Geochemical constraints on the genesis of Au mineralization at the Wangzhuang deposit, South Qinling, China: Insights from fluid inclusion, trace elements, and H–O–S–Fe–Pb isotopic studies

Abstract

The Zhen’an–Xunyang Basin, a rifted basin in the South Qinling orogenic belt, hosts significant Hg–Sb–Au deposits, including Gongguan, Qingtonggou, and recently discovered Carlin-type gold deposits like the Wang Zhuang deposit. The Wang Zhuang gold deposit, characterized by bedding-parallel fault zone control, providing the setting to study the genetic mechanisms of Au deposits. Current assessments indicate a potential Au resource of 2.57 tons within the Wangzhuang gold deposit, highlighting promising prospects. The alteration and mineralization processes of the Wangzhuang deposit are divided into three stages: (Ⅰ) arsenopyrite–quartz vein stage, (Ⅱ) quartz veins with arsenian pyrite stage, and (Ⅲ) quartz-carbonate stage. The initial two stages, reflecting the principal phase of ore formation, are closely interconnected, while Stage III encompasses a subsequent phase of mineralization characterized by relatively weaker alteration and mineralization. This study reveals magmatic involvement in ore formation through H–O–S–Fe–Pb isotope analyses. Fluid inclusion analyses of quartz reveal that the Au mineralization is associated with medium– to low–temperature (112–330 °C) and medium– to low–salinity (1.3–11.4 wt% NaCl equivalent) fluids enriched with organic components. The H–O isotopes of quartz show δD_V-SMOW values ranging from −83.8 ‰ to −75.4 ‰, decreasing from early to late mineralization stages. S isotopic analyses of pyrite and arsenopyrite reveal δ³⁴S values of 8.4 ‰ to 10.9 ‰ in arsenopyrite, significantly lower than the δ³⁴S values of 35.6 ‰ to 60.4 ‰ in the surrounding strata, indicating a non-stratigraphic sulfur source. Fe isotopes suggest that δ⁵⁶Fe values range from 0.2 ‰ to 2.1 ‰ before mineralization and 0.6 ‰ to 2.0 ‰ during mineralization, indicating Fe was primarily derived from adjacent strata. Pb isotopic ratios of pyrite and arsenopyrite exhibit the following ranges: ²⁰⁸Pb/²⁰⁴Pb from 39.700 to 40.368, ²⁰⁷Pb/²⁰⁴Pb from 15.977 to 16.073, and ²⁰⁶Pb/²⁰⁴Pb from 18.543 to 18.811. When integrated with Pb isotopic data from the Jinlongshan and Gongguan deposits in the same region, the isotopic evolution trend aligns with that of Late Jurassic granites, indicating that Pb originated from a mixture of magmatic and stratigraphic sources. In summary, magmatic fluids mobilized metals from Cambrian strata, serving as the primary ore-forming source. Fluid mixing and changes in physical and chemical conditions likely facilitated gold precipitation. The Au–Hg–Sb deposits in this region may share a common ore-forming process, with host rock characteristics and the distinct properties of Au, Hg, and Sb determining their segregation.