Geology and geochemistry of the Early Permian Axi low-sulfidation epithermal gold deposit in North Tianshan (NW China)

The Axi gold deposit containing 12 Mt of ore with an average Au grade of 5 g/t is the largest low-sulfidation epithermal gold deposit in Chinese Tianshan. It is hosted by Late Devonian to Early Carboniferous continental arc volcanic rocks in the Tulasu volcanic-sedimentary Basin in North Tianshan (NW China). The orebody is controlled by annular and radial faults associated with volcanic crater, and later NW-trending faults. Four stages of ore-formation could be identified based on petrographic studies: (I) chalcedony; (II) gold-bearing pyrite-quartz veins and breccias; (III) gold-bearing marcasite-quartz-carbonate veins; and (IV) quartz +/- barite-calcite veins. Electrum precipitated in stage II (285-195 degrees C, logfS(2) = -6.7 to -13) and stage III (190-95 degrees C, logfS(2) = -15.8 to -25.6). The chalcedony formed in stage I is strongly enriched in LREE (La/Yb = 4.5-36.2) compared with later gold-bearing pyrite-quartz veins and quartz +/- barite-calcite veins (La/Yb = 1.2-2.0). O-H isotopic data indicate that fluid mixing is a major mechanism causing electrum precipitation in stage II. Hydrothermal breccias consisting of chalcedony, altered wall-rocks and gold-bearing pyrite-quartz veins (stage II) are cemented by gold-bearing marcasite-quartz-carbonate assemblages (stage III), which implies that brecciation plays an important role in stage III ore-formation. Re-Os isotopic analyses of gold-bearing pyrite samples yield an Re-187/Os-188-Os-187/Os-188 isochron age of 299 +/- 35 Ma with an initial Os-187/Os-188 ratio of 0.57 +/- 0.37. K-Ar ages of four intensively sericitized andesitic wall rocks range between 286 and 293 Ma. These Early Permian ages are much younger than the Late Devonian-Early Carboniferous host volcanic rocks, but overlap the age range of postcollisional granitoid in the Tulasu Basin. The Re-Os isotopic compositions and the ages suggest that mantle-derived components were involved in the Axi ore-forming system in a postcollisional setting. (C) 2017 Elsevier B.V. All rights reserved.