Mineralogy and Distribution of Platinum-group Mineral (PGM) Placer Deposits of the World

97 Pages·10.505 MB·English

by  Cabri L.J., Harris D.C., Weiser T.W.

Explor. Mining Geol., Vol. 5, No. 2, pp. 73-167, 1996
Copyright © 1996 Canadian Institute of Mining, Metallurgy and Petroleum
Pergamon Printed in Great Britain. All rights reserved
PII: S0964-1823(96)00012-8 0964-1823/96 $15.00 + o.oo
Mineralogy and Distribution of Platinum-group Mineral (PGM) Placer Deposits of the World
LOUIS J. CABRI
Canada Centre for Mineral and Energy Technology
555 Booth Street, Ottawa, Ontario, Canada, K1A 0G1
DONALD C. HARRIS*
Geological Survey of Canada
601 Booth Street, Ottawa, Ontario, Canada, K1A0E8
THOROLF W. WEISER
Federal Institute for Geosciences and Natural Resources
Stilleweg 2, 30655 Hannover, Germany
Received January 18, 1996; accepted March 26, 1996.
^Present address: 23 Bentworth Crescent, Nepean, Ontario, Canada, K2G fXl
Panning for platinum, Tapitangan River, Banggi Island, North Borneo, Malaysia
Explor. Mining Geol., Vol. 5, No. 2, 1996
Abstract—Mineralogical studies of platinum-group minerals (PGM) made during the period 1970-
1995 are compiled in this paper for PGM obtained from platinum- and gold-bearing placer deposits
of fifteen countries. A total of 3399 quantitative electron microprobe analyses of 37 different PGM
are presented, together with analytical data plotted on compositional diagrams. The data reflect the
dominance of Pt-Fe alloys (1723 analyses) and Os-Ir-Ru-Pt alloys (976 analyses) in the placers. The
platinum-group element (PGE) alloys, as well as some other PGM, generally occur as grains less
than one millimetre in size. The Pt-Fe alloys, containing some Cu and Ni, are characterized by a wide
range of Pt:(Fe, Cu, Ni) ratios and, where sufficient data are available, often show a bimodal popu­
lation at about 16-17 and 25 at.% (Fe, Cu, Ni). Iridium, osmium, and rhodium occur in trace quan­
tities in over 96% of the analyzed grains, and all three elements were never found to be in concen­
trations below their detection levels simultaneously. The large data base for the hexagonal alloys
shows a well-developed trend parallelling the irascibility gap in the Os-Ir-Ru ternary system. New
analyses have narrowed further the width of this miscibility gap, which we postulate to be due to the
formation of some alloys at higher temperatures.
Important features of the PGM alloys studied include the presence of inclusions of other PGM
(as well as some undefined PGM), PGE-bearing minerals, spinels, silicates, and more rarely sulfides
such as chalcopyrite, bornite, and pentlandite, as well as their crystallographic orientation and tex­
tures. Alteration and replacement features (when present) are minor, not pervasive, and usually
restricted to a limited number of PGM. The sum of the features obtained by detailed mineralogical
characterization of the placer PGM is consistent with a high temperature intrusive origin. Evidence
of minor secondary alteration that is sometimes found (e.g., irarsite, tulameenite) is probably asso­
ciated with later processes such as serpentinization. The presence of PGE-oxides and hydroxides (?),
all of which are still incompletely characterized, is ascribed to surficial weathering processes. In
many cases the placer PGM can be traced to the source intrusive rocks (e.g., zoned ultramafic com­
plex, ophiolite). However, correlation with source rocks is sometimes not possible due to deep
weathering, lack of detailed geological maps, or inacessible field areas. Copyright © 1996
Canadian Institute of Mining, Metallurgy and Petroleum.
CONTENTS
ABSTRACT......................................................................................................................................74
INTRODUCTION............................................................................................................................76
ANALYTICAL TECHNIQUES............................................................. .......................................76
NOMENCLATURE OF PLATINUM-GROUP ELEMENT (PGE) Al .I .OYS...............................78
NOMENCLATURE OF PLATINUM-IRON ALLOYS...................................................................78
MINERALOGY OF PLATINUM-GROUP MINERALS (PGM) IN PLACER DEPOSITS.........78
ORIGIN OF PGM IN ELUVI ALAND ALLUVIAL PLACER DEPOSITS.................................81
DESCRIPTIONS OF INDIVIDUAL PGM PLACER DEPOSITS................................................82
Canada
Yukon, Florence Creek.................................................................................................................82
Northwest Territories, Liard River...............................................................................................83
British Columbia
Atlin Area..................................................................................................................................84
Quesnel River Area...................................................................................................................84
Tulameen River Area................................................................................................................84
Alberta, Edmonton, North Saskatchewan River..........................................................................89
Saskatchewan, near North Battleford, North Saskatchewan River.............................................90
Burma
Chindwin River Area....................................................................................................................90
Nawngkettha.............................................................................................................................90
Nawngpusawng.........................................................................................................................91
Myenga.......................................Г............................................................................................91
Mineralogy and Distribution of PGM Placer Deposits of the World • L.J. Cabri et al. 75
Manawtha.................................................................................................................................94
Tanga........................................................................................................................................95
Kyain........................................................................................................................................95
Indawgyi Lake.........................................................................................................................95
Wunbe In..................................................................................................................................95
Malaysia
Borneo, Sabah Province..............................................................................................................95
Indonesia
Borneo, South Kalimantan, Riam Kanan....................................................................................96
Papua New Guinea.....................................................................................................................97
Russia
Urals..............................................................................................................................................98
Sissert, Omutnaya River...........................................................................................................98
Sissert........................................................................................................................................98
Kushvinskij...............................................................................................................................99
Nizhni Tagil..............................................................................................................................99
Nevyansk.................................................................................................................................100
Western Chukot...........................................................................................................................100
Thrkey ' -
Kars Province, Eastern Anatolia, Ortakale River.......................................................................101
United States
Alaska, Fox Gulch......................................................................................................................101
California....................................................................................................................................102
Australia
Tasmania.....................................................................................................................................102
Brazil
Minas Gerais, Itabira..................................................................................................................103
Colombia
Choco Region, Rio Condoto Area..............................................................................................103
El Paso....................................................................................................................................104
Viravira....................................................................................................-.............................104
Delfin.......................................................................................................................................Ю5
Nevado....................................................................................................................................105
Ecuador
Esmeraldas Province, Santiago River.........................................................................................110
Ethiopia
Birbir River, Joubdo Stream.......................................................................................................Ill
Sierra Leone
Freetown Complex......................................................................................................................114
South Africa
Transvaal and Orange Free State................................................................................................117
COMMON ACCESSORY PGM....................................................................................................118
UNDEFINED PGM AND PGM OXIDES.....................................................................................118
CONCLUSIONS.............................................................................................................................118
ACKNOWLEDGMENTS..............................................................................................................120
BIBLIOGRAPHY...........................................................................................................................122
APPENDIX A — REFERENCES FOR LOCATION MAP OF PGM PLACER DEPOSITS......129
APPENDIX В — TABLES OF ELECTRON MICROPROBE ANALYSES OF PGM...............132
76 Explor. Mining Geol., Vol. 5, No. 2, 1996
Introduction community today. The purpose of the present paper is to
compile all the authors’ previously published and new data
\ Until the early 20th century, placer deposits containing
on PGM found in placer (and eluvial) deposits in the context
platinum-group minerals (PGM) were the principal source
of their worldwide distribution, in the hope that this will
of platinum-group elements (PGE) for nearly two hundred
provide a useful database in the areas of PGE geochemistry
years, and often were mined as a by- or co-product of plac­
and exploration for PGE deposits. Many placer deposits
er gold. In some gold placer operations, mercury amalga­
have been examined by other workers, for which localities
mation is a common practice for gold extraction and this
and comprehensive reference lists are given in this report
imparts a silver metallic luster to the gold that can be easily
(the data are not included). However, it is acknowledged
mistaken for PGM, or the PGM can be overlooked because
that there may be other published data on platinum-bearing
of mercury amalgamation. Placer mining is reported to have
placer deposits of which we are not aware, especially for
begun in Colombia in the mid-1700s, followed by Russia in
China and the former U.S.S.R. The worldwide localities of
1822-24. With the discovery and exploitation of PGE in Cu-
the placer deposits which we and others have examined are
Ni sulfide ores in the early 20th century, production from
shown in Figure 1. References to the various PGM placer
alluvial deposits decreased to the extent that approximately
localities, with an indication as to whether analyses or other
99% of current primary PGE production comes from sulfide
data are available, are listed in Appendix A.
ores. The placer deposits which once were the world’s
This paper provides the reader with the largest single
largest producers of placer PGE are closely related to long
source of mineralogical and petrological data on PGM and
tectonic belts containing numerous, small, PGE-bearing,
related minerals found in placer deposits, which should
zoned ultramafic complexes of the “Alaskan” or “Uralian”
improve our understanding of their mineralogy, geochem­
type, or other related intrusive rocks. The even smaller PGE-
istry, and genesis.
bearing dunitic cores of these ultramafic complexes usually
do not exceed about five square kilometers in area as cur­
rently exposed, e.g., Tagil [5.3 km2], Tulameen [5 km2],
Analytical Techniques
Joubdo [3.5 km2], Condoto [2 km2], and Omutnaya [1.7
km2]. The best examples of metallogenic belts associated The basic tool used for analysis of the PGE is the elec­
with rich PGE-bearing placers are the 1000 km-long belt in tron microprobe. Several different instruments were used
northwestern Colombia and the 600 km-long belt in the over the twenty five-year period reported on in this paper.
Urals, Russia. Published data on the recovery of PGE from The first data we collected were at the Mines Branch (now
placer mining are few, and suffer imprecise mineralogical Canada Centre for Mineral and Energy Technology —
information, as it is only in recent years that the PGM have CANMET) with a Materials Analysis Company (MAC)
been better characterized. The principal PGM occurring in model 400 electron microprobe. Later, data were collected
placers are the Pt-Fe and Os-Ir-Ru-Pt alloys, though other using a JEOL model 733 Superprobe at CANMET. At the
liberated PGM are found occasionally. The PGM generally Geological Survey of Canada (GSC), a CAMECA
occur as grains less than 1 millimetre in size, but larger Camebax, and, more recently, an SX-50 was employed. At
grains and nuggets are also found. These alloys, especially the Federal Institute for Geosciences and Natural Resources
the Pt-Fe alloys, often contain numerous inclusions of (BGR) of Germany, a Siemens ELMISONDE was used
known and undefined PGM, as well as silicates, spinels, and originally, and, later, a CAMECA Camebax. In addition, dif­
base metal sulfides. The PGM alloys, in some cases, also ferent models of Scanning Electron Microscopes (SEM)
show evidence of secondary PGM in the form of rims and were employed to characterize and image the PGM.
cross-cutting veinlets. In general terms, the external mor­ Minerals were generally studied as polished sections, but
phology and size of the PGM alloys is dependent on the dis­ were also mounted as SEM stubs to examine their morphol­
tance travelled. The largest grains, with well-preserved crys­ ogy and to provide qualitative analyses.
tal faces, are usually found nearer the source rocks. As the instruments evolved, many different correction
During the past 25 years, the authors have studied a programs were used to convert the X-ray intensities to con­
number of gold- and platinum-bearing placer deposits centrations. For example, the early computer program of
worldwide, and published some of the results in scientific Rucklidge and Gasparrini (1969) was revised and updated
journals (e.g., Cabri and Feather, 1975; Cabri and Harris, several times. However, well-characterized standards have
1975; Harris, 1974; Harris and Ballantyne, 1994; Harris and always been used, usually compounds synthesized from
Cabri, 1991; Weiser and Schmidt-Thome, 1993). These pure elements. In addition, care was taken to correct any
studies have resulted in the characterization of many new enhancements of primary X-ray lines by secondary lines of
platinum-group mineral (PGM) species, in the redefinition other elements (see, for example, Crocket and Cabri, 1981).
of other PGM species, and in the establishment of nomen­ The modem analysis of minor to trace elements using
clature for some platinum-group element (PGE) systems microprobes is far superior compared to results from the
(e.g., the Os-Ir-Ru-Pt alloys and the Pt-Fe alloys). This older instruments. We have not attempted to compare trace
nomenclature (Cabri and Feather, 1975, and Harris and elements analysis from one laboratory to another using dif­
Cabri, 1991), which was approved by the Commission of ferent microprobes.
New Minerals and Mineral Names of the International One of the major factors in trace element analysis is
Mineralogical Association, is in use by the mineralogical the counting time. Because analytical routines were set up
Mineralogy and Distribution of PGM Placer Deposits of the World * L.J. Cabri et al. 77
°
я* 15*-
/ - Alaska \ -■ I
(1-14) Yukon “ч ;<ir ~ f'-jz-ST’:
f :• nwt ' Л 1 ■ Finland 5
*(1)^
Г' f *oUrate
—' , ‘1‘19>
.. r- Alta . "
ВС-A» (1Л2) Sasic, "'W fit, 1ЛЛ-Я'4^ . •
(1-2|
Great Britain^ ip \ * ?' ^Mongolia
J‘Oregon ^'i,C^^Germany<l)-^i f/'''""' ' ^.Q4(1-2)
Cоa-lвif)o rnia•^•O) (1-2t) _F( ria-гn)Sce о-ЗY-Sлw 'iVfee' r>ra wnd (t■)? * ~
/"f'
Colombia •-"~ \ < *5 Ethiopia -_'L^ K {-^ .«M-r a7-l^aJ,ysia (1)
Ecdua-3do) r /*: e; \ Л~ч 1 / ZairS*' >,/ '•••v6 4ч .\ «о ^r-^fXw RT •
(1-2) 4 — ■ I(nмdoгneгsi'a- **^ <\•'
D Madagascar ч O'
-;4 V? 0B-r9az)il Y*o y'' " (1-4) - A(u1s-6tr)a lia % Ne(1w- 5C)aledonia
South Africa •ii О /
(1-6)
4 -о-
mb
Tasmania
WORLDWIDE DISTRIBUTION d-в) С.-
OF PGM PLACER DEPOSITS
Fig. 1. Worldwide distribution of PGM placer deposits. The solid circles represent localities of worldwide PGM placer deposits examined
by the authors, the open circles represent localities studied by other workers. The numbers represent the references listed in Appendix A.
to analyze up to 20 elements on the same spot, the count­ In Table B.l of the Pt-Fe alloys, the p/f ratio represents
ing time was usually limited to 10 or 20 seconds for each the atomic sum of the PGE (p) relative to the atomic sum of
element in order to analyze the large number of grains Fe, Cu and Ni (f). This ratio was used to calculate the atom­
being studied. Most of the analyses plotted in the diagrams ic per cent (Fe,Cu,Ni) used in plotting the compositions of
throughout this paper are mean values of several analyses, the Pt-Fe alloys.
but where a large number of grains were analyzed, each More recently, Secondary Ion Mass Spectrometry
point represents a single analysis. Unfortunately, it is not (SIMS) has been used to measure the 187Os/186Os ratios of
practical to indicate whether a given point represents a sin­ osmium-bearing minerals occurring within PGM nuggets
gle analysis or the average of several analyses. We realize and grains (Hattori et al., 1991, 1992; Hattori and Cabri,
that not all the grains are homogeneous, but a study of 1992). Details of the analytical methodology for these in-
homogeneity and zoning is best suited for a separate situ isotopic determinations are given by Hattori et al.
SEM study. (1991). Determination of Os isotope ratios is based on the
Microprobe analyses for all of the PGM that we have decay of 187Os (one of seven Os isotopes) to produce 187Re
analyzed over the years are tabulated in Appendix B. with a half-life of 4.56 x 1010 years (Luck and Allegre,
Although some of the data for the common Pt-Fe and Os-Ir- 1983). The 187Os/186Os of the bulk Earth was 0.805 at 4.55
Ru-Pt alloys and for many other PGM have been previous­ Ga, and the present value is approximately 1.04 (Allegre
ly published (references are given), the majority are report­ and Luck, 1980). Because most Os resides in the mantle, the
ed here for the first time. For most of the Pt-Fe and Os-Ir- values of the bulk Earth are close to the mantle values
Ru-Pt alloys, results are presented in various types of dia­ (Allegre and Luck, 1980, Walker et al., 1989, 1991). On the
grams, e.g., frequency diagrams showing the atomic per other hand, crustal rocks have high and varied Re/Os and
cent (Fe,Cu,Ni) of the Pt-Fe alloys, and quaternary diagrams 1870s/1860s ratios, with an estimated average ratio of about
illustrating compositions and mineral species of the Os-Ir- 30 (Palmer and Turekian, 1986, Palmer et al., 1988). As a
Ru-Pt alloys. These diagrams are presented throughout the result of the large difference in 187Os/186Os ratios between
text for the various localities discussed. For many of the mantle and crustal rocks, in-situ isotopic analysis of osmi­
accessory minerals where a large number of analyses exist, um-bearing minerals is a powerful method for determining
ternary-quaternary diagrams are given to show their compo­ the source of the PGM in question, especially because
sitional variations. For accessory minerals with a limited it is unbiased by subjective interpretation of textures
number of analyses, the reader is referred to Appendix B. and geochemistry.
78 Explor. Mining Geol., Vol. 5, No. 2, 1996
Nomenclature of are imprecisely defined (Harris and Cabri, 1991). Analytical
Platinum-group Element (PGE) Alloys results of natural Os-Ir-Ru-Pt alloys from our large database
have enabled the boundaries in these alloy systems to be
In 1992, the Nomenclature Subcommittee of the
established more accurately. For example, recent data from
ommission on New Minerals and Minerals Names
the North Saskatchewan River, Edmonton, Alberta, permit
(CNMMN) reviewed the naming of members of mineral
refinement of the miscibility gap between rutheniridosmine
solid-solution series. Earlier proposals for the nomenclature
and iridium (Fig. 2). Because the miscibility gap probably
of the Os-Ir-Ru-Pt system (Harris and Cabri, 1973) were
narrows at higher temperatures, these data suggest that the
used as a basis for discussions. The recommendations of the
PGM originated from rocks that formed at slightly higher
subcommittee,Approved by the CNMMN, were subsequent­
temperatures than those at which most such alloys formed.
ly published in several journals by Nickel (1992). The
Ultramafic rocks have not been found in Alberta, but recent
nomenclature of alloys in the ternary systems Os-Ir-Ru, Os-
exploration efforts have found kimberlitic rocks in the Peace
Ir-Pt, Ru-Ir-Pt, Ir-Ru-Rh, Ir-Os-Rh and Pd-Ir-Pt has been
River area, located some 400 km northwest of Edmonton.
revised and reviewed by Harris and Cabri (1991). In Figure
2, the ternary systems Os-Ir-Ru and Ir-Os-Pt are illustrated, Nomenclature of Platinum-iron Alloys
using the 50% rule for nomenclature currently approved by A nomenclature for the platinum-iron alloys, based on a
the CNMMN. The minerals in the Os-Ir-Ru-Pt system are study of both natural and synthetic alloys, was published by
either cubic (iridium and platinum) or hexagonal (osmium, Cabri and Feather (1975) following approval by the
ruthenium and rutheniridosmine). CNMMN of the International Mineralogical Association.
Synthesis work in the pure Os-Ir and Ru-Ir systems has The nomenclature allows for minor amounts of Ir, Pd, Rh,
shown that a miscibility gap exists, although its boundaries Os, Ru that often substitute for Pt, and also minor amounts
of Cu, Ni, Sb that substitute for Fe. In addition, the nomen­
clature discussed older names that are no longer acceptable
Ru
(e.g., “polyxene”, “ferroplatinum”), and also suggested an
approved standard method for referring to alloys when only
compositional data are available. The nomenclature for plat­
inum-iron alloys includes the following minerals:
1. Native platinum is defined to be a. face-centered
cubic alloy with > 80 mol.% Pt. Ferroan platinum is
defined as a variety of native platinum (Pt,Fe) with 20 to 50
mol.% Fe.
2. Isoferroplatinum is a primitive cubic alloy, with a
composition usually near Pt3Fe, but with no precise compo­
sitional limits.
3. For Pt-Fe alloys with compositions in the range of 20
to 50 mol.% Fe, for which the crystal structure has not been
determined, the mineral is referred to by the general name
platinum-iron alloy or Pt-Fe alloy.
4. Tetraferroplatinum is a tetragonal alloy with a
composition near PtFe, but for which the compositional lim­
its are not exactly defined.
It should be noted that many Pt-Fe alloys in the litera­
ture with compositions close to Pt3Fe are erroneously called
isoferroplatinum although lacking X-ray data which would
support a primitive cubic structure. The difference between
the two crystal structures can be seen on X-ray powder dif-
fractograms through the absence or presence of a few weak
reflections. It should be emphasized that the absence on a
diffractogram of the weak reflections required by the primi­
tive cubic crystal structure of isoferroplatinum is not
unequivocal proof of the mineral’s identity as ferroan plat­
inum. This is because of the poor diffracting properties of
these alloys, which require confirmation of the structure by
single crystal methods.
Pt
Mineralogy of Platinum-group Minerals (PGM) in
Placer Deposits
Fig. 2. The quaternary system Os-Ir-Ru-Pt and its nomenclature.
The shaded areas represent slight modifications to the miscibility A listing of the mineral species reported in our studies
gap proposed by Harris and Cabri (1991) because of new data.. of PGM placer deposits is given in Table 1. The most com-
Mineralogy and Distribution of PGM Placer Deposits of the World • L.J. Cabri et al. 79
Table 1. Reported platinum-group minerals
1 | | | | | | | | 1 | 1 1 1 1 1 | 1 1 1
es
ori
Mineral Formula Australia, Tasmania Brazil, Itabira Burma, Chindwin River Canada, Alberta Canada, British Columbia Canada, North West Territ Canada, Saskatchewan Canada, Yukon Columbia, Choco Ecuador, Santiago River Ethiopia, Jourbdo Indonesia, Kalimantan Malaysia, Sabah Papua, New Guinea Russia, Urals Russia, West Chukot Sierra Leone South Africa Turkey, Ortakale River U.S., Alaska U.S., California
I
Arsenopalladinite Pds(As,Sb)3 •
Atheneite (Pd,Hg)3As 0
Bowieite Rh2S3 0 0 0 0 0 0
Braggite (Pt,Pd,Ni)S 0 ф • 0 0
Cooperite PtS © 0 0 ® © 0 0 ©
Cuproiridsite CuIr2S4 0 © О
Cuprorhodsite CuRh2S4 a © 0 0 0 0
Erlichmanite OSS2 0 0 © 0 0 0 •
Ferronickelplatinum (Ni,Fe)Pt\ 0 "
Genkinite (Pt,Pd)4Sb3 0
Geversite PtSb2 0 0
Hollingworthite RhAsS 0 0 0 ® 0 0 0 0
Hongshiite PtCu • 0
Irarsite IrAsS 0 © 0 0 ® 0 0 0 0 0 0
Iridarsenite IrAs2 \ 0
Iridium Ir © © • © • © © © • 0 • • • О
Isoferroplatinum Pt3Fe • 0 ©
Isomertieite PdnSb2As2 © 0
Kashinite Ir2S3 0 © 0 0 О
Keithconnite Pd3Te 0 0
Kotulskite PdTe 0
Laurite RuS2 © 0 0 © 0 0 © © 0 0 0 0 0 О
Malanite CuPt2S4 0 0 0 0
Mertieite-II Pd8(Sb,As)3 О
Osmium Os © 0 © © © © 0 0 0 0 © © • © 0 0 • • О
Platarsite PtAsS 0
Platinum Pt • • • • •
Pt-Fe alloy Pt-Fe 0 © © • © • • • • • • • 0 • • • © • •
Rhodplumsite Rh3Pb2S2 0
Ruthenarsenite RuAs 0
Rutheniridosmine (Os,Ir,Ru) © © • © © •
Ruthenium Ru © • • • • • 0
Sperry lite PtAs2 © • 0 • • © 0 •
Stibiopalladinite Pd5Sb2 0 0 0
Tetraferroplatinum Pt Fe • 0 © • •
Tolovkite IrSbS 0 0
Tulameenite Pt2FeCu © © © © • 0 0
Platinian Copper Cu,Pt 0
Gold Au • © • • • © © • • • ©
Rhodian Pentlandite (Fe,Ni,Rh)9S8 ®
Rhodian Pyrrhotite (Fe,Rh)S 0
Undefined PGM 0 0 0 0 0 © 0 0 0 0 0 0
Undefined PGE-oxides 0 0 0 0
• Single grains © single grains and inclusions, exsolutions, etc.
О inclusion, exsolution etc. © © probable
80 Explor. Mining Geol., Vol. 5, No. 2, 1996
mon species are the Os-Ir-Ru-Pt and the Pt-Fe alloys. In the tionally made possible the characterization of PGE oxides
Os-Ir-Ru-Pt system, the species are osmium, ruthenium, and hydroxides alluded to in earlier studies (cf. Cabri et al.,
rutheniridosmine, iridium and rare platinum. The Pt-Fe sys­ 1981). Native gold, which can contain platinum-group ele­
tem is represented by platinum, isoferroplatinum (Pt3Fe) and ments in solid solution, is also common in PGM placers.
tetraferroplatinum (PtFe). The identity of isoferroplatinum, Composite plots of the Os-Ir-Ru-Pt and Os-Ir-Rh-Pt
though reported by other workers from several localities, is alloys which we have analyzed are shown in Figures 3 and
doubtful without X-ray confirmation of the primitive cubic 4, together with their localities. In the second part of this
structure. Tulameenite is often found as rims and replace­ paper, individual plots of alloys are presented for each local­
ments of other Pt-Fe alloys. ity, together with a brief site description. As illustrated in
The P(jM alloys usually occur as discrete grains, and Figure 3, most of the Os-Ir-Ru-Pt alloys plot in the compo­
sometimes as intergrowths. The alloys invariably contain sitional field of hexagonal osmium, with fewer occurring as
inclusions of many other PGM, as well as sulfides, silicates hexagonal ruthenium or hexagonal rutheniridosmine, or as
and oxides. Some rater PGM inclusions include sperrylite, cubic iridium. Very few of these alloys contain platinum as
hongshiite, arsenopalladinite, braggite, cooperite, mertieite- the third major element, when calculated as atomic per cent.
II, tolovkite, bowieite, stibiopalladinite, rhodplumsite, Our overall data base shows an interesting trend from Os to
hollingworthite, genkinite, kashinite, malanite, palladium, Ru for the hexagonal alloys which appears to follow the
and also many unidentified species. Laurite, and to a lesser boundary of the miscibility gap. In contrast, the alloys that
extent, erlichmanite, are common inclusions in Os-Ir-Ru-Pt plot in the field of cubic iridium, where iridium is the major
alloys. Irarsite may occur either as idiomorphic crystals, or, element, are usually more platinum-rich and show no trend
in many PGE alloys, as rims suggesting later replacement. lines. We have found three localities (Burma, Colombia and
Chromite is the most common non-PGM inclusion, espe­ Urals) where some Os-Ir-Ru-Pt alloys contain Rh as the
cially in the Pt-Fe alloys. Other non-PGM inclusions include third major element when calculated as atomic per cent.
magnetite, olivine, pyroxenes, micas, and sulfides such as These localities are listed in Table B.10; the compositions,
bornite, chalcopyrite, pentlandite, and cubanite. The recent which plot in the Os and Ir fields, are shown in Figure 4.
capability of oxygen analysis using light-element detectors
(e.g., Weiser, 1991; Legendre and Auge, 1993a) has addi­
■ Australia, Tasmania Ethiopia
о Brazil Indonesia
о Burma Malaysia
о Canada, Alberta Papua New Guinea
a Canada, B.C. Russia, Urals
© Canada, NWT Sierra Leone
a Canada, Yukon Turkey
л Colombia US, Alaska
в Ecuador US, California
Fig. 3. Composite plot of all Os-Ir-Ru-Pt alloys analyzed by the Fig. 4. Composite plot of all Os-Ir-Rh-Pt alloys analyzed by the
authors, showing localities. authors, showing localities.
Mineralogy and Distribution of PGM Placer Deposits of the World • L.J. Cabri et al. 81
Origin of PGM in Eluvial and Alluvial Placer temperature transport and deposition of Au-Pd-Pt grains in
Deposits South Devon, England. However, there is a clear contrast
Mertie (1969) discusses platinum placers in great detail, between the mineralogy of these grains (dendritic textures
dividing them into seven different types (e.g., residual and elu­ and related zoning with no inclusions) and the PGM grains
vial placers, stream placers, etc.). He concludes that placers and nuggets that we have studied. The latter nuggets show
containing PGE are commonly derived from dunite or serpen- complex, sometimes oriented inclusions and intergrowths,
tinite, in which the PGE are sparsely and irregularly distributed. as well as inclusions of minerals derived from the host pri­
In non-glaciated regions, the original lodes conceivably can be mary ultramafic rocks. A second example of nugget forma­
discovered by tracing the alluvial deposits upstream; however, tion in the surficial environment is provided by Cassedanne
even though the country rock may be recognized, workable and Alves (1992), who report PGE nuggets in alluvial
lodes are rarely located. Mertie (1969) also reports that plat­ deposits in central Minas Gerais, Brazil. These authors
inum alloys (as well as gold) “rarely migrate far downstream describe “palladium-rich platinum nuggets” which have dis­
from their bedrock sources, unless they are so fine-grained as to tinct morphological features (reniform, mamillary, coral-
be moved by swift water or floated by surface tension”. loidal, and dendritic) and which show no marks of abrasion.
“Generally, however, ordinary detrital grains of platinum or Although the present paper does not discuss in detail
gold work rapidly downward through alluvial deposits, and the origin of the PGM grains and nuggets that we have stud­
come to rest either near, on, or within bedrock”. Thus, except ied, it should be emphasized that textural, chemical, and
for unusual situations (e.g., glaciation), “placers of the precious mineralogical evidence are all important to understanding
metals may be assumed to lie within a few miles of their their genesis. One striking feature is the common presence
bedrock sources” (Mertie, 1969). of intergrown non-PGM minerals in nuggets, especially of
The origin of PGM in eluvial and alluvial placer deposits chromite. In some localities it has been shown that the
became a controversial subject when Augustithis (1965) sug­ chemistry of the chromite, and of associated olivine inclu­
gested that “nuggets of native Pt formed in the lateritic soil” sions, matches with corresponding minerals in a nearby
which capped the Joubdo ultramafic complex in Ethiopia, in dunite body, suggesting that the nuggets are detrital in ori­
contrast to the conclusions of many earlier studies (e.g., gin (cf., Nixon et al., 1990; Slansky et al., 1991). In addition,
Duparc and Molly, 1928). This problem was critically exam­ the wide variety of PGM inclusions, some rare and exotic,
ined by Cabri and Harris (1975), who discussed certain incon­ some occurring as very thin geometrically-oriented exsolu­
sistencies in interpretation of the data by Augustithis (1965). A tion lamellae or as droplet-shaped inclusions, and some as
similar surficial genesis has been suggested by several authors idiomorphic crystals, all of which occur in the matrices of
for a variety of deposits, including the PGM-bearing laterites Pt-Fe or Os-Ir-Ru-Pt alloys, can only be explained by a pri­
in Sierra Leone (e.g., Bowles, 1986, 1988) and the mary origin. For example, in Madagascar, the variety and
Witwatersrand Au-U-PGM paleoplacers (e.g, Cousins, 1973). characteristics of PGM and silicate inclusions in Pt-Fe alloy
Although the idea of surficial genesis was given impetus by nuggets indicate that they were originally formed in a mag-
studies in theoretical and experimental PGE geochemistry, the matic environment (Legendre and Auge, 1992).
results published to date are limited to compounds such as On the other hand, sedimented particles of quartz and
pure Pt and Pd, which are quite unrelated to the diverse and layers of goethite, which are foreign to the source intrusive
chemically complex PGM that occur in placers. Although it is rocks of placers, are sometimes associated with PGM
clearly important to document the geochemical behavior of the nuggets, but clearly formed as late rims in the surficial envi­
PGE in solution, the mineralogy of PGE should also be con­ ronment, as indicated by their different texture and mineral-
sidered in such geochemical studies. For example, it has been ogy (e.g., Cabri et al., 1981; Cabri and Genkin, 1991). The
known for some time (as summarized in Cabri and Laflamme, larger size of PGE alloys that are found in placers, compared
1981) that sulfides and sulfarsenides contain trace amounts of to lode deposits, has often been cited as “proof’ that the
PGE. Mineralogical studies suggest that PGE are dispersed nuggets grew in the placers (e.g., Cousins and Kinloch, 1976;
through oxidation and dissolution of such minerals in the sur­ Bowles et al., 1994). However, Cabri and Genkin (1991)
ficial environment, rather than by dissolution of PGM nuggets, have described a 10 mm-wide aggregate of tulameenite and
which consist principally of PGE alloys. chromite in serpentine from a lode deposit in the Nizhni
The complexly intergrown mineralogy and chemistry of Tagil dunite massif, Urals, and crystal aggregates of Pt-Fe
many PGE grains and nuggets, their detailed mineralogical alloys greater than 13 mm in size have been found in the Pt-
characteristics, and their correlation with host-rock petrolo­ rich dunite core of the Alto Condoto zoned ultramafic com­
gy, all argue for a primary source for PGE grains and plex in Colombia (Salinas et al., 1992; Salinas and Tistl,
nuggets. Such evidence is not discussed by those postulating 1991). Unfortunately, rich lode deposits, such as at Krutoy
formation of PGE grains and nuggets in the surficial envi­ Log (Mertie, 1969), were all exploited in the past, and little
ronment (e.g., Bowles, 1986, 1988; Cousins 1973), who textural or mineralogical data are now available. However, it
describe only the morphology of the nuggets. An apparent was known by the early workers that rich PGE accumula­
exception is the study of Leake et al. (1991), the first exam­ tions did occur within ultramafic rocks, although they were
ple to our knowledge to show clear evidence of formation of few in number and were widely separated in the host intru­
PGE alloys in the surficial environment. Leake et al. (1991) sions (Mertie, 1969). Therefore, such primary deposits were
documented textural and chemical evidence for low- difficult to find, as is also reported to be the case in recent
82 Explor. Mining Geol., Vol. 5, No. 2, 1996
studies (e.g., Tistl, 1994). Clearly, the evidence for primary Canada
mineralization generally has been removed by the erosion of Yukon: Florence Creek
enormous quantities of source rocks.
Source of sample: The Florence Creek PGM weri
One fruitful approach to understanding the origin of
obtained from a Au-Pt-bearing heavy mineral concentrate
PGM grains and nuggets is through in-situ measurements of
that was donated by Bob Wonga of Whitehorse, Yukor
certain isotope ratios, as first documented by Hattori et al.
Territory. Twelve grains ranging from 1.8 x 1.4 mm to 30(
(1991, 1992) and Hattori and Cabri (1992). The first study
x 700 цт were recovered from the concentrate (Ballantynt
showed that erlichmanite nuggets from Sierra Leone have
and Harris, 1991).
,S7Os/1S6Os ratios comparable with mantle values, whereas
Geological setting: Locally, the area is underlain b)
Pt-Fe nuggets had slightly higher ratios. Although the data
hornblende granodiorite and porphyritic quartz monzonite
forthe Pt-Fe nuggets suggest that some later geochemical
granitoid rocks. Regional aeromagnetic data do not reveal
contamination occurred, the ratios are still more than ten
strong magnetic features, which are often characteristic ol
times less than those of the continental crust and black
ultramafic-mafic rocks.
shales. In the study by Hattori and Cabri (1992), Os-isotope
Description of PGM and related minerals:
ratios were determined in osmium and osmium-bearing
Microprobe analyses show that all grains are Pt-Fe alloys
PGM for samples from worldwide placers associated with
with some close to PtFe in composition (Fig. 5, Table B.l).
five Alaskan-type ultramafic complexes (Tulameen in 3
Recent X-ray powder diffraction data for a Pt-Fe alloy grain
British Columbia, Choco in Colombia, Nizhni Tagil and
with 24-25 at. % (Fe, Cu, Ni) indicate a disordered face-cen­
Omutnaya in Russia, Joubdo in Ethiopia, and Goodnews
tred cubic mineral (i.e., ferroan platinum), not isoferroplat­
Bay in Alaska) and with five Alpine-type intrusions (Atlin,
inum as reported earlier (Ballantyne and Harris, 1991). One
Ruby Creek, and Cariboo in British Columbia, Adamsfield
ferroan platinum grain contained four drop-like sulfide inclu­
in Tasmania, Teshio and Onnebetsu in Japan, and in
sions of bomite-digenite (50 |лт diameter), which in turn
Borneo). All PGM analyzed from all localities had isotopic
host inclusions of (Pt,Pd)S (braggite?) and an undefined Pt-
ratios consistent with the mantle 1870s/1860s value. This
remarkable result fully supports existing mineralogical and
petrological evidence that PGE mineralization initially
formed within ultramafic intrusive rocks, but subsequently
Ш Florence Creek
was concentrated mechanically in placers as grains and
nuggets through weathering.
In summary, most modem mineralogical and petrologi­
cal studies of PGE-bearing grains and nuggets from world­
wide sources have confirmed that the origin of PGM is due
to high-temperature mineralization related to mafic and ultra­
mafic intrusions, including the following areas: Colombia \
\
(Salinas et al., 1992, Tistl, 1994), Fifield, Australia (Johan et
al., 1991a, 1991b, Slansky et al., 1991), Tulameen, Canada 1 i 1 1 1 i l 1 1 l 1 1 ■ 1 1 1 1 1 1 r 1 1 r
(Nixon et al., 1989, 1990), Indonesia (Zientek et al., 1992), io ti 12 13 и ie 16 it ie 10 20 212223242620272820303132 33Э4Э8
AL % (Fe.Cu.NI)
and Madagascar (Legendre and Auge, 1992). This interpre­
(Bu»d on total 4 atom*)
tation is supported by Os isotopic studies which have con­
Fig. 5. Frequency diagram showing at.% (Fe,Cu,Ni) contents of Pt-
firmed that placer platinum-group minerals consistently have
Fe alloys from Florence Creek, Yukon.
187Os/186Os ratios within the range of accepted mantle signa­
tures (Hattori et al., 1992; Hattori and Cabri, 1992).
Descriptions of Individual PGM Placer Deposits
The following section of the paper contains chemical
data and brief descriptions of placer deposits which we have
studied. In a few cases we were involved in sample collec­
tion and investigations of the placers. Generally, however,
samples were obtained directly from collections or through
colleagues. The mineral abundances cited below therefore
may be biased either by sample collection or by methods of
concentration of the rare PGM. This is especially important
when making comparisons with bulk geochemical data,
which, ideally, should be determined only on the basis of
mining operations over many years (cf. Mertie, 1976). The
Fig. 6. Backscattered electron image (BEI) of a Pt-Fe alloy with an
descriptions given below are accompanied by analytical
inclusion of bomite and digenite that contains inclusions of
data, figures, and photomicrographs of nuggets and PGM Pt,Pd,Cu,S (braggite?) and an undefined Pt-Rh-S from Florence
(both unpublished and previously published). Creek, Yukon.