Andrei Y. Barkov

The mechanism of charge compensation in Cu-Fe-PGE thiospinels from the Penikat layered intrusion, Finland

Abstract Thiospinels of Cu-(Fe) and platinum-group elements (PGE) are relatively abundant in the Kirakkajuppiua PGE deposit of the Peuikat layered complex. Finland. In actinolite-clinochlore rock that is nearly base-metal sulfide-free and relatively poor in chromite, the thiospinels occur as subhedral or anhedral grains (up to 0.4 mm). They are members of the cuprorhodsite-ferrorhodsite and cuprorhodsite-malanite series, relatively poor in cuproiridsite. and display considerable grain-to- grain variations in Cu. Fe. Pt. and Rh. Strong negative Fe-Cu. Pt(+Ir)-Fe. Rh-Cu. and Rh-Pt and strong positive Pt(+Ir)-Cu and Rh-Fe correlations in these thiospinels are indicative of a coupled substitution: Fe-for-Cu substitution in the tetrahedral (A) sites causes an excess in formal positive charge, which is compensated by Rh-for-(Pt+Ir) substitution hi the octahedral (B) sites. Probable valence states in the Fe-free and Fe-rich end-members of the solid-solution series at Penikat are Cu+[Rli3+(Pt,Ir)4+] S4- and (FefoCu0.5+)Rh2S42-. and these suggest the heterovalent substitution scheme AFe3+ + 2 BRh3+ → ACu+ + 2 BPt4+(+2 Ir4+) to incorporate Fe in the ferrorhodsite-rich end member.

Osmian hollingworthite and rhodian cobaltite- gersdorffite from the Lukkulaisvaara layered intrusion, Russian Karelia

I|OLLINC;WORTHrrE, ideally RhAsS, was first described by Stumpt] and Clark (1965) from the Bushveld igneous complex. Since then it has been reported from the same and other occurrences in the Bushveld (Genkin et al., 1966: Stumpfl, 1972; Tarkian and Stumpfl, 1975); Werner Lake, Canada (Rucklidge, 1969); Hitura (ll~ikli et al., 1976), Penikat (Alapieti and Lahtinen, 1986; llalkoaho et al., 1990; Halkoaho, 1994), Siikak~im~i (H~inninen et al., 1986), Finland; placers from the Urals (Begizov et al., 1976), a mafic-ultramafic complex, Kola Peninsula (Distler and Laputina, 1981), Cu-Ni sulphide deposits of Noril sk (Genkin and Evstigneeva, 1986) and alpine-type complexes, Russia (Distler et al., 1986); Stillwater layered complex (Volborth et al., 1986); Shetland ophiolites (Tarkian and Prichard, 1987; Prichard and Tarkian, 1988); Osthammeren, Norway (Nilsson, 1990); Two Duck Lake, Canada (Ohnenstetter et al.. 1991); l,ukkulaisvaara, Mt. Generalskaya and Imandrovsky layered intrusions, NW Russia (Barkov and Lednev, 1993; Barkov et al., 1994; 1995b), although in only a few cases are Os-rich hollingworthites recorded (Stumpfl, 1972; Distler and Laputina, 1981; Tarkian and Prichard. 1987: Ohnenstetter et al., 1991). Hollingworthite is a cubic mineral with a cobaltitetype structure, isostructural with two other platinumDepartment o f Geosciences and Astronomy, University o f Oulu, FIN-90570 Oulu, Finland

Unique W-rich alloy of Os and Ir and associated Fe-rich alloy of Os, Ru, and Ir from California

Abstract 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.

Tatyanaite, a new platinum-group mineral, the Pt analogue of taimyrite, from the Noril'sk complex (northern Siberia, Russia)

Tatyanaite, a new mineral from Noril9sk (Siberia), is the Pt analogue of taimyrite. It is defined as the member(s) of the tatyanaite-taimyrite solid-solution series with Pt > Pd. Tatyanaite solid solution occurs in massive sulphide ore, which consists of chalcopyrite and subordinate pentlandite, pyrrhotite, and cubanite (or isocubanite). It occurs as central zones of large, elongate grains (up to ∼1 mm) and as aggregates of smaller grains associated with Ag-Au alloys. The associated minerals include unusually Pt-rich taimyrite [(Pd 1.25 Pt 0.86 )(Cu 0.85 Ni ) (Sn 1.01 Sb 0.02 )], atokite-rustenburgite, paolovite, froodite, sperrylite, maslovite, and galena. Cryptic zoning (Pt increases and Pd decreases toward the centre) and polysynthetic twins are characteristic. In reflected light, tatyanaite is pink with lilac tinge in air. Bireflectance is weak to distinct, from brownish pink to pinkish lilac. Anisotropy is distinct to moderate, from light brown to dark blue. Reflectance percentages in air and (in oil) are, for R 1 and R 2 , 470 nm 42.8, 44.1 (32.8, 33.3), 546 nm 49.5, 50.0 (37.6, 38.8), 589 nm 51.8, 54.6 (38.9, 39.9), and 650 nm 55.6, 56.8 (41.6, 44.2). It is ductile; the microhardness is VHN 20 = 292-348, mean of 327 kg/mm 2 . The average of nine electron-microprobe analyses gave Pt 45.38, Pd 19.53, Cu 10.62, Ni 0.15, Fe 0.03, Sn 23.02, Sb 0.27, sum 99.0 wt.%, corresponding to [(Pt 4.76 Pd 3.75 ) ∑8.51 Cu 0.48 ] ∑8.99 (Cu 2.94 Ni 0.05 Fe 0.01 ) ∑3.00 (Sn 3.96 Sb 0.05 ) ∑4.01 [or to (Pt 1.19 Pd 0.94 )(Cu 0.85 Ni 0.01 Fe ) (Sn 0.99 Sb 0.01 )]. The powder pattern is similar to that of synthetic Pd 9 Cu 3 Sn 4 , and, by analogy with the latter, it was indexed for an orthorhombic cell with a = 7.89(1) A, b = 4.07(1) A and c = 7.73(1) A, and V = 248(1) A 3 . The three strongest lines in the pattern are 2.283 (10, 212), 2.163 (4, 203) and 1.369 (3, 323). Tatyanaite-taimyrite formed from a late-stage liquid rich in noble metals, Cu and Sn.

Sr-Na-REE titanates of the crichtonite group from a fenitized megaxenolith, Khibina alkaline complex, Kola Peninsula, Russia : first occurrence and implications

An extensive, nearly continuous and hitherto unreported solid-solution series is observed in Sr-Na-REE titanates in a fenitized megaxenolith, Khibina alkaline complex, Kola Peninsula, Russia. These titanate minerals, related to crichtonite, landauite and davidite-(Ce,La), may represent potentially new members of the crichtonite group. They display compositional zoning, and are associated with diverse oxide minerals, albite and alkali feldspar. Our compositions (EMP) suggest that the “large cations” (Sr, Na, K, and the rare-earth elements, REE), along with minor Ca, occupy fully the A ( M 0 ) site in their structure; in contrast, Ca enters dominantly the B ( M 1 ) site, probably via a coupled substitution of the type [(Ca 2+ +Ti 4+ ) = (Zr 4+ +Fe 2+ )]. The extensive Na + -for-Sr 2+ substitution observed at the A site is coupled with a Ti 4+ -for-Fe 2+ substitution at the C ( M 3 - M 4 - M 5 ) site. The tetrahedral T ( M 2 ) site is dominated by Fe. Among the REE, only the light REE are present in substantial amounts (up to 7.37 wt.% REE 2 O 3 ); they are positively intercorrelated, indicating an ordered distribution at the A site. The incorporation of the REE 3+ , which probably replace Sr 2+ and K + , is controlled by a corresponding decrease in Zr 4+ (and by relative increase in divalent Mn) at the B site in order to maintain charge balance. The observed presence of up to 1.87 wt.% Cr 2 O 3 in the Sr-Na-REE titanate minerals at Khibina indicates that rocks of mafic affinity were the protolith for the mineralized megaxenolith. The high Na contents of these minerals are clearly related to the geochemical environment ( i.e. , Na-metasomatism). The contrasting association of Cr and Nb (up to 1.06 wt.% Nb 2 O 5 ) in these minerals undoubtedly involves derivation from two different sources. We suggest that the Ti-(Fe)-Nb-REE oxide mineralization formed in the megaxenolith as a result of interaction of a pre-existing mafic rock(s), probably Proterozoic rocks of the Imandra-Varzuga Supergroup, with metasomatizing oxidizing fluids of alkaline affinity.

New data on PGE alloy minerals from a very old collection (probably 1890s), California

Abstract 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 <0.5-1 wt%); they appear to be primary. An Fe-enrichment (up to 4.6 wt%) is observed in lamellae of Ir-Ru-Os alloy exsolved from the Fe-poor Os-Ir-Ru alloy host, and also in a rim-like alteration-induced phase developed along the margin in some of the grains of Ir-Ru-Os alloy. Much greater levels of Fe (up to 19.1 wt%), incorporated via the substitution mechanisms: Fe → Ru, Fe → (Os + Ru), and Fe → Ir, were documented in three grains of Fe-Os-Ru-Ir alloys, which attain Fe-dominant compositions, i.e., hexaferrum. These Fe-Os-Ru-Ir alloys and associated exotic phases enriched in unconventional elements, such as W-(Mo)-bearing rutheniridosmine, (Os,Ir)5(W,Mo) and newly recognized (Ir,Os)5(W,Mo), appear to be secondary, formed under conditions of low fugacities of O2 and S2 as a result of interaction of primary Os-Ir-Ru alloys with a reducing fluid phase. No grains of Pt-Fe alloys were found; these only occur as Pt3Fetype (isoferroplatinum or Fe-rich platinum) and Pt2Fe-type inclusions (<50 μm), enclosed in a matrix of Ir- or Os-dominant alloys rich in Ru. The Pt2Fe alloy appears to be a compositional variant of Fe-rich platinum, possibly reflecting a lower limit of Pt content possible in the mineral platinum. An Au-Ag alloy, ranging up to Au0.99, precipitated pseudomorphously by a subsolidus reaction between a residual Au-Ag-rich melt and exsolution-induced inclusions of the Pt-Fe alloy phases. Micro-inclusions of olivine, hosted by a ternary alloy Os0.33Ru0.33Ir0.30, are extremely rich in Mg (Fo95.1-95.4), probably reflecting high-temperature reaction involving chromite or magnesiochromite. The alloy grains from the old collection were likely derived from a mineralized zone of ultramafic rocks, rich in chromitemagnesiochromite and poor in overall S, in the Trinity ophiolite complex of northern California.

Menshikovite, Pd-Ni arsenide and synthetic equivalent

Abstract The average compositions of menshikovite (Pd3Ni2As3) from the type locality (Lukkulaisvaara layered intrusion, Russian Karelia) and a Pd-Ni arsenide synthesized at 450°C, are very close. The observed ∑Metal:As ratio is close to 5:3, as in the case of previously described arsenides from the Stillwater and Two-Duck Lake intrusions. The compositions suggest that these unnamed Pd-Ni arsenides and the synthetic phase are in fact menshikovite.

New and unusual Pd-Tl-bearing mineralization in the Anomal'nyi deposit, Kondyor concentrically zoned complex, northern Khabarovskiy kray, Russia

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 pegmatitic and 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) association and various species of platinumgroup 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] and a 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.

Melonite from Kingash and Kuskanak, Eastern Sayans, Russia, and the extent of Bi-for-Te substitution in melonite and synthetic Ni(Te,Bi)2–x

Abstract We describe occurrences of palladoan melonite in intimate intergrowths with cobaltite-gersdorffite from the Neoproterozoic dunite-wehrlite-gabbro complexes of Kingash and Kuskanak, Eastern Sayans, Russia. The observed compositional trends of melonite are consistent with the overall variations examined on the basis of numerous literature sources. The levels of Bi in NiTe2 are normally limited to ≤0.25 Bi atoms per formula unit (apfu), under natural conditions. Greater levels (≤0.5 Bi apfu) are associated with the (Pd + Pt) enrichment in the palladoan varieties. The telluride-sulfarsenide intergrowths probably formed at Kingash and Kuskanak late in the crystallization history of the ore zones, from microdroplets of residual melt rich in semimetals (Te, Bi, As) and noble metals (Pd and Ag), below the solidus of the enclosing gabbroic rocks and within a narrow range of temperatures (500-550°C). On the basis of our observations made on specimens of melonite and synthetic Ni(Te,Bi)2-x (x = 0.6), we infer that the limit of incorporation of Bi into a melonitetype phase is ≤0.5 Bi apfu.