Assemblages and genesis of platinum-group minerals in low-sulfide ores of the monchetundra deposit,Kola Peninsula,Russia

New data on the composition, assemblages, and formation conditions of platinum-group minerals (PGM) identified in platinum-group element (PGE) occurrences of the Monchetundra intrusion (2495 +- 13 to 2435 ± 11 Ma) are described. This intrusion is a part of the Paleoproterozoic pluton of the Monche-Chuna-Volch’i and Losevy tundras located in the Pechenga-Imandra-Varzuga Rift System. The rhythmically layered host rocks comprise multiple megarhythms juxtaposed to mylonite zones and magmatic breccia and injected by younger intrusive rocks in the process of intense and long magmatic and fluid activity in the Monchetundra Fault Zone. The primary PGM and later assemblages that formed as a result of replacement of the former have been identified in low-sulfide PGE occurrences. More than 50 minerals and unnamed PGE phases including alloys, Pt and Pd sulfides and bismuthotellurides, PGE sulfarsenides, and minerals of the Pd-As-Sb, Pd-Ni-As, and Pd-Ag-Te systems have been established. The unnamed PGE phases—Ni₆Pd₂As₃, Pd₆AgTe₄, Cu₃Pt, Pd₂NiTe₂, and (Pd, Cu)₉Pb(Te, S)₄—are described. The primary PGM were altered due to the effect of several mineral-forming processes that resulted in the formation of micro- and nanograins of Pt and Pd alloys, sulfides, and oxides, as well as in the complex distribution of PGE, Au, and Ag mineral assemblages. New types of complex Pt and Pd oxides with variable Cu and Fe contents were identified in the altered ores. Pt and Pd oxides as products of replacement of secondary Pt-Pd-Cu-Fe alloys occur as zonal and fibrous nanoscale Pt-Pd-Cu-Fe-(±S)-O aggregates.