Abstract

The 2,430 Ma Burakovsky layered complex, located in Karelia, Russia, is one of the largest layered Fennoscandian intrusions. It consists, from the base to the top, of a thick ultramafic part dominated by dunite and peridotite overlain by a layered mafic part that consists of alternating layers of pyroxenite, wehrlite and gabbro (Transitional Banded Zone, TBZ), and a thick sequence of gabbroic rocks [Gabbronorite Zone (GNZ), Pigeonite Gabbronorite Zone (PGNZ), Magnetite Gabbronorite–Diorite zone (MGDZ), which predominates. The Main Chromite Horizon, located at the interface between the ultramafic part and the layered series, hosts laurite and Os–Ir alloy grains observed as tiny inclusions in chromite crystals. The Transitional Banded Zone and Gabbronorite Zone are characterized by disseminated sulfide blebs hosted by clinopyroxene and plagioclase. The blebs consist of a chalcopyrite – pyrrhotite – pentlandite assemblage locally associated with PGM found as discrete grains in sulfides, in clinopyroxene, but more commonly around the margin of sulfide grains. Platinum-group minerals consist of dominant Pd and Pt arsenides and sulfarsenides and Pt–(Pd–Bi) tellurides (sperrylite, platarsite, palladoarsenide, kotulskite, moncheite) with minor amounts of native Pt. The expression of the mineralization is consistent with the late crystallization of an immiscible semimetal-rich liquid that forms discrete PGM grains around sulfide blebs and in veinlets injected in the adjacent silicates. The PGM were also observed in association with secondary phases, dominated by epidote replacing primary base-metal sulfides. This feature, commonly described in other layered intrusions, is here mainly observed around the platinum-group minerals – base-metal sulfides association hosted by plagioclase. The existence of compositional heterogeneities in primary plagioclase likely acts as a target for the development of alteration.

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