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Combined structural, compositional and osmium-isotope data on selected Pt-Fe nuggets from economically important placer deposits closely linked to clinopyroxenite-dunite massifs of the Siberian Platform (Kondyor, Inagli, Guli) and the Middle Urals (Nizhny Tagil), Russia, are presented for the first time. Pt-Fe alloys investigated are ferroan platinum (space group Fm3m) with a composition close to Pt 3 Fe. This emphasizes the necessity of an X-ray study in identifying the particular Pt-Fe alloy species. Less common are compositions such as Pt 2 Fe and an intimate intergrowth of Pt 3 Fe 2 and PtFe. Other platinum-group minerals (PGM) observed in ferroan platinum include a diversity of Os-Ir-Ru alloys, PGE sulfides [laurite, malanite, cuproiridsite, cooperite, and an unnamed base metal -(Ir,Pt) monosulfide], PGE sulfarsenides (hollingworthite, irarsite), Pt-Pd tellurides (moncheite, telluropalladinite) and stibiopalladinite. This suite of PGM is consistent with those from other zoned or UralianAlaskan-type massifs. However, unusually Ru-rich alloys included in ferroan platinum of Guli are characteristic of PGM derived from an ophiolite source and underline the transitional signature of the Guli massif between zoned-and ophiolite-type complexes. Pd-rich ferroan platinum nuggets indicate a derivation from clinopyroxenite source-rock, whereas Ir-rich Pt-Fe alloys suggest a chromitite source. The presence of numerous Os-Ir-Ru exsolution lamellae in ferroan platinum are indicative of a hightemperature origin of the PGM. The first Os-isotope data from Os-rich minerals from chromitites and placers closely associated to the Kondyor and Inagli massifs reveal low 187 Os/ 188Os values with a very narrow range, indicative of a common mantle source of the PGE, implying that the PGM are of primary origin. Disintegration of parent ultramafic source-rocks and short-range mechanical transport of liberated PGM formed the placers. Os-isotope model ages in the range of 340 to 355 Ma constrain the formation age of the Kondyor and Inagli massifs of the Aldan Province at the southeastern part of the Siberian Craton, and closely match those from the Guli massif (370 Ma) of the Maimecha-Kotui Province at the northern part of the Siberian Craton.

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Abstract:The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs (up to 1.5 cm in size), often strongly deformed and fractured, immersed in a matrix of silicates (mainly chlorite and talc). Chromite is characterized by high Cr# (0.80-0.86), high Fe 2+ # (0.70-0.94), and low TiO 2 (av. = 0.18 wt %) consistent with variation trends of spinels from metamorphic rocks. The chromitite contains a large suite of accessory phases, but only irarsite and laurite are believed to be relicts of the original igneous assemblage, whereas most accessory minerals are thought to be related to hydrothermal fluids that emanated from a nearby felsic intrusion, metamorphism and weathering. Rutile is one of the most abundant accessory minerals described, showing an unusually high Cr 2 O 3 content (up to 39,200 ppm of Cr) and commonly forming large anhedral grains (>100 µm) that fill fractures (within chromite nodules and in the matrix) or contain micro-inclusions of chromite. Using a trace element geothermometer, the rutile crystallization temperature is estimated at 550-600 • C (at 0.4-0.6 GPa), which is in agreement with P and T conditions proposed for the regional greenschist to low amphibolite facies metamorphic peak of the area. Textural, morphological, and compositional evidence confirm that rutile did not crystallize at high temperatures simultaneously with the host chromitite, but as a secondary metamorphic mineral. Rutile may have been formed as a metamorphic overgrowth product following deformation and regional metamorphic events, filling fractures and incorporating chromite fragments. High Cr contents in rutile very likely are due to Cr remobilization from Cr-spinel during metamorphism and suggest that Ti was remobilized to form rutile. This would imply that the magmatic composition of chromite had originally higher Ti content, pointing to a stratiform origin. Another possible interpretation is that the Ti-enrichment was caused by external metasomatic fluids which lead to crystallization of rutile. If this was the case, the Cedrolina chromitites could be classified as podiform, possibly representing a sliver of tectonically dismembered Paleoproterozoic upper mantle. However, the strong metamorphic overprint that affected the studied chromitites makes it extremely difficult to establish which of the above processes were active, if not both (and to what extent), and, therefore, the chromitite's original geodynamic setting.

We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73-77 in the banded type to 81-86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd-Rh-As and Pd(Pt)-(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite-erlichmanite series and irarsite (8%). Os-Ir alloy, geversite, an unnamed Pt-PdBi-Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir-Os, Os-Ir, Pt-Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh) 2 S 4 , braggite, unnamed BM-Ir and BM-Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.

This paper reviews a database of about 1500 published and 1000 unpublished microprobe analyses of platinum-group minerals (PGM) from chromite deposits associated with ophiolites and Alaskan-type complexes of the Urals. Composition, texture, and paragenesis of unaltered PGM enclosed in fresh chromitite of the ophiolites indicate that the PGM formed by a sequence of crystallization events before, during, and probably after primary chromite precipitation. The most important controlling factors are sulfur fugacity and temperature. Laurite and Os–Ir–Ru alloys are pristine liquidus phases crystallized at high temperature and low sulfur fugacity: they were trapped in the chromite as solid particles. Oxygen thermobarometry supports that several chromitites underwent compositional equilibration down to 700 °C involving increase of the Fe3/Fe2 ratio. These chromitites contain a great number of PGM including—besides laurite and alloys—erlichmanite, Ir–Ni–sulfides, and Ir–Ru sulfarsenides formed by increasing sulfur fugacity. Correlation with chromite composition suggests that the latest stage of PGM crystallization might have occurred in the subsolidus. If platinum-group elements (PGE) were still present in solid chromite as dispersed atomic clusters, they could easily convert into discrete PGM inclusions splitting off the chromite during its re-crystallization under slow cooling-rate. The presence of primary PGM inclusions in fresh chromitite of the Alaskan-type complexes is restricted to ore bodies crystallized in equilibrium with the host dunite. The predominance of Pt–Fe alloys over sulfides is a strong indication for low sulfur fugacity, thereby early crystallization of laurite is observed only in one deposit. In most cases, Pt–Fe alloys crystallized and were trapped in chromite between 1300 and 1050 °C. On-cooling equilibration to ~900 °C may produce lamellar unmixing of different Pt–Fe phases and osmium. Precipitation of the Pt–Fe alloys locally is followed by an increase of sulfur fugacity leading to crystallize erlichmanite and Ir–Rh–Ni–Cu sulfides, occurring as epitaxic overgrowth on the alloy. There is evidence that the system moved quickly into the stabilization field of Pt–Fe alloys by an increase of the oxygen fugacity marked by an increase of the magnetite component in the chromite. In summary, the data support that most of the primary PGM inclusions in the chromitites of the Urals formed in situ, as part of the chromite precipitation event. However, in certain ophiolitic chromitites undergoing annealing conditions, there is evidence for subsolidus crystallization of discrete PGM from PGE atomic-clusters occurring in the chromite. This mechanism of formation does not require a true solid solution of PGE in the chromite structure.

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The chemical and osmium-isotope composition of platinum-group minerals (PGM) [e.g., laurite-erlichmanite (RuS 2-OsS 2), ruarsite-osarsite (RuAsS-OsAsS) series and Os-Ir alloy (Os,Ir)] from variably altered podiform chromitites of the Kraubath and Hochgrössen dunite-harzburgite massifs are reported for the first time. These massifs, the largest dismembered mantle relics in the Eastern Alps of Austria, were interpreted as a strongly metamorphosed ophiolite sequence, which forms part of the Speik Complex. Unaltered podiform chromitites from both localities display negatively sloped chondrite-normalized platinum-group element (PGE) patterns. The highly altered podiform chromitite at Kraubath is dominated by less refractory PGE (PPGE: Pd, Pt, Rh) over refractory PGE (IPGE: Os, Ir and Ru). The chemical composition of chromite varies from a Cr# [100*Cr/(Cr + Al)] of 74 to 87 and a Mg# [100*Mg/(Mg + Fe 2+)] of 44 to 61, values typical of podiform chromitites from the mantle section of an ophiolite. The PGM assemblage in the unaltered podiform ores is dominated by laurite (43% and 75% of all PGM at Kraubath and Hochgrössen, respectively). Sperrylite, PtAs 2 , is the most abundant PGM (61%) in the altered chromitite, whereas minerals of laurite-erlichnmanite series are subordinate (4%). At Kraubath, Os-bearing PGM (laurite, erlichmanite, ruarsite and Os-Ir alloy) occur as (a) single grains and (b) complex polyphase assemblages. At Hochgrössen, laurite and Os-Ir alloy are present as solitary grains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation multiple-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) revealed low 187 Os/ 188 Os and ␥Os t=0 values, indicative of a subchondritic source of the PGE in the mantle. Combined with less radiogenic Os isotopic values measured by negative thermal ionization mass spectrometry (N-TIMS), 187 Os/ 188 Os ranges from 0.11580 to 0.12437, and ␥Os t=0 values, §