This is the first report of occurrences of platinum-group minerals (PGM) and unnamed phases of platinum-group elements (PGE) from the Pados-Tundra ultramafic complex of Paleoproterozoic age, Kola Peninsula. The PGM occur as individual inclusions or intergrowths (≤1–10 μm) hosted by cores of zoned grains of chromite–magnesiochromite (Chr) in chromitite of the Dunite block in the eastern portion of the complex. Osmium-poor laurite is abundant, commonly as intimate intergrowths with clinochlore (>80% laurite grains), followed by Os- and Ir-dominant alloys (i.e., the minerals osmium and iridium, respectively); unnamed phases of ruthenium selenide [RuSe2] and rhodium telluride [RhTe] are rare. Selenium-rich laurite, not reported previously, likely belongs to an inferred RuS2–RuSe2 series. Native Ru forms micro-(nano)-spherules in framboid-type aggregates, in association with relict laurite, which are surrounded by a skeletal grain of clinochlore; the host is Chr. The framboidal texture of nanophases of ruthenium, hitherto unreported in PGE alloys, likely formed as a result of deposition from a H2-bearing fluid, involving a reaction of desulfurisation and reduction of laurite. A highly S-deficient environment is indicated; a low sulfur fugacity, presumably below the Os–OsS2 buffer, caused the observed coexistence of a low-Os variety of laurite with Os-dominant alloy. An ultimate loss of total S is implied to have decreased the S/Se ratio, which resulted in the precipitation of Ru-dominant sulfoselenide–selenide phases, likely from an oxidizing fluid at a late stage of mineralization. We also infer that the laurite–clinochlore intergrowths crystallized relatively late, from microvolumes of an H2O-bearing fluid of contrasting composition, enriched in Ru, S, and lithophile elements.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.