altered to hydrated iron oxide
phases (e.g., leucoxene). These more altered samples have correspondingly lower magnetic
susceptibilities. The destruction of primary magnetite by hydrothennal alteration within the
Anthony Road Andesite has previously been reported by Sharpe (l993b, 1994).
Hornblende andesite that has altered magnetite (and correspondingly low magnetic
susceptibility) shows a broader range in major and trace element levels than hornblende
andesite with relatively fresh magnetite (and high magnetic susceptibility; Table 7.4). The
immobile element ratios P205ffi02 and TilZr plot as straight lines which pass through the
origin on bivariate plots for both altered and unaltered sample sets, implying that these
elements have been immobile and that the two sample sets are related (Figure 7.3; Barrett
and MacLean, 1994). The greater range of major and trace element levels in the hornblende
andesite with altered magnetite phenocrysts is best shown on bivariate plots with an
immobile element ratio on the abscissa (e.g., K20, CaO, MgO versus TilZr; Figure 7.4).
The geochemistry indicates that the differing magnetic properties are not the result of primary
geochemical differences. The destruction of magnetite phenocrysts during alteration to
chlorite, leucoxene, pyrite and goethite corresponds to a lowering of the magnetic
susceptibility. The suppression of magnetisation during hydrothennal alteration is common
under both reducing and oxidising conditions (Sweetkind et aI., 1993). The alteration of
magnetite is interpreted to have occurred during hydrothennal and diagentic alteration.
131
A 5 ..
o ~
B
o U ~
c
o bJ)
~
4
3
2
1
• Magnetic ... Non-magnetic
••
• • •..• .
•...........• ... &6
..•
.. 1/&
o 12
.. ...
.. ..
10 • Magnetic ... Non-magnetic
8
6 ... .... ...
... .&
4
2
o 10
•
""..... ....• •.......
• •• .,.•
... ...
8 • ...
Magnetic Non-magnetic
6 ....
4 .&
.&
.&
...
2
... ...
o 10 15
Ti/Zr 20 25
Figure 7.4 Bivariate plots of magnetic and non-magnetic hornblende andesite of the Anthony Road Andesite. A. K20 versus TilZr. B. CaO versus TilZr. C. MgO versus TiJZr.
Data courtesy of Aberfoyle Resources Limited.
132
Table 7.4 Representative major element compositions for magnetic and non-magnetic hornblende andesite of the Anthony Road Andesite. Data from Aberfoyle Resources Limited.
Magnetic Non-magnetic Sample No. 624356 624357 624430 624433
Si02 63.54 62.67 60.32 63.2 Ti02 0.42 0.42 0.47 0.46 A1203 15.59 15.32 15.61 15.89 FeO* 6.89 6.95 8.18 7.73 MnO 0.10 0.11 0.16 0.15 MgO 3.60 3.56 4.75 2.88 CaO 2.35 3.36 5.65 4.00 Na20 3.56 4.10 4.26 3.22 K20 3.77 3.32 0.36 2.26 P20S 0.18 0.18 0.22 0.21
TOTAL 100 100 100 100
TilZr 18.22 17.42 21.44 19.24
7.4 Discussion
It is possible that three alteration events have affected the Anthony Road Andesite: 1.
Cambrian syn~volcanic alteration (diagenetic and hydrothermal); 2. regional metamorphism
accompanying major deformation and 3. localised syn-tectonic alteration.
Emplacement on or near the seafloor during the Cambrian exposed the hornblende andesite
association to diagenetic alteration on burial. On initial emplacement, the volcanics would
have been fractured, porous and permeable, allowing easy access for seawater. Reactions
occurring during diagenetic alteration include hydration, alkali exchange between volcanic
glass (which is thermodynamically unstable) and seawater, dissolution of glass and
precipitation of cements in pore spaces (Bischoff and Dickson, 1975; Stolz, 1992). The
albite - chlorite ± epidote ± sericite ± carbonate ± hematite alteration assemblage observed in
the upper hornblende andesite association may be the result of low-temperature diagenetic
alteration.
Alteration associated with intense hydrothermal activity is commonly pervasive and
destructive. of primary rock textures. The silica - sericite ± chlorite ± pyrite alteration
observed at "pyrite corner" and around Leech Hill is interpreted to be Cambrian and
hydrothermal in origin because the altered rocks have been subsequently cleaved during
regional metamorphism, and the alteration is intense and of limited extent. Hydrothermal
alteration was responsible for the replacement of hornblende phenocrysts by chlorite, silica
and calcite and for the pervasive replacement of groundmass by chlorite, sericite, calcite and
133
pyrite. Pyrite in the Mount Read Volcanics is considered to be hydrothennal in origin (Offler
and Whitford, 1992).
Regional metamorphism to prehnite - pumpellyite facies has overprinted earlier alteration
across the study area (Turner, 1968; Eastoe et al., 1987; Deer et aI., 1992). Minerals fonned
in VHMS hydrothermal alteration zones are also those stable in this grade of regional
metamorphism (Jolly, 1978; Barrett and MacLean, 1994). IQ, the· hornblende andesite
association, albite alteration of plagioclase phenocrysts, development of pumpellyite, rare
fibrous actinolite and chlorite, albite and carbonate throughout the groundmass, together
with the widespread replacement of groundmass, and pre-existing chlorite, by epidote are
interpreted to be of regional metamorphic origin.
7.4.1 Timing of alteration
Constraints on the timing of alteration events in the Anthony Road Andesite are provided by
absolute age dating of the volcanics, alteration studies from elsewhere in the Mount Read
Volcanics and by correlation of the main defonnation event in the area with the main phase
of the Devonian defonnation across western Tasmania.
Absolute age dating indicates emplacement of the Anthony Road Andesite occurred in the
late Middle Cambrian at around 502.2±3.5 Ma (Perkins and Walshe, 1993). From
emplacement, the volcanics were susceptible to alteration resulting from interaction with
seawater and diagenetic alteration. The Anthony Road Andesite was an active volcanic centre
that would have focussed both heat and fluid flow through the volcanic pile, resulting in
hydrothennal alteration. Hydrothennal alteration could have continued for a prolonged time
period following emplacement. Seawater/diagenetic and hydrothermal alteration that
occurred synchronous with and closely following emplacement of the Anthony Road
Andesite can thus be considered Cambrian in age.
Structural defonnation correlated with the Tabberabberan Orogeny and associated regional
metamorphism affected the Mount Read Volcanics during the Devonian (Williams, 1979;
Seymour, 1980; Corbett, 1992; Crawford et aI., 1992). Cleavage development and
metamorphism to prehnite-pumpellyite facies observed in the volcanics of the study area can
be correlated with the Devonian defonnation events.
134
7.4.2 Comparison with alteration elsewhere in the Mount Read Volcanics
From handspecimen and thin section study, it is apparent that alteration of the Anthony Road
Andesite is mineralogically similar to but not as intense as the alteration observed close to the
massive sulphide deposits elsewhere in the Mount Read Vo1canics. In alteration haloes to the
orebodies, feldspars are commonly destroyed as Na20 and CaO have been removed from
the rocks by fluids enriched in FeO*, MgO and K20 (Gemme}l and Large, 1992; Large,
1992). In general in the Anthony Road Andesite, primary feldspar phenocrysts, though
albitised and variably replaced by sericite and chlorite, are commonly still preserved. Local
exceptions to this observation do exist (e.g., pyrite corner silica - sericite - pyrite alteration
zone) where alteration is intense and pervasive.
The alteration index (AI) of Ishikawa et al. (1976) 1 increases with an increase in alteration
intensity and proximity to massive sulphide mineralisation. At the Hellyer deposit, the AI
increases from 36 in unaltered andesite on the periphery, to 91 in the siliceous core zone of
the alteration pipe beneath the deposit (Gemmell and Large, 1992). The AI in the hornblende
andesite association of the Anthony Road Andesite varies between 18 and 62. The lack of
higher AI values indicates that Na20 and CaO have not been as strongly depleted in the
vo1canics sampled as they commonly are in massive sulphide deposit alteration zones
(Large, 1992).
The pervasive, texturally destructive, silica - sericite ± chlorite ± pyrite alteration at "pyrite
corner" is similar to alteration in the siliceous core at the Hellyer deposit and probably also
formed at high water/rock ratios by high temperature (> 300°C), low pH fluids (Gemmell
and Large, 1992; Large, 1992).
In the Mount Read Vo1canics, hydrothermally altered vo1canics close to mineralisation show
strong depletion in Na20 and CaO and enrichment in K20 (Eastoe et al., 1987; Gemmell and
Large, 1992; Large, 1992). These vo1canics now commonly contain only -0.1-1.0 wt%
Na20 and due to K20 enrichment, have K20/Na20 ratios> 2.5 (Eastoe et aI., 1987;
Gemmell and Large, 1992). In contrast, the hornblende andesite association maintains CaO
levels from 0.5 to 7.8 wt% (mean 4.59 wt%), Na20 from 0.77 to 8.03 wt% (mean 4.41
wt%) and Sr from 75 to 983 ppm (mean 504 ppm), indicating that CaO and Na20 have not
been leached. The mean K20INa20 ratio for the andesites is 0.68 (Appendix E).
1 AI =100 (MgO+K20) / (Na20 +CaO+MgO+K20 )
135
7.5 Conclusions
Data from hand specimens, drillcore and thin sections combined with facies analysis and
structural studies have been used to document alteration· within the hornblende-andesite
association of the Anthony Road Andesite. In this chapter, the alteration has been described
in terms of specific mineral assemblages that may be related to distinct alteration phases
within the Anthony Road Andesite. The results have proved som.Ywhat inconclusive due to -::
the wide distribution of a limited number of dominant alteration minerals which can be
formed in both hydrothermal and regional metamorphic environments.
Alteration throughout the homblende-andesite association has resulted in the development of
new mineral assemblages accompanied by geochemical changes and the partial to complete
destruction of any volcanic glass and primary phenocryst phases.
The replacement of magnetite by leucoxene, goethite and chlorite has caused local reductions
in the magnetic susceptibility of the hornblende andesite association. The alteration of
magnetite is interpreted to have occurred during hydrothermal and diagenetic alteration.
136�
l-3 = ~ r-c
~ ~
z ~
(1 =
~
~
"CS
~
f""!'"
~
S"�
"'1
00
=
~
"'1
0 rIJ. =
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Chapter 8�
The Lake Newton Ironstone�
8.1 Introduction
Ironstone that occurs in the upper Anthony Road Andesite on the shore of Lake Newton,
western Tasmania (Figure 8.1), is referred to here as the Lake Newton Ironstone. Clasts of
ironstone also occur in andesitic-dacitic volcaniclastic sandstone in the Howards Anomaly
area and in polymictic volcanic breccia in the bed of Tyndall Creek (Lynchford Member)
close to the Anthony Road (Figure 8.1).
Ironstone has been reported from two other localities in the Mount Read Volcanics: the
Henty Mine (Yeats, 1989; Halley and Roberts, 1997) and the Comstock area near
Queenstown (MacDonald, 1991). All three localities are close to Cambrian mineralisation.
The Lake Newton Ironstone is < 500 m away from and possibly also stratigraphically
equivalent to low grade disseminated Ag and anomalous base metal mineralisation at
Howards Anomaly. Deformed clasts of ironstone are also present where barite-base and
precious metal-rich veins outcrop in Tyndall Creek. At the Henty Au Mine, ironstone
associated with calcite occurs along strike from massive pyrite (Halley and Roberts, 1997).
Ironstone and siliceous chert occur stratigraphically above the Lyell Comstock Pb-Zn
deposit, Mount Lyell mine area (Corbett and Solomon, 1989; MacDonald, 1991), although
the extent and volume of ironstone are not well documented.
The occurrence of ironstone in the study area is significant as ironstones are commonly
found at the same stratigraphic level as VHMS mineralisation (Large, 1992). In addition,
bacteria similar to bacteria found in modem seafloor iron-rich deposits (Boyd and Scott,
1994) have been identified in the Lake Newton Ironstone, and by analogy, provide
constraints on the temperature and depositional environment.
In this chapter, the Lake Newton Ironstone is described in terms of the field relationships
with the surrounding volcanic facies, petrography, and major and trace element
geoche~stry.
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8.1.1 Terminology
The Lake Newton Ironstone is essentially composed of iron and silica (FeO*l+SiOz > 95
wt%). Terminology for such iron-silica rocks is diverse, and includes exhalite, ironstone,
tuffaceous exhalite, jasper, jaspilite, tetsusekiei, ochre, umber and ferruginous chert. In most
sedimentary classifications, an ironstone is a rock containing >15 wt% Fe (James, 1966;
Adams et al., 1985) and on this basis, most analysed samples of the Lake Newton unit are
ironstones (FeO* 9.93 - 31.31 wt%). The term ironstone is also preferred in being largely
descriptive. Terms such as exhalite are genetic, implying formation from hydrothermal fluids
which have exhaled~ and are inappropriate unless that origin can be proven.
8.2 Field relationships and associated alteration
On the shore of Lake Newton, the Lake Newton Ironstone outcrops at Jasper Point in the
upper Anthony Road Andesite (Figure 8.1). Outcrop is limited by vegetation and by
fluctuating water levels in Lake Newton. In this area, ironstone was observed in two
circumstances: 1. as matrix in a polymictic volcanic breccia (matrix-supported andesitic
breccia; Plate 8.1) and 2. as diffusely laminated ironstone (Plate 8.2).
Ironstone forms the matrix of a clast-supported polymictic volcanic breccia unit comprising
blocky to angular clasts (5 to 10 cm across) of hornblende andesite and rhyolite (TSz 88
3/94). Although ironstone predomin;mtly occurs in the matrix, the clasts are commonly
weakly hematite altered and contain disseminated hematite (TS 76-4/94). Hematite alteration
is common in the Anthony Road Andesite only at this stratigraphic level.
In some outcrops, ironstone occurs in diffusely laminated intervals less than one metre thick
directly overlying the polymictic volcanic breccia with the ironstone matrix. The diffusely
laminated ironstone comprises quartz and hematite, and cross-cutting quartz veinlets (1-3 cm
wide) are common. Beds are less than 5 mm in thickness.
Clasts of ironstone have also been observed in outcrop in the Howards Anomaly area and in
Tyndall Creek. Ironstone clasts at Howards Anomaly occur in strongly hematite-carbonate
altered andesitic-dacitic volcaniclastic sandstone at a similar stratigraphic level to ironstone on
the shore of Lake Newton. Clasts of ironstone occur in the Lynchford Member of the
Tyndall Group in Tyndall Creek, beside the Anthony Road. The Lynchford Member and
1 FeO* =FeO+Fez0 3 2 TS = Thin section
139
volcaniclastic units in the upper Anthony Road Andesite are interpreted to interfinger
(Chapter 4) and it is likely that all the ironstone occurrences are at a similar stratigraphic
level.
8 .3 Ironstone mineralogy and texture
The Lake Newton Ironstone is deep red to purple and compos6d of varying proportions of
quartz (silica) and hematite. Hematite is commonly fine grained and amorphous though
euhedral grains ranging from, 30 Jlm (TS 624406) to 360 Jlm (TS 89a-3/94), have been
observed. Megaquartz (Kneller et al., 1968) and microcrystalline quartz composed of equant
to elongate grains, occur throughout the ironstone. Three textures are common: 1. spherical
globules composed of hematite rims and microcrystalline quartz centres; 2. hematite
fIlaments enclosed by microcrystalline and megaquartz; and 3. diffuse laminae composed of
varying proportions of hematite and microcrystalline quartz. Spherical globules and filaments
have been observed in ironstone forming the matrix to polymictic volcanic breccia and in
diffusely laminated ironstone.
The globules are ovoid with diameters varying from 50-70 Jlm (e.g., TS 624406).
Microcrystalline quartz grains, 7-10 Jlm across, are enclosed within the hematite rim.
Aggregates of up to 10 interconnected globules are common whereas single globules are
uncommon.
Single strand filaments 10-40 Jlm wide and up to 2 mm in length (e.g., TS 82-3/94; Plate
8.3) are composed predominantly of hematite with minor microcrystalline quartz. The
fIlaments are straight or wavy forms and some are branched. Filaments display no preferred
orientation. The hematite-dominated filaments are enclosed by microcrystalline quartz (-10
Jlm grain diameters).
Diffusely laminated ironstone is distinct from ironstone which forms the matrix to the
polymictic volcanic breccia unit. Alternating beds are composed of varying proportions of
hematite and microcrystalline quartz (Plate 8.2; TS 89a-3/94). The laminae range from 200
Jlm up to 2.8 mm in thickness. Hematite predominates throughout the majority of laminae
where it 0Gcurs as a mat of interconnected to dispersed grains.
140�
Plate 8.1 Ironstone occurs as the matrix of a polymictic volcanic breccia on the shore of Lake Newton ("Jasper Point"). v=volcanic c1ast of hornblende andesite or rhyolite. (GR 38061OE, 5358425N)
~:.
Plate 8.2 Photomicrograph of diffusely laminated ironstone which outcrops on the shore of Lake Newton (TS 89a-3/94, plane polarised light, magnification x5). Hematite and quartz grains predominate. (GR 380665E, 5358520N)
Plate 8.3 Photomicrograph of ftlamentous texture observed in the Lake Newton Ironstone (TS 82-3/94; plane polarised light, magnification x5). (GR 380625E, 5358440N)
141�
8.3.1 Microbial fossils
The single strand filamentous textures present in the diffusely bedded Lake Newton
Ironstone (Plate 8.3) are similar to bacterial mats found at hydrothermal vent sites on the
modem seafloor (e.g., Juniper and Fouquet, 1988; Binns et al., 1993; Boyd and Scott,
1994) and also recognised in ancient ironstones (e.g., Duhig et al., 1992) where they have
been replaced by silica and hematite deposited from hydrotherrp.al fluids. Amorphous iron
oxides recovered from active, low temperature (20° to 30°C), vent sites in the Woodlark
Basin, Papua New Guinea, contain iron-oxidising bacteria (Boyd and Scott, 1994) that can
survive in deep water, metabolising ferrous iron as an energy source (Juniper and Fouquet,
1988; Shock, 1994). The comparable filamentous textures in the Lake Newton Ironstone
could have been produced by microbial and low temperature hydrothermal activity.
8 •4 Ironstone geochemistry
Ironstones display wide compositional ranges, particularly in Si02, FeO*, Al203 and MnO,
and classification schemes are based on these oxide and base or precious metal
concentrations (Bonatti, 1975; Hekinian et al., 1993).
8 •4 •1 Analytical methods
Diffusely laminated ironstone samples were analysed by ANALABS Pty. Ltd. Major
elements and many trace elements (Ba, As, Cr, Zr, V, Nb, Y, Sr, Rb, Sb) were determined
by XRF. Other trace elements (Cu, Pb, Zn, Ag, Ni, Co, Bi, Cd, Hg) were determined using
AAS techniques. Rare earth elements (REE) were determined by rCP-MS. Au was
determined by fire assay (detection limit 0.008 ppm). Two additional diffusely laminated
ironstone samples were analysed at the University of Tasmania. Major elements and trace
elements (Y, Rb, Ni, Nb, Zr, Sr, Ba, Sc, V, Cr) for these were determined by XRF
following the methods of Norrish and Chappell (1977). REE analyses were determined
using the ion-exchange XRF technique of Robinson et al. (1986). Major and trace element
analyses are listed in Table 8.1 whereas REE analyses are listed in Table 8.2.
142�
Table 8.1 Major and trace element analyses of the Lake Newton Ironstone and altered and unaltered hornblende andesite and quartz-feldspar rhyolite of the Anthony Road Andesite.
Lake Newton Ironstone Andesite RhyoIite Altered Unaltered Altered Unaltered
Sample no. 10-2/95 11-2195 12-2/95 13-2195 15-2/95 16-2195 18-2195 89a-3/94 82-4/94 14-2195 HA3229.9 9-2195 71-4/94 (wt%) Si02 89.20 67.10 75.40 87.80 87.90 85.70 79.20 82.39 84.55 65.21 57.92 57.45 72.17 Ti02 0.01 0.04 0.05 0.02 0.04 0.05 0.Q7 0.01 0.01 0.55 0.69 0.79 0.31 Al203 0.27 0.66 1.13 0.35 0.75 0.97 2.66 0.23 0.29 16.23 18.99 22.03 15.24 Fe203 9.93 31.91 22.39 11.23 10.35 11.99 16.1 I 16.35 13.74 7.84 7.04 7.99 2.58 MnO 0.01 0,0] 0.03 0.01 0.03 0.03 0.03 0.01 0.01 0.22 0.07 0.12 0.04 MgO 0.04 0.12 0.19 0.05 0.11 0.13 0.50 0.02 0.01 1.43 2.55 l.l0 0.75 CaO 0.Q7 0.08 0.03 0.02 0.1 I 0.16 0.04 0.03 0.14 2.31 3.94 3.21 0.27 Na20 0.05 0.05 0.14 0.06 0.10 0.15 0.80 <0.03 <0.03 3.25 8.00 4.37 5.40 K20 0.Q3 0.10 0.19 0.08 0.11 0.13 0.1 I 0.05 0.02 2.67 0.52 2.74 3.06 P205 0.01 0.02 0.01 0.01 0.Q3 0.02 0.Q3 0.02 0.02 0.30 0.27 0.18 0.17
L.O.I. 0.13 0.36 0.41 0.40 0.33 0.36 0.65 0.27 0.19 2.05 2.77 2.77 1.27 Total 99.75 100.45 99.97 100.Q3 99.86 99.69 100.20 99.38 98.98 100.00 100.00 100.00 100.00 Trace Elements (ppm) Se na na na na na na na 2 4 na 23 na 13 V 65 129 129 82 50 129 235 95 269 223 201 232 86 Cr <5 <5 <5 6 9 <5 <5 <2 2 19 47 7 12 Co <5 <5 6 <5 <5 <5 <5 na na 37 na 15 na Ni <5 <5 <5 <5 <5 <5 <5 5 3 15 15 <5 6 Cu 5 4 <4 <4 33 9 <4 na na 68 na 8 na Zn 10 18 26 II 25 26 54 na na 193 na 142 na As 10 30 25 21 42 34 15 na na 132 na 10 na Rb 5 9 10 <5 II 6 8 2 I 103 12 100 63 Sr 21 25 22 9 39 55 43 6 30 568 589 710 275 y 6 8 5 3 8 7 5 5 4 28 22 27 18 Zr 16 15 32 13 22 39 30 36 24 157 185 171 147 Nb <3 3 <3 <3 <3 <3 <3 5.3 2.2 9 10 8 8.7 Ag <2 <2 <2 <2 <2 <2 <2 na na <2 na <2 na Cd <2 <2 <2 <2 <2 <2 <2 na na <2 na <2 na Sb 14 40 23 27 25 43 16 na na 30 na 14 na Ba 77 111 135 79 82 77 63 43 52 1059 185 1603' 2153 Au <0.008 <0.008 <0.008 <0.008 <0.008 <0.008 <0.008 na na <0.008 na <0.008 na Hg 0.06 0.08 0.05 0.08 0.1 0.15 0.06 na na 0.1 na 0.06 na Pb <5 <5 <5 <5 184 59 <5 na na 43 na 7 na Bi <20 <20 <20 <20 <20 <20 <20 na na <20 na <20 na
Ti!Zr 3.75 15.99 9.37 9.22 10.90 7.69 13.99 1.67 2.50 2l.l8 22.42 27.79 12.83 P2051Ti02 1.00 0.58 0.26 0.55 0.83 0.42 0.36 2.00 2.00 0.55 0.39 0.23 0.55 Si02IFeO* 8.98 2.10 3.37 7.82 8.49 7.15 4.92 5.04 6.15 8.32 8.23 7.19 27.99 Fe203+Si02 99.13 99.01 97.79 99.03 98.25 97.69 95.31 98.74 98.29 73.05 64.96 65.44 74.75 na not analysed
Table 8.2 Rare earth element data for the Lake Newton Ironstone and unaltered/altered andesite and rhyolite from the Anthony Road Andesite.
Andesite Rhyolite Ironstone
Unaltered Altered Unaltered Altered
Sample no. HA3229.9 14-2/95 71-4/94 9-2/95 10-2/95 11-2/95 12-2/95 13-2/95 15-2/95 16-2/95 18-2/95
La 45.7 74.1 67.7 30.4 5.3 11.3 9.3 2.2 ·7.94 10.1 10.1
Ce 93.4 141 121.8 56.2 8.8 23 16.8 3.3 14.3 16.9 20.3
Pr lOA 15.6 12.7 6.4 0.86 2.35 1.6 0.4 1.56 1.95 1.87
Ni 34.4 56.6 43.2 22.5 3 6.7 5 1.4 5.6 7 6.3
Srn 6.8 10.3 6.68 4.8 0.6 1.5 0.3 1.2 1.2 1.1
Eu 1.37 2.79 1.72 1.5 0.14 0.67 0.22 0.09 0.29 0.31 0.29
- Gl 4.4 7.9 3.87 3.6 0.6 1.5 0.7 0.3 1.2 1.1 0.9
t� Dy 3.3 5.2 3.23 3.4 0.5 1.4 0.8 0.4 1.1 1 0.7
Er 1.9 2.6 2.15 2.3 0.4 1.2 0.6 0.4 1.1 0.8 0.5
Yb 1.8 2.4 2.13 2.2 0.8 1.5 0.7 0.8 2.3 1.7 0.7
(La/Sm)N 4.2 4.5 6.4 4.0 5.6 4.7 5.9 4.6 4.2 5.3 5.8
(SmlDy)N 3.4 3.3 3.4 2.3 2.0 1.8 2.1 1.2 1.8 i~",
2.0 2.6
(DylYb)N 1.2 1.4 0.99 1.0 0.41 0.61 0.75 0.33 0.31 0.38 0.65
(La/Yb)N 17.2 20.9 21.5 9.4 4.5 5.1 9.0 1.9 2.3 4.0 9.8
8.4.2 Major and selected trace element geochemistry
The diffusely bedded Lake Newton Ironstone is dominantly composed of silica and iron
oxides (Si02+FeO* > 95.31 %; mean 98.14%; Table 8.1). The relative proportions of FeO*
oxide and silica vary significantly, with a range of Si02 from 67.10 to 89.20 wt% and a
range of FeO* from 9.93 to 31.91 wt %. Ah03 is the only other oxide which exceeds 1
wt% (in two samples) with a maximum value of 2.66 wt% (Tablec8.1):
Trace element concentrations are consistently low with many abundances close to the
detection limits for the analytical techniques used. Zr, Ba, Y, Rb, Sr and Cr concentrations
are several times lower than in the host volcanic facies (Table 8.1). Lake Newton Ironstone
samples all contain low to undetected concentrations of base and precious metals (Zn 10 to
54 ppm; Pb <5 to 184 ppm; Ag <2 ppm; Au <0.008 ppm; Table 8.1).
8.4.3 Rare earth element geochemistry
Total REE abundances for ironstone samples (Table 8.2) are relatively high with
concentrations well above the detection limits for the analytical techniques used.
The REE patterns for the ironstone samples are characterised by light REE and heavy REE
enrichment with lower middle REE concentrations (Figure 8.2). Enrichment of the light REE
is indicated by (La/Sm)N of 4.2 to 5.9 whereas heavy REE-enrichment is indicated by
(DylYb)N of 0.75 to 0.31 (Table 8.2). Ironstone samples have similar light REE-enriched
REE patterns to the host volcanic facies (although at lower concentrations). REE patterns for
the ironstone are different in two samples (13 and 16) which have a negative Ce anomaly,
and one sample (11) has a positive Eu anomaly (Figure 8.2).
8.4.4 Geochemical characteristics of the Lake Newton Ironstone
Ironstone in general consists of components from two main sources: hydrothermal and non
hydrothermal. The main non-hydrothermal components in the ironstone could include
components of clastic input and/or components precipitated from seawater.
In the Lake Newton Ironstone samples, Ti02, MgO and Na20 correlate positively with
Al203 (Figure 8.3). These components are abundant in clasts of hornblende andesite and
rhyolite in the polymictic volcanic breccia and may have been derived via clastic input from
this source.
145
••
Iron in ironstones is widely interpreted as being of hydrothermal origin, leached at depth by
hydrothermal fluids, based on analogues on the modem day seafloor (Sigurdsson, 1977),
and its concentration relative to clastic components can discriminate between deposits of
hydrothermal and non-hydrothermal origin (Bonatti et aI., 1972; Adachi et aI., 1986). All
samples from the Lake Newton Ironstone plot within the hydrothermal field on the Fe
(Ni+Co+Cu)10-Mn diagram employed by Bonatti (l975;,Figure 8.4) and in the
hydrothermal field defmed on a Fe-AI-Mn plot by Adachi et al. (1986; Figure 8.5).
The FeO*- and Si02-rich nature of the diffusely bedded ironstone suggests that the major
elements of the Lake Newton Ironstone are primarily hydrothermal in origin and that only a
minor clastic component is present.
Two samples (13 and 16) exhibit slight negative Ce anomalies (Figure 8.2) that may be
attributed to convective mixing of hydrothermal fluid with seawater (cf. Barrett et aI., 1990;
Binns et aI., 1993). Seawater is depleted in Ce relative to the other lanthanides due to its
removal in Ce02, as Ce3+ changes valence state to Ce4+ (Brookins, 1989). As hydrothermal
precipitates settle out of the water column, they scavenge REE from seawater (German et al.,
1990) and ultimately can acquire REE patterns similar to seawater (Brookins, 1989; German
et al., 1990).
100�
Lake Newton Ironstone� ____ 10�
- . + - 15
.....-- 11 ---.& - 16 ~:: ..
_....-. 12 - -0- . 18 +-"" .... , ~. ". .. ~~ ' e 13+ ..'.. ..
~
s .. "�.8 I::
10 . .~-. +�•.. ~ -.~
.--. .."• +. -'. '._.. ~ -..::':.~,-?to.- ". ...,. ,
",.~ .-.. . . ...... -/'. • '\lo... of- • • .'. -.. • -6-6.. :...:-. • _ ..'.~.'iJ --.: ..... _•.'. -' ..,, ..." ':.""...... - -- ........- ".
1 I i i i i i i i , i i , I , , i i i i i I i i i a i i , , i i i I
La Ce Pr Nd Srn Eu Od Dy Er Yb
Figure 8.2 Chondrite-normalised rare earth element plot for samples of the Lake Newton Ironstone. Chondrite normalising values are from Boynton (1984).
••
0.08 [ I I 1 I-I�
QOO •�
0.06
0.05 t- •• ~
~ 0.04 I- ..�
SN
0.03 l-
o.m� I ,.t · 0.01 _
o I I I I I
o 0.5 1 1.5 2 2.5 3
0.6 t- .-~- \- I I
05 •
~ 0.4� ~
o 0.3 Ib/)
~
0.2 ... •
0.1 r .• •
o I .. I I I I I
o 0.5 1 1.5 2 2.5 3
I I I I 1
0.8� .l ~ 0.6
~ 0.4 Z
0.2 • •• o I ... I· I I I I
o 0.5 1 1.5 2 2.5 3
A120 3 wt%
Figure 8.3� Major element plots of Ti02. MgO and Na20 versus A1203 for samples from the Lake
Newton Ironstone (n=9).
147�
(Ni+Co+Cu)10
Lake Newton ironstones (Sample population =9)
Hydrothermal field
Fe� Mn
Figure 8.4� (Ni+Co+Cu)lO-Fe-Mn triplot for the Lake Newton Ironstone. Diagram and fields are from Bonatti (1975; in Binns et al. 1993).
Al
Non-hydrothermal field
Ironstones from Lake Newton (Sample population = 9)
-- .... .... : ~Hydw'h"",," field
I / ~
Fe� Mn
Figure 8.5� Fe-Al-Mn triplot forhydrothermal and non-hydrothermal ironstones. Diagram design and� fields are from Adachi et al. (1986).�
148
Sample 11 has a distinct positive Eu anomaly (Figure 8.2). Positive Eu anomalies in
hydrothermal deposits result from the scavenging ofEu from plagioclase feldspar by reduced
hydrothermal fluids at depth in the volcanic pile (Barrett et aI., 1990; Klinkhammer et al.,
1994). Samples of Fe-rich deposits close to hydrothermal vent sites on the Southern
Explorer Ridge, northeastern Pacific, have positive Eu anomalies, whereas sediment samples
distal to the vent sites do not (Barrett et al., 1990). Positive Eu anomalies in REE patterns
also occur in Fe-rich samples from active vent sites on the ¥ast Pacific Rise at 21°N
(Michard et aI., 1983), on the Mid-Atlantic Ridge (Campbell et aI., 1988) and in the,
Escanaba Trough (Campbell et al., 1993).
Eight of the nine ironstone samples display a negative Eu anomaly in their REE pattern.
Negative Eu anomalies suggest that either: 1. the hydrothermal fluid was sufficiently
oxidised so that Eu in the fluid was present dominantly as Eu3+ and thus was not leached
from plagioclase feldspar by deep hydrothermal fluids (Brookins, 1989) or; 2. the
hydrothermal fluid re-equilibrated or mixed with seawater which typically displays a slight
negative Eu anomaly (Brookins, 1989).
All Lake Newton Ironstone samples show marked heavy REE-enrichment. Deep seawater
also exhibits heavy REE-enrichment although in general, seawater and ironstone REE
patterns are not well matched (Elderfield and Greaves, 1982; Brookins, 1989). Other
processes which can explain enriched heavy REE patterns are: 1. complexation of REE by
anions (HC03-) in the circulating hydrothermal fluid; 2. substitution of heavy REE for Fe3+,
or, 3. ion-exchange during alteration of heavy REE-rich minerals such as hornblende or
clinopyroxene in the volcanic pile.
Klinkhammer et al. (1994) have shown that the REE patterns of fluids from 21l1N on the
East Pacific Rise are dominated by the effects of ion exchange reactions between
hydrothermal fluids and plagioclase in the volcanic pile. The REE patterns of fluids from the
East Pacific Rise correspond well with partition coefficient data from hydrothermal
plagioclase in basalts when plotted on an Onuma diagram (Onuma et al, 1968; Klinkhammer
et aI., 1994). REE analyses of the Lake Newton Ironstone and partition coefficients for
clinopyroxene, hornblende, plagioclase and apatite are listed in Table 8.3 and plotted on an
Onuma diagram in Figure 8.6. The REE plotted were assumed to be in a +3 oxidation state
except for Eu which was assumed to be in a +2 oxidation state (Rollinson, 1993). The three
representative samples (11, 13, 16) of the Lake Newton Ironstone show very similar trends,
from Er to La, which correspond well with the trend of plagioclase and differ only with
respect to Eu (Figure 8.6). This similarity suggests that the REE patterns in the Lake Newton
Ironstone were controlled, to an extent, by REE-liberating ion exchange reactions with
plagioclase at depth.
149
Table 8.3 Mineral/melt partition coefficient data for plagioclase, clinopyroxene, hornblende and apatite in basaltic to dacitic liquids (Rollinson, 1993). Ionic radius (Angstroms) are for eight-fold co-ordination. It is assumed that the REE are in a +3 oxidation state except for Eu which is +2.
Plagiodase1 Clinopyroxene1 Hornblende1 Apatite2 Ionic Radius3
La 0.1477 0.056 0.5442 14.50 1.160�
Ce 0.0815 0.092 0.8430 21.10 1.143� .....
Pr 1.126
NO 0.0551 0.230 1.3395 32.80 1.109
Srn 0.0394 0.445 1.8035 46.00 1.079
Eu 1.1255 0.474 1.5565 25.50 1.250
Qj 0.0310 0.556 2.0165 43.90 1.053
Dy 0.0228 0.582 2.0235 34.80 1.027
Er 0.0202 0.583 1.7400 22.70 1.004
Yb 0.0232 0.542 1.6420 15.40 0.985
*Mineral/melt partition coefficient data are for basaltic melts except for apatite which are for dacitic melts. Data sources are (1) Fujimaki et al. (1984), (2) Fujimaki (1986) and (3) Shannon (1976).
100
Apa~it~ 11 _
......0- -'D-o _ • _ Lake Newton
.........- Ironstone
10 "'-. -./~ " .-' -"'. - .. ...
~
~ Lake Newton trl>Fl~One..... "'--'•..~_-----m'u • ".
S Q) A - lIr- ---6_ .- .. - -. - m- ,.)It Thalanga - _•~ _ 0 U .... ~- --- --s:: 1 -..---_ ..... -' - -_m- - --.. A. Hornblend~ 0 ..0 "m- - -m- -- "-.- .. .~ -1+ - - + . - -+- . + It-p.,
'O
Clinopyroxene - ...
0.1
+
0.01 Srn La Fn
1 1.05 1.1 1.15 1.2 1.25 1.3
Radius (VIII)
Figure 8.6 Onuma diagram of partition coefficients (mineral/melt) for hornblende, plagioclase, clinopyroxene (Fujimaki et al., 1984) and apatite (Fujimaki, 1986) against ionic radius in eight-fold coordination (Shannon, 1976; Onuma et al., 1968). Chondrite normalised REE for the Lake Newton Ironstone and Thalanga ironstone (Duhig et al., 1992) are also shown. Normalising factors for the REE are those of Boynton (1984).
150
Ironstone samples with the lowest REE abundances (samples 10 and 13) also have the
lowest abundances of Ti02, A1203, FeO*, MgO, K20, Rb and Sr and two of the highest
Si02IFeO* ratios (Tables 8.1, 8.2). This could indicate that the ironstone samples with
higher REE abundances have gained a large proportion of these REE from the non
hydrothermal sources.
8.5 Discussion
8.5.1 Origin of the Lake Newton Ironstone
Filamentous textures in the diffusely laminated ironstone are interpreted as microbial in
origin and demonstrate, based on modern seafloor analogues, that the ironstone was
deposited on or within metres of the seafloor. Geochemical data indicate that the ironstone is
largely of hydrothermal origin with only minor components derived via clastic input from the
host volcanic rocks. The quartz-hematite mineralogy of the ironstone indicates that the fluid
was oxidised whereas the presence of microbial filaments indicates low fluid temperatures.
Such low-temperature, Fe-rich hydrothermal fluids are actively exhaling at many localities on
the present day seafloor (e.g., Woodlark Basin, Papua New Guinea, Boyd and Scott, 1994;
and Southern Explorer Ridge, northern Pacific, Barrett et al., 1990).
REE compositions of the Lake Newton Ironstone record mixing of seawater and
hydrothermal fluid. The Onuma diagram (Figure 8.6), and a positive Eu anomaly in sample
11 (Figure 8.2), are consistent with leaching of REE from plagioclase in the volcanic pile by
hydrothermal fluids. Negative Ce anomalies in two samples (13 and 16; Figure 8.2) suggest
scavenging of REE from seawater following the precipitation of iron oxyhydroxides from
the hydrothermal fluid.
8.5.2 Model for ironstone formation
Based on field relationships, textural studies, geochemical data and the characteristics of
modem day analogues, a two-stage model is proposed for the formation of the Lake Newton
Ironstone (Figure 8.7).
Stage I
Iron- and silica-rich, < 100°C hydrothermal fluids rose along fractures through the volcanic
pile and precipitated to form and/or replace the matrix to polymictic volcanic breccia near the
top of the Anthony Road Andesite.
151
i
ii
~ Hydrothermal fluids .. Ironstone
A Andesite /1 -t Rhyolite v po Polymictic breccia
1\J\
Diffusely laminated ironstone
\
Figure 8.7 Schematic illustration of the two stages of Lake Newton Ironstone formation. i. Hydrothermal fluids rise along faults and fractures in polymictic breccia and precipitate ironstone. ii. Hydrothermal fluids reach the seafloor, interact and cool, and precipitate to form diffusely laminated ironstone.
Stage IT
Iron- and silica-rich hydrothermal fluids continued to rise through the volcanic pile. On
reaching the seafloor, the hydrothermal fluid interacted with seawater and precipitated
diffusely laminated ironstone.
8.5.3 Geochemical comparison with modern seafloor and ancient
VHMS-related ironstones ,.
Ironstones of exhalative origin are widely distributed in both modem and ancient geological
settings, especially volcanic settings.
Modern seafloor ironstones
Iron oxide and silica-rich deposits occur in modem continental margin, back-arc, mid-ocean
ridge and intraplate settings at sites near active submarine volcanism (Binns et al., 1993;
Boyd et al., 1993). Muds rich in FeO*, Si02 and Al203 have also been documented on the
bottom of the crater lake of Soufriere volcano, West fudies (FeO* -16-28 wt%; Si02 -30-45
wt%; Al203 -10-16 wt%; Sigurdsson, 1977) and on the floor of Santorini caldera, Greece
(FeO* -32-52 wt%; Si02 -6-27 wt%; Bostrom and Widenfalk, 1984).
Modem-day exhalites include Fe-Si-Mn oxides from the Franklin Seamount, Woodlark
Basin (Binns et al., 1993), Fe-Si oxides from DSDP Leg 32, northern Pacific (Adachi et al.,
1986) and nontronitic sediments from the Galapagos hydrothermal mound, DSDP Leg 70
(Barrett et al., 1988). The modem seafloor Fe-Si-rich deposits are enriched in MnO, MgO,
CaO, Na20, K20 and P20S relative to the Lake Newton Ironstone (Figure 8.8), reflecting
the greater clastic component in the modem examples.
Ancient VHMS-related ironstones
In VHMS-prospective terranes, ironstones form one of three types of ore-equivalent
horizons, along with sulphide-bearing sedimentary units, and pyritic and carbonaceous
shales (Figure 8.9; Large, 1992). These ironstones are predominantly composed of iron
silica±manganese oxides occurring as jasper, hematitic siltstone and quartz-hematite±quartz
magnetite rocks (Franklin et aI., 1981; Large, 1992). The ironstones display variable
relationships with massive sulphide mineralisation, and may directly overlie or be laterally
equivalent to the orebody (Duhig et al., 1992; Large, 1992).
The Lake Newton Ironstone is compared with VHMS-related ironstones (or their
equivalents) in Figure 8.8. Unfortunately, geochemical data are not available for ironstone at
153�
the Henty Mine or Comstock. The Fe-Si-rich deposits used in the comparison include the
tetsusekiei, Kuroko (Kalogeropoulos and Scott, 1983), chemical layer of Main Contact Tuff,
Noranda (Kalogeropoulos and Scott, 1983), average umber composition, Cyprus (Elderfield
et aI., 1972), the Arakapas Fe-mudstone, Cyprus (Robertson, 1978), the Key Anacon,
Bathurst (Saif, 1983), the Thalanga ore horizon (Duhig et al., 1992) and ironstones within
the Trooper Creek Formation, Mount Windsor Volcanics, Queensland (Duhig et al., 1992).
'c
Most ancient VHMS-related Fe-Si-rich deposits are enriched in MnO, MgO, CaO, Na20,
K20 and P20S relative to the Lake Newton Ironstone (Figure 8.8) possibly reflecting a
higher proportion of clastic components in these deposits. Ironstones from Thalanga and the
Trooper Creek Formation display comparable major element composition to the Lake
Newton Ironstone (Figure 8.8). The similarities in major and REE composition of the
Thalanga ironstones and the Lake Newton Ironstone suggest that they contain similar
proportions of hydrothermal and clastic components and record similar processes of
formation.
REE patterns of the Thalanga ironstones, ironstones in the Trooper Creek Formation, and
the Lake Newton Ironstone are compared on a chondrite normalised REE diagram in Figure
8.10. The REE patterns for Thalanga ironstone and sample 11 of the Lake Newton Ironstone
are closely similar, although overall, the Thalanga ironstones have lower REE abundances
(Figure 8.10). Of the Lake Newton Ironstone samples, sample 11 is the only one with a
positive Eu anomaly, a feature which is common for the Thalanga ironstones, but
uncommon for the Trooper Creek Formation ironstones.
8.6 Conclusions
The Lake Newton Ironstone was deposited in the Middle to Late Cambrian and outcrops on
the shore of Lake Newton in two circumstances: 1. as matrix in polymictic volcanic breccia
and 2. as diffusely laminated ironstone. It is a relatively pure hydrothermal precipitate,
containing only minor locally-incorporated clastic detritus, formed within metres of or on the
seafloor. Single strand microbial filaments similar to modern day iron-oxidising bacteria are
visible in thin section. The fluids which precipitated the ironstone also caused local Fe
enrichment of the host volcanic rocks. From bacteria-bearing analogues on the modern
seafloor, the fluids which formed the Lake Newton Ironstone were most likely low
temperature (20 0 to 30°C) and oxidising. The hydrothermal fluid which formed the ironstone
interacted with the volcanic pile at depth and also with seawater upon reaching the seafloor.
The Lake Newton Ironstone is geochemically very similar to ironstone in the ore horizon to
the Thalanga massive sulphide deposit, Queensland.
154
Figure 8.8
100
10
5 ()i> 0.1 0. OIl o
, ,~
I 'll I I, ~
• '''' '\,l\ J~" l!
.Il \
\\ .•1' l .....6._ •• ""-';;l.. ,J,.. \ ~_ .. -a. _......._. ":"\00.t'l .~ .•"•. .' ....""-
'" . ". ". '.."; '. ~.' ...... 0. ..-~ ,.~ 0.01 ":
Lake Newton Ironstone --0'- 11-2/95
'··9·· 13-2/95
'··6·· 16- 2/95
0.001
lOOn a I Seafloor ironstones
10 ., ~!I /'.)
- -a. - Galapagos Leg 70
....... Nth Pacific Leg32
1 1 5 8 0.1 ..
8OIl
\i t " 5\ _-a)~ •.•..~ , , .Jt. 'I ,.
\'i .J" ..... 1 ',.. J ., ' .. l' '] \. , ".~ " 1'("I" ." \ .1 ~• .".'. ,'I , •••••"1!!1 ll-'!'-I' .-' , ._ ' • •,;' .' I L '.' , 0~I\!. v ,~I'''. III I,~ f ,
ii l • .a ~
......, Franklin 5eamount
o ~ 0.01 1
i' or-tr :.... " .. ~.
0.001 I I I i i • , , I I I I
100
10
, "I~. , . i
~.i~""~\\.' ,.... .
Mt Windsor Volcanics - "D' - Thalanga Ore Horizon (537)
...... Thalanga Ore Horizon (517)
••-A•• Trooper Creek Fonnation (527)
i... ~l ..,~. ~-~
8 Cl) 0. OIl
0.1
4' <;.I•',.''''.~
.
,,~ ,... ... 11,~ L • r\,,~. .. '.'J..., It .'. ....~._ 11II"~ ";.1...,......h... __F"'" ,. .
.3 om r-..L.' JI:
...... ~ ~ _ .
0.001
100
Hydrothermal iron-formations --a.- Tetsusekei chemical layer
....... Noranda Contact Tuft 10
-··Ar·· Bathurst Key Anacon
Cyprus - Umber
5 _ Cyprus - Arakapas Mudstone
8 8OIl
.3 0.1
om 9 § Cl <i]
~ ~ ~ u ~ ~ ~ '" « ~
Plots comparing selected major elements from the Lake Newton Ironstone and ironstones from the Mount Windsor Volcanics (Duhig et aI., 1992), modem seafloor ironstones from Galapagos (Barrett et aI., 1988), North Pacific (Adachi., 1986) and Franklin Seamount (Binns et aI., 1993), tetsusekei and Noranda Contact Tuff (Kalageropoulos and Scott, 1983), Bathurst Key Anacon (Saif, 1983), Cyprus Umber (Elderfield et aI., 1972) and the Arakapas Mudstone (Robertson, 1978). Diagram modified from Duhig et al. (1992).
-- ---
v
quartz-hematite ±" sulfide·bearing " 'I magnetite exhaliteepiclastic� massive sulfide
- n O Z????22?T?> -.- J .,,?Z2?Z2?????211ZI-!� 'I or jasper --- V -.: ~i:j~.~..~~
ui.~::.:A.·J.· ·,· 't ,':..:.:.:..:.:.:.1;;····,······ .. ' ~ H J'1AD,J..1� -\ IJ'l
pyritic A A A bedded A A�
siltstone Acarbonate-sulfide�A A� A ~ lens'
Figure 8.9� Variations in ore equivalent horizons associated with Australian VHMS� deposits; based on Mount Chalmers, Thalanga, Rosebery and Hellyer.� From Large (1992).�
100 ,� i
- --- - Lake Newton ....... ,� -_.•-- Thalanga
B 10 .. --.'-- --.-'-...� ---.-_.~ "... .-.' -,� .... ......•......� \ ...•... ,...�
• I·~
,.' ,~
•...__.....'_.
0.1 I · i • I I i' I I i i' i i I I' I • I • I • I 100~ , i ,
Lake Newton Ironstone - --- - 1.1-2/95.......� • 13-2/95
""-L.=:t� ... Q)�
• 16-2/95 .-::: 10� .=-..... - ..
Mount Windsor Volcanics .{j� --... ..., _.-.'li'.__ ,_ .......�-...... --- .... - -. - - - -S17(THALANGA)-- ~ ,.- ..8 -, '~A.•.A.I__ _•• , ----t"-TH40(TROOPERCKFM)
I~
---S27(TROOP.CKFM)
~
0.1 I i i ,� , , i' i , i i' I I i I I I La Ce Pr Nd� Srn Eu Gd Dy Er Yb
Figure 8.10� Chondrite-normalised rare earth element plot comparing the Lake Newton Ironstone with ironstones from the Mount Windsor Volcanics. Top plot highlights similarities. Data from Thalanga and the Trooper Creek Formation is from Duhig et al. (1992). Chondrite values are from Boynton (1984).
156
Chapter 9�
Synthesis�
9 •1 Fades architecture of the Basin Lake area
The internal facies· architecture of the study area has been established using lithofacies
characteristics and geometry in each of the major lithostratigraphic units (Yolande River
Sequence, Anthony Road Andesite, Tyndall Group, Dwen Conglomerate; Figure 9.1).
The Anthony Road Andesite is dominated by lavas and shallow intrusions that comprise
coherent and autoclastic facies (in situ hyaloclastite, peperite and redeposited hyaloclastite).
The internal facies architecture of the Anthony Road Andesite is complex. Characteristics of
the three volcanic associations strongly reflect magma properties and the submarine
intrabasinal environment of emplacement. Submarine lavas, high-level sills and shallowly
intrusive to partly emergent domes occur throughout. Redeposition of autoclastic products
has produced large volumes of breccia and sandstone, though these have not been
transported far, being commonly poorly sorted, weakly stratified and composed of large,
angular clasts.
The composition and facies architecture of the Yolande River Sequence, Tyndall Group and
Dwen Conglomerate are distinctly different from those of the Anthony Road Andesite. This
is due in part to their derivation from extrabasinal or shallow intrabasinal sources, spatially
removed from the site of deposition. The volcaniclastic lithofacies of the Yolande River
Sequence and Tyndall Group occur as laterally continuous, moderately to well sorted, and
graded beds deposited from mass flows. Mass-flow units in the Yolande River Sequence
and Tyndall Group are syn-eruptive or immediately post-eruptive and contain large volumes
of juvenile volcaniclastic (principally pyroclastic) detritus. Rhyolite in the Tyndall Group
was extruded synchronous with volcaniclastic sedimentation and is overlain by an apron of
autoclastic breccia.
The detritus which comprises the Dwen Conglomerate is markedly different from that of the
Yolande River Sequence and Tyndall Group, being ofPrecambrian non-volcanic derivation
and having been significantly reworked prior to final deposition. Upward coarsening in the
Dwen Conglomerate and an increase in the occurrence of tractional bedforms reflect a change
to shallower water conditions.
157
--------
Middle Owen
Owen Newton Creek Conglomerate Sandstone
Jukes Conglomerate
Zig Zag Hill FormationTyndall�
Group�
Mount Julia ~I:l Memberu 0 o ',c ..... ro8 §o 0 Lynchfordu~
Member
-monornict breccia (rhyolitic)
-pyroxene-feldspar-phyric basaltic andesite
Anthony • , . I " , . I ~ .' : " .' l'·
Road� Andesite�
;. '.1\ :'. ' ••<:;.
: : • • ~ I :., :,.. ~. ;' ....
,-blocky peperite
!(andesitic breccia facies)'
rin si~ ~yalocl~stite . (andesltIc breCCla facles)
hornblende andesite massive facies
-black mudstone (throughout)
-black mudstone Yolande� River�
-vclastic turbidites Sequence
-intrusions
.'<::::>0 "00' <::::>·0 :'.':. oQq' aO···· . '0; OOQo:·.· i:".:" .:.: ;.:· 0'00 . 0···· '. 00· 0 .,~. 0" ' ..: " .' ... ,0 <::::> .. '0 0 . 00 '. ..:........ .. " .�O ..0:~Q :':" '.C>() O:OO·~.··()~O·~·b. ·.~·O·:·po;·O··:··~96":66o .... 00 0-'0 00 ~''5':' ·~o··O.. ,o. ..C> 0 .... ' .. '.
(.)~~.;:;·:/:~;·:/::~:'{7::·~·~;~~:·:·:/~Ct:;~~:·~;·\:~·~;:.:;~::r::~\~.r:~::.~:·:·::::·:;:·tf~~::~~;·):·~~;;·:·i.r:·:?:::;.~
. '0'00 9'.°60:000'0'.00 o~do.'o'o OoQ<),iOo:oocio: 0.:0: -o·o~~~;·~·b· ...o::oo: '.'0';;':-0
0<>·0.' o. 6~'C:-: . '0' ?~.l"-.'p' Cy. '. o' ·.A.C::(oO·· . 0s:::J ~ 0'",",--, '-J . ~,<;:]",",--"
.C;;~ '0 . 00' . 6 c:'0~· 6 '0' O' (j o .. 0 . ·O.o.<:~:.o:
t>'~ C>. ~-D 7"0 :-. -:--:- o~' 0:- .. o· .---:- 6-:- ~ 0 :-0 . o· .' ~~o. l;) D" ' .. - :::::;~ ~ 'V> ~. ~ ...:It> .. i.:J ·.OQ c>. ~.
o . '. o' o. . . O' '0" o' ..:o····oo.(j·:o· .0·0·· ·0.C> .0.0 .c>··o·
,. .' c: .:c:;,'.':.X~ '.. ';:'. ~ ..-:,"7' ''- .':: ..:: .~ cc, :..'CA:'" ,:.:. V ...{7 ... ':'-./. 17 u··· t:J .... ~ <::J. : ". 17' ..", . ~. ::'.:' '..-:.-:.':'-":':':::~;.::'~ ':.'.;:: ~:.:~:::: ..c:j..::::::~.:.:: ~.{::. ~"f ~
&::J 17· ..D. f::J. [7 ,.. '6" .17·.·.· .
t;o. ----D.· .. · .. e>:." ·-·.·.~D·.~~::
- • "7" .:~~
e..----"'" :---- --;-------;..-=-~
= .=. == = = ~
==.
Figure 9.1 Cartoon of the interpreted facies architecture of the Basin Lake area.
9.2 Geological history and fades architecture of the study area
The Yolande River Sequence, Anthony Road Andesite, Tyndall Group and Owen
Conglomerate within the study region were emplaced within a submarine setting. Formation
of each of the four successive major lithostratigraphic units in the area is outlined below.
."
Phase 1. The Yolande River Sequence (Figure 9.2)
The Yolande River' Sequence records below-wave-base sedimentation in a deep marine
setting. Components in volcaniclastic units were sourced from explosive dacitic eruptions at
subaerial or shallow marine, extrabasinal or basin-margin vents and transported by
voluminous turbidity currents. Volcaniclastic units are dominated by components that have
undergone little reworking prior to final deposition and are syn-eruptive or closely post
eruptive. Background suspension sedimentation formed numerous non-volcanic black
mudstone units which represent significant time intervals between the deposition of rapidly
emplaced volcaniclastic turbidites.
Seafloor topography would have been minimal during deposition of the Yolande River
Sequence as the dominant sedimentation processes (mass flows) tend to infill and bury
topographic features rather than construct them.
Phase 2. The Anthony Road Andesite (Figure 9.3)
The Anthony Road Andesite comprises an intrabasinal submarine dome, lava and intrusive
complex emplaced through and onto the Yolande River Sequence. Lavas and shallow
intrusions of basaltic andesite, hornblende andesite and rhyolite, including both coherent and
autoclastic fades, comprise the majority of the Anthony Road Andesite. Black mudstone,
limestone and ironstone are minor components. The dominance of lavas and syn-volcanic
intrusions indicates the overall proximal character of the preserved fades assemblage. The
presence of interbedded black mudstone throughout, plus an apparent total absence of
tractional bedforms, along with the character of enclosing volcaniclastic facies in the Yolande
River Sequence and Tyndall Group, indicate deposition within a wholly below-wave-base
setting.
159�
Yolande River Sequence ~ ,,~l) Inferred source 2. (. ~~_--:--......J~
..~ ...... , ",,/
I.Depositional setting
,...... 0\ o
1. intermediate-felsic extrabasinal active volcanoes 2. eruption column 3. pyroclastic flow
4. volcaniclastic turbidites 5. hemipelagic black mud
Figure 9.2 Depositional setting, processes and inferred sources for the Yolande River Sequence (4-5).
Anthony Road Andesite
-0\-�
1. subaerial volcanoes 2. active intrabasinal submarine andesitic-dacitic polygenetic volcano
3. submarine rhyolitic lava domes (hyaloclastite aprons) 4. submarine andesitic lava 5. hemipelagic black mud
Figure 9.3 Interpreted emplacement processes and setting for the Anthony Road Andesite (2-5).
The Anthony Road Andesite extends along a 7 km strike length, suggesting the presence of
numerous, intermittently active, volcanic centres. Lavas and shallow intrusions of the
hornblende andesite association have been emplaced throughout the stratigraphic history of
the Anthony Road Andesite; the basaltic andesite and rhyolite associations are restricted to
the upper stratigraphic levels. The Anthony Road Andesite is upwards of 1 km thick with
single units ranging from 2 to 3 m (e.g. black mudstone) to 150 m (e.g. rhyolite dome)
thick.
Throughout emplacement of the Anthony Road Andesite, the topography on the seafloor
would have been highly variable and constantly changing. Submarine lavas and domes
would have formed topographic highs, with aprons of related volcaniclastic deposits, as
topography was created.
Phase 3. The Tyndall Group (Figure 9.4)
The Tyndall Group represents a change from proximal, intermediate, intrabasinal volcanism
(Anthony Road Andesite) to a succession dominantly composed of the products of
extrabasinal or shallow intrabasinal, felsic explosive eruptions. In the study area, submarine
intrabasinal rhyolite lavas are restricted to the Zig Zag Hill Formation. The Tyndall Group is
dominated by breccia, conglomerate and sandstone facies rich in feldspar and quartz crystals
with abundant rhyolite and dacite clasts. The occurrence of clasts of welded ignimbrite in the
study area and the dominantly dacitic to rhyolitic provenance of the Tyndall Group suggest
the presence of felsic caldera volcanoes in the source area. The Comstock Formation is
largely syn-eruptive, comprising breccia and crystal-rich sandstone full ofjuvenile volcanic
particles, whereas the Zig Zag Hill Formation is post-eruptive, dominated by conglomerate
and sandstone composed of well rounded, significantly reworked clasts. The two formations
are the depositional record of the change from active volcanism to denudation of the source
volcanic terrain. The source terrain is not exposed.
The abundance of mass-flow bedforms and the lack of well developed tractional sedimentary
structures typical of shallow water facies indicate that the Tyndall Group was deposited in a
submarine below-wave-base setting. The volcaniclastic mass-flow units in the Mount Julia
Member and Zig Zag Hill Formation are thick, coarse, and lacking in finer interbeds which
may suggest a shallower and more proximal depositional setting than for the Yolande River
Sequence.
162�
--T-----~---\---
Tyndall Group
Inferred source
Depositional setting
1. extrabasinal subaerial active explosive felsic caldera volcanoes 2. eruption column 3. pyroclastic flow 4. submarine volcaniclastic mass�
flow deposits 5. submarine rhyolitic lava dome/flow 6. limestone 7. extinct submarine andesitic-dacitic volcanic edifice�
Figure 9.4 Depositional setting, processes and inferred sources for the Tyndall Group during the emplacement of the Comstock Formation (4-6).
The Tyndall Group is the youngest lithostratigraphic unit in the Mount Read Volcanics and
elsewhere it is separated from the older Mount Read Volcanics by an unconformity. The
Tyndall Group conformably overlies the Anthony Road Andesite in the Basin Lake area and
this may be due to deposition in a relatively deep, central part of the basin, not affected by
uplift or granite intrusion.
Phase 4. The Owen Conglomerate (Figure 9.5)
The Dwen Conglomerate conformably overlies the Tyndall Group within the study area but
is unconformable elsewhere in the Mount Read Volcanics (Corbett, 1992). The Dwen
Conglomerate records a dramatic change, from dominantly volcanic provenance in the
Tyndall Group below, to dominantly Precambrian metamorphic basement provenance. The
provenance change may reflect progressive denudation of the inactive volcanic source terrain
and uplift ofthe Tyennan Region to the east of the basin (Turner, 1989).
An overall upwards coarsening and increase in tractional bedforms occurs throughout the
Dwen Conglomerate. Facies of the Dwen Conglomerate record deposition in an alluvial or
submarine fan setting with an upward shallowing from below-wave-base (lower Newton
Creek Sandstone) to above-wave-base (upper Newton Creek Sandstone, Middle Dwen
Conglomerate) conditions (Banks and Baillie, 1989). Conglomerates are well sorted and the
clast population is well rounded indicating significant reworking in a high energy
environment prior to final deposition. The abundance of pebble to cobble· conglomerate
throughout the Dwen Conglomerate suggests that topographic relief was sharp and that
volumes of detritus were high.
9.3 Mafie-intermediate suceessions in the Mount Read Volcanics
South of the Henty Fault Zone major basaltic to andesitic volcanic successions occur
towards the top of the Yolande River Sequence in the Lynch Creek area south of
Queenstown (Lynch Creek basalts; Dower, 1991) and in the upper Central Volcanic
Complex at Crown Hill (Crown Hill Andesite; Corbett, 1992; Figure 9.6). Both the Lynch
Creek basalts and the Crown Hill andesite are directly overlain by the Tyndall Group
(Corbett, 1992). The Lynch Creek basalts belong to suite IT and suite lIT whereas the Crown
Hill Andesite belongs to suite IT (Crawford et al., 1992).
North of the Henty Fault Zone, basaltic to andesitic successions are present near Rosebery,
the Pinnacles and Hellyer. In the Rosebery area, the Sterling River Andesites occur within
164�
the Central Volcanic Complex (Corbett, 1992). At Pinnacles/Hollway Rivulet, andesites
belonging to suite III of Crawford et al. (1992) occur in the upper Central Volcanic Complex
(Coutts, 1990; McKibben, 1993). The Que-Hellyer Volcanics occur near the base of the
Mount Charter Group which is considered to be equivalent to the Yolande River Sequence
south of the Henty Fault (Corbett and Komyshan, 1989; Corbett, 1992). The Que-Hellyer
Volcanics of the Mount Charter Group are up to 1 km thick and comprise suite I basalts and
andesites overlain by suite III basalts (Hellyer Basalt; Corbett, 19,92; Crawford et al., 1992).
A temporal change from suite I to suite ITI (calc alkaline to shoshonitic) occurs in the Que
Hellyer Volcanics whereas a change from suite IT to suite ill occurs in the Anthony Road
Andesite (Figure 9:6). The Que-Hellyer Volcanics are overlain by thick black mudstone
(Que River Shale) and a dacitic to rhyolitic volcaniclastic sequence (Southwell Subgroup;
Corbett, 1992). Correlates of the Tyndall Group overlie the Southwell Subgroup (Corbett,
1992).
The hornblende andesite association of the Anthony Road Andesite (suite IT; Crawford et al.,
1992) has similar major and trace element geochemistry to andesites of the Lynch Creek
basalts and the Crown Hill andesite (Crawford et al., 1992; Sharpe, 1994). The basaltic
andesite association (suite lIT; Crawford et al., 1992) is geochemically similar to some
basaltic/andesitic units of the Lynch Creek basalts, and the Hollway Andesite and Hellyer
Basalt north of the Henty Fault (Jack, 1989; Coutts, 1990; Dower, 1991; Crawford et aI.,
1992). The Lynch Creek basalts and the Anthony Road Andesite are the only mafic
intermediate units in the Mount Read Volcanics in which both suite 11 and suite III
compositions occur in close spatial association (Figure 9.6; Crawford et al., 1992; Sharpe,
1994). In addition, the Lynch Creek basalts occur within the Yolande River Sequence and
comprise geochemically distinct associations that resemble the hornblende andesite and
basaltic andesite associations of the Anthony Road Andesite (Corbett, 1992; Sharpe, 1994).
It appears that compositionally similar, basaltic to andesitic volcanism represented by the
Anthony Road Andesite, Lynch Creek basalts and Que-Hellyer Volcanics was broadly
synchronous with the accumulation of the Yolande River Sequence (Western volcano
sedimentary sequences) but centred at widely separated locations in the Mount Read
Volcanics (Figure 9.6). The rhyolite association of the Anthony Road Andesite is
geochemically distinct from all other units in the Mount Read Volcanics belonging to suite IT
and having Si02 up to 76 wt%.
165�
Inferred source Owen Conglomerate
Depositional setting
,~ ..
1. uplifted and exposed Precambrian basement (Mountains) 2. alluvial plain
3. prograding alluvial fans 4. deep water facies 5. buried Cambrian volcanics
Figure 9.5 Depositional setting and inferred sources for the Owen Conglomerate (2-4).
a� Owen Conglomerate
Tyndall Group Ml Cripps Subgroup =Tyndall Group
Hdlycr Basalt
lI.1i.\t'd Sequcn~
North<rn Cenlr.li Vo~c:mic
Complex
North and West of the Henty Fault South and East of the Henty Fault (SC = Sock Creek: QRS =Que River Shale)
b
DJ Suite I I:!LJ Suite II� Suite IV 0 Suite V
Figure 9.6� a. Relationships of major rock associations and lithostratigraphic units across the Mount Read Volcanics (Modified after Corbett, 1992). b. Distribution of geochemical suites across the Mount Read Volcanics (Modified after Crawford et a, 1992).
9.4 Importance of the Lake Newton Ironstone
The three localities in the Mount Read Volcanics where ironstone is recorded (Lake Newton,
this study; Henty Mine, Halley and Roberts, 1997; arid Comstock near Queenstown,
MacDonald, 1991) are all spatially associated with Cambrian mineralisation.
The Lake Newton Ironstone is a relatively pure hydrothermal pre,cipitate formed at or within
metres of the seafloor from low temperature «lOODC) oxidised fluids. The fluids which
precipitated the ironstone also caused local Fe-enrichment of the host volcanic rocks in the
upper Anthony Road Andesite. The Lake Newton Ironstone occurs close to the contact
between the Anthony Road Andesite and the Tyndall Group, at a similar stratigraphic level to
mineralisation at the Henty Au mine (Halley and Roberts, 1997). This stratigraphic horizon
is thus prospective for VHMS mineralisation throughout the Basin Lake area.
9.5 Analogous modern volcanic settings
Studies of the Mount Read Volcanics in the Basin Lake area have provided information
relating to sedimentation processes, depositional environment and provenance. The
comparison of the ancient, altered volcanic succession in the study area with modern
analogues, in terms of both palaeogeography and geochemistry, is an important procedure in
refining interpretations of the volcanic processes, depositional environment and tectonic
setting.
9.5.1 Palaeogeographic analogues
A modern day palaeogeographic analogue for the Mount Read Volcanics of the Basin Lake
area occurs in the Taupo Volcanic Zone, North Island of New Zealand (Figure 9.7). The
Taupo Volcanic Zone extends NE across the North Island of New Zealand and contains an
onshore component 200 km long x 60 km wide and an offshore component which extends a
further 50 km into the Bay of Plenty (Graham et aI., 1995; Wilson et aI., 1995). Volcanic
activity is related to subduction of oceanic crust of the Pacific Plate beneath continental crust
of the Australian Plate (Graham et aI., 1995). The zone has been active from the late
Pliocene - Quaternary (Wilson et al., 1995).
The onshore section of the Taupo Volcanic Zone is an analogue for the source volcanic
terrain of the Yolande River Sequence and Tyndall Group. The Taupo Volcanic Zone
contains at least eight known calderas, and subaerial andesitic-dacitic composite cones
168�
(Figure 9.7; Cole, 1990; Wilson et aI., 1995). At least 34 separate caldera-forming
ignimbrite eruptions have been recognised (Wilson et aI., 1995). Rhyolite has been the
dominant magma type erupted (>15 000 km3) with lesser andesite, basalt and dacite (Wilson
et al., 1995). Explosive eruptions have formed ash fall deposits and thick welded and non
welded ignimbrites (Cole, 1990; Houghton et al., 1992).
The offshore section of the Taupo Volcanic Zone (Bay of Plenty) is an analogue for the
depositional basin in which the Mount Read Volcanics of t£e Basin Lake area were
deposited. Large volumes of pyroclasts have been delivered into the Bay of Plenty via direct
fallout, pyroclastic flows which entered the sea, and through transport by rivers and streams
(Walker, 1979; WilSon et aI., 1995). Similar processes are interpreted during deposition of
the Yolande River Sequence and the Tyndall Group. Active submarine intrabasinal
volcanism also occurs offshore from the Taupo Volcanic Zone at the composite volcanoes of
White Island and Mayor Island, and at the Whakatane Seamounts (Figure 9.7; Cole, 1990;
Hedenquist et al., 1993). This active intrabasinal submarine andesitic to dacitic volcanism is
a partial analogue of the proximal intrabasinal volcanism of the Anthony Road Andesite.
The volcanic rocks of the Taupo Volcanic Zone do not make good geochemical analogues
for the Basin Lake area. Magma compositions in the two provinces are geochemically
distinct. In particular, post-collisional volcanism characterised by shoshonitic basaltic
andesites, the basaltic andesite association of the Anthony Road Andesite, is not reported in
the Taupo Volcanic Zone.
9.5.2 Geochemically similar volcanic settings
The high-K calc-alkaline affmities of the hornblende andesite and rhyolite associations of the
Anthony Road Andesite, the shoshonitic affinities of the basaltic andesite association of the
Anthony Road Andesite and the calc-alkaline nature of the Tyndall Group rhyolites have
been discussed in Chapter 6.
Amphibole phenocrysts of the hornblende andesite association of the Anthony Road
Andesite overlap in K20 content with those of high-K calc-alkaline andesites from
Stromboli, Aeolian Arc (Crawford et aI., 1992). Major element and REE abundances of this
association also resemble those of the Stromboli lavas (Ellam et al., 1988), and of the
Nevados de Payachata Volcanic Region, Andes, Chile (Davidson et aI., 1990). P20S
abundances are similar to those in high-K calc-alkaline andesites from Java, Sunda Arc
(Whitford et al., 1979) and (LaIYb)N are similar to the most enriched high-K andesites from
Eastern Srednogorie, Bulgaria (Manetti et aI., 1979). Thus, the hornblende andesite
association includes compositions that are similar to those of lavas erupted in modem arc
settings, including both intraoceanic arcs and continental margin arcs.
169
174°� 176° . "~and
dJ 36°
Depositional analogue \
/.... /::: i
/Bay of / .� Plenty / : .W)fite Island�
:" Source�
analogue�38°
.? ~
.§'1J40°� '§
~
C:5
100 km
Figure 9.7� The Taupo Volcanic Zone, North Island of New Zealand (Modified after Graham et aI., 1995). Rhyolitic caldera volcanoes dominate the zone with lesser andesitic volcanic centres (filled circles; Graham et aI., 1995). Palaeogeographic source and depositional analogues for the Basin Lake area are shown.
170
The basaltic andesite association of the Anthony Road Andesite shows distinctive P20S- and
light REE-enrichment, having shoshonitic affinities. Crawford et al. (1992) recognised that
the shoshonitic rocks of the Mount Read Volcanics are geochemically similar to lavas
generated by post-collisional volcanism. Examples occur at Mount Hagen and Cape Nelson,
in Papua New Guinea (Jakes and Smith, 1970; Mackenzie and Chappell, 1972) and the Mus
depression in Eastern Anatolia, Turkey (Pearce et aI., 1990). Post-collisional shoshonitic
dykes in the Northwestern Alps, Italy (Venturelli et aI., ~:?84) and from northern
Karakorum, China (Pognante, 1990) most closely approximate the major and trace element
contents in the basaltic andesite association.
The geochemistry of the Anthony Road Andesite is analogous to volcanics erupted in
modern arc (hornblende andesite and rhyolite associations) and post-collisional settings
(basaltic andesite association) whereas rhyolites in the Zig Zag Hill Formation of the Tyndall
Group are highly evolved and geochemically comparable to calc-alkaline rhyolites erupted in
modern continental margin arc settings (Ewart, 1979; Middlemost, 1985).
Many of the geochemical analogues for the Basin Lake area occur on land, some on very
high mountains (e.g., the Andes and the Alps), and do not make good palaeogeographic
analogues.
171�
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