Apatite textures, composition, and Sr isotope signatures: Constraints on the genesis of gold at Zaozigou deposit, West Qinling, Central China

Abstract Apatite records ore genesis information in various hydrothermal mineral systems. They exhibit complex textures and diverse elemental and isotopic compositions over multiple generations. This study uses integrated petrographic, elemental, and isotopic data to analyze the properties, origin, and evolution of mineralizing fluids and ore-forming processes in a gold (Au) deposit with a complex tectonothermal history. The Zaozigou deposit is one of the top Au-producing deposits in China, the genesis of which has long been controversial. Based on cathodoluminescence, we classified the apatite crystals from the Zaozigou deposit into four groups (Ap1, Ap2, Ap3, and Ap4). Ap1 and Ap2 coexist with magmatic minerals, while Ap3 and Ap4 are associated with ore-related hydrothermal minerals. All four types exhibit homogeneous BSE intensity and distinct cathodoluminescence colors and elemental compositions (e.g., REE+Y, U, Th, Sr, Mn, Fe, Mg, Cl, F, and S). Strontium isotope data indicate low 87Sr/86Sr ratios (Ap1: 0.709854–0.711636, Ap2: 0.71024–0.711707) in pre-Au ore apatite. In contrast, syn-Au ore apatite shows high 87Sr/86Sr ratios (Ap3: 0.712022–0.713108, Ap4: 0.711656–0.71357). These alterations in elemental and Sr isotope compositions are indicative of modifications in fluid composition and nature, as well as the pivotal role played by metasomatic alteration in Au deposition. This study distinguishes between the initial magmatic and subsequent auriferous phases. Apatite chemistry and Sr isotope signature reveal that low-salinity, moderate-temperature carbonaceous crustal metamorphic fluids enriched in sulfur, strontium, and fluorine are responsible for Au deposition at the Zaozigou deposit through fluid-rock interactions. The auriferous fluids, Au, and associated metals are likely derived from crustal metamorphic systems, possibly from the West Qinling Paleozoic strata and/or South Qinling volcano-sedimentary basement rocks. The findings of this work suggest a supracrustal orogenic model for the Zaozigou Au deposit.