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The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa H.F.J. Theart Department of Earth Sciences, University of Pretoria, Pretoria, South Africa E-mail: [email protected]

C.D. de Nooy Anglovaal Mining Limited, P O Box 1212, Florida, 1710, South Africa E-mail: [email protected]

ABSTRACT Platinum Group Elements (dominated by palladium) are present in the Ni-Cu-Co bearing massive sulphide ore of the Uitkomst Complex, Mpumalanga Province, South Africa. Mineralogical investigations were conducted in two different parts of the ore body and it was found that these elements are contained in fine-grained mineral phases that are largely attributed to five mineral types based on composition and optical properties. Most of these phases fall within the quaternary system Pd-Bi-Te-Sb. Mineral assemblages of this composition are generally regarded as indicative of relatively low temperature (probably less than 490°C) conditions for the formation of the associated massive sulphide ores. Although the mineral phases reported are similar, there are significant differences in the relative abundances of individual mineral phases between the two areas. The distribution of the individual phases recorded for the deeper part of the ore body indicates an increase in Type II (michenerite) grains towards the base of the zone sampled, an antipathetic relationship between Type I (merenskyite) and Type II (michenerite) phases, and a concentration of Type III (testibiopalladite) grains at the top and the base of the zone. The Platinum Group Mineral grains are preferentially associated with pyrrhotite and generally located along grain boundaries. The relative abundances of the Platinum Group Elements in the massive sulphide body are similar to that of other Ni-sulphide deposits such as Sudbury and Noril’sk, but differ significantly from that of the Merensky Reef, UG-2 Reef, and Platreef ore bodies of the Bushveld Complex. Introduction This paper reports on the results of two applied mineralogical investigations conducted to determine the nature of the Platinum Group Minerals (PGMs) and gold present within the Massive Sulphide Body (MSB) of the Uitkomst Complex. These investigations were conducted for the optimisation of the extractive metallurgical process, but also provide important new information on the characterization of the PGMs of this apparent satellite intrusion of the Bushveld Complex. The Nkomati Mine, a joint venture between Anglovaal Mining Limited and Anglo-American Corporation, is currently exploiting the MSB deposit. Mining commenced at the end of 1996. Sulphide mineral concentrates, containing nickel, copper, cobalt and Platinum Group Elements (PGEs), are produced at the mine and are toll treated elsewhere. Although the mine is currently regarded as South Africa’s only primary nickel producer, the sale of PGEs represents an important source of byproduct revenue. The two mineralogical investigations were conducted either by or under the direct supervision of the second author during the exploration phase and immediately following commencement of mining. The Geology of the Uitkomst Complex The Uitkomst Complex (Figure 1) is a layered ultramafic-

mafic intrusion (Gauert et al., 1995). It is situated in the escarpment area of the Mpumalanga Province of South Africa, some 45kilometres west of Barberton and 35kilometres east of Machadodorp. Igneous rocks belonging to the Complex concordantly intruded sedimentary rocks of the lower part of the Transvaal Supergroup in a shoot-like manner and plunge at about 4.5° to the north-west. De Waal et al. (in press) suggested that the Uitkomst Complex is related to the Bushveld Complex in both composition and age, and reported a UPb zircon age for the Complex of 2044 +/- 8 Ma, improving on the earlier Rb-Sr (biotite) model age of 2025 Ma reported by Kenyon et al. (1986). Following the recommendation of A. L. Wilson (personal communication, 1996), the igneous layering within the Uitkomst Complex is divided into two groups, referred to as the Main Group and the Basal Group. These two groups display large differences such as the much greater abundance of sulphides and xenoliths of the country rocks in the Basal Group and the highly altered nature of the upper part of the Basal Group when compared to the overlying units of the Main Group. The consistent thickness of the individual lithostratigraphic units of the Complex along the central axis, over a distance exceeding 12km, is remarkable. The stratigraphy is summarised in Table 1. The nomenclature used for the unit names, with minor modification, was

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Figure 1. Location and surface geology of the Uitkomst Complex.

introduced by exploration geologists working for Inco in the 1970’s, and was reported in Kenyon et al. (1986), Gauert et al. (1995) and Gauert (1998). The geology in the vicinity of the Nkomati Mine is illustrated in an idealised cross-section of the Uitkomst Complex (Figure 2). In this area, the upper part of the Complex has been eroded and only the lower units have been preserved. Wagner (1929) reported the presence of sulphide mineralization related to the Uitkomst Complex. He described it as a platinum occurrence on the farm Uitkomst, contained in a highly altered pyroxenite sill, associated with magmatic Ni-Cu-Fe sulphides resembling the ores of Sudbury in Canada. Extensive exploration has since confirmed the existence of both massive and disseminated sulphide ores. The disseminated sulphide ores of the Complex occur as discrete sub-horizontal zones hosted within specific Table 1. Lithostratigraphic units of the Uitkomst Complex. Group Main

Basal

Unit Gabbronorite Upper Pyroxenite Peridotite (harzburgite) Massive Chromitite Chromititic Peridotite Lower Pyroxenite Basal Gabbro

Average Thickness (m) 262 66 264 (0-6) 35 37 3.5

lithostratigraphic units of the Complex. These ore bodies are referred to here as the Basal Mineralized Zone (BMZ) within the Basal Gabbro Unit, the Main Mineralized Zone (MMZ) within the Lower Pyroxenite Unit and sulphide-rich zones within the Chromititic Peridotite Unit (PCMZ). A Massive Sulphide Body (MSB), discovered on the farm Slaaihoek, is situated in the sedimentary rocks and granite/gneiss underlying the intrusion. Chromitite ores occur within the Chromititic Peridotite and Massive Chromitite Units of the Complex. The chromitite ores also contain low-grade mineralization of base metal and PGE-bearing sulphides. Figure 2 shows the relative positions of the different ore bodies. Three distinct zones are identified in the MSB, namely, an Upper Stringer Zone comprising subhorizontal sulphide veins in predominantly sedimentary host rocks, the Massive Sulphide Zone, and a Lower Stringer Zone formed by irregular veins in the granitic footwall of the deposit. The reported ore resource of the Table 2. The originally reported in-situ resource of the MSB (Anglovaal Minerals, 1995) Ore Tons Zone x 106 MSB 2.3

Ni % 2.69

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Cu % 1.40

Co % 0.13

Pt Pd Rh Au ppm ppm ppm ppm 1.70 4.29 0.21 0.19

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Figure 2. Cross-section, looking north-west, through the central part of the Uitkomst Complex with the different orebodies projected onto the same plane.

MSB is given in Table 2. Pyrrhotite, pentlandite and chalcopyrite are the dominant sulphide minerals in the ore. Since the formation of the MSB, the ore body has been modified by the intrusion of sills and deformation along a thrust zone. This resulted in three discrete lenses indicated in a long section of the ore body (Figure 3). Thrusting of the rocks within the Uitkomst Complex has caused duplication of the igneous stratigraphy, thickening of units and the displacement of sub-vertical features such as strike-slip faults. The most prominent feature of this kind is known as the Basal Shear Zone, which is probably the local expression of a regional thrust plane located near the base of the Transvaal Supergroup. It is located within chemical sediments (dolomite and chert) of the Malmani Subgroup close to the contact with the underlying Black Reef Quartzite Formation. A well-developed mylonitic foliation fabric is present in the shear zone that parallels the regional dip of the sedimentary rocks. Mylonitic rocks belonging to the Basal Shear Zone range in thickness from 0.2m to 3m. Hornsey (1999) confirmed earlier indications (as derived from unpublished long sections constructed before 1996) that the Basal Shear Zone formed as a result of south-east directed compression and estimates the minimum amount of displacement to be about 300m. He suggests its episodic reactivation and an intricate effect on diabase intrusions, based on underground

mapping in the vicinity of the MSB. Furthermore, he demonstrated that the Basal Shear Zone post-dates the emplacement of the MSB and that it is related to shear zones observed higher up in the Complex. Exploration results of the MSB indicated PGE enrichment (especially palladium) towards the western end of the ore body and in the Lower Stringer Zone, when compared to the Massive Sulphide Zone, or more so, the Upper Stringer Zone. The PGE-bearing mineral phases and their mode of occurrence are documented in the following paragraphs for two parts of the MSB. The localities of these areas of investigation are indicated in Figure 3. The deeper, PGE-enriched western part of the ore body is represented by a borehole intersection (SH99) with palladium and platinum grades of up to about 23 ppm and 8 ppm, respectively. A log of the relevant portion of borehole SH99 is provided in Figure 4 where the positions of the samples are also indicated. Channel samples collected underground in the exposed (by mining) eastern part of the ore body (over a width of about 3m) are used to reflect the mineralization of the Upper Stringer Zone and possibly remobilised sulphide veins. Figure 5 is a geological map of the sidewall of the tunnel at the locality where the channel samples were collected. In this area, palladium and platinum grades of up to 12 ppm and 4 ppm, respectively, were determined for individual samples.

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Figure 3. Long-section, looking north-east, through the Massive Sulphide Body (below 1050m AMSL) of the Uitkomst Complex (adapted from Hornsey, 1999). The projected positions of the sample localities are shown as: (a) Borehole SH99 area, and (b) The channel sample area.

Methods of investigation Thirty-two polished sections were prepared for the MSB ore intersected by borehole SH99. For the second area (Upper Stringer Zone), two polished sections were prepared of each of thirty-two channel samples that had been taken at 10cm intervals. All polished sections were examined in detail using reflected light microscope techniques. The relative abundances of the component minerals in these polished sections were determined using a point counting technique. For this, the entire polished section is covered by a grid pattern, using a click-stop stage mounted on the rotating stage of the optical microscope. The mineral falling under the cross

hair at each of the approximately 400 points of the grid is identified and recorded. The results are calculated to obtain area percentages assumed to be equivalent to volume percentages. Volume percentages are converted to mass percentages by multiplying with published specific gravity values of the minerals with corresponding compositions. 2 The entire surface (625mm ) of each polished section was methodically scanned at 200 times magnification and all potential PGM particles were marked for subsequent microprobe analysis using the diamondtipped scriber on the microscope. For each PGM particle, the following details were recorded during the optical microscope investigation: optical

Table 3. Relative abundances of PGMs and gold (Borehole SH99) Type I II III IV V VI VII Total

Provisional Composition Number mineral of grains Merenskyite (Pd,Ni)(Te, Bi)2 869 Michenerite PdTe(Bi,Sb) 153 Testibiopalladite PdTe(Sb,Bi) 45 Sperrylite PtAs2 21 Temagamite Pd3HgTe3 13 Mainly tellurides 7 Gold (Au,Ag) 55 1163

Volume % 77.1% 8.3% 7.4% 5.5% 0.4% 0.1% 1.1% 100%

Table 4. Proportions of PGMs hosted by the component minerals in borehole SH99 Host mineral Pyrrhotite Pentlandite Chalcopyrite Silicates/carbonates Magnetite Other minerals

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Mass % 65.2 7.5 6.1 15.8 5.2 0.1 100.0

Relative % PGMs 60.4 20.5 5.5 13.2 0.1 0.3 100.00

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Table 5. Relative abundances of the PGMs in channel samples Type Provisional Composition Mineral

of

Number Volume Volume %

%

grains

Ia Ib Ic IV II V

Merenskyite (Pd,Ni)(Te,Bi)2 191 (low-Pt) Merenskyite (Pd,Pt,Ni)(Te,Bi)2 (intermediate-Pt) 80 Merenskyite (Pd,Pt,Ni)(Te,Bi)2 (high-Pt) 206 Sperrylite PtAs2 29 Michenerite PdTeBi 33 Temagamite Pd3HgTe3 1 Other 5

46.3

545

100.0

}

15.6

31.5 5.6 1.0 0.01 0.09

93.4

5.6 1.0 0.01 0.09 100.0

properties, average diameter, host mineral, relationship to host mineral, associated minerals and probable mineral species. The relative volume of each PGM particle was obtained from the square of the average diameter of the particle. For this study the specific gravities of the different minerals (PGM) were assumed to be equivalent and therefore the mass percentages are assumed equivalent to the volume percentages. The EDS (energy dispersive spectrometer) facility on a JEOL JXA-840A scanning electron microscope at Anglovaal Mining Limited’s Mineralogical Laboratory was used to semi-quantitatively identify the constituents of each PGM phase and of the associated minerals. The component elements and their apparent relative abundances (as indicated by the EDS spectrum) were recorded. In order to identify the mineral species and also to examine variations in composition, semiquantitative analyses were done on the larger PGM particles. The particles were analysed using an accelerating voltage of 15kV, a specimen current of 1nA and counting times of 60 seconds. Standard metallic reference materials were used. Chemical analyses of Ni, Cu, Co in whole rock samples are based on ICP emission spectroscopy following nitric-bromine dissolution. Pt, Pd, Au and Ag were determined by ICP emission spectroscopy following fire assay lead collection. Sulphur was determined using the Leco Combustion Method. The chemical analyses were conducted at the Anglovaal Research Laboratory. Results for the borehole SH 99 area The Upper Stringer Zone, the Massive Zone, and the Lower Stringer Zone were identified in the intersection of borehole SH99 in the PGE-enriched western part of the MSB. The chemical variation through the ore body is depicted by concentration versus depth graphs in Figure 6. Because of the relative scarcity of PGEs in the Upper Stringer Zone, polished sections were examined

Figure 4. Geological log of borehole SH99 (as logged by Mr. J. Wilton, Avmin Ltd.) showing the position of the individual samples plotted in Figure 6.

from only the Massive Zone and the Lower Stringer Zone. Predominant sulphide and other opaque minerals Pyrrhotite, pentlandite and chalcopyrite are the dominant sulphide minerals of the MSB in this borehole (see Table 4). Pyrrhotite typically makes up between 50 and 80% (by mass) of the polished sections, pentlandite between 5 and 10%, and chalcopyrite between 1 and 13%. Pyrite was observed in only four of the thirty-two samples. Between 1 and 10% magnetite is present in most of the samples which also have minor amounts of a cobaltite-gersdorffite mineral ((Co,Ni,Fe)AsS), sphalerite and galena. Trace amounts of other minerals including PGMs and other, often tellurium-bearing, phases such as hessite (Ag2Te), altaite (PbTe), coloradoite (HgTe) and tellurobismuthite (Bi2Te3), are also present. The pyrrhotite occurs as coarse-grained, anhedral, interlocking particles. The pentlandite is present as clusters and stringers of granular particles within the pyrrhotite. The grain size of the pentlandite typically ranges between 50 and 100µm in diameter. The pentlandite less commonly occurs as fine-grained (usually less than 20µm in length) exsolution “flames”

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Figure 5. Geological map of the western sidewall of the Lens 1 Centre Drive (mapped by Mr. M. Segwapa, Nkomati Mine) at the locality of the channel samples of the Upper Stringer Zone.

within the pyrrhotite. The chalcopyrite occurs as anhedral blebs within the pyrrhotite displaying a wide range in grain size (less than 50µm to more than 10mm). Platinum Group Mineral types The investigation of the PGM grains and gold is based on the diagnostic study of about 1100 individual grains and semi-quantitative analyses of some selected grains. The different mineral types are defined below. The grains were provisionally classified as mineral types because of the semi-quantitative nature of the analyses. Type I (merenskyite [(Pd,Pt)(Te,Bi)2 ]) Semi-quantitative electron microprobe analyses were obtained from 121 grains belonging to this type. The average composition (weight percentages) of these grains is 20% palladium, 62% tellurium, 13% bismuth and 4% nickel. Iron was detected in the spectra of many of the particles and the presence of mercury was detected in three of the grains. These grains display optical and chemical features similar to the mineral merenskyite. The average composition of Type I grains may be expressed as (Pd0.70Ni0.26Fe0.04)(Te1.81Bi0.24). A solid solution between melonite (nickel end-member) and merenskyite (palladium end-member) has been reported (Cabri and Laflamme 1976, Shvedov et al., 1997). Cabri and Laflamme (1976) reported similar amounts of bismuth in merenskyite samples from Sudbury.

Type II (michenerite [PdBiTe]) Fifty-eight grains are assigned to this mineral based on their semi-quantitative analyses and optical properties. The grains contain approximately 25% palladium, 35% tellurium, 36% bismuth and 4% antimony (average weight percentages). Antimony, which may substitute for bismuth in the crystal lattice, shows the largest variation in concentration. The optical and chemical properties of these grains are similar to that of the mineral michenerite and the average composition may be written as Pd(Bi0.81Sb0.15)Te1.19. Similar amounts of antimony in michenerite are present in Sudbury ores (Cabri and Laflamme, 1976) Type III (testibiopalladite [PdTe(SbTeBi)]) Sixteen grains, containing 26% palladium, 37% tellurium, 20% antimony and 17% bismuth (average weight percentages), are correlated with the mineral testibiopalladite. Bismuth may substitute for antimony in the crystal lattice of these grains as suggested by Kim and Chao, (1991) for natural examples. The average composition of these grains may be written as PdTe1.17(Sb0.66Bi0.33). The Type III grains have significantly higher antimony contents than the Type II grains. Chen et al.(1993)reportedtestibiopalladite from the Thompson Mine, (Thompson Nickel Belt, Manitoba) with compositions indicating solid solution between testibiopalladite and michenerite.

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Figure 6. Compositional variation in samples from borehole SH99.

Type IV (sperrylite [PtAs2]) Only platinum and arsenic were detected in a group of grains classified as sperrylite. The composition of these grains is close to the ideal composition of sperrylite. Type V (temagamite [Pd3HgTe3 ]) Eight grains classified as temagamite were analysed. On

average these grains consist of 31% palladium, 22% mercury and 44% tellurium (weight percentages) and small amounts of Ni and Fe. The average composition for this rare mineral may be written as (Pd2.53 Ni0.30Fe0.17)Hg0.96Te2.96 comparing well with temagamite compositions reported by Edgar et al. (1989) from the Rathbun Lake deposit, Ontario although no nickel and iron are present in the analyses reported. Beaudoin

Table 6. Host minerals to the PGMs in channel samples (Relative abundance of the dominant sulphide minerals are shown in brackets in the “all PGMs” column) Host mineral Pyrrhotite Chalcopyrite Pentlandite Pyrite Magnetite Gangue Total Number of grains

All PGMs 82.7 10.8 0.6 1.9 0.5 3.6 100.0 543

(69.3) (4.0) (10.4) (2.4) (6.2) (7.8)

Type Ia 93.0 5.5 0.2 0.0 0.8 0.5 100.0 191

Type Ib merenskyite 54.2 35.3 0.0 0.0 0.6 10.0 100.0 80

Type Ic 82.6 5.1 1.3 5.9 0.0 5.1 100.0 206

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Type IV sperrylite 74.2 20.5 1.0 0.4 0.0 3.9 100.0 29

Type II michenerite 97.6 2.4 0.0 0.0 0.0 0.0 100.0 33

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Figure 7. PGM contents versus depth in borehole SH99 (relative volume percentages).

et al. (1990) reported small amounts of nickel and iron in temagamite from a Cu-Ni-PGE massive sulphide deposit associated with a peridotitic, gabbroic sill at Blue Lake (Quebec). Type VII (gold [Au,Ag]) The composition of the gold grains was not accurately determined but it appears that the gold contains between about 10 and 15% of silver in solid solution. Relative abundances of the PGMs Grains belonging to Type I (merenskyite) are by far the dominant PGMs in the samples examined and represent about 80% of the PGMs detected (Table 3). Other minerals including Type II (michenerite), Type III (testibiopalladite) and Type IV (sperrylite) each comprise between 5 and 10% of the total amount of PGMs identified. Gold makes up about 1% of the total volume of precious-metal-bearing minerals observed in the samples. Variation in the PGM assemblage with depth During the investigation it was noted that there are

variations in the relative abundances of the individual PGMs through the MSB intersected by borehole SH99 (Figure 7). In most of the polished sections Type I grains (rich in tellurium) are dominant with fewer grains belonging to Type II (rich in both bismuth and tellurium) and much less belonging to the other minerals. However, in the sections examined from the top and bottom of the MSB, Type III, which is enriched in Sb and Te, dominates. There is also a zone towards the top of the MSB, which contains a relatively high proportion of Type II grains. The gold and sperrylite “spikes” that are obvious in Figure 7 represent single, coarse-grained particles. PGM grain size distribution In general, the PGM particles display a wide variation in grain size (Figure 8) conforming to a log-normal distribution. The largest particle observed was a Type IV (sperrylite) grain with an average diameter of about 150µm. Although this single particle only makes up 0.086% of the total number of PGM grains detected, it comprises 5.0% of the total volume of PGM grains.

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H.F.J. THEART AND C.D. DE NOOY Mode of occurrence of PGMs The PGMs mostly occur as irregular to sub-rounded particles enclosed in the sulphide minerals or, less commonly, in the silicate minerals. Just over 60% (by volume) are contained within pyrrhotite particles (Table 4). This proportion is only slightly lower than the average relative abundance of the pyrrhotite. About 20% of the total PGMs occur within pentlandite particles. In the polished sections examined, pentlandite only makes up 7.5% of the opaque minerals, indicating preferential concentration of the PGMs in pentlandite. The proportion of the PGMs hosted by chalcopyrite and silicate minerals (5.5% and 13%) is similar to the relative abundances of the chalcopyrite and silicate minerals (6% and 16%). Although magnetite makes up, on average, about 5% of the ore, only about 0.1% of the PGMs are hosted by magnetite particles. A few PGM grains were observed within sphalerite and gersdorffite. The PGMs frequently occur as composite grains with other PGMs and/or Agtelluride (hessite), Pb-telluride (altaite), Hg-telluride (coloradoite) or Bi-telluride (tellurobismuthite). Argentian (silver-rich) pentlandite is closely associated with some of these composite PGM occurrences. Results for the Upper Stringer Zone in the area investigated by channel samples Samples used in the second investigation represent two sub-zones, referred to as the “upper sub-zone” and the

295

Table 7. Grain boundray relationship of the PGMs in channel samples Mode of occurrence Enclosed within mineral grain

Along grain boundaries between adjacent grains

Total

pyrrhotite chalcopyrite pentlandite pyrite magnetite silicates pyrrhotite-pyrrhotite pyrrhotite-magnetite pyrrhotite-silicate pyrrhotite-chalcopyrite pyrrhotite-pentlandite pyrrhotite-pyrite chalcopyrite-silicate other

Relative % 24.7 4.5 0.5 0.5 0.2 1.0 27.6 17.4 9.6 1.1 3.4 0.5 5.1 3.8 100.0

“lower sub-zone”. The upper sub-zone is formed by sulphide veins that may have been remobilised in the Basal Shear Zone, whereas the lower sub-zone may be regarded as part of the Upper Stringer Zone in this part of the ore body, where the massive zone is not present. These samples were collected from underground where the sulphide mineralization was exposed in the mine development over a vertical width of about 3m.

Figure 8. Grain size distribution of the PGMs and gold in borehole SH99. SOUTH AFRICAN JOURNAL OF GEOLOGY

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Figure 9. Chemical composition of channel samples of the Upper Stringer Zone.

This section may not reflect the entire mineralised zone. Graphs depicting the chemical composition of the samples, relative to the distance from the top of the channel shown in Figure 5, are provided in Figure 9. The ”upper sub-zone” displays extensive fracturing (evidence of brittle deformation) at its base and also in samples from the centre of this sub-zone. Notwithstanding the fact that the total amount of sulphide in the upper sub-zone is less than within the lower sub-zone, the upper sub-zone is richer in Pt and Pd (weighted average Pt+Pd+Rh+Au = 11.45 ppm) than the lower subzone (weighted average Pt+Pd+Rh+Au = 3.72 ppm). Rhodium grades are significantly higher in the lower sub-zone than in the upper sub-zone. The distribution of pyrrhotite, magnetite and djerfisherite is provided in Figure 10. Predominant sulphide and other opaque minerals The dominant opaque minerals in these sections are similar to those described for the samples from borehole SH99 and only the differences will be mentioned here. Between 0.1% and 14% (with an average of 2%) pyrite is present in all of the polished sections examined (Table 6) and occurs as euhedral crystals ranging in size between 5 and 200µm. The pyrite crystals, particularly the larger grains, frequently contain inclusions of magnetite, chalcopyrite and gangue.

Minor amounts of violarite were noted and where it is present it replaces pentlandite in very small veins and patches. Isolated particles of galena were observed and chemically confirmed. Most of the grains are nodular in shape and display an intense blue colour under the microscope, in contrast to the normal light bluish colour of galena. Djerfisherite (K6(Cu,Fe,Ni)25S26Cl), forms rounded, drop-like particles enclosed by pyrrhotite in samples from the top of the lower sub-zone (Figure 10). A few grains enclosed in chalcopyrite are also present. The grains are frequently surrounded by a halo of pentlandite. The typical size of the djerfisherite grains is between 10 and 100µm. The djerfisherite is present in a zone which has a significantly lower magnetite content than the zone immediately above and which contains 9 to 14% magnetite, but no djerfisherite. This mineral has also been reported from the PGE-rich massive sulphide deposit at the Oktyabrsky Mine near Noril’sk in Russia, where it occurs in an assemblage dominated by chalcopyrite with lesser galena and pentlandite and also containing froodite, Ag- and Pb-tellurides, Fe-rich argentopentlandite and cassiterite (Barkov et al. 1997). PGE and gold mineralogy The investigation of the PGMs is based on about 545 grains that were identified in the thirty-two polished

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Figure 10. Distribution of pyrrhotite, magnetite and djerfisherite in channel samples of the Upper Stringer Zone.

sections of the channel samples representing the Upper Stringer Zone. The grains investigated were assigned to similar compositional types as discussed for borehole SH99. Table 6 summarises the results of the second investigation and it is evident that Type I grains are the most abundant (93.4% by volume) of all the PGMs observed followed by Types IV and II. As the Pt content of the Type I grains varies between 1.2% and 16.3%, three sub-classes are introduced, namely, a sub-class with a low platinum content (Type Ia), one with an intermediate platinum content (Type Ib) and a high platinum sub-class (Type Ic). It should also be noted that Ni was recorded in all the Type I grains analysed from the channel samples. Harney and Merkle (1990) empirically confirmed substitution between platinum and palladium and also between tellurium and bismuth in the minerals michenerite, kotulskite, merenskyite and froodite. PGM grain size distribution More than 75% (by volume) of the PGM grains examined in this suite of samples have diameters smaller than 25µm (Figure 11) and the mode of the log-normally distributed population falls within the 15 to 20µm size

class. The three sub-classes of the Type I (merenskyite) grains have very similar grain size distributions. The size distributions obtained for Type IV (sperrylite) and Type II (michenerite) grains are based on a relatively small number of grains. However, the sperrylite grains appear to be slightly larger than the merenskyite grains, whereas all of the observed michenerite grains are smaller than 15µm in diameter. Mode of occurrence of PGMs The results of the investigation indicate that 83% (by mass) of the PGMs are hosted by pyrrhotite (Table 7). The average pyrrhotite content of the ore is 69% indicating that there is some preferential association of the PGMs with pyrrhotite. About 11% of the PGMs have chalcopyrite as the dominant host mineral. The Type Ib “merenskyite” and the sperrylite appear to be preferentially associated with chalcopyrite. Pentlandite and pyrite only host a very small proportion of the PGMs. A detailed examination of the mode of occurrence of the PGMs shows that 31.4% (by mass) of the PGMs are completely enclosed within the host mineral grain

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Figure 11. Grain size distribution (Volume %) of the PGMs in the channel samples.

(Table 7). Of these enclosed PGMs, almost 80% (by mass) occur within pyrrhotite while 15% occur within chalcopyrite. The remaining 68.6% of the PGMs occur along the grain boundaries between adjacent mineral grains. The majority (65%) occur along the boundaries between adjacent pyrrhotite grains or along the boundaries between a magnetite grain and the surrounding pyrrhotite. Discussion The Uitkomst Complex has been genetically related to the much larger Bushveld Complex (de Waal et al. 2001, in press). However, when the PGM assemblage of the massive sulphide ore of the Uitkomst Complex is compared to the Merensky- and UG-2 and Platreef ores of the Bushveld Complex, some remarkable differences are apparent. Ores of the Merensky Reef from different areas display variation in the relative abundance of individual PGM minerals, but the assemblages are dominated by PGE-sulphide minerals, such as braggite, cooperite and laurite (Schwellnus et al., 1976; Kingston and El-Dosuky, 1982; Mostert et al., 1982). Sperrylite was reported as the most abundant mineral only in the Western Platinum Mine area investigated by Brynard et al. (1976). The PGE-bearing phases of the UG-2 Reef are more complex, frequently displaying the effects of hydrothermal activity that probably altered the PGM phases of the reef (Penberthy and Merkle, 1999, Peyerl, 1983, and Kinloch and Peyerl, 1990). Although these authors recorded the presence of sulpharsenides and

bismuthotellurides, similar mineral phases as reported for the Merensky Reef predominate (McLaren and de Villiers, 1982) with sulphides representing more than 90 volume percent of the PGM assemblage in unaffected rocks (Penberthy and Merkle, 1999). The prominence of Pt-Fe alloys is a notable feature in some areas of the UG-2 Reef, where it is affected by replacement pegmatoid (Penberthy at al. 2000). The Platreef of the Bushveld Complex has some features that are similar to the Basal Group of the Uitkomst Complex. These include a higher content of xenoliths and sulphide minerals in the former. Notwithstanding these similarities, the PGM assemblage of the Platreef differs significantly from the MSB. Isoferroplatinum, sperrylite, cooperite, merenskyite, and semi metal alloys are characteristic of the Platreef, and the ores display a Pt/Pd ratio of 1:1 (Lee, 1996). The only PGM assemblage reported from the Bushveld Complex that is somewhat similar to the Uitkomst massive sulphide assemblage was found in a PGE-enriched horizon of the Upper Zone as reported by Harney and Merkle (1990). In these rocks the assemblage consists of merenskyite, michenerite, froodite and sperrylite. The mineral assemblage from the ores of the Uitkomst Complex displays some similarities with the palladium-dominated PGM assemblage reported for the nickel sulphide ores of the Sudbury intrusion (Cabri and Laflamme, 1976). This assemblage is dominated by michenerite, moncheite, sperrylite, sudburyite, froodite, insizwaite and merenskyite. It is interesting to note that most of the samples analysed by Cabri and Laflamme

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H.F.J. THEART AND C.D. DE NOOY (1976) were collected from, or close to, the footwall contact of the intrusion. The predominance of merenskyite, sobolevskite, moncheite, froodite and michenerite in the PGM assemblages of the coppernickel-PGE deposits in the footwall of the Sudbury intrusion (Farrow and Watkinson, 1997) is perhaps the most significant. In these cases the associated sulphide ores occur in geological settings, relative to the parent intrusion, that is somewhat similar to that of the MSB of the Uikomst Complex. In such an environment, the igneous, immiscible sulphide phase, responsible for the mineralization, attained hydrothermal characteristics. The substitution described above for Types Ia, Ib and Ic of merenskyite is empirically confirmed in the results of Harney and Merkle (1990). These authors reported substitution in the minerals michenerite, kotulskite, merenskyite and froodite between platinum and palladium and also between tellurium and bismuth. Hoffman and MacLean (1976) suggested that the melting temperature of michenerite is strongly dependent on the respective tellurium and bismuth concentration, varying from 489° C for the bismuth end-member to 501° C for the tellurium end-member. It may be concluded from this that the PGM assemblage of the MSB formed under conditions favouring the formation of the minerals in the quaternary system Pd-Bi-Te-Sb. This must include both physical conditions, such as temperature and pressure, as well as the chemical concentrations of the relevant elements. The mineral assemblage probably formed under lower temperatures (probably below 490°C, according to the experimental work of Hoffman and MacLean, 1976) than those which would have favoured the PGE-sulphide minerals of the unaltered Merensky and UG-2 Reefs. Such conditions of formation may have been more similar to those applicable to the formation of the alteration PGM assemblage in the Merensky and UG-2 Reefs, as reported by Harney and Merkle (1990), Penberthy and Merkle (1999) and Penberthy et al. (2000). The PGM assemblage of the Uitkomst massive sulphide ore and the footwall Cu-Ni-PGE deposits of the Sudbury area (Farrow and Watkinson,1997) probably formed under similar conditions. Conclusions The palladium-enriched nature of the MSB is clearly demonstrated in the data presented. The average Pt : Pd ratio of the SH99 intersection (Figure 6) is 1 : 3.4, and the channel samples have a ratio of 1 : 2.3 (Figure 9). This is further confirmed by the average composition of the MSB (Table 2), which is based on 23 surface borehole intersections, with a Pt : Pd ratio of 1 : 2.5, that is very similar to that of the channel samples. When this is compared to the Pt : Pd ratios of the PGMs from the two localities investigated (about 1 : 34 for SH99 and 1 : 13 for the channel samples) it is evident that Pt is not fully accounted for in the PGMs reported. This may have been caused by the “nugget” effect of sperrylite, as alluded to in its grain size distribution (Figures 8 and 11), which would imply that the number of samples

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investigated was insufficient. An alternative or additional possibility is that some of the platinum is contained as solid solution in one or more of the major sulphide minerals such as chalcopyrite, pentlandite or pyrrhotite. The latter possibility was not investigated in the studies reported here. As no rhodium-bearing PGMs were detected during this investigation, it is assumed that this element, together with elements such as iridium, ruthenium and osmium, is contained in solid solution within the sulphides. The latter has also been reported for the massive sulphide ores from the Talnakh ore junction at Noril’sk, Siberia (Distler and Kunilov, 1994). In general, similar PGM assemblages have been identified in the two parts of the ore body investigated, but significant differences were observed in the relative abundance of the minerals. Grains belonging to Type I (merenskyite) dominated the PGM assemblages of the samples studied in both areas and it would be reasonable to suggest that this is a general feature of the massive sulphide ore of the Uitkomst Complex. As may be seen in figures 8 and 11, the grain size (mode in the class interval 21 to 30µm) of the PGMs examined from borehole SH99 is significantly larger than the grains examined from the channel samples (mode: 10 to 15µm). Other differences are the lack of a distinct platinum-rich variety of merenskyite (Type Ib and Ic) in the samples from SH99 and the relatively low antimony content of “michenerite” grains from the channel samples. Most of the PGMs (90-95% by volume) fall in the compositional quaternary Pd-Bi-Te-Sb system. Palladium was only detected in tellurium-bearing phases (mainly merenskyite, michenerite and temagamite). In samples from SH99, the grains belonging to these phases do not contain significant amounts of platinum together with the palladium. Sperrylite is the only platinum-bearing phase identified in SH99. It is possible that early removal of sperrylite grains during the formation of the minerals may explain the absence of platinum (and arsenic) from the Pd(Te,Bi,Sb) minerals. In contrast, both the Type Ib and Type Ic merenskyite grains from the channel samples contain platinum. Testibiopalladite (Type III) grains are concentrated near the top and bottom of the MSB studied in SH99. This would imply a concentration of antimony towards the edges of the MSB. The PGMs are nearly always present in polyminerallic associations such as the merenskyite and michenerite (Type I and II, respectively) with other tellurides such as silver telluride and lead telluride. These associations indicate contemporaneous crystallization of all of the tellurides, bismuthides, antimonides and bismuth tellurides. The association of merenskyite and michenerite is attributed to partial solid solution on the PdTe2-PdBiTe join of the Pd-Te-Bi system (Shvedov et al. 1997). Differences in the chemical and mineralogical characteristics of the upper and lower sub-zones of the Upper Stringer Zone investigated in the channel samples may indicate that these sulphide ores formed under

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different conditions. In particular, the lower magnetite content of the lower sub-zone when compared to the upper sub-zone may indicate a lower oxygen fugacity during the formation of the former. The presence of djerfisherite at the top of the lower sub-zone is possibly related to the distribution of the magnetite. From a metallurgical point of view, the PGMs are rather fine-grained (about 75% by mass of the PGM grains are smaller than 60µm for the samples from SH99 and smaller than 25µm for the channel samples). This may contribute to losses during beneficiation of the ore. In addition, 83% by mass of the PGMs have pyrrhotite as the dominant host mineral. Since the pyrrhotite is largely rejected during the current beneficiation process, unliberated PGMs may report with the enclosing pyrrhotite to the tailings fraction. Fortunately, the location of about 70% of the PGMs along mineral grain boundaries should facilitate liberation of the PGM particles during milling and result in recoveries higher than those indicated by the small grain size and the close association of the PGMs with pyrrhotite. In contrast, PGMs hosted by silicate and carbonate minerals will potentially be lost to the tailings. Acknowledgements We thank Anglovaal Mining Limited and the Nkomati Mine for permission to publish the results of this investigation. Archie Adlington-Corfield is acknowledged for the SEM analyses, Werner Glatthaar for assistance with the examination of the polished sections and the late Mack Segwapa for collection of the channel samples. M. Kendrick and B. G. Askes draughted some of the figures. Professors S. A. de Waal, R. K. W. Merkle and J. P. R. de Villiers, Dr. S M. C. Verryn and Mr. R. A. Hornsey kindly reviewed early versions of the paper. The first author wishes to especially thank Prof. R. K. W. Merkle for his encouragement and introduction to the literature on the PGMs. Dr. L J Cabri and Prof. A. L. Wilson reviewed the paper on behalf of the South African Journal of Geology. References Anglovaal Minerals Ltd. (1995). Official press release, 19 July. 1p. Barkov, A.Y., Laajoki K.V.O., Gehor, S. A., Yakuvlev Y.N. and Taikinaaho O. (1997). Chlorine-poor analogues of djerfisherite-thalfenisite from Noril’sk, Siberia and Salmagorsky, Kola Peninsula, Russia. Canadian Mineralogist, 35, 1421-1430. Beaudoin, G., Laurent, R. and Ohnenstetter, D. (1990). First report of platinum-group minerals at Blue Lake, Labrador Trough, Quebec. Canadian Mineralogist, 28, 409-418. Brynard, H.J., de Villiers, J.P.R. and Viljoen, E.A. (1976). A mineralogical investigation of the Merensky Reef at the Western Platinum Mine, near Marikana, South Africa. Economic Geology, 71, 1299-1307. Cabri, L.J. and Laflamme J.H.G. (1976). The mineralogy of the platinumgroup elements from some copper-nickel deposits of the Sudbury area, Ontario. Economic Geology, 71, 1159-1195. Chen, Y, Fleet, M.E. and Pan, Y. (1993). Platinum-group minerals and gold in arsenic-rich ore at the Thompson-Mine, Thompson Nickel Belt, Manitoba, Canada. Mineralogy and Petrology, 49, 127-146. de Waal, S. A., Maier, W. D., Armstrong R. A. and Gauert C. D. K. (2001, in press). The age and parental magma of the Uitkomst Complex, South Africa. Canadian Mineralogist. Distler V.V. and Kunilov V. E. (1994). Geology and Ore deposits of the

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