Ore genesis of the Zuojiazhuang gold deposit in the West Qinling Orogen: constraints from pyrite trace elements and multi-isotope analyses

The Zuojiazhuang gold deposit is located in the northwestern part of the Devonian Feng-Tai basin in the West Qinling Orogen. Gold ore bodies are mainly hosted in the EW striking shear zone in the Hejiazhuang pluton (247 Ma). The quartz-sulfide ore (lode gold), as micro-disseminated gold hosted in the Devonian sediments, is quite different from other gold deposits in the basin. Fluid inclusion study indicated that the ore fluid in the Zuojiazhuang gold deposit can be characterized as a middle-low temperature (122-305 ℃), low salinity (1.2%-11.8% NaCleq) and epizonal (2.4 km) CO2-NaCl fluid system. Hydrogen-O isotope analysis (δDH2O=-88.8‰--81.1‰; δ18OH2O=-0.4‰-+7.6‰) shows that the ore fluid is metamorphic fluid at origin, with magmatic and meteoric water mixing during upward movements. Lead isotope distribution indicates the metal source came from shallow upper crust and orogene belt. Sulfur isotopic composition was concentrated between 11.4‰ and 13.4‰, which is in agreement with sedimentary-hosted gold deposit in the basin and Devonian orogenic gold deposit around the world. In-situ trace elemental analysis of auriferous pyrite shows that the ore fluid was enriched in Au, As, Cu, Sb, Ag, Pb, Bi, etc. Moreover, similar trace elemental enrichment in the Devonian sediments in the Feng-Tai basin, especially at the boundary of Middle to Upper Devonian, has been widely reported. Therefore, we hypothesize that the Devonian sediments could be the reasonable metal source for the Zuojiazhuang deposit. The mechanism of gold deposition in the Zuojiazhuang deposit could be attributed to the sharp decrease of fluid pressure and subsequent fluid boiling, which caused rapid precipitation of minerals inside the tensile fracture zone to form the lode gold deposit. The Zuojiazhuang deposit may be defined as epizonal orogenic gold deposit as it is compared with typical orogenic gold around the world. © 2019, Editorial Office of Earth Science Frontiers. All right reserved.