Primary and alteration assemblages of platinum-group minerals from the Ognit complex, lrkutskaya oblast, Eastern Sayans, Russia

Two associations of platinum-group minerals (PGM) and other rare compounds are reported from the Ognit ultramafic-mafic complex of Neoproterozoic age, located at the southern margin of the Siberian Craton in the East Siberian metallogenic province, Eastern Sayans, Russia. The first association includes Ru-Os disulphides (laurite-erlichmanite series), Ru-Os diarsenides (anduoite-omeiite) and sulpharsenides (platarsite-osarsite-ruarsite), which are strongly enriched in Ir-Ru-Os (i.e. most refractory and classified as the iridium subgroup, IPGE, of the platinum-group elements, PGE). These PGM likely crystallized compara-tively early in the crystallization history. They exhibit a closer affinity to primary grains of base-metal sulphides (BMS), mostly pentlandite, subordinate chalcopyrite, and troilite, which points to reducing conditions in a late magmatic environment, in which droplets of sulphide melt had separated. The inferred saturation in As in this melt likely promoted the formation of As-based spe-cies, e.g., droplet-like or subhedral grains of orcelite, locally mantled by unnamed PtCu5. The primary BMS were remobilized to various degrees during deuteric alteration. The second association is more diverse, and mainly consists of the Pd-rich PGM: palladium bismuthotellurides and antimonides, such as phases of the type Pd(Bi,Te,Sb)(1+x) merenskyite, michenerite, mertieite-II and stibiopalladinite, naldrettite, polarite, urvantsevite, froodite, and unnamed PGM: Pd2Ge and Pd2+x Cu1-x(Sb,Sn), which likely is a Sb-dominant analogue of cabriite or taimyrite. Various species of ore minerals are present, including heazlewoodite, maucherite, cobaltite, Se-bearing galena, shandite, parkerite, graphite, native bismuth, Ag-rich alloys and compounds (native silver and hes-site), auricupride, and altaite. In addition, awaruite and native copper, which are fairly abundant, formed during serpentinization at extremely low levels of sulphur and oxygen fugacities, from H-2-H2O-bearing fluids and via transformation reactions involving primary BMS (pentlandite and chalcopyrite). The potential exists for new mineral species to be discovered in the Ognit complex.