Geology, mineralization, and fluid inclusion characteristics of the Skrytoe reduced-type W skarn and stockwork deposit, Sikhote-Alin, Russia

The Skrytoe deposit (> 145 Kt WO3, average grade 0.449% WO3) in the Sikhote-Alin orogenic system (Eastern Russia) is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in a late to post-collisional tectonic environment after cessation of active subduction. It is localized within a mineralized district of reduced-type skarn W and veined Au (+/- W) deposits and occurrences related to the Early Cretaceous ilmenite-series plutonic suite. The deposit incorporates large stockworks of scheelite-bearing veinlets related to propylitic (amphibole, chlorite, quartz) and phyllic (quartz, sericite, albite, apatite, and carbonate) hydrothermal alteration. The stockwork cuts flat-lying mafic volcanic rocks and limestone partially replaced by pyroxene skarn that host the major W orebodies. Scheelite is associated with pyrrhotite and/or arsenopyrite, with minor chalcopyrite and other sulfide minerals; the late phyllic stage assemblages hosts Bi and Au mineralization. The fluid evolution included low-salinity moderate-temperature, moderate-pressure (similar to 370-390 degrees C, similar to 800 bars) methane-dominated carbonic-aqueous fluids that formed post-skarn propylitic alteration assemblages. Then, at the phyllic stage, there has been an evolution from methane-dominated, moderate-temperature (330-360 degrees C), low-salinity (< 12.3 wt% NaCl equiv.) fluids forming the early quartz-sericite-albite-arsenopyrite assemblage, through lower temperature (290-330 degrees C) methane-dominated, low-salinity (similar to 9-10 wt% NaCl equiv.) fluids forming the intermediate quartz-sericite-albite-scheelite-pyrrhotite assemblage, to yet lower temperature (245-320 degrees C) CO2-dominated carbonic-aqueous low-salinity (similar to 1-7 wt% NaCl equiv.) fluids forming the late quartz-sericite-sulfide-Bi-Au assemblage. Recurrent fluid immiscibility (phase separation) and cooling probably affected W solubility and promoted scheelite deposition. The stable isotope data support a sedimentary source of carbon (delta C-13(fluid) = similar to-21 to -10 parts per thousand), a magmatic source for water (delta O-18(H2O) = +7.4 to +7.7 parts per thousand), and dominantly crustal-derived source of sulfur (delta S-34 = -4.6 to -2.9 parts per thousand) in the hydrothermal fluids. This is consistent with the development of larger, longer crystallizing crustal intermediate to felsic magma chambers in the late to post-collisional tectonic environment, with their protracted magmatic evolution advancing magmatic differentiation and partitioning of W into magmatic-hydrothermal fluid. The dominating role of the crustal-derived magmatic water, sulfur, and carbon appears to be an important feature of reduced W skarn deposits related to ilmenite-series granitoids.