Abstract New results (compositional data and reflectance values) are reported for some rare sulfides enriched in K, Tl and Pb, which are related to djerfisherite, thalfenisite, bartonite, a “Cl-bearing bartonite”, or chlorbartonite, and also for shadlunite, from the Noril'sk and Salmagorsky complexes, Russia. Our observations and comparisons with relevant data in the literature imply that: (1) bartonite is probably a S-dominant (or Cl-free) analogue of djerfisherite; and a “Cl-bearing bartonite” and chlorbartonite are probably compositional variants of the djerfisherite–bartonite series. (2) The most probable formulae of bartonite and djerfisherite are (K, Me 2+ ) 6 (Fe,Cu,Ni) 25– x S 26 (S,Cl) and (K, Me 2+ ) 6 (Fe,Cu,Ni) 25– x S 26 (Cl,S), where 0 ≤ x ≤ 5, respectively. (3) Two independent substitution mechanisms probably operate in the natural series. A coupled substitution [ Me 2+ + S 2– ↔ K + + Cl – ] is reflected by an observed deficit in K, accompanied by the incorporation of Me 2+ (Pb, Fe, or Ni) in the K site. Another mechanism is inferred to be [2Fe 3+ + 〈 ↔ 3Fe 2+ ], which assumes the existence of vacancy-type defects at the Me site. Thus, the second mechanism could possibly control the existing variations of Σ(Fe, Cu, Ni) in the range of ∼21 to 25 a.p.f.u., documented in djerfisherite- and bartonite-type minerals. The minerals analysed from Noril'sk, which are free of Cl and related to bartonite and to a Tl-dominant analogue of bartonite (unnamed species), probably crystallized from microvolumes of late fluid rich in K and Tl, under conditions of relatively low oxygen fugacity in the environment. Uniform contentss of Fe and Cu, observed in coexisting phases of normal (Cl-bearing) djerfisherite and bartonite (or Cl-free analogue of djerfisherite) at Salmagorsky imply that they reached equilibrium with regard to the distribution of these elements during crystallization. These phases probably formed as a result of fluctuations in the ratios of sulfur and chlorine fugacity in a fluid at a postmagmatic hydrothermal stage.
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 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.
We report new results of multiple EMP analyses of sorosite from the Baimka Au–PGE placer deposit, western Chukotka, northeastern Russia. It occurs as (1) relatively large (0.2–0.4 mm) euhedral, cryptically zoned hexagonal grains, with Fe–Nirich zones in the core, (2) large irregular or skeletal grains, and (3) elongate subhedral to euhedral (hexagonal), or anhedral micrograins (≤20 m). They all are hosted by native tin (Sb-bearing), and are associated with stistaite Sn1+xSb and herzenbergite SnS. The Fe content of these zoned grains decreases from the center (3 wt.%) toward the edge (≤0.05 wt.%). The Ni content also decreases from the core (1.2 wt.%) to the periphery ( 415°–227°C, the latter being the eutectic temperature in the binary system Cu–Sn. A mineralized mafi c rock (gabbro, gabbro–diorite, or clinopyroxenite) associated with the Yegdegkychsky gabbro – diorite – syenite – monzonite complex in the placer area could be a potential primary source for the sorosite-bearing mineralization.
The Paleoproterozoic Lyavaraka ultrabasic complex is one of several dunite–harzburgite–orthopyroxenite bodies exposed as shallow plutonic complexes in the Serpentinite Belt, Kola Peninsula, Russia. Lyavaraka and the other complexes are anorogenic, formed in a stable within-plate environment in the interval 2.5–2.4 Ga as members of a large igneous province formed in the Sumian cycle of igneous activity. This geotectonic setting accounts for the shallow emplacement of the strongly magnesian komatiitic magma in the Fennoscandian Shield. We recognize three stages of crystallization of the Al-undepleted magma, present as dislocated blocks. Zone I is the ultrabasic core-like zone in which olivine predominates. Orthopyroxene is the major mineral in Zone II, and Zone III contains the most evolved ultrabasic rocks in which recurrent olivine coexists with Cpx + Pl. Primocrysts of hypermagnesian Opx (Mg# 91–93) nucleated in central areas of Zone II as olivine (Mg# 89.1–90.3) was forming in Zone I. In Zone III, olivine grains of a second generation (Mg# 74.5–75.8) formed after the primocrystic Cpx (Mg# up to 88.0) appeared. The recurrence of olivine is attributed to the progressive buildup in fO2 as a result of degassing and conversion of Fe2+ to Fe3+, well documented in our earlier studies of oxide parageneses.
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 μm 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)Σ2.00(Rh0.49Ni0.36Ru0.12)Σ0.97Ir0.95Sb2.08 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 Fdm (#227), a = 11.6682(8) Å, V = 1588.59(19) Å3, and Z = 16. A least-squares refinement of X-ray powder-diffraction data gave a = 11.6575(5) Å and V = 1584.22(19) Å3. The strongest five reflections in the powder pattern [d in Å(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 = 11.66(2) Å]. The crystal structure of fleetite was solved and refined to R = 0.0340 based upon 153 reflections with Fo > 4σ(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.
Une grande variete de mineraux du groupe du platine est associee aux sulfures de metaux de base dans le gisement a Ni-Cu-EGP de Wellgreen, au Yukon. Plusieurs solutions solides sont presentes dans ce gisement: (1) melonite palladifere - merenskyite - moncheite, (2) testibiopalladite - michenerite, (3) sudburyite - kotulskite - sobolevskite - (Ni,Pd)(Te,Sb,Bi) 1 + x (imgreite paliadifere - melonite palladifere), et (4) breithauptite - sudburyite. Les mineraux du groupe du platine et les phases porteuses des EGP incluent sperrylite, stibiopalladinite ou mertieite II (ou les deux), geversite, alliages Pt-Pd-Fe-(Cu) (tetraferroplatine enrichi en Cu-Ni, et platine natif ou isoferroplatine), froodite (?), hollingworthite, laurite, iridium natif, cobaltite-gersdorffite enrichi en Rh, ullmannite palladifere, et un alliage inhabituel contenant Re-Ir-Os-Ru. Nous resumons ici les caracteristiques importantes des mineraux du groupe du platine dans le gisement de Wellgreen: leur taille est infime dans la plupart des cas, les antimono- et bismuthotellurures de Pd-(Pt)-Ni demontrent une grande etendue en composition, ceux-ci contiennent une proportion importante de nickel, et y est developpee une solution solide complexe et non courante impliquant sudburyite, kotulskite, sobolevskite and Me(Te,Sb,Bi) 1 + x .
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