A metamorphic devolatilization model for the granitoid-hosted Wulong lode gold deposit, Liaodong Peninsula, Eastern North China Craton: Evidence from geology, fluid inclusions, and O-Sr-Li isotopes

Abstract

The genesis of the gold deposits in the eastern North China Craton (NCC) has long been disputed. Many would relate the gold mineralization to magmatism. The Wulong in the Liaodong Peninsula is a classic example in the eastern NCC and was considered to be of magmatic hydrothermal origin previously. Nevertheless, we propose a metamorphic devolatilization model to explain its genesis based on geological, fluid inclusion and O-Sr-Li isotopic study. The geological profiles, temperatures, salinities, and O isotopes of the ore-forming fluids at Wulong show no spatial variations over a 600 m vertical depth interval. These evidence convincingly prove its vertically extensive feature, which is similar to orogenic gold deposits, but different from magmatic hydrothermal systems. The initial Sr isotopes of the ore-forming fluids (0.71435 to 0.71578) fall into the range of metabasalts from the Liaohe Group (the Paleoproterozoic basement), indicating that metabasalts are the probable sources. Lithium and its isotopic modelling further suggest that metamorphic devolatilization of metabasalts (with chain silicates Ch, phyllosilicates Ph, and quartz Qtz as endmembers; Ch, Li ≈ 5 ppm, δ7Li ≈ 8‰; Ph, Li ≈ 63 ppm, δ7Li ≈ −1‰; Qtz, Li ≈ 10 ppm, δ7Li ≈ 30‰) could account for the Li concentrations (5–41 ppm) and δ7Li value of the fluid (21.4‰) of the Wulong gold deposit. Rayleigh fractionation in a closed system led to large Li isotopic variation of the ore-forming fluids (21.4 to 11.0‰). The metamorphic devolatilization model based on the vertically extensive feature, Sr isotopic constraint and Li isotopic modelling is in striking contrast with previous qualitative isotopic studies that support a magmatic hydrothermal origin for the Wulong and other granitoid-hosted gold deposits in the eastern NCC. Integrated with geological characteristics and radiogenic isotopes, this study here shows that Li isotopes can provide new constraints on the genesis of orogenic gold deposits.