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The exposures, structure, mineralogy, and composition of unusual sulfide-bearing troctolites from the Yoko-Dovyren layered intrusion in the northern Baikal area (Russia) are described in detail for the first time. The troctolite succession (referred to as the Konnikov Zone) is characterized by the presence of pegmatoid poikilite sulfides and sulfide dissemination with diverse PGE mineralization. The former are dominated by pyrrhotite-troilite products of exsolution of monosulfide solid solution (mss), and the latter is composed mostly of cubanite-chalcopyrite assemblages produced from an intermediate Ni-Cu-Fe solid solution (iss). The positive covariations between the contents of sulfur and chalcogens (Se, Te) along with the sublinear dependence of the Pd, Pt, Au, and Cu contents on the Te contents indicate a sulfide control of the distribution of these elements in troctolite cumulates. According to the sulfide-normalized contents of these elements in rocks, the average "100% sulfides" in the samples are subdivided into two groups: (1) strongly depleted in PGE, Au, Cu, and Te and (2) with 10-50-fold enrichment in them. This division is consistent with the morphological and mineralogical differences between the groups. Of genetic significance is the fact that the mss assemblages are somewhat poorer in PGE and Te than the primitive sulfides from the Dovyren basal zone, whereas the assemblages with predominant copper sulfides are significantly richer in these elements. This fact is confirmed by LA-ICP-MS data on the trace-element composition of the sulfide phases. The established specific features indicate a limited scale of fractionation of immiscible sulfides during the solidification of the troctolite cumulates. The formation of PGE- and Te-rich assemblages can be related to the course of crystallization of a sulfide precursor similar to the most primitive sulfide liquid. This is consistent with the known laws of crystallization of sulfide systems and explains the abnormally high S/Te ratios in pegmatoid troctolites enriched in mss products. Thus, sulfide melts act as an agent that transports precious metals and chalcogens in the troctolite cumulate area. This conclusion requires specification of the physical mechanisms and parameters (rheology, permeability, wettability by sulfides of different phases, etc.) of the cumulus medium favoring the spatial separation of a monosulfide solution and Cu-containing PGE-rich fractions with their subsequent infiltration and deposition at the boundaries of critical low permeability. [ABSTRACT FROM AUTHOR]

Abstract—: The paper presents newly acquired data on concentrations of chalcophile elements and chalcogens (Se and Te) in sulfide-bearing rocks and Cu-Ni ores from the bottom portion of the Yoko-Dovyren Massif, northern Baikal area, Russia. Positive covariations between Pd, Pt, Au, S, and Te in the samples highlight sulfide control on the behavior of these elements, which was related to the redistribution of essentially Fe-Ni sulfide liquids at a magmatic stage. The character of relationships between Pd, Pt, Te, Cu, and S in the rocks led us to distinguish two groups of genetic trends: the first group combines samples from the chilled zone, plagioperidotites, and olivine gabbronorites that compose underlying sills in the central part of the intrusion, and the second one comprises poor and high-grade ores in the northeastern termination of the intrusion (Ozernyi Prospect). We put forward the hypothesis that the first-group trends reflect different degrees of accumulation of crystallization products of the most primitive sulfide liquids, whereas the trends of the second group pertain to sulfide matter significantly depleted in Cu, Te, and PGE. New data on Fe, Ni, Cu, Co, Se, Te, Zn, Mo, Ag, Cd, Sb, Pb, Rh, PGE, and Au concentrations in sulfides from the chilled gabbronorite and ores of the Baikalskoe deposit are presented. Results of thermodynamic modeling (with the COMAGMAT-5 program package) of sulfide saturation in the intercumulus of a primitive orthocumulate are used to reproduce the composition (Cu, Pd, Pt, Au, and Te) of the parental sulfide liquid. The model concentrations of noble metals in the sulfide are demonstrated to be one to two orders of magnitude higher than the concentrations in the "average sulfide" estimated by LA-ICP-MS. More realistic estimates for the composition of the parental sulfide liquids can be obtained by normalizing the bulk concentrations of these elements to 100% sulfide mass. These estimates are in good agreement with results from thermodynamic simulations. [ABSTRACT FROM AUTHOR]

This study tests the application of chalcophile elements such as nickel, copper and the platinum-group elements as indicators of nickel-sulphide prospectivity in komatiites from terranes of the Karelian Craton in northern Finland and Norway. Major element abundances reflect volcanic processes associated with the emplacement dynamics of ultramafic lavas, whereas the variable chalcophile element concentrations record the ore-forming process, mainly as an anomalous metal depletion and enrichment relative to the calculated background. Geochemical data from this study indicate that Paleoproterozoic komatiites in the Pulju Greenstone Belts and Archean komatiites in the Enontekiö area are prospective for nickel-sulphide mineralisation. Conversely, on the basis of the present dataset, ultramafic rocks from the Palaeoproterozoic Karasjok Greenstone Belt display lower prospectivity for nickel-sulphides, although potential exists if high-volume flow conduits and channels within the large volcanic flow field could be identified. [ABSTRACT FROM AUTHOR]