Geology, mineralogy, and genesis of PGE mineralization in the South Sopcha massif, Monchegorsk complex, Russia

Abstract

New data are reported on the localization and genesis of PGE mineralization at the South Sopcha deposit situated in the southern framework of the Monchegorsk pluton. Disseminated PGE-Cu-Ni mineralization, the thickness of which in particular boreholes exceeds 100 m, is hosted in the zone of alternating peridotite, pyroxenite, norite, and gabbronorite. The PGE grade does not exceed 1–2 gpt with Pd/Pt = 3–4 at Ni and Cu contents from 0.2 to 1.5 wt %. The PGE contents up to 4–6 gpt and Pd/Pt = 4–8 are noted at local sites of hydrothermally altered rocks. Another type of PGE mineralization is established in the outcrops of the southeastern marginal group of the massif. Pyroxenite, norite, and gabbronorite fragments are incorporated here in the gabbroic matrix, making up a complex zone of magmatic breccia complicated by mylonites and late injections. Elevated PGE contents (1.0–6.5 gpt) are detected in all types of rocks in the zone of brecciation, mainly in the matrix. Platinum-group minerals (PGM) occur in association with magmatic and late sulfides, amphibole, mica, and chlorite. PGM vary in composition depending on the petrographic features of rocks. In rocks of the layered series and in pegmatoid pyroxenite PGM are extremely diverse comprising PGE compounds with As, Sb, Bi, Te, Se, and S. In the brecciated rocks of the marginal group, Pd bismuthotellurides (mainly merenskyite), sperrylite, hollingworthite, and Pd- and Rh-bearing cobaltite and gersdorffite are predominant. The PGE mineralization in rocks of the layered series and pegmatoid pyroxenite was formed from the magmatic melt enriched in volatiles and with subsequent transformation of PGE assemblages under the influence of hydrothermal fluids at a lower temperature. In gabbroic rocks of the marginal group, PGM are associated with the latest sulfides (chalcopyrite, bornite, chalcocite), forming separate grains and thin veinlets in hydrothermally altered rocks. The gabbroic melt affected incompletely crystallized rocks of the layered series by formation of contact-type PGE mineralization, deposition and redeposition of ore matter.