Hydrothermal metallogeny of the Shanggong gold deposit, east Qinling: Study on the stable isotope geochemistry

Abstract

The Shanggong Au deposit is a typical fault-controlled orogenic-type lode gold deposits. Its ore-forming process includes the early, middle and late stages. The δ18O values of ores and altered rocks are higher than those of unaltered rocks, suggesting that the rocks have extracted 18O from fluids during fluid-rock interaction. Nineteen δ18Ow values, ranging from 4. 2‰ - 13. 4‰ and averaging 8. 1‰, eight δDw values, ranging from - 66‰ - - 88‰ and averaging - 78‰ and a δ13Cankerite value (1. 5‰), suggest that the early stage fluids derived was from metamorphic devolatilization of carbonate-bearing strata. As for late stage, three δ18Ow values, ranging from - 2. 0‰ - - 0.6‰, a δDw value ( - 56‰), and three δ13Cankerite, ranging from - 1.6‰ - - 2. 2‰, show that the fluids were dominated by meteoric water. Three middle stage samples yield δ18O w values of 1. 9‰ - 4. 5‰ and δ 13CCO2 values of - 1. 2‰ - 0. 5‰, exactly between the values of the early and late stages. This strongly indicates that the middle stage was characterized by a mixing phase of metamorphic and meteoric fluids. The δDW values for middle stage range from - 113‰ to - 94‰, lower than those of the early and late stages, which can be interpreted as being resulted from large-scale precipitation of sulfides. 28 negative δ34S values for the middle stage sulfides ( - 19. 2‰ to - 6. 3‰) suggest a contribution of biogenic matter to ore-forming fluid-system. Theoretical synthesis shows that none of the main geologic units in the Xiong'er Terrane, i. e. the Taihua Supergroup, the Xiong'er Group and the Yanshanian granitoids, and the underlying lower crust and the mantle, could be considered as the source of ore fluids for the Shanggong Au deposit, although it was suggested in previous studies. Instead, a source which meets the isotopic constraints, is a carbonaceous carbonate-sandstone-shale-chert (CSC) sequence in the Guandaokou and Luanchuan Groups in the south of the Xiong'er Terrane. During the Mesozoic collision between the Yangtze and North China continents, the Guandaokou and Luanchuan Groups underthrusted beneath the Xiong'er Terrane along the north-dipping Machaoying A-type subduction zone, and then devolatilized through metamorphism, leading to the development of the Shanggong ore-forming fluid-system. Therefore, the CMF (collisional orogeny, metallogeny and fluid flow) model could be applied to the interpretation of the hydrothermal metallogenesis of the Shanggong Au deposit.