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 380‐320d̀C for the E‐stage, 300‐220d̀C for the M‐stage and 200‐120d̀C 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., SO42‐, Cl1, 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 isotope 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 δ18OW values, from 4.2 to 13.4 %o, averaging 8.1 %o, 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). ...

The Shanggong gold deposit, Eastern Qinling Orogen, China: Isotope geochemistry and implications for ore genesis

“…Two possibilities can be considered to explain the d 13 C decrease. One is an input of low-d 13 C fluids, which is consistent with the O and H data on the hydrothermal fluids and the conclusions obtained from fluid inclusion studies (Chen et al, 2006) that indicate the late involvement of meteoric water. Another possibility is the continuing metamorphic Ankerite from ankerite cement of breccia ore (DF2-0) 0.0 M+L 3…”

“…More specifically, the Shanggong Au deposit is comparable to the orogenic disseminated Au deposits as defined in Bierlein and Maher (2001). In this paper, we report the results of new C, S data and published Pb and Sr isotopic studies of the Shanggong Au deposit, which we integrated with D-O systematics and fluid inclusion studies presented in a previous study (Chen et al, 2006). The integrated isotopic data enable us to propose a model of ore genesis in the context of regional geodynamics and associated magmatism.…”

“…Thus, a possible source of lead is in the upper crust. Chen et al (2006) found that the ore fluid's dD and d 18 O values were higher than the Xiong'er Group and that d 18 O values systematically increase from unaltered volcanic rocks (6.7-8.0‰), through altered volcanic rocks (8.5-8.9‰) to ores (12.7‰), suggesting that 18 O is added to the wall rocks during fluid-rock interaction. Calculated 18 O W values for E-stage fluids average 8.1‰, which is in the range of metamorphic water (Fig.…”

“…6 the data plot in the magmatic box and below the metamorphic water box. Chen et al (2006) concluded that the E-stage fluids show characteristics of magmatic and/or metamorphic fluids. Calculated values of d 18 O w of L-stage fluids range from À2.4‰ to À1.1‰.…”

“…Hydrogen and oxygen isotope systematics and the geochemistry of ore fluids from the Shanggong Au deposits have been reported by Chen et al (2006) and are summarized below. In this contribution, we present new S, C isotopic data, integrated with published Pb and Sr data.…”