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

• Published in: Mineralium deposita 2017-08, Vol.52 (6), p.883-902
• Main Authors: Jin, Xiao-ye, Li, Jian-wei, Hofstra, Albert H., Sui, Ji-xiang
• Format: Article
• Language: English
• Subjects: Analytical errors; Antimony; Argon; Arsenopyrite; Chalcopyrite; Copper; Copper compounds; Cratons; Deposits; Diorite; Earth and Environmental Science; Earth Sciences; Enargite; Exposure; Fault zones; Faults; Fluids; Galena; Geochemistry; Geochronology; Geology; Gold; Halos; Inclusions; Lead; Magma; Mathematical analysis; Metallogenesis; Mineral deposits; Mineral Resources; Mineralization; Mineralogy; Minerals; Plateaus; Pyrite; Quartz; Radiometric dating; Rock; Rocks; Sediment; Sphalerite; Subduction; Subduction (geology); Sulfides; Sulfur; Sulfur isotopes; Sulphides; Sulphur; Temperature; Temperature effects; Tourmaline; Triassic; Veins (geology); Yields
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-016-0710-8

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 (2 σ ) on the host quartz diorite porphyry and the sericite 40 Ar/ 39 Ar plateau ages of 249.1 ± 1.6 and 249.0 ± 1.5 Ma (2 σ ) 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 δ D H2O values of −70 to −45‰ and δ 18 O H2O of 5.8 to 9.7‰, while quartz yields calculated δ 18 O H2O values of 5.1∼5.7‰. Hydrothermal tourmaline in quartz-sulfide-tourmaline veins has δ 11 B 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 δ 34 S 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, howev