Platinum Mineralization Research Articles

Applications of Conditional Simulation to a Positively Skewed Platinum Mineralization

Any mine planning requires careful prediction of both the head grade andtonnage ofmineralization. There are various methods of interpolation that attempt to provide reasonable estimatesat unsampled locations. All of these give realizations that are unduly smooth and extremevalues that occur in reality are not reflected in these estimates. Such methods, therefore,provide limited scope for accurate risk assessment. An alternative approach that is rapidlygaining popularity is the method of conditional simulation. This approach attempts to reproduceboth the grade distributions of the sample data as well as its spatial variability. In this paper,a case study is presented on a platinum mineralization to demonstrate and compare sequentialGaussian and sequential conditional simulation techniques and to quantify and discuss therelevant sensitivities.

Yixunite and Damiaoite—A Twin of New Native Alloys of Indium and Platinum from the Yanshan Mountains

AbstractYixunite and damiaoite were found in a cobalt‐ and copper–bearing platinum ore vein of a contact metasomatic deposit. The chief ore minerals are bornite, chalcopyrite, magnetite and carrollite. The platinum minerals include moncheite, sperrylite, daomanite, cobalt malanite and cooperite. Yixunite and damiaoite occur as immiscible globules, 1.0 to 2.0 mm in diameter. Yixunite is always in the central part of a globule. It is opaque with metallic lustre, bright white colour and black streak. HM=5.8; VHN50 = 634 kg/mm2 (573–681 kg/mm2); insoluble in HCl, HNO3, HF or H3PO4; no cleavage; no magnetism. Density is hard to measure because of small grain size. Calculated density = 18.21 g/cm3. Reflective colour is bright white with a yellowish tint. Isotropic. The mean analytical results (ranges) (%) are: Pt 82.8 (81.8–83.6), In 16.4(15.6–17.1) and total 99.2. The empirical formula (based on 4 atoms) is Pt2.993 In1.007. The five strongest lines of X–ray diffraction (hki, d, I) are 111, 2.30 (100); 200, 1.99(60); 202, 1.411 (40); 311, 1.203 (80); 222, 1.151(40). Space group: Pm3m with a = 0.3988(3) nm and Z= 1. Damiaoite occurs as single globules or was exsolved from yixunite. Opaque with metallic lustre; bright white colour with black streak; HM = 5.3; VHN50 = 485 kg/mm2 (434–529 kg/mm2); insoluble in HCl, HNO3, HF or H3PO4; cleavage: no; magnetism: no. Density: hard to measure because of small grain size. Calculated density = 10.95 g/cm3. Reflective colour is bright white with a yellowish tint. Isotropic. The mean analytical results (ranges) (%) are Pt 45.6 (45.4–46.0), In 53.5 (52.4–53.9), total 99.1. The empirical formula (based on 3 atoms) is P1.002 In1.998. The six strongest lines of X–ray diffraction (hkl. d. I) are 220,2.25 (100); 311,1.92 (60); 400,1.59 (60); 422,1.299 (80); 440,1.125 (60); 620,1.006 (70). Space group: Fm3m with a = 0.6364 (3) nm and Z = 4.

97/01904 First finding of platinum minerals in coals

97/01904 First finding of platinum minerals in coals

Application of scanning proton microprobe and neutron activation analysis to studying the platinum distribution of a low-gradeplatinum deposit

In the ores of low-grade platinum deposits, the platinum concentration is very low(average Pt content <1 μg. g~(-1)), and the platinum minerals often occur as micron grains,thus making it difficult to study the platinum distribution oniy by routine tools, such aselectron microprobes or optic microscopes. As a result, the research and exploitation ofthese deposits in our country is hampered by the lack of more powerful tools.

Dunite of the Ioko-Dovyrensk massif

The dunite zone of the Ioko-Dovyrensk massif is 15 km long and 870 m thick. The dunites are fresh and possess physicochemical properties that make them a suitable raw material for the production of magnesiasilicate refractories. The weathering crust can be a source of molding sand. The North-Baikal Geological Prospecting Expedition (GPE) and the Locomotive and Car Repair Plant (Ulan-Ude) have tested four technological samples of olivine sands, three of which tests gave positive results. The creation of a base for the production of magnesia-silicate refractories in Siberia will make it possible to reduce the import of raw materials and even to export domestic products. The possibility of practical use of the dunites, the discovery of a low-sulfide metallic platinum mineralization, and the considerable growth of the prospected resources of nickel, copper, and cobalt make the development of the Ioko-Dovyrensk massif a quite promising undertaking.

Contribution to the petrogenesis of the platinum palladium-bearing black dunites of the Acoje block, Zambales ophiolite, Philippines: selenium and sulfur-contents, clinopyroxenes, initial weathering

Abstract Black serpentinites (BS), hosting the NiCu sulfide, platinum and palladium mineralisation in the Acoje ophiolite block in contact with chromitite concentrations, were investigated in comparison to the non-mineralised green serpentinites (GS), concerning their selenium and sulfur bulk rock contents, their secondary clinopyroxene, pyroaurite and hematite formation. Se/S-ratios in the black serpentinites range from 3.4 × 10 −3 to 4.8 × 10 −4 . They are in the same order of magnitude as Se/S-ratios analysed in individual sulfides (Orberger and Taxel 1991) and consistent with the suggested mantle Se/S-ratios of 10 −4 (Eckstrand R., Goldschmidt Conference, Baltimore, 1988). A positive correlation of selenium and platinum/palladium is found for this ophiolitic environment. This correlation is proposed for high PtPd potential magmatic sulfide deposits in layered intrusions (Eckstrand 1988); thus, this study confirms the magmatic character of the NiCu-sulfide mineralisation proposed by Abrajano and Pasteris ( Tectonophysics 168 , 65–100, 1989). Two types of diopsides characterise mainly the black serpentinites. Diopside I is present in green and black serpentinite and shows CaO contents of about 21 wt%, and Cr 2 O 3 contents of 0.54–0.92. Diopside II is mainly present in black serpentinite. It is extremely CaO-rich (24.0–25.4 wt% CaO) and significantly lower in Cr 2 O 3 (0.04–0.54 wt%). Initial weathering is mainly observed in BS and is expressed in the formation of pyroaurite and locally Fe-oxide and -hydroxide, surrounding the base metal sulfides, and the formation of Fe-rich “serpentines” (up to 11 wt%), forming large black platy aggregates in the clay fraction of BS. Pyroaurite-formation buffers pH and retains PtPd during weathering, whereas hematite and goethite formation provide sufficiently oxidising conditions for a PtPd-transport. The black platy aggregates in the clay fraction of the black serpentinites might be responsible for its black color.

Experimental investigation of the hydrothermal geochemistry of platinum and palladium: I. Solubility of platinum and palladium sulfide minerals in NaCl/H 2SO 4 solutions at 300°C

Approximately 150 silica tube experiments were used to determine the solubility of Pt and Pd sulfides in NaCl/H2SO4 solutions at 300°C. Oxidation state was controlled by equilibrium between aqueous sulfate and coexisting solids. Buffering assemblages included Pd + Pd4S, Pd4S + Pd16S7, Pd16S7 + PdS, Pt + PtS, and PtS + PtS2. Solution pH was controlled by the dissociation of H2SO4 (0.1 or 1.0 m), adjusted for ion pairing. Salinities ranged from 0.01 to 3.0 m NaCl. Under these conditions, measured Pt and Pd solubilities varied over a wide range ( 1000 ppm), indicating dissolution as chloride complexes of high ligand number. Detailed interpretation of the results gives equilibrium constants for the following reactions: Pd(s) + 2H+ + 4Cl− + 12O2(g) = PdCl42− + H2O (a1) Pt(s) + 2H+ + 3Cl− + 12O2(g) = PtCl3− + H2O, (a2) logK(300°C, Psat) = +12.65 ± 0.5 (reaction al) and +14.84 ± 0.3 (reaction a2). Our experimentally derived constants are in fair (Pt) to excellent (Pd) agreement with previously published estimates based on extrapolation of low-temperature data. The temperature dependence of reaction (a1) is described by the following polynomial (valid between 25 and 300°C): logKal = 22.53 − 7.398e − 02T + 1.433e-04T2 − 1.034e − 08T3 (T = °C). Our study indicates that chloride complexes may contribute significant solubilities (> 1 ppb) of Pt and Pd at low to moderate temperature, but only under conditions which are highly oxidized (hematite stable), highly saline (>3 m NaCleq), and/or unusually acidic (pH < 4). In the presence of aqueous sulfide, Pt-group element (PGE) solubilities as chloride complexes are extremely low (<1 ppt) and are probably less than the total contribution from other aqueous species (e.g., bisulfide, hydroxy, or ammonia complexes). However, the very strong temperature dependence of reaction (a1) suggests that much higher PGE mobility may be expected at supercritical temperatures. PGEs dissolved as chloride complexes may be deposited in response to reduction, increase in pH, dilution, and/or temperature decrease. Of these, reduction is the most effective mechanism and explains the common association of anomalous PGEs with strongly reduced horizons within sedimentary basins.

The Geordie Lake Intrusion, Coldwell Complex, Ontario; a palladium- and tellurium-rich disseminated sulfide occurrence derived from an evolved tholeiitic magma

The Geordie Lake intrusion is a small body of sulfide-bearing mafic rocks that has intruded contemporaneous syenites of the Coldwell alkaline complex, northwestern Ontario. The intrusion consists of alternating layers of troctolite containing dendritic olivines and ophitic gabbros. Cumulate rocks are absent and the rocks are interpreted to be a series of rapidly cooled intrusions of tholeiitic basaltic magma. Compositional data are presented for olivine, plagioclase, alkali feldspar, clinopyroxene, amphibole, magnetite, and ilmenite. These data, together with whole-rock major and trace element analyses, demonstrate that the Geordie Lake magma was relatively evolved at the time of emplacement. The formation of secondary amphiboles and biotite together with low equilibration temperatures calculated for coexisting Fe-Ti oxides demonstrates that extensive deuteric subsolidus reequilibration has occurred.Sulfides consist principally of disseminated chalcopyrite associated with minor bornite, pyrite, millerite, cobaltite, siegenite, sphalerite, and galena. Tellurides (hessite, melonite, altaite) and platinum-group minerals are associated primarily with disseminated chalcopyrite and are rare in late-stage massive chalcopyrite. The platinum-group minerals are dominantly palladium tellurides and arsenides. Rare sperrylite is the only platinum mineral present.Textural relationships suggest that rapid cooling of the magma and crystallization of olivine and magnetite induced the early separation of an immiscible sulfide melt. This melt remained as a liquid until the silicate host rocks had cooled to subsolidus temperatures. During the final stages of crystallization of the silicate magma the sulfide melt and residual water-rich fluids were trapped together within pore spaces. Crystallization of Ti-poor magnetite, tellurides, and platinum-group minerals from the sulfide melt occurred at low temperatures (<550 degrees C). The fugacities of sulfur and tellurium prevailing during crystallization of the sulfide melt are estimated from predominance diagrams. Pyrrhotite, pentlandite, and Ir-, Os-, and Rh-based minerals are absent from the sulfide assemblage. The predominance of Pd and Cu tellurides and sulfides, coupled with low Pt(Pt + Pd) and high Cu/(Cu + Ni) ratios, suggests that the Geordie Lake platinum-group minerals-sulfide assemblage crystallized from a relatively evolved sulfide melt associated with an evolved tholeiitic magma.

The Ethiopian mining sector

The Ethiopian Constitution of 1928 vests in the state ownership of all mineral resources. However, the mining code of 1944 recognized the right of individuals and private companies to explore for and develop those resources. The results were quite impressive; Ethiopia became a producer of gold, platinum, copper and other minerals. A revised mining code in 1971 further extended the rights of individuals and companies to develop the mineral resources of the country. In 1974, the newly established revolutionary government eliminated private ownership of mines and established instead state-owned mining companies to develop the mineral resources of the country. In time, the demand for government funding by other sectors of the economy left little for the mining sector and it went into decline. To correct that problem, the Government of Ethiopia in 1989 decided to again make private ownership in the mining sector legal. This paper describes some of the recent initiatives to open the mining sector to private enterprise.

Platinum mineralization in the Alaskan-type intrusive complexes near Fifield, N.S.W., Australia. Part 2. Platinum-group minerals in placer deposits at Fifield

The Fiield Platinum Province, New South Wales, Australia, contains Alaskan-type mafic-ultramafic complexes with notable PGE mineralization. This mineralization has evolved in several stages ranging from high- to low-temperature hydromagmatic, with weathering and latertic concentration followed by erosion and placer accumulation in alluvial channels. This last stage has furnished the largest quantity of platinum from the area. PGM nuggets were collected in a corridor, south and southeast of Fifield, joining the leads exploited in the past and continuing northwards. The following PGM association was observed in the nuggets: isoferroplatinum, native osmium, osmiridium, iridosmine, laurite and bowieite. Isoferroplatinum is the matrix mineral in the nuggets, while (It, Os, Pt) alloys occur as exsolutions. The nuggets sometimes carry inclusions of euhedral chromite. Two sulphidic phases-laurite and bowieite-are rare and occur embedded as trapped early formed crystals in isoferroplatinum. This PGM association is entirely different, when compared to the PGM in pegmatoid clinopyroxenites (“P-units”) described previously (Johan et al., 1989), both as to the nature of the mineral species and to the chemistry of isoferroplatinum common to both occurrences. This proves conclusively that the source for the alluvial nuggets must be of a different nature from the presently established hard rock mineralization. The co-existence of iridosmine and osmiridium exsolutions in isoferroplatinum proves a high temperature origin for the nuggets, and excludes a low temperature origin through in situ overgrowth from aqueous solutions in either the laterite or the erosive profile. The presence of chromite inclusions of a certain diagnostic composition indicates that the original host rock for the nuggets was a chromitite within dunite. The chemistry of the coexisting alloy phases provides a temperature estimate of about 800 °C, which is compatible with Alaskan complexes in Alaska and the Urals. The textural study of the nuggets shows different rates of cooling from nugget to nugget, with many textures relating to heterogeneous nucleation in metallurgical processes.

Mineralogy of PGE in the mafic-ultramafic massifs of the Kola region

During the last few years nearly 50 minerals and unnamed platinum-group element (PGE) phases have been identified by the authors in nickel-bearing mafic-ultramafic massifs of the Kola region. About 20 established PGE minerals have been found whilst more than 20 previously undescribed and unnamed PGE phases can be reported. In addition high concentrations of these elements in sulfarsenides of the cobaltitegersdorffite series have been established. The PGE phases form tiny (5–70 µm, seldom larger) mono- and polymineral inclusions in the main sulfides of the nickel-copper mineralization; at the contacts with silicates and within the silicates themselves. The details of the composition of platinum mineralization in various massifs, its formation under magmatic and hydrothermal conditions and wide isomorphism in certain groups of minerals have been established. Specific features of PGE mineralization of the region as a whole are: the predominance of bismuth tellurides and arsenides of palladium and platinum, a highly subordinated role of their sulphides and higher concentrations of several PGE in sulfarsenides.

Economic mineral resources: activities in 1990

The most exciting discovery in mineral exploration in Greenland in 1990 was announced by Platinova Resources Ltd. and Corona Corporation. A single packsack drill hole intersected a gold-platinum rich zone in the layered mafic Kap Edvard Holm complex of East Greenland (Fig. 1). The mineralised zone consists of an upper gold-rich section overlying a platinum-rich section. Gold values averaged 2.6 ppm over 1.5 m while the bottom section of the hole yielded 3.4 ppm platinum. Surface sampling indicates that the platinum mineralisation extends 1.5 to 2 m below the bottom of the drill hole. The mineralised zone occurs in a well-defined layered sequence which can be followed for more than 10 km.

Platinum mineralisation potential of the Longwood Igneous Complex, New Zealand

The Longwood Igneous Complex is situated at the southern tip of the South Island of New Zealand. The Complex is 32 km long and up to 12 km wide and is part of a more extensive north-trending belt of late Paleozoic volcanic, volcaniclastic and intrusive rocks. The complex is comprised of an extensive series of layered gabbros (Pahia Layered Series) together with diorites, trondjhemite and granitoids. Exploration by Sigma Resources has located significant platinum-in-drainage anomalies which are derived from a 10 km x 1 km area of largely plagioclase-rich, olivinebearing cumulate rocks within the Pahia Layered series. Grains of platinum and palladium sulphide, arsenide and alloy minerals have been identified in placer deposits derived from this area. A total of 1,500 ounces of platinum was reported to have been recovered as a by-product of historic placer gold mining of these deposits. Platinumgroup element mineralisation in float rocks (up to 3.0 ppm Pt and 3.3 ppm Pd) and significant platinum-in-soil anomalies (peak value 0.47 ppm Pt) have been found within this area. The mineralised troctolite and olivine-gabbro float and adjacent exposures are broadly similar to the anorthosite-troctolite-olivine gabbro zones within the Banded Series of the Stillwater Complex. The mineralised rocks are leucocratic plagioclaseolivine adcumulates with minor clinopyroxene oikocrysts, primary amphibole and sparse (<0.5%) sulphide. Sulphide minerals are pyrrhotite, chalcopyrite, pyrite and secondary copper sulphides. The similarity of this mineralisation style and its host rocks to the JM Reef of the Stillwater Complex and AP and PV mineralisation of the Penikat Intrusion highlights the excellent potential for discovery of a large stratiform platinumgroup element deposit in the Pahia Layered Series.

Interpretation of soil nitrite-ammonium ratios with possible application to pedogeochemical exploration for platinum mineralization

Abstract The nitrogenous compounds ammonia, ammonium, nitrite, nitrate, and diatomic nitrogen play important roles in the geochemical cycle of nitrogen in the near-surface environment. Ammonia or ammonium (ammine species), depending on the pH, is the stable reduced species, nitrate is the stable oxidized species, and nitrite is a metastable intermediate species. Nitrogen is considered unimportant in the present context because of the impracticality of attempting to detect subtle variations in diatomic nitrogen abundances in Earth's highly nitrogenous atmosphere. Platinum is known to be an effective catalyst of the ammine-nitrite oxidation step. It is shown in laboratory experiments, and field studies involving rocks and soil that the nitrite-ammonium ratio is higher in the presence of Pt than in its absence. In addition, Pd also catalyses this oxidation reaction, but only one-fourth as effectively as Pt. Ammonium and nitrite are significantly more mobile in the weathering environment than are the platinoid elements, suggesting that the nitrite-ammonium ratio my have some value as a pathfinder for platinum group element (PGE) mineralization. Soils collected across a platiniferous vein at Crescent Peak, Clark County, Nevada, serve to illustrate the potential of this technique. Copper, Zn, and Au in soils indicate the presence of mineralized bedrock. Platinum and Pd values are quite low, but the high nitrite to ammonium ratio above the mineralized zone suggests the presence of platinoid elements in underlying bedrock. Background nitrite-ammonium ratios do not exceed 0.6, whereas soils above the PGE-bearing rock have values greater than 6.0. Preliminary bedrock data from the Stillwater Intrusive, Montana, indicate nitrite-ammonium ratios in unmineralized anorthosite and mineralized J-M Reef similar to those of barren and mineralized soils, respectively, at Crescent Peak. This result not only supports the Crescent Peak findings but suggests the possibility of applying this methodology to lithogeochemical exploration.

Platinum mineralization in the Alaskan-type intrusive complexes near Fifield, New South Wales, Australia part 1. platinum-group minerals in clinopyroxenites of the Kelvin Grove prospect, owendale intrusion

Several Alaskan-type complexes intrude metasedimentary rocks of the Cambro-Ordovician Girilambone Group near Fifield, New South Wales, Australia. These intermediate to ultramafic circular intrusives, emplaced during the Devonian, are composed of monzodiorite, gabbro-norite, clinopyroxenite, hornblendite, peridotite and dunite. Several types of PGE mineralization associated with these complexes are known. A new type described here occurs within pegmatoid, biotite- and magnetite-poor clinopyroxenites (“P-units”) which form irregular lenses and vein-like bodies within biotite- and magnetite-rich, fine grained clinopyroxenites. The mineralized "P-units" are not enriched in base metal sulphides with respect to the surrounding barren clinopyroxenites.

Platinum group metals in common plants of northern forests: developments in analytical methods, and the application of biogeochemistry to exploration strategies

Abstract All platinum-group metals (PGM) in ashed plant tissues, from an area of platinum mineralization in Saskatchewan, have been measured by neutron activation analysis of a NiS fire assay bead. Concentrations of up to 1350 ppb (ng/g) Pd, 880 ppb Pt, 49 ppb Rh, 37 ppb Ru, 24 ppb Ir, and 15 ppb Os occur in ashed twigs of black spruce ( Picea mariana ), indicating that the ratios of PGM uptake are about the same as those occurring in the bedrock. Plants growing on drift-covered diabase, known to have about 100 ppb PGM, contain up to 77 ppb Pt in ash, demonstrating the potential value of biogeochemical methods in helping to delineate platiniferous zones. Twigs of black spruce, jack pine, and labrador tea appear to be the optimum sample media in this environment. A rapid multi-element neutron activation analysis by direct irradiation of tissue permits measurement of 0.05 ppb Ir in dry material, or 2 ppb Ir in ashed samples. In view of the normally very low concentrations of Ir in plants, any detectable Ir probably indicates enrichment of PGM in the substrate. Comparison of several wet-chemical analytical methods indicates that the optimum (cost-effective) technique for detecting 1–2 ppb Pt, Pd, or Rh in 2-g samples of ash is by Te co-precipitation (following fusion and dissolution), and analysis by ICP-MS with sample introduction by electrothermal vaporization. However, care must be taken to ensure complete dissolution of all PGMs in the ash.

PIXE analysis of mineral matter in thin sections of human lung

Abstract It is postulated that insoluble mineral residues in the lungs of deceased miners may provide a quantitative measure of the integrated lifetime dust exposure. For epidemiological surveys rapid instrumental techniques are required to analyse representative samples of lung tissue. Particle-induced X-ray emission (PIXE) has been evaluated for analysis of microtomed slices of wax-embedded lung and lymph node (Hilar gland) tissue from deceased miners. The 50 μm slices, mounted on Mylar backings and placed in a He atmosphere, were irradiated using 3.2 MeV protons. PIXE analysis provided adequate sensitivity for key mineral elements including Si, Cr and Ti. The porous, nonuniform nature of lung tissue made it impossible to measure the tissue mass in the irradiated area, preventing the calculation of mass concentrations. Instead, biological sulphur was used as an internal standard, assuming that the fraction of S in soft, fat-free tissue is constant. Results are presented for lung and lymph node tissue from gold, chrome, copper, platinum and asbestos miners. Si mineral residues in lymph node tissue were found to be concentrated by a factor 50 relative to lung. Cr residues were clearly observed in the chrome miner's lung, but no excess of Cu was present in the copper miner's lung. There is evidence of preferential Si removal relative to Ti. Results warrant further development of PIXE for scanning of large numbers of lung samples prepared in the above manner.

Platinum-group minerals from the Kambalda nickel deposits, Western Australia

The relative abundance, compositional variability, and geologic occurrence of platinum-group minerals have been determined by a study of samples from a number of geologic environments in the Kambalda nickel deposits. These results have been integrated with findings from a previous study by Hudson and Donaldson (1984) of platinum-group minerals in amalgamation residues from the gold recovery circuit of the Kambalda nickel mill, to give an overall picture of the nature and distribution of platinum-group minerals at Kambalda.The major platinum minerals are sperrylite and moncheite, and the major palladium minerals are sudburyite, merenskyite, stibiopalladinite, palladoarsenide, michenerite, and testibiopalladite; palladium also occurs in solid solution in palladian melonite. Irarsite is the only iridium mineral recognized, and no discrete phases have been observed for the other platinum-group elements. Sperrylite occurs within massive pyrrhotite-pentlandite ores and is particularly abundant in ores that are rich in chalcopyrite. A high proportion of the Pt in the Kambalda nickel deposits is believed to occur as sperrylite, but not all sperrylite is sufficiently coarse in grain size to enable gravity concentration. Nevertheless, sperrylite is the dominant platinum-group mineral recovered in gravity concentrates.Discrete palladium minerals are most abundant in stringers of sulfide in the footwall to the ore, in crosscutting sulfide veins, or in reaction zones associated with hydrothermal veins and porphyries. Within the massive and matrix ores, coarse discrete palladium minerals are rare, and it is concluded that palladium occurs in solid solution or as finely dispersed submicroscopic grains in sulfide minerals, predominantly pentlandite. The occurrence, within the ore zones, of Pt as sperrylite and Pd dispersed in pentlandite is believed to reflect a primary magmatic distribution. However, the presence of sudburyite, moncheite, merenskyite, michenerite, testibiopalladite, and palladian melonite in stringers and reaction zones indicates that their formation may be related to postmagmatic processes, in particular metamorphic segregation of sulfides and the interaction of ore sulfides with younger hydrothermal fluids.

Platinum-group element-chromitite associations in the Bushveld Complex

New and recently published platinum-group element data of chromite-rich layers in the Bushveld Complex are evaluated. Chondrite-normalized platinum-group element plots of these data show that trends for chromitite layers differ from that of the Merensky reef. The data furthermore indicate remarkable consistencies in compositional trends for many kilometers along strike of individual layers, suggesting that the conditions of formation of chromitite layers and their associated platinum-group element mineralization were very uniform over large areas of the magma chamber. Significant differences in the proportions of platinum-group elements within a particular layer are only evident on a regional scale.The platinum-group minerals of the middle group of chromitite layers in the Marikana area of the western Bushveld are mainly sulfides and arsenides. Laurite is the most common platinum-group mineral and is mostly enclosed by chromite. A variety of other platinum-group minerals occur in close association with base metal sulfides. Extremely low bulk sulfur content in the chromitite layers is ascribed to a postcumulus loss of sulfur. Postcumulus loss of sulfur and the existence of laurite enclosed within chromite casts doubt on the validity of any petrogenetic models for platinum mineralization in chromitite-rich layers based on partition coefficients and the R factor.

Sulfide and platinum mineralization in the Merensky Reef: evidence from hydrous silicates and fluid inclusions

The base metal sulfides of the Merensky Reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates. Biotite and phlogopite crystallized in the vicinity of sulfides from a volatile-enriched highly fractionated intercumulus melt. Amphibole, chlorite, and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase. Biotite is often accompanied by zircon, rutile, and quartz. Accessory quartz hosts a complex suite of H2O-NaCl-(CaCl2)-CO2-CH4 fluid inclusions which have thus far not been described from the Merensky Reef. The earliest fluid inclusion compositions are NaCl-(H2O) with less than 10 vol.% water; CO2 coexisting with a halite daughter crystal and brine; and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations. The maximum trapping temperature is around 730° C at 4 to 5 kb pressure. Later inclusion generations are H2O-NaCl, CO2-H2O, and pure CO2 and CH4. The presence of Cl-rich fluids during the intercumulus stage of the crystallizing Merensky Reef is directly related to the mode of sulfide precipitation. Prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as H2S. Sulfur saturation causes reaction of H2S with oxides of the silicate melt to form a sulfide melt plus water. During reaction the magma is enriched in water until a separate fluid unmixes. It carries all compounds with high fluid/melt partition coefficients, as well as metals capable of forming OH- and Cl-complexes. Precious metals are assumed to have fractionated into the Cl-rich fluid as Cl-complexes rather than being dissolved in the sulfide melt. During the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets. The model proposed explains the distribution pattern of platinum-group minerals in the Merensky Reef better than any orthomagmatic mineralization concept offered so far.