World-class Xincheng gold deposit: An example from the giant Jiaodong gold province

The Jiaodong gold deposits are currently the most important gold resources (with Au reserves of >4000 t) in China, and the leading gold-producing country globally (with Au production of similar to 428 t in 2013). Jiaodong is also considered as perhaps the only world-class to giant gold accumulation on the planet where relatively young gold ores (ca. 130-120 Ma) were deposited in rocks that are 2 Ga older. The Xincheng world-class high-grade gold deposit, with a proven reserve of >200 t gold, is one of the largest deposits in the giant gold province of the Jiaodong Peninsula. It is located in the northwestern part of the Jiaobei Uplift, and hosted by ca. 132-123 Ma Xincheng quartz monzonites and monzogranites. Ore zones are structurally controlled by the NE-trending and NW-dipping Jiaojia Fault and subsidiary faults, and are mainly restricted to the footwall of the fault. The dominant disseminated- and stockwork-style ores are associated with strong sericitization, silicification, sulfidation and K-feldspathization, and minor carbonate wallrock alteration halos. The four mineralization stages are pyrite-quartz-sericite (stage 1), quartz-pyrite (stage 2), quartz-polysulfide (stage 3) and quartz-carbonate (stage 4). Gold occurs dominantly as electrum, with lesser amounts of sulfide-hosted native gold and rare native silver and argentite, normally associated with pyrite, chalcopyrite, galena and sphalerite: the latter with proven resources of about 105 t Ag, 713 t Cu, and 5100 t S. There are three types of ore-related fluid inclusions: type 1 aqueous-carbonate (H2O-CO2), type 2 aqueous (liquid H2O + vapor H2O), and type 3 CO2 (liquid CO2 and vapor CO2) inclusions. Homogenization temperatures range from 221 to 304 degrees C for type 1 inclusions, with salinities of 2.4-13.3 wt.% NaCl eq., and bulk densities of 0.858-1.022 g/cm(3). The delta S-34(CDT) values of hydrothermal sulfides are 4.3-10.6 parts per thousand and delta O-18 values of hydrothermal quartz have a median value of 13.0 parts per thousand. delta D values of fluid inclusions in hydrothermal quartz have a median value of -75 parts per thousand. Calculated delta O-18(water) has a median value of 5.2 parts per thousand. The timing of gold mineralization at the Xincheng gold deposit is younger than 123 +/- 1 Ma, and likely between 120.9 and 119.9 Ma. A minerals system genetic model for the probable epizonal orogenic Xincheng deposit suggests an initial medium temperature, CO2-rich, and low salinity H2O-CO2 deeply sourced metamorphic ore fluid associated with dehydration and decarbonization of subducting Paleo-Pacific lithosphere. The Jiaojia Fault constrained the migration of ore-forming fluids and metals at the brittle-ductile transition. Fluid immiscibility, caused by episodic pressure drops, led to significant high-grade gold deposition in the giant Xincheng gold deposit. (C) 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.