Magmatic-hydrothermal origin of the early Triassic Laodou lode gold deposit in the Xiahe-Hezuo district, West Qinling orogen, China: implications for gold metallogeny

The Xiahe-Hezuo district in the West Qinling orogen contains numerous Au-(As-Sb) and Cu-Au-(W) deposits. The district is divided into eastern and western zones by the Xiahe-Hezuo Fault. The western zone is exposed at a shallow level and contains sediment-hosted disseminated Au-(As-Sb) deposits, whereas the eastern zone is exposed at a deeper level and contains Cu-Au-(W) skarn and lode gold deposits within or close to granitic intrusions. The Laodou gold deposit in the eastern zone consists of auriferous quartz-sulfide-tourmaline and minor quartz-stibnite veins that are structurally controlled by fault zones transecting the Laodou quartz diorite porphyry stock and enveloped by potassic and phyllic alteration. Both the veins and alteration halos commonly contain quartz, sericite, tourmaline, pyrite, and arsenopyrite, with minor galena, sphalerite, chalcopyrite, tetrahedrite, and enargite. Gold occurs mainly as invisible gold in pyrite or arsenopyrite and locally as inclusions less than 50 μm in diameter. The zircon U-Pb age of 247.6 ± 1.3 Ma (2σ) on the host quartz diorite porphyry and the sericite 40Ar/39Ar plateau ages of 249.1 ± 1.6 and 249.0 ± 1.5 Ma (2σ) on two ore-related hydrothermal sericite samples are within analytical errors of one another. At the formation temperature (275 °C) inferred from microthermometric measurements of fluid inclusion, sericite and tourmaline yield calculated δDH2O values of −70 to −45‰ and δ18OH2O of 5.8 to 9.7‰, while quartz yields calculated δ18OH2O values of 5.1∼5.7‰. Hydrothermal tourmaline in quartz-sulfide-tourmaline veins has δ11B of −11.2 to −0.9‰ (mean of −6.3‰) that are similar to the values of magmatic tourmaline (−8.9 to −5.5‰ with a mean of −6.8‰) in the host quartz diorite porphyry. The δ34S values of sulfide minerals range from −5.9 to +5.8‰ with a mean of −0.6‰ that is typical of magmatic sulfur. Pyrite from hydrothermally altered quartz diorite porphyry and quartz-sulfide-tourmaline veins have relatively homogeneous lead isotopic compositions, compatible with granitic intrusions in the district. The geochronological and isotopic data combined support a magmatic origin for the Laodou gold deposit, most likely formed from fluids exsolved from the Laodou quartz diorite porphyry or associated intrusive phases at deeper levels beneath the stock. Orogenic and Carlin-like gold deposits in the West Qinling orogen have been commonly thought to have formed from metamorphic fluids. This study, however, highlights the role of magmatic-derived fluids in the formation of lode gold deposits. Synthesis of geochronological, geological, and geochemical data on magmatic rocks and ore deposits in and surrounding the Xiahe-Hezuo district indicates that gold mineralization predominantly occurred within a subduction-related magmatic arc prior to collision between the Yangtze and North China cratons that produced the West Qinling orogen.