Decoupling of Au and W mineralization in the Xiangzhong Metallogenic Province (South China): Insights from the Xingfengshan Au–W deposit

Abstract

The Xiangzhong Metallogenic Province (XZMP) in South China is one of the most important Au–Sb–W metallogenic domains in the world. Gold and W mineralization in XZMP are commonly coupled, however, the Xingfengshan Au–W deposit is characterized by skarn W mineralization that is overprinted by sheeted quartz vein-hosted Au mineralization. Garnet U–Pb dating provides a maximum age limit for W mineralization at 214.8 ± 3.2 Ma (MSWD = 1.8), while biotite Ar–Ar dating indicates Au mineralization occurred at 205.2 ± 0.4 Ma (MSWD = 0.96), confirming that Au and W mineralization were decoupled. This is supported by the elevated temperature of the Au mineralizing event demonstrated by chlorite geothermometry (W = 291–346 °C vs. Au = 351–416 °C). Arsenopyrite, the main Au-hosting mineral, can be divided into three generations — Apy-1, Au-enriched Apy-2, and Apy-3. Gold occurs as invisible Au in Apy-1 and Apy-2, but as visible Au coexisting with Apy-3. The abundant chlorite, biotite, and ilmenite inclusions in Apy-1 and Apy-2, and low δ³⁴S_V-CDT values (Apy-1 = −8.82 to −8.44 ‰, Apy-2 = -9.57 to −7.69 ‰) suggest that they formed by fluid–rock interaction. Apy-3 is enriched in trace elements (e.g., Co, Ni, Cu) and has lower δ³⁴S_V-CDT values (−12.56 to −12.03 ‰), indicating the involvement of late-stage oxidized magmatic-hydrothermal fluids in its formation. This fluid remobilized invisible Au from Apy-2 and precipitated it as visible Au associated with Apy-3. The close spatiotemporal relationship between mineralization at Xingfengshan and the Baimashan composite pluton, the S isotope composition of pyrite (−4.94 to 1.18 ‰), and the metallogenic affinity of granites together suggest that the W and Au mineralization are genetically associated with the Longcangwan two-mica granite and Longtan biotite granite of the Baimashan composite pluton, respectively. This study highlights the complexity of regional Au–Sb–W mineralization in the XZMP.