Ore geology, fluid geochemistry and genesis of the Shanggong gold deposit, Eastern Qinling Orogen, China

Abstract

The Shanggong Au deposit in the Xiong'er Terrane, East Qinling, has reserves of about 30 t Au, making it one of the largest orogenic-type Au deposits hosted in volcanic rocks in China. The deposit is hosted in the andesitic assemblage of the Xiong'er Group of 1.85∼1.4 Ga. Three stages of hydrothermal ore-forming processes are recognized, Early (E), Middle (M) and Late (L), characterised by quartz-pyrite, polymetallic sulfides and carbonate-quartz, respectively. Homogenization temperatures of fluid inclusions are between 380-320°C for the E-stage, 300-220°C for the M-stage and 200-120°C for the L-stage. The composition of fluid inclusions changed from CO2-rich in the E-stage to CO2-poor L-stage. The M-stage fluid has the highest contents of cations and anions (e.g., SO4 2-, Cl-, K+), the highest (K+Na)/(Mg+Ca) and lowest CO2/H2O ratios, which probably resulted from CO2 phase separation. This, together with the alkaline and reducing conditions, as indicated by highest pH and lowest Eh values, is most conducive to the deposition of polymetallic sulfides and native elements such as Au, Ag and Te. H-O is otope systematics indicate that ore fluids evolved from deep-sourced through to shallow-sourced, with the M-stage being a mixing phase of these two fluid-systems. Nineteen δ18 OW values, from 4.2 to 13.4 ‰, averaging 8.1 ‰, suggest that the E-stage fluids derived from metamorphic devolatilization of sedimentary rocks at depth. Comparison of the H-O isotope systematics between the Shanggong deposit and the main lithologies in the Xiong'er Terrane, shows that neither these nor the underlying lower crust and mantle, or combinations thereof, could be considered as the source of ore fluids and metals for the Shanggong Au deposit. Instead, a source which meets the isotopic constraints, is a carbonaceous carbonate-sandstone-shale-chert (CSC) sequence, which is present in the Guandaokou and Luanchuan Groups in the south of the Xiong'er Terrane. This conclusion is supported by thirteen high δ18O values of the Meso-Neoproterozoic strata south of the Machaoying fault, and the high δ18OW values calculated for their possibly metamorphic fluids. It can be also supported by previous observation that the Guandaokou and Luanchuan Groups were underthrust beneath the Xiong'er Terrane, during the Mesozoic collision between the Yangtze and Sinokorean continents. Available isotope ages, together with geological field data, constrain the timing of the Au metallogenesis between 250∼110 Ma. This metallogenesis and associated granitic magmatism, can be related to the Yangtze-Sinokorean continental collision that resulted in the formation of the Qinling Orogen. This collision event progressed from early compression (Triassic to Early Jurassic), through middle compression-to-extension transition (Late Jurassic to Early Cretaceous), to late extension (Cretaceous). These three stages in the evolution of the Qinling Orogen form the basis of an ore genesis model that combines collisional orogeny, metallogeny and fluid flow (CMF model). These three evolutionary stages correspond to the three-stages of ore-forming fluids of the Shanggong Au deposit. We conclude that the formation of the Shanggong Au deposit is a result of the Mesozoic northward intracontinental A-type subduction along the Machaoying fault during Yangtze-Sinokorean continental collision, which led to the metamorphic devolatilization of the CSC sequence, thereby providing both fluids and metals.