Geology, mineralization, fluid inclusion, and stable isotope characteristics of the Sinyukhinskoe Cu-Au skarn deposit, Russian Altai, SW Siberia

Abstract

The Sinyukhinskoe deposit (>100 t Au with average 9.2 g/t Au) is situated in the Altaid orogenic system and is part of the regional system of Devonian-Carboniferous Cu-Au to Au-Cu porphyry, skarn and other deposits. Productive plutonic suites were emplaced in a Devonian subduction-related continental magmatic arc setting, with its further evolution toward a late- and post-collisional environment in the late Devonian-Carboniferous. The deposit is related to a magnetite-series, I-type multiphase pluton, with the evolution of the intrusive phases from early gabbroic through diorite-granodiorite to the late granitic phases. The igneous rocks belong to medium-K calc-alkaline series or are transitional from medium- to high-K calc-alkaline varieties. Prior to granitic phases, this evolution was interrupted by the intrusion of intramineral high-K mafic dikes exhibiting a shoshonitic affinity. The deposit is characterized by the development of abundant andradite-rich garnet and wollastonite in prograde and retrograde skarns, locally together with magnetite and bornite. The subsequent propylitic (epidote-chlorite-amphibole-albite-calcite-quartz, locally with clinozoisite and prehnite) alteration assemblages bear more intense Cu mineralization (chalcopyrite, bornite, locally tennantite) and high fineness (~940–910‰) native gold. Later phyllic (quartz-sericite to quartz-sericite-chlorite) alteration assemblages comprise the most intense intermediate-fineness (910–880‰) native gold and Cu (bornite, chalcocite) mineralization that is followed by low-fineness (820–760‰) native gold and telluride minerals. The retrograde skarn was formed from a high-salinity (22–23 wt% NaCl-eq.), hot (450 ± 50 °C), high-pressure (2.5 ± 0.5 kbar) homogenous magmatic-hydrothermal fluid, under its possible direct separation from crystallizing magma. After intrusion of intramineral mafic dikes, propylitic alteration assemblages were formed from a moderate salinity (15–18 wt% NaCl-eq.) homogenous aqueous fluid at the temperatures of 380–400 °C. This fluid could be supplied by crystallizing magma, with its further mixing with heated meteoric waters. Fluid inclusions in phyllic alteration minerals indicate a new influx of homogenous, high-salinity (18 wt% NaCl, 22 wt% KCl, 10 wt% CaCl2), high-pressure (1.3 ± 0.1 kbar), aqueous-chloride fluid at lower (~280 °C) temperatures. Elevated Bi, Hg, Te contents, as well as δ34S values close to meteorite standard (0 ± 1.5‰) suggest at least partial fluid sourcing from a mantle-derived magma at the phyllic alteration stage.