Hydrothermal metallogeny of the Shanggong gold deposit, east Qinling: Radiogenic isotope geochemistry

Abstract

The Shanggong Au deposit, hosted in a continental volcanic sequence of the Xiong'er Group in the Xiong'er Terrane, East Qinling, is a fault-controlled orogenic-type lode gold deposit. In previous studies its ore-forming fluids and metals were interpreted as being originated from the crystalline basement of the Taihua Supergroup (namely, primary source-bed), the Xiong'er Group (secondary source-bed), and/or the Yanshanian (Jurassic-Cretaceous) granitoids. The lower crust and the mantle beneath the Xiong'er terrane were also suggested as the source. However, these interpretations are not spported by the radiogenic isotope data presented in this paper. Instead, the radiogenic isotope data, including Sr and Pb, show that the ore-forming fluids and metals have been sourced from a sedimentary sequence of the Guandaokou and Luanchuan Groups outcroped in the south of the ore-hosting terrane. Metamorphic devolatilization of the sedimentary sequence during its north-dipping intracontinental subduction along the Machaoying fault resulted in the development of ore-forming fluid-system and the formation of the Shanggong gold deposit. The 40Ar/39Ar dating of the early-stage quartz yields an age of 222. 8 Ma±24.9 Ma. Rb/Sr isochron dating for alteration minerals or rocks formed in the early, middle and late stages yield ages of 242 Ma±10 Ma, 165 Ma±7 Ma and 113 M±6 Ma, respectively. This shows that the metallogenesis occurred in the period of 250 Ma ∼ 100 Ma, and was coeval with continental collision between the Yangtze and North China plates.