Common Platinum-group Minerals Research Articles

(Digital Presentation) Present Status and Future Prospects of Large Area Growth and Devices of Two-Dimensional Materials

2D noble metal di- chalcogenides (NMDC or NMC) materials have attracted enormous research interests due to their novel physical and chemical properties with confined dimensions. Here we present two of our experimental works in investigating the growth of PtS and PtSx. (i) Platinum monosulfide as one of the most common platinum-group minerals has been less studied due to either the low purity in the natural product or the extremely high-pressure conditions for synthesis. Recently, platinum monosulfide (PtS) 2D membranes have emerged as rising-star materials for fundamental Raman and X-ray photoelectron spectral analysis as well as device exploration. However, a large-area homogeneous synthesis route has not yet been proposed and released. In this communication, we report a facile metal sulfurization strategy for the synthesis of a 4-inch wafer-scale PtS film. Enhanced characterization tools have been employed for thorough analysis of the crystal structure, chemical environment, vibrational modes, and atomic configuration. Furthermore, through theoretical calculations the phase diagram of the Pt–S compound has been plotted for showing the successful formation of PtS in our synthesis conditions. Eventually, a high-quality PtS film has been reflected in device demonstration by a photodetector. Our approach may shed light on the mass production of PtS films with precise control of their thickness and homogeneity as well as van der Waals heterostructures and related electronic devices. (ii) PtSx, as a more recent 2D material that aroused extensive research interests, has been applied in electronic and optoelectronic devices because of its excellent electronic characteristics and commendable stability. However, the synthesis of large-scale uniform PtSx remains a challenge. Here we studied a modified chemical vapor deposition (CVD) method to grow large-scale uniform PtSx films on SiO2/Si and c-plane sapphire substrates, which can avoid some disadvantages of the mechanical exfoliation method in the device fabrication. The large area uniform PtSx films are achieved through Pt sulfurization using H2S gas, replacing the sulfur powder in traditional methods, as the sulfur source. The material characterizations successfully proved that our modified CVD method is feasible and repeatable, and the PtSx film can be synthesized on different substrates. The PtSx photodevices show good photoresponse properties with a wide spectrum response range. The PtSx/sapphire device shows a better photoresponse in terms of the detectivity (9.17 × 109 Jones) and the responsivity (0.31 A W−1) than that of the PtSx/SiO2/Si device in our experiment, as well as one-year air stability, which may be caused by the different PtSx film quality due to a lattice mismatch difference between the PtSx and the different substrates. These results offer the design and synthesis technology for large-scale, uniform, and stable PtS and PtSx film, as well as an example of photoresponse-enhanced devices which could be generalizable to other transition metal dichalcogenides (TMDCs) and devices for future development and applications. Figure 1

Geology, sulfide mineralogy and petrogenesis of the Angstberg Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany): A potential Ni-Cu exploration target in Central Europe?

Germany’s largest known magmatic Ni-Cu-(PGE) sulfide enrichment is associated with the gabbroic Angstberg intrusion that is one of several small-scaled, Ni-Cu sulfide-bearing intrusions in the northern Bohemian Massif (Central European Variscan Belt). The intrusions are part of a Middle Devonian to Early Carboniferous, mafic-(ultramafic) magmatic system that emplaced within Cadomian granodiorites of the Lausitz Block. The Angstberg intrusion represents an exposed sub-vertical, dike-like body (320 × 60 m in plan view) and comprises olivine-gabbronorite, olivine-gabbro, gabbro, olivine-hornblende-gabbro, and hornblende-pyroxene-gabbro. The hornblende-bearing and hornblende-free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their Ti, LREE, HREE and volatile contents. Fractionation probably took place in deeper-seated sections of the magmatic system, possibly within staging magma chamber(s). Overall enriched incompatible trace-element patterns suggest that the Angstberg lithologies are linked with intra-plate magmatism, likely associated with a transtensional setting in the course of the Variscan orogeny. The previously suggested Devonian age of the intrusion is confirmed by an 40Ar-39Ar biotite age of 373.4 ± 0.9 Ma for the olivine-gabbro. The magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine-richest parts of the olivine-gabbronorite and olivine-hornblende-gabbro lithologies. In addition, some sulfide-matrix breccias (up to 70 modal % sulfides) occur directly on the footwall contact of the intrusion to granodiorite. Both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite, and accessory PGE, Au, and Ag phases. Nickel-rich bismuthotellurides of the melonite–merenskyite solid-solution series and froodite are the most common platinum-group minerals. Disseminated sulfides are characterized by high Ni, moderate Cu, and low Pt and Pd tenors (∼4–9 wt% Ni, ∼2–6 wt% Cu, ∼200–800 ppb Pt, ∼200–1000 ppb Pd). Sulfide-matrix breccias feature comparable Ni, but lower Cu, Pt and Pd tenors (∼4–6 wt% Ni, ∼0.3–1 wt% Cu, ∼10–100 ppb Pt, ∼40–200 ppb Pd). Fractional crystallization and wall-rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the Angstberg intrusion. Sulfur isotope compositions of pyrrhotite-pentlandite-chalcopyrite assemblages (−1.4 to −0.2‰ VCDT) propose a mantle-origin of S, whereas addition of crustal S cannot be completely ruled out as the S isotope values of the sulfides overlap with those of the granodiorite (−7 to +4‰ VCDT). Olivine- and sulfide-textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine-rich crystal mush during magma rise to upper levels. The sulfide-matrix breccias also resulted from entrained sulfide melts, or from gravity-driven downward percolation of sulfide melts from stratigraphically higher, now-eroded levels. In both cases, the formation of sulfide-matrix breccias most likely was amplified by in-situ thermomechanical erosion of the granodioritic wall-rock, caused by flash-boiling of thermally dehydrating biotite. Despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern Bohemian Massif, the Angstberg intrusion, as well as other neighboring magmatic sulfide occurrences, may represent a new exploration target for Ni and Cu in Central Europe.

Platinum-group minerals in the Skouries Cu-Au (Pd, Pt, Te) porphyry deposit

The Skouries deposit is a platinum-group element (PGE) enriched Cu-Au porphyry system located in the Chalkidiki peninsula, Greece, with associated Ag, Bi and Te enrichment. The deposit is hosted by multiple porphyritic monzonite and syenite intrusions, which originated from a magma chamber at depth. An initial quartz monzonite porphyritic intrusion contains a quartz–magnetite ± chalcopyrite–pyrite vein stockwork with intense potassic alteration. The quartz monzonite intrusion is cross cut by a set of syenite and mafic porphyry dykes and quartz–chalcopyrite–bornite ± magnetite veins which host the majority of the Cu and Au mineralisation. Late stage quartz–pyrite veins, with associated phyllic alteration crosscut all previous vein generations. Electron microprobe and scanning electron microscopy shows that the PGE are hosted by platinum-group minerals (PGM) in the quartz-chalcopyrite–bornite ± magnetite veins and within potassic alteration assemblages. The PGE mineralisation in Skouries is therefore part of the main high temperature hypogene mineralisation event. Platinum-group minerals at Skouries include: sopcheite [Ag4Pd3Te4], merenskyite [(Pd,Pt)(Te,Bi)2] and kotulskite [Pd(Te,Bi)], with rare telargpalite [(Pd,Ag)3Te], isomertieite [Pd11Sb2As2], naldrettite [Pd2Sb], testibiopalladite [PdTe(Sb,Te)] and sobolevskite [PdBi]. The most common platinum-group mineral is sopcheite. The PGM in Skouries are small, 52 µm2 on average, and occur as spherical grains on the boundaries between sulphides and silicates, and as inclusions within hydrothermal quartz and sulphides. These observations support a “semi-metal collector model” whereby an immiscible Bi-Te melt acts as a collector for PGE and other precious metals in high temperature hydrothermal fluids. This mechanism would allow the formation of PGM in porphyries without Pt and Pd fluid saturation.

Laurite and Associated PGM In the Stillwater Chromitites: Implications For Processes of Formation, and Comparisons With Laurite In the Bushveld and Ophiolitic Chromitites

Abstract Chromitites found in layered intrusions and ophiolite complexes are generally enriched in platinum-group elements (PGE), especially IPGE ( i.e ., Ir, Os, Ru)-bearing platinum-group minerals (PGM), and the chromitites are usually poor in base metal sulfide (BMS) minerals. The most common PGM observed is laurite [Ru(Os,Ir)S 2 ], but how the laurite formed is not clearly understood. To address this problem we compare the differences in the composition and shape of PGM in the nine chromite layers (A to K) in the Stillwater Complex, Montana and then extend the study to examine laurites from the Bushveld Complex and ophiolites. The most common PGM in the Stillwater chromitites is laurite, predominantly enclosed in chromite grains. In a few cases the laurite is accompanied by rarer and smaller PGM, including malanite [CuPtRh(±Ir)S] and Pt-Pd-sulfides. Interstitial to the chromite grains the PGM assemblage is quite different, dominantly PPGE ( i.e ., Pt, Pd, and Rh)-bearing, including Pd-Pb, Pt-Pd tellurides, sperrylite, platarsite, minor laurite, and one grain of Pd-Ge. The PGM grains enclosed in chromite formed by a different mechanism to the PGM grains outside chromite. During the crystallization of the chromite the magma was sulfide undersaturated and Ru, Os, Ir, and Rh partitioned into chromite thereby enriching the chromitite layers in IPGE. As the cumulate pile cooled, the fractionated silicate liquid became saturated in a BMS liquid and this migrated among the chromite grains. With further cooling the chromite grains sintered to form larger grains and in some case incorporated small grains of the BMS, which was converted to laurite by the exchange of IPGE and Fe plus Ni between the chromite and the BMS. In contrast, the BMS that was not included in chromite exsolved to form pentlandite, pyrrhotite, and PGM. The shape and composition of the PGM within the chromite grains in the Stillwater chromitite layers is not uniform. Upper and lower layers contain laurites with rounded shapes and an Os content of 7–8%. In the sulfide inclusion-poor middle G layer, the laurites have 5% Os and a predominantly euhedral shape. It is likely that both rounded and euhedral laurites formed by subsolidus ejection of PGE from the chromite as it cooled and recrystallized. The rounded laurite formed in a more BMS and S-rich environment, whereas the euhedral laurite formed in an S-poor environment. Traces of Rh in laurite, PPGM, and BMS inclusions associated with laurite are most abundant in the uppermost layer K, suggesting that the upper-layer chromitites contained more PPGE in solid solution on crystallization. The average size of the laurite grains increases upwards from an average area of 6 μm 2 in layer A to 21 μm 2 in layer K. The larger size of the laurites from higher layers in the intrusion may be the result of them having had a longer period to cool, further from the basal contact. Rutile inclusions are most abundant in chromitite layer B and could be the result of a greater degree of contamination of the magma in the lower layers. Comparison of the shape of Stillwater laurites with those in the Bushveld Complex chromitites reveals similarities with the Bushveld chromitites, as they also contain both euhedral and rounded laurites that are commonly associated with smaller PPGM. Ophiolitic laurites entirely enclosed in chromite are predominantly euhedral and sometimes zoned. Chromitites from ophiolites are generally PPGE-poor and although ophiolitic laurites also form composite PGM with other smaller PGM, these are usually Os- and Ir-rich rather than PPGE-rich. These laurites have a more variable and greater range of Os concentrations than those from the Stillwater and Bushveld Complexes. Most ophiolitic laurites probably formed by crystallizing directly from magma, but it is possible that some formed by diffusing from the chromite in a low f S 2 environment.

Ophiolitic chromitites from the Kızılyüksek area of the Pozantı-Karsantı ophiolite (Adana, southern Turkey): Implication for crystallization from a fractionated boninitic melt

Ophiolitic rocks are widely distributed in Turkey. One type, the Pozantı-Karsantı ophiolite from southern Turkey, contains a large number of chromitite deposits located mostly in the mantle peridotites and close to the Moho transition zone dunite and cumulate dunites. Cr-spinel grains from the chromitites are represented by high Cr# [100×Cr/(Cr+Al)=68–81], and their Mg# [100×Mg/(Mg+Fe2+)] range from 54 to 71. Gallium and Co contents vary between 18 and 32ppm and 185–266ppm, respectively, and they show negative correlation with Cr#. A detailed optical investigation reveals that the Cr-spinel grains contain silicate, platinum-group mineral (PGM) and base metal sulfide (BMS) inclusions. Single phase inclusions of amphibole are the only hydrous silicate phases in the investigated chromitites, and they contain low TiO2 (<0.43wt.%). Olivine, with high Fo (~96) and NiO contents (0.48–0.68wt.%), and clinopyroxene, with low TiO2 (<0.1wt.%), Al2O3 (<2.84wt.%) and Na2O contents (<0.4wt.%) were also observed as primary silicate inclusions. Chromitites contain low concentrations of total platinum-group elements (PGE) ranging between 32 and 162ppb, with an average value of 93ppb. Primitive mantle-normalized PGE diagrams show almost flat to positive slopes from Os to Rh (RhN/OsN=0.99 to 8.5) and negative slope from Rh to Pt and Pd. All samples show marked positive Ru anomalies. Consistent with the geochemical data, Ru, Os, and Ir bearing PGE sulfide (laurite-erlichmanite solid solution series [(Ru, Os)S2–(Os, Ru)S2] phases) are the most common PGM detected in the investigated chromitite samples. They show a narrow range of Os-Ru substitution [Ru#; Ru/(Ru+Os)=0.72–0.97], indicating no erlichmanite in the PGM paragenesis. In addition to the most common PGM laurite, several osmium (Os, Ir), iridium (Ir, Os), irarsite, and one single grain of speryllite (PtAs2) were detected as magmatic inclusions in Cr-spinel. Three unknown PGE/PGE–BME (base metal element) phases were also detected in Cr-spinel grains with compositions that correspond to the chemical formulas of (Os, Ru, Ir, Rh, Fe, Pd)2S5, Ir(Rh,Pt,Ni,Cu)S3, and (Ir, Rh, Ru)2(Ni, Cu)S3, respectively. The high Cr# and low Ti content of Cr-spinel grains and amphibole inclusions with low Ti content as hydrous phases in Cr-spinel grains require a hydrous melt depleted in incompatible trace elements for the formation of investigated chromitites; therefore, we suggest a fore arc tectonic environment for the generation of Kızılyüksek chromitites. The presence of Os-Ir alloys and Ru-rich laurites implies that Cr-spinel crystallization took place at relatively high temperature (1100–1300°C) and low ƒ(S2) (between −1 and −3) conditions. Major and trace element compositional variations of Cr-spinel, wide variation of RhN/OsN ratios of the chromitites and depletion of Os in the chromitites compared to Ir and Ru may imply that Kızılyüksek chromitites crystallized from a variously fractionated boninitic melt.

MAGMATIC AND POST-MAGMATIC SIGNIFICANCE OF CHROMITITE AND ASSOCIATED PLATINUM-GROUP MINERALS (PGM) IN THE EASTERN KHOY OPHIOLITIC COMPLEX (NW IRAN)

The Khoy ophiolites are one of the largest Iranian ophiolitic complexes and are characterized by a very complicate geology, consisting of several ophiolites with different ages and geodynamic settings of formation. Few small lenticular and irregular bodies of podiform and schlieren type chromitites have been recognized in the Khoy ophiolites, occurring in the serpentinized mantle peridotites. They have massive and nodular textures. According to their chromite composition, the Khoy chromitites can be classified as Al-rich and fall clearly in the field of podiform chromitites. The magmatic chromite composition of the podiform chromitites of the Khoy ophiolites suggests that they crystallized from a MORB type melt in an extensional regime. The Khoy chromitites were altered during ocean floor metamorphisms and no magmatic silicates were preserved. The composition of chlorite occurring in the silicate matrix or in contact with chromite has been used as a geothermometer. The estimated temperatures possibly reflect the effects of low-grade metamorphism. The Khoy chromitites have very low total platinum group elements (PGE) concentration with a predominance of Os + Ir + Ru over Rh + Pt + Pd. Consistently with the geochemical data, they contain few platinum group minerals (PGM) of Ru, Ir and Os. Laurite is the most common PGM along with a minor Ir-Cu sulfide, possibly cuproiridsite, osmium and erlichmanite. Most of the PGM are magmatic in origin. They crystallized in a narrow range of temperature, around 1000 o C, at relatively low sulfur fugacity and in the absence of fluids. This observation is consistent with formation of the chromitites in a mid-oceanic ridge (MOR) rather than in a supra-subduction zone (SSZ).

Genesis of the Jinchuan PGE deposit, China: evidence from fluid inclusions, mineralogy and geochemistry of precious elements

The Jinchuan deposit is a platinum group element (PGE)-rich sulfide deposit in China. Drilling and surface sampling show that three categories of platinum group element (PGE) mineralization occur; type I formed at magmatic temperatures, type II occurs in hydrothermally altered zones of the intrusion, and type III in sheared dunite and lherzolite. All ore types were analyzed for Os, Ir, Ru, Rh, Pd, Pt and Au, as well as for Cu, Ni, Co and S. Type I ore has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios of <7 and relatively flat chondrite-normalized noble metal patterns; the platinum group minerals (PGM) are dominated by sperrylite and moncheite associated with chalcopyrite, pyrrhotite and pentlandite. Type II has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 40 to 330 and noble metal distribution patterns with a positive slope; the most common PGM are sperrylite and Pd bismuthotelluride phases concentrated mostly at the margins of base metal sulfides. Type III ores have the highest (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 240 to 710; the most abundant PGM are sperrylite and phases of the Pt–Pd–Te–Bi–As–Cl system. It is concluded that the Jinchuan deposit formed as a result of primary magmatic crystallization followed by hydrothermal remobilization, transport, and deposition of the PGE.

Mineralogy of Ni-Cu-Platinum-Group Element Sulfide Ore in the 800 and 810 Orebodies, Copper Cliff South Mine, and P-T-X Conditions during the Formation of Platinum-Group Minerals

The 800 and 810 orebodies are two major deposits in the Copper Cliff South mine, located at opposite contacts between the quartz diorite of the Copper Cliff offset dike and metapelite of the McKim Formation. The Copper Cliff South mine is one of the richest Cu-Ni- platinum-group elements (PGE) deposits in the Copper Cliff offset dike. The ore is composed of variable proportions of pyrrhotite, chalcopyrite, and pentlandite and occurs as massive sulfides, semimassive sulfides with inclusions of quartz diorite and wall rocks, net-textured sulfides, disseminated sulfides, and veins. Sphalerite and magnetite are also present in minor amounts. The primary magmatic minerals in the quartz diorite are plagioclase, quartz, amphibole, and biotite. The most common platinum-group mineral (PGM) in the Copper Cliff South mine is sperrylite (PtAs 2 ). It occurs in every textural type of ore, typically enclosed by chalcopyrite, pyrrhotite, pentlandite, cobaltite, and hydrous silicates such as chlorite, amphibole, and epidote. Sperrylite may be homogeneous, zoned (with Sb-rich cores and Sb-poor rims), associated with hollingworthite (RhAsS) and cobaltite, or it may contain inclusions. The Pd-bearing minerals are froodite (PdBi 2 ) and michenerite (PdBiTe), which are most commonly enclosed by silicates (chlorite, biotite, quartz) or chalcopyrite. Hollingworthite invariably occurs in the center of zoned cobaltite in veins or in disseminated sulfides. Argentian gold (Au 0.56 Ag 0.44 ) occurs as small grains in chalcopyrite and in a veinlet with parkerite (Ni 2 Bi 3 S 2 ). The most abundant Ag mineral is hessite (Ag 2 Te), which is present in almost every sample with galena, gersdorffite, and locally PGM. Volynskite (AgBiTe 2 ) occurs as a zoned grain with tsumoite (BiTe) in the center. The most common trace minerals spatially and genetically associated with PGM are tsumoite, galena, gersdorffite, cobaltite, hessite, melonite, and native Te. The Copper Cliff South mine area was affected by more than one hydrothermal event, deformation, and metamorphism, all of which affected the primary igneous textures and mineralogy of the rocks. The first hydrothermal event was responsible for the extensive remobilization of metals, including some PGE. This is revealed by the presence of PGM in large veins in quartz diorite and metapelite and by chlorite, amphibole, epidote, biotite, and quartz that often enclose PGM. The temperature at the time of this event was estimated in samples containing garnet and biotite in equilibrium to be 327° to 540°C. Pressure, estimated using amphibole chemistry, may have been as high as 3 to 4 kbars. The fluid responsible for the transport of metals was very rich in Cl and oxidizing. The fluid composition, temperature, and/or pressure changed with time, suggested by the presence of zoned minerals, including sperrylite, gersdorffite, and cobaltite. Based on the textural relationships, sperrylite precipitated before tellurides and galena, and after or at the same time as michenerite. A second hydrothermal event produced calcite-bearing veinlets that cut larger veins.

PGE AND Ag MINERALIZATION IN A BRECCIA ZONE OF THE PRECAMBRIAN NUASAHI ULTRAMAFIC-MAFIC COMPLEX, ORISSA, INDIA

The Nuasahi ultramafic–mafic complex, in Orissa, India, contains high concentrations of platinum-group elements (Pt up to 9.6 ppm, Pd up to 17.9 ppm) and Ag (up to 6.6 ppm) in a chromiferous sulfide-rich breccia zone. The plutonic complex occurs within an Archean belt of supracrustal rocks. The complex consists of an ultramafic unit in fault contact with and overlain by gabbros whose stratigraphic top is characterized by a zone of brecciation. Lode chromite deposits occur in the ultramafic suite and are currently being mined. Magnesiochromite compositions in the lodes are refractory and show little variation within the ultramafic suite (Cr 2 O 3 ~60%, Al 2 O 3 ~12%, MgO ~15%). Sudburyite, (Pd,Bi)Sb, members of the michenerite–testibiopalladite series, and irarsite–hollingworthite (IrAsS–RhAsS) are the common platinum-group minerals (PGM) in the chromite–sulfide assemblage of the breccia zone. Sudburyite is preferentially included within coarse-grained ferrian chromite, along with numerous inclusions of chalcopyrite, pyrrhotite, rutile, ilmenite and oriented lamellae of clinochlore. The distinctive noble-metal composition (high Pd, Pt and Ag with high Cu) and dominant Pd–Bi–Te–Sb-bearing PGM in the mineralized portion indicate its evolved nature. Mineralization developed by magmatic processes; a hybrid magma resulting from partial incorporation of serpentinized chromite-bearing ultramafic rocks and chromitite in the younger gabbroic magma was emplaced within the pre-existing breccia zone. The breccia zone developed during brittle deformation along a NNW-trending shear zone.

PLATINUM-GROUP MINERAL ASSEMBLAGES AND CHROMITE COMPOSITION IN THE ALTERED AND DEFORMED BACURI COMPLEX, AMAPA, NORTHEASTERN BRAZIL

In the Bacuri complex, Amapa, Brazil, there is good evidence for Pd mobility both during serpentinization and lateritization. In non-lateritized, serpentinized samples, Pd bismuthide occurs with chalcopyrite in chlorite-filled veins that cross-cut serpentine and chromite probably only a few tens of micrometers from the source of the platinum-group elements (PGE). Low Pt/Pd values of less than one are characteristic of magmatic values, and sporadic higher values of up to 4 indicate partial removal of Pd. The highest Pt/Pd values (up to 26) occur in laterites, where Pt remains but Pd has been extensively removed. The PGE are concentrated in non-lateritized rock-types including chromitite, serpentinite containing disseminated chromite, and sulfide-bearing serpentinite. The highest whole-rock PGE concentrations are 166 ppb Pt and 609 ppb Pd. Osmium, Ir and Ru concentrations in the chromitite can be attributed to the presence of laurite and irarsite, whereas sulfide-bearing serpentinite contains sperrylite. Pd–Bi tellurides, including sobolevskite and michenerite, are the most common platinum-group minerals. They commonly occur with pentlandite and (less common) pyrrhotite, typically partially altered to millerite, magnetite, pyrite, nickeloan pyrrhotite, maucherite and gersdorffite, all of which form less than 1% of these rocks. Chromitite in folded layers 3–4 m thick has Cr/(Cr + Al) values of 69.0–84.1, Mg/(Mg + Fe2+) values of 27.6–51.4, and TiO2 values of up to 1.69%, compositions most typical of chromite from a stratiform complex of continental origin.

Stratigraphical distribution of platinum-group minerals in the Eastern Layered Series, Rum, Scotland

Abundant and diverse platinum-group minerals (PGM) occur throughout the Tertiary layered intrusion on Rum, Scotland. In this paper we document the distribution of PGM within the Eastern Layered Series (ELS) on Rum, which comprises 16 alternating units of olivine-dominant feldspathic peridotite grading to plagioclase-dominant allivalite. The PGM occur in six main chrome-spinel layers in the Eastern Layered Series and are clearly associated with minor concentrations of interstitial sulphides. Common PGM phases include: Pd–Cu alloys, Pt–Fe alloys, native Pt, laurite, moncheite, sperrylite, isomertiete, cooperite and braggite along with a large number of other less common arsenide, bismuthotelluride and sulphide phases. Analyses of the discrete chromitite layers yield up to ΣPGE + Au 2618 ppb. Although present throughout the 750-m-thick ELS, there are clear stratigraphical changes in the PGM assemblage. The presence of PGM in the ELS on Rum are interpreted as being caused by mantle melting associated with the proto-Icelandic `hot spot', followed by localised concentration because of the combined effects of magma mixing, sulphide-silicate liquid immiscibility and fractional crystallisation. Most of the PGM are magmatic in origin but some grains show evidence of hydrothermal alteration.

Alluvial platinum‐group minerals from the Manampotsy area, East Madagascar

Two platinum‐group mineral (PGM) concentrates collected from the same placer in the Manampotsy area located 120 km southeast of Antananarivo, Madagascar, were studied. Nuggets, which ranged in size from 0.1 to 1 mm, were characteristically rounded but a complex aggregate was also observed. Except for the latter which was multiphase (composed of laurite, Ir‐Os and Pt‐Fe alloys), they consisted of isoferroplatinum (Pt3Fe) with variable contents of Cu, Rh, Pd, Ir and Os in solid solution. Two varieties of inclusions were encountered within the nuggets, PGM and silicates, but no base‐metal sulphide was found. The most common PGM inclusions were Os‐Ir‐(Ru) alloys occurring as laths corresponding either to trapped minerals or to exsolution. Other minerals, occurring either isolated or as complex assemblages, were: (1) alloys — unnamed Pt3Cu and (Pd,Pt,Au)2Cu; (2) sulphides — laurite, erlichmanite, cooperite, braggite, kashinite, cuprorhodsite and several unnamed species; (3) sulfarsenides — hollingworthite; (4) tellurides — keithconnite and unnamed (Rh,Pd)3Te, and telluro‐arsenide — vincentite? Complex PGM assemblages, forming rounded multiphase aggregates, resulted from fractional crystallization in a closed system of PGE‐rich liquid droplets, mainly composed of Pd, Pt, Ir, Rh, Au, Cu, Ni, Te, S, trapped in isoferroplatinum grains at high temperature. Two types of silicate inclusion were found: (1) inclusions consisting of one or two minerals (amphibole, albite or two‐phase amphibole‐clinopyroxene and amphibole‐K‐feldspar inclusions; and (2) mixed inclusions of glass (Si, Al, and alkali‐rich) and amphiboles. No inclusion exceeded 50 μm in size. The nature and characteristics of the inclusions indicate that the nuggets formed in a magmatic environment. The presence of a large variety of PGM included in the Pt‐Fe nuggets suggests that the original mineralization consisted principally of Pt (as Pt3Fe, the only mineral found in the concentrates) with lesser amounts of other PGE (dominantly Pd, with Os, Ru, Rh). Mineralogical data indicate that this mineralization could have been formed in an ‘Alaskan‐type’ complex.

Compositional variations and reflectance of the common platinum-group minerals

A detailed study of the compositional variations of 22 different platinum-group minerals, of Os-Ir-Ru- and Pt-Fe alloys, Pt-Pd- and Ru-Os-Ir sulfides, RhIr-Pt sulfarsenides and Pd-Pt tellurides from various deposits resulted in verification of extensive solid solution series and element substitution within different mineral groups. On the basis of representative new microprobe analyses and related spectral reflectance curves of the platinum-group minerals investigated significant correlations between chemical composition and reflectance are discussed.

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.

Mineralogy of platinum-group minerals in the Witwatersrand, South Africa

Approximately 80 percent of the platinum-group mineral grains in the Precambrian Witwatersrand conglomerate reefs are (Ir, Os, Ru) alloys. Osmium, iridosmine, osmiridium, and iridium are most abundant; rutheniridosmine, ruthenosmiridium, and ruthenian iridium are less abundant. Zoned grains are common, with Os-rich cores enclosed by (Ir, Pt)-rich outer zones filled with exsolution lamellae of (Pt, Fe) alloy. Discrete grains of this alloy proved to be isoferroplatinum of a composition close to Pt 3 Fe. In addition, grains ranging in composition between Pt 3 Fe and platiniridium are present.Sperrylite commonly occurs as an outer coating on all the above grain types. Sperrylite and isoferroplatinum make up 15 to 20 percent of the platinum-group mineral grains and account for nearly all the Pt present. Hollingworthite, iridarsenite, and ruthenarsenite occur in variable quantities (1-5%) as coatings on alloy grains, which in turn may be coated with sperrylite.Euhedral and subhedral inclusions in the common platinum-group mineral grains are michenerite, moncheite, laurite, geversite, and two unnamed minerals: rhodium sulfide and (Pt, Rh, Ru) alloy. Free grains of geversite, moncheite, sudburyite, and stibiopalladinite are very rare. The last two may be intergrown.Occasionally, partial or complete alteration of (Ir, Os, Ru) alloys has resulted in mixtures of erlichmanite, irarsite, osarsite, and phases that could not be identified.The evidence suggests that the (Ir, Os, Ru) and (platinum-group elements, Fe) alloys and their inclusions underwent little or no alteration during weathering, transportation, and burial, or even during later metamorphism of the reefs when sperrylite, holling-worthite, and related phases probably formed from less stable platinum-group minerals, possibly tellurides and antimonides.