U-Pb ages of apatite, sulfur isotopes of sulfides and possible origin of the Dashui hematite-rich, sulfide-deficient gold deposit in the West Qinling orogen, Central China

Abstract

The Dashui gold  (Au)  deposit is one of the largest Au deposits in West Qinling orogenic belt and unique in that its metallic minerals are dominated by hematite but sulfides are extremely scarce. The age and genesis of the Dashui Au deposit, however, remain poorly constrained. To resolve these issues, here we present robust apatite U-Pb dating results under the context of detailed textural and compositional studies. The ore mineral assemblages at Dashui consist of fine-grained quartz, hematite, kaolinite, apatite, native Au, and trace amounts of pyrite. The presence of hematite and gold as inclusions fully enclosed in quartz and apatite suggests that the former minerals are of primary origin rather than, as generally considered, a product of supergene oxidation of Au-bearing pyrite. Cathodoluminescence imaging reveals that the ores contain two types of apatite. CL-bright apatite was likely inherited from the magmatic wall rocks, whereas CL-dark apatite precipitated coevally with the ore minerals. In-situ LA-ICP-MS U-Pb isotope analysis of the CL-dark apatite suggests that the Dashui deposit formed at 217.2 ± 4.1 Ma, contemporaneously with the main stage of regional Au mineralization in the West Qinling orogenic belt that formed under the Triassic subduction-collisional tectonic setting. Given than Au mineralization at Dashui is temporarily and spatially associated with intermediate to felsic porphyry intrusions, and that the ore sulfides have S isotopes consistent with a magmatic-hydrothermal origin, the deposit can be broadly classified as a Carlin-like Au deposit. Nevertheless, the hematite-rich rather than pyrite-rich ore assemblages indicate an ore-forming environment that was more oxidized than that of typical Carlin-type deposits.