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U N I V E R S I TAT I S O U L U E N S I SACTAA




OULU 2008

A 516

Risto Kaukonen

SULFIDE-POOR PLATINUM-GROUP ELEMENT DEPOSITSA MINERALOGICAL APPROACH WITH CASE STUDIES AND EXAMPLES FROM THE LITERATURE

FACULTY OF SCIENCE,DEPARTMENT OF GEOSCIENCES,UNIVERSITY OF OULU

A 516

ACTA

Risto K

aukonen

A C T A U N I V E R S I T A T I S O U L U E N S I SA S c i e n t i a e R e r u m N a t u r a l i u m 5 1 6

RISTO KAUKONEN

SULFIDE-POOR PLATINUM-GROUP ELEMENT DEPOSITSA mineralogical approach with case studies and examples from the literature

Academic dissertation to be presented, with the assent ofthe Faculty of Science of the University of Oulu, for publicdefence in Auditorium GO101, Linnanmaa, on November21st, 2008, at 12 noon

OULUN YLIOPISTO, OULU 2008

Copyright © 2008Acta Univ. Oul. A 516, 2008

Supervised byProfessor Tuomo AlapietiProfessor Eero Hanski

Reviewed byProfessor Fernando GervillaProfessor Bernhard Saini-Eidukat

ISBN 978-951-42-8953-8 (Paperback)ISBN 978-951-42-8954-5 (PDF)http://herkules.oulu.fi/isbn9789514289545/ISSN 0355-3191 (Printed)ISSN 1796-220X (Online)http://herkules.oulu.fi/issn03553191/

Cover designRaimo Ahonen

OULU UNIVERSITY PRESSOULU 2008

Kaukonen, Risto, Sulfide-poor platinum-group element deposits. A mineralogicalapproach with case studies and examples from the literatureFaculty of Science, Department of Geosciences, University of Oulu, P.O.Box 3000, FI-90014University of Oulu, Finland Acta Univ. Oul. A 516, 2008Oulu, Finland

AbstractSulfide-poor deposits of platinum-group elements (PGE) occur in two main types: silicate-typeand oxide-type. In the silicate-type mineralization PGE form discrete platinum-group minerals(PGM) that occur as inclusions in various silicate minerals. In the oxide-type mineralization PGMmay have different modes of occurrence. They may be associated with silicates or they may occuras inclusions in chromite, magnetite or ilmenite, for example. In some cases they may even beassociated with base metal sulfides.

The approach chosen in this work is mainly a mineralogical one. PGM parageneses, theirmodes of occurrence and associations with other minerals were studied from different deposits.These are then compared to some well-recorded examples of PGE deposits.

The case studies presented, the Duluth Complex in Minnesota, U.S.A., the HanumalapurComplex in Karnataka, India, and the Penikat Layered Intrusion in northern Finland, are examplesthat illustrate the multitude of possibilities regarding PGE mineralization versus the traditionalapproach where any significant quantities of PGE are supposed to occur only in association withbase metal sulfides.

As the traditional orthomagmatic and hydrothermal models cannot explain the genesis of somesulfide-poor PGE occurrences, a new theory of PGE mineralization was developed. This “redoxtheory” is an attempt at explaining the association of PGE with various oxide minerals, mostimportantly chromite.

Keywords: oxide-type, PGE, PGM, redox theory, silicate-type, sulfide-poor

AcknowledgementsThe biggest thanks go to the late Professor Tuomo Alapieti who was my supervi-sor and mentor and got me interested in the platinum-group mineralogy in the firstplace. Before his demise he saw and approved my dissertation in its almost finalform. I would like to express my gratitude to Professor Bernhard Saini-Eidukatand Prof. Fernando Gervilla who reviewed the manuscript and gave valuable com-ments and suggestions for improvements. I would also like to thank Professor EeroHanski for his comments and supervising the final stages of this work.

I am indebted to Dr. Steven Hauck and Mr. Mark Severson for helping meduring my time in Duluth, Minnesota. I would also like to express my gratitude tothe Minnesota Department of Natural Resources and Lehman Company forgranting access to the Duluth Complex drill core samples and the Foundation ofOutokumpu Oy for providing part of the funding for my stay in the USA.

I am very grateful to Professor Devaraju for hosting me and Prof. Alapietiduring our visits to India in which we gathered sample material for this thesis andfor our long and fruitful research collaboration. I would also like to thank Mr.Lalgondar and the Karnataka Department of Mines and Geology for grantingaccess to the drill core samples from the Hanumalapur Complex.

I am deeply indebted to the staff of the Institute of Electron Optics, especiallyto Mr. Olli Taikina-aho and Dr. Seppo Sivonen for their help with the analyticalinstruments used in this work. I also wish to thank Mrs. Sari Forss for making topquality polished thin sections, Mrs. Kristiina Karjalainen for finalizing several ofthe figures of this thesis and Mrs. Stacy Saari for English language corrections.

Finally, I would like to thank all those close to me, family and friends, whosupported me in various visible and invisible ways.

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PrefaceFor the past twenty-five or more years there has been an on-going debate about theformation of stratiform platinum group element (PGE) mineralization related tomafic layered intrusions. During that time, two fundamentally different schools ofthought have evolved, both trying to explain PGE genesis by emphasizing diffe-rent geological and geochemical processes. One of these schools of thought focu-ses on the role of base metal sulfides acting as gathering agents for the PGE (and iscalled “magmatists” or “downers” due to migration of PGE downwards in strati-graphy according to their argument). The other school of thought emphasizes therole of magmatic hydrothermal fluids as gathering factors of the PGE. Thus theyare called “hydrothermists” or “uppers” because their line of reasoning supportsthe idea of PGE migrating upwards in stratigraphy within a given intrusion.

Both of these groups have been able to present more or less solid proof tovarious facts that may have something to do with the formation of many differentPGE deposits. But each deposit is indeed different and hence their formationscannot necessarily be explained by following just one school of thought. ManyPGE deposits are clearly associated with magmatic base metal sulfide occurrencesand indeed the platinum group elements (Ru, Rh, Pd, Os, Ir and Pt) are generallychalcofile elements by their geochemical affinity. Likewise, there are goodexamples where the PGE are not associated with base metal sulfides, but havebeen mobilized by fluid activity quite as their chemical properties such assolubility in hydrothermal fluids and ability to form certain types of chemicalcomplexes suggest.

These two schools of thought have been dictating the direction of PGEexploration globally with varying success since the late 1970’s. Somewhat recentdiscoveries, however, suggest that there are deposits that require a fundamentallydifferent approach than either one of the above mentioned schools of thought alonecan provide.

A geoscientist once said: “The explanations for natural phenomena should beas simple as possible, but no simpler!” This has been the leading idea in theprocess of preparing this thesis.

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ContentsAbstractAcknowledgements

Preface

Contents1 Introduction 112 Overview of the theories regarding PGE occurrences 13

2.1 Compaction of the cumulate pile ............................................................ 132.2 Orthomagmatic theory ............................................................................ 132.3 Hydrothermal theory ............................................................................... 152.4 Other theoretical considerations.............................................................. 16

3 Analytical techniques used in the case studies 173.1 SEM/EDS and platinum-group minerals ................................................ 18

4 Case studies 214.1 Background and sampling....................................................................... 214.2 Duluth Complex ...................................................................................... 22

4.2.1 Introduction................................................................................... 224.2.2 Geology of the South Kawishiwi Intrusion .................................. 244.2.3 Cryptic variation of the rock-forming minerals ............................ 264.2.4 Geology of the PGE mineralization in Du-15............................... 354.2.5 PGE Mineralogy ........................................................................... 37

4.3 Hanumalapur Complex ........................................................................... 434.4 Introduction ............................................................................................. 43

4.4.1 Geology of the Hanumalapur Complex ........................................ 444.4.2 Geochemistry and mineralogy of the main rock-forming

minerals......................................................................................... 464.4.3 Types of PGE mineralization in the Hanumalapur Complex ....... 504.4.4 PGE mineralogy of the Hanumalapur Complex ........................... 50

4.5 Penikat Layered Intrusion ....................................................................... 564.5.1 Introduction................................................................................... 564.5.2 Geology of the Penikat Intrusion and its PGE occurrences .......... 564.5.3 Silicate-type PGE mineralization in the SJ Reef .......................... 62

5 Examples from the literature 655.1 The Bushveld Complex........................................................................... 65

5.1.1 The UG-2 chromitite..................................................................... 69

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5.1.2 Other chromitite layers in the Bushveld Complex........................ 715.1.3 The hortonolitic dunite pipes ........................................................ 72

5.2 Other examples of sulfide-poor PGE occurrences .................................. 735.2.1 Chromitites of the Koitelainen Intrusion, Finland ........................ 735.2.2 Chromitites of the Stillwater Complex, Montana, U.S.A. ............ 74

6 Discussion 776.1 Importance of the case studies in understanding the formation

of sulfide-poor PGE mineralization ........................................................ 776.2 The role of chromite in the formation of PGE reefs ............................... 81

6.2.1 Thermodynamic Considerations ................................................... 846.2.2 Why all chromitites are not enriched in PGE?.............................. 86

6.3 Formation of silicate-hosted PGE mineralization ................................... 876.4 Case studies revisited .............................................................................. 89

6.4.1 Chromite theory applied to the PGE mineralization in the Duluth Complex............................................................................ 89

6.4.2 Formation of the PGE mineralization of the HanumalapurComplex........................................................................................ 92

6.5 Implications for exploration.................................................................... 937 Summary and Conclusions 95

References

Appendices

10

1 IntroductionMost of the world’s demand for platinum-group elements (PGE) such as rut-henium, rhodium, palladium, osmium, iridium and platinum is supplied by extrac-ting these metals from stratiform ore deposits related to mafic layered intrusions.Most often the platinum-group minerals (PGM, i.e. PGE-bearing minerals) occuras so minute grains that they are completely invisible to the naked eye. Hence,from the point of view of exploration, it is imperative to understand how thesedeposits form in order to be able to look for them in the right places and thus havea better chance of locating them.

Figure 1 presents a general location map of those stratiform PGE deposits thatwill be referred to in this dissertation. Most of the world’s production comes fromSouth Africa, namely the famous Merensky Reef and UG2 deposits, which extendlaterally over the entire Bushveld Complex as thin bands. At the time of writing,other mines with PGE as their primary products only occur in North America inthe Stillwater Complex, Montana, U.S.A. and Lac des Iles Intrusion, Ontario,Canada and in Africa at the Great Dyke in Zimbabwe. The Norils’k Intrusion ofRussia and the Sudbury structure of Ontario, Canada, host deposits that producesignificant quantities of PGE as byproduct from essentially base metal sulfidemines. New PGE mines may be opened within the next few years at the PortimoComplex and Konttijärvi and Ahmavaara Intrusions in Northern Finland. Thiswould make them the very first PGE mines in Europe. The Duluth Complex ofMinnesota, U.S.A., Penikat Intrusion of Finland and the Hanumalapur Complex ofKarnataka, India, all possess identified and recognized PGE potential that will bediscussed further in this dissertation.

There are several theories regarding the genesis of stratiform PGEmineralization, two of which are perhaps more widely known and predominant.The following several chapters will provide an overview of these currentlyprevailing theories, a few distinct case studies will be presented and compared tosome examples from the literature. The case studies will be from the DuluthComplex and the Hanumalapur Complex. Some of the places mentioned in Fig. 1will be used as examples from the literature. Then finally, if (or rather when) theprevailing theories do not work properly or would be misleading or inadequate fordirecting exploration, a modified or a new theory will be proposed.

11

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12

2 Overview of the theories regarding PGE occurrences

There are several fundamentally different theories that attempt to provide an exp-lanation on how platinum deposits have formed. Since this dissertation focuses onstratiform PGE deposits related to mafic layered intrusions, only the theoriesregarding the formation of such deposits will be briefly discussed here.

2.1 Compaction of the cumulate pile

Vermaak (1976) and von Gruenewaldt (1979) used the Merensky Reef of theBushveld Complex as an example when presenting the idea that reef-type PGEdeposits are formed when the cumulate pile below the forming reef is compactedand the intercumulus melt within it, which is enriched in PGE, sulfides and volati-les, is compressed upwards. This upward compression, however, will be stoppedby a layer of previously crystallized plagioclase that floats on the heavier melt.

The crystallization of plagioclase from the magma causes the residual melt tobe enriched in iron and magnesium and correspondingly depleted in silica, whichleads to the crystallization of chromite and orthopyroxene. These in turn willdeplete FeO from the melt causing a substantial reduction in the solubility of sulfurin the melt. Because of this, liquation is likely to take place. In addition to the basemetals (iron, nickel and copper) the immiscible sulfide melt will also scavenge thePGE and some silver, bismuth, tellurium, tin and antimony from the residualsilicate melt (von Gruenewaldt 1979).

2.2 Orthomagmatic theory

Because of the highly chalcophile nature of PGE, their partitioning coeffients aremuch higher for sulfide melt than for silicate melt (Naldrett 1981). It has also beendemonstrated that as the proportion of sulfide melt increases, there will eventuallybe significant depletion of chalcophile elements in the silicate magma (Campbell& Naldrett 1979). Hence, according to the widely supported orthomagmaticmodel, stratiform PGE deposits originate through sulfide saturation and conse-quent segregation of an immiscible sulfide melt which will scavenge PGE from thesilicate magma. Several authors have argued that sulfide saturation required toform a PGE reef in a basic layered intrusion occurs when two magmas of different

13

composition mix in one way or another (cf. Campbell et al. 1983, Irvine et al.1983, Naldrett et al. 1986, Campbell & Turner 1986).

One such mixing process could be the jetting of a new pulse of magma of acertain composition followed by settling of that new magma and subsequentmixing with the older magma that was already in the chamber (Campbell et al.1983). At the early stages of fractional crystallization when the crystallizingphases are mainly olivine and bronzite1, increased fractionation causes a decreasein the density of the magma. If there were a new pulse of the same magma eruptinginto the chamber at this stage, it would be denser than the earlier (now partiallyfractionated) magma and would hence settle at the bottom of the chamber.However, if this new pulse of magma were to erupt with sufficient force, i.e. as ajet, it would go all the way through the less dense ambient magma and beginsettling only after gravity overcame the jet’s initial momentum. The jetting causesthe new magma to mix with the older magma and form a hybrid melt which willthen settle on the floor of the magma chamber. The result is a layer of warmprimitive magma overlain by cooler, more fractionated magma.

When plagioclase becomes a cumulus phase, fractionation causes the densityof the (initially tholeiitic) magma to increase and eventually the density of thefractionated magma becomes greater than that of the parent magma. If there is nowa new pulse of the original primitive magma entering the chamber, it will risethrough the denser fractionated magma and during ascent mix turbulently with it toform a hybrid layer on top of the magma chamber. The result is a layer of hotprimitive magma on top of a layer of cooler and more fractionated magma.

The most favorable conditions for a sulfide melt to equilibrate with as large avolume of silicate melt as possible are most attainable when a new pulse of hotmagma (the density of which is slightly less than that of the fractionated magma)enters a density stratified chamber. Under these conditions the new pulse will riseto its own density level and spread laterally to form its own density stratified layer.This layer will lose heat rather rapidly to the cooler layer above it. If the densitydifference between the two magmas is small, the density stratification forming inthe hybrid layer will destabilize as it cools. This in turn will allow any immisciblesulfides that have formed to equilibrate with a large volume of silicate melt.However, if the density difference between the two magmas is large, the density

1.= according to the current pyroxene nomenclature (subcommittee on pyroxenes, 1989) this shouldread “enstatite”, but the term “bronzite” was retained here because of the original text of Campbell etal. (1983) and because that term is still more readily understood by the intended audience of this workthan the current term.

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stratification shall not be destabilized by cooling and the sulfides will not be ableto equilibrate with a large volume of silicates (Campbell et al. 1983).

2.3 Hydrothermal theory

The hydrothermal theory emphasizes the role of magmatic hydrothermal fluids,particularly chlorine-bearing ones. Such fluids function as collecting agents of thePGE (e.g., Stumpfl & Tarkian 1976, Ballhaus & Stumpfl 1985 and 1986, Boudreauet al. 1986, Volborth et al. 1986, Boudreau & McCallum 1992, Rudashevsky et al.1992). Evidence to show the significance of the fluid phase includes the commonoccurrence of pegmatoids associated with PGE occurrences and the presence ofwater-bearing silicates, graphite and chlorapatite. Together they suggest the fluidto have been composed of components of the C-O-H-S-Cl system, namely CO2,CH4, H2O, H2S and particularly various chloride complexes that would have beenthe carriers of PGE (e.g., Stumpfl & Rucklidge 1982, Stumpfl & Ballhaus 1986,Ballhaus & Stumpfl 1985, 1986, Boudreau et al. 1986, Boudreau & McCallum1992).

It has been proposed (e.g., Ballhaus & Stumpfl 1986, Boudreau et al. 1986)that chlorine-bearing magmatic hydrothermal fluids that were derived from theintercumulus melt below the PGE mineralization would have played a major rolein the formation of the Merensky Reef of the Bushveld Complex and the J-M Reefof the Stillwater Complex, for example. Fluids will migrate upwards within thestratigraphy of a crystallizing layered intrusion until they encounter a melt that isnot yet saturated with fluid. In this case the fluid and the melt will mix which maycause sulfide liquation (Boudreau & McCallum 1992). The upward migration offluids may also cease if they collide with a layer, such as a plagioclase adcumulate,which would be solid enough to halt the migration (Vermaak 1976). If neither ofthe above processes happens, the fluids may migrate across the entire intrusion andescape through the ceiling (Boudreau & McCallum 1992).

Boudreau & McCallum (1992) have also proposed that chlorine-bearing fluidsmight even dissolve PGE from crystalline sulfide minerals and transport themelsewhere, mainly upwards in stratigraphy. Further support for the fluid theorieshas also been sought by studying the fluid inclusions of intercumulus quartz fromthe Merensky Reef. Inclusions have been found to contain sodium chloride, waterand carbon dioxide among other substances. Some polyphase fluid inclusions havebeen reported to contain up to six different solid phases and also a liquid andgaseous phase (Ballhaus & Stumpfl 1986).

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The solubility of platinum and palladium into likely magmatic fluids has beenthe focus of some experimental studies (e.g., Wood 1987, 1991, 2002, Wood et al.1989, Mountain & Wood 1988, Sassani & Shock 1990, Hsu et al. 1991, Gammonset al. 1992). According to these investigations, at temperatures of 300°C platinumand palladium are soluble as chloride complexes only under extremely oxidizing,saline and/or acidic conditions. This type of fluid would, however, cause a verydifferent kind of alteration in the rocks than what has been documented. Hence, atlow temperatures (300°C) hydroxide and/or bisulfide complexes are consideredmore important than chloride complexes as transporting agents of the PGE(Mountain & Wood 1988, Wood et al. 1989).

At higher temperatures (in supercritical fluids) the solubility of PGE aschloride complexes may be a lot greater than at low temperatures and significanteven in less oxidizing conditions. In mafic layered intrusions at the time of theiremplacement, the prevailing conditions are such that chloride complexes could bevery important transporting agents of the PGE (Sassani & Shock 1990). PGE willbe liberated from the chloride complexes if the prevailing conditions change fromoxidizing to reducing, pH increases and/or temperature drops (Gammons et al.1992).

2.4 Other theoretical considerations

Cawthorn (1999) pointed out several problems regarding the theories reviewedabove and how they fail to provide a comprehensive model for the genesis of stra-tiform PGE deposits. He particularly emphasized that all current models fail toexplain (or simply overlook) the association of PGE with chromitite layers. Teigler(1999) concluded that the physico-chemical conditions during the formation of theLG6 chromite of the Bushveld Complex must have been very stable and that chro-mite precipitation was the governing control of PGE concentration.

In reality there can probably be no single theory to explain how all stratiformPGE deposits form, but several theories may need to be applied to explain all thefeatures of a given deposit. Features may exist that cannot be properly explainedby current theories and require a completely new approach.

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3 Analytical techniques used in the case studies

Various analytical techniques were applied to different types of samples in order toobtain a plethora of relevant data. All mineralogical data and most of the whole-rock data were gathered at the University of Oulu, Institute of Electron Optics.Some of the noble metal analyses were performed at Activation Laboratories inToronto, Canada, but most of them were done at the Geochemical Laboratory ofthe Geological Survey of Finland in Espoo.

The silicate and oxide mineralogical data were obtained at the Institute ofElectron Optics, University of Oulu, Finland, with a JEOL Superprobe JXA-733(EPMA) utilizing wave length dispersive spectrometry. Platinum-group mineralswere analyzed mostly with the same basic hardware, but utilizing energydispersive spectrometry. For most PGM from the Duluth Complex, a JEOL JSM-6400 scanning electron microscope (SEM) equipped with a LINK eXL energydispersive spectrometer (EDS) was used. Atomic ratios were calculated with theZAF-4 program, which performs the necessary corrections for over-lapping peaksof different elements. With this equipment, including the software as well as thehardware, it has proved possible to obtain acceptable analyses of grains as small asca. 1.5 microns in diameter.

Standard analytical conditions included an acceleration voltage of 15 kV and asample current of 15 nA on the EPMA and 1.5 nA on the SEM. The standards usedwere jadeite for Na, MgO for Mg, Al2O3 for Al, wollastonite for Si and Ca,tugtupite for Cl, orthoclase for K, pyrite for sulfur, synthetic InAs for arsenic, PbTefor tellurium and lead, HgTe for mercury and pure metallic standards for the otherelements encountered in this investigation. Due to the small size of most of thePGM, a focused beam with a diameter of about 1 micron had to be used in mostanalyses. For most silicates and oxides, whenever possible, a larger 10 micronelectron beam was used. Plagioclase and apatite analyses were performedpreferably with a 20 µm beam, whereas augite analyses represent a subsoliduscomposition as they were measured between orthopyroxene lamellae with afocused beam.

Whole-rock (XRF) analyses were performed at the same institute usingpowder pellets and a Siemens SRS 303AS XRF-analyzer.

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3.1 SEM/EDS and platinum-group minerals

Identification of specific PGM, even though it can be a very laborious task requi-ring special expertise with both mineral chemistry and electron microscopy, is veryimportant because it provides vital information for the potential exploitation andrefining processes, but also because it can provide information that is relevant tounderstanding the formation of the mineralization. The advantages of the SEM/EDS method that was applied in this study are: (1) it is possible to obtain analysesfrom very small grains, (2) qualitative analyses only take a few seconds to providean identification of the mineral and (3) quantitative analyses are obtained relati-vely quickly with reasonable accuracy as all elements are being analyzed simulta-neously. The main disadvantage of the method is a substantial potential for errorparticularly with very small grains as a lot of interpretation is often required fromthe analyst to eliminate the x-ray peaks that come from the surrounding mineralsand hence the element totals may often be quite low. There is also a problem withtrace elements, as they cannot be detected with this method.

Another problem related to PGM in general arises when trying to measuretheir dimensions. Silicates are transparent in thin section and thus it is possible todetermine how the grain is positioned relative to its crystal axes. However, PGMare opaque and generally so tiny that even their reflective properties are virtuallyimpossible to determine. With electron optical means, it is easy to get accuratedimensions from any grain from the back-scattered electron image. The problem ishow to utilize these data as they are somewhat incomplete and rather arbitrarybecause one cannot know exactly what was measured as any grain is always athree-dimensional object. In a polished (thin) section only some arbitrary two ofthe three dimensions are seen. This problem is also illustrated in Fig. 2. In practicesome people have chosen to report grain sizes as areas in µm2 (X µm x Y µm = Zµm2), and others as equivalent circle diameter (Penberthy & Merkle 1999) whichis the diameter of a circle with the same area as that of the measured grain (2 xarea/). Yet others report the grain size as the average of two dimensions, maximumand minimum ((X + Y)/2) or simply as one maximum dimension (µm). Here thegrain sizes are presented as maximum dimension (µm), (for a rectangular grain,the long side was shown in the graph (although the short side was also measured),not the hypotenuse). This method was chosen over the others because there are somany variables that cannot be determined in any way. If one measures random Xand Y dimensions of a randomly oriented mineral grain from a random section,which could be from the center or from the edge of the grain, and takes averages of

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each two measurements, one is bound to lose the large grains in the process. Eventhough this method might exaggerate the proportion of the largest grains slightly,the other methods are likely to omit them altogether, which is equally misleading.In the end it is probably not so important how the measuring was done as long asconsistency is maintained in the process and the method described along with theresults so that the reader can “calibrate” them properly for comparisons withsimilar charts constructed using some other method.

Fig. 2. With a scanning electron microscope it is impossible to determine from arandom cross-section which way the grain is positioned and how large or small thegrain actually is. For example a square cross-section could represent a small cube (A),a large octahedron (B) or a section of a tetragonal grain (C). Likewise a rectangularsection (D-F) can be derived from different kinds of grains with very differentdimensions. With amoeboidal grains the possibilities are virtually infinite.

A

D

CB

E F

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4 Case studies

4.1 Background and sampling

The investigation of mafic layered intrusions has a proud and successful history ofseveral decades at the University of Oulu, Finland. The fundamental goal of thescientific research is to understand the processes involved in the formation of thevarious PGE occurrences related to these types of rocks and thus provide valuableinformation to direct mineral exploration. This thesis is part of the on-goinginvestigation of stratiform PGE mineralization related to mafic layered intrusionsall over the world.

Parts of two mafic layered intrusive complexes were examined in detail forthis thesis. They are the South Kawishiwi Intrusion of the Duluth Complex innortheastern Minnesota, U.S.A. and the Hanumalapur Complex in Karnataka,India. Both locations contain highly anomalous PGE mineralization, the formationof which is not readily explainable by the prevailing theories found in ore geologytext books. For comparative purposes, a few samples were also studied from asimilar type of PGE mineralization in the Penikat Intrusion, Northern Finland.

Samples for this study were obtained during a number of visits to the studyareas in the U.S.A. and India. Initial sampling from the Duluth Complex was doneby Dr. Alapieti in the early 1990’s and some results of these studies were alreadypresented by Kaukonen (1994, 1995). Additional data are presented here in orderto fill the major remaining gaps. Additional sampling was done in 1995–96 by theauthor. All samples from the Duluth Complex are drill core samples. The samplesfrom the Hanumalapur Complex are mainly drill core samples. The selection ofthe drilling sites, however, was made based on analysis of outcrop samples. Thesampling criteria used for the drill core were: 1) each rock type should be sampled,2) when possible, (i.e. there was a long monotonous sequence in the core), theinterval between any two consecutive samples should be roughly similar, 3)lithological anomalies, such as disseminated sulfides or chromite seams, should besampled.

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4.2 Duluth Complex

4.2.1 Introduction

The Duluth Complex (DC) in northeastern Minnesota, is the third largest knownmafic layered igneous complex in the world, only surpassed in size by theBushveld Complex of South Africa and the Great Dyke of Zimbabwe. Formerly, itwas thought that the Dufek Complex of Antarctica might be bigger than the DC,but it has been shown to be significantly smaller than previously anticipated(Ferris et al. 1998). The DC is a composite tholeiitic mafic complex that wasemplaced into comagmatic flood basalts associated with the Mesoproterozoic (1.1Ga) Midcontinent Rift System. The complex is a multiple intrusive body, a“patchwork” of intrusions that is traditionally divided into an older anorthositicseries and a younger troctolitic series, both of which comprise several smallersubintrusions (Fig. 3). These subintrusions could roughly be thought to correspondto megacyclic units typical of many other layered complexes. According to recentage determinations from various intrusions of the DC, intrusive activity spanned aperiod of about 11 Ma, 1107–1096 Ma (Paces & Miller 1993).

Even though the Lake Superior region has been the subject of countlessgeologic studies for the past two centuries (Wold & Hinze 1982), there were notmany scientific publications about the DC until Grout (1918) introduced the term“lopolith” to the geologic literature. Later work, however, showed that the DCactually does not fit into Grout’s lopolith definition. There are plenty of relativelyrecent papers describing the geology of the DC, e.g., a series of papers by variousauthors in a Minnesota Geological Survey publication edited by Sims & Morey(1972), Weiblen & Morey (1980) and Miller & Ripley (1996) to name a few. Herethe emphasis is focused on PGE mineralization, so for a broader description of thegeology of the entire DC, the reader is referred to the publications mentionedabove.

One of the subintrusions of the troctolitic series, the South KawishiwiIntrusion, which is located at the western edge of the DC (Fig. 3), was studied indetail because it contains an interesting oxide-related PGE mineralization, theformation of which is not yet fully understood. Detailed studies were made fromdrill core samples and included investigations of mineral chemistry andpetrography of the main rock-forming minerals and determinations andinvestigations of the PGE minerals found from some samples. These are alsocompared with results reported by others.

22

Fig.

3. G

ener

aliz

ed g

eolo

gic

map

of t

he D

ulut

h C

ompl

ex (m

odifi

ed a

fter

Mill

er &

Wei

blen

199

0).

ME

SO

PR

OTE

RO

ZOIC

PA

LEO

PR

OTE

RO

ZOIC

Kew

eena

wan

Sup

ergr

oup

SE

DIM

EN

TAR

Y R

OC

KS

Bay

field

and

Oro

nto

Gro

ups

INTR

US

IVE

RO

CK

S

Oth

er in

trusi

ve ro

cks

ofth

e D

ulut

h C

ompl

ex

Sou

th K

awis

hiw

i Int

rusi

on

Loga

n si

llsV

OLC

AN

IC R

OC

KS

Nor

th S

hore

Vol

cani

c G

roup

Biw

abik

& G

unfli

nt Ir

on F

orm

atio

ns

Rov

e &

Virg

inia

For

mat

ions

Ani

mik

ie G

roup

N

OR

THK

ILO

ME

TER

S

Mod

ified

from

Mill

er a

nd W

eibl

en (1

990)

ON

TAR

IOC

AN

AD

A

MIN

NE

SO

TAU

.S.A

900

890

920

470

910

480

ELY TW

O H

AR

BO

RS

GR

AN

D M

AR

AIS

060

30

Du-

15x

DU

LUTH

Arc

hean

Roc

ks

Arc

hean

Roc

ks LAK

E S

UP

ER

IOR

23

4.2.2 Geology of the South Kawishiwi Intrusion

The footwall rocks, which bound the South Kawishiwi Intrusion (SKI) in anarcuate shape from north to west, are dominantly granitic rocks of the ArcheanGiants Range Batholith (Fig. 4). Some parts of the southwestern footwall arebounded by the Pokegama Quartzite, the Biwabik Iron-formation and the VirginiaFormation, all of which are members of a group of Paleoproterozoicmetasediments collectively referred to as the Animikie Group (Sims & Morey,1972).

Fig. 4. A close-up view of figure 3 showing a part of the western margin of the DuluthComplex. Drill holes Du-15 and BL-95-1 are also marked on the map (modified afterSeverson, 1988).

The following description of the geology of the South Kawishiwi Intrusion islargely based on the thorough work of Severson (1994). To the south the SKI isbounded by the Partridge River Troctolite Series (PRTS). The actual contactbetween the two igneous packages is somewhat ambiguous as there are nooutcrops in the area. According to Severson (op.cit.), the contact between the twointrusions is best illustrated by a “heterogeneous zone” that marks the end of the

LAKE SUPERIOR

DULUTH COMPLEX

CANADA

Duluth

Ely

MAP LEGEND

Base metal mineralization

South Kawishiwi Intrusion

Intrusive Rocks of the Duluth complex

Virginia Formation

Biwabik Iron Formation

0 50 km

10 km 0

920

47045'

GIANTS RANGE BATHOLITH

Du-15

Modified after Listerud and Meineke, 1977, Morey and Cooper, 1977; Watowich et. al., 1981; Severson 1988.

N

BL-95-1 x

x

24

PRTS. The typical igneous stratigraphy present in the SKI is usually clearly visiblein the drill holes north of the “heterogeneous zone” whereas to the south theigneous stratigraphy generally bears more resemblance to the PRTS. Anotherfactor contributing to the general confusion regarding the nature of the contact isthe presence of a major fault zone in the area, the Grano Fault, that cross-cuts the“heterogeneous zone”. The contacts of the SKI and the overlying rocks to the easttend to be obscure (Miller & Ripley 1996).

Investigations of a large number of drill core samples were done in order toillustrate and understand the internal stratigraphy of the South KawishiwiIntrusion. These studies revealed that the SKI consists of at least 17 units thatcorrelate along a 30-km strike length (Severson, op.cit.). Most of the 17 units arenot equally represented throughout the SKI, however. The overall stratigraphicpattern indicates that the SKI is not only layered vertically but alsocompartmentalized horizontally along its strike length. The SKI displays similarigneous features to the Duluth Complex as a whole in the sense that it is a multipleintrusive body with each individual unit exhibiting a finite spatial extent withoverlap between different units occurring in a myriad of patterns (Severson,op.cit.).

The basal unit of the SKI is a heterogeneous mixture of sulfide-bearing olivinegabbros and norites, of which the latter rock type is commonly encountered at thebasal contact probably due to assimilation of footwall rocks into the basic magma.This unit grades upward into the dominantly troctolitic Basal Heterogeneous Zonethat is the main sulfide-bearing zone of the SKI. At least three ultramafic-troctolitic packages are present within the SKI, and the U3 Unit is the lowest ofthese. Massive oxide zones are commonly found within this unit as well asrecognizable xenoliths of the Biwabik Iron-Formation (BIF), which areencountered along strike in areas adjacent to Birch Lake. Disseminated sulfidesare regularly present within U3 and almost all significant PGE occurrences thathave been found are associated with this unit. These PGE occurrences are usuallycharacterized by relatively high chromium values, although Cr content can be highin this unit regardless of the PGE content. The textural and mineralogical featuresin the unit suggest that the overlying massive oxide-rich rocks could be refractorymaterial from BIF incorporated into basic magma. Directly above the U3 Unitthere is a pegmatitic unit (PEG), which is overlain by a texturally homogeneous,sulfide-barren anorthositic-troctolitic unit referred to as the AT-T unit. The U2 isthe middle ultramafic-troctolitic package of the SKI, which is similar to theoverlying U3 Unit, except that massive oxide layers are usually absent. Higher in

25

the stratigraphy is the uppermost package of alternating ultramafic and troctoliticlayers referred to as the U1. It is similar to the U2 Unit. The upper part of the SKIcomprises rather homogeneous, thick rock units which are composed of troctolites,olivine gabbros and anorthosites.

Common textures of the rocks of the SKI are presented in Fig. 5. Plagioclaseis almost always a cumulus mineral, sometimes even enclosed in large olivineoikocrysts. At the very bottom of the intrusion, however, olivine displays a verydistinctive cumulus texture, with plagioclase filling the interstices. In places augiteand ilmenite also form large poikilitic crystals enclosing mainly lath-shapedplagioclase crystals. Another common texture is presented in the middle right ofFig. 5 where intercumulus pigeonite has reverted to augite and orthopyroxene andencloses both plagioclase and olivine.

4.2.3 Cryptic variation of the rock-forming minerals

Whenever possible, (i.e. the rocks of the given layered intrusion are not veryaltered) it is important to study the cryptic variation of the main rock-formingminerals such as plagioclase, olivine and pyroxenes. They can provide valuableinformation about the crystallization history of the intrusion, which in turn formsthe basis for understanding the formation of the mineralization hosted by theintrusion. The Duluth Complex is generally very well preserved and thus providesan excellent environment for this kind of examination.

When this investigation was initiated in 1994 there were little analytical datapublished from the Duluth Complex regarding the cryptic variation of the rock-forming minerals. Previously Chalokwu & Grant (1990) had studied the silicatemineralogy of the Partridge River Intrusion, but such data from the SouthKawishiwi Intrusion were even scarcer. Previous investigations had generallyconcentrated on the sulfide occurrences and little attention had been paid to thesilicate chemistry. Since 1994 the situation has changed remarkably as there arenow numerous publications from various locations more or less devoted tosystematic studies of silicate mineralogy (e.g., Kaukonen 1995, Lee & Ripley1996, Hauck et al. 1997a).

Cryptic variation of the South Kawishiwi Intrusion was studied from twoseparate drill holes. The drill holes in question are Duval-15 (Du-15) and BirchLake-95-1 (BL-95-1). The lengths of these drill holes are some 750 and 500meters, respectively. The former was selected because of the highly anomalousPGE values found in it. The latter was picked due to it being both new at the time

26

of selection and the fact that the initial core logging suggested it as being relativelyrepresentative of the stratigraphy of the entire intrusion, which the Du-15 certainlyis not.

Fig. 5. A) - Poikilitic olivine enclosing zoned plagioclase. B) - Cumulus olivine enclosedin plagioclase oikocrysts. C) - Poikilitic augite enclosing lath-shaped plagioclasecrystals. D) - Interstitial inverted pigionite enclosing both plagioclase and olivine. E) -Poikilitic ilmenite enclosing plagioclase crystals. F) - Same as on the left, reflectedlight. Width of all figures is about 5 mm. Bottom right is under reflected light and allothers are taken with crossed nicols.

A B

C D

E F

27

Figures 6–11 present the cryptic variation graphs of plagioclase, olivine, ortho-and clinopyroxe, apatite, ilmenite and magnetite from both drill holes. At firstglance the corresponding graphs of the two drill holes may seem rather different,but a closer look reveals striking similarities. The reader is advised to take note ofthe depth scale as BL-95-1 is distinctively shorter than Du-15. Essentially, for thesection that is common to both drill holes, extending roughly to 525 meters, thecryptic variation graphs are rather similar. The crystallization trends are morereadily visible on the graphs representing BL-95-1 where from about 440 metersupwards, the crystallization trend is normal, (i.e. plagioclase, olivine and pyroxesshow anorthite, forsterite and enstatite depletion, respectively) whereas below thatdepth the trends are reversed. However, in Du-15 this reversed trend does notcontinue all the way down to the bottom of the hole, but instead the highestanorthite, forsterite and enstatite values are found rather near the contact zone.Numerical data of some microprobe analyses presented in Figs 6–11 can be foundin Appendices 1–6. The lithostratigraphic column of Du-15 was adapted fromKaukonen (1995) and the construction of the BL-95-1 lithostratigraphic column isbased on a drill core log by Severson (1995). Due to the scale of the figure, somegeneralizations in presenting the rock types had to be made. For example, the”heterogeneous zone” of BL-95-1 is actually composed of several relatively thinalternating layers of troctolite and olivine gabbro with occasional xenoliths of ironformation.

The changes in the cryptic variation trends are interpreted to be caused byassimilation of silica-rich rocks from the footwall. Hence the crystallizationconditions at the bottom of the South Kawishiwi Intrusion were much moredisturbed than higher in the stratigraphy. Also the assimilation of cold silica- andiron-rich rocks had a profound impact on both the composition and temperature ofthe crystallizing magma, which probably contributed to reversing thecrystallization trend of the main rock-forming silicates. The small-scale changes,i.e. the “zig-zag” pattern between two or three consecutive samples, is in turninterpreted to be normal rhythmic variations common to these type of rocks.

28

Fig.

6. C

ompo

sitio

ns o

f ol

ivin

e, p

lagi

odas

e an

d ap

atite

in d

rill

hole

DU

-15

with

a g

ener

aliz

ed li

thol

ogy

(lith

olog

y fr

om K

auko

nen

1995

)

80

Oliv

Fo%

Oliv

NiO

wt.%

Plag

An%

Plag

Fe

wt.%

Apa

t Cl w

t.%A

pat F

wt.%

0 80

2040

600

0 0,

20,

40,

60

13

26

02

4

0

800

600

400

200

Sul

fide-

bear

ing

zone

2040

60

Troc

tolit

e

Oliv

ine

gabb

ro

Gab

bro

Con

tact

zon

e

Depth in drill hole (m)

0,2

0,4

0,6

29

Fig.

7. C

ompo

sitio

ns o

f pyr

oxen

es in

dri

ll ho

le D

U-1

5 w

ith a

gen

eral

ized

lith

olog

y (li

thol

ogy

from

Kau

kone

n 19

95).

Sul

fide-

bear

ing

zone

2040

6080

Opx

En

%

Troc

tolit

e

Oliv

ine

gabb

ro

Gab

bro

Con

tact

zon

e

0

0,2

0,4

0,6

0,2

0,4

0,6

0,8

020

4060

03

12

0,6

00,

20,

4

TiO

2 wt.%

TiO

2 wt.%

MnO

wt.%

Cpx

En%

MnO

wt.%

0

800

600

400

200


30

Fig.

8. C

ompo

sitio

ns o

f ilm

enite

and

mag

nelit

e as

wei

ght

perc

en in

dri

ll ho

le D

U-1

5 w

ith a

gen

eral

ized

lith

olog

y (li

thol

ogy

from

Kau

kone

n 19

95).

81

24

Ilm M

gO

Mag

n M

gOM

agn

Tio 2

Mag

n A

l 2O3

0

800

600

400

200


34

010

86

42

812

1520

Sul

fide-

bear

ing

zone

02

6

Troc

tolit

e

Oliv

ine

gabb

ro

Gab

bro

Con

tact

zon

e

0

0

416

06

24

05

10

Mag

n C

r 2O

3M

agn

V 2O

3

31

Fig.

9. C

ompo

sitio

ns o

f ol

ivin

e, p

lagi

odas

e an

d ap

atite

in d

rill

hole

DU

-15

with

a g

ener

aliz

ed li

thol

ogy

(lith

olog

y fr

om S

ever

son

1995

).

80

Oliv

Fo%

Oliv

NiO

wt.%

Plag

An%

Plag

Fe

Wt.%

Apa

t F w

t.%A

pat C

l wt.%

0 80

2040

600

0 0,

20,

40,

60

13

20

24

0

600

500

300

41

3

100

200

400

Het

erog

enen

eous

Sul

fide-

bear

ing

zone

Troc

tolit

e

Oliv

ine

gabb

ro

Ano

rthos

ite

2040

60


0,2

0,4

0,6

32

Fig.

10.

Com

posi

tions

of p

yrox

enes

in d

rill

hole

BL-

95-1

with

a g

ener

aliz

ed li

thol

ogy

(lith

olog

y fr

om S

ever

son

1995

).

80

TiO

2 wt.%

0 1

00

00,

20,

4

0

600

300

200

100

400

500

0,5

2040

601

00,

51,

50,

6

Opx

En%

MnO

wt.%

Cpx

En%

Ti

O2 w

t.%M

nO w

t.%

Het

erog

enen

eous

Sul

fide-

bear

ing

zone

2040

60

Troc

tolit

e

Oliv

ine

gabb

ro

Ano

rthos

ite


0,2

0,4

0,6

33

Fig.

11.

Com

posi

tions

of i

lmen

ite a

nd m

agne

lite

as w

eigh

t per

cent

in d

rill

hole

BL-

95-1

with

a g

ener

aliz

ed li

thol

ogy

(lith

olog

y fr

omS

ever

son

1995

).

6

Mag

n M

gO0

00

02

4

0

600

300

200

100

400

500

51

6

Ilm M

gOM

agn

Al 2O

3M

agn

TiO

2M

agn

V 2O

3M

agn

Cr 2

O3

Het

erog

enen

eous

12

39

63

1015

202

34

50

Sul

fide-

bear

ing

zone

02

4

Troc

tolit

e

Oliv

ine

gabb

ro

Ano

rthos

ite


34

4.2.4 Geology of the PGE mineralization in Du-15

Platinum-group minerals have so far been found only from the western margin ofthe Duluth Complex (Hauck et al. 1997b). More recent reports of the on-goingPGE exploration in the general area (e.g., Marma 2002, Shafer et al. 2003) haverevealed that there are several different types of PGE mineralization, some ofwhich include PGE associated with (1) stratabound/structurally controlled zonesof disseminated sulfides, (2) structurally controlled zones of massive sulfides and(3) stratabound/structurally controlled zones of massive/semi-massive oxide zoneswith or without base metal sulfides (Hauck et al. op.cit.). The PGE mineralizationintersected by drill hole Du-15 belongs to the last group as it is essentially astructurally controlled zone of massive/semi-massive oxides with no or very littledisseminated base metal sulfides.

Although slightly higher PGE values have been found from other localitieswithin the DC, the PGE mineralization in Du-15 contains the highest PGE valuesfound from the SKI to date. These values (adapted from Sabelin et al. 1986) arepresented in Fig. 12. They are found from oxide-rich rocks at about 10 metersabove the footwall contact. The common oxide texture here (Fig. 13) referred to asthe “two-in-one” texture by Severson (1994), (because microscopically it looks asif two thin sections were superimposed on top of each other) was interpreted byAlapieti (1991) to have been produced by sintering of oxide minerals in a fashiondescribed by Hulbert & von Gruenewald (1985).

The PGE mineralization intersected by drill hole Du-15 is locatedstratigraphically in the lowermost ultramafic-troctolitic package, the U3 Unit. Thisunit is close to the bottom contact of the intrusion and contains abundant xenolithsfrom the footwall Biwabik iron-formation. Kaukonen (1995) pointed out that thehighest Pt+Pd values almost coincide with the most primitive olivine and veryanorthitic plagioclase values. This is also illustrated in Fig. 12. However, nodistinctive correlation can be made between silicate compositions and PGEcontents due to the limited amount of whole-rock PGE data.

35

Fig.

12.

Lef

t - P

d, P

t and

Cr

conc

entr

atio

ns in

a s

ectio

n of

Du-

15. R

ight

- C

orre

spon

ding

S, N

i and

Cu

conc

entr

atio

ns a

ccom

pani

edby

An

and

Fo p

erce

ntag

es o

f pla

gioc

lase

and

oliv

ine

(PG

E, C

r, S

, Ni a

nd C

u gr

aphs

afte

r Sab

elin

et a

l. 19

86; A

n an

d Fo

gra

phs

afte

rK

auko

nen

1995

).

740

739

738

737

736

735

734

733

732

731

730

729

01

2S,

Ni,

Cu

wt.%

4050

6070

80A

n an

d Fo

%

An

FoSN

iC

u

740

739

738

737

736

735

734

733

732

731

730

729

010

0020

0030

0040

0050

0060

0070

0080

0090

00


Pt a

nd/o

r Pd

(ppb

)

01

23

45

6C

r w

t.%

Cr

Pt +

Pd

Pd Pt

36

4.2.5 PGE Mineralogy

There are a number of reports that present data on PGE mineralogy of the Du-15mineralization, e.g., Sabelin & Iwasaki (1986), Sabelin et al. (1986) Mogessie etal. (1991) and probably the most comprehensive, Komppa (1998, 2002).According to all these papers, various alloys, most notably those between Pt andFe, are the most common platinum-group minerals in this occurrence. Unlike Du-15, BL-95-1 is a privately owned drill core and thus a very limited amount of datahave been publicly available from it.

For this study most of the PGE data regarding Du-15 were gathered withpermission from the original notes of Komppa (1998, 2002). Some minorreinterpretations and refinements were made in the process. A few polished thinsections were scanned from BL-95-1 for PGM and these data are presented forcomparison. Selected analyses of PGM from BL-95-1 are presented in Appendix7.

Figures 14 and 15 present back-scattered electron images of some of theplatinum-group minerals found in Du-15 and BL-95-1, respectively. The imagesshow that at least in these two drill holes, the PGM in the SKI seem to beassociated with a wide range of rock-forming minerals rather than with just basemetal sulfides.

Figures 16 (A-C) and 17 (A-C) illustrate the associations, grain sizes anddistributions of PGM in oxide rocks of Du-15 and BL-95-1, respectively. Thefigures from the two drill holes look rather different. One factor used to explain thedissimilarities is the difference in the amount of data. Du-15 has a rather large setof data and hence statistical errors are diminished whereas for BL-95-1, theamount of data is very limited, which would cause the potential statistical errors tobe exaggerated. That alone, however, is not enough to explain such largedifferences particularly with the associations and the PGM parageneses betweenthe two drill holes.

Despite the differences, Figs 16 and 17 show that the PGM parageneses inboth of these occurrences are rather unusual when compared with many wellknown PGE deposits. Sperrylite (PtAs2), for example, is a rather minor mineralphase here and the Pt-Pd sulfides, cooperite (PtS) and braggite ((Pd,Pt,Ni)S), thatare common in many other PGE deposits, have not been encountered in thislocality at all.

37

Fig. 13. A) - A typical ”two-in-one” texture displaying oxide grains enclosed inplagioclase. The observed texture is interpreted as due to sintering. Width of the figureis about 5 mm, crossed nicols. B) - Same as (A) but under reflected light. C) - Anotherexample of sintered oxides, this time with a fair amount of pleonaste (gray). Width ofthe figure is about 5 mm, plane polarized light (white is plagioclase and olivine, black ismagnetite and ilmenite). D) - Same as (C) but under reflected light. E) - Anotherexample of oxide textures, black = magnetite, gray = pleonaste; white = silicates; planepolarized light. Width of the figure is about 1.3 mm. F) - Same as (E) but under reflectedlight. The top right corner is a close-up of the area marked with the rectangle: thecircular dark spots are pleonaste and the dark lamellae are Mg-spinel.

A

C

B

D

E F

38

Fig. 14. Back-scattered electron images of some PGM from DU-15. A) - A compositegrain of (A) tulameenite (Pt2FeCu), (B) sperrylite (PtAs2), (C) guanglinite (Pd3As) and (D)irarsite (IrAsS) in silicate. B) - (E) Isoferroplatinum (Pt3Fe) and (F) zvyagintsevite(Pd3Pb) in silicate. C) - Irarsite (white) and pentlandite in a patch of pleonaste inmagnetite. D) - Irarsite (white) and laurite (RuS2, light gray) in pleonaste. E) - At the topof the image there is a bright PGM grain composed of several phases that are probablysobolevskite (PdBi), irarsite and electrum (AgAu). The PGM are enclosed in a rim ofsecondary magnetite surrounding a grain of altered chalcopyrite. F) - A grain ofzvyagintsevite in a crack in magnetite.

C

E F

BA

D

39

Fig. 15. A) - A PGM grain composed of several phases located at a border betweenorthopyroxene (black) and chalcopyrite (darkest gray): (A) paolovite (Pd2Sn), (B)froodite (PdBi2), (C) sobolevskite (PdBi) and (D) native silver. B) - (E) a composite PGMgrain of sobolevskite and sperrylite (PtAs2) and (F) native silver enclosed in base metalsulfides. C) - A fairly large grain of paolovite (white) at silicate (black) - sulfide (gray)border. D) - A close up of the same. E) - A crack in olivine (black) is filled withsecondary magnetite (gray) that encloses several small grains of maslovite (PtBiTe). F)- Several grains of apparently Te-bearing sobolevskite, roughly Pd(Bi0.7Te0.3), enclosedin magnetite.

F

B

D

E

C

A

40

Fig. 16. Association (A), grain sizes (B) and distribution (C) of PGM in the oxide rocksof drill hole DU-15.

18%

38%

23%

16%

6%

< 2 μm

< 5 μm

< 10 μm

< 20 μm

> 20 μm

24%

13%

48%

15%

Silicate

Oxide

Sulfide

Sil-ox border

26%

26%10%

7%

5%

11%

8%

4%5% PtFe alloys

PdCu alloys

Pd-Bi-Te minerals

Pd-Ag sulfides

Pd-Pb minerals

Pd-Te-As-Sn minerals

Other Pt minerals

Other Pd minerals

Ru, Rh and Ir minerals

A

B

C

41

Fig. 17. Association (A), grain sizes (B) and distribution (C) of PGM in drill hole BL-95-1.

The data for BL-95-1 are from a very different stratigraphic and structural level,much higher than that of the Du-15. This probably means that even though thebulk rocks may be very similar, the PGE mineralization that they host were formedthrough different processes or at least one of them has undergone a process that

A10%

30%

20%

25%

13%3%

Silicate

Oxide

Sulfide

Sil-sulf border

Sil-ox border

Ox-sulf border

B8%

68%

13%

5% 5%

< 2 μm

< 5 μm

< 10 μm

10-20 μm

> 20 μm

C36%

9%11%

11%

14%

18%Sobolevskite

Sperrylite

Maslovite

Paolovite

Other Pt minerals

Other Pd minerals

42

had a significant effect on the PGM paragenesis. Because the mineralization inDu-15 contains significantly higher total PGE concentrations and in light of theavailable data seems to be the more peculiar of the two occurrences, the emphasisof this investigation will be put on that mineralization. Both of these occurrenceswill be revisited later in chapter 6 with a discussion regarding the formation ofoxide-hosted PGE mineralization.

4.3 Hanumalapur Complex

4.4 Introduction

The late Archean Hanumalapur Complex in Karnataka, India (for location seeFigs. 1 and 18) is one of the mafic-ultramafic complexes (MUC) that form themain lithology of the Hegdale-Gudda Formation (HGF). As depicted on the map(Fig. 19), the HGF occurs in several segmented blocks occupying the interdomaltroughs and border zone between the basement tonalite and supracrustal sequenceof the Kur Gudda and Tuppada Halli Formations. It extends discontinuously in aNE-SW direction over a strike length of more than 40 km. Detailed investigationsof all the MUC’s of the HGF have revealed that only the Hanumalapur Complexshows evidence for hosting a potential ore-grade PGE deposit.

Exploration and geological investigations have been going on in the generalarea ever since the discovery of titaniferous magnetite deposits in 1916 by Smeethand Sampath Iyengar. Studies in the area were mainly focused either on thesemagnetite deposits or the small chromite deposits which were subsequentlydiscovered in some of the ultramafic bodies on the eastern side of the area. Boththe Karnataka State Department of Mines and Geology and the Geological Surveyof India have repeatedly carried out exploration on V-Ti magnetite and chromitedeposits in the area. However, except for the exploratory mining of the MasanikereV-Ti magnetite, the investigations have not resulted in the opening of any mines inthe area.

First evidence for PGE mineralization in the area was finally discovered in theearly 1990’s as laurite grains were identified from a chromitite band. As such, thediscovery represents the first potential PGE occurrence identified in India(Devaraju et al. 1994a,b, Alapieti et al. 1994, Radhakrishna 1996).

43

4.4.1 Geology of the Hanumalapur Complex

The Hanumalapur Complex has been studied extensively by the KarnatakaPlatinum Project, a development-aid collaboration project of the Department ofGeosciences, University of Oulu, and the Department of Studies in Geology,Karnatak University, at Dharwad, together with the Karnataka State Department ofMines and Geology. This examination has revealed that the complex is a narrowsill-like body, ca. 3.5 km long and around 300 m wide. It has been metamorphosedto upper greenchist through lower amphibolite facies and the correspondingdeformation has completely destroyed the original cumulus textures as well as theprimary mineralogy. Only some chromite cores are partially preserved in places.The complex is bound on both sides by tonalite-trondhjemite gneiss (Fig. 19) andthe contact between the two comprises a shear zone. It also displays a rough zonedstructure composed of a central, 100–150-m-thick ultramafic zone, and outergabbroic zones.

Fig. 18. Close-up view of Fig. 1 centered on the study area in Karnataka, India. Legend:1) Hegdale Gudda Formation, 2) Oxide deposits, 3) Ultramafic lenses, 4) TuppadahalliFormation, 5) Quartzite, 6) Basement granitoids (a = trondhjemite, b = polyphasegneiss), 7) Dolerite dykes.

760 00 760 05750 50

TavarekereTavarekere

+ + + +

+ + + +

+ +

+ + +

++

+

+

+ +

+ + +

+ + +

+ +

+

+

+ ++

+ + +

+

+

+

+ + +

+ + + + +

+

+

+

+

+

+ +

+ +

+

+ + +

+

+ +

+

++

+

+ +

+

+

+

+ +

+

+ + +

+ +

+ + +

+ +

+

+

++

+

+ +

++ + +

+

+ +

+ .

+ + + + + + + +

+ + + + +

+ +

+ +

+

+

+ +

+

+ +

+

+

+

Baktanakatte

Rangapura

Tuppadahalli

Jamapura

Ubrani

Badigund Hanumalapur

..

.

.

..

.

.

..

.

..

..

..

.

..

.. ... .

.. .

.. .

.. .

.. .

.. .

.. . .

..

.. .

.. . .

. ..

. .

..

..

. .

.. .

.. .

.

. .

.. .

.

. .

.. .

..

.

.

. .

.. ..

. .

.

.

.

.

.

.

.

.

.

.

.

.

3 3km0 Heggadehalli

50130

D D D

750 55

D

D

DD

123

456a.

..

D

. .......

.... .........

.... ........

............ ........ ........ ........

... .

........ ........ ....

...

.... ........ ........ ....

........

....

....

....

... .

................

.... ..............

. .

....

..

.

.

........

... .

. ...

.

......

...

..

. ...

.. ....

.

..

.

....

.

.

.

.

.

.

.

.. .

..

..

+7Gijikatte

Masanikere

Ragibasavanahalli

Nellihankalu

Tavarekere

+

Gagodahalli

Db

..

.. .

.

44

Fig. 19. Close-up view of the small rectangular area in Fig. 18, presenting the variousrock types of the Hanumalapur Complex.

45

4.4.2 Geochemistry and mineralogy of the main rock-forming minerals

Due to the pervasive metamorphism and deformation, the list of major rock-forming minerals of the complex is rather short, composed generally of amphibole,chlorite and magnetite along with the more confined occurrences of carbonate,epidote and talc. Results of selected microprobe analyses of the major rock-forming minerals are presented in Appendix 8. The analyses show that chlorite isgenerally rather Al-rich with varying Mg/Fe ratios and as such it can be morespecifically termed ripidolite. Similar variations occur in amphiboles where thelargest compositional differences are also related to the Mg/Fe ratio and the end-members are magnesio- and ferrohornblende, respectively. In addition to these twosilicate groups, the largest compositional differences occur in magnetite andchromite. In the former, the main distinction is the presence or absence ofchromium in the mineral structure, whereas in the latter the compositionaldifferences may be reflected in varying concentrations of Al and Mg. In extremecases, best illustrated by certain outcrop samples (Fig. 20), chromite may display aclearly zoned texture. Generally the chromium-bearing magnetites wereinterpreted to be altered chromites. However, as Cr is usually found only in thevery center of a given grain if at all, it is often impossible to tell whether thepresent magnetite grain has ever been a grain of chromite. Ideally, magnetitegrains may display a weakly zoned texture with a broken outer rim of puremagnetite, an inner rim of Cr-bearing magnetite having a Cr2O3 content of 1–10weight% and a core of Cr-rich magnetite or Fe-rich chromite with a Cr2O3 contentof up to 20 weight%.

Figures 21 and 22 present graphs of the whole-rock XRF analyses for majorand selected minor and trace elements, respectively, of the samples from DrillHole-1 (for location see Fig. 19). Overall, the two most common minerals in therocks are chlorite and amphibole which occur in various proportions throughoutthe intrusion. Changes in Al2O3, MgO and CaO contents are usually attributed tochanges in the compositions of either or both of these minerals. Major differencesin silica and iron are generally caused by an increase or decrease in the amount ofmagnetite in the rock whereas changes in Na2O and K2O are attributed to albiteand biotite, respectively.

46

Fig. 20. A) – In some samples the chemical zoning in chromite is strong enough to beclearly visible with an ore microscope. Width of figure is about 3.5 mm. B) - A back-scattered electron image of the same chromite crystal with a slightly highermagnification. The core of the chromite grain contains more Al and Mg than the rim butthe Cr-content is rather constant throughout the grain.

B

A

47

Fig.

21.

Who

le r

ock

chem

istr

y of

maj

or e

lem

ents

in d

rill

hole

1.

040

80Si

O2

05

10T

iO2

010

2030

Al2

O3

040

80F

e2O

3*0

2040

MgO

010

20C

aO0

12

34

5N

a2O

00,

30,

6M

nO0

0,3

0,6

V

300

250

200

150

100500


= C

hrom

ite-a

mph

ibol

e sc

hist

(py

roxe

nite

?)

= C

hrom

ite-c

hlor

ite s

chis

t (br

onzi

tite

?)

=Am

phib

olite

(ga

bbro

?)

=Chl

orite

sch

ist (

bron

zitit

e ?)

= So

il co

ver/

not d

rille

d

= C

hlor

ite±c

arbo

nate

±epi

dote

sch

ist (

nori

te ?

)

= A

lbite

-epi

dote

-am

phib

olite

(le

ucog

abbr

o ?)

48

Fig.

22.

Who

le r

ock

chem

istr

y of

sel

ecte

d m

inor

and

trac

e el

emen

ts in

dri

ll ho

le 1

.

01

23

S %

040

080

0C

l ppm

02

46

8C

r %

00,

10,

20,3

Ni %

01

2C

u %

01

23

Pd

ppm

01

23

45

Pt

ppm

00,

20,

4A

u pp

m0

24

6P

GE

+A

u pp

m

300

250

200

150

100500


= C

hrom

ite-a

mph

ibol

e sc

hist

(py

roxe

nite

?)

= C

hrom

ite-c

hlor

ite s

chis

t (br

onzi

tite

?)

=Am

phib

olite

(ga

bbro

?)

=Chl

orite

sch

ist (

bron

zitit

e ?)

= So

il co

ver/

not d

rille

d

= C

hlor

ite±c

arbo

nate

±epi

dote

sch

ist (

nori

te ?

)

= A

lbite

-epi

dote

-am

phib

olite

(le

ucog

abbr

o ?)

49

4.4.3 Types of PGE mineralization in the Hanumalapur Complex

The most promising assays from outcrop samples from the Hanumalapur Complexhave yielded concentrations of over 6 ppm of total PGE+Au. Gold, however, isinvariably low with the best concentrations not exceeding 300 ppb. Most outcropsamples that were taken for PGE analysis were picked from rocks with abundantvisible chromite. The analytical results from these samples were then used forselecting the drill locations. Finally, the classification into different types of PGEmineralization is based essentially on data derived from the investigation andanalysis of drill core samples, even though to date the highest assay values are stillderived from outcrop samples.

The principles used for defining the type of PGE mineralization include (i) thewhole-rock concentration of each PGE, (ii) mineral paragenesis of the main rock-forming minerals, (iii) proportion of base metal sulfides in the rock and (iv) PGE-mineralogy of the rock. Essentially, these same principles have been appliedpreviously to the Penikat Intrusion by Halkoaho et al. (1990) and Halkoaho(1994). In the Hanumalapur Complex, each type of PGE mineralization has itsown distinctive characteristics in all of the above four categories and they aresummarized in Table 1. This division of the Hanumalapur Complex PGEmineralization is adapted from Kaukonen (2000).

4.4.4 PGE mineralogy of the Hanumalapur Complex

Due to the small grain size, which is commonly < 5 µm (as shown in Fig. 23A),and the limitations of the energy dispersive analytical method and hardware, theactual analyses are usually semi-quantitative and hence some interpretation isrequired to identify the mineral. Also, the chemical formulae of many PGM are sosimilar that the differences often fall within reasonable analytical errors. It is alsopossible that there are undetermined solid-solution series between several possibleend-members. This is important to bear in mind when interpreting the analyticaldata and naming the mineral phases. All these problems become very evident,particularly when dealing with the various Pd-Sb and Pd-Te-Bi minerals.

50

Table 1. Characteristic features of the various types of PGE mineralization in theHanumalapur Complex (after Kaukonen 2000).

So far over 650 PGM grains have been analyzed from DH-1 (drill hole 1) and asshown in Fig. 23B they have been grouped into sperrylite, other Pt±Rh minerals,Pd-Te-Bi minerals, Pd-Sb±As minerals, other Pd minerals and Ru-Os-Ir minerals.Gold grains have also been documented. This division is based on statistics,chemical affinities of PGE and the association of certain minerals with some of themajor rock-forming minerals. Figure 23C gives a generalized representation ofhow the PGM relate to other minerals. Selected PGM analyses of theHanumalapur Complex are presented in Appendix 9.

Sperrylite (PtAs2) is by far the most common Pt-bearing mineral in theanalyzed samples. Sperrylite grains are often euhedral or subhedral and they areusually associated with silicates.

Other Pt±Rh minerals include moncheite (PtTe2), cooperite (PtS), braggite((Pt,Pd)S) and platarsite (PtAsS), all of which tend to occur in silicates.Hollingworthite (RhAsS) is also presented in this group mainly for the sake ofsimplicity, but also because it may contain significant amounts of Pt and/or Ir andthus fits reasonably well into this group.

About one third of all PGM grains found in DH-1 so far are kotulskite (PdTe)and the majority (over 80 %) of them are hosted by silicates. A general observationmade during this study indicates that as the occurrence of the PGM gets morefrequent, its average grain size also increases. This statistical feature is easilyrecognizable from the occurrence of kotulskite in DH-1. Although a large portionof the kotulskite grains still fall within the 2–5 µm range, the occurrence of much

Type of mineralization

Main rock-forming minerals

Characteristic PGM Basemetal sulfides Accessory minerals

Silicate-hostedPd-mineralization

chlorite, amphibole

kotulskite, variousPd-Te-Bi minerals,sperrylite

absent or very scarse

magnetite, carbonate

Silicate-hostedPt-mineralization

chlorite, epidote(carbonate)

sperrylite, kotulskite, various Pd-bearing minerals

absent or very scarse

amphibole, biotite

Oxide-hostedPt-mineralization

chlorite, Cr-rich magnetite

Various Ru, Os and Ir-bearing minerals, sperrylite

absent none really

Sulfide-hostedPGE-mineralization

amphibole ± carbonate

merenskyite, michenerite (kotulskite, sperrylite)

chalcopyrite, pyrrhotite (pentlandite)

chlorite, epidote, biotite, magnetite

51

larger grains does indeed become more frequent. The kotulskite grains in DH-1usually contain several percents of both Sb and Bi.

Merenskyite (PdTe2) and michenerite (PdTeBi) are also common, butcompared to kotulskite, their association with sulfides is more apparent, as they areoften found either inside of base metal sulfides or at the border between a sulfidegrain and some other mineral (silicate, carbonate or oxide). In addition to these,the Pd-Te-Bi mineral group contain a few grains of keithconnite (Pd3-xTe),testibiopalladinite (Pd(Sb,Bi)Te) and some additional unnamed phases.

Fig. 23. Grain sizes (A), distribution (B) and association (C) of platinum-group mineralsin drill hole 1.

58%10%

14%

4%

9%

6%Silicate

Oxide

Sulfide

Silicate-oxide border

Silicate-sulfide border

Other

C

8%

55%

22%

10%

5%

< 2 μm

< 5 μm

< 10 μm

10-20 μm

> 20 μm

A

B14%

3%

52%

14%

3%

5%8% Sperrylite

Other Pt±Rh minerals

Pd-Te-Bi±Sb minerals

Pd-Sb±As minerals

Other Pd minerals

Ru-Os-Ir minerals

Gold

52

Fig. 24. Association (A) and distribution (B) of PGM in silicate-hosted Pdmineralization. (C) and (D) present the same for the oxide-hosted PGE mineralization,respectively. (E) and (F) display the corresponding information for the sulfide-hostedPd mineralization, respectively.

33%

29%

5%

19%

4%3% 7%

Mertieite-I and II

Kotulskite

Other Pd-minerals

Sperrylite

Hollingworthite

Other PGM

Gold

90%

4% 6%

Silicate

Sulfide


68%

19%

13%

Oxide

Silicate


47%

15%3%

6%

6%

2%

12%

9%

Sulfide

Oxide

Silicate

Carbonate



Sulfide-carbonate border

Others

29%

3%

15%18%

9%

9%

3%

15% Sperrylite

Platarsite

Hollingworthite

Anduoite

Ruarsite

Irarsite

Osarsite

Pd-minerals

47%12%

12%

13%

17% Merenskyite

Michenerite

Kotulskite

Other Pd-minerals

Gold

A B

C D

E F

Sulfidehosted

Oxidehosted

Silicatehosted

53

The most common Pd-Sb±As minerals in DH-1 are mertieite-II (Pd8Sb3),stibiopalladinite (Pd5Sb2) and mertieite-I (Pd5(Sb,As)2). They most often occur asinclusions in silicates. Other minerals in the Pd-Sb±As mineral group includesudburyite (PdSb), isomertieite ((Pd,Cu)11(Sb,As)4), and other unnamed phases.The small group of “other Pd-minerals” comprises temagamite (Pd3HgTe3),paolovite (Pd2Sn), sopcheite (Ag4Pd3Te4) and some unnamed phases.

In DH-1, minerals containing Ru and/or Os and/or Ir are generally closelyassociated with oxide minerals, namely chromite or Cr-rich magnetite. Theminerals in this group are laurite (RuS2), anduoite (RuAs2), ruarsite (RuAsS),erlichmanite (OsS2), osarsite (OsAsS) and irarsite (IrAsS).

The highest gold values from DH-1 are a few hundred ppb. Gold forms alloyswith silver, and the metal ratios vary from nearly pure gold to about 90% silver.The grain sizes are usually very small. Gold grains are most often associated withbase metal sulfides, but they also occur in magnetite and in silicates. Often whengold is associated with silicates, it forms composite grains with some PGM.

Figures 23A-C present cumulative data, i.e. all different types of occurrencescombined. Figures 24A-F display the associations and distributions of PGMwithin different types of mineralizations. These data are from DH-1 only, but datafrom outcrop samples and preliminary investigations of other drill cores support it.A silicate-hosted Pt mineralization is not represented because the respective thinsections were virtually devoid of PGM grains even though the lead fire-assayanalysis yielded more than 4.3 ppm of Pt and over 0.6 ppm of Pd. That can also beconsidered evidence for uneven distribution of PGE (and PGM) in the rock. Datafor the silicate-hosted Pd mineralization, oxide-hosted PGE mineralization andsulfide-hosted Pd mineralization are presented in Figures 24A-B, 24C-D and 24E-F, respectively.

Figures 25A-B present the minimum and maximum mantle- and chondrite-normalized PGE values for the Hanumalapur Complex, and for comparisoncorresponding averages for UG2 and the Merensky Reef of the Bushveld Complexand the SJ Reef of the Penikat Intrusion. Because this graph is based on only 13drill core samples, it is likely to change as more data become available. However,even now it is evident that the highest values for the Hanumalapur Complexcorrelate rather well with the Bushveld values whereas the chromite type SJ Reefvalues are so much higher that they are in a league of their own.

54

Fig.

25.

Man

tle-n

orm

aliz

ed (A

) and

C1

chon

drite

-nor

mal

ized

(B) P

GE

gra

phs

of th

e H

anum

alap

ur C

ompl

ex w

ith th

e P

enik

at S

J R

eef

and

the

UG

2 an

d M

eren

sky

Ree

f va

lues

of

the

Bus

hvel

d C

ompl

ex s

how

n fo

r co

mpa

riso

n. D

ata

for

SJ

is f

rom

Ala

piet

i & L

ahtin

en(2

002)

, for

UG

2 an

d M

eren

sky

Ree

f fr

om B

arne

s &

Mai

er (

2002

), fo

r m

antle

fro

m M

cDon

ough

& S

un (

1995

) an

d fo

r C

1 ch

ondr

itefr

om N

aldr

ett (

1981

).

Ni

Os

IrR

uR

hPt

PdA

uC

u

0,010,1110100

1000

1000

0

1000

00

Mantle normalized

Han

umal

apur

min

imum

Han

umal

apur

max

imum

Han

umal

apur

ave

rage

SJ c

hrom

ite ty

pe

SJ s

ulfi

de ty

pe

UG

2

Mer

ensk

y R

eef

Os

IrR

uR

hPt

PdA

u

0,00

1

0,010,1110100

1000

C1 chondrite normalized

Han

umal

apur

min

imum

Han

umal

apur

max

imum

Han

umal

apur

ave

rage

SJ c

hrom

ite ty

pe

SJ s

ulfi

de ty

pe

UG

2

Mer

ensk

y R

eef

AB

55

4.5 Penikat Layered Intrusion

4.5.1 Introduction

The Penikat Layered Intrusion in northern Finland (Fig. 26) has been studiedextensively, and there are numerous papers dealing with the PGE mineralization itcontains. The similarities to the Hanumalapur Complex are evident. They bothcontain essentially silicate-hosted PGE mineralization with negligible base metalsulfides, and the association of chromite (or Cr-rich magnetite) is rathercharacteristic to these occurrences. Both have also undergone upper greenschist tolower amphibolite facies metamorphism.

A few samples from the Penikat Layered Intrusion were studied and theresults of those were compared to the two main case studies. The samples wereselected from a highly anomalous section of drill hole KI-86 and represent adistinct silicate-type PGE mineralization with Pt+Pd concentrations of up toalmost 20 ppm.

4.5.2 Geology of the Penikat Intrusion and its PGE occurrences

The 2440 Ma Penikat Layered Intrusion forms part of a discontinuous belt ofPaleoproterozoic mafic layered intrusions that is about 300 km long extendingfrom Tornio on the Swedish border eastward across Finland and all the way to thehill of Näränkävaara by the Russian border and into Russia (Alapieti & Lahtinen1986, Iljina & Hanski 2005). In addition, there are several other mafic layeredintrusions of the same age scattered in the Fennoscandian Shield that have beengenetically correlated with the above-mentioned Tornio-Näränkävaara belt(Alapieti et al. 1990).

The present surface expression of the Penikat intrusion is 23 km long and 1.5to 3.5 km wide. It was broken into five westward-dipping blocks by tectonicmovements that took place during the Svecokarelian orogeny. These aredesignated as the Sompujärvi, Kilkka, Yli-Penikka, Keski-Penikka and Ala-Penikka blocks (Fig. 27). Their borders are bound by east-west-trending faults(Alapieti & Lahtinen 1986, 2002).

56

Fig. 26. Locations of the 2.44 Ga mafic layered intrusions in the Fennoscandian Shield.The Penikat intrusion is located close to the city of Kemi (after Alapieti & Lahtinen,2002).

KoitelainenKoitelainen

... .... ..RUSSIA

Barents Sea

NORWAY

FINLAND

Gulf of Bothnia

Caledonides

0 100 km50

Paleozoic alkaline rocksJothnian sedimentary rocks(1300-1400 m.y.) Proterozoic granitoidsPalaeoproterozoic metamorphosed

and igneous rocksLayered intrusions (2.4-2.5 Ga)

Archaean rocks

Oulanka

Burakovo

KukkolaTornio

Oulu

PenikatKemi

Portimo

Keivitsa

Pana Tundra

MURMANSK

Akanvaara

Koitelainen

Monchegorsk

Imandra

Generalskaya

Lake

Arctic Circle

Koillismaa

White SeaN

Caledonides

Mt.

Onega

Fedorova TundraSWEDEN

sedimentary

... .. .. . ..

.. ..

.. .

..

.

... ..

.

. . ...

... ... .... .. ......

.

NO RWAY

SWEDENFINLAND

RUSSIA

Tsohkkoaivi.. ..

. .

. . .

.. ......

.. ..

.. ... .. .

.. .. ...... ..

.. .

..

. ..

57

Fig. 27. Structure and stratigraphy of the Penikat intrusion (after Alapieti & Lahtinen,2002).

The internal structure of the Penikat intrusion may be divided into three principalunits: the Marginal series, the Layered series and the granophyre. The Marginalseries separates the intrusion from the underlying granitoids, with which it has avery sharp contact. It is composed of a fine-grained chilled margin, subofiticgabbroic rocks and bronzite cumulates. The Layered series is composed of fivealternating sequences of ultramafic, gabbroic and anorthositic rocks that have a

0 5 km

Late Archean/earlygranitoidsPaleoproterozoic

Megacyclic unit IMegacyclic unit IIMegacyclic unit IIIMegacyclic unit IVMegacyclic unit V

Marginal series

PV ReefAP Reef

SJ Reef is located at thecontact between the megacyclicunits III and IV

LAYERED INTRUSION

Proterozoic metasedimentaryand metavolcanic rocks

LEGEND

Loljunmaa dyke

40o Layering

SOMPUJÄRVIBLOCK

YLI-PENIKKABLOCK

KESKI-PENIKKABLOCK

ALA-PENIKKABLOCK

40o

40o

o40

o40

o55

o70N

KILKKA BLOCK

58

rather regular lithological repetition which is mappable almost throughout theentire area of the intrusion. These sequences have been interpreted as megacyclicunits which each represent an influx of a new pulse of magma into the chamber.The granophyres, which is on top of megacyclic unit V, is visible only in someplaces below hanging-wall supracrustal rocks (Alapieti & Lahtinen 1986).

Starting from the bottom of the layered series, megacyclic unit I begins with a100- to 350-meter-thick bronzitite layer, which is lithologically uniform for theentire length of the intrusion. The rocks of this layer are composed of feldspathicbronzite ± chromite orthocumulates, in which plagioclase and augite occur asintercumulus minerals. These grade upwards rather rapidly to gabbroic rocks asplagioclase and augite become cumulus phases. Megacyclic units II and IIIresemble each other greatly as they both begin with heterogeneous ultramafic partsthat are composed of alternating layers of peridotitic and pyroxenitic rocks withsome chromitite interlayers. Both units also have gabbroic upper parts, that arecomposed of gabbronorites, i.e. plagioclase-augite-bronzite cumulates withinterlayers of peridotitic, websteritic and anorthositic rocks. Often, a pegmatiticlayer is encountered just below the base of megacyclic unit IV. This layer iscomposed of plagioclase and sometimes also chromite, that occur as cumuluscrystals accompanied by large (several centimeters in diameter) oikocrysts ofaugite and bronzite. Megacyclic unit IV begins generally with a 10-20-meter-thicklayer of ultramafic rock, although in some places this layer may be as thin as 10cm. This is followed by a thick sequence of gabbroic and anorthositic cumulateswith marked rhythmic layering in the lower parts of the sequence, homogeneousgabbronorites in the middle and a distinctive increase in the proportion ofplagioclase in the upper parts. Another characteristic feature of this unit is theunusual behavior of chromium, since the amount of this element decreases at thebase of the ultramafic layer compared to its abundance in the plagioclase-rich unitbelow. Megacyclic unit V begins with a 3- to 7-meter-thick layer of bronzititefollowed by a sequence of plagioclase-bronzite-(ilmenomagnetite) cumulatescontaining intercumulus augite and plagioclase-bronzite-augite-(ilmenomagnetite)cumulates. Above these rocks, orthopyroxene is absent and the rocks in theuppermost part of unit V are plagioclase-augite-(ilmenomagnetite) cumulates(Alapieti & Lahtinen 1986).

There are some low concentrations of platinum group elements associatedwith certain chromite-rich layers in megacyclic units I, II and III. However, themost important PGE mineralization in the Penikat intrusion occurs in megacyclicunit IV (Alapieti & Lahtinen 1986). Since each of these occurrences continue

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through the entire length of the intrusion and can be located in different blocks attheir corresponding structural levels, they have been termed PGE reefs (Halkoaho1993).

Stratigraphically, the lowermost of the PGE reefs is the Sompujärvi Reef (SJ-Reef), which is located at the contact between megacyclic units III and IV, some500–1000 meters above the base of the intrusion. The mineralization is rathererratic in distribution as PGM (although usually concentrated in the basal portionof megacyclic unit IV) can occur in the overlying peridotitic cumulates as well asin the gabbronoritic cumulates at the top of megacyclic unit III, either separately oreven simultaneously. Correspondingly, the thickness of the mineralization variesconsiderably from a few decimeters up to several meters with the average beingabout one meter (Halkoaho 1993).

Alapieti & Lahtinen (1986) described that the SJ mineralization is for the mostpart almost totally sulfide-free as the sulfur content of the mineralized rocks isoften below the detection limit of 10 ppm. Halkoaho et al. (1989a, 1990a) andHalkoaho (1993), however, divide it very roughly into two types: the base metalsulfide type and the chromite type. They mention that this division is not entirelyaccurate because in some drill holes PGM have been found to be concentrated insilicate rocks which contain neither base metal sulfides nor chromite.

The Ala-Penikka PGE Reefs, AP I and AP II, are located ca. 250–450 and350–450 meters above the base of megacyclic unit IV, respectively. They areknown to extend over the entire length of the Penikat intrusion. The AP I PGEReef is rather erratic in its distribution. It is generally ca. 20–40 cm thick althoughin places it can attain a thickness of ca. 20 meters. It is also closely associated withdisseminated base metal sulfides with both the sulfides and the PGM occurring inthe interstices of the cumulus (silicate) framework. The PGE mineralization in theAP II Reef greatly resembles that of the AP I Reef with the exception that it isgenerally depleted in base metal sulfides. However, in some places the base metalsulfide content does rise up to ca. 2 vol. % with a PGE content of some tens ofppm (Halkoaho 1989, 1993, Halkoaho et al. 1989b, 1990b).

The Paasivaara (PV) PGE Reef is mainly located in the transition zonebetween megacyclic units IV and V. This transition zone is approximately 40 mthick and has a varying lithology and cumulus stratigraphy. The PGM areassociated with disseminated sulfides that are rather ubiquitous in this reef andoccur most often in the interstices of the cumulus plagioclase framework (Huhtelinet al. 1990). The PGE mineralization of the PV Reef is similar to the other PGEoccurrences in the Penikat intrusion, erratic in its distribution as PGE-bearing

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layers can be encountered as much as 30 meters below the main enrichment andoccasionally even in the lowest bronzititic cumulates of unit V. The averagethickness of the main mineralized zone, however, is approximately 1 meter(Alapieti & Lahtinen 1986).

The PGE mineralogy of the Penikat Layered Intrusion has been studiedextensively after the initial report of Alapieti & Lahtinen (1986). Since then, thesubject was given more attention by Halkoaho (1989, 1993) and particularly byTörmänen (1995).

Overall, well over a thousand PGM grains have been analyzed and ca. 40different platinum-group minerals have been identified from the Penikat intrusion.The most important of them are sperrylite, various Pd-Sb-As minerals and variousPd-Te-Bi minerals. PGE sulfides such as cooperite and braggite and alloys such asisoferroplatinum are common in some famous world-class PGE deposits, mostnotably the sulfide-hosted Merensky Reef and Stillwater deposits (Brynard et al.1976, Kinloch 1982, Kinloch & Peyerl 1990, Volborth et al. 1986). These mineralsare, however, rather rare in the Penikat intrusion (Törmänen, op.cit.).

More often than not, PGM are found to occur in association with alteredsilicates, although the association with base metal sulfides is somewhat commontoo. Halkoaho (1993) made an interesting observation that of all PGM, only lauriteoccurs as inclusions in chromite. Some other PGM may occur in silicate inclusionsin chromite, although not directly in chromite.

Halkoaho (op.cit.) suggested that the PGE mineralization associated with basemetal sulfides in the SJ Reef was largely a product of a magma mixing process(described earlier as “orthomagmatic theory”). However, another explanation isrequired to account for the chromite- and silicate-hosted PGE mineralization andHalkoaho (op.cit.) concluded that they are products of remobilization andredistribution of the PGE by a fluid phase.

Halkoaho et al. (1989b, 1990b) and Halkoaho (1993) presented that magmatichydrothermal fluids were particularly involved in the formation of the AP I and IIPGE Reefs. They interpreted the AP Reefs to have formed from an upward-migrating fluid-enriched intercumulus melt, in which PGE, S, Ni, Cu and relatedelements occurred in the fluid phase. This residual melt was trapped by a relativelythick layer of plagioclase-bronzite mesocumulate at its lower contact to form theAP I Reef. The AP II reef was formed at a slightly higher stratigraphic level as aconsequence of the fluids’ ability to bypass the barrier layer in places (Halkoaho,op.cit.).

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Huhtelin et al. (1990) interpreted the PV Reef to be a “classic” example of themagma mixing process. As the fifth magma pulse entered the chamber, the oldermagma was already partially consolidated. Sulfide precipitation took place as thenew magma mixed turbulently with the older residual magma. This turbulentmixing and convection allowed base metal sulfide droplets to “scavenge” the PGEfrom a large amount of silicate melt (cf. Campbell et al. 1983, Irvine et al. 1983,Naldrett et al. 1985). Then cooling of the magma and lessening of the turbulentconvection caused the sulfides and the associated PGM to sink within the reef.

4.5.3 Silicate-type PGE mineralization in the SJ Reef

The few samples of the SJ Reef that were studied in detail in this investigationwere taken from drill hole KI-86, which penetrates the SJ Reef at the Keski-Penikka Block (see Fig. 27 for location). The highest assay value yielded a3PGE+Au concentration of 19.49 ppm with a complete absence of base metalsulfides or chromite in the rock. The rock is essentially a chlorite-amphibole rock,completely altered during metamorphism with no traces of any primary textures.Analytical data of this chlorite and other minerals from the SJ Reef are presentedin Appendix 10. The composition of the chlorite in the SJ Reef is similar to that ofthe chlorite in the PGE-rich rocks of the Hanumalapur Complex.

The PGE are present as distinct PGM phases that occur as inclusions in eitherchlorite or amphibole or in between the two silicate phases. The occurrences ofPGM are illustrated in Figs 28A-B.

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Fig. 28. A) Mertieite-II and kotulskite grains in amphibole. The dark greybackground ischlorite.

Fig. 28. B) Hollingworthite grains (white) forming an arc in chlorite (backgound). Thegrey mineral is magnetite.

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Due to the small number of samples available for this study, there is really no pointin presenting any statistical diagrams. The two main PGM phases that wereencountered are hollingworthite (RhAsS) and mertieite-II (Pd8(Sb,As)3). Inaddition, only isolated odd grains of isomertieite and kotulskite were found. Hencethe relatively high Pt concentration (up to 10.3 ppm) of these samples remains amystery as Pt is most often located in sperrylite, although no sperrylite grains wereencountered in this case. Presumably, it is also possible that Pt is contained in PGEsulfides such as braggite and/or cooperite as is the case with the chromite-typePGE mineralization of the SJ-Reef (Alapieti & Lahtinen 2002), but again, no suchgrains were encountered.

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5 Examples from the literatureThe following examples from the literature are well-known and well-documentedsulfide-poor PGE occurrences. They have been chosen based on their geologicalsignificance as well as similarities to the case studies presented earlier in this workto shed more light on the enigma of the formation of such deposits.

5.1 The Bushveld Complex

The Bushveld Complex in South Africa (Fig. 29) is by far the largest mafic layeredintrusive complex in the world. It has an areal extent of ca. 65 000 km2 and athickness of 7–9 km. It has a wide range of lithologies from dunites andpyroxenites to anorthosites on one end and pure oxide layers on another end withjust about any and all conceivable intermediate varieties in between (Eales &Cawthorn 1996). The mafic rocks of the Bushveld Complex are extremely wellpreserved, having undergone only minimal structural disturbance or alteration(Cawthorn et al. 2002), even though they are 2060 Ma old (Walraven et al. 1990).Because the rocks are so well preserved, the Bushveld Complex provides a verygood place to study various aspects of igneous petrology. The sheer size of theBushveld Complex is enough to make it a geological formation with world-wideinterest, but even more so are the various types of ores that it hosts that really drivethe scientific and economic interest.

Stratigraphically, the gabbroic and ultramafic rocks of the Bushveld Complexare divided into four zones based on the varying proportions of the different rock-forming minerals. These zones are: the Lower Zone, the Critical Zone, the MainZone and the Upper Zone (McBirney 1984, Sharpe 1986). In addition to these,there is a separate Marginal Zone, the rocks of which have only been exposed onthe Eastern Limb (Fig. 29) below the ultramafic rocks. These rocks are mainlycomposed of fine-grained gabbronorite that supposedly represents the chilledmargin of the complex (Wager & Brown 1968).

The thickness of the Lower Zone is up to ca. 1800 meters (Hess 1989) and therock types represented are mainly bronzitite, dunite and harzburgite. Theoccurrence of chromite varies greatly in this unit. It is a fairly common accessorymineral, but in places it can form its own, almost monomineralic layers, some ofwhich are being exploited as chromium ores (Sharpe 1986).

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The lower part of the Critical Zone is usually defined as the horizon wherecumulus chromite is so abundant that it forms its own well-defined layer and theamount of intercumulus plagioclase increases to several percent (Sharpe, op.cit.).The thickness of the Critical Zone is about 1200 m and the rock types range fromchromitites to anorthosites. The significance of this zone is dictated by the vastamounts of economically important ores contained within it. In addition tochromium, the Critical Zone hosts two very significant PGE-bearing layers, theUG-2 and the Merensky Reef, from which a great deal of the world’s platinum isbeing produced.

The Main Zone is by definition the largest zone with a thickness in excess of3000 meters. It is generally being defined by the occurrences of oxidic layers orrather the lack of them as chromite is no longer encountered above the CriticalZone whereas magnetite is not introduced until in the Upper Zone. Hence the rocktypes of the Main Zone include rather homogeneous anorthosites, norites andgabbronorites. Layering is generally poorly developed and oxide minerals andolivine are absent (Hess 1989).

The rocks of the Upper Zone are characterized by the occurrence of magnetiteas a cumulus mineral. The thickness of the zone is ca. 2500 m and it has beenfurther divided into three subzones. The middle subzone displays the reappearanceof olivine as a cumulus phase. This time, however, it is distinctively more iron-richthan in the lower units. In the uppermost subzone, apatite, and in placesamphibole, occur as cumulus minerals (McBirney 1984, Sharpe 1986). Magnetite,which is the most important mineral of the Upper Zone, is vanadium-rich andtitanium-poor in the lower parts of the zone, while in turn in the upper parts it is V-poor and Ti-rich. Cryptic variation within the zone is also evident, as olivine ispure fayalite in the uppermost rocks.

Even though the individual blocks of the Bushveld Complex have theirdifferences, certain units comprise very similar layers, the compositions of whichare alike in different blocks. Because many layers are mostly monomineralic andhomogeneous, one can conclude that the differentiation of the crystallizingBushveld magma was very extensive (McBirney 1984).

The lower parts of the intrusion are mainly composed of a complex series ofbronzitites, gabbros, anorthosites and some economically important chromitites.Some of these units are repeated as cyclic sequences in which modal and crypticvariations are repeated in regular saw-tooth patterns. At the base of each cycle,there is an abrupt change into a more olivine-rich assemblage that is oftenaccompanied by a chromitite interlayer at its lower level. This is followed by a

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distinctive increase in the proportion of orthopyroxene, the composition of whichis intially rather magnesian but becomes progressively more iron-rich upward. Thecyclic units as a whole become more iron-rich upward and at the same time, theyalso become progressively thinner until they eventually disappear near the base ofthe Main Zone (McBirney, op.cit.).

The general consensus among the researchers of the Bushveld Complex is thatthere must have been at least two different parental magmas, an ultramafic one andan anorthositic one. Evidence for this has been gathered from all around thecomplex by studying, among other things, the geochemistry of the dikes of theMarginal Zone and at the bottom of the complex. Isotopic and rare earth elementinvestigations have proved, however, that there has been a third basaltic type ofmagma involved (Sharpe 1986).

No undifferentiated end-members representing the parental magmas havebeen found from the Bushveld Complex, so the classification is generally based onisotopic investigations of rocks of the chilled margin of the Marginal Zone. Theultramafic parental magma (the pyroxenite of the Marginal Zone) is regarded asboninitic in composition (Hatton & Sharpe 1989). It has probably been rich in SiO2

(52–56 %), MgO (10–16 %) and chromium (600–1500 ppm). The basaltic parentalmagma was likely close to tholeitic basalt in composition (i.e. SiO2 ca. 49–52 %,MgO ca. 6–10 %). The anorthositic magma was added rather late in the history ofthe complex, and hence, by that time, the floor rocks were already quite hot. Thismeans that there are no chilled rocks of the anorthositic magma to be found, or atleast none have been found despite extensive searching (Sharpe 1986).

There are two distinctively different types of economic platinum-groupelement mineralizations in the Bushveld Complex. In the Merensky Reef, the PGEare associated with base metal sulfides, whereas in the UG-2, they are associatedwith chromite. There are also other distinctive layers – for example, the LG-6chromitite (e.g., Teigler 1999) – that have anomalously high PGE values, thoughPGE have not been extracted from them.

The famous Merensky Reef is a classic example of a sulfide-hosted PGEdeposit. It has been widely studied, and the orthomagmatic model of PGE oreformation is also based on studies of the Bushveld Complex. Because thisparticular study concentrates on sulfide-poor PGE deposits, the attention will befocused on the UG-2 and other chromitite-associated PGE occurrences within theBushveld Complex.

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5.1.1 The UG-2 chromitite

The occurrence of platinum-group elements in the UG-2 chromitite was firstreported by Hall & Humphrey in 1908. Wagner (1925, 1929) reportedconcentrations of up to 12 pennyweights per short ton, which translates to ca. 20ppm of total PGE. Apparently, the attention was then shifted to the MerenskyReef, which was discovered in the middle of the 1920’s (Hall 1932). Littleinformation regarding the PGE of the UG-2 was published for several decades.

The UG-2 chromitite layer contains ca. 50 percent of the total platinum-groupelement resources of the Bushveld Complex (von Gruenewaldt, 1977). Accordingto McLaren & De Villiers (1982), its average noble metal content is about 7 ppm.Of the noble metals, Pt is the most abundant followed by Pd, Ru, Rh, Ir and Au, inthat order. Several mines and mining companies have extracted PGE from the UG-2 chromitite (Vermaak 1995).

On a regional scale, the UG-2 layer can be followed almost continuouslyalong strike in both the eastern and the western portions of the Bushveld Complex.On the western part, it lies stratigraphically ca. 15-220 meters below the MerenskyReef, whereas on the eastern part, the corresponding distance varies between 145–370 meters. The thickness of the UG-2 layer varies considerably from 15 cm toover 2.5 meters (McLaren & De Villiers 1982).

The platinum-group mineral data from McLaren & De Villiers (op.cit.)indicate that the most abundant PGM in the UG-2 chromitite are laurite, braggite,cooperite, some unnamed sulfides of Pt-Ir-Rh-Cu and Pt-Pb-Cu, and alloys of Pt-Fe, Pd-Cu and Pd-Pb. The grain size data of the same investigation indicate thatthe majority of PGM grains are ca. 2-3 µm in diameter. Another feature recordedby McLaren & De Villiers (op.cit.) is the general association of PGM with basemetal sulfides, although they commonly may occur along grain boundaries and asinclusions in silicates and chromite.

There are several studies regarding the distribution of platinum-groupelements within the UG-2 chromitite. Figure 30 is based on the data by Hiemstra(1985). This data correlate very well with most of the similar figures by McLaren& De Villiers (op.cit.), but not as consistently with those of Gain (1985) from theFarm Maandagshoek. The lower peak seems to be consistently located at thebottom contact of the layer. The position of the higher peak, however, varies sothat it is either located in the middle or at the upper contact of the main chromititezone. The figures in the papers quoted above (particularly those of Gain) seem toindicate that the thinner the chromitite layer is at a given location, the more likely

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the upper PGE anomaly is located at the upper contact of the layer. Conversely, thethicker the chromitite layer, the more likely the upper PGE anomaly is to be foundnear the middle of it.

Penberthy & Merkle (1999) investigated the lateral variations in the PGEconcentration as well as the mineralogy of the UG-2 chromitite. They reported thatthe predominant PGM in a typical UG-2 chromitite are various PGE sulfides, suchas cooperite, braggite and laurite. These are generally associated with base metalsulfides either as inclusions or at grain boundaries. Parts of the UG-2 chromititelayer are associated with a replacement pegmatoid. There the PGM paragenesis issomewhat different, consisting predominantly of laurite and various PGE±Fealloys and other non-sulfide PGM. However, the association of PGM with basemetal sulfides is still roughly as evident as it is in the case of the normal UG-2chromitite. A third type of chromitite within the UG-2 layer reported by Penberthy& Merkle (op.cit.) is a cataclastic chromitite, which contains abundant fracturesand faults. Their presence apparently made the area more prone to the affects offluid activity. Evidence for this is displayed by the presence of large amounts ofhydrous silicates, quartz and calcite cementing fractured chromite grains. In theserocks, the PGM paragenesis consists mainly of non-sulfide minerals such as Pt-Fealloy and PGE sulfarsenides.

Penberthy & Merkle (op.cit.) interpreted that the base metal sulfideassemblage of the UG-2 chromitite was formed generally as a result of post-magmatic loss of Fe and S due to equilibration of chromite and sulfides, combinedwith fluid activity. They explained the formation and close association of PGMwith base metal sulfides to be the result of expulsion of PGE from the base metalsulfides during cooling. The lateral variations in the PGE mineralogy are generallythought to have been a result of local disturbances, such as potholes, faulting or thepresence of a replacement pegmatoid, and in some cases, they appear to resultfrom poorly-defined hydrothermal activity.

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Fig. 30. Distribution of noble metals as a function of stratigraphic height within the UG-2 chromitite layer. Data from Hiemstra (1985).

5.1.2 Other chromitite layers in the Bushveld Complex

Lee & Parry (1988) reported whole-rock PGE concentrations in excess of 6 and1.5 ppm from the Middle and Lower Group chromitites, respectively. Theyconclude that the chromitites are enriched in platinum-group elements comparedto the surrounding silicate host rocks. With reference to the Bird River sill inCanada, they further conclude that the controls on the occurrence of PGE-enrichedchromitites are an intrinsic aspect of the evolution of the primary magma.

Scoon & Teigler (1994) studied the sulfide-poor PGE occurrences related tothe Lower and Middle Group chromitites of the Bushveld Complex. They

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concluded that the concentration of PGE is controlled by chromite, but the entireprocess is not fully understood. They proposed a combination of localized sulfursaturation and direct nucleation of platinum-group minerals. The fundamentalcause for all this would be magma-mixing that drives the melt into the chromitefield, hence triggering the crystallization of large quantities of chromite.

Some interesting observations were made by Teigler (1994), who describedthe PGE distribution and the chemistry of chromite from the Lower Group 6chromitite of the Bushveld Complex. He concludes that the prevailingcrystallization conditions must have been relatively stable during chromiteprecipitation as the observed compositional changes in chromite are negligible.This is also reflected by the relatively constant PGE content over the entirethickness of the layer. Thus, the observed PGE pattern in the LG6 chromitite israther different from that of the UG-2, where generally two distinctive peaks havebeen recorded (e.g., McLaren & De Villiers 1982, Hiemstra 1985).

Teigler (op.cit.) made another interesting observation regarding therelationship of chromite and the PGE. He concluded that there appears to be adistinctive correlation between the modal proportion of chromite and total PGEcontent of the rock as massive chromitite contains almost three times as much PGEas a rock with roughly even amounts of chromite and orthopyroxene. This rock inturn contains roughly three times as much PGE as an orthopyroxenite withaccessory chromite. Hence, he made a fundamental suggestion that theconcentration of PGE in the LG6 chromitite was controlled essentially byprecipitation of chromite.

5.1.3 The hortonolitic dunite pipes

The hortonolitic dunite pipes of the Bushveld Complex present yet anotherinteresting environment with specific relevance to the theme of this investigation.In these peculiar rocks, the PGE are associated with chromite or olivine as well aswith base metal sulfides. Platinum concentrations of up to 2000 ppm wereencountered in the Onverwacht pipe (see Fig. 29 for location) in a sample near thecontact with a chromite xenolith (Wagner 1929, Stumpfl & Rucklidge 1982).

However, not all of these pipes are rich in PGE. Wagner (1925) was able toestablish that only the dunites that contain hydrous silicates, phlogopite andhornblende, also contain platinum. Consequently, he attributed the high PGEconcentrations of the dunites to high fluorine content of the hydrous silicates of therock and concluded that ”platinum did not separate from a dry melt”. Initial

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petrographic studies by Wagner (op.cit.) indicated that PGM occurred asinclusions in olivine and also in chromite even though chromite was clearly thefirst mineral to crystallize. This petrographic evidence led him to conclude thatcrystallization of PGM began and ceased before crystallization of iron-rich olivineand the crystallization of chromite must have continued at least for some timeafter the PGM had ceased to form. More recent and comprehensive studies bymodern electron optical methods, however, have revealed that at least part of thePGE mineralization postdates the formation of dunites as PGM have been found inmicrofractures and intergranular spaces along with base metal sulfides andgraphite (Stumpfl & Rucklidge 1982). Rudashevsky et al. (1992) confirmed theconclusions of their predecessors by establishing that different PGM phases weresyngenetic with different silicates and other main or accessory minerals.

Even though these hortonolitic dunite pipes have long since been mined out,they have left us with a very valuable scientific legacy. The careful petrographicinvestigations referred to above have provided descriptions which prove that somePGM indeed occur as inclusions in olivine and chromite and hence must havecrystallized directly from a silicate melt at a very high temperature.

5.2 Other examples of sulfide-poor PGE occurrences

5.2.1 Chromitites of the Koitelainen Intrusion, Finland

The Koitelainen Intrusion is one of the 2.44 Ga mafic layered intrusions innorthern Finland (Fig. 26). It spans an area of about 400 km2 with a maximumthickness of ca. 2.3 km (Mutanen, 1981, 1997). Virtually all “standard” types oflayering are present at Koitelainen with rock types ranging from massivechromitites and peridotites all the way to anorthosites accompanied by agranophyre on top. A detailed description of the general geology of the intrusioncan be found from Mutanen (1989, 1997).

There are several anomalous stratiform occurrences of PGE in the KoitelainenIntrusion. Mutanen (1992) reports that the PGE are generally associated withoxide minerals such as chromite, magnetite and ilmenite. A vast majority of thetotal known PGE+Au reserves of the Koitelainen Intrusion is concentrated into theupper part of the layered series, into magnetite gabbro. The massive anddisseminated sulfides in the lower part of the intrusion are PGE+Au poor and,according to Mutanen (op.cit.), there actually seems to be a peculiar negativecorrelation between the occurrences of PGE and base metal sulfides.

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5.2.2 Chromitites of the Stillwater Complex, Montana, U.S.A.

The 2.7 Ga old Stillwater Complex in Montana, U.S.A. (Fig. 31) is a major sourceof PGE in North America. Mining has taken place for nearly 20 years and about5% of the world’s PGE are produced from the J-M Reef. The J-M Reef is similarto the Merensky Reef of the Bushveld Complex in the sense that the PGE areessentially associated with base metal sulfides (Todd et al. 1982).

Good overviews of the general geologic features of the Stillwater Complex aregiven by Jackson (1961) and Todd et al. (1982). The ultramafic zone of thecomplex contains several chromite seams which have been given letterdesignations A-K from stratigraphically lowest to highest, respectively (Jackson etal. 1954, Jackson, 1968). Many of these seams have been reported to containplatinum-group minerals and Talkington & Lipin (1986) reported total PGEconcentrations of over 15 ppm from the A seam. By far the most common PGM inthese chromite seams is laurite, which commonly occurs as small (< 20 µm)inclusions inside chromite. The Pt- and Pd-bearing minerals that have beenreported from these seams (e.g., Talkington & Lipin 1986, Page 1971) occurgenerally in interstitial silicates.

The observations of Talkington & Lipin (1986) reveal that the chromite seamsusually occur near the bottom of a cyclic unit. Hence, they propose that theprecipitation of chromite and platinum-group elements is somehow associatedwith the beginning of a new cycle, or more specifically an influx of new magmainto the chamber. However, they also point out that Pt-, Pd- and Rh-minerals donot precipitate coevally with Ru-, Os- and Ir-minerals since they do not occur asinclusions in chromite but only interstitially. According to the whole rock PGEanalyses, the order of abundance of the individual PGE is Pd > Pt > Rh > Ru > Ir(Os was not determined). Because Pd, Pt and Rh are all more abundant than Ru,the lack of Pt-, Pd- and Rh-bearing minerals as inclusions in chromite is regardedas a problem when trying to understand the entire sequence of processes by whichthe mineralization was formed. Hence, Talkington & Lipin (op.cit.) were only ableto speculate that perhaps some sulfide-related or hydrothermal process wasresponsible for the distribution of Pt, Pd and Rh.

74

Fig. 31. Generalized geologic map of the Stillwater Complex, Montana, U.S.A. (modifiedfrom Zientek . 2002). The dotted line represents the J-M Reef.

Kilometres

80 4

Montana

Stillwater Complex

560000 570000 580000 590000

Upper Banded series (Upper anorthosite, Upper gabbro, and Upper mixed zones)Middle Banded series (Middle anorthosite zone)Middle Banded series (Middle mixed, Middle gabbro, and Lower mixed zones)Middle Banded series (Lower anorthosite zone)Lower Banded series (Lower gabbro and Norite zones)Bronzitite zonePeridoitite zone and Basal series

N

5025000

5035000

5030000

5035000

5030000

5025000

Kilometres

80 4

Montana

Stillwater Complex

560000 570000 580000 590000

Upper Banded series (Upper anorthosite, Upper gabbro, and Upper mixed zones)Middle Banded series (Middle anorthosite zone)Middle Banded series (Middle mixed, Middle gabbro, and Lower mixed zones)Middle Banded series (Lower anorthosite zone)Lower Banded series (Lower gabbro and Norite zones)Bronzitite zonePeridoitite zone and Basal series

N

5025000

5035000

5030000

5035000

5030000

5025000

75

6 Discussion

6.1 Importance of the case studies in understanding the formation of sulfide-poor PGE mineralization

The case studies represent examples of several different types of sulfide-poorplatinum-group element occurrences. The PGE mineralization in the SouthKawishiwi Intrusion of the Duluth Complex is unique in many ways. First, itsdistribution seems to be very erratic and discontinuous as the occurrence of thePGE seems to be confined to the oxidic xenoliths assimilated from the underlyingBiwabik Iron Formation. Second, the rocks of the SKI and the Duluth Complex ingeneral are very fresh and unaltered and hence it is easier to construct andunderstand the geologic history of the intrusion, compared to the HanumalapurComplex, for example, which has undergone a very thorough metamorphism.Third, the mineral assemblage of the PGE-mineralized rock is somewhatuncommon as there is abundant pleonaste in the xenoliths. This, however, is anatural consequence of the recrystallization of blocks of iron formation that wereassimilated into a tholeiitic magma. Finally, the PGE mineralogy is somewhatdifferent from many well-known PGE deposits as the proportion of various alloys(such as isoferroplatinum and tulameenite to name a few) is rather large. ThePenikat Intrusion was originally included in the literature examples. However, itwas added to the case studies because its silicate-type mineralization resemblesthat of the Hanumalapur Complex and a few samples were readily available forstudy. This made a ”hands-on” investigation possible.

From an economic point of view, the erratic and discontinuous distribution ofthe PGE mineralization is a negative thing and the mineralization being limited tothe oxidic xenoliths effectively destroys any hope of economic exploitation ofsuch a deposit with current refining techniques. However, scientifically, this PGEmineralization is extremely interesting and is proving to be the key to manypreviously unanswered questions regarding the formation of PGE occurrences.

Normally, the pervasive metamorphism of the Hanumalapur Complex wouldbe regarded as a significant drawback when trying to understand the initial orprimary formation of sulfide-poor PGE deposits. However, when combined with astudy of the completely unmetamorphosed rocks of the Duluth Complex andcoupled with the examples from the literature, the high degree of metamorphicalteration becomes an asset. The graphs illustrating the distribution of the variousPGE in the different PGE occurrences of the complex (Figs. 24 A-F and 32)

77

present evidence that IPGE (Ir, Os and Ru) have a greater affinity for chromititesthan the PPGE (Pt, Pd and Rh). Next, due to chromite’s high resistance tometamorphism, PGE that were primarily trapped inside chromite grains will not beremobilized as long as the host chromite grain is intact. However, during suchthorough metamorphism, the PGE that initially were not trapped inside chromitegrains are likely to be remobilized and redistributed. It is also evident from thefigures that the PPGE are generally more mobile than the IPGE, as demonstratedby Barnes et al. (1985). In other words, Ru-, Ir- and Os-bearing minerals are oftentrapped inside chromite or magnetite.

Fig. 32. Hosts of some PGM in drill hole 1 of the Hanumalapur Complex. Different typesof mineralizations are characterized by different PGM. The pie charts tell that sperrylite,kotulskite and Pd-Sb-As minerals are typical PGM in silicate-hosted mineralization, Ru-Os-Ir minerals are typical in oxide-hosted mineralization whereas merenskyite andmichenerite are the most common PGM in sulfide-hosted PGE mineralization.

Silicate

Oxide

Sulfide



Other

Sperrylite Pd-Sb-As minerals Ru-Os-Ir minerals

Kotulskite Merenskyite Michenerite

78

Table 2 presents the relative abundances of various PGM species in the casestudies along with the UG2 and Merensky Reef of the Bushveld Complex forcomparison. The data for BL-95-1 and the silicate-type SJ Reef of the PenikatIntrusion are rather limited and hence possibly somewhat distorted. However, thesimilarities between the silicate-type SJ and the silicate-type Pd mineralization ofthe Hanumalapur Complex as well as those of the UG2 and the oxide-type PGEmineralization of the Hanumalapur Complex are rather evident. Mineralogically,the Duluth occurrences are distinctively different as they carry large proportions ofPGE plumbates and stannides and, in the case of Du-15, also alloys. Nevertheless,the Merensky Reef is commonly regarded as the stratotype of a base metal sulfidePGE deposit. None of the case studies, however, bears even a slight resemblanceto it, neither when looking at the PGE mineralogy nor when looking at the generalrock types. Hence, the mechanisms by which the case occurrences were formedmust have been fundamentally different from the ore forming processes thatproduced the Merensky Reef.

The data and the interpretations based on the PGE mineralization of theHanumalapur Complex also provide valuable clues to locating other similar PGEmineralization in metamorphosed mafic layered complexes elsewhere in theworld. This is highlighted by the fact that analogous formations in Late Archean orPalaeoproterozoic terrains anywhere in the world are generally extensivelymetamorphosed.

79

Tabl

e 2.

Rel

ativ

e ab

unda

nces

of

vari

ous

plat

inum

-gro

up m

iner

al s

peci

es i

n so

me

PG

E d

epos

its. D

ata

for

Bus

hvel

d is

com

pile

dfr

om K

inlo

ch (

1982

), M

cLar

en &

De

Villi

ers

(198

2), P

enbe

rthy

& M

erkl

e (1

999)

and

Caw

thor

n et

al.

(200

2). -

= a

bsen

t, *

= ra

re, *

* =

com

mon

, ***

= d

omin

ant P

GM

spe

cies

.

Min

eral

spe

cies

Du-

15B

L-95

-1H

anum

alap

urS

ilica

te ty

pe P

tH

anum

alap

urS

ilica

te ty

pe P

dH

anum

alap

urO

xide

type

Han

umal

apur

Sul

fide

type

Pen

ikat

SJ

Sili

cate

type

Bus

hwel

dU

G2

Bus

hwel

dM

eren

sky

Ree

f

Pt-F

e al

loys

***

--

--

--

***

Pd

allo

ys**

*-

--

--

--

*

Pd/

Pt s

ulfid

es*

--

--

--

***

***

Sper

rylit

e*

****

***

***

-*

***

Pd/

Pt-T

e±B

i min

eral

s**

***

****

**

***

**

**

Pd-

Sb±

As

min

eral

s*

-*

***

-**

***

*-

Ru-

Os-

Ir m

iner

als

*-

--

***

--

***

*

Rh

min

eral

s*

-*

***

-**

**-

PG

E-P

b/S

n/H

g m

iner

als

***

***

--

--

--

-

Au-

Ag

allo

ys-

-*

*-

**-

-*

80

6.2 The role of chromite in the formation of PGE reefs

Wagner (1925) first proposed that the crystallization of vast amounts of chromitemust have in some way contributed to the high concentrations of PGE found fromthe hortonolitic pipes of the Bushveld Complex. Ever since the discovery of thehortonolitic rocks, the role of chromite acting as a possible concentrator of PGEhas been a subject of constant debate. The actual mechanism, however, has yet tounveil itself.

Naldrett and Cabri (1976) proposed that the PGE would incorporate in thespinel structure. This theory was dismissed in a later article by Naldrett & vonGruenewaldt (1989) after Stockman and Hlava (1984) had pointed out thatmicroscopic studies would suggest that chromite actually nucleates around smallparticles of laurite or metallic alloy. Hence, Naldrett & von Gruenewaldt (1989)concluded that chromite does not concentrate Pt and Pd effectively.

Cawthorn (1999) discussed potential geochemical models regarding theformation of PGE reefs and pointed out that the current popular and widelyaccepted theories fail to explain the association of PGE with chromitites. Teigler(1999) further suggested that chromite precipitation is the governing control on theconcentration of PGE in the LG6 chromitite of the Bushveld Complex, yet he didnot provide a proper explanation why that is so. Scoon & Teigler (1994)emphasized the importance of localized sulfur saturation and concluded that thesole nucleation of chromite might change the solubility of sulfur at the locus of theformation of the massive chromitite, thus triggering the crystallization ofmagmatic platinum-group minerals.

Based on the case studies and the examples from literature described earlier inthis thesis, an attempt is being made to provide an explanation to the chromitecontrol of PGE precipitation in mafic layered intrusions. This approach, however,is somewhat different from the more traditional sulfur-related approaches as theemphasis is placed on redox conditions on a local scale in the magma.

Figures 33A and B illustrate the process by which chromitite-hosted PGE-reefs are envisaged to form. The core idea of this hypothesis is that when a layer ofchromitite is forming, the microenvironment, or more specifically the prevailingredox conditions in the immediate vicinity of the chromitite will be such thatplatinum-group elements are likely to form discrete mineral phases. As thecrystallization of chromite continues, the chromitite layer will become thicker andthe PGM will be trapped inside the chromitite. This would obviously mean that thePGM inside the chromitite are actually magmatic cumulus minerals and hencesulfide liquation would not be required.

81

Fig. 33. A schematic illustration of the process by which chromitite-hosted PGEmineralizations are envisaged to form. (A) Early stage of chromite crystallization. (B)Situation after prolonged chromite crystallization and convection.

Chromite crystallizing

PGM forming in the narrowzone between the melt and thecrystalline chromitite layer

( A )

As the chromitite layerthickens the PGM crystalsget trapped inside of it

The prevailing redox conditions of the outerpart of the thickening chromitite layer continueto “attract” PGE from the melt

( B )

Double diffusive convection




82

Fig. 34. Various PGM enclosed in Cr-rich magnetite. All samples are from theHanumalapur Complex, Karnataka, India. Images A,B and C are from drill core samplesand images D, E and F are from outcropsamples. The rock type in each image is amagnetite-chlorite rock in which the magnetite is distinctively Cr-rich with up to about10 wt.% Cr2O3.

Evidence for this type of process is found from mineralogy in the form of mineralparageneses and textures as well as modes of occurrence. It seems that Ru-, Os-and Ir-bearing minerals are proportionately more common in chromitite-related

Hollingworthite

Irarsite

Ruarsite

Hollingworthite+

Irarsite

Anduoite

Irarsite

Sperrylite

Hollingworthite

HollingworthiteSperrylite Sperrylite

Mertieite-II Mertieite-II

OsarsitePalladoarsenide

Hollingworthite

A B

C D

E F

83

mineralization than in others, although Pd-, Rh- and Pt-bearing minerals occur inchromitite-hosted deposits as well. Another striking feature is that more often thannot, the PGM grains in chromitites are rather euhedral. It is also very common tofind them as inclusions in chromite grains. This not only applies to the Ru-Os-Irgroup of PGM but to the Pd-Rh-Pt group as well, as illustrated in Fig. 34. Anothernoticeable feature is the general scarcity or even absence of base metal sulfides inchromitite-hosted PGE mineralization. However, as there are to date noexperimental data available on this particular issue, the evidence presented abovedo allow room for speculation and hence remain inconclusive.

6.2.1 Thermodynamic Considerations

There are a number of papers that present experimental studies of the behavior ofplatinum-group elements in various magmatic processes. Most of them are moreor less concentrating on the effect of the sulfur fugacity, which is probably aconsequence of the popular idea that PGE occur together with base metal sulfides.However, as showed by Amossé et al. (1990), the solubility of PGE in silicate meltis affected by both ƒO2 and ƒS2.

The process begins essentially with crystallization of an oxide phase, which inmost cases is chrome spinel. The crystallization of spinel is greatly governed bytemperature and ƒO2, of which the latter also influences the oxidation state of iron.The concentration of chromium in the magma determines which kind of spinel isformed, as Cr3+ is very insoluble in silicate melt and readily enters the spinelstructure as early in the crystallization process as possible (Hill& Roeder 1974).

The experimental results of Amossé et al. (1990) showed that PGE candissolve in high-temperature silicate melts even in the absence of any fluid phase.They also concluded that an increase in the oxygen fugacity accounts for the earlyprecipitation of ruthenium, iridium and osmium, and according to theirexperiments platinum-rich alloys can coexist with chromite. Their experimentsalso showed that platinum (and apparently also palladium and rhodium) have ahigh solubility at sulfur fugacities that are buffered by base-metal sulfides. So thesolubility of PGE in silicate melt is greatly affected by both ƒO2 and ƒS2. Ingeneral terms, it can be said that higher ƒO2 decreases the solubility and higher ƒS2

increases the solubility of PGE in the melt.Eventually the temperature decreases, the chemical balance of the melt

changes and chromite crystallization ceases which in turn spells a decrease in ƒO2

as well as a relative increase in Fe2+/Fe3+ ratio and silica content in the magma. All

84

of those factors together lead to crystallization of various silicate phases such asolivine, pyroxenes and plagioclase, until much later in the crystallization sequencewhen the Fe2+/Fe3+ ratio of the magma has decreased low enough to allow for anoxide phase to crystallize again. Crystallization of silicate minerals causes anincrease in the ƒS2 and a higher sulfur fugacity allows for higher PGEconcentrations to be soluble in the melt. Should ƒS2 increase enough, animmiscible sulfide melt will form and whatever PGE were left in the silicate meltafter the initial high ƒO2 conditions are likely to enter the sulfide melt at this stage.However, if ƒS2 fails to rise high enough for a sulfide melt to separate, thenaccording to the redox-theory, the governing factors for PGE precipitation shouldprobably be ƒO2 together with temperature.

The experimental work of Andrews and Brenan (2002) showed that if bothlaurite (RuS2) and Ru-Ir-Os alloy are to crystallize from a sulfur-undersaturatedmagma at high ƒS2, a relatively oxidized, low-FeO magma is required. They alsopostulated that sulfide saturation does not preclude a magmatic origin for laurite ifthe sulfide saturation postdated laurite formation and entrapment in chromite.These conclusions agree well with the redox-theory.

Tredoux et al. (1995) proposed that PGE could be present in a naturalchromite-forming system at high temperatures as tiny submicroscopic “metalclusters” consisting of a few hundred atoms of the PGE in the metallic state. Dueto their physico-chemical properties, these clusters would coalescence to formplatinum-group minerals, which would subsequently be entrapped in chromite orother coevally crystallizing minerals. Garuti et al. (1999) applied this theory intheir interpretation of the PGM assemblage of the Ray-Iz Ultramafic Complex inthe Polar Urals, Russia, and concluded that with the presence of a fluid phase,chromite crystallization can take place at a low temperature and under high ƒS2.They also suggested that the final observed PGM compositions are more of areflection of a late stage ƒS2 than the conditions that were prevailing at the initialstage of crystallization.

All these experimental data and other observations are well in accordance withthe redox-theory, which predicts and requires an increase in oxygen fugacity toreduce the solubility of PGE in the melt and form various solid phase PGM. Yet itallows an increase in ƒS2 to take place to alter the initial mineralogy during coolingor even a sulfide saturation to happen and subsequent immiscible sulfide liquid toform.

85

6.2.2 Why all chromitites are not enriched in PGE?

Some chromitites, particularly some podiform chromitites associated withophiolites, contain only trace amounts of PGE. If PGE are enriched intochromitites by the redox scavenging process directly from magma, then why allchromitites do not contain PGE?

The redox scavenging process is very delicate because the area or volumewhere it takes place is small compared to the overall volume of a given magmachamber. Hence one of the requirements for the process to work efficiently isconvection. Convection is necessary to feed the redox area continuously withPGE-undepleted magma. This redox area in question is envisioned to be aninteractive horizon between the chromitite and the magma. With prolongedconvection the PGE can be scavenged from a large volume of magma. There isreally no reason why the redox process should not work with podiformchromitites. It is just that the process cannot be anywhere near as efficient as inlarge layered intrusions due to the lack of prolonged convection.

The Kemi Intrusion in Northern Finland is famous for its unusually thickstratiform chromitite. Yet that particular chromitite is notoriously poor of PGE.How does that fit into the redox model?

The Kemi chromitite is enigmatic in many ways. It is very unusual to form astratiform chromitite layer that thick, over 100 m in places. If the model of Alapietiet al. (1989) explaining the formation of the Kemi Intrusion is completely valid,then according to the redox theory, there should be at least anomalous PGEconcentrations in the chromitite. Unpublished data of Alapieti & Lahtinen (1989)show PGE values for the main chromitite to be ca. 270 ppb. PGE concentrations inthe upper chromitite, ca. 130 m above the main chromitite, vary from 413 to 523ppb and the highest assay values were obtained from a sulfide-bearing silicate-richlayer in the middle of the main chromitite with a PGE concentration of 548 ppb.The main chromitite is distinctively enriched in Ru compared to other PGE,whereas the upper chromitite is clearly Pt and Rh dominant and the sulfide-bearing, silicate-rich layer is Pd dominant. Kojonen et al. (2005) reported PGEconcentrations of higher than 400 ppb from the main chromitite. Their analysesindicate that Ru is the most dominant of the PGE followed by Os and Ir, whereasPt, Pd and Rh concentrations are invariably very low. Somewhat recently, anapparent feeder channel was discovered from the Kemi Intrusion that is composedessentially of chromite. Hence a theory was developed suggesting that the bulk ofthe chromitite was brought to the chamber in suspension, as “crystal mush”, rather

86

than having it crystallized in-situ from the magma in the chamber (O Leinonen,2001 pers. comm., Kemppainen 2002). This model would not contradict the redoxtheory and would essentially explain the relative scarcity of PGE in the Kemichromitite.

6.3 Formation of silicate-hosted PGE mineralization

Presently, geological literature includes only a few reports of essentially silicate-hosted PGE mineralization. These mainly include studies of the Penikat LayeredIntrusion in Northern Finland (e.g., Alapieti & Lahtinen 1986, Alapieti et al. 1990,Halkoaho 1989, 1993) and the recent reports of the Hanumalapur Complex inKarnataka, India (Alapieti et al. 1996, Kaukonen 2000, Kaukonen et al. 2002).

Halkoaho (1993) distinguished four different types of PGE mineralization inthe SJ Reef of the Penikat Intrusion: 1) chromite type, 2) composite chromite basemetal sulfide type, 3) base metal sulfide type and 4) PGE-only type. The formationof the PGE-only type mineralization is probably still not thoroughly understood,but Halkoaho (op.cit.) concluded that evidently some mechanism other thanmagma mixing is needed to explain its formation, and proposed that volatileactivity may explain the origin of the mineralization.

The present knowledge of the silicate-type PGE mineralization is ratherlimited as the known occurrences are few and far between. Hence one must becareful not to rush into seemingly easy conclusions and neglect other possibilities.Even a direct magmatic origin, i.e. cumulus crystallization from a silicate meltshould not be ruled out. Wagner’s (1925) observations of PGM inclusions inolivine in the hortonolitic rocks of the Bushveld Complex may be interpreted asevidence that PGM indeed may be cumulus minerals enclosed in olivine“oikocrysts” regardless of the size and the shape of the olivine grains. If theaverage diameter of PGM is about 5 µm, then an olivine grain with a diameter of0.5 mm would indeed easily qualify as a huge oikocryst compared to the PGM itencloses. The difference is the same when a 0.5 mm olivine grain is enclosed by a5 cm grain of poikilitic plagioclase. In terms of fractional crystallization it isconsidered normal that olivine usually precedes plagioclase in the crystallizationsequence. Sometimes olivine crystals, euhedral or not, may even get trapped inlarge intercumulus plagioclase grains. That is basically a proven naturalphenomenon and common textbook knowledge since the early 20th century(Bowen 1928). Why would it not be possible that some minerals might precedeolivine in a similar fashion? Sometimes chromite does precede olivine, but

87

chromium is an abundant element compared to the PGE and usually chromitecrystals are readily observable at least with the aid of an ore microscope. Theobservation of PGM, however, often requires a scanning electron microscope. Thefact that PGM are usually tiny and hard to observe should not limit us to think thatthey could not have petrological characteristics such as a place of their own in thegeneral fractional crystallization sequence of minerals.

Yet another possibility for the origin of silicate-type PGE mineralization isvolatile transfer (Cox et al. 1979) in a relatively early magmatic stage when theintrusion is still at least partially molten. Such a process would require a magmaticfluid phase to scavenge PGE from a silicate melt and distribute them as PGMsomewhere in the solidifying mass. As such, a fluid phase would be likely to riseupwards in stratigraphy. It would be logical to predict that the PGM would bedistributed either when the fluid encounters a solid layer which it cannot penetratein its present form and composition or a layer where redox conditions change tofavor solidified PGM over dissolved PGE in a fluid phase.

On the other hand, the hydrothermal theory might provide at least a partialexplanation in the sense that it is quite possible that some sort of volatile activityhas taken place and remobilized and redistributed the PGE to their currentlocations. However, whether those fluids were magmatic or metamorphic, is stilldebatable. If they were magmatic, similar mineralization, with at least anomalousconcentrations of PGE, should probably be found in relatively unaltered intrusionselsewhere. To date there have been no reports of such occurrences. If the fluidswere of a metamorphic origin, similar mineralization should only occur inmetamorphosed intrusions. The PGE-only mineralization of the SJ Reef of themetamorphosed Penikat Intrusion bears such similarities to the silicate-hostedPGE mineralization of the Hanumalapur Complex that an argument is being madein favor of metamorphic origin of the fluids that remobilized and redistributed thePGE.

Of course there is still the orthomagmatic theory of Campbell et al. (1983), i.e.magma mixing that needs to be considered here too. The formation of a silicate-hosted PGE mineralization, such as that in the Hanumalapur Complex, cannot bereadily explained by magma-mixing alone, because the thorough metamorphismhas completely obliterated all primary textures and thus destroyed all evidence foror against it. It is plausible, however, that magma mixing can be an initial phase inthe PGE concentration, stage 1, and that sulfides and/or PGE will be remobilizedduring metamorphism, i.e. stage 2, and redistributed elsewhere.

88

6.4 Case studies revisited

6.4.1 Chromite theory applied to the PGE mineralization in the Duluth Complex

Mogessie et al. (1991) proposed that late magmatic Cl-bearing fluids played a keyrole in transporting the PGE. Kaukonen (1995) concluded that as the PGE-richfluids were migrating upwards they bumped into a layer that was far morecrystalline than the surrounding material and had such redox conditions that thePGE were removed from the fluid phase and reacted with the little sulfur (±Te, Se,Bi) that was left in the melt in the zone, whereas most of the fluid kept onmigrating through the zone.

Although the olivine and plagioclase compositions are very primitive in thePGE-rich rocks of this mineralization, a more important factor contributing to theformation of the PGE mineralization could be the presence of assimilated andcompletely recrystallized material from the underlying Biwabik iron formation.Because the mineralization occurs so low in the stratigraphy, it is questionablewhether there were enough PGE in the material below, which could have beenaccumulated into such high concentrations if collected by upward-migratingfluids. Also, the absence of a continuous reef speaks against the fluid theory. Onthe other hand, orthomagmatic theory fails to explain this enigmatic PGEmineralization due to both the very small amount of base metal sulfides and theabsence of a continuous PGE reef.

Additional evidence against the tradional orthomagmatic model can begathered by looking at the PGE mineralogy (Fig. 16). Even though about half ofthe PGM are somehow associated with base metal sulfides, PGE sulfides arecompletely absent from the Du-15 samples and in fact various alloys are the mostcommon types of PGM. Even though there are experimental data that indicate thattetraferroplatinum (PtFe) can coexist with pyrrhotite at 850°C (Makovicky et al,1990), it is doubtful that such an extraordinary PGM assemblage as that of the Du-15 had simply just crystallized from a sulfide melt in a “traditional” manner. It istoo rare and unique in characteristics for that to have happened, and hence someother previously unrecognized process must have been involved.

Because the characteristics of the PGE mineralization in question are sounique, it is evident that the current prevailing theories regarding the formation ofPGE deposits are not completely applicable to this deposit. In order to explain theformation of this mineralization, a completely new approach was chosen.

89

The theory now proposed involves interaction between the oxide-rich blocksscavenged from the footwall and the surrounding magma (Fig. 35). Xenoliths ofthe Biwabik iron formation are rather common in the SKI near the footwallcontact. Because they were mainly composed of magnetite (at the time ofassimilation) and hence denser than the magma, they either sunk to the bottom ofthe chamber or were already scavenged from the bottom of the SKI. Apparentlysome xenoliths contained more quartz, which would have allowed them to float onthe magma at least with the aid of convection since some of these blocks do occurat a higher stratigraphic level as well. Because thermal diffusion is far more rapidthan chemical diffusion, the silicate material in the block would have melted atleast partially because of the heat of the surrounding magma. However, that heatwould not have been sufficient to melt the magnetite in the block. In time the blockwould lose some silica and gain alumina from the Al-rich magma in which it was“swimming”. This alumina reacted with magnetite to form pleonaste or hercyniteand the sintering texture now present in the rock.

So why are the PGE values high in or near these oxide-rich blocks? Inaccordance with the chromite theory, it is now proposed that the microenvironmentin the magma in the immediate vicinity of the xenoliths of banded iron formationhad such favorable redox conditions that platinum-group minerals wouldcrystallize. It means that PGM were effectively crystallized from whatever melt,silicate or sulfide, happened to be around a xenolith at any given moment as itwas “swimming” in the magma. The seeming sulfide association is merely aresult of trapped sulfide liquid filling the pore spaces.

It is actually the Duluth Complex and in particular the Du-15 drill hole wherethe above-mentioned “chromitite theory” or perhaps rather “redox theory” firstoriginated. It is essentially an evolution of the “floating block” idea presented here.Moreover, the oxide mineral probably does not have to be chromite. Magnetite, forexample, would have the same effect, i.e. it would make the local redox conditionsfavorable for PGM to crystallize. The Du-15 samples obviously fit the theoryrather well except for maybe the lack of Ru-Os-Ir minerals which are often closelyassociated with oxide-hosted PGE mineralization. One possibility explaining theirabsence is that they were already fractionated prior to the emplacement of theSouth Kawishiwi Intrusion. Deep-seated differentiation is an apparent andrecognized phenomenon that has played a major role in the evolution of the Mid-Continent Rift magmas (e.g., Miller & Weiblen 1990, Miller & Ripley 1996).

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Fig. 35. Schematic illustration of the formation of the PGE mineralization intersected bydrill hole Du-15 in the South Kawishiwi Intrusion of the Duluth Complex, Minnesota,USA. A) Early stage - xenoliths of banded iron formation are scavenged from thefootwall. B) Favorable redox conditions around the immediate vicinity of the BIFxenoliths cause PGE to be scavenged from the magma into the recrystallization frontsof the xenoliths.

The BL-95-1 mineralization is perhaps a better example of an oxide-hosted PGEmineralization than the Du-15 in the general sense that it bears more reef-likefeatures and displays a PGM assemblage and association that could predictablyhave been formed primarily according to the redox theory explained above. Here

Convection

Magma



Blocks of BIF“swimming”in the magma

( A )

Convection

Magma



PGE-rich rimforms around the BIF xenoliths

( B )

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too the main reason for the lack of Ru-Os-Ir minerals would be the deep-seatedfractionation that caused their depletion.

Shafer et al. (2003) studied Re-Os isotopes of the South Kawishiwi Intrusion.All their samples had high γOs values (165-456) indicating crustal contamination.This supports the idea that assimilation of the Bibawik Iron Formation is the mainsource of the oxide-rich rocks in the area.

The redox theory is an attempt at explaining the close relationship betweenPGE and chromitite or other oxide rocks that are commonly found in mafic layeredintrusions. According to the theory, PGM will crystallize directly from any type ofmelt as long as the local microenvironment is favorable. Such favorablemicroenvironment or more precisely, redox potential, is provided bycrystallization of oxide minerals such as chromite or magnetite.

As the redox theory by definition assumes PGM crystallization directly from asilicate melt, the concentration of PGE within the magma is of utmost importancewhen evaluating potential exploration targets. In general the more fractionated themagma is, the less PGE it will contain. Thus, in the case of the Duluth Complex,the fact that deep-seated fractionation has played a major role in its magmaticevolution, would make it a rather unattractive target for PGE exploration, becausea large portion of the PGE were already fractionated or “redoxed” out of themagma before its emplacement.

6.4.2 Formation of the PGE mineralization of the Hanumalapur Complex

No single theory can adequately explain the formation of the different types ofPGE occurrences that occur in the Hanumalapur Complex. There may be onetheory that can explain both the silicate types, but a different theory is needed forthe sulfide type and yet another to explain the oxide type. Given the extensive andpenetrative metamorphism that the rocks have undergone, the reconstruction oftheir mineralization history is by no means a simple task. The following, however,is an attempt at organizing the events in chronological order starting from theoldest and working through the various processes.

It is envisioned that the first type of PGE mineralization that formed in theHanumalapur Complex was the oxide-hosted type. It was formed simultaneouslyas chromite was crystallizing through the redox process that was described earlier.The PGM assemblage of the oxide-hosted type at Hanumalapur is ratherconventional with a large proportion of Ru-Os-Ir minerals and some Rh-Pd-Pt

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minerals present as well. Evidently, the bulk PGE concentration is not very highbecause sulfide saturation took place and consequently crystallization of oxideminerals (or in this case chromite) ceazed and the remaining PGE (much morethan had fractionated into the chromitite as descrete PGM) entered the sulfidemelt.

There is probably little reason not to explain the sulfide-hosted type to haveformed essentially according to the popular orthomagmatic theory. The base-metalsulfides and the more conventional PGM are present with the possible exception ofPt-Pd sulfides which seem to be rather rare at Hanumalapur. The only minordiscrepancy is that the bulk PGE concentrations of the sulfide samples are notquite as high as one might expect.

The oxide- and sulfide-hosted PGE mineralization are probably of a magmaticorigin. This can be sufficiently established even after the metamorphism that hasaltered the mineralogy of the rocks and wiped out most of the primary textures. Isit possible that such a thorough process left the primary mineralization untouched?It probably is not. Hence it is postulated that the silicate-hosted PGEmineralization formed due to redistribution of PGE during metamorphism, andmoreover, the bulk of the PGE concentration was probably leached from thesulfide-hosted mineralization.

6.5 Implications for exploration

The task of an exploration geologist is to discover rock formations that containhigh concentrations of some valuable metal, preferably in large enough quantitiesto make mining economical. Sometimes the geologist may be told to look forcertain kinds of deposits in which the metals occur in such form that they arerelatively easy to extract with current technology. However, the extraction of themetals is more of an engineering problem and the geologist should not ignore arich mineralization only because it is not of the type he was told to look for.Recovering PGE from tiny PGM particles scattered among silicates with barelytraces of base metal sulfides in the rock may indeed be challenging. Preliminaryresults from refining experiments of the Hanumalapur ore (P. Mörsky, pers. comm.2007) are very encouraging, however.

For several decades exploration for PGE has focused on certain types of rocksthat contain significant quantities of base metal sulfides. Every now and thenpromisingly high concentrations of PGE have indeed been discovered, and

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because they happened to occur in rocks that contained a lot of base metal sulfides,the theory that PGE go hand in hand with base metal sulfides seemed to work.

The case studies along with the examples from the literature presented in thiswork demonstrate that the occurrence of PGE is not necessarily tied to theoccurrence of base metal sulfides. In some cases the fact that they happen to occurtogether is probably more a coincidence than an established genetic relationship.Therefore the scope of PGE exploration, when targeted to a given layered intrusionor a similar rock formation, should be expanded from the mere sulfide-bearingrocks to include other areas of the intrusion as well, particularly any chromitites ormagnetite rocks, and at least in metamorphic terrains also some of the silicaterocks. A possibly important and easily attainable mineralogical piece ofinformation that could indicate the presence of a PGE mineralization could be thecomposition of chlorite. If chlorite is a major rock-forming silicate mineral in arock and it is Al- and Mg-rich like the chlorites in the case studies presented in thisthesis, then it might be a good idea to do some whole-rock analyses for platinum-group elements.

Another thing to consider is the composition, or more specifically, the degreeof fractionation of the magma where a given intrusion was crystallized. Assumingthe redox theory of PGE concentration process is correct, then the morefractionated the magma is, the less PGE it will contain. When applied to thegeological time scale, it can be generally interpreted to mean that older intrusionsare more likely to contain high concentrations of PGE than younger formationsdue to fractionation of elements on a planetary scale. This leads one to focus moreon rocks dating from the Archean to Paleoproterozoic than on younger formationsfor future exploration targets.

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7 Summary and ConclusionsIn light of the case studies presented here and the examples from literature, it isevident that at least anomalous or even ore-grade sulfide-poor platinum-groupelement mineralization occur commonly in many mafic layered intrusions. Inthese occurrences the platinum-group minerals are typically associated with eitheroxide or silicate minerals whereas base metal sulfides are scarse or completelyabsent.

Oxide-hosted and silicate-hosted PGE mineralization have characteristic PGMassemblages that are different from each other and different from typical sulfide-hosted PGM assemblages. Common PGM in oxide-hosted PGE mineralizationinclude a large proportion of minerals containing Ru, Os and/or Ir, although theymay still contain significant concentrations of Pt, Pd and/or Rh minerals. ThePGM occur generally as inclusions in oxide or silicate minerals or along grainboundaries between oxides and/or silicates but not with sulfides.

The most important PGM in silicate-hosted PGE occurrences include a varietyof Pd-Pt tellurides and antimonides. As both base metal sulfides and oxideminerals are scarce in the silicate-type mineralization, PGM occur typically asinclusions in silicates or in cracks or bordering two or more silicate grains.

Oxide-hosted PGE mineralization is envisioned to form through a process thatbegins with crystallization of some oxide mineral, usually chromite. According tothis theory, the microenvironment, namely the redox conditions, in the immediatevicinity of the oxide grains is favorable for platinum-group minerals to form. Asthe crystallization of the oxide minerals continues they will eventually form a layerthat will continue to thicken as long as the crystallization of the oxide mineral inquestion continues. With the thickening of the oxide layer, the crystallization frontof the PGM, or the redox front, will also move, as the favorable microenvironmentis envisioned to occur only at or near the oxide-liquid border. In order to get anykind of significant concentration of PGE into the oxide horizon, prolongedconvection in the magma chamber is required to continuously feed the redox frontwith undepleted magma. Evidently experimental data are still needed to prove thistheory either right or wrong.

The formation of stratiform silicate-hosted PGE mineralization may, at least tosome extent, be explained by the commonly known hydrothermal theory withsome minor modifications. It is evident that the PGE were remobilized andredistributed at some stage in the history of a given intrusion. The argument madehere suggests that the actual cause and event of the remobilization and subsequent

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redistribution is related to regional metamorphism rather than the initialmagmatism. Hence, the fluids involved were metamorphic rather than magmatichydrothermal. Metamorphism also breaks up the initial mineral structures byopening voids for fluids and PGE to migrate into. One of the case studiespresented here, the Hanumalapur Complex of Karnataka, India, and to some extentthe Penikat Intrusion of Northern Finland, represent rocks that have undergone arather penetrative greenschist to amphibolite facies metamorphism. Currently,there are no known examples of silicate-hosted PGE mineralization in anunmetamorphosed terrain.

The aim of this work is not to disparage the importance of sulfide-hosted PGEmineralization, but rather to point out that there are other types of PGEoccurrences of potential economic importance as well. An attempt was also madeto explain the formation of these other types of deposits. The understanding of thegeology of the various types of PGE mineralization should be the guideline fordirecting PGE exploration worldwide. It is hoped that the recognition of new typesof PGE mineralization will lead to new discoveries in the future.

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105

Appendices

107

Appendix I Selected plagioclase analyses from Du-15

1 2 3 4 5 6 7 8

SiO2Al2O3CaOFeONa2OK2OTotal

53.1130.1212.67

0.343.920.37

100.53

52.3930.1212.76

0.273.980.28

99.80

52.8130.2312.910.153.980.30

100.38

53.0129.5712.27

0.174.380.36

99.76

52.9929.9812.29

0.204.150.2999.9

52.7529.6312.67

0.234.090.27

99.64

53.4729.6411.98

0.134.470.33

100.02

52.1129.6712.58

0.224.020.34

98.94

SiAlCaFeNaK

2.3971.5990.6130.0140.3420.022

2.3841.6130.6200.0110.3500.016

2.3881.6080.6250.0050.3480.016

2.4081.5830.5980.0050.3880.022

2.4021.6010.5960.0080.3640.016

2.4001.5910.6180.008

0.360.016

2.4191.5810.5820.0050.3920.016

2.3901.6040.6170.0080.3580.022

1 = DU-15-392 = DU-15-703 = DU-15-99.54 = DU-15-6805 = DU-15-7506 = DU-15-10747 = DU-15-1119.58 = DU-15-1401

An = 63An = 63An = 63An = 59An = 61An = 62An = 59An = 62

9 10 11 12 13 14 15 16


50.1831.4114.29

0.223.310.20

99.61

51.8530.6313.12

0.353.710.32

99.98

53.1130.0112.670.293.980.33

100.39

52.0230.0612.91

0.113.860.29

99.25

49.5832.0415.13

0.282.860.17

100.06

50.9430.9413.75

0.253.590.22

99.69

53.6329.8512.65

0.224.220.34

100.91

51.8530.7813.52

0.174.210.05

100.58

SiAlCaFeNaK

2.2981.6950.7020.0080.2920.012

2.3581.6390.6390.0140.3280.016

2.4001.5960.6140.0110.3480.016

2.3781.62

0.6310.005

0.340.016

2.2651.7240.7410.0110.2520.010

2.3291.6640.6730.0080.3180.010

2.4081.581

0.610.0080.3680.022

2.3451.6410.6550.0050.3700.006

9 = DU-15-150110 = DU-15-155011 = DU-15-1617.512 = DU-15-176013 = DU-15-200014 = DU-15-205015 = DU-15-210016 = DU-15-2402


109

Appendix I continued. Selected olivine analyses from Du-15

1 2 3 4 5 6 7 8

SiO2TiO2Al2O3V2O3Cr2O3MgOCaOMnOFeONiOZnONa2OK2OTotal

36.56n.d.n.d.0.04n.d.

26.000.010.42

37.700.26n.d.0.02n.d.

101.01

36.00n.d.n.d.n.d.n.d.

23.09n.d.0.44

41.410.140.110.04n.d.

101.23

35.440.010.02n.d.

0.0222.89

n.d.0.48

41.36n.d.

0.07n.d.n.d.

100.29

36.40n.d.n.d.n.d.0.07

26.25n.d.0.46

38.65n.d.0.050.04n.d.

101.92

35.270.01n.d.n.d.n.d.

22.320.010.42

42.180.07n.d.

0.05n.d.

100.33

36.290.01n.d.

0.010.05

25.920.050.54

36.510.02n.d.n.d.n.d.

99.40

36.28n.d.n.d.n.d.n.d.

26.670.080.33

36.810.260.130.04n.d.

100.60

36.740.01n.d.n.d.n.d.

27.740.030.31

36.210.160.04n.d.n.d.

101.24

SiTiAlVCrMgCaMnFeNiZnNaK

1.016----

1.077-

0.0100.8770.005

---

1.017----

0.972-

0.0100.9780.0030.0020.004

-

1.012----

0.974-

0.0120.988

-0.002

--

1.006----

1.081-

0.0100.893

-0.0020.004

-

1.011----

0.954-

0.0101.0110.002

-0.004

-

1.021----

1.0860.0020.0140.858

----

1.009----

1.1060.0020.0080.8550.0050.0030.004

-

1.010----

1.1370.0020.0070.8330.003

---

1 = DU-15-392 = DU-15-703 = DU-15-99.54 = DU-15-6805 = DU-15-7506 = DU-15-1119.57 = DU-15-20008 = DU-15-2050

Fo = 55Fo = 50Fo = 50Fo = 55Fo = 49Fo = 56Fo = 56Fo = 58

110

Appendix I continued. Selected apatite analyses from Du-15

1 2 3 4 5 6 7 8

P2O5Ce2O3La2O3MgOCaOMnOFeOSrOBaOFClH2OSumO=F.ClTotal

42.320.290.130.01

53.76n.d.0.1

0.210.053.551.45n.d.

101.871.82

100.05

42.550.30n.d.0.06

53.500.040.19n.d.n.d.2.621.210.29

100.761.38

99.38

42.530.280.240.01

53.560.050.110.430.012.811.640.10

101.771.56

100.21

42.270.730.170.03

52.010.080.21n.d.n.d.

1.961.650.46

99.571.20

98.37

41.980.310.110.02

53.180.110.130.230.022.101.620.41

100.221.25

98.97

42.640.28n.d.

0.0253.77

0.060.080.190.062.750.910.31

101.071.36

99.71

43.180.310.060.02

53.31n.d.

0.150.23n.d.

3.091.500.01

101.861.64

100.22

44.300.370.070.04

54.250.040.20n.d.n.d.

2.770.840.36

103.241.35

101.89

PCeLaMgCaMnFeSrBaFClOH

5.7780.019

--

9.287-

0.0100.019

-1.8110.395

-

5.8870.019

-0.0109.3600.0100.029

--

1.3510.3340.315

5.8490.0190.019

-9.3100.0100.0100.039

-1.4430.4510.107

5.9460.0400.0200.0109.2490.0100.030

--

1.0300.4650.506

5.8840.020

--

9.4220.0200.0200.020

-1.0960.4520.452

5.8800.019

--

9.3980.0100.0100.020

-1.4160.2510.334

5.8920.019

--

9.206-

0.0190.019

-1.5780.4100.013

5.9560.019

-0.0109.2300.0100.029

--

1.3900.2250.386

1 = DU-15-702 = DU-15-99.53 = DU-15-6804 = DU-15-7505 = DU-15-8506 = DU-15-10747 = DU-15-1119.58 = DU-15-2225

111

Appendix II Selected orthopyroxene analyses from Du-15

1 2 3 4 5 6 7 8


53.640.350.82n.d.n.d.

20.671.210.47

22.760.170.070.02n.d.

100.18

53.580.310.93n.d.0.03

19.851.020.35

24.24n.d.n.d.n.d.n.d.

100.31

53.410.260.84n.d.0.07

19.831.520.50

24.21n.d.n.d.0.02n.d.

100.66

53.900.310.97n.d.n.d.

21.641.480.34

22.110.10n.d.n.d.n.d.

100.85

52.810.130.70n.d.n.d.

18.691.010.51

25.48n.d.n.d.n.d.n.d.

99.33

53.340.310.97n.d.

0.1220.43

1.440.42

24.52n.d.n.d.n.d.n.d.

101.55

52.710.300.89n.d.

0.0418.97

1.010.53

25.11n.d.n.d.n.d.n.d.

99.56

54.490.131.00n.d.n.d.

22.281.010.36

22.180.050.010.01n.d.

101.52


2.0000.0090.036

--

1.1490.0490.0160.7100.0040.002

--

2.0030.0090.041

--

1.1070.0400.0110.757

----

1.9970.0070.036

--

1.1050.0610.0160.757

----

1.9870.0090.044

--

1.1890.0580.0110.6820.002

---

2.0070.0050.032

--

1.0600.0410.0160.811

----

1.9780.0090.040

-0.0051.1290.0580.0130.759

----

1.9980.0090.041

--

1.0730.0410.0160.795

----

1.9910.0050.044

--

1.2140.0400.0110.6780.002

---

1 = DU-15-392 = DU-15-703 = DU-15-99.54 = DU-15-6805 = DU-15-7506 = DU-15-8507 = DU-15-10748 = DU-15-1850

En, Wo, Fs = 60.2En, Wo, Fs = 58.1En, Wo, Fs = 57.5En, Wo, Fs = 61.6En, Wo, Fs = 55.4En, Wo, Fs = 58En, Wo, Fs = 56.2En, Wo, Fs = 62.8

2.572.13.173.012.142.982.152.07

37.239.839.435.442.43941.635.1

113

Appendix II continued. Selected clinopyroxene analysesfrom Du-15

1 2 3 4 5 6 7 8


52.430.762.350.11n.d.

13.4420.54

0.319.640.07n.d.0.29n.d.

99.94

52.550.701.960.030.03

12.7420.24

0.2412.77

0.010.030.27n.d.

101.57

52.320.591.90n.d.n.d.

13.1820.93

0.2310.96

n.d.n.d.

0.27n.d.

100.38

52.640.641.920.130.02

13.6320.88

0.319.060.04n.d.

0.26n.d.

99.53

52.270.722.120.090.02

12.2520.79

0.2612.21

0.090.020.30n.d.

101.14

53.550.361.51n.d.n.d.

13.8221.67

0.259.210.01n.d.

0.23n.d.

100.61

52.610.581.87n.d.

0.0312.8620.84

0.2011.10

0.07n.d.

0.300.01

100.47

52.260.752.460.020.09

13.6621.24

0.279.840.02n.d.

0.33n.d.

100.94


1.9550.0200.1030.005

-0.7460.8200.0090.3000.002

-0.022

-

1.9530.02

0.085--

0.7060.8070.0070.398

--

0.018-

1.9560.0160.085

--

0.7340.8380.0070.341

--

0.018-

1.9670.0180.0850.005

-0.7590.8350.0090.283

--

0.018-

1.9490.0200.0940.005

-0.6810.8310.0090.3810.002

-0.022

-

1.9790.0110.067

--

0.7620.8570.0090.284

--

0.018-

1.9660.0160.081

--

0.7160.8350.0070.3460.002

-0.022

-

1.9360.0200.107

-0.0050.7540.8430.0090.305

--

0.022-

1 = DU-15-392 = DU-15-703 = DU-15-99.54 = DU-15-6805 = DU-15-7506 = DU-15-8507 = DU-15-10748 = DU-15-1119.5

En, Wo, Fs = 40En, Wo, Fs = 36.9En, Wo, Fs = 38.4En, Wo, Fs = 40.4En, Wo, Fs = 36En, Wo, Fs = 40En, Wo, Fs = 37.7En, Wo, Fs = 39.6

43.942.243.844.543.9454444.3

16.120.817.815.120.114.918.216

114

Appendix III Selected ilmenite analyses from Du-15

1 2 3 4 5 6 7 8

SiO2TiO2Al2O3V2O3Cr2O3Fe2O3FeOMgOCaOMnONiOZnONa2OK2OTotal

0.0150.51

0.06n.d.n.d.3.74

41.362.10n.d.0.33n.d.n.d.n.d.n.d.

98.11

n.d.49.01

n.d.n.d.0.039.75

41.381.25n.d.0.46n.d.n.d.n.d.n.d.

101.88

n.d.50.980.020.160.094.69

41.702.11n.d.0.38n.d.n.d.n.d.n.d.

100.13

n.d.49.08

0.01n.d.n.d.6.76

42.930.49n.d.0.33n.d.n.d.n.d.n.d.

99.60

n.d.51.20

0.02n.d.n.d.

4.4343.59

1.07n.d.

0.54n.d.n.d.n.d.n.d.

100.85

n.d.49.50

n.d.n.d.n.d.

6.1241.99

1.13n.d.

0.44n.d.

0.06n.d.n.d.

99.24

n.d.50.35

n.d.n.d.

0.196.81

41.192.09n.d.


100.99

n.d.51.05

0.050.180.014.73

42.271.61n.d.

0.560.21n.d.n.d.n.d.

100.67

SiTiAlVCrFe3+

Fe2+

MgCaMnNiZnNaK

-1.9260.006

--

0.1401.7550.158

-0.015

----

-1.819

---

0.3621.7090.092

-0.018

----

-1.909

-0.0060.0060.1731.7350.156

-0.015

----

-1.871

---

0.2561.8220.037

-0.015

----

-1.916

---

0.1671.8140.081

-0.024

----

-1.885

---

0.2311.7780.085

-0.018

-0.003

--

-1.870

--

0.0060.2551.7000.154

-0.015

----

-1.9050.0060.006

-0.1791.7530.119

-0.0240.009

---

1 = DU-15-392 = DU-15-703 = DU-15-6804 = DU-15-7505 = DU-15-8506 = DU-15-10747 = DU-15-1119.58 = DU-15-1850

115

Appendix III continued. Selected magnetite analyses fromDu-15

1 2 3 4 5 6 7 8


n.d.3.321.001.070.30

59.8134.06

0.04n.d.0.16n.d.n.d.n.d.n.d.

99.76

n.d.4.111.790.890.33

58.5335.06

0.28n.d.0.010.07n.d.n.d.n.d.

101.07

0.047.891.650.911.56

48.3136.93

0.54n.d.

0.210.120.10n.d.n.d.

98.26

n.d.6.002.810.830.61

52.4636.360.25n.d.


99.53

n.d.8.302.560.961.52

46.7138.08

0.20n.d.

0.310.09n.d.n.d.n.d.

98.73

n.d.3.251.501.550.97

57.1732.910.36n.d.

0.080.24n.d.n.d.n.d.

98.03

0.042.051.151.651.52

60.7733.05

0.08n.d.

0.090.13n.d.

0.01n.d.

100.54

n.d.5.812.770.942.82

51.2636.21

0.36n.d.

0.35n.d.

0.02n.d.n.d.

100.54

SiTiAlVCrFe3+

Fe2+

MgCaMnNiZnNaK

-0.0970.0460.0320.0091.7211.0880.002

-0.005

----

-0.1150.0810.0270.0091.6511.1000.016

--

0.002---

0.0030.2280.0740.0270.0461.3931.1850.030

-0.0070.0050.002

--

-0.1700.1270.0270.0181.4891.1480.014

-0.007

----

-0.2380.1150.0270.0461.3371.2140.011

-0.0090.002

---

-0.0960.0700.0470.0281.6651.0650.021

-0.0020.007

---

0.0030.0590.0500.0500.0451.7341.0470.005

-0.0020.005

---

-0.1640.1210.0270.0851.4401.1310.020

-0.011

----

1 = DU-15-702 = DU-15-99.53 = DU-15-6804 = DU-15-7505 = DU-15-8506 = DU-15-20507 = DU-15-21008 = DU-15-2225

116

Appendix IV Selected plagioclase analyses from BL-95-1

1 2 3 4 5 6 7 8

SiO2Al2O3CaOFeONa2OK2OTotalSiAlCaFeNaK

51.0330.7413.97

0.183.860.23

100.012.3261.6550.6820.0080.3400.010

52.0629.8812.89

0.274.330.23

99.662.3751.6070.6310.0110.3840.010

51.7230.6013.890.333.810.33

100.682.3441.6330.6750.0140.3320.016

51.9630.0012.56

0.244.270.38

99.412.3771.6150.6150.0080.3800.022

49.9531.2314.54

0.353.540.16

99.772.2901.6870.7140.0140.3140.012

52.0330.4913.07

0.234.070.23

100.122.3621.6310.6360.0080.3600.010

50.3230.9814.69

0.203.360.28

99.832.3031.6730.7210.0080.2980.016

52.2530.1612.88

0.354.100.27

100.012.3731.6170.6280.0140.3600.016

1 = BL95-1/9 An = 662 = BL95-1/112 An = 623 = BL95-1/211 An = 664 = BL95-1/331 An = 605 = BL95-1/425 An = 696 = BL95-1/586 An = 637 = BL95-1/735 An = 708 = BL95-1/853 An = 63

9 10 11 12 13 14 15 16


50.8330.8613.54

0.333.460.34

99.36

50.4331.3914.04

0.243.200.22

99.52

51.4530.4413.420.363.900.31

99.88

51.2631.1913.66

0.233.370.36

100.07

50.6731.1914.34

0.173.370.16

99.90

53.3629.9212.44

0.224.360.05

100.35

52.7629.5712.51

0.243.950.29

99.32

53.3229.3012.05

0.194.700.17

99.73

SiAlCaFeNaK

2.3281.6680.6630.0140.3080.022

2.3081.6940.6880.0080.2860.012

2.3461.6390.6550.0140.3460.016

2.3301.6710.6660.0080.2940.022

2.3111.6780.7020.0050.2960.010

2.4081.5890.6020.0080.3800.006

2.4071.5890.6110.0080.3500.016

2.4211.5670.5870.0080.4140.010

9 = BL95-1/91110 = BL95-1/1089.511 = BL95-1/113812 = BL95-1/126913 = BL95-1/1419.114 = BL95-1/144915 = BL95-1/1486.416 = BL95-1/1533


117

Appendix IV continued. Selected olivine analyses fromBL-95-1

1 2 3 4 5 6 7 8


34.56n.d.n.d.0.02n.d.

25.85n.d.0.53

39.380.130.07n.d.n.d.

100.54

33.480.05n.d.0.02n.d.

22.06n.d.0.71

42.350.100.07n.d.n.d.

98.84

33.86n.d.n.d.n.d.n.d.

24.070.070.53

42.050.100.06n.d.n.d.

100.74

34.74n.d.

0.04n.d.n.d.

26.07n.d.

0.5237.74

0.160.010.010.05

99.34

35.21n.d.n.d.

0.030.02

25.02n.d.

0.5339.01

n.d.n.d.n.d.n.d.

99.82

34.95n.d.n.d.n.d.

0.0124.62

0.050.58

38.580.020.060.04n.d.

98.91

37.20n.d.n.d.n.d.

0.0231.64

n.d.0.38

30.750.270.11n.d.n.d.

100.37

36.600.01n.d.n.d.n.d.

28.630.030.27

34.310.060.040.02n.d.

99.97


0.979----

1.092-

0.0120.9330.0030.002

--

0.9840.002

---

0.966-

0.0181.0410.0020.002

--

0.973----

1.0290.0020.0121.0090.0020.002

--

0.990----

1.107-

0.0120.8990.003

---

1.001----

1.060-

0.0120.927

----

1.002----

1.0520.0020.0140.925

-0.0020.004

-

1.006----

1.276-

0.0080.6960.0070.002

--

1.011----

1.178-

0.0070.7920.002

---

1 = BL95-1/9 2 = BL95-1/112 3 = BL95-1/331 4 = BL95-1/5065 = BL95-1/7356 = BL95-1/9117 = BL95-1/12698 = BL95-1/1533

Fo = 54Fo = 48Fo = 50Fo = 55Fo = 53Fo = 53Fo = 65Fo = 60

118

Appendix IV continued. Selected apatite analyses fromBL-95-1

1 2 3 4 5 6 7 8

P2O5MgOCaOMnOFeOFClH2OSumO=F.ClTotal

42.27n.d.

54.900.060.242.351.650.31

101.781.36

100.42

42.17n.d.

55.180.050.272.460.990.42

101.541.26

100.28

43.930.08

52.110.100.242.571.610.22

100.861.45

99.41

43.210.05

54.090.080.252.571.460.27

101.981.41

100.57

42.600.02

53.790.050.232.671.450.20

101.011.45

99.56

43.510.05

54.01n.d.

0.142.081.560.47

101.821.23

100.59

43.030.03

54.200.040.181.511.340.78

101.110.94

100.17

43.160.02

53.69n.d.0.151.651.410.69

100.771.01

99.76

PMgCaMnFeFClOH

5.823-

9.5660.0100.0291.2090.4550.336

5.817-

9.6360.0100.0391.2680.2730.459

6.0110.0199.0360.0100.0291.3180.4400.242

5.8980.0109.3520.0100.0291.3130.3990.289

5.874-

9.3880.0100.0291.3770.4010.222

5.9460.0109.357

-0.0191.0660.4280.507

5.9570.0109.496

-0.0290.7830.3700.847

5.979-

9.411-

0.0200.8550.3890.757

1 = BL95-1/3792 = BL95-1/5063 = BL95-1/7354 = BL95-1/9575 = BL95-1/1089.56 = BL95-1/11847 = BL95-1/14498 = BL95-1/1533

119

Appendix V Selected orthopyroxene analyses from BL-95-1

1 2 3 4 5 6 7 8


50.830.140.700.05n.d.

21.410.910.55

23.85n.d.0.06n.d.n.d.

98.50

50.690.230.940.010.06

20.171.160.47

24.370.040.010.06

-98.21

51.020.140.740.04

-21.251.030.44

23.190.110.050.04

-98.05

51.550.170.83n.d.

0.0922.10

1.150.39

22.28n.d.n.d.n.d.

0.0198.57

51.640.350.820.040.03

19.901.110.49

24.550.120.05n.d.n.d.

99.10

52.090.320.920.02n.d.

20.531.140.41

22.87n.d.

0.050.05n.d.

98.40

53.740.331.170.020.05

24.801.530.39

18.380.150.02n.d.

0.01100.59

53.740.140.920.07n.d.

23.571.020.38

20.14n.d.0.040.05n.d.

100.07


1.9490.0050.032

--

1.2230.0370.0180.765

-0.002

--

1.9550.0070.042

--

1.1580.0490.0160.7850.002

-0.004

-

1.9590.0050.032

--

1.2160.0420.0140.7450.0020.0020.004

-

1.9570.0050.037

-0.0051.2500.0480.0110.707

----

1.9700.0090.037

--

1.1330.0460.0160.7840.0050.002

--

1.9830.0090.041

--

1.1640.0460.0140.727

-0.0020.004

-

1.9610.0090.048

--

1.3490.0590.0130.5620.004

---

1.9810.0050.040

--

1.2960.0400.0110.620

--

0.004-

1 = BL95-1/92 = BL95-1/1123 = BL95-1/3314 = BL95-1/5065 = BL95-1/7356 = BL95-1/9117 = BL95-1/12698 = BL95-1/1486.4

En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs =

60.458.160.762.357.760.168.566.3

1.832.462.12.392.342.372.992.04

37.839.437.235.339.937.528.531.7

121

Appendix V continued. Selected clinopyroxene analysesfrom BL-95-1

1 2 3 4 5 6 7 8


49.900.682.530.070.11

13.1421.49

0.349.68n.d.n.d.0.270.03

98.24

49.970.461.53n.d.n.d.

12.1921.26

0.3712.23

n.d.n.d.0.27n.d.

98.28

50.240.681.880.08n.d.

13.2020.47

0.4010.57

0.200.040.30n.d.

98.06

49.630.732.000.12n.d.

13.2621.39

0.2310.58

n.d.n.d.0.29n.d.

98.23

50.700.792.69n.d.

0.0513.2621.88

0.209.600.090.010.32n.d.

99.59

50.790.652.120.05n.d.

13.1121.32

0.1910.87

0.020.060.31n.d.

99.49

51.900.602.240.050.11

14.4022.510.177.930.13n.d.

0.250.03

100.32

51.990.552.53n.d.n.d.

13.9022.02

0.248.480.09n.d.

0.43n.d.

100.23


1.9090.0210.115

-0.0050.7500.8810.0110.310

--

0.018-

1.9340.0140.070

--

0.7020.8810.0120.395

--

0.018-

1.9300.0190.0830.005

-0.7550.8430.0140.3390.0070.0020.024

-

1.9080.0210.0930.005

-0.7600.8800.0070.339

--

0.024-

1.9130.0230.118

--

0.7450.8840.0070.3040.002

-0.022

-

1.9250.0180.096

--

0.7400.8660.0070.344

-0.0020.022

-

1.9280.0180.098

-0.0050.7970.8950.0040.2450.004

-0.018

-

1.9320.0160.112

--

0.7710.8780.0070.2640.002

-0.032

-

1 = BL95-1/92 = BL95-1/1123 = BL95-1/3314 = BL95-1/5065 = BL95-1/7356 = BL95-1/9117 = BL95-1/12698 = BL95-1/1486.4

En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs = En, Wo, Fs =

38.635.53938.438.537.941.140.3

45.444.543.544.545.744.446.245.9

162017.517.115.717.612.613.8

122

Appendix VI Selected ilmenite analyses from BL-95-1

1 2 3 4 5 6 7 8


0.0250.580.070.400.164.97

41.741.79n.d.0.390.19n.d.n.d.n.d.

100.31

n.d.49.84

n.d.0.110.016.22

42.910.79n.d.0.50n.d.n.d.n.d.n.d.

100.38

0.0750.61

0.050.140.065.91

41.701.85n.d.0.58n.d.0.01n.d.n.d.

100.98

n.d.49.86

0.040.200.083.99

43.070.670.080.42n.d.

0.06n.d.n.d.

98.47

0.0350.69

0.020.280.094.33

41.681.870.090.380.10n.d.

0.06n.d.

99.62

n.d.48.740.030.16n.d.

7.8239.932.05n.d.

0.220.02n.d.n.d.n.d.

98.97

0.0150.94

0.050.300.203.23

40.742.43n.d.

0.450.250.060.02n.d.

98.68

n.d.49.83

0.010.160.105.86

42.940.71n.d.

0.50n.d.

0.110.03n.d.

100.25

SiTiAlVCrFe3+

Fe2+

MgCaMnNiZnNaK

-1.8940.0060.0180.0060.1851.7380.132

-0.0150.006

---

-1.881

-0.006

-0.2351.7990.057

-0.021

----

0.0031.884

-0.006

-0.2201.7260.137

-0.024

----

-1.917

-0.0060.0060.1531.8400.0520.0030.018

-0.003

--

-1.909

-0.0120.0060.1631.7460.1380.0060.0150.003

-0.006

-

-1.849

-0.006

-0.2971.6850.155

-0.009

----

-1.9290.0060.0120.0060.1211.7140.181

-0.0180.0090.003

--

-1.882

-0.0060.0060.2231.8030.054

-0.021

-0.0030.006

-

1 = BL95-1/92 = BL95-1/1123 = BL95-1/3314 = BL95-1/5475 = BL95-1/7356 = BL95-1/9117 = BL95-1/12698 = BL95-1/1533

123

Appendix VI continued. Selected magnetite analyses fromBL-95-1

1 2 3 4 5 6 7 8


n.d.5.352.661.162.32

51.0235.06

0.36n.d.0.230.090.080.10n.d.

98.43

n.d.3.231.731.081.10

59.1734.34

0.10n.d.0.090.040.08n.d.n.d.

100.96

n.d.8.062.951.040.29

48.9538.01

0.34n.d.

0.320.160.060.04n.d.

100.22

n.d.0.780.730.320.10

66.2031.76

0.02n.d.

0.060.100.02n.d.n.d.

100.09

n.d.4.072.652.262.66

51.9034.06

0.28n.d.

0.180.060.10n.d.n.d.

98.22

n.d.1.470.581.340.56

63.6432.05

0.12n.d.

0.030.280.050.030.03

100.18

0.110.190.310.900.08

67.3931.47

n.d.n.d.n.d.

0.11n.d.

0.02n.d.

100.58

0.060.900.580.900.97

64.2831.610.11n.d.n.d.

0.010.18n.d.n.d.

99.60

SiTiAlVCrFe3+

Fe2+

MgCaMnNiZnNaK

-0.1540.1190.0370.0691.4651.1210.021

-0.0070.0020.0020.010

-

-0.0910.0770.0310.0311.6781.0810.005

-0.0020.0020.002

--

-0.2270.1300.0310.0091.3751.1880.020

-0.0110.0040.0020.004

-

-0.0230.0320.0090.0051.9081.0160.002

-0.0020.002

---

-0.1180.1190.0690.0831.4951.0900.016

-0.0050.0020.002

--

-0.0420.0270.0410.0191.8291.0250.007

--

0.0090.002

--

0.0050.0050.0140.0270.0051.9371.005

---

0.002---

0.0030.0260.0280.0280.0281.8601.0180.007

---

0.005--

1 = BL95-1/92 = BL95-1/1123 = BL95-1/4254 = BL95-1/6545 = BL95-1/7356 = BL95-1/9577 = BL95-1/13168 = BL95-1/1533

124

Appendix VII Selected PGM analyses from BL-95-1

Sample Analysis # Mineral Elements ELMT% ATOM% Formula Surroundings

1269 RJK-504 Maslovite PtBiTeTOTAL

38.3328.1932.4298.94

33.56023.04043.400100.000

PtBiTe Sulfide

1269 RJK-510 Paolovite PdSnPtTOTAL

63.6634.263.03100.95

66.29531.9871.719100.001

Pd2Sn Silicate

1269 RJK-512 Paolovite PdSnTOTAL

62.6336.3298.95

65.79434.206100.000

Pd2Sn Sulfide

1269 RJK-517 Paolovite PdSnTOTAL

64.7835.96100.74

66.77233.228100.000

Pd2Sn Sil.-sulf. margin

1442 RJK-521 Tetraferro-platinum

PtFeNiTOTAL

80.5716.113.2999.97

54.52038.0757.405100.000

PtFe Silicate

1442 RJK-526B Majakite PdNiAsCuPtTOTAL

41.8621.4128.966.801.18100.21

31.28328.99330.7318.5130.480100.000

PdNiAs Sulfide

1442 RJK-539 Sobo-levskite

PdTeBiTOTAL

34.3712.8550.3697.58

48.59315.15636.251100.000

PdBi Oxide

1442 RJK-541 Sobo-levskite

PdTeBiTOTAL

36.3814.7947.9999.16

49.74116.85433.405100.000

PdBi Sil.-sulf. margin

125

Appendix VIII Selected analyses of rock-forming mineralsfrom DH-1

1 2 3 4 5 6 7 8

SiO2TiO2Al2O3V2O3Cr2O3MgOCaOMnOFeO(tot)NiOZnONa2OK2OTotal

47.220.399.960.090.01

12.6112.67

0.0614.23

n.d.0.021.310.11

98.68

54.230.052.620.07n.d.

17.1712.03

0.2211.26

0.070.100.320.02

98.16

51.840.036.680.010.02

16.8811.54

0.209.390.020.091.060.03

97.79

26.900.09

18.330.09n.d.

17.57n.d.

0.1023.36

0.030.10n.d.n.d.

86.57

27.660.02

18.87n.d.

0.0722.790.010.30

16.770.21n.d.

0.01n.d.

86.71

26.55n.d.

21.43n.d.

0.0321.16

0.010.14

17.310.180.060.010.02

86.90

38.510.03

27.57n.d.0.060.02

23.050.037.31n.d.n.d.0.02n.d.

96.60

38.450.07

27.750.01n.d.

0.0223.83

0.047.31n.d.n.d.n.d.n.d.

97.48


6.8510.0441.7080.017

-2.7281.9700.0091.726

--

0.3660.018

7.7100.0090.445

--

3.6411.8290.0261.3420.0090.0090.086

-

7.349-

1.124--

3.5681.7540.0261.116

-0.0090.290

-

5.6920.0134.5730.025

-5.539

-0.0134.129

-0.013

--

5.649-

4.544--

6.939-

0.0492.8610.037

---

5.416-

5.147--

6.433-

0.0252.9530.0250.012

--

6.152-

5.183---

3.944-

0.979----

6.0910.0105.177

--

0.0104.0450.0100.971

----

1 = DH-1-32.45/Amphibole2 = DH-1-45.32/Amphibole3 = DH-1-101.75/Amphibole4 = DH-1-32.45/Chlorite5 = DH-1-45.32/Chlorite6 = DH-1-101.75/Chlorite7 = DH-1-88.25/Epidote8 = DH-1-101.75/Epidote

126

Appendix VIII continued. Selected analyses of rock-forming minerals from DH-1

1 2 3 4 5 6


007n.d.0.05n.d.0.03

67.3930.46

n.d.n.d.n.d.0.04n.d.n.d.n.d.

98.04

n.d.0.140.171.17

13.6352.5430.46

0.01n.d.0.250.110.23n.d.n.d.

98.71

n.d.51.87

n.d.0.330.03n.d.

45.050.06n.d.0.68n.d.0.08n.d.n.d.

98.10

n.d.50.54

0.010.47n.d.

2.2842.23

0.24n.d.

2.510.090.170.04n.d.

98.58

MgOCaOMnOFeOCO2Total

MgCaMnFeC

1.1651.20

0.451.78

43.9798.57

0.0581.8410.0140.0582.014

0.1653.310.09n.d.43.9797.53

0.0081.9320.002-2.029

SiTiAlVCrFe3+

Fe2+

MgCaMnNiZnNaK

0.003-

0.005--

1.9911.000

---

0.002---

-0.0050.0090.0370.4181.5270.984

--

0.0090.0020.007

--

-2.003

-0.013

--

1.9350.006

-0.031

-0.003

--

-1.947

-0.019

-0.0861.8110.018

-0.1080.0030.0060.006

-

1 = DH-1-43.05/Magnetite2 = DH-1-93.40/Magnetite3 = DH-1-32.45/Ilmenite4 = DH-1-45.32/Ilmenite5 =DH-1-32.45/Carbonate6 =DH-1-101.75/Carbonate

127

Appendix IX Selected PGM analyses from DH-1


43.05 DEV50 Sudburyite PdSbTOTAL

49.4551.56101.01

52.32047.680100.000

PdSb Ox.-Sil. margin

43.05 DEV53 Paolovite PdSnTOTAL

64.5836.17100.75

66.57133.429100.000

Pd2Sn Sulf.-Carb. margin

45.60 DEV113A Ruarsite SAsRuOsIrTOTAL

13.2134.6633.8517.411.40100.53

31.49835.35525.5976.9950.556100.001

RuAsS Oxide

45.60 DEV125 Anduoite AsRuOsTOTAL

52.7722.1024.3599.22

67.01320.80412.18299.999

RuAs2 Oxide

45.60 DEV137A Hollingw. SAsRhPtTOTAL

14.2036.5537.4510.7798.97

32.81136.14326.9564.091100.001

RhAsS Oxide

45.60 DEV138 Irarsite SAsRhIrTOTAL

12.1826.9910.2050.90100.27

34.39932.6358.97823.988100.000

IrAsS Oxide

84.55 DEV152A Laurite SAsRuRhIrTOTAL

33.906.0052.121.203.8297.04

62.7594.75330.6140.6911.184100.001

RuS2 Ox.-Sil. margin

101.75 DEV213 Kotulskite PdSbTeBiTOTAL

42.815.1646.045.3599.36

48.4125.09843.4133.077100.000

PdTe Silicate

129

Appendix IX continued. Selected PGM analyses from DH-1


101.75 DEV217 Mertieite-I AsPdSbTOTAL

2.6070.4127.03100.04

3.78372.04824.170100.001

Pd5(Sb.As)2 Silicate

101.75 DEV221 Sperrylite SAsRhPtTOTAL

0.7642.511.0955.65100.01

2.67763.9711.19432.158100.000

PtAs2 Silicate

101.45 DEV272 Mertieite-II PdSbTOTAL

69.0630.4999.55

72.15627.844100.000

Pd8Sb3 Silicate

101.45 DEV273 Sperrylite AsPtTOTAL

41.7857.9199.69

65.25934.741100.000

PtAs2 Silicate

95.00 DEV319 Stibiopall. PdSbTOTAL

69.1331.21100.34

71.70828.292100.000

Pd5Sb2 Silicate

94.63 DEV353C Temagamite PdTeHgTOTAL

33.3542.9424.71101.00

40.54143.52615.93299.999

Pd3HgTe3 Silicate

80.80 DEV436 Merenskyite PdTeBiTOTAL

25.8657.3715.7298.95

31.65458.5539.794100.001

PdTe2 Carb.-Sulf. margin

80.60 DEV442A Michenerite PdTeBiTOTAL

23.1831.3044.6299.10

32.19536.25031.556100.001

PdTeBi Sil.-Sulf. margin

130

Appendix X Analyses of rock-forming minerals from KI-86

1 2

SiO2TiO2Al2O3V2O3Cr2O3MgOCaOMnOFeO(tot)NiOZnONa2OK2OTotal

32.81n.d.

14.14n.d.0.02

31.280.030.099.240.260.05n.d.0.01

87.93

57.82n.d.0.07n.d.n.d.

21.8213.62

0.074.930.030.070.040.01

98.48


6.320-

3.218--

8.982-

0.0121.4930.0350.012

--

7.951-

0.017--

4.4712.0080.0080.570

-0.0080.016

-

1 = ki-86/chlor2 =ki-86/amph

131

Appendix X continued. Analyses of platinum-groupminerals from KI-86


KI-86 TTA1 Mert.-II AsPdSbTOTAL

2.4864.3623.4090.24

3.98572.86523.150100.000

Pd8(Sb,As)3 Silicate

KI-86 TTA2 Kotulskite PdTeBiTOTAL

31.5523.3415.3170.20

53.65133.09313.256100.000

PdTe Silicate

KI-86 TTA3 Hollingw. SAsRhIrTOTAL

13.6125.8930.169.8679.52

38.26330.92126.2264.58999.999

RhAsS Silicate

KI-86 TTA4 Isomert. AsPdSbTOTAL

7.7767.7414.8090.31

12.02873.86614.106100.000

(Pd,Cu)11(Sb,As)4 Silicate

132


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OULU 2008

A 516

Risto Kaukonen

SULFIDE-POOR PLATINUM-GROUP ELEMENT DEPOSITSA MINERALOGICAL APPROACH WITH CASE STUDIES AND EXAMPLES FROM THE LITERATURE

FACULTY OF SCIENCE,DEPARTMENT OF GEOSCIENCES,UNIVERSITY OF OULU

A 516

ACTA

Risto K

aukonen

sulfide-poor platinum-group element deposits a ...jultika.oulu.fi/files/isbn9789514289545.pdf · in...

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