Geology, mineralization, and fluid inclusion characteristics of the Agylki reduced tungsten (W-Cu-Au-Bi) skarn deposit, Verkhoyansk fold-and-thrust belt, Eastern Siberia: tungsten deposit in a gold-dominant metallogenic province

Abstract The Agylki deposit is a large (>100 Kt WO3, possibly underexplored) and high-grade (average 1.46% WO3, 2.79% Cu, 0.5 g/t Au, and 0.03-0.04% Bi) reduced W skarn deposit situated in the Verkhoyansk fold-and-thrust belt, the latter formed after a passive continental margin, part of the Verkhoyansk-Kolyma gold-dominant metallogenic province. The belt comprises intrusion-related Au mineralization and roughly parallels the other main metallogenic belts of the region hosting numerous gold (Au, Au-Bi, Au-Sb, etc.) and W-Sn deposits. The deposit could be part of a broader regional-scale magmatic-hydrothermal system comprising nearly coeval intrusion-related reduced W skarn deposits and vein/stockwork Au(±W) deposits. The deposit is related to a swarm of Late Jurassic-Early Cretaceous monzodiorite-porphyry, granodiorite- and granite-porphyry dikes that intrude a Triassic terrigenous sequence. The dikes comprise weakly reduced-weakly oxidized (ilmenite-titanite-bearing), high- to medium-K calc-alkaline, metaluminous to weakly peraluminous, I-type granitoid rocks. The deposit is represented by a single lenticular flat-lying to shallow-dipping mineralized zone that was formed by the replacement of a limestone horizon. This may correspond to a distal level in an integrated structural model of W skarn deposits that occurs well above causative pluton. The deposit bears signatures of reduced W skarn deposits, with dominant pyroxene skarn containing pyrrhotite and scheelite. Propylitic alteration assemblages (amphibole-epidote-chlorite-calcite-quartz with abundant pyrrhotite, scheelite and minor chalcopyrite) overprint skarn and are in turn overprinted by zones of phyllic alteration (quartz-sericite-Fe-carbonate) comprising abundant apatite and arsenopyrite followed by scheelite, chalcopyrite and other sulfides as well as Bi, Te and Au minerals. Fluid inclusions data indicate formation of retrograde (mainly pyroxene-quartz-amphibole) skarn from aqueous, dominantly magnesian-sodic-chloride, moderate-salinity (14.0-17.3 wt.% NaCl-eq.), high pressure (1.4±0.2 kbars), moderately-hot (∼400-500°C) fluid. Propylitic alteration assemblages were formed from homogenous, low-carbonic, lower salinity (8.8-13.7 wt.% NaCl-eq.), lower pressure (1.1-0.8 kbars) and lower temperature (∼300-400°C) Ca-enriched fluid. Early phyllic alteration assemblages were formed from a boiling fluid, with separation of carbonic-rich (particularly methane-enriched) gaseous fluid and coexisting low-carbonic aqueous fluid at ∼320-280°C and ∼0.9±0.1 kbar. Late phyllic alteration assemblages were formed from homogenous, low-carbonic, moderate-salinity (14.0-17.8 wt.% NaCl-eq.) low-temperature (250-200°C) fluid.