The Baogutu gold deposit in west Junggar, NW China: An epizonal intrusion-related gold deposit

Abstract

The Baogutu gold deposit (∼4 Mt ore @ 5.5 g/t Au) in West Junggar, is hosted by Early Carboniferous volcano-sedimentary rocks and Late Carboniferous dioritic dikes. Both the dikes and gold mineralization are controlled by NE-trending faults. Two diorite dikes in direct spatial association with gold-bearing quartz veins have LA–ICP–MS zircon U–Pb ages of 316 ± 5 Ma and 314 ± 3 Ma, respectively. The emplacement ages of the dikes are a little older than reported gold mineralization ages (312–311 Ma), which is consistent with their crosscutting relationships. Gold mineralization occurs in quartz veins and stockworks in altered tuff, tuffaceous siltstone, silty mudstone, and dikes. Three gold-bearing vein stages have been identified: (I) pyrite–quartz veins in the deep part of the system; (II) stibnite–quartz veins and (III) native arsenic–quartz veins at shallower-level. Fluid inclusion study shows that the pyrite–quartz veins of stage I formed at temperatures between 345 and 265 °C from fluids with salinities of 5.1–9.1 wt% NaCl equiv, whereas the native arsenic–quartz veins of stage III formed at ∼200 °C from fluids with salinities of 1.4–3.9 wt% NaCl equiv. The ore-forming pressure is estimated to be ∼50–62 MPa, corresponding to mineralization depth of ∼3.3–4.0 km. Oxygen and hydrogen isotope compositions of the fluids (δ18OH2O = 7.9 to 11.2 ‰, δD = -88 to −96 ‰) indicate that the initial hydrothermal fluids have a magmatic source, with involvement of meteoric water in the late stage. The in-situ lead isotopic compositions of pyrite in stage I pyrite–quartz veins are similar to those of bulk sulfide minerals in the Baogutu porphyry Cu deposit, and close to the porphyry stocks, whereas the stibnite and native arsenic in stage II and III quartz veins have more radiogenic lead isotopic signatures due to water–rock exchange with the wall rocks. The Baogutu gold deposit shows affinities to epizonal gold deposits in intrusion-related gold systems (IRGS). The IRGS model suggests that, sheeted, disseminated and vein-type Au–As–Bi mineralization might be found hosted in dioritic to granitic stocks and proximal strata, and vein-type Au–As–Sb mineralization in distal faults.