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

Abstract 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 ∼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 (H 2 O–CO 2 ), type 2 aqueous (liquid H 2 O + vapor H 2 O), and type 3 CO 2 (liquid CO 2 and vapor CO 2 ) inclusions. Homogenization temperatures range from 221 to 304 °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 δ 34 S CDT values of hydrothermal sulfides are 4.3–10.6‰ and δ 18 O values of hydrothermal quartz have a median value of 13.0‰. δ D values of fluid inclusions in hydrothermal quartz have a median value of −75‰. Calculated δ 18 O water has a median value of 5.2‰. 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, CO 2 -rich, and low salinity H 2 O–CO 2 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.