Add time:08/12/2019         Source:sciencedirect.com

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 °C, logfS2 = −6.7 to −13) and stage III (190–95 °C, logfS2 = −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 187Re/188Os–187Os/188Os isochron age of 299 ± 35 Ma with an initial 187Os/188Os 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.

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