New constraints on genesis of the Qiangma gold deposit, southern North China Craton: Evidence from rutile U-Pb dating, pyrite trace element composition and S-Pb isotopes

The Xiaoqinling district, situated at the southern North China Craton, is the second largest gold province in China. The genesis of gold deposits in this district, however, has been debated for decades because of lacking unequivocal constraints on mineralization age and gold source(s). In this study, we present ore-related hydrothermal rutile U-Pb dating, pyrite trace element compositions, and S-Pb isotopes of the Qiangma Au deposit that is one of the largest gold deposits in the Xiaoqinling district, to provide new constraints on genesis of the regional gold mineralization. The Qiangma Au deposit is formed by four paragenetic stages consisting of milky quartz-coarse-grained pyrite, grey quartz-fine-grained pyrite, grey quartz-polymetallic sulfides, and quartz-calcite. The gold mineralization is accompanied by intensively hydrothermal alteration assemblage comprising quartz, sericite, sulfides, and calcite. Hydrothermal rutile texturally intergrown with gold-bearing pyrite and sericite yields a U-Pb age of 128.1 +/- 7.4 Ma (2 sigma, MSWD = 0.9), representing the timing of gold mineralization at Qiangma. This age is consistent with the ages of regional felsic to mafic magmas and metamorphic core complexes as results of late Mesozoic tectonic reactivation of the North China Craton. Petrographic investigation and LA-ICP-MS trace element analysis show that gold mainly occurs as native gold with minor amounts of lattice gold within pyrite, which has no correlation with As and Te. Pyrite from the four stages have sulfur isotopes mainly between 1.8 and 5.8 parts per thousand (mean of 3.7 parts per thousand), suggesting a magmatic affiliation. Pyrite from quartz-polymetallic sulfides stage has lead isotopes of 17.51-17.71 for Pb-206/Pb-204, 15.39-15.49 for Pb-207/Pb-204, and 38.01-38.53 for Pb-208/Pb-204, which are distinctly different from those of Mesoproterozoic clastic and carbonate rocks and late Mesozoic granites, but are broadly consistent with late Mesozoic subcontinental lithospheric mantle derived mafic dikes. Collectively, our new lines of evidence suggest that the Qiangma Au deposit, as well as some gold deposits in the Xiaoqinling district, is most likely related to mantle-derived magmas generated in an extensional setting related to thinning and destruction of subcontinental lithospheric mantle keel beneath the North China Craton.