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ARTICLE

Geochemistry and mineralogy of platinum-group elements

at Hartley Platinum Mine, Zimbabwe

Part 2: Supergene redistribution in the oxidized Main Sulfide Zone of the Great Dyke,

and alluvial platinum-group minerals

Received: 27 February 2001 / Accepted: 1 November 2002 / Published online: 23 January 2003 Springer-Verlag 2003

Abstract The behaviour of platinum-group elements(PGE) in the exogenic cycle was examined in profiles ofoxidized Main Sulfide Zone (MSZ) ores, in which the

general metal distribution patterns of the pristine MSZare grossly preserved. However, at similar Pt grades,significant proportions of Pd have been lost from thesystem. This indicates that Pd is more mobile than Ptand is dispersed in the supergene environment. Sperry-lite and cooperite/braggite are stable in the oxidizedMSZ. In contrast, the (Pt,Pd)-bismuthotellurides, com-mon in pristine MSZ ores, have disintegrated, and ill-defined (Pt,Pd)-oxides or (Pt,Pd)-hydroxides haveformed. The assemblage of detrital PGM present in theMakwiro River close to the Hartley Platinum Mine in-dicates further mineralogical changes. Sperrylite largelyremains stable whereas most cooperite/braggite grains

have been partly altered or completely destroyed. Grainsof Pt-Fe alloy are ubiquitous in the alluvial sediments.Most likely, these grains are neo-formations that formedeither from pre-existing, unstable PGM or via a solutionstage under low-temperature conditions.

Keywords PGM Oxidized Main Sulfide Zone Placers Great Dyke Hartley Platinum Mine Zimbabwe

Introduction

In the Great Dyke of Zimbabwe, economic concentra-tions of platinum-group elements (PGE) are restricted tosulfide disseminations of the Main Sulfide Zone (MSZ),which is found in pyroxenitic host rocks some metersbelow the transition between the lower Ultramafic andthe upper Mafic Sequence of the Great Dyke. Pristine,sulfide-bearing MSZ ores mined underground are themajor focus of mining and were described in Part 1 ofthis study (Oberthu r et al. 2003). Near-surface oxidizedMSZ ores have a large economic potential with an es-timated resource of 400 Mt of ore (Prendergast 1988);however, all previous attempts to extract the PGE fromthis ore type proved uneconomic due to low PGErecoveries achieved by conventional metallurgicalmethods.

Thermodynamic data and field studies have demon-strated that the PGE are variably mobile in the supergeneenvironment (e.g. Wagner 1929; Fuchs and Rose 1974;Bowles 1986, 1995; Cook et al. 1992; Wood et al. 1992;Evans et al. 1994; Auge et al. 1995; Cabri et al. 1996; Hey1999; Oberthu r et al. 1999, 2000; Evans and Spratt 2000).However, PGE mobilities, the processes of PGE redis-tribution, and PGM disintegration versus neoformationin the supergene environment are much debated and casesof both dispersion and concentration have been proposedby the above authors.

The present study is based on samples from exposuresof oxidized ores of the MSZ created in the course ofmining at Hartley Platinum Mine. The present studyfocuses on the redistribution of the PGE and PGM inthe supergene environment. Direct relationships betweenprimary and secondary PGE/PGM mineralization areestablished through the investigation of samples frompristine MSZ ores (see Part 1, Oberthu r et al. 2003) andfrom weathered MSZ surface exposures as well as de-trital PGM from recent alluvial sediments in a river closeto the Hartley Platinum Mine.

Miner Deposita (2003) 38: 344355DOI 10.1007/s00126-002-0338-8

Thomas Oberthu r Thorolf W. Weiser Lothar Gast

Editorial handling: O. Thalhammer

T. Oberthu r (&) L. GastFederal Institute for Geosciencesand Natural Resources (BGR),Stilleweg 2, 30655 Hannover, GermanyE-mail: thomas.oberthuer@bgr.de

T.W. WeiserRischkamp 63, 30659 Hannover, Germany

K. KojonenGeological Survey of Finland,Betonimiehenkuja 4, 02150 Espoo, Finland

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Geological setting

The Great Dyke layered intrusion is of linear shape andtrends over 550 km NNE at a maximum width of about11 km, and cuts Archean granites and greenstone beltsof the Zimbabwe craton (Worst 1960). Stratigraphically,the layered series of the Great Dyke is divided into alower Ultramafic Sequence of dunites, harzburgites,

olivine bronzitites and pyroxenites, with narrow layersof chromitite at the base of cyclic units, and an upperMafic Sequence mainly consisting of a variety of pla-gioclase-rich rocks (norites, gabbronorites, olivinegabbros). Economic concentrations of PGE, Ni, and Cuin the form of disseminations of mainly intercumulussulfides are found in the Main Sulfide Zone (MSZ)hosted in pyroxenites, some meters below the transitionfrom the Ultramafic to the Mafic Sequence (Prendergastand Wilson 1989).

Production of PGE from the MSZ commenced at theHartley Platinum Mine in the Darwendale Subchamberin 1996. At the Hartley Platinum Mine, the MSZ out-

crop is on the western flank of the Great Dyke andtrends in a NNE direction (Fig. 1). The MSZ dips about18E and pristine sulfide MSZ ores were mined from100 m below surface downwards. As from late 1997,oxidized MSZ ores were excavated in a number of openpits following the outcrop of the MSZ (Fig. 2). How-ever, mining of oxidized MSZ ores was terminated earlyin 1999 due to insufficient, low recoveries, followed bythe suspension of underground mining later that year.The mine is currently under care and maintenance.

Previous work

Geochemical and mineralogical characteristics of thepristine MSZ reported in Oberthu r et al. (2003) will beused for comparison. Weathered, oxidized MSZ oreswere investigated by Wagner (1929), who reportedsperrylite and cooperite in ores from the Old Wedza mine(close to Mimosa mine; see Fig. 1). Evans et al. (1994),Oberthu r et al. (1999, 2000), and Evans and Spratt (2000)studied oxidized MSZ ores and found Pt grades similarto those of the pristine MSZ; however, they agree that alarge proportion of the primary PGE-carriers includingPGM has been destroyed and that their PGE contentsare now distributed either in iron-hydroxides, or smec-

tites, or as discrete PGE-oxides or -hydroxides.Gordon (1923) prospected for gold and platinum

in rivers along the Great Dyke close to Shurugwi (Sel-ukwe Subchamber) and found some anomalous Ptcontents by fire assay in the concentrates. A first re-connaissance study on alluvial PGM originating fromthe Great Dyke using modern mineralogical techniqueswas conducted by Oberthu r et al. (1998) who reportedrare grains of sperrylite, Pt-Fe alloy, Os-Ir-Ru alloy,atheneite, and isomertieite in heavy mineral concentratesfrom various rivers flowing along the Great Dyke.

Samples and methods

Surface and underground locations of the samples studied areshown in Fig. 2. Two vertical profiles across the oxidized MSZ,1.0 m (pit 130; HOP-10x) and 1.8 m (pit 5; HOP-20x) long, weresampled about 5 and 15 m below surface in the open pits. In ad-dition, 1-m-wide composite samples (HOP-30x) across the oxidizedMSZ were taken in two open pits (panels 102 and 120) at depths of3, 8, 12, 16, 20, 25, and 30 m below surface. The whole rocksamples were analyzed for major and trace elements by XRF(BGR), and PGE contents were determined by INAA after Ni-sulfide extraction (Actlabs, Canada). In addition, heavy mineralconcentrates were prepared from the samples of oxidized MSZ(HOP-10x: 1-kg samples; HOP-20x: 4.8-kg samples; compositesamples HOP-30x: 1.4 kg each).

At two localities along the Makwiro River, about 200 kg ofgravel and sand was sieved and panned to produce heavy mineralconcentrates. These were further treated at the BGR laboratories toproduce a final concentrate of the heaviest heavy minerals, which

Fig. 1 Generalized geology of the Great Dyke, and locations ofplatinum mines and prospects

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were studied under a binocular. Grains of interest were hand-picked and transferred to an SEM sample holder. The surfacemorphology of the grains and their chemistry were studied using anSEM with an attached EDS system. Finally, polished sections ofthe detrital PGM grains were prepared. Polished and thin sectionswere made from the rock and concentrate samples. The sectionswere investigated by reflected light microscopy and SEM/EDS.Mineral analyses were performed using a Cameca Camebax elec-tron microprobe at the analytical conditions described in Oberthu ret al. (2003).

Results

Oxidized MSZ: petrographic and geochemical aspects

The oxide ores in the open pits form competent layers,light to dark brownish in color (Fig. 3), and locally showsome greenish or bluish staining caused by secondary

Fig. 2 Part of the HartleyPlatinum Mine concession areashowing geographical features,underground workings as atJune 1999, surface outcrop ofthe MSZ, localities of MSZsurface samples, and samplepoints of the stream sediments

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Cu- and Ni-minerals. The orthopyroxenes of the py-roxenites show only incipient alteration, but the inter-stitial network is filled by iron-hydroxides and brownishsmectites. All samples of this study, taken down to 30 mbelow surface, are pervasively oxidized with regard tothe sulfides. Relict sulfides, transected and surroundedby iron-hydroxides, are rare.

Geochemical profiles of oxidized MSZ resemblethose of pristine MSZ sequences with respect to theirgeneral shapes and Pt grades. However, the elementdistributions show a wider dispersion, in part proba-bly also due to the larger sample widths of 20 cm.In profile, HOP-20x (Fig. 4), the decoupling of

Pd fi Pt fi Au, is well discernable, but the peaks of Niand Cu are not as pronounced as in profiles of pris-tine MSZ (c.f. Oberthu r et al. 2003; Fig. 3). Notably,a variable proportion of the Pd is "missing" relative toPt when compared with the pristine MSZ ores.Whereas average Pt/Pd ratios of 1.30 characterizepristine sulfide MSZ (Brown 1998a), the two profilesof oxidized MSZ investigated have Pt/Pd ratios of2.05 and 2.26, respectively, and the average Pt/Pdratio of the composite samples is 2.41. Similar profilesfrom Ngezi, Unki, and Mimosa have Pt/Pd ratiosranging from 2 to 3.5 (Oberthu r 2002, unpublisheddata). The increased Pt/Pd ratios corroborate the

findings of, for example, Wagner (1929), Fuchs andRose (1974), and Evans et al. (1994) that Pd is moremobile than Pt and is dispersed in the supergene en-vironment. It is recalled in this context (see Part 1)that most of the Pd is hosted in pentlandite (Weiseret al. 1998; Oberthu r et al. 2000), whereas Pt isdominantly present in the form of discrete PGM inthe pristine MSZ. Moreover, worth mentioning is thatcementation zones or enrichment horizons in the oxi-dized MSZ ores were not observed during open pitmining at Hartley Platinum Mine (R. Brown, personal

communication) or in the course of extensive explo-ration drilling in the Ngezi project area (H. Wilhelmij,personal communication).

Oxidized MSZ: ore mineralogy

The samples are characterized by an interstitial networkfilled by iron-hydroxides and brownish smectites, andrare relict sulfides, mainly pyrrhotite, surrounded byrims of iron-hydroxides. Pentlandite, the major carrierof Pd in the pristine MSZ ores, is generally destroyed,although relict shapes of this mineral, now replaced byiron-hydroxides, are discernable. Microprobe analysesshow that a large proportion of the Ni and Cu is hosted

Fig. 4 Profile HOP-20x (9 samples, each 20 cm wide) acrossoxidized MSZ showing the distribution patterns of Cu and Ni (inppm), Pt, Pd, and Au (in ppb)

Fig. 3 Panoramic view of open pit workings of the MSZ at HartleyPlatinum Mine, panel 102, view south, February 1999. MSZhorizon dips about 18E and is conspicuous due to its dark-brownweathering. Overlying websterite and gabbronorites in the hangingof the MSZ and to left side of photo have whitish weathering colors

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in smectites and not in the iron oxides/hydroxides. Oremicroscopic studies reveal that discrete PGM are ex-tremely rare in polished sections (Fig. 5a). Therefore,heavy mineral concentrates were prepared from the ox-ide MSZ samples. Altogether 807 discrete PGM and 613gold grains were extracted from the concentrates.Sperrylite grains are most common (58%), followed bycooperite/braggite (35%) and Pt-Fe alloy grains (3.6%).

Relict (Pt,Pd)-bismuthotellurides (3.3%) were found in afew samples only. These PGM proportions (by number)differ considerably from those found in the pristine MSZores (see Oberthu r et al. 2003), which are dominated byPt-rich (55%) and Pd-rich bismuthotellurides (16%),followed by sperrylite (11%), cooperite/braggite (11%),hollingworthite/platarsite/irarsite [RhAsS/PtAsS/IrAsS](2%), and some rarer PGM.

Fig. 5 Photomicrographs,scanning electron microscope(SEM), and backscatterelectron images (BEI) of PGMin oxidized MSZ. a Idiomorphicsperrylite (light gray) withroundish inclusion of pyrrhotiteand chalcopyrite (dark gray) insitu in weathered MSZ oreconsisting of iron-hydroxides(medium gray) and smectites(dark gray). BEI, sampleHOP-05, ps 5663a. b Sperrylitegrain from heavy mineralconcentrate. SEM image,

sample HOP-102, grain 2350.c Porous grain of Pt-Fe alloysurrounded by a mixture ofsecondary oxides/hydroxidesand silicates. BEI, concentratesample HOP-206a, ps 5910a.d Braggite group grain showinginternal inhomogeneity of Pt-rich (lighter) and Pd-rich(darker) areas. Polished section,BEI, sample HOP-103, ps5657c. e Grain of michenerite(white, center) in disintegration.Alteration rim (gray) ofprobable (Pt,Pd)-oxide/hydroxide phases shows Pd, Cu,

and Fe as major elements.Polished section, BEI, sampleHOP-206a, ps 5910a.f Colloform, banded grain ofPGE-oxide/hydroxide phasewith shrinkage cracks. Polishedsection, BEI, sample NGZ 1Cfrom Adit A, Ngezi concession,ps 5711b

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Sperrylite mostly shows euhedral crystal shapes(Fig. 5a, b). Cooperite/braggite, in contrast, are usuallypresent as splinters of irregular shapes with clean sur-faces (Fig. 5c). In general, the sperrylite and cooperite/braggite grains show no distinct features of alteration.Under the binocular, most grains have silver-white sur-faces; however, some grains have very thin, black surfacecoatings. Internally, the braggite grains often display amosaic of Pt-rich and Pd-rich areas when viewed in theBSE mode in polished sections (Fig. 5d). Grain sizes ofthe PGM range from 50400 lm for hand-picked grains.However, sizes down to 1 lm were also observed ofPGM grains attached to or included in various gangueminerals in polished sections of the concentrates. Inaddition, some porous grains of Pt-Fe alloy were de-tected (Fig. 5c), which probably represent replacementsof some other precursor PGM of unknown chemicalcomposition. Notably, Schneiderho hn and Moritz(1939) showed texturally similar porous grains of nativePt from oxidized Merensky reef and proposed that thesegrains represent relicts of former sperrylite or cooperitegrains.

The formation of mineralogically and chemicallyill-defined PGE-oxides/hydroxides was observedaround relict, disintegrating (Pt,Pd)-bismuthotellurides(Fig. 5e). These alteration phases are generally porous atvarious degrees. Chemically, the alteration phases arequite inhomogeneous and are characterized by the rel-ative loss of Bi and Te, an upgrade of Pt and/or Pdcontents (e.g. from ca. 3035 at% Pt in moncheite to 6070 at% Pt in the alteration rims; oxygen not analysed),and substantial gains in Fe and Cu (up to some wt%).Occasionally, strongly elevated contents of Se and V (upto some wt%) are present. In addition to the alterationrims around disintegrating (Pt,Pd)-bismuthotellurides,

individual, colloform grains of (Pt,Pd)-oxide/hydroxidephases were detected (Fig. 5f). Although up to 20 ele-ments were analyzed, totals of only 8090 wt% wereachieved, indicating that these phases are oxides orhydroxides. Notably, Evans and Spratt (2000) alsodescribed various (Pt,Pd)-oxides/hydroxides from theZinca prospect and reported similar low analytical to-tals. Gold grains (40300 lm) have shapes (filigree,hooked, platy with crystal faces) that far more resemblegold from primary deposits rather than rounded, detritalgold.

A distinct vertical zonation with respect to the dis-crete PGM grains extracted and their proportions was

noted within the profiles. From the four samples ofprofile HOP-10x, 142 PGM grains (66 sperrylite and 76cooperite/braggite) were extracted. The ratio of sperry-lite to cooperite/braggite grains increases from 1:3 at thebottom to 9:1 on top of the profile. Similarly, the ninesamples of profile HOP-20x (407 PGM grains) demon-strate a steady relative increase of the ratio sperrylite tocooperite/braggite from bottom to top, with cooperite/braggite grains being present in the lower portion of theprofile only (Fig. 6). The varying proportions of thesetwo PGM probably reflect an original feature of the

pristine sulfide MSZ as shown in Fig. 9 of Oberthu r et al.(2003). Furthermore, some relict (Pt,Pd)-bismuthotellu-ride grains, slightly altered along their rims or nearlycompletely disintegrated (Fig. 5e), are present in thecenter of the profile, i.e. on and above the Pt peak. RarePt-Fe alloy grains are commonly porous (Fig. 5c) and

were only found on and below the Pt peak in profileHOP-20x. The concentrates of the composite samples(HOP-30x) mainly contained varying proportions ofsperrylite and cooperite/braggite grains only, indicatingthat pervasive oxidation of the MSZ penetrates down toat least 30 m below surface in the open pits at HartleyPlatinum Mine. Indeed, with the exception of a few relictgrains of Pd-rich (Pt,Pd)-bismuthotellurides, the ores ofoxidized MSZ only contain discrete grains of Pt-richPGM.

Detrital PGM, Makwiro River

The headwaters of the Makwiro River are some 25 kmNE of Selous in Archean granite-greenstone terrain.From there, the river flows in a westerly direction andcrosses the Great Dyke some 15 km north of Selous,where it changes direction towards the south, mainlyflowing on rocks of the Great Dyke on its western side,close to the contact with Archean granites (see alsoFig. 2). The heavy mineral concentrates from theMakwiro River are mainly made up of grains of chro-mite (>90%), magnetite, rutile, ilmenite, zircon,

Fig. 6 Mineralogical variation of discrete PGM grains (n=numberof PGM grains) in profile HOP-20x across oxidized MSZ (comparewith Fig. 4) as found in heavy mineral concentrates made fromsamples of 4.8-kg weight each

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monazite, rare pyrite, and pentlandite, as well as thephase [Ni6FeCu], identified as a Cu-bearing awaruite byXRD analyses. Notably, this awaruite was found tocarry up to 19.96 at% Pt (substituting for Ni).

We recovered 162 PGM and 271 gold grains from theconcentrates by panning (Table 1). The Pt grade of thegravel was calculated to be around 15 ppb Pt by takinginto account both the grain sizes and the Pt contents ofthe PGM recovered. The detrital PGM assemblage dis-tinctly contrasts with that of both the pristine and theoxidized MSZ (Table 1). Sperrylite (47%) and Pt-Fealloys (33%) predominate in the gravels, followed bycooperite/braggite (15%). Other PGM comprise threegrains of potarite [Pd,Hg] and one grain of atheneite[(Pd,Hg)3As].

Pt-Fe alloy grains are mainly equidimensional,usually rounded, and they display smooth, convex,polished, silver-white surfaces when viewed under abinocular microscope (Fig. 7a). Internally, the Pt-Fegrains are generally compact and chemically homoge-neous (2526 at% Fe). Intergrowths and inclusions arerare. Sperrylite is mostly present in the form of well-

crystallized grains. Grain surfaces occasionally displayetch pits (Fig. 7b), and overgrowths of native Pt arealso common (Fig. 7c). Partial replacements ofsperrylite grains or overgrowths of pure Pt are alsoobvious in polished sections (Fig. 7d). Cooperite/braggite grains are inconspicuous in general. Somegrains, however, are distinct by features of externalcorrosion and partial internal leaching along irregularcorrosion channels. A number of porous PGM grains(Fig. 7e) essentially consist of Pt (9599 at% Pt,12 at% Pd). Potarite and atheneite form irregular,chemically homogeneous grains. Internally, the ath-eneite grain shows a foam pattern characterized by

darker and lighter areas due to slightly varying con-tents of Hg (Fig. 7f). PGM grain sizes range from 65480 lm (mostly between 150 and 200 lm), those of thegold grains from 50750 lm.

Summary and discussion

Oxidized MSZ

Thermodynamic data and field studies have demon-strated that the platinum-group elements are variablymobile in supergene environments; however, the pro-cesses of PGE redistribution and their mobilities in the

supergene environment are much debated and cases ofboth dispersion and concentration have been proposed.Near-continuous underground and surface exposuresof the MSZ allowed us to investigate the behaviour ofthe PGE and PGM in the exogenic environment indetail.

Based on a thermodynamic approach, Mountain andWood (1988) suggested that Pt and Pd may be mobile ina variety of environments. They found that the stabilityfield of sperrylite is much larger than that of Pt-sulfides,even down to 25 C, which explains the common pres-ence of sperrylite in placers. Wood and Vlassopoulos(1990) investigated the dispersion of Pt, Pd, and Au

around PGE mineralization in Quebec, and observedthat Pt and Au seemed to be dispersed mainly in par-ticulate form by mechanical processes, whereas Pd isdispersed mainly in true solution and is ultimatelydrained away. However, the above authors also com-ment that experimental data on the dissolution in vari-ous media and the transport of Pt, Pd, and Ausimulating natural processes are largely missing.

In the oxidized MSZ ores at Hartley Platinum Mine,the sulfide aggregates have been replaced by iron hy-droxides. Considerable amounts of Ni and Cu are pre-sent in smectites. Pt/Pd ratios of the oxide ores areelevated relative to their sulfide counterparts, underlin-

ing that Pd is more mobile than Pt in the supergeneenvironment. Pentlandite, the major carrier of Pd in thepristine MSZ, has been destroyed in the course of oxi-dation of the MSZ ores. The discrete PGM grains pre-sent at Hartley Platinum Mine are Pt-phases (mainlysperrylite and cooperite/braggite) which are regarded asrepresenting relict phases of the primary MSZ ores. Thisis underlined by the similar frequencies and verticaldistribution patterns of these minerals in pristine andoxidized MSZ (see Fig. 9 in Oberthu r et al. 2003, andFig. 6 of this study). However, it must be recalled that(Pt,Pd)-bismuthotellurides predominate (71%) in thepristine MSZ, and sperrylite plus cooperite/braggite to-

gether make up 22% of the discrete PGM only. There-fore, about 70% of the Pt and nearly 100% of the Pd ofthe pristine MSZ are not present in a particulate form inthe oxidized MSZ. Evans et al. (1994) proposed thatPt-Fe alloys formed from (Pt,Pd)-bismuthotellurides inoxidized MSZ ores from the Zinca area. The presentstudy identified a number of porous Pt-Fe grains inoxidized ore (Fig. 5c), which may support the view ofEvans et al. (1994). However, the small number of Pt-Fealloy grains identified indicates that an additional pro-cess must have taken place and that a large proportion

Table 1 Relative proportions by number and in % of discrete PGMgrains in pristine and oxidized MSZ, Hartley Platinum Mine, andof detrital PGM grains from the Makwiro River

PGM type Locality

Sulfide MSZ Oxide MSZ Makwiro Rivern=181 n=807 n=162

(Pt,Pd)(Bi,Te)a

% 71 3.3 PtAs2 % 11 58 47(Pt,Pd,Ni)S % 11 35 15Pt and Pt-Fe alloys % 1 3.6 33PGE-AsSb % 2 0.1 Others % 4c xd 5e

Gold (n) 26 613 271

a(Pt,Pd)(Bi,Te)=(Pt,Pd)-bismuthotelluridesbPGE-AsS=PGE-sulfarsenides (see text)cPtSnS (5x), RuS2 (3x)dUnspecified (Pt,Pd)-oxides/-hydroxidesePdHg (5x), (Pd,Hg)3As (2x), unspecified Pd-arsenide (1x)

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of the Pt and Pd must be present in some other miner-alogical form.

Evidence has mounted in recent years that PGE-oxides or PGE-hydroxides exist in the oxidized zone orin laterites of many deposits in the world (e.g. Weiser1990; Auge and Legendre 1994; Auge et al. 1995; Jedwab1995; Salpe teur et al. 1995; Hey 1999; Evans and Spratt2000; Oberthu r et al. 2000). These phases are incon-

spicuous in reflected light and, therefore, may have beenoverlooked in previous studies.

Recent work of Evans and Spratt (2000) and Obe-rthu r et al. (2000) has identified a number of these ill-defined PGE-oxides/hydroxides in ores of the MSZ,where these phases occur in two major modes. Relict(Pt,Pd)-bismuthotelluride grains have rims of low-reflecting, porous, (Pt,Pd)-oxide/hydroxide phases which

Fig. 7 Photomicrographs,scanning electron microscope(SEM), and backscatterelectron images (BEI) ofplatinum-group minerals fromthe Makwiro River. a Well-rounded grain of Pt-Fe alloy.SEM image, grain 1007. b Well-crystallized sperrylite grain withcrystallographically orientatedetch pits. SEM image, grain

4413. c Sperrylite grain withthin overgrowth of nativeplatinum. SEM image, grain4423. d Sperrylite (gray) withthin, discontinuous rim ofchemically pure native platinum(white). Polished section, BEI,ps 5981. e Porous grain ofnative platinum. Polishedsection, BEI, ps 5981. fGrain ofatheneite [(Pd,Hg)3As] showinginternal foam texture. Lighterareas have slightly higher Hgcontents. Polished section, BEI,ps 5981

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occasionally show shrinkage cracks. Microprobe ana-lyses of these grains indicate that they are oxides orhydroxides. Their alteration largely takes the form ofremoval of Te and Bi and addition of oxygen (O, OH),accompanied by an overall upgrading of the Pt contentsof the neoformed phases relative to their precursorminerals. In a second manner, individual, occasionallydistinctly zoned grains of (Pt,Pd)-oxide/hydroxidephases are found interstitially to silicates and in partintergrown with secondary phases. These grains mayrepresent relicts of disintegrated sulfide aggregates in-cluding Pd-bearing pentlandite, or precipitates fromdissolved PGE which were mobilized by supergenefluids.

A first attempt to quantify the problem of themissing Pt and Pd contents not present as distinctminerals was made by Evans and Spratt (2000). Theseauthors re-investigated oxidized MSZ samples from theZinca area and identified cooperite/braggite andsperrylite grains as well as various Pt-oxide/hydroxidephases. The latter made up about 60 wt% of the Ptphases and could therefore largely account for the

missing Pt (ca. 70% at Hartley Platinum Mine),whereas the fate of much of the Pd is still unknown.Although the presence of Pt and Pd in the form of themineralogically and chemically ill-defined PGE-oxides/hydroxides has been substantiated at Hartley PlatinumMine, more work is needed to chemically characterizethese phases. Furthermore, the relative proportions of Ptand Pd present as distinct PGM, hosted by (Pt,Pd)-oxide/hydroxide phases and also by iron-hydroxides orby smectites, still have to be evaluated.

Apparent grain sizes of the PGM, measured in pol-ished sections, in general range from

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the increasing ratio of sperrylite to cooperite/braggitefrom pristine via oxidized MSZ to the fluvial environ-ment. The high proportion of Pt-Fe alloy grains isconspicuous. Possible sources of the Pt-Fe alloy grainscomprise the following: (1) they are direct descendantsfrom the MSZ. This is improbable as they are rare inpristine and oxidized MSZ ores, and a prolonged con-centration and upgrading of these grains in the fluvialenvironment appears unlikely. Furthermore, Pt-Fe alloygrains should be present in similar proportions relativeto sperrylite as in the MSZ, which is not the case (seeTable 1); (2) the Pt-Fe alloy grains originate from dis-seminated occurrences in other rock units of the GreatDyke, e.g. chromitites. This possibility is not supportedby the present work as no PGM were found furtherupstream in the Makwiro River, and Pt-Fe alloy grainshave not been reported from chromitites of the GreatDyke (Germann and Schmidt 1999; Oberthu r 2002); and(3) Pt-Fe alloy grains represent neo-formations thatcame into existence in the course of weathering of theMSZ ores and the concomitant supergene redistributionof the ore elements. Observations in the oxidized MSZ

ores (porous Pt-Fe alloy grains; Fig. 5c) and of PGM inferralitic soils of Madagascar (Salpeteur et al. 1995) arein support of hypothesis (3), but do not fully explain thequantity of Pt-Fe alloys present. At the current state ofknowledge, it must be speculated that some of the Pt-Fealloy grains are true neo-formations that formed in thesupergene environment. Their precursor phases eitherare pre-existing, unstable PGM (note Pt coatings andpartial replacements of cooperite/braggite and also ofsperrylite; Fig. 7d), or may even have formed via a so-lution stage under low-temperature conditions. Futuremicroprobe work aims at the chemical characterizationof the various Pt and Pt-Fe alloy phases, and it is hoped

that some indications of the processes responsible fortheir formation will be obtained from the analyses.

The rare Pd-Hg and Pd-Hg-As phases, which are notpresent in the MSZ ores, are regarded as neo-formationswhich probably formed from dispersed elements duringsupergene processes. The generally large sizes of thePGM (65480 lm) and gold grains (50750 lm) com-pared to those from the MSZ ores are proposed tomainly reflect sedimentological sorting processes, assmaller grains were probably removed in suspension.Some of the gold grains found in the concentrates mayalso originate from the granite-greenstone terrainsdrained by the Makwiro River.

Metallurgical implications

The recovery of PGE from pristine, sulfide MSZ was86% for Pt and 90% for Pd at Hartley Platinum Mine(Rule 1998). Problems encountered in flotation areubiquitous talc, present in larger quantities in shear zonesof different size and scale, the finely dispersed nature ofthe sulfides, and the small grain sizes of the PGM. Pre-vious tests at processing oxidized MSZ ores achieved Pt

recoveries of 23.4% by gravity concentration and 28%by flotation only (Menell and Frost 1926). Prendergast(1990) states that all early attempts at processing oxideores showed Pt recoveries below 50% by either method.In contrast, flotation recoveries of about 70%, slightlylower than those of the sulfide MSZ, were assumed whenoperating the open pits and working oxidized MSZ ore atHartley Platinum Mine. However, total PGE recoveriesdropped to about 50% when these ores were mixed withsulfide MSZ ores at about equal proportions, demon-strating that PGE recoveries from the oxidized MSZ oreswere much lower than expected. As a consequence, openpit mining was terminated early in 1999.

Metallurgical test work performed by Zimplats onpervasively oxidized MSZ ores (from surface down toabout 1015 m) from the Ngezi Project achieved re-coveries of 1530% only. These results indicate thatprobably only relict sperrylite and cooperite/braggitegrains were recovered, as also suggested by the presentmineralogical study.

Evidently, the PGE contents not recovered from thepervasively oxidized ores either are present in the form

of PGE-oxides/hydroxides, or are dispersed in iron-hydroxides or in smectites, at still unknown relativeproportions. Characterization of the mineralogical sitingof the PGE and quantification of the proportions ofPGE-bearing phases are needed to develop new metal-lurgical processes which will make the vast resources ofPGE-bearing oxidized oresnot only of the GreatDyketechnologically available.

Conclusions

The present study revealed new details on the mineral-ogical siting and distribution of the PGE in the oxidizedMSZ, and on the redistribution of the PGE and the fateof the PGM in the exogenic environment. Major find-ings comprise the following:

1. The redistribution of the PGE in the exogenic cyclewas studied in the oxidized MSZ ores at HartleyPlatinum Mine, which showed pervasive oxidationdown to at least 30 m below surface. In profiles ofoxidized MSZ, the general metal distribution andzoning patterns of the pristine MSZ are grosslypreserved. However, at similar Pt grades, some Pdhas been lost from the system as shown by the

increasing Pt/Pd ratios from pristine (1.3) to oxidized(22.4) MSZ. This indicates that Pd is more mobilethan Pt and was partly removed, probably in solution(ground- and/or surface waters), in the exogenicenvironment.

2. Regarding the PGE mineralogy of the oxidized MSZ,it was found that sperrylite and cooperite/braggiteare stable minerals. Major changes from pristine tooxidized MSZ comprise the disintegration of the(Pt,Pd)-bismuthotellurides and the concomitant neo-formation of chemically and mineralogically

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ill-defined (Pt,Pd)-oxides/-hydroxides. Furthermore,there are indications that at least some porous grainsof Pt-Fe alloy are formed in situ from unknownprecursor PGM in the oxidized ores.

3. Processing of oxidized MSZ ores using conventionaltechniques resulted in unsatisfactory PGE recoveries.Therefore, novel methods have to be developed forthis ore type. These have to be based on a betterchemical and mineralogical characterization of the(Pt,Pd)-oxides/-hydroxides, and on a precise knowl-edge of the proportions of PGE hosted in discretePGM, being present as (Pt,Pd)-oxides/-hydroxides,or found in a dispersed form in iron-oxides/-hydrox-ides, or in smectites.

4. The assemblage of detrital PGM in the MakwiroRiver close to the mine indicates that mineralogicalchanges continue in the exogenic cycle. Sperrylite isrelatively stable in this environment, whereas mostcooperite/braggite grains have been altered or com-pletely destroyed. (Pt,Pd)-oxides/-hydroxides werenot observed, probably due to the fact that theporous and friable grains were mechanically de-

stroyed and/or removed in suspension. UbiquitousPt-Fe alloy grains make their appearance in thealluvial sediments. Most likely, the Pt-Fe alloy grainsare neo-formations that formed either from pre-existing, unstable PGM, or via a solution stage underlow-temperature conditions.

Acknowledgements Sincere thanks go to G.L. Holland and R.Brown, chief geologists of BHP/Hartley Platinum Mine, as well asP. Vanderspuy, H. Wilhelmij, and H. OKeeffe of Zimplats, fortheir continuous support of our fieldwork in Zimbabwe, editorialsuggestions, and for permission to publish this paper. K. Kap-penschneider of Hartley Platinum Mine kindly provided digitalmaps. Laboratory work was ably performed by J. Lodziak (elec-tron microprobe), C. Wo hrl (SEM), K. Souto Otero (map draw-ings), A. Weitze, C. Laurisch, and S. Pietrzok (image processing),P. Schlu ter and S. Schwarz (data evaluation and drawings). Criticalreviews by Johan Kruger, Wolfgang Maier, Frank Melcher, andOskar Thalhammer considerably improved the paper.

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