Zircon U-Pb age and Hf isotope compositions of the wall rocks in the Sarshuoke gold-copper polymetallic deposit in Altay, Xinjiang: Implications for the ore-forming conditions

The Sarshuoke gold-copper polymetallic deposit is hosted in the intermediate andestic-acidic volcanic-subvolcanic rocks of the Devonian Ashele Formation. Gold-copper(Au-Cu) mineralization mainly occurs at the shallow level(less than 100m in depth), and the shapes of veins and lenticular Au-Cu orebodies are significantly constrained by the NNW-trending faults. Au-Cu minerals exist mainly in hydrothermal altered rocks with some in the quartz veins, and the wall rocks are the pyritization-phyllicization rhyolite porphyry. Gold mainly occurs in fine-grained pyrite and other sulfides and quartz. The lead-zinc(Pb-Zn) ores are mainly distributed at lower level at ca.100m under the surface. The grade of Pb-Zn increases with the increasing depth. The grade of Pb-Zn ranges from 3% to 5%, and the highest grade could reach up to 12%. Au-Cu polymetallic mineralization is related closely to rhyolite porphyry. Zircon U-Pb dating shows that the wall rock of the deposit, i.e., rhyolite porphyry, crystallized at 386.2±1.5Ma. Zircon Lu-Hf isotope analysis yielded pronounced depleted Hf isotope composition with ε_Hf(t) ranging from 7 to 13, mostly 9-10. The data obtained by the authors, in combination with regional geology and previous studies, reveal that the rhyolite porphyry was formed on an active continental margin related to plate subduction in late Early Devonian. Pb-Zn mineralization is genetically related to the rhyolite porphyry igneous activity. Furthermore, Au-Cu mineralization most possibly took place at the late stage and was ge-netically related to volcanic hydrothermal effects on the basis of Au-Cu-bearing rhyolite porphyry.