Highly atypical mineralization involving Pd-Pt, Au-Ag, REE, Y, Zr, U, Th, and Cl-F-enriched minerals is found in zones with base metal sulfides (BMS; ~5 vol.% to 20 vol.%) in the eastern portion of the Oktyabrsky deposit in the Norilsk complex (Russia). The overall variations in Mg# index, 100 Mg/(Mg + Fe2+ + Mn), in host-rock minerals are 79.8 → 74.1 in olivine, 77.7 → 65.3 in orthopyroxene, 79.9 → 9.2 in clinopyroxene, and An79.0 → An3.7. The span of clinopyroxene and plagioclase compositions reflects their protracted crystallization from early magmatic to late interstitial associations. The magnesian chromite (Mg# 43.9) trends towards Cr-bearing magnetite with progressive buildups in oxygen fugacity; ilmenite varies from early Mg-rich to late Mn-rich variants. The main BMS are chalcopyrite, pyrrhotite, troilite, and Co-bearing pentlandite, with less abundant cubanite (or isocubanite), rare bornite, Co-bearing pyrite, Cd-bearing sphalerite (or wurtzite), altaite, members of the galena-clausthalite series and nickeline. A full series of Au-Ag alloy compositions is found with minor hessite, acanthite and argentopentlandite. The uncommon assemblage includes monazite-(Ce), thorite-coffinite, thorianite, uraninite, zirconolite, baddeleyite, zircon, bastnäsite-(La), and an unnamed metamict Y-dominant zirconolite-related mineral. About 20 species of PGM (platinum group minerals) were analyzed, including Pd-Pt tellurides, bismuthotellurides, bismuthides and stannides, Pd antimonides and plumbides, a Pd-Ag telluride, a Pt arsenide, a Pd-Ni arsenide, and unnamed Pd stannide-arsenide, Pd germanide-arsenide and Pt-Cu arseno-oxysulfide. The atypical assemblages are associated with Cl-rich annite with up to 7.54 wt.% Cl, Cl-rich hastingsite with up 4.06 wt.% Cl, ferro-hornblende (2.53 wt.% Cl), chlorapatite (>6 wt.% Cl) and extensive solid solutions of chlorapatite, fluorapatite and hydroxylapatite, Cl-bearing members of the chlorite group (chamosite; up to 0.96 wt.% Cl), and a Cl-bearing serpentine (up to 0.79 wt.% Cl). A decoupling of Cl and F in the geochemically evolved system is evident. The complex assemblages formed late from Cl-enriched fluids under subsolidus conditions of crystallization following extensive magmatic differentiation in the ore-bearing sequences.
A shell-like polycrystalline grain (ca. 1 mm) of W-(Mo)-bearing Os-Ir alloy (11.4.18.6 wt% W; up to 1.5% Mo) is present in a very old collection (probably the 1890s) of tiny nuggets from Trinity Co., California. An extensive compositional series [(Os0.43-0.80Ir0.28-0.05) W0.12-0.18], and inverse Ir-Os correlation, are observed; the mean composition [Os0.676W0.153Ir0.124Fe0.021Mo0.015Ru0.011; Σatoms = 1], based on results of 50 electron-microprobe analyses, displays a ratio (Os + Ir):W of 5:1. The observed variations and element correlations suggest that (W + Mo) contents are controlled by Ir, and incorporated via the following substitution scheme: [(W + Mo) + Ir] ↔ Os. The X-ray diffraction data indicate that the W-rich alloy has a hexagonal close-packed structure, related to that of osmium and allargentum, with a = 2.7297(4) Å, c = 4.3377(6) Å, and V = 27.99(1) Å3; the c:a ratio is 1.59. The probable space-group is P63/mmc, and Z = 2; the calculated density is 21.86(1) g/cm3. The W-rich alloy is associated with an Os-Ru-Ir alloy rich in Fe (7.0.9.7 wt%), which exhibits atomic Fe ↔ [Os + Ru] and Ir ↔ [Os + Ru] mechanisms of substitution. We suggest that these W-(Mo)- and Fe-rich alloys formed by metasomatic alteration of a primary Os-Ir-Ru alloy, associated with mineralized ultramafic-mafic rocks of ophiolite afinity. A fluid phase may well have remobilized and transported W, Mo, and Fe. The W-rich alloy likely crystallized from a reducing fluid under conditions of low fugacities of O2 and S2, thus promoting the observed siderophile behavior of W and Mo. These unusual W-(Mo)- and Fe-rich alloy grains were likely derived, as a placer material, from the Trinity ophiolite complex of northern California.
We describe occurrences of platinum-group minerals (PGM) and an uncommon mineral enriched in Cl, and provide a brief review of Cl-bearing minerals associated with basic–ultrabasic complexes. An unusual phosphohedyphane-like phase (~30 µm), close to CaPb4(PO4)3Cl, occurs in one of the PGM-bearing veins of massive sulfides in the Monchepluton layered complex, Kola Peninsula, Russia. These veins consist of varying amounts of pyrrhotite, pentlandite, chalcopyrite, pyrite and accessory grains of galena; they are fairly abundant in the heavy-mineral concentrate, as are small (<0.1 mm) grains of PGM: michenerite, sperrylite, Bi-enriched members of the merenskyite–moncheite series and kotulskite, also rich in Bi. The PGE mineralization is attributed to a low-temperature deposition at the hydrothermal stage. The pyromorphite–phosphohedyphane solid solution likely formed as a secondary phase under conditions of a progressive build-up of oxygen fugacity via oxidation reactions of a precursor grain of galena and involving Ca, as an incompatible component of the sulfides, in a medium of residual fluid enriched in Cl.
An unusual phase rich in Cl (78.4 wt.% Pb, 19.2% Cl) and close in composition to penfieldite [Pb 2 Cl 3 (OH)] was found as a ~5 m inclusion in chalcopyrite, in a spatial association with platinum-group minerals, in a sulfide-poor (≤5 vol.% of base-metal sulfides) enstatite orthocumulate of the Merensky Reef, Bushveld layered complex, South Africa.This seems to be the first reported occurrence of a Pb-Cl-(OH) compound in mafic-ultramafic rocks.The associated platinum-group minerals are members of the braggite series, cooperite (which forms large intergrowths with braggite: up to 0.5 mm in the longest dimension), members of the rustenburgite-atokite and merenskyite-moncheite series, zoned laurite, and an unknown stannosulfide of Pt, the likely chemical formula of which is PtSnS.The stannosulfide probably formed at a hydrothermal stage from microvolumes of a latestage fluid or liquid.The Cl-rich phase precipitated from a late-stage solution rich in Cl, or formed as a result of replacement of a precursor mineral (probably galena) by an aqueous hydrochloric solution at a very low temperature, at the final stage of hydrothermal alteration.
Abstract We describe the new species ognitite, NiBiTe, and a Co-rich variety of maucherite, hitherto unreported; both were discovered in the Ognit ultramafic complex of Neoproterozoic age in Eastern Sayans, Russia. The mean composition of ognitite ( n = 7) is: Ni 17.05, Fe 0.07, Cu 0.14, Pd 0.14, Te 32.53, Bi 49.64, total 99.57 wt.%, corresponding to: (Ni 1.11 Cu 0.008 Fe 0.005 Pd 0.005 ) Σ1.13 Bi 0.90 Te 0.97 (Σ atoms = 3 apfu). Ognitite is trigonal, space group P 3 m 1 [ R 1 = 0.0276 for 81 reflections with F o > 4σ( F o )]. The unit-cell parameters derived from the single-crystal X-ray diffraction data are: a = 3.928(1) Å, c = 5.385(1) Å and V = 71.95(4) Å 3 , with Z = 1. The c : a ratio is 1.37. The powder X-ray diffraction data obtained on the same fragment used for the single-crystal study are: a = 3.9332(4) Å, c = 5.3920(6) Å and V = 72.24(1) Å 3 . Ognitite exhibits the brucite-type structure with edge-sharing NiTe 3 Bi 3 octahedra forming sheets parallel to (0001). It is related to melonite, but is distinct compositionally by the extreme Bi-enrichment (melonite and its synthetic analogue contain <0.4 Bi apfu), and structurally as Bi and Te are ordered at two distinct sites, leading to the loss of the centre of symmetry in ognitite. At more than 9 wt.% Co, or ~2 apfu Co, the core of Co-rich maucherite [(Ni,Co) 11 As 8 ] in a zoned crystal, which is surrounded by Co-depleted orcelite, far surpasses the norm (≤1 and up to 3.9 wt.% Co). The unit-cell parameters of the Co-rich maucherite are: a = 6.85(2) and c = 21.83(5) Å, which are based on results of synchrotron micro-Laue diffraction. The host rock consists of serpentine, clinochlore (Mg# 95–97) and skeletal chromite. We favour the metastable crystallisation of fluid-saturated globules of a sulfide–arsenide melt to explain the anomalous compositions of ore minerals at Ognit. These anomalies seem consistent with rapid cooling in a fluid-enriched system, possibly related to late-stage degassing of the magma, as reflected in a prominent metasomatic aureole at the contact with the enclosing gneissic rocks.
Abstract New Pd-Tl-(Bi,As)-rich compounds are described from the Anomal'nyi Cu-PGE deposit, Kondyor alkaline ultramafic complex, northeastern Russia. They occur in vein-type settings associated with bodies of 'kosvite' (magnetite-rich clinopyroxenite) in the dunite-(peridotite) core, in pegmatiticand micaceous rocks. The ore zones are substantially enriched in phlogopite; they consist of diopside, disseminated titaniferous magnetite and fluorapatite (Sr-bearing). The observed assemblages of ore minerals include sulfides of the chalcopyrite-bornite-(secondary chalcocite) associationand various species of platinum-group minerals (PGM) [isomertieite or arsenopalladinite rich in Sb, mertieite-II, mertieite-I, sobolevskite, kotulskite, merenskyite, zvyagintsevite, palarstanide, paolovite, sperrylite, maslovite or moncheite, hollingworthite and unnamed species of PGM] anda Ag-Au alloy. The oxides [Pd 4 (Bi,Te,Tl)O 6 and Pd 4 (Tl,Bi,Te)O 6 ] probably formed in situ by oxidation reactions at the expense of the associated PGM intergrowths. These involve palladium bismuthide-thallide phases, Pd 5 (Tl,As,Bi)and Pd 5 (As,Tl,Bi), which are documented here for the first time. A fairly evolved environment, enriched in Cu, Pd, Te, Bi, Pb and volatile components, is indicated for the vein-type deposit at Anomal'nyi.
We document a new and unusual occurrence of patterns of protruding spheroidal weathering developed in a dunitic rock of the Pados-Tundra mafic-ultramafic complex of Early Proterozoic age, Kola Peninsula, Russia.It provides an example similar to that reported recently from a mineralized harzburgite in the Monchepluton layered complex in the same region.These patterns are genetically different from common results of "normal spheroidal weathering" sensu stricto.The spheroidally weathered dunite at Pados-Tundra consists of a high-Fo olivine, Ol (Fo 87.5 ), which is, in fact, not altered.Accessory grains of aluminous chromite are present.Relief spheroids (1.5 to 4 cm in diameter; up to ~5 vol.%) are distributed sparsely and heterogeneously.They are hosted by the olivine matrix and composed of talc, Tlc, and tremolite, Tr, (Mg# = 95-96) formed presumably at the expense of orthopyroxene, Opx, (i.e., pre-existing oikocrysts) during a deuteric (autometasomatic) alteration.In contrast, oikocrystic Opx (En 86.0 ) is quite fresh in related spheroids at Monchepluton, in which only minor deuteric alteration (Tlc + Tr) are observed.We infer that (1) the ball-shaped morphology of the weathered surface is a reflection of the presence of oikocrysts of Opx, which crystallized after Ol at the magmatic stage; they were entirely replaced by the deuterically induced Tlc + Tr at Pados-Tundra.(2) Differential rates of weathering are implied for rock-forming minerals in these ultramafic rocks, with a higher resistance of Opx vs. Fo-rich Ol, and Tlc + Tr vs. Fo-rich Ol. (3) The ball-like shape of the large spheroids, produced by magmatic processes, may likely represent an additional factor of their higher stability to weathering in the superficial environment.Similar patterns can be expected in other mafic-ultramafic complexes, especially in layered intrusions.
Tetra-auricupride, ideally AuCu, represents the only species showing the coexistence of Au with an elevated level of Pt, as in the case of a detrital grain studied structurally for the first time, from an ophiolite-associated placer at Bolshoy Khailyk, western Sayans, Russia. We infer that tetra-auricupride can incorporate as much as ~30 mol. % of a “PtCu” component, apparently without significant modification of the unit cell. The unit-cell parameters of platiniferous tetra-auricupride are: a 2.790(1) Å, c 3.641(4) Å, with c/a = 1.305, which are close to those reported for ordered AuCu(I) in the system Au–Cu, and close also to the cell parameters of tetraferroplatinum (PtFe), which both appear to crystallize in the same space group, P4/mmm. These intermetallic compounds and natural alloys are thus isostructural. The closeness of their structures presumably allows Pt to replace Au atoms so readily. The high extent of Cu + Au enrichment is considered to be a reflection of geochemical evolution and buildup in levels of the incompatible Cu and Au with subordinate Pt in a remaining volume of melt at low levels of fO2 and fS2 in the system.
We describe a potentially new species of a platinum cupride–stannide mineral (PCSM) of composition Pt(Cu0.67Sn0.33). It occurs in a placer deposit in the River Bolshoy Khailyk, southern Krasnoyarskiy kray, Russia. A synthetic equivalent of PCSM was obtained and characterized. The PCSM occurs as anhedral or subhedral grains up to 15 μm × 30 μm in association with various platinum-group minerals, Rh–Co-rich pentlandite and magnetite, all hosted by a placer grain of Cu–Au–Pt alloy. Synchrotron micro-Laue diffraction studies indicate that the PCSM mineral is tetragonal and belongs to the inferred space-group P4/mmm (#123). Its unit-cell parameters are a = 2.838 (3) Å, c = 3.650 (4) Å, and V = 29.40 (10) Å3, and Z = 1. The c:a ratio calculated from the unit-cell parameters is 1.286. These characteristics are in good agreement with those obtained for specimens of synthetic Pt(Cu0.67Sn0.33). A review on related minerals and unnamed phases is provided to outline compositional variations and extents of solid solutions in the relevant systems PtNi–PtFe–PtCu, PdCu–PdHg–PdAu, PdHg–PtHg, and AuCu–PtCu. The PCSM-bearing mineralization appears to be related genetically with an ophiolitic source-rock of the Aktovrakskiy complex of the western Sayans. The unnamed phase likely crystallized from microvolumes of a highly fractionated melt rich in Cu and Sn.
