Mantle-derived gold scavenged by bismuth-(tellurium)-rich melts: Evidence from the mesozoic wulong gold deposit in the north china craton

摘要

Mesozoic gold deposits associated with large-scale cratonic destruction of the North China Craton (NCC) made up a giant gold province that is unique among ancient cratons worldwide. The Au sources of these deposits are highly debated among Archean to Proterozoic strata, Mesozoic granitoid plutons, subducting slab-derived fluids, and mantle-derived fluids. Here we chose the Wulong gold deposit, a typical lode gold deposit in the NCC, to constrain the sources of gold and sulfur. Sulfides from auriferous quartz veins in the Wulong gold deposit have restricted ?33S (-0.1 to +0.2%o), without diagnostic signature of Archean sedimentary sulfur. Their mantle-like ?34S (-0.9 to +3.3%o) and restricted Pb isotopic ratios are remarkably different from those of sulfides from Paleoproterozoic strata and Mesozoic granitoid plutons. Sulfides from the Mesozoic doleritic dykes in the Wulong district have nearly identical ?34S (+1.7 to +2.0%o) to that of auriferous quartz veins, indicating that they were derived from similar mantle sources. We interpret that the sulfur and gold of the Wulong gold deposit were likely sourced from mantle-derived fluids released from devolatilization of the hydrated, subcontinental lithospheric mantle (SCLM). However, the enrichment of Au is not directly linked to an anomalously Au-rich source in the SCLM beneath the NCC which contains 0.39 ppb Au on average. Neither Mesozoic igneous rocks (0.22 ppb Au on average) nor Paleoproterozoic strata (0.19 ppb Au on average) in the Wulong district are enriched in Au. In addition, sulfides from auriferous quartz veins are not enriched in Au (commonly < 1 ppm Au), indicating that the mantle-derived fluids are likely Au-undersaturated during the deposition of sulfides. On the other hand, positive correlation of Au and Bi of auriferous quartz veins and close association of native gold with native bismuth and Bi-Te-S minerals consistently indicate that Au enrichment is likely related to Bi-(Te)-rich melts. The Bi-(Te)-rich melts may have precipitated from the Au-undersaturated auriferous fluids and scavenged Au from the fluids, leading to the Au enrichment. The insignificant fractionation of ?34S among sulfides and typical reduced assemblage of native bismuth, jose?ite and pyrrhotite in auriferous quartz veins consistently indicate that the occurrence of Bi-(Te)-rich melts is related to the reduction of fluids. This study demonstrates that Au in the mantle-derived fluids can be scavenged and significantly enriched by Bi-(Te)-rich melts, even if the fluids were Au-undersaturated, leading to the formation of giant lode gold deposits.