Geological and geochemical constraints on the genesis of the Xiqianluzi Pb–Zn polymetallic deposit in Beishan, Gansu Province, NW China

Abstract

The Xiqianluzi Pb–Zn polymetallic deposit is located in the southeast of the southern Beishan belt, whereas the ore‐forming metal sources are still unclear. Its Pb–Zn ore bodies mainly occur in the Changcheng system Qianluzigou Group metamorphic clastic rocks, showing zonation of Cu–Zn–Pb metals from deep to surface. The ore structures are mainly banded, massive and veined, which have the characteristics of syngenetic sedimentation. Au mineralization in silty slate is obviously controlled by WNW–ESE‐trending faults, and occurs as veined or lenticular type. The Pb, Zn, Cu, and Au are mainly enriched in quartz or siliceous veins. The biotite granodiorites are enriched in Si, Na, Rb, Ba and K, with A/CNK values less than 1.1, and depleted in Nb, P, Ti. They belong to calc‐alkalic series and weak peraluminous I‐type granite, formed in a post‐collision extensional environment and characterized by crust–mantle mixed source. The sulphide minerals in the Pb–Zn ores have δ34S values in the range of 17.7‰–21.0‰ with average of 19.2‰, reflecting seawater sulphate origin of sulphur. The sulphur of Pb–Zn ores was likely transformed from S6+ to S2− by thermochemical sulphate reduction. The Pb isotopic compositions of sulphide minerals in the Pb–Zn ores have 206Pb/204Pb ratios of 16.837–17.001, 207Pb/204Pb ratios of 15.437–15.557, and 208Pb/204Pb ratios of 36.377–36.889. In contrast, sulphide minerals in Au ores have δ34S values (5.4‰–7.6‰, with average of 6.5‰) lower than those of sulphide minerals in the Pb–Zn ores. The δ34S values of sulphide minerals in the Au ores are consistent with those of sulphide minerals in the Late Permian–Middle Triassic magmatic hydrothermal gold deposits in the southern Beishan belt, which imply a magmatic source for sulphur. Sulphide minerals in the Au ores have higher lead isotopic ratios, including 206Pb/204Pb of 18.249–18.325, 207Pb/204Pb of 15.583–15.598, and 208Pb/204Pb of 38.023–38.366, compared to those in the Pb–Zn ores. Geological and isotopic features indicate that the Xiqianluzi Pb–Zn polymetallic deposit comprises two mineralization types; namely, its Pb–Zn ores can be considered as Mesoproterozoic sedimentary exhalative type, while its Au ores are magmatic origin. These show that Au mineralization was closely related to the magmatic hydrothermal activities, and thus Au ores can be classified as Late Permian–Early Triassic magmatic hydrothermal Au mineralization.