Compounds of Ru-Se-S, alloys of Os-Ir, framboidal Ru nanophases, and laurite-clinochlore intergrowths in the Pados-Tundra complex, Kola Peninsula, Russia

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 mu m) 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 H-2-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.