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The Palaeoproterozoic Ni–Cu sulphide deposits of the Pechenga Complex, Kola Peninsula, occur in the lower parts of ferropicritic intrusions emplaced into the phyllitic and tuffaceous sedimentary unit of the Pilgujärvi Zone. The intrusive rocks are comagmatic with extrusive ferropicrites of the overlying volcanic formation. Massive lavas and chilled margins from layered flows and intrusions contain <3–7 ng/g Pd and Pt and <0·02–2·0 ng/g Ir, Os and Ru with low Pd/Ir ratios of 5–11. The abundances of platinum group elements (PGE) correlate with each other and with chalcophile elements such as Cu and Ni, and indicate a compatible behaviour during crystallization of the parental magma. Compared with the PGE-depleted central zones of differentiated flows (spinifex and clinopyroxene cumulate zones) the olivine cumulate zones at the base contain elevated PGE abundances up to 10 ng/g Pd and Pt. A similar pattern is displayed in intrusive bodies, such as the Kammikivi sill and the Pilgujärvi intrusion. The olivine cumulates at the base of these bodies contain massive and disseminated Ni–Cu-sulphides with up to 2 μg/g Pd and Pt, but the PGE concentrations in the overlying clinopyroxenites and gabbroic rocks are in many cases below the detection limits. The metal distribution observed in samples closely representing liquid compositions suggests that the parental magma became sulphide saturated during the emplacement and depleted in chalcophile and siderophile metals as a result of fractional segregation of sulphide liquids. Relative sulphide liquid–silicate melt partition coefficients decrease in the order of Ir > Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphide mass ratio) are high and of the order of 104–105, and they indicate the segregation of only small amounts of sulphide liquid in the parental ferropicritic magma. In differentiated flows and intrusions the sulphide liquids segregated and accumulated at the base of these bodies, but because of a low silicate–sulphide mass ratio the sulphide liquids had a low PGE tenor and Pt/Ir and Cu/Ir ratios similar to the parental silicate melts. During cooling the sulphide liquid crystallized 40–50% of monosulphide solid solution (mss) and the residual sulphide liquid became enriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. The Cu-rich sulphide liquid locally assimilated components of the surrounding S-rich sediments as suggested by the radiogenic Os isotopic composition of some sulphide ores (γOs > 100). Most of the massive and breccia ores represent mixtures of mss and residual Cu-rich sulphide liquid whereas chalcopyrite-rich veins formed when the Cu-rich sulphide liquids were squeezed out into the surrounding sediments. [ABSTRACT FROM PUBLISHER]

Abstract: New in-situ trace element and hydrogen isotopic data of hydromagmatic amphiboles from ferropicritic gabbro–wehrlite intrusions and lava flows of the Palaeoproterozoic Pechenga Greenstone Belt (Russia) provide insights into the controversial source of volatiles that are implicated in Precambrian ultramafic magmatism. The intrusions mainly contain oikocrystic kaersutites and pargasites, whereas lava flows generally include idiomorphic Fe-kaersutites and Fe-pargasites. All orthomagmatic amphiboles display slightly LREE-enriched chondrite-normalised patterns. Primitive mantle-normalised patterns of amphiboles from the intrusions display positive Nb–Ta, moderate negative Th–U and distinctive negative Sr–Li anomalies. Conversely, Fe-kaersutites and Fe-pargasites from the lava flows display undepleted Th, U, Sr and Li signatures. Boron and Cl enrichment is primarily observed in secondary edenites, which locally rim orthomagmatic kaersutites. In-situ δD measurements of hydromagmatic amphiboles from the gabbro–wehrlite intrusions display values ranging between −70‰ and −170‰, and total water contents between 1 and 3 wt.%. Conversely, primary hydromagmatic amphiboles from the Pechenga flows display lighter δD compositions ranging between −100‰ and −270‰, and lower (<2 wt.%) total water contents. The Ti– and Nb–Ta-enriched nature of hydromagmatic amphiboles from the intrusions and lava flows indicates that the source of volatiles in the Pechenga ferropicrites could be either a metasomatised subcontinental mantle or a volatile-bearing plume. Positive Nb–Ta/La anomalies in amphiboles from the intrusions rule out crustal contamination with granitoid crust during petrogenesis and emplacement as the main source of the volatiles. Local B–Cl enrichment suggests that some samples may have interacted with hydrothermal fluids. The range of in-situ δD values in hydromagmatic amphiboles is consistent with the presence of magmatic water. The lighter hydrogen isotopic composition and lower water contents of the primary igneous amphiboles from the lava flows compared to amphiboles from the intrusions indicate that the lava flows degassed upon emplacement. Previous studies investigating the role of volatiles in Precambrian ultramafic magmatism preferentially collected data from thin lava flows, whereas thicker flows or intrusions have been generally neglected. We emphasise the necessity to re-evaluate the controversial role of volatiles in the petrogenesis of ultramafic magmas by investigating evidence from thicker flows or intrusions, which may have retained their full volatile budget. [Copyright &y& Elsevier]