Petrology and Ni-Cu-Cr-PGE Mineralization of the Largest Mafic Pluton in Europe: The Early Proterozoic Burakovsky Layered Intrusion, Karelia, Russia

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Abstract
The largest mafic pluton in Europe (area = 630 km2, thickness = 5 to 7 km) is the early Proterozoic (2445 ± 4 Ma; Pb-Pb) Burakovsky Layered Intrusion (BLI). It is located in the southern part of Russian Karelia, in the SE part of the Baltic Shield, within an Archean granite-greenstone terrain. The BLI is overlain by Quaternary deposits, and our present understanding of its character, composition, and internal structure is based on geophysical surveys and the nature of the rocks at depth, as sampled by diamond drill core. In order to better understand the petrogenesis of the BLI, we present geologic, petrographic, mineral-chemical, and whole-rock chemical analyses from throughout the stratigraphic sequence. The BLI is a lopolith-like body and is divided into two major units: the Layered Series (LS), which exhibits layering that is discordant to the contact, and the Border Group (BG), with layering that conforms to the contact surface. The LS constitutes most of the BLI and consists of, from bottom to top: the ultramafic zone (UZ), 3 to 3.5 km thick, with a lower dunite (01 ± Chr) and an upper peridotite subzone (01 ± Chr, 01 + Opx ± Chr, and rare Chr cumulates); the pyroxenite zone (PZ) (Opx ± Chr ± 01, Opx + Cpx ± Chr ± 01), 0.2 km thick; the gabbronorite zone (GZ), 1.1 km thick and composed of a lower banded subzone (Opx, Opx + Cpx ± Chr, Opx + Pig ± Cpx cumulates) and an upper massive subzone (Opx + Cpx + Pig and Pig cumulates); the pigeonite-gabbronorite zone (PGZ) (Pig + inverted Pig, Pig-Aug + Pig cumulates), 1.2 km in thickness; and the magnetite-gabbrodiorite zone (MGZ) (Ti-Mt + Pig + Pig + Cpx), 0.8 km thick. A quite unusual feature of the BLI is an irregular conformable body of clinopyroxenite, in the lower part of the PZ, which consists of inverted Pig-Aug with a non-cumulate texture. The pyroxenites contain occasional relict cumulate structures. The pyroxenes contain small oval-shaped quartz-carbonate inclusions. We speculate that the clinopyroxenites may be the result of subsolidus metasomatism by fluids likely generated within the intrusion. Economic mineralization is represented by chromite, Ti-V-Mt, and platinum-group minerals (PGMs). Chromite mineralization is well developed in the upper peridotite subzone of the UZ. The largest chromite seam—the main chromite horizon (MCH)—is 3 to 4 m thick and is situated at the top of the UZ, in contact with the PZ. Potential reserves of Ni, in both sulfides and olivine, are present in the UZ. Syngenetic platinum-group-element (PGE) mineralization is observed in chromitite layers in the layered subzone of the GZ and in the rocks of the Border Group. In the MCH, Os, Ir, and Rh occur as small inclusions of sulfides in chromite; these inclusions have compositions that would place them within the aurite-erlichmanite and isoferroplatinum-awaruite series. Low-sulfur PGE mineralization (mainly Pd and Pt) occurs in the banded subzone of the GZ and in sulfide-bearing rocks of the BG. PGMs consist of Pd- and Pt-bearing tellurides and bismuthides: moncheite, merenskyite, froodite, sobolevskite, and kotulskite. Epigenetic PGE mineralization also occurs in tectonic breccias. The BLI is similar in many respects to other early Proterozoic layered mafic intrusions (i.e., the Bush veld, Monchetundra, and Koilismaa intrusions), especially in terms of the general rock sequence and economic mineralization. The main differences are the very thick dunite cumulates (comprising half of the intrusion) and the relatively minor proportion of pyroxenerich and plagioclaserich cumulates. Another contrasting feature is the predominance of refractory PGE (Os, Ir, and Rh), over Pt and Pd, in the MCH. This observation leads to the interpretation that the MCH has a non-cumulate (deuteric hydrothermal?) origin.