Geological and isotopic constraints on ore genesis, Huangjindong gold deposit, Jiangnan Orogen, southern China

Abstract

The Huangjindong gold deposit, with gold resources of ∼80 t, is hosted in Neoproterozoic slates along the Chang-Ping Fault in the Jiangnan Orogen, southern China. The orebodies are dominated by auriferous arsenopyrite-pyrite-quartz-(scheelite) veins; quartz-rich breccias that are cemented by late arsenopyrite, pyrite, muscovite, and quartz; auriferous stibnite-quartz veins; and disseminated gold in arsenopyrite-pyrite-sericite-quartz altered rocks. Neither significant alteration zoning nor ore-element zoning are recognized within the orebodies, although the orebodies at shallower crustal levels are slightly more stibnite-rich. At the regional scale, the first-order control on the distribution of gold deposits is the NNE- to NE-trending crustal-scale, sinistral strike-slip Chang-Ping Fault. As in other gold provinces globally, this controlling fault is not gold mineralized. The gold occurrs in WNW to E-W-trending zones of thrusts and fracture arrays associated with tight ‘locked-up’ asymmetrical folds. The broadly E-W-trending orebodies formed independently within approximately NE-trending fault-bounded corridors defined by the first-order Chang-Ping, and second-order Ni-Wan and Keng-Shang/Chang-Long Faults. Hydrothermal activity was initiated at the ductile-brittle transition, but evolved to brittle-dominated deformation, all within a transpressional environment.Oxygen and hydrogen isotope compositions of fluids (δD = −44.4‰ to −34.2‰; δ18O = +8.6‰ to +9.5‰) calculated from gold-related hydrothermal quartz and hydrothermal sericite, respectively, are consistent with a metamorphic source. Sulfur isotope compositions of the gold-bearing arsenopyrite and pyrite (median values between −8.0‰ and −6.0‰) are generally distinct from δ34S of the regional Neoproterozoic slates in the Jiangnan Orogen, but overlap the compositions of pyrites in global sediment-hosted orogenic gold deposits. Lead isotope compositions of the gold-bearing arsenopyrite and pyrite (206Pb/204Pb: 17.049 to 18.417; 207Pb/204Pb: 15.263 to 15.712; 208Pb/204Pb: 37.597 to 39.220) suggest that the lead was sourced from metamorphic rocks involved in the orogenic events. Therefore, both the stable and radiogenic isotopic data indicate that ore-forming fluids and metals were derived from a metamorphic source, although the precise nature of that source is unclear.On the basis of these geological and isotopic data, Huangjindong is interpreted as an orogenic gold deposit. The gold ore bodies show classic structural controls, being adjacent at the province scale to the crustal-scale Chang-Ping Fault but located in thrusts and associated tight anticlinal fold hinges that lie in corridors between markedly oblique, relatively brittle fault arrays that parallel the Chang-Ping Fault.