Mineralogy Of Platinum-group Elements Research Articles

Mineralogy of Platinum-Group Elements and Gold in the Ophiolite-Related Placer of the River Bolshoy Khailyk, Western Sayans, Russia

We describe assemblages of platinum-group minerals (PGM) and associated PGE–Au phases found in alluvium along the River Bolshoy Khailyk, in the western Sayans, Russia. The river drains the Aktovrakskiy ophiolitic complex, part of the Kurtushibinskiy belt, as does the Zolotaya River ~15 km away, the site of other placer deposits. Three groups of alloy minerals are described: (1) Os–Ir–Ru compositions, which predominate, (2) Pt–Fe compositions of a Pt3Fe stoichiometry, and (3) Pt–Au–Cu alloys, which likely crystallized in the sequence from Au–(Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu2, to PtAu4Cu5. The general trends of crystallization of PGM appear to be: [Os–Ir–Ru alloys] → Pt3Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt–Fe phase) → Pt–Au–Cu alloys. We infer that Rh and Co mutually substitute for Fe, not Ni, and are incorporated into the pentlandite structure via a coupled mechanism of substitution: [Rh3+ + Co3+ + □ → 3Fe2+]. Many of the Os–Ir–Ru and Pt–Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt–Ir–Ni-rich alloys, and rarer phases like Cu-rich bowieite and a Se-rich sulfarsenide of Pt. The accompanying pyroxene, chromian spinel and serpentine are highly magnesian, consistent with a primitive ultramafic source-rock. Whereas the alloy phases indicate a highly reducing environment, late assemblages indicate an oxygenated local environment leading to Fe-bearing Ru–Os oxide (zoned) and seleniferous accessory phases.

The mineralogy and mineral associations of platinum group elements and gold in the Platreef at Zwartfontein, Akanani Project, Northern Bushveld Complex, South Africa

The mineralogy of the platinum-group elements (PGE), and gold, in the Platreef of the Bushveld Complex, was investigated using an FEI Mineral Liberation Analyser. Polished sections were prepared from 171 samples collected from two boreholes, for the in-situ examination of platinum group minerals (PGM). PGM and gold minerals encountered include maslovite (PtBiTe, 32 area% of total PGM), kotulskite (Pd(BiTe), 17 %), isoferroplatinum (Pt3Fe, 15 %), sperrylite (PtAs2, 11 %), cooperite (PtS, 5 %), moncheite (PtTe2; 5 %), electrum (AuAg; 5 %), michenerite (PdBiTe; 3 %), Pd alloys (Pd, Sb, Sn; 3 %), hollingworthite ((Rh,Pt)AsS; 2 %), as well as minor (all <1 area% of total PGM) merenskyite (PdBiTe2), laurite (RuS2), rustenburgite (Pt0.4Pd0.4Sn0.2), froodite (PdBi2), atokite (Pd0.5Pt0.3Sn0.2), stumpflite (PtSb), plumbopalladinite (Pd3Pb2), and zvyagintsevite (Pd3Pb). An observed association of all PGM with base metal sulfides (BMS), and a pronounced association of PGE tellurides, arsenides and PdP Hutchinson and McDonald Appl Earth Sci (Trans Inst Min Metall B) 114:B208–224, 2008).

Review of platinum-group element distribution and mineralogy in chromitite ores from southern Iran

The platinum-group element (PGE) potential of four chromite mining districts in southern and south-eastern Iran was investigated using geochemical and mineralogical methods. A total of 20 mines and prospects were sampled in the Neyriz ophiolite, the Abdasht and Sikhuran complexes in the Esfandagheh district, and the Faryab district. Chromitite and dunite were analysed for major, trace element and PGE geochemistry, and the platinum-group element mineralogy (PGM). In all mining districts, chromites analysed from mantle tectonite, probable transition zone and cumulate zone chromitites are refractory, low in TiO2 (<0.3wt.%) as well as Cr- and Mg-rich, with Cr# [100∗Cr/(Cr+Al)] ranging from 70 to 81 and Mg# [100∗Mg/(Mg+Fe2+)] ranging from 40 to 83. Associated olivine is highly magnesian (Mg# 89–98) and Ni-rich (0.1–1.0wt.% NiO).PGE concentrations range from <100 to >5000ppb, with median values around 200ppb in all complexes investigated. The 187Os/188Os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial Os. Mantle-normalised Pt/Ir ratios are generally low (<0.2), but are up to 0.5 in small chromite pods in the harzburgitic mantle section of the Neyriz ophiolite. PGM, mainly laurite–erlichmanite, mostly occur as small (<1 to 30μm) inclusions in chromite. They are associated with hydrous silicates (calcic amphibole), base metal sulphides (Ni- and Cu-sulphides) and other PGM, such as cuproiridsite, Ni–Ir sulphides, and Os–Ir alloys. PGMs interstitial to chromite are represented by irarsite, hollingworthite, Ru–Os–Ir alloys, tolovkite, RhNiAs, Pt–Fe alloy, probable Ru-rich oxides, and others.In the Sikhuran complex, pods of sulphide-bearing chromitite (0.2–1.2% sulphides) occur in a dunite zone interpreted as an ultramafic cumulate zone. Chromites in chromitite are characterised by high Cr# (75–81) and intermediate Mg# numbers (45–65); in associated dunite, chromites (Cr#, 74–80) are more Fe-rich (Mg#, 29–50). Chromitite and dunite carry up to 5.2ppm total PGE and are characterised by high normalised Pt/Ir (up to 5), Pt/Pd<1, and radiogenic Os (moderately suprachondritic initial Os). The interstitial sulphides are dominated by pentlandite that carries on average 0.45% Co, 25ppm Os, 11ppm Ir and 8ppm Rh. Palladium and Pt concentrations in pentlandite are below the detection limit of the Laser Ablation–ICP-MS technique. Pentlandite is associated with rare grains of Pd- and Pt-minerals such as stibiopalladinite and Pt–oxide, and with Cu-rich phases.Large degrees of partial melting in a supra-subduction zone geotectonic setting, probably in a two-stage melting process, produced boninitic melts that crystallised refractory, Cr–Mg rich and Ti-poor chromite. In some cases, sulphide saturation was achieved during and after chromite formation. Saturation during chromite crystallisation is evidenced by Ni–Cu-rich sulphide droplets preserved in some chromitites. Sulphur saturation after chromite crystallisation governed the formation of Ni-rich sulphide liquids that exsolved monosulphide solid solution, which later crystallised to form PGE-bearing pentlandite. Platinum and Pd did not partition into pentlandite but crystallised as Pt- and Pd-rich PGM associated with the pentlandite.

Geochemistry and mineralogy of platinum-group elements (PGE) in chromites from Centralnoye I, Polar Urals, Russia

The Polar Urals region of northern Russia is well known for large chromium (Cr)-bearing massifs with major chromite orebodies, including the Centralnoye I deposit in the Ray-Iz ultramafic massif of the Ural ophiolite belt. New data on platinum (Pt)-group elements (PGE), geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium (Ir) values. These values indicate the predominance of ruthenium–osmium–iridium (Ru–Os–Ir)-bearing phases among the platinum-group mineral (PGM) assemblage that is typical of mantle-hosted chromite ores. Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains. The most abundant PGM found in the chromite is erlichmanite (up to 15 μm). Less common are cuproiridsite (up to 5 μm), irarsite (up to 4–5 μm), and laurite (up to 4 μm). The predominant sulfide is heazlewoodite, in intergrowth with Ni–Fe alloys, sporadically with pentlandite, and rarely with pure nickel. Based on the average PGE values and estimated Cr-ore resources, the Centralnoye I deposit can be considered as an important resource of PGE.

Enrichment of PGE through interaction of evolved boninitic magmas with early formed cumulates in a gabbro–breccia zone of the Mesoarchean Nuasahi massif (eastern India)

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103–3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 103; komatiite: Cu/Pd ratio ∼8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.

PGE mineralization in marginal sulfide ores of the Chineisky layered intrusion, Russia

We have undertaken a detailed study of platinum group element (PGE) mineralogy and geochemistry of disseminated sulfides associated with the marginal zone of the Chineisky layered mafic intrusion. Towards an intrusive contact the marginal zone reveals a gradual progression from gabbro-gabbronorites towards monzodiorite. Sulfides occur in all the rocks of the marginal zone including exocontact sandstone. They occur mainly as pyrrhotite, chalcopyrite, and pentlandite and show progressive enrichment in Cu towards the intrusive contact. In the same direction, PGE mineralogy reveals the following systematic changes: (1) size of PGE mineral grains decreases from 50 µm up to 1 µm; (2) the association of Pd minerals with Ni and Co arsenide and sulfarsenides becomes stronger; and (3) the composition of PGE minerals changes for palladium: Pd-Sn → Pd-As → Pd-Sb → Pd-Te → Pd-Bi; for platinum: Pt-Fe + PtAs2 → PtS + PtAs2 → PtAs2. This zoning pattern is interpreted as the result of fractional crystallization of an immiscible sulfide melt, with the residual liquid, enriched in Cu, PGE, and volatile elements, being expelled towards the periphery of the intrusive body. PGE minerals also decompose in an oxidation zone. The most stable of them are paolovite and sperrylite, which both accumulated in placers derived from the massif. Pd is removed from the decomposed minerals, and then absorbed by brown iron and goethite in the oxidation zone.

Diversity of platinum-group minerals in podiform chromitites of the late Proterozoic ophiolite, Eastern Desert, Egypt: Genetic implications

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan–African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter. Platinum-group element (PGE) mineralization has been recently reported in podiform chromitites from the late Proterozoic Pan–African ophiolite of the Eastern Desert of Egypt. The populations of platinum-group minerals (PGM) in the studied CED and SED chromitites are quite distinguishable; they are mainly sulfides (Os-rich laurite) in the former, and Os–Ir alloy in the latter. Sulfarsenides and arsenides are also found in subordinate amounts from both chromitites. The most abundant base metal sulfides (BMS) in the Eastern Desert chromitites of Egypt are millerite, heazlewoodite, pentlandite, chalcopyrite and pyrite. The sulfur fugacity and temperature conditions are the main controller of PGE mineralogy in the host chromitite at the initial stage within the upper mantle. Os-rich laurite is stable at high sulfur fugacity and low temperature conditions, whereas Os–Ir alloy is stable at lower sulfur fugacity and higher temperature conditions. The diversity of PGE mineralogy combined with the differences in petrological characteristics of chromian spinels from CED chromitites to SED ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which, in turn may be attributed to different tectonic settings. During the post-magmatic processes, i.e., serpentinization, the primary PGM, e.g., Os-rich laurite and Os–Ir alloy, can be modified at low temperatures to secondary PGM.

Distribution and mineralogy of platinum-group elements in altered chromitites of the Campo Formoso layered intrusion (Bahia State, Brazil): control by magmatic and hydrothermal processes

Polyphase, penetrative hydrothermal metasomatism in chromitites of the Campo Formoso layered intrusion produced spectacular chromite – ferrian chromite zoning and transformed the primary intercumulus silicates into a chlorite – serpentine – carbonate – talc assemblage. Alteration did not substantially modify the composition of chromite cores and the distribution of platinum-group elements (PGE) through the sequence of chromitite layers, which still are consistent with magmatic fractionation processes. Texture and composition of laurite and Os–Ir–Ru alloys included in chromite cores indicate that these PGM were not altered, and are probably magmaticin origin. In contrast, the PGM located in the intergranular chlorite matrix (laurite, Ir–Ru–Rh sulfarsenides and Pt–Pd compounds with Sb, Bi and Te) display evidence of hydrothermal reworking. These PGM are intimately intergrown with low-temperature Ni-sulfides. The paragenesis suggests that the Ni-sulfides-PGM assemblage formed at the expenses of unknown PGM precursors, which must have been originally present in the intercumulus silicate matrix. Mechanism of formation involves a sequence of dissolution-precipitation events controlled by variation of redox conditions during chromite alteration. The presence of a secondary ore mineral assemblage consisting of galena, bismuthinite, native antimony, and various Pb–Sb compounds suggests a possible contribution of fluids derived from the adjacent granite.