Sulfide-platinum metal mineralization of Khudolaz complex Malyutka altered gabbro massif: hydrothermal influence to the mineral association types

The results of study of sulfide assemblages in altered gabbro Malyutka massif of South Urals Khudolaz differentiated complex were presented in this paper. This massif is a small olivine-hornblende gabbro stock containing Cu-Ni ores located in the footwall. The platinum group minerals (PGM) - michnerite and sperrylite - were found in the pyrrhotite, chalcopyrite, and pentlandite for the first time. We determined a significant influence of hydrothermal alteration on the ore occurrence. The ore forming process was subdivided into three types of the stages or assemblages: 1) magmatic type (pyrrhotite ± pyrite-chalcopyrite-pentlandite ± violarite + magnetite assemblage containing Rh-sperrylite and Pt-michnerite); 2) early hydrothermal type (pyrite ± pyrrhotite-chalcopyrite-violarite ± pentlandite + magnetite assemblage containing high Sb-michnerite); 3) late hydrothermal type (pyrite-chalcopyrite assemblage, PGE-mineralization not determined). Hydrothermal fluids were responsible for a small Fe and Cu loss from sulfides and further redeposition of these elements. Nickel and Co contents were increased in newly formed minerals such as pyrite and violarite. The first two types of assemblages were characterize of different michnerite composition: magmatic michnerite contained Pt (up to 3.2 %) and no Sb, whereas early hydrothermal michnerite displayed extremely high-Sb contents (up to 9.7 %) and a lack of Pt.