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—The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.

Author(s): Barkov, AY; Bindi, L; Tamura, N; Martin, RF; Ma, C; Winkler, B; Shvedov, GI; Morgenroth, W | Abstract: Fleetite, Cu2RhIrSb2, a new species of platinum-group mineral (PGM), was discovered intergrown with an Os–Ir–Ru alloy in the Miass Placer Zone (Au–PGE), southern Urals, Russia. A single grain 50 lm across was found. Osmium, ruthenium, and iridium are the main associated minerals; also present are Pt–Fe alloys, laurite, Sb-rich irarsite, Rh-rich tolovkite, kashinite, anduoite, ferronickelplatinum, heazlewoodite, PGE-bearing pentlandite and digenite, as well as micrometric inclusions of forsterite (Fo93.7), chromite–magnesiochromite, and Mg-rich edenite. In reflected light, fleetite is light gray; it is opaque, isotropic, non-pleochroic, and non-bireflectant. We report reflectance values measured in air. A mean of seven point-analyses (wavelength-dispersive spectrometry) gave Cu 13.93, Ni 8.60, Fe 0.10, Ir 28.07, Rh 7.91, Ru 1.96, Sb 39.28, total 99.85 wt.%, corresponding to (Cu1.41Ni0.58Fe0.01)R2.00(Rh0.49Ni0.36Ru0.12)R0.97Ir0.95Sb2.08 on the basis of six atoms per formula unit, taking into account the structural results. The calculated density is 10.83 g/cm3. Single-crystal X-ray studies show that fleetite is cubic, space group Fd3m (#227), a = 11.6682(8) A, V = 1588.59(19) A3, and Z = 16. A least-squares refinement of X-ray powder-diffraction data gave a = 11.6575(5) A and V = 1584.22(19) A3. The strongest five reflections in the powder pattern [d in A(I)(hkl)] are: 6.70(75)(111), 4.13(100)(220), 3.52(30)(311), 2.380(50)(422), 2.064(40)(440). Results of synchrotron micro-Laue diffraction experiments are consistent [a ¼ 11.66(2) A]. The crystal structure of fleetite was solved and refined to R ¼ 0.0340 based upon 153 reflections with Fo . 4r(Fo). It is isotypic with Pd11Bi2Se2 and best described as intermetallic, with all metal atoms in 12-fold coordination. Fleetite and other late exotic phases were formed by reaction of the associated alloy phases with a fluid phase enriched in Sb, As, and S in circulation in the cooling ophiolite source-rock. The mineral is named after Michael E. Fleet (1938–2017) in recognition of his significant contributions to the Earth Sciences.

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.

Chromitite zones associated with ultramafic units of the Lysanskiy layered complex of dunite–peridotite–gabbro composition could well represent the primary source for the placers bearing platinum-group minerals (PGM) of the entire drainage of the River Sisim and its tributaries, the rivers Ko and Seyba, eastern Sayans. Alluvial gold present in the placers of River Seyba, as elsewhere in the Sisim Placer Zone, reflects mineralisation during a recent period of tectonic activity. We focus on the PGM in the Seyba suite, and in particular on the attributes of pentlandite enriched in platinum-group-elements (PGE) and the compositionally similar and recently defined ferhodsite, which were trapped in host grains of Os–Ir–Ru alloy. Both minerals formed from small volumes of fractionated Fe–Ni–Cu melt considerably enriched in the PGE. In the Seyba suite, as in several others, the amounts of PGE in ferhodsite exceeds that in pentlandite, which results in a greater proportion of vacancies than in pentlandite.

Tamuraite, ideally Ir5Fe10S16, occurs as discrete phases (≤20 μm) in composite inclusions hosted by grains of osmium (≤0.5 mm across) rich in Ir, in association with other platinum-group minerals in the River Ko deposit of the Sisim Placer Zone, southern Krasnoyarskiy Kray, Russia. In droplet-like inclusions, tamuraite is typically intergrown with Rh-rich pentlandite and Ir-bearing members of the laurite–erlichmanite series (up to ~20 mol.% “IrS2”). Tamuraite is gray to brownish gray in reflected light. It is opaque, with a metallic luster. Its bireflectance is very weak to absent. It is nonpleochroic to slightly pleochroic (grayish to light brown tints). It appears to be very weakly anisotropic. The calculated density is 6.30 g·cm−3. The results of six WDS analyses are Ir 29.30 (27.75–30.68), Rh 9.57 (8.46–10.71), Pt 1.85 (1.43–2.10), Ru 0.05 (0.02–0.07), Os 0.06 (0.03–0.13), Fe 13.09 (12.38–13.74), Ni 12.18 (11.78–13.12), Cu 6.30 (6.06–6.56), Co 0.06 (0.04–0.07), S 27.23 (26.14–27.89), for a total of 99.69 wt %. This composition corresponds to (Ir2.87Rh1.75Pt0.18Ru0.01Os0.01)Σ4.82(Fe4.41Ni3.90Cu1.87Co0.02)Σ10.20S15.98, calculated based on a total of 31 atoms per formula unit. The general formula is (Ir,Rh)5(Fe,Ni,Cu)10S16. Results of synchrotron micro-Laue diffraction studies indicate that tamuraite is trigonal. Its probable space group is R–3m (#166), and the unit-cell parameters are a = 7.073(1) Å, c = 34.277(8) Å, V = 1485(1) Å3, and Z = 3. The c:a ratio is 4.8462. The strongest eight peaks in the X-ray diffraction pattern [d in Å(hkl)(I)] are: 3.0106(26)(100), 1.7699(40)(71), 1.7583(2016)(65), 2.7994(205)(56), 2.9963(1010)(50), 5.7740(10)(45), 3.0534(20)(43) and 2.4948(208)(38). The crystal structure is derivative of pentlandite and related to that of oberthürite and torryweiserite. Tamuraite crystallized from a residual melt enriched in S, Fe, Ni, Cu, and Rh, these elements were incompatible in the Os–Ir alloy that nucleated in lode zones of chromitites in the Lysanskiy layered complex, Eastern Sayans, Russia. The name honors Nobumichi Tamura, senior scientist at the Advanced Light Source of the Lawrence Berkeley National Laboratory, Berkeley, California.

The lopolithic Pados-Tundra layered complex, the largest member of the Serpentinite belt&ndash, Tulppio belt (SB&ndash, TB) megastructure in the Fennoscandian Shield, is characterized by (1) highly magnesian compositions of comagmatic dunite&ndash, harzburgite&ndash, orthopyroxenite, with primitive levels of high-field-strength elements, (2) maximum values of Mg# in olivine (Ol, 93.3) and chromian spinel (Chr, 57.0) in the Dunite block (DB), which exceed those in Ol (91.7) and Chr (42.5) in the sills at Chapesvara, and (3) the presence of major contact-style chromite&ndash, IPGE-enriched zones hosted by the DB. A single batch of primitive, Al-undepleted komatiitic magma crystallized normally as dunite close to the outer contact, then toward the center. A similar magma gave rise to Chapesvara and other suites of the SB&ndash, TB megastructure. Crystallization proceeded from the early Ol + Chr cumulates to the later Ol&ndash, Opx and Opx cumulates with accessory Chr in the Orthopyroxenite zone. The accumulation of Chr resulted from efficient cooling along boundaries of the Dunite block. The inferred front of crystallization advanced along a path traced by vectors of Ol and Chr compositions. Grains and aggregates of Chr were mainly deposited early after the massive crystallization of olivine. Chromium, Al, Zn and H2O, all incompatible in Ol, accumulated to produce podiform segregations or veins of chromitites. This occurred episodically along the moving front of crystallization. Crystallization occurred rapidly owing to heat loss at the contact and to a shallow level of emplacement. The Chr layers are not continuous but rather heterogeneously distributed pods or veins of Chr&ndash, Ol&ndash, clinochlore segregations. Isolated portions of melt enriched in H2O and ore constituents accumulated during crystallization of Ol. Levels of fO2 in the melt and, consequently, the content of ferric iron in Chr, increased progressively, as in other intrusions of the SB&ndash, TB megastructure. The komatiitic magma vesiculated intensely, which led to a progressive loss of H2 and buildup in fO2. In turn, this led to the appearance of anomalous Chr&ndash, Ilm parageneses. Diffuse rims of Chr grains, abundant in the DB, contain elevated levels of Fe3+ and enrichments in Ni and Mn. In contrast, Zn is preferentially partitioned into the core, leading to a decoupling of Zn from Mn, also known at Chapesvara. The sulfide species display a pronounced Ni-(Co) enrichment in assemblages of cobaltiferous pentlandite, millerite (and heazlewoodite at Khanlauta), deposited at &le, 630 &deg, C. The oxidizing conditions have promoted the formation of sulfoselenide phases of Ru in the chromitites. The attainment of high degrees of oxidation during crystallization of a primitive parental komatiitic magma accounts for the key characteristics of Pados-Tundra and related suites of the SB&ndash, TB megastructure.

Megacryst-bearing alkaline basic rocks of Cenozoic age occur in scattered centers in western Europe, from the Rhine graben to southern Spain. We focus on two volcanic fields, La Garrotxa in northeastern Spain, and in particular the Pomareda and Roca Negra monogenetic cones in the Olot suite, and the Chuquet Genestoux occurrence, Puy-de-Dome, France. The explosive emplacement of the basanitic magma favored the entrainment of xenoliths of clearly crustal and mantle origin in both suites. We seek to characterize the megacrysts of scapolite and andesine. The sulfate-bearing scapolite crystallized in space group I 4/ m rather than the expected P 4 2 / n , likely because of disorder in the sulfate groups. The megacrysts in both suites are part of an igneous mantle-derived assemblage, to judge from δ 18 O values, supported by δ 13 C and δ 34 S values of the sulfate-bearing scapolite. We document signs of flash melting in both scapolite and plagioclase megacrysts from Pomareda. A white rim of up to 2 mm on the scapolite megacrysts consists of skeletal individuals of calcic plagioclase; these crystals contain wollastonite, silvialite, diopside, and myriads of vacuoles. The andesine megacryst also underwent melting at its margin, to a sieve-textured assemblage of neoformed labradorite, glass, and free silica. The flash melting of individual megacrysts encapsulated in basanitic melt, documented here for the first time, has produced metastable products. Progress of the melting reactions was arrested by a thermal quench upon eruption.

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&nbsp, → 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 (&gt, 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.

Chromite orebodies within the harzburgitic upper mantle have been investigated in several ophiolitic complexes, with a focus here on New Caledonia. The relationship between harzburgite and a dunite envelope around pods of chromitite points to a passive metasomatic transformation. The replacement of orthopyroxene by olivine leads to the necessary mobilization of Cr 3+ , not accepted by the neoformed olivine. A symplectitic intergrowth of enstatite + chromite at the interface between harzburgite and dunite may result from a silica-enriched fluid phase. Chromite in the orebodies contains micro-inclusions of olivine, clinopyroxene, orthopyroxene (rare), pargasite, aspidolite and nepheline. The olivine is hypermagnesian (up to 97 mol% Fo) and Ni-rich (0.25–1.05 wt%) and crystallized from a reducing fluid phase, not a magma. The pyroxenes and pargasite also are highly magnesian. The presence of Na in the environment accounts for pargasite, aspidolite and anomalously sodic nepheline. A Ni–Cu alloy is present. Chromite in the orebodies contains fluid inclusions, whereas chromite disseminated in harzburgite does not. The mixed fluid contains H 2 O (5 wt% equivalent of NaCl), with CO 2 and CH 4 in a 10 to 1 molar ratio. Values of δ 13 C VPDB (−22.1‰ for CO 2 and −28.1‰ for CH 4 , respectively) are typical of shallow marine hydrothermal systems. Experiments reveal that the precipitation of chromite from a fluid phase requires a reducing fluid and a temperature of ≤1050 °C. The appearance of olivine in the envelope results from the incongruent dissolution of enstatite in the fluid. The reaction is proven experimentally to be very efficient at a modest temperature (750 °C) and a low confining pressure (1.5 kbar). It leads to a shrinkage of the solid fraction, making the reaction self-propagating. Large volumes of harzburgite can be efficiently processed in an open, fluid-dominant reducing system. Relevant experiments show that the dissolution will increase in efficiency at higher temperatures and pressures. We do not negate the widely accepted explanation of chromitite formation by localized melt–harzburgite or melt–lherzolite interaction, but do point out that on the grounds of efficiency, such a process should be of secondary importance.

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 [Pd4(Bi,Te,Tl)O6 and Pd4(Tl,Bi,Te)O6] probably formed in situ by oxidation reactions at the expense of the associated PGM intergrowths. These involve palladium bismuthide-thallide phases, Pd5(Tl,As,Bi)and Pd5(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.

Dunite&ndash, harzburgite&ndash, olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84&minus, 89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30&ndash, 35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo&le, 89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula.

The Parker phlogopite mine, located near Notre-Dame-du-Laus, Quebec, 74 km north of Ottawa, is well known among mineral collectors for its centimetric euhedral crystals of black spinel. Among the dozens of phlogopite mines active in the early 1900s in the Mont-Laurier&ndash, Bancroft corridor in the Central Metasedimentary Belt of the Grenville Province, the Parker mine is exceptional because of the association of forsterite + spinel with phlogopite. Euhedral crystals of these minerals are found &ldquo, frozen&rdquo, in a carbonate matrix. The carbonate dike and segregations are associated with spinel-rich dunite that contains accessory diopside, phlogopite, and pargasite, as well as ilmenite and apatite. The interstitial melt crystallized to calcite + dolomite. Hematite appeared as flakes in the melt owing to net loss of hydrogen, and the spinel underwent oxidation-induced exsolution. Our spinel crystal entrapped a domain of carbonate during growth. It also entrapped globules of boundary-layer melt that crystallized to a carbonate + sulfate + phosphate + silicate + oxide assemblage. Such globules, where present in the cumulate, are more pristine than in the coarse crystal of spinel, i.e., less affected by a hydrothermal overprint. We contend that the carbonate melt ultimately formed by the hydrous melting of marble, as supported by oxygen-isotope data on all major minerals. Melting occurred 1140 million years ago, at a time of tectonic relaxation following the Shawinigan compressive stresses.

Ferri-fluoro-katophorite is the second species characterised involving the rootname katophorite in the sodium–calcium subgroup of the amphibole supergroup. The mineral and its name were approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification, IMA2015-096. It was found in the Bear Lake diggings, Bancroft area, Ontario, Canada, where coarse euhedral crystals of amphibole, phlogopite, sanidine solid-solution (now coarsely exsolved to microcline perthite), titanite, augite, zircon and fluorapatite crystallised from a low-viscosity silicocarbonatitic magma of crustal origin. Greenish grey prismatic crystals of ferri-fluoro-katophorite generally protrude from the walls into a body of coarsely crystalline calcite, but they also occur away from the walls, completely enclosed by calcite. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement is: A(Na0.55K0.32)Σ0.87B(Na0.79Ca1.18Mn2+0.03)Σ2.00C(Mg3.29Mn2+0.02Fe2+1.19Fe3+0.31Al0.09Ti4+0.08Li0.02)Σ5.00T(Si7.39Al0.61)Σ8.00O22W[F1.23 (OH)0.77]Σ2.00. Ferri-fluoro-katophorite is biaxial (–), with α = 1.640(2), β = 1.652(2), γ = 1.658(2), 2Vmeas. = 68.9(2)° and 2Vcalc.. = 70.1°. The unit-cell parameters are a = 9.887(3), b = 18.023(9), c = 5.292(2) Å, β = 104.66(3)°, V = 912.3(6) Å3, Z = 2 and space group C2/m. The strongest ten lines in the powder X-ray pattern [d values (in Å) I (hkl)] are: 2.708, 100, (151); 2.388, 74, (131); 3.139, 72, (310); 8.449, 69, (110); 2.540, 65, ($\bar{2}$02); 2.591, 53, (061); 2.739, 47, ($\bar{3}$31); 2.165, 45, (261); 3.279, 44, ($\bar{2}$40); 2.341, 43, ($\bar{3}$51).

We describe assemblages of platinum-group minerals (PGM) and associated PGE&ndash, 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&ndash, Ir&ndash, Ru compositions, which predominate, (2) Pt&ndash, Fe compositions of a Pt3Fe stoichiometry, and (3) Pt&ndash, Au&ndash, Cu alloys, which likely crystallized in the sequence from Au&ndash, (Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu2, to PtAu4Cu5. The general trends of crystallization of PGM appear to be: [Os&ndash, Ru alloys] &rarr, Pt3Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt&ndash, Fe phase) &rarr, Pt&ndash, 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+ + □ &rarr, 3Fe2+]. Many of the Os&ndash, Ru and Pt&ndash, Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt&ndash, 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&ndash, Os oxide (zoned) and seleniferous accessory phases.

We report the results of a mineralogical investigation of placer samples from the upper reaches of the Sisim watershed, near Krasnoyarsk, in Eastern Sayans, Russia. The placer grains are predominantly Os–Ir–(Ru) alloys (80%) that host various inclusions (i.e., platinum-group elements (PGE)-rich monosulfide, PGE-rich pentlandite, Ni–Fe–(As)-rich laurite, etc.) and subordinate amounts of Pt–Fe alloys. Analytical data (wavelength- and energy-dispersive X-ray spectroscopy) are presented for all the alloy minerals and the suite of micrometer-sized inclusions that they contain, as well as associated grains of chromian spinel. The assemblage was likely derived from chromitite units of the Lysanskiy mafic–ultramafic complex, noted for its Ti–(V) mineralization. In the Os–Ir–(Ru) alloys, the ratio Ru/Ir is ≤1, Ir largely substitutes for Os, and compositional variations indicate the scheme [Ir + Ru] → 2Os. In contrast, in the laurite–erlichmanite series, Ir and Os are strongly and positively correlated, whereas Ir and Ru are negatively correlated; Ru and Os are inversely correlated. These compositions point to the scheme [Os2+ + 2Ir3+ + □] → 4Ru2+ or alternatively, to Os2+ + Ir2+ → 2Ru2+. We deduce a potential sequence of crystallization in the parental rock and address the effects of decreasing temperature and increasing fugacity of sulfur and arsenic on the assemblage. Inclusions of Ti-rich minerals in the alloy grains are consistent with the Lysanskiy setting; the complete spectrum of chromite–magnesiochromite compositions indicates that an important part of that complex was eroded. A localized fluid-dominated micro-environment produced the unique association of laurite with monazite-(Ce), again considered a reflection of the special attributes of the Lysanskiy complex.

Pegmatites are extremely coarse-grained and heterogeneous rocks in which quantitative measurements of mineral proportions and chemical compositions of the whole rock are virtually impossible to acquire. Thus, conventional criteria such as bulk compositions and modal mineralogy used for the classifications of igneous rocks simply cannot be applied for pegmatites. An alternative is to use the mineralogical and chemical attributes of K-rich feldspars, the only mineral that is omnipresent in pegmatites. We have used this approach to test a possible discriminant among four groups of pegmatites on the basis of major petrological features, such as the abundance of quartz, feldspars, micas and phosphates. Group I is represented by relatively flux-poor, and silica-poor pegmatites, in most cases with hypersolvus feldspars, devoid of quartz and with minor biotite, which are common in rift settings as in the Coldwell Alkaline Complex in northwestern Ontario, Canada. Group II comprises relatively flux-poor, silica-rich pegmatites with quartz, subsolvus feldspars and biotite as major primary minerals, typically occurring in the asymmetric collisional Grenville Orogeny. Group III comprises relatively flux-rich, silica-rich P-poor pegmatites with quartz, subsolvus feldspars, and muscovite as the major primary minerals. Finally, group IV consists of relatively flux-rich, silica-rich, P-rich pegmatites with the same previous major minerals as in group III but with abundant phosphates. Group III and IV are found in most symmetric collisional orogens, such as in the Eastern Brazilian Pegmatite Province as the result of the collision of cratons mainly formed by igneous and metamorphic rock of Archean and Early Proterozoic age. We have selected specimens of blocky perthitic K-rich feldspar from the inner part of thirty-one pegmatites belonging to these four categories occurring worldwide to cover a wide range of mineralogy, geological age, geotectonic setting and geographical positions. Concentrations of major elements (Si, Al, K, Na, Ca, Fe, Mg, Mn, Ti and P) were obtained by X-ray fluorescence (XRF), and those of minor and trace elements (P, Fe, Li, Ge, Ga, Rb, Sr, Ba, Tl, Pb, Y, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were established by laser-ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS), in areas free of coarse Na-feldspar veins or patches. We show that the four groups have very different average values of the minor and trace elements. However, only the cations occupying tetrahedral sites, particularly the Fe and P, are sufficiently immobile to show distinct differences among pegmatites. Hence, we propose a P–Fe diagram to discriminate among the four groups of pegmatites, as a possible criterion with which to classify pegmatites.

Most geoscientists are convinced that minerals of both niobium and tantalum contain those elements in the 5 + state only. In fact, there are several well-documented minerals containing Nb 0 , Nb 2 + , Nb 4 + and Nb 5 + , and tantalum follows a similar pattern. Both elements can exhibit a siderophile, a chalcophile and a lithophile character, as well illustrated empirically, and as documented via XANES and XPS spectra. Of the XPS spectra that we report here on ferroniobium of metallurgical grade, pyrochlore, samarskite, niocalite, synthetic edgarite (FeNb 3 S 6 ) and NbTe 2 , only the samarskite and niocalite contain only one valence state of Nb (5 +). Vladimir Vernadsky understated the situation when he placed Nb and Ta in the “dispersed” category. Columbite-group minerals are most unusual in exhibiting variable mutual disorder involving (Nb,Ta) and (Fe,Mn). Oddly, samples must be heated to become ordered. We raise the possibility that at the stage of primary crystallization, the relevant magmas contained mixed valences of Nb, Ta, Fe and Mn, which made a disordered distribution along the octahedrally coordinated cations energetically more acceptable. Owing to the relative insolubility of members of this solid-solution series in the ambient aqueous fluid, they have remained metastably frozen in a partly or completely disordered state.

As Richard H. Jahns used to say, granitic pegmatites are prone to stew in their own juice. The early juice acts as a catalyst in transforming the early-formed disordered K-feldspar to microcline below 450°C. As it becomes progressively enriched in fluorine during the system’s evolution, the exsolved fluid becomes increasingly aggressive, and can create cavities by dissolution. It eventually leaks into the country rock, mostly from the apical parts of zoned pegmatites. There, it interacts with plagioclase in any lithology it encounters. As the plagioclase is highly strained and kinetically stuck, it is consumed, and the fluid quickly becomes more aluminous, more acidic and Ca-bearing. We envision a convecting fluid phase cycling around the pegmatite body, and a reaction path that brings the acidified fluid back into the heart of the pegmatite. There, it is aggressive toward the microcline perthite, which is replaced by cleavelanditic albite + muscovite + quartz, mostly in the range 250–400°C, a process that enlarges the original cavities and creates new ones. Miarolitic pegmatites of either NYF and LCT affiliation contain clear signs of the presence of cations brought in from outside the system, mostly Ca (causing a miniflood of Ca), but also Sr, Fe, Mg, Sc, Li and B. Other elements are efficiently recycled internally. The recurrent cycling of an aqueous fluid has major ramifications concerning the economic aspects of granitic pegmatites. In particular, the cycles of hydrothermal enrichment in a large LCT pegmatite can give rise to zones enriched in micas of the lepidolite series, pollucite, tantalite-(Fe) and tantalite-(Mn), i.e. , super-LCT deposits.

Rare high-temperature and low-pressure contact-metamorphic assemblages are known to occur in the Guneyce–Ikizdere area, eastern Black Sea, Turkey. The emplacement of plutonic rocks of the calc-alkaline Rize batholith has caused the development of assemblages belonging to the spurrite–merwinite subfacies of the sanidinite facies. We provide compositional data for spurrite, rustumite, hillebrandite, tilleyite, cuspidine, vesuvianite, monticellite and gehlenite, and the secondary calc-silicates foshagite, dellaite, killalaite, bicchulite, hydroxylellestadite and two mineral species first discovered in these skarns, defernite and trabzonite. Our data point to the possible existence of a fluorine-rich variant of rustumite and a hydroxyl-dominant variant of cuspidine. These minerals are associated with familiar skarn minerals. The temperatures achieved are considered to be in the range 800–850°C on the basis of published phase-equilibrium results. The unusually high temperatures of contact metamorphism next to compositionally intermediate magmas may well reflect the presence of an underplating of basic magma in the vicinity and the efficient transfer of heat by the rising fluid phase. The relatively high concentrations of Cl and F encountered in these skarn assemblages are consistent with hydrothermal activity leading to Kuroko-type mineralization in this orogenic setting.

We report results of electron-microprobe analyses for detrital grains of chromian spinel ( n ≈ 800) and associated minerals recovered from modern fluvial and buried paleochannel (Pt–)Au placer deposits ( n = 30) of mostly Holocene age distributed throughout British Columbia. The majority of analyzed spinel-group minerals are members of the chromite–magnesiochromite solid-solution series, and extend to Fe 3+ -rich compositions of chromian spinel. The values of mg# [100Mg/(Mg + Fe 2+ )] and cr# [100Cr/(Cr + Al)] in grains from all 30 placer samples occupy a relatively narrow range, 58–82 and 68–92, respectively, consistent with a mafic to ultramafic source. Four types of core-to-rim patterns of zoning have been identified: 1) a weak decrease in Cr coupled with a strong decrease in mg# at typically constant cr# and fe 3+ #, as a result of limited chromian spinel – liquid fractionation and subsolidus Fe 2+ –Mg exchange with ferromagnesian silicates; 2) a sharp decrease in Cr and cr# and increase in fe 3+ # [100Fe 3+ /(Al + Cr + Fe 3+ )] with limited decrease in mg#, reflecting magmatic fractionation; 3) a significant increase in Cr and cr# with weak to strong decrease in mg#, reflecting late-stage interaction with a Cr-rich magma followed by variable enrichment in Fe 2+ during cooling; and 4) a strong decrease in Cr and cr# with a small increase in mg# and fe 3+ #, likely due to interaction with a more oxidized melt and subsolidus re-equilibration with an aluminous phase (Al-rich residual liquid or amphibole). Values of mg# in placer silicate grains (olivine, clinopyroxene, either Na–Al-poor or Na–Al–Ti-rich, rare aluminous orthopyroxene, edenitic amphibole, among others) and in micro-inclusions in chromian spinel attain 90–95. We evaluated the provenance of chromian spinel grains in the placers using the global spinel database and a new one focusing on Alaskan-type intrusions. Chromian spinel in the Atlin area and a majority of Dease Lake placers exhibits a pronounced Cr–Al trend indicative of an ophiolitic affinity (Slide Mountain and Cache Creek oceanic terranes). In certain placers in the Tulameen area, the chromian spinel defines a Cr–Fe 3+ trend indicating derivation from the Tulameen Alaskan-type intrusion in the Quesnellia island-arc terrane. Placer deposits in central British Columbia (Manson Creek – Cariboo region) display a mixed heritage involving both ophiolitic and Alaskan-type sources. The recognition of an Alaskan-type mafic to ultramafic source for some of the Cr–PGE-bearing placers in central British Columbia underscores the potential for bedrock mineralization in an area where such intrusions are presently poorly represented.

The emplacement of anorogenic magmas, be they mantle-derived or crust-derived and silica-undersaturated or silica-oversaturated, marks a period of rifting or tectonic relaxation and apparent quiescence. In a given area, such magmatism commonly recurs episodically, and can yield even more strongly alkaline products than in the first cycle, in spite of the depletion that resulted from that episode of melting. Anorogenic magmatism is said to be punctuated where it recurs, in response to a triggering mechanism. The second cycle reflects an influx of heat and a fluid phase responsible for the fertilization of the depleted source-rock. In cases of an anorogenic stage after a major collision, the first cycle of magmatism, yielding an AMCG suite, arises by gravity-induced sinking of lithosphere and the diapiric rise of an asthenospheric mantle; renewed magmatism may involve localized and renewed detachment as late as 200 m.y. after the collision. Where the hiatus is much longer, as in Nigeria, we appeal to a propagating zipper-like zone of extension, possibly related to rotation of a crustal block. The economic ramifications of punctuated anorogenic magmatism are important; the second-generation magmas may well crystallize products that are mineralized in the high-field-strength elements and any other elements enriched in the source rocks. Such a model would account for the rich deposits of alluvial columbite, zircon and cassiterite associated with the Younger Granites of Nigeria.

The El Muerto granitic pegmatite, of Grenville (Neoproterozoic) age, part of the Oaxacan Complex in southern Mexico, contains an exceptional development of HFSE-enriched accessory phases. We report here compositional data and textures involving the exotic minerals of this suite, in particular allanite-(Ce), perrierite-(Ce), monazite-(Ce), thorite, fergusonite-(Y), aeschynite-(Y), yttrium-rich betafite, polycrase-(Y), uranopolycrase, niobian rutile and a variant of thorutite. This pegmatite has the earmarks of a NYF suite, but is devoid of F. El Muerto is a member of an anorogenic anorthosite – mangerite – charnockite – granite (AMCG) suite, understood to be emplaced as a result of gravitational collapse of the lower crust following a collision. In common with other suites of anorogenic affiliation, we attribute the unusual enrichment in HFSE to a stage of metasomatic fertilization of a source in the lower to middle crust prior to its melting.

The alkali feldspars in members of the epizonal Kwandonkaya complex, one of the Younger Granite anorogenic plutons in north-central Nigeria, became frozen in a state of arrested reaction as the complex cooled. High sanidine solid-solution, the liquidus phase expected in porphyritic rocks in early ring-dykes and in cone sheets, has given way to exsolution-induced assemblages involving more ordered species, owing to interaction with an aqueous fluid as a necessary catalyst. The rocks now contain grains of orthoclase perthite, possibly zoned structurally, and structurally intermediate microcline; well-ordered microcline is rather sparse. The three intrusive centers, Buri, Jubga–Ziem and Panshanu, show a progression in terms of the time available for equilibration and the activity of H 2 O in the melt. The feldspars of this metaluminous complex define a transition from a hypersolvus to a subsolvus texture. We document clear signs of magma degassing in miarolitic pockets in an apical region at the Jubga–Ziem intrusive center. The peraluminous compositions that result, leading to unusual clay-mineral structures surrounding the insoluble accessory phases, are an expression of loss of alkalis due to degassing, and not an attribute of the felsic magma in this association. The prevalence of orthoclase and intermediate microcline in perthite at Kwandonkaya, either separately or coexisting, is considered representative of other centers of Mesozoic A-type granite and syenite emplacement in Nigeria.

The meteorite impact that created the 32-km-wide Late Carboniferous West Clearwater structure, situated about 125 km east of the Hudson Bay arc, northern Quebec, formed a sheet of impact melt now exposed on a central ring of islands. We describe the textures and mineralogy of two xenoliths of partially metamorphosed limestone. These blocks represent recrystallized remnants of a cover of Middle Ordovician limestone, now completely eroded in the area. The fossiliferous dolomite-bearing limestone recrystallized to granoblastic calcite + periclase marble when the blocs landed in the sheet of impact-generated melt. The silica content of the marble led to the formation of a sanidinite-facies assemblage of unusual nesosilicates such as spurrite, merwinite, and monticellite. Aluminian srebrodolskite, Ca 2 (Fe 3+ ,Al) 2 O 5 , is found in material with a composition similar to “Portland cement clinker”. Phase equilibrium and stratigraphic data constrain the minimum peak conditions of metamorphism at T > 815°C, P X (CO 2 ) > 0.1. Retrograde mineral assemblages indicate that X (CO 2 ) dropped to less than 0.1 shortly after the peak of metamorphism. Portlandite and brucite formed during the late hydration of the assemblage upon cooling.

Five placer grains of Pt–Fe alloy, ca. 0.5 to 2 mm, roundish in shape, and hitherto unconfirmed from this locality, were recovered from a placer deposit in Burwash Creek, Kluane area, Yukon. Our results of multiple electron-microprobe analyses (EMP) display the presence of elevated levels of Pd (0.49–5.84, mean 2.38 wt.%), Rh (1.05–1.94, mean 1.50 wt.%), Ir ( 45.7–47.5 Mg 37.5–48.0 Fe 4.9–16.8 , mg # = 100Mg/(Mg + Fe 2+ ) in the range 69.0–90.7], plagioclase (Ab 57.5–60 An 35.1–39.9 Or 2.6–4.8 ), and sodic–calcic amphiboles (0.6 and 1.1 Na apfu ), some of which are rich in Cl (0.72 and 1.17 wt.%, or 0.17 and 0.36 Cl apfu , respectively). The analyzed inclusions of PGM are members of the bowieite Rh 2 S 3 – kashinite Ir 2 S 3 solid-solution series, miassite (Rh,Pt,Pd) 17 S 15 , cooperite PtS, isoferroplatinum Pt 3 Fe (or Fe-rich platinum), vasilite (Pd,Cu,Pt) 16 S 7 , keithconnite Pd 20 Te 7 , an unusual Pt–Pd–Ni–Cu alloy ( i.e. , Pd-rich platinum: Pt 47.7–52.8 Pd 29.3–37.9 Ni 6.3–8.0 Cu 6.1–7.5 ), and a sulfotelluride of Pd, Pd 11 Te 2 S 2 (?), which may represent a new isomertieite-type compound. The trace-element composition of the silicate melt inclusions determined using laser-ablation ICP–MS shows a notable enrichment in large-ion lithophile elements. We infer that the reported association of PGM and the trace-element composition of silicate melt inclusions observed at Burwash Creek probably correspond to an Alaskan–Uralian-type mineralization; considered less likely is a relation with a Triassic ultramafic–mafic complex (sill-like) of so-called Kluane-type, also developed in the placer area.

Patterns of compositional zoning are documented in nine grains of magnesiochromite–chromite (Mgc–Chr), 0.3–0.4 mm in size, subhedral or subrounded, recovered from six placer deposits in British Columbia. Their core zones correspond to Mgc or magnesian Chr, mantled by zones of Chr. A rim of Zn-rich chromite (9.1% ZnO) occurs in one grain. In most of these grains, Fe 2+ increases and Mg decreases toward the margin. In contrast, an “anomalous” rim, slightly richer in Mg than the core, is developed in one grain, in which an increase in Mg (≤2% MgO) is accompanied by a corresponding increase in Fe 3+ , Al, minor Ti, and by a decrease in Fe 2+ and Cr. In the other zoned grains, contrasting trends are documented for Fe 3+ and Cr, which decrease or increase during crystallization toward the margin. Aluminum covaries with Fe 3+ and Cr, as a reflection of different mechanisms of coupled substitution. In general, various factors may have exerted a control over the observed zoning, such as 1) loss of Mg during subsolidus re-equilibration between the Mgc–Chr and coexisting olivine, 2) the regime and variations in levels of oxygen fugacity, f (O 2 ), 3) a re-equilibration between the Mgc–Chr and a late melt enriched in Cr, and (4) accumulation of Al in a remaining melt or loss of Al during a re-equilibration at a lower temperature. The Zn was probably introduced into the rim by a reaction involving a late-stage fluid phase. The rim richer in Mg may reflect a diffusion-controlled mechanism of growth resulted from a late-stage reaction with an isolated portion of the more oxygenated melt rich in Fe 3+ , Al and Ti and relatively depleted in Cr. Two types of potential source-rocks, ophiolitic and Alaskan-type, are evaluated for these placer grains of zoned Mgc–Chr.

We describe an unusual pattern of zoning in an amalgam of Au and Ag; it occurs as peripheral zones in a 0.3 × 0.15 mm grain in a placer deposit associated with the Tulameen – Similkameen river system, British Columbia. This amalgam formed at the expense of primary Au–Ag alloy, which is present as relics in the core: Au 54–55 Ag 45–46 (Hg-free; not zoned). Abundances of Au gradually increase outward and complement decreasing levels of Ag in the amalgam. Values of Au/Ag increase from 1.1–1.2 in the core to 1.8 at the margin of the grain. The observed range of compositions, (Au 1.50–1.87 Ag 1.06–1.40 ) ∑2.81–3.00 Hg 1.00–1.19 , is limited. Two possibilities are discussed to explain the formation of the zoned amalgam. (1) This phase could be natural, formed by reaction between primary Au–Ag alloy and a late melt or oxidizing fluid, which may have remobilized and transported Hg as a result of destabilization and oxidation of cinnabar. If so, the observed zonation would reflect an increasing activity of Au species (and decreasing activity of Ag) during crystallization. (2) This phase may have formed under supergene conditions, as a result of anthropogenic introduction of Hg into placer gravels during historic mining operations in this region; we favor this hypothesis. Zoning in the amalgam may have been controlled principally by electrochemical factors related to the process of “self-electrorefining”. Gold is more “noble” than silver, so that ions of Au probably migrated and were preferentially deposited closer to the margin of the grain. This mechanism is similar to that responsible for the “secondary” rim rich in Au (poor in Ag) in placer grains of Au–Ag alloy, as reported from other localities worldwide.

We report results of multiple electron-microprobe analyses of nine grains of alloy minerals, 2–5 mm in size, rich in the platinum-group elements (PGE), from a unique, very old collection (~1890s) of placer material from Trinity Co., California. Osmium, iridium, ruthenium, and rutheniridosmine are the principal alloy species poor in Fe (typically

We report results of detailed electron-microprobe analyses (EMP) of fourteen grains of Pt–Fe alloy, ca. 0.4 to 1 mm, principally droplet-shaped or roundish, from Florence Creek, Yukon. We also describe new occurrences of Pt–(Pd)–Fe alloy from Arch Creek (up to 11 wt.% Pd) and Canadian Creek, Yukon. An extensive compositional series is observed at Florence, in which values of ∑PGE/(Fe + Cu + Ni) vary from 2.2 to 5.4. Minor Ir and Os display a contrasting behavior in this series. The observed levels of Ir are generally greater than those of Os. The Ir correlates negatively with Pt (the correlation coefficient, R, is −0.81; 140 EMP point-analyses); in contrast, the correlation between Os and Pt is slightly positive (R = 0.64). Probably, at a higher temperature of crystallization, Ir readily replaces Pt in the structure of a Pt–Fe alloy. As a result, Ir did not attain its maximum, and the observed inclusions of exsolution-induced Ir-dominant alloy, hosted by the Pt–Fe alloy, are moderately enriched in Ir (52–54 at.%). An inclusion of erlichmanite (4.2 wt.% Rh) and a partial rim of Au–Ag alloy (Au 0.80 Ag 0.18 ) were observed. Droplet-like inclusions of highly aluminous silicate minerals, hosted by grains of Pt–Fe alloy, are rich in Na, K and in Cl (up to 2 wt.%). Veinlets and micrograins of an Os-rich alloy (86–92 wt.% Os) are intimately associated with micrograins of nearly pure albite, Ab 97.8 An 1.7 Or 0.5 , and with an intermediate member of the chamosite–clinochlore series. The observed association chamosite + albite + native osmium, infilling a cavity in a host grain of Pt–Fe alloy, was likely deposited from a late Na–H 2 O–(Cl)-bearing fluid phase at a postmagmatic–hydrothermal stage, at a temperature less than 700°C. The Os and Na were possibly mobilized and transported as Na–(Os)-rich complexes related to the sodium hydroxy- or oxo-osmates. The precipitation of the metallic Os phase could be related to a change in physicochemical conditions, e.g., in fluid pressure or redox potential, leading to instability of these Na–(Os)-rich hydroxy- or oxo-osmate complexes. We suggest that a mineralized zone rich in chromite–magnesiochromite and hosted by an Alaskan–Uralian-type complex is the likely provenance for grains of Pt–Fe alloy recovered in Florence Creek.

We document the first natural occurrence of oscillatory zoning in hematite, due to cryptic variations in tin content. Our studies bear on a placer grain ( ca. 0.5 × 0.7 mm) from a deposit located in Canadian Creek, Yukon, characterized by electron-microprobe analysis (EMP) and Raman spectroscopy. This grain was found in association with ferberite and hubnerite, which form an extensive and nearly continuous series (with up to 10–12 mol.% MgWO4 in ferberite), bismuthinite, daubreeite (?) [BiO(OH)], and tetradymite (Bi2Te1.9S1.1), which displays a minor deviation from stoichiometry owing to S-for-Te substitution. The compositional data, obtained via EMP traverses oriented across oscillatory zones (up to 10 μm thick, diffuse, and “bright” in back-scattered electron imaging) indicate that this zoning is a reflection of covariations in Fe, Sn, and, to a lesser degree, Al. The “bright” oscillatory zones are enriched in Sn, up to 2.26% SnO2, and in Al, up to 0.42% Al2O3, and are correspondingly poorer in Fe. The Sn-enrichment observed in hematite could be related to a solid-solution series from α-Fe2O3 toward a mixed-valence compound Sn2O3, yielding a mechanism (Sn4+ + Sn2+) ↔ 2 Fe3+. However, a regime of low values of oxygen fugacity would be required to stabilize Sn2+, which seems unlikely in this system. Instead, a series could extend from α-Fe2O3 toward SnO2 (cassiterite). Presumably, Sn4+ enters octahedral sites; a substitution Sn4+-for-Fe3+ is combined with a reduction of Fe3+ to Fe2+ in order to maintain charge balance. We prefer this mechanism, and suggest that the incorporation of minor Al3+ may occur to minimize lattice strain due to the incorporation of Sn4+ and Fe2+. This grain of Sn-bearing hematite may have been derived, along with the W- and Bi-rich minerals, from a mineralized zone associated with a fluid-rich environment, probably a hydrothermal deposit (possibly Au-bearing), which is related to Cretaceous felsic magmatism developed in the placer area.

We have conducted electron microprobe (EMP) analysis of 158 grains of platinum-group minerals (PGM; 0.1–1 mm in size) from 11 placer samples collected from Holocene fluvial placers and buried paleochannel placers at various localities in British Columbia. These grains principally comprise Pt-Fe-(Cu) alloy minerals: Fe-rich platinum [ΣPGE:(Fe + Cu + Ni) = 3.6–7.6], Pt3Fe-type alloy (isoferroplatinum or Fe-rich platinum), subordinate “Pt2Fe”-type alloy (probably, a compositional variant of Fe-rich platinum) and the tulameenite-tetraferroplatinum series. Less-abundant are iridium [Ir-dominant Ir-Os-(Pt) alloy] and osmium [Os-dominant Os-Ir-(Pt) alloy]. Ruthenium [Ru-dominant Ru-Ir-Os alloy] occurs as a single grain. One of these Pt-Fe alloy grains is unusually zoned; its core zone is: Pt74.0Fe20.4Cu1.9Ir1.5Rh1.1Pd1.0Os0.08Ru0.01Ni0.01 (in at%) [ΣPGE:(Fe + Cu + Ni) = 3.5], and its rim zone is: Pt78.5Fe15.5Cu1.7Ir1.5Rh1.4 Pd1.2Ni0.15Os0.06Ru

The crystal structures of four amphiboles from the Bear Lake diggings, in the Bancroft area of Ontario, fluorotremolite, fluororichterite and two samples of potassian fluor-magnesiokatophorite, have been refined to R indices in the range 1.5–2.2% using ~1380 unique reflections measured with Mo K α X-radiation. The crystals used in the collection of the X-ray intensity data were subsequently analyzed by electron and ion microprobes, and formulae were calculated using the Fe3+ and Fe2+ contents established by crystal-structure refinement. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. Difference-Fourier maps through the A (2/ m ) site show that the A ( m ) site is occupied and the A (2) site is vacant in all crystals. Occupancy of the A ( m ) site, and not the A (2) site, by Na imposes strong constraints on the possible patterns of short-range order in these amphiboles. The site populations of the A ( m ), M (4) and O(3) sites are in accord with the local arrangement A ( m )(Na + K)–O(3)F– M (4)Na, with lesser amounts of A ( m )(Na + K)–O(3)OH– M (4)Na and major to minor amounts of A □–O(3)OH– M (4)Ca. The coupling of elements to specific short-range-ordered arrangements in these crystals suggests that the overall chemical variability of amphiboles is strongly constrained by local bond-valence restraints in addition to overall electroneutrality.

Dikes of albitite cross-cut two massifs of serpentinized lherzolite in the Pyrenees, France. At Espechere (Hautes-Pyrenees), they are corundum-bearing, whereas at Urdach (Pyrenees-Atlantiques), some dikes contain corundum, others do not. The majority of the minerals described here occur in both types of albitite. Albite is predominant, associated with muscovite and biotite, and with chlorite, epidote, zircon, titanite, thorite, pyrochlore ( s.l. ), aeschynite-(Ce), ferrocolumbite, rutile, ilmenite and magnetite as accessory phases. The epidote shows very high Sr contents (3 to 6 wt%, and up to 8.8 wt% SrO). However, certain minerals seem more specific to one type of dike. In corundum-bearing albitite, ilmenorutile, samarskite ( s.l. ), manganocolumbite, fersmite and fergusonite-(Y) are found, whereas in corundum-free albitite, allanite-(Ce), chevkinite-(Ce), apatite and monazite-(Ce) are developed. There are also subtle differences between corundum-bearing albitites of Urdach and Espechere: the composition of samarskite ( s.l. ) varies from samarskite-(Y) (Urdach) to ishikawaite (Espechere), but the two may be altered to calciosamarskite. Likewise, the composition of pyrochlore ( s.l. ) shows a strong variation, from weakly light-REE-enriched betafite and pyrochlore (Urdach) to Pb-, U-rich pyrochlore ( s.s. ) (Espechere). The textures, dominantly pegmatitic, reflect a rapid magmatic crystallization, large crystals of albite predating the fine-grained carriers of Ca, Sr, Fe, Mg, Zr, Ti, Y, REE, U and Th.

Des filons d'albitite pegmatitique recoupent deux massifs de Iherzolite serpentinisee, a Urdach et Espechere, sur le versant nord des Pyrenees occidentales en France. Ces roches presentent des caracteres mineralogiques qui les rapprochent des assemblages a feldspath alcalin ± corindon ± zircon formant des xenolithes, voire des megacristaux isoles dans de nombreuses provinces basaltiques alcalines. Les donnees geochimiques et isotopiques attestent d'une origine mantellique pour ces roches, dont la composition globale a Si-Al-Na dominants presente de fortes ressemblances avec celles de certaines inclusions vitreuses observees a l'echelle microscopique dans les xenolithes de peridotite ramones par les basaltes alcalins. Leurs carateres geochimiques et les donnees disponibles en petrologie experimentale menent a l'hypothese que ces roches ont cristallise a partir d'un liquide produit selon un tres faible degre de fusion partielle (

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) A, c = 4.3377(6) A, and V = 27.99(1) A3 ; 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/cm 3 . 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 ultramafi c-mafi c rocks of ophiolite affi nity. A fl uid phase may well have remobilized and transported W, Mo, and Fe. The W-rich alloy likely crystallized from a reducing fl uid 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.

Various platinum-group minerals (PGM), phases rich in the platinum-group elements (PGE), and unusual species occur in the Kirakkajuppura area, Penikat layered complex, Finland. The PGM are intimately associated with actinolite and clinochlore. The base-metal sulfides [chalcopyrite, millerite, bornite, chalcocite, ferroan chalcocite(?), digenite or roxbyite: Cu9S5] are in minor or trace amounts. The observed PGM and rare species are: vysotskite–braggite series [PdS–(Pt,Pd)S], zvyagintsevite [Pd3Pb], a zvyagintsevite-like phase [(Pd,Pt)3(Pb,Cu)], unnamed Pd–Pb oxide [Pd7PbO8 or Pd3.5Pb0.5O4], various Cu–Fe–PGE thiospinels (large grains and unusual micro-aggregates), laflammeite [Pd3Pb2S2], laurite–erlichmanite series [(Ru,Os,Ir)(S,As)2 –(Os,Ru,Ir)(S,As)2], irarsite–hollingworthite series [(Ir,Rh,Ru)As1− x S1+ x – (Rh,Ir,Ru)As1− x S1+ x ; x ≤ 0.2], konderite-like Pb–(Pd)-rich sulfides and a Fe-dominant analogue of konderite [(Cu,Ni,Fe)3+ x (Fe,Pb)(Rh,Ir,Pd)8− x S16; x < 0.5], unnamed Rh1− x (Ni,Fe,Cu)2+ x S3 (0 ≤ x ≤ 0.1), a Pd–Pt–Cu alloy [Pt-rich, Cu-rich palladium: Pd89.42–91.22Pt5.94–6.16Cu2.62–4.64], skaergaardite–hongshiite [(Pd0.92 Pt0.10)∑ 1.02Cu0.98], keithconnite [Pd20(Te,Bi)7], plumbian keithconnite [Pd20(Te6Pb)], unnamed Pd5(As,Te,Sn,Pb)2, Pd-rich, Cu-rich gold [Au76.1–81.1Pd13.7–16.0Cu6.1–6.3Ag1.2–3.4], Zn-rich copper (natural?), unnamed Pb4O(VO4)2, and an undefined Pb-rich iron silicate (hydrous?). In addition, unnamed (Th,Ca)(V5+,Si,P)O4 and associated baddeleyite, barite, and a phosphatian huttonite or thorite occur in a pegmatitic gabbro from Kirakkajuppura. The Kirakkajuppura PGE mineralization probably formed by efficient hydrothermal remobilization and transport of PGE from their “primary” zones [Pd–Pt zones associated with the low-sulfide SJ reef and Ru–Os-mineralized chromitite zones], resulting in the local deposition of the extremely S-poor, PGE-rich assemblage, which is intimately associated with hydrous silicates and is located near the northern end of the Penikat complex.

The plutonic rocks in the Kwandonkaya complex, located within the NYG province of Nigeria, have some hypersolvus granites composed mainly of orthoclase microperthite and interstitial annite. These are inferred to have formed from a relatively F-poor, and relatively dry felsic melt. During cooling, Al–Si order was not completely achieved when the inversion of sanidiness to orthoclase and exsolution occurred. A majority of the granites contain intermediate to low microcline with annite to siderophyllite. The samples were incipiently modified in the subsolidus at very low fluid–rock ratios. Drusy granites result from resurgent boiling and volatile loss, which produced orthoclase-dominant feldspar and zoned zinnwaldite, with microcline lining cavities, whereas late loss of volatiles resulted in low microcline and zinnwaldite and metasomatism associated with cassiterite-topaz mineralization. Mica composition in both types of drusy granite is similar and seems to have been fluid-buffered. Albitization was rock-buffered and resulted in variable degree of Al–Si order in K-feldspar and mild modification of mica composition. Key factors affecting both the degree of Al–Si order of K-feldspar and mica compositions at Kwandonkaya seem to be the degree of volatile build-up and loss, and extent of fluid–rock interactions.

In mixed-layer illite/smectite containing >80% illite, the presence of an expandable component and changes in its abundance are difficult or impossible to detect with the X-ray diffraction (XRD) method using conventional polar interstratification compounds, e.g., solvation with ethylene glycol. Although high-grade diagenetic and very low-grade metamorphic (VLGM) samples do not expand in response to ethylene glycol treatment, the illite crystallinity (IC) improves significantly with increasing diagenetic/metamorphic grade. Improvement of IC in anchizonal to epizonal samples previously was thought to be related primarily to increase in crystallite thickness and decrease in the number of stacking faults. The present study reexamines the contribution of expandable components to IC in shale and slate samples from the Gaspe Peninsula for both the

A single-pass flow-through experiment was conducted to induce serpentinization, and to relate serpentine mineralogy to reaction progress during open-system alteration of San Carlos olivine by air-saturated water. The fluid flowed at 300 °C and 300 bars for 1368 hours through crushed olivine contained in a tubular titanium reaction cell. Chemical analysis of the fluids indicated a steady-state composition with respect to Mg and pH after 550 hours, which was interpreted to reflect saturation with brucite and one or more serpentine-group minerals. The alteration products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Nucleation and growth characteristics of serpentine phases on the fracture surfaces of olivine were studied by TEM using platinum/carbon replica techniques. After reaction, olivine fragments displayed clear signs of dissolution, and growth of neoformed phases. Brucite and magnetite were distributed uniformly through the reaction cell, although magnetite was more abundant. There is, however, a distinct zonal distribution of the serpentine phases from the inlet to the outlet of the reaction cell. Lizardite is the dominant serpentine phase at the inlet, whereas fibrous serpentines are predominant near the outlet. Lizardite crystals are more numerous (but smaller) in the middle of the cell, and are topotactically aligned along a single preferential crystallographic orientation of the olivine. Cylindrical chrysotile formed predominantly on the surfaces of lizardite, conical or polygonal serpentine. The morphology and sorting of the serpentine phases through the reaction cell suggest that crystallization of lizardite was controlled by heterogeneous nucleation on olivine under supersaturated conditions, and crystallization of chrysotile by nucleation on lizardite at higher levels of supersaturation.

We describe an ilmenite-bearing aplitic syenite dyke in the roof zone of the Tibchi granite, exposed at Kalato, in the Tibchi ring-complex, northern Nigeria. Inclusions of ferrocolumbite, rutile and ixiolite in the ilmenite are inferred to have been trapped at the magmatic stage. The main mafic mineral is annite. Compositionally, the ilmenite, rutile and ferrocolumbite have near-end-member compositions. A positive correlation between Sc and Ta/(Ta+Nb) indicates that Sc behaved incompatibly as ferrocolumbite grew. Such entrapped accessory minerals may well have formed by local saturation at the ilmenite-melt and annite-melt interface. During and after their crystallization, the melt reached saturation in H2O and degassed. A second generation of ilmenite enriched in Mn and Zn replaced the primary ilmenite along fractures and grain margins. As fO2 began to increase, composite blebs and rinds of ‘ferropseudobrookite’, rutile and hematite began to develop by oxidation-induced exsolution in the primary ilmenite. Incorporation of Nb, Ta, Sc and Si in the ‘ferropseudobrookite’ may well have stabilized it at Kalato. Ultimately, it is transformed to hematite + rutile. The IMA-sanctioned view that the solid solution between pseudobrookite and Ti3O5 is complete, and thus that ‘ferropseudobrookite’, as an intermediate member of the series, does not merit species status, needs to be re-evaluated.

Laflammeite, Pd 3 Pb 2 S 2 , is a new parkerite-type sulfide discovered in the Penikat layered complex, Finland. It occurs in altered pyroxenite in the Kirakkajuppura deposit in association with actinolite and clinochlore, accessory chromite, various platinum-group minerals [vysotskite, zvyagintsevite, cuprorhodsite – (ferrorhodsite) – malanite, laurite – erlichmanite, irarsite, keithconnite, unusual Pb–(Pd)-rich thiospinels of Cu, Fe, Rh and Ir, unnamed Pd 9 PbO 10 and Rh(Ni,Fe,Cu) 2 S 3 ], Pd-rich gold, and minor base-metal sulfides (chalcopyrite, bornite and millerite). Laflammeite occurs as individual subhedral to euhedral crystals up to 0.3 mm in size, and in intergrowths with Pt–Ni-poor vysotskite. Laflammeite is opaque and cream-colored with a brownish tint in reflected light in air. The anisotropy is weak, from brownish gray to grayish brown. The bireflectance is weak; pleochroism was not observed. Reflectance percentages in air (and in oil) are, for R 1 and R 2 , 45.3, 46.2 (31.55, 32.45) at 470 nm, 46.5, 47.55 (32.6, 33.75) at 546 nm, 47.7, 48.8 (33.9, 35.05) at 589 nm, and 49.6, 50.0 (36.0, 36.3) at 650 nm. Fine twins are present. Microhardness values VHN 50 are 156 and 185; the average is 171 kg/mm 2 . Cleavage is {010}, perfect. The average results of 26 wavelength-dispersion electron-microprobe analyses are Pd 39.16, Ir 1.08, Pb 52.01, S 7.90, total 100.15 wt.%, corresponding to (Pd 2.96 Ir 0.05 ) ∑3.01 Pb 2.02 S 1.98 (basis: 7 atoms per formula unit). The powder pattern was indexed on a monoclinic cell with a 11.521(11), b 8.294(10), c 8.321(6) A, β 134.38(5)°, V 568.3(6) A 3 . The space group is probably C 2/ m , with Z = 4. D calc is 9.41 g/cm 3 . The strongest lines in the pattern [ d in A(I)( hkl )] are: 5.953(6)(001), 4.144(10)(020), 2.917(9)(220), 2.413(8)(022), 2.365(7)(422) and 2.082(5)(040). Laflammeite likely crystallized from a hydrothermal fluid or a late-stage liquid in an environment poor in overall sulfur and rich in the PGE , Pb and volatiles.

Menshikovite, ideally Pd3Ni2As3, is a new mineral species found in two localities in Russia. It occurs in a pod of coarsegrained, entirely altered gabbronorite rich in sulfides (up to 30 vol.%), hosted by a microgabbronorite, in the Vostok deposit, Lukkulaisvaara intrusion, in Karelia. There, menshikovite (up to 0.2 mm) is associated with chalcopyrite, pentlandite, calcic amphibole (mostly actinolite), clinochlore, merenskyite, kotulskite, sobolevskite, michenerite, hollingworthite, hessite, and a Re-rich sulfide. Menshikovite (up to 0.1 mm) also occurs in mineralized quartz–feldspar sandstones, which are metasomatically altered and recrystallized. They are located at the lower (exo)-contact of the Chiney intrusion in Siberia, and contain amphibo le(s), mica(s), chlorite, disseminated or massive sulfides (mostly chalcopyrite), Pd-rich maucherite, nickeline, sperrylite, mertieite -II or stibiopalladinite (or both), isomertieite, majakite, and michenerite. We also report a third occurrence of menshikovite from th e Oktyabr’ deposit, Noril’sk complex, in Siberia. In reflected light, menshikovite is pink and opaque. A cleavage is not observed. There is no evidence of bireflectance. The anisotropy is weak, from light gray to brownish gray. Reflectance percentages in air (and in oil) are, for R 1 and R2, at 470 nm 48.4, 50.2 (38.45, 39.3), at 546 nm 51.2, 53.2 (41.0, 41.8), at 589 nm 53.2, 55.3 (42.3, 43.3), and at 650 nm 56.6, 58.7 (46.6, 47.8). Fine twins are present. The average value of microhardness VHN 40, 50, 65 is 517.1 kg/ mm 2 ; it has Mohs hardness of ~5 and is brittle. The average of two sets of multiple wavelength-dispersion electron-microprobe analyses of menshikovite from Lukkulaisvaara gave Pd 48.18, Pt 0.07, Ni 17.77, Fe 0.07, As 33.91, the total is 100.0 wt.% (not normalized), corresponding to Pd2.99(Ni2.00Fe0.01)2.01As2.99 (basis: atoms = 8). Compositions of menshikovite from Chiney vary from Pd3.00(Ni1.98Co0.01)1.99As2.99 to Pd3.15(Ni1.97Co0.01)1.98As2.86. A solid-solution series probably exists between menshikovite and majakite [i.e., (Pd,Ni)2As of the -Pd2As type]. The powder pattern of menshikovite from Lukkulaisvaara was indexed on a hexagonal cell with a 8.406(4), c 6.740(4) A, and V 412.4(6) A 3 . The strongest eight lines in this pattern [ d in A (I)(hkl)] are 2.626(10)(112), 2.477(10)(202), 2.429(8)(300), 2.283(7)(301), 2.150(6)(103), 1.978(7)(113), 1.818(7)(400) and 1.757(6)(401). Menshikovite formed in volatile-rich environments relatively late in the crystallization history of the layered complexes. The name honors Yurii P. Men’shikov, of the Kola Science Center, Apatity, Russia.

International audience; The Ampandrandava phlogopite deposit, located near Beraketa in southern Madagascar, and others like it in the region, in both the Androyen and Anosyen domains, were created by the crystallization of pods and dikes of crustally derived silicocarbonatitic melt. The anatectic reaction seems to have involved regional marble with C and O isotopes shifted toward mantle values and, at Ampandrandava, in particular, input from an evaporitic sequence. This hypothesis accounts for the importance of magmatic anhydrite in the pegmatitic pods and the presence of chlorine in the fluorapatite and phlogopite. The pods and dikes were emplaced in phlogopite-bearing clinopyroxenite; the aggressive carbonate-sulfate melt digested the host locally, and the diopside, phlogopite and fluorapatite crystallized quickly owing to a vapor pressure quench rather than a thermal quench. The high-temperature anatectic event occurred after the final episode of regional metamorphism, at a time of exhumation of deep crust, tectonic quiescence accompanying post-orogenic collapse, and anorogenic magmatism once collision and deformation had ceased.

An unusual phase rich in Cl (78.4 wt.% Pb, 19.2% Cl) and close in composition to penfieldite [Pb 2Cl3(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), member s 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 replaceme nt of a precursor mineral (probably galena) by an aqueous hydrochloric solution at a very low temperature, at the final stage of hydrothermal alteration.

Pegmatites at the East Kemptville Sn-Zn-Cu-Ag deposit occur within a F- and P-rich, 370 Ma topaz-muscovite leucogranite, the most evolved phase of the chemically zoned Davis Lake Pluton. Structural observations and geochronology indicate that the leucogranite intruded into an active shear/fault zone environment. Pegmatites are preferentially located near the roof zone of the intrusion and include: (1) quartz-feldspar pods (≤ 1–2 m) aligned parallel to a foliation in the leucogranite. Such pegmatites have marginal aplites or may be cored by aplites; (2) aplite-pegmatite zones with well-developed crenulate and unidirectional solidification textures; and (3) muscovite-bordered miarolitic cavities lined with quartz-feldspar ± cassiterite. Associated with areas of pegmatites are irregular- to cuspate-shaped zones enriched in muscovite. Mineralogically the pegmatites are simple, consisting of clear to black quartz, albite, microcline perthite and muscovite; topaz may be enriched in aplites coring pegmatites and coarse euhedral cassiterite is rare.Chemically the leucogranites marginal to pegmatites are similar to the host leucogranite, but some aphanitic felsic dykes indicate extreme differentiation, as reflected by REE depletion and P-enrichment. Enrichment of pegmatitic feldspar in P2O5 (to 1 wt. %) indicates that melts contained 2–3 wt. % P2O5, whereas muscovite chemistry (to 5 wt. % F) reflects the F-rich nature of the melt. Trace and REE contents of pegmatitic feldspars are consistent with local segregation of volatile-rich melts to form pegmatites rather than extreme crystal-chemical fractionation.Fluid inclusion studies of pegmatitic quartz and cassiterite indicate the presence of a highly saline brine (40 wt. % eq. NaCl; Na > K > Fe > Mn > Ca > Sr) of magmatic origin. Isochores modelled for a 40 wt. % eq. NaCl fluid constrain pegmatite formation at 550–600°C, thus the depressed solidus is consistent with the volatile-rich (F, P, H2O) nature of the melt. Stable isotopes demonstrate that a magmatic fluid (δ18OH2O = +5·5 to +10‰, δDH2O = −33 to −41‰ for 450–500°C) equilibrated with the pegmatites and the system cooled abruptly at c. 425°C.The low volume of pegmatite at East Kemptville suggests that the melt was not near volatile saturation, instead pegmatite generation is interpreted to have resulted from rapid decompression related to the active shear zone setting of the granite. Although feldspar chemistry reflects a local segregation model, the chemistry of aphanitic dyke rocks indicates that the leucogranite did evolve into a more fractionated melt. The local presence of cassiterite in pegmatites and miarolitic cavities indicates that locally, saturation of Sn occurred, but not throughout the EKL where elevated Sn contents are attributed to infiltration of mineralising fluids (i.e. deuteric alteration).

Rare Pd–Ag tellurides, telargpalite and the phase Pd 6AgTe4 occur in pods

Zoned tungstenoan molybdenite occurs in a fenite (~80 modal % nepheline) within a strongly heterogeneous fenitized megaxenolith (~0.2 km) enclosed by nepheline syenite, in the Khibina alkaline complex, Kola Peninsula, Russia. Molybdenite forms euhedral crystals (~50‐60 � m in length), enclosed by pyrrhotite, and abundant veinlets among silicates. The crystals are zoned with respect to tungsten, which is strongly enriched in the central zone. This area is 5‐6 � m across and contains up to 5.85 wt.% W (0.05 atoms per formula unit). Tungsten-rich zones of the molybdenite veinlets contain the same concentration of W as the central zone of the euhedral crystals, implying similar conditions of formation. There is thus a substantial solid-solution between molybdenite and tungstenite. We attribute the outward zoning to W-poor molybdenite in terms of increasing fugacity of oxygen and decreasing fugacity of sulfur, apparently in the presence of a reduced H 2S‐CH 4-rich fluid of subalkaline character. The existence of locally reducing conditions in the fenite(s) at Khibina also is implied by the adjacent occurrences of titaniu mand niobium-rich sulfides and a graphite-like material.

We report on the existence of a newly recognized extensive solid-solution series between taimyrite and tatyanaite, its Pt-dominant analogue. Copper probably occupies a separate site. Substitution involving Pd and Cu also is important, such that taimyrite–tatyanaite forms a solid-solution series with cabriite, which is structurally closely related to taimyrite and tatyanaite. These platinum-group minerals ( PGM ) occur in base-metal sulfide ( BMS ) ores in the Oktyabr’sky deposit, Noril’sk complex, Siberia, Russia. The taimyrite–tatyanaite is present as single grains and aggregates, associated with a heterogeneous Ag–Au alloy, and also as zoned intergrowths of Pt–Pd–Cu stannides. These intergrowths commonly, but not necessarily, occur within chalcopyrite and typically display the following sequence of crystallization, based on textural and compositional data: atokite–rustenburgite → tatyanaite → Pt-rich taimyrite → taimyrite poorer in Pt → late-stage Ag–Au alloys. Subsolidus re-equilibration of the atokite–rustenburgite core with the Cu-rich host is considered unlikely to produce the peripheral zones of the Cu-rich PGM around this core. These zoned PGM probably formed from a trapped liquid rich in noble metals, Cu, and Sn. Decrease in temperature and increase in activity of Cu in the remaining liquid seem to have been the main factors to control the order of crystallization of the PGM in the zoned intergrowths.

The new mineral species edgarite, FeNb3S6, was discovered in a feldspar-rich fenite, in a fenitized xenolith enclosed by nepheline syenite of the Khibina alkaline complex, Kola Peninsula, northwestern Russia. It occurs as platy inclusions (up to 0.15 mm) in Ti-(V)-rich pyrrhotite and ferroan alabandite, and as dark gray aggregates of platy grains located on the surface of the pyrrhotite. The associated minerals include Ti-(V)-rich marcasite, Mn-Fe-rich wurtzite-2H, corundum, nearly end member phlogopite, rutile, monazite-(Ce), and a graphite-like material. Edgarite is soft (VHN5;10= 135–205 kg/mm2), distinctly bireflectant, and has a strong anisotropy. Its reflectance in air (and in oil) (R1 and R2 in percent, respectively) is: 470 nm: 28.1, 40.2 (13.0, 24.2), 546 nm: 27.4, 39.3 (12.3, 22.7), 589 nm: 27.0, 38.5 (12.2, 21.7), and 650 nm: 27.0, 36.9 (12.4, 20.3). The composition is Nb 52.87, Fe 10.12, V 0.36, Mn 0.10, Ti 0.04, S 35.86, sum 99.35 wt%, which corresponds to (Fe0.96V0.04Mn0.01)Σ1.01Nb3.03S5.95 (basis: Σ atoms=10). By analogy with synthetic FeNb3S6, the X-ray powder pattern of edgarite was indexed on a hexagonal cell, a=5.771(1), c=12.190(6) A, and V=351.6(3) A3, Dcalc is 4.99 g/cm3. The space group is most probably P6322, with Z=2. The strongest lines of the pattern [d in A (I, hkl)] are: 6.11 (8, 002), 3.04 (6, 004), 2.88 (5, 110), 2.606 (8, 112), 2.096 (10, 114), 1.665 (8, 300), 1.524 (6, 008), 1.126 (7, 322), and 1.027 (6, 414). Edgarite appears to have formed at a very late or final stage of metasomatism, after the main event of fenitization, from a highly reduced, subalkaline S-C-H-rich fluid, which may have remobilized Nb as a result of destabilization of oxide minerals. These reducing conditions promoted the chalcophile behavior of lithophile elements (Nb, Ti, V and Mn) on a local scale in the fenite.

In the Kwandonkaya Complex, an A-type metaluminous to peraluminous granite complex in northern Nigeria, the presence of Fe2+-rich ferromagnesian phases (fayalite, hedenbergite and amphibole) at the initial and annite at final stages of crystallisation indicates relatively reduced melts throughout (