Multi-Stage Magmatic-Hydrothermal Sulfide-PGE Mineralization of the Khudolaz Complex (South Urals)

For the first time, from the standpoint of magmatism and subsequent hydrothermal–metasomatic alteration, sulfide and platinum-metal mineral assemblages of rocks of ore-bearing intrusions of the Khudolaz Complex have been characterized. Four types of assemblages have been identified: (1) pentlandite–chalcopyrite–pyrrhotite in the form of drop-shaped and interstitial disseminations with inclusions of sperrylite, moncheite, michenerite, merenskyite, froodite; (2) complex amoebalike inclusions with the composition pyrite ± pyrrhotite–chalcopyrite–violarite ± pentlandite with inclusions of Sb–michenerite, sudburyite, and borovskite; (3) newly formed pyrite–chalcopyrite veins and patches in intensely metasomatized rocks; (4) additionally-formed euhedral pyrite disseminations in metasomatites along zones intersected by dolerite dikes. The formation of these sulfide–platinum-metal assemblages occurred in three stages: (1) magmatic and late magmatic ( type 1 ), 2) hydrothermal–early medium temperature and late low-temperature ( types 2, 3 ), (3) repeated hydrothermal–low–medium temperature ( type 4 ). It is shown that the crystallization of sulfide minerals occurred in a wide temperature range (about 1000–200°C). PGE minerals separated at the late magmatic stage during cooling of a highly fractionated sulfide or immiscible chalcogenide melt and during decomposition of sulfide solid solutions ( T ~ 650–300°C). At the early hydrothermal stage ( T ~ 300–250°C), pyrrhotite was replaced by pyrite; pentlandite, by violarite; a significant amount of Ni and Co from primary sulfides was inherited by secondary sulfides. Primary chalcopyrite was mainly replaced by silicates (chlorite, amphibole, etc.). It is suggested that the antimony minerals of Pd (including high-antimony michenerite with Sb up to 0.46 apfu) could have crystallized from an Sb-enriched hydrothermal fluid. Host rocks could have been an additional source of antimony in the fluid. At the late hydrothermal stage ( T < 200°C), significant dissolution of primary sulfide and platinum-metal phases occurred with redeposition in the upper parts of massifs and in host rocks. The recurring hydrothermal process ( T ≤ 200°C) was associated with emplacement of dikes of the Ulugurtau complex and new redeposition of sulfides along zones of fluid action.