The superlarge Malmyzh porphyry Cu-Au deposit, Sikhote-Alin, eastern Russia: Igneous geochemistry, hydrothermal alteration, mineralization, and fluid inclusion characteristics

Abstract

The Malmyzh porphyry Cu-Au deposit cluster (>10 Mt Cu-eq.) is the largest in eastern Russia. It is located in the turbidite terrane and was formed in a transform continental margin setting. The deposit is related to a magnetite-series, dominantly low-K calc-alkaline diorite-tonalite igneous suite, with a higher potassium content in the younger (quartz monzonite to granodiorite) intrusive phases. These metaluminous to peraluminous I-type igneous rocks exhibit slightly elevated Sr/Y and have combined subduction-related to post-subduction geochemical signatures. The magma was emplaced in a series of separated local magmatic “porphyry centres” that vary in petrologic features and relative abundance of different igneous rocks as well as in the dominant development of different hydrothermal alteration and mineralization types. The deposit comprises quartz-K-feldspar ± biotite ± chalcopyrite stockworks in zones of potassic alteration, followed by quartz-magnetite ± chlorite and then by quartz-sulfide-magnetite (with chalcopyrite and bornite) stockworks in zones of propylitic alteration. The latter is partially overprinted by quartz-sericite-chalcopyrite and quartz-sericite-pyrite stockworks formed during phyllic alteration stage. The most productive Cu mineralization is related to a later (quartz-sulfide-magnetite) hydrothermal event during the propylitic alteration stage, and to an early (quartz-sericite-chalcopyrite) event during the phyllic alteration stage. Further development of phyllic alteration caused a dilution of Cu grades. Minor Au, Bi and Te mineralization accompanied Cu mineralization. Potassic alteration assemblages formed from a homogenous high-salinity (57–78 wt% NaCl and 13–12 wt% KCl), high-temperature (>525–535 °C), sodic-potassic aqueous-chloride fluid, under a pressure of 500 ± 100 bars. At the propylitic stage, the early quartz-magnetite-chlorite assemblage was formed from a lower temperature (480 ± 5 °C), sodic-dominant aqueous-chloride fluid, under near-critical conditions, and at a similar pressure (~500 bars). This was followed by two episodes of the most intense Cu and Au deposition that occurred under overlapping temperatures (~380 to 250 °C) but from compositionally distinct (sodic-potassic- to sodic-calcic-dominant) boiling to homogenous fluids during the end of the propylitic alteration stage and the beginning of the phyllic alteration stages, respectively. The different cycles of fluid exsolution likely corresponded to degassing of different magmatic intrusions during a multistage magmatic evolution. Corresponding fluid pressure decrease (to ~250 bars and lower) indicates a transition from lithostatic to hydrostatic conditions.