Ore geology, fluid inclusion and 40Ar/39Ar geochronology constraints on the genesis of the Yingchengzi gold deposit, southern Heilongjiang Province, NE China

Abstract

The Yingchengzi gold deposit, located at the eastern margin of the Zhangguangcai Range, southern Heilongjiang Province, NE China, is the only high-grade gold deposit in the region. The deposit is hosted in mylonitized Paleozoic granite and metavolcanic rocks and is structurally dominated by brittle–ductile shear zones and faults. The orebodies consist of gold-bearing quartz veins and gold-bearing altered mylonites. The hydrothermal ore-forming process can be divided into early, middle and late stages, characterized by quartz–pyrite veins, quartz–gold–polymetallic sulfide veins, and quartz–carbonate veins or veinlets, respectively. Ore minerals and gold mainly occurred in the middle stage.Four types of fluid inclusions were identified in the vein minerals based on petrography and laser Raman spectroscopy: NaCl–H2O (W-type), CO2–H2O (C-type), pure CO2 (PC-type) and less common calcite-bearing (S-type) inclusions. The early stage minerals contain C-type and rare PC-type fluid inclusions which have homogenization temperatures of 262–400°°C, and low salinities of 2.20–8.51wt.% NaCl equiv. Only the middle stage minerals contain all four types of fluid inclusions with homogenization temperatures of 173–316°°C, and salinities of 2.20–12.42wt.% NaCl equiv. The late stage quartz and calcite crystals contain only W-type fluid inclusions, which have homogenization temperatures of 104–207°°C and salinities of 1.05–6.58wt.% NaCl equiv. Fluid boiling and the mixing caused the rapid precipitation of sulfides and gold. These indicate that the ore-forming fluid system evolved from CO2-rich, metamorphogenic to CO2-poor, due to the boiling and input of meteoric water. The trapping pressures estimated from C-type fluid inclusions are 98–245MPa in the main ore-forming stage, giving a reasonable estimate that an alternating lithostatic–hydrostatic fluid system controlled a fault-valve activity at a depth of 9–10km.40Ar/39Ar dating of hydrothermal sericite from gold-bearing altered rock shows that gold mineralization occurred at ~248Ma. We assume that the Yingchengzi gold deposit was formed in a Late Permian–Early Triassic continental collision regime following closure of the Paleo-Asian Ocean. The regional geology, ore geology, fluid inclusions, mineralization age and tectonic setting infer that the Yingchengzi gold deposit is a mesozonal orogenic gold system, making it the first orogenic gold deposit discovered in the Zhangguangcai Range, NE China.