Abstract
The Axi gold deposit is one of the largest epithermal deposits in northwestern China and is characterized by multistage generations of pyrite associated with gold mineralization. Genetic models for superjacent epithermal Au and porphyry Cu systems involve either contemporaneous or episodic mineralization, implying different metal sources and ore-forming events. Nine pyrite separates from disseminated ores associated with pyrite-sericite-quartz alteration in the Axi deposit yield a Re–Os isochron age of 350 ± 10 Ma (MSWD = 5.1, initial 187Os/188Os (IOs) = 0.20 ± 0.11). This age likely reflects a mixed contribution of two subtypes of disseminated pyrite (Py1) based on zoned pyrite crystals. The isochron age of 353 ± 6 Ma (MSWD = 1.6, IOs = 0.107 ± 0.021, n = 6), excluding samples with pyrite cores, overlaps previous ages for andesite from the Dahalajunshan Formation (ca. 356 Ma), indicating that the early mineralization event occurred shortly after the formation of host rocks (ca. 353 Ma). Pyrite from disseminated ores shows little variation of sulfur isotopic compositions (+ 2.9 to + 4.0‰) and has high Cu, Co, Ni, and V contents, which favors a magmatic-hydrothermal origin. Pyrite grains from gray quartz veins yield a younger Re–Os isochron age of 332 ± 8 Ma (MSWD = 0.22, n = 4). Vein-hosted pyrite shows characteristics of a mantle-derived magma source (IOs: 0.171 ± 0.024; δ34S: − 0.10~+ 3.10‰), which result from the intrusion-driven convective hydrothermal circulation accompanying fluid exsolution and metal remobilization from basement and wallrocks. Our data demonstrate that the Axi gold deposit was formed during two temporally separated hydrothermal events. This model of ore formation may apply to other epithermal-porphyry deposits within the Axi–Tawuerbieke district: disseminated Au and porphyry-style Au ± Cu mineralization occurred shortly after magmatic activity (356–353 Ma) whereas epithermal-style Au mineralization occurred at ca. 332 Ma.
Published Version
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