Deciphering ore-forming processes in the Daping orogenic gold deposit, SW China: constraints from in situ sulfur isotope and pyrite trace elements

Schematic sketch illustrating the fluid-rock reaction progression at the Daping gold deposit : (a) A 3D model showing the spatial distribution of ore bodies. (b) and (c) depict the mineral assemblages and ore vein relationships in stages Ⅰ and Ⅱ of the BSP section. (d) Enlarged view of (c) highlighting sulfur isotopic variations and Co-Ni zoning in stage Ⅱa pyrite. (e) and (f) show the mineral assemblages and vein relationships in stages Ⅰ and Ⅱ of the JHJKC section. (g) Enlarged view of the Sd-Py ± Qz ± Gn ± Cpy vein in (f), illustrating sulfur isotopic variations and As zoning in pyrite. • Integrated sulfur isotope and pyrite microtexture reveal dynamic redox evolution and multi-stage mineralization in the Daping gold deposit. • Oscillatory zoning of As in pyrite, coupled with δ 34 S variations linked to fluid pressure fluctuations, supports As-enrich gold incorporation in pyrite. • Siderite precipitation and As-induced lattice modification of pyrite jointly promoted invisible gold enrichment. Orogenic gold deposits, characterized by intricate fluid–rock interactions and geochemical signatures, pose significant challenges for exploration. The Daping gold deposit, a prominent orogenic gold deposit in Southwest China, is primarily hosted in Neoproterozoic diorite and Silurian sedimentary carbonates. This unique geological setting provides an ideal opportunity to examine the evolution of mineralization processes across contrasting host lithologies. In this study, we integrate in situ sulfur isotope analyses, pyrite geochemistry, and siderite mineralogical observations to elucidate gold transport, precipitation, and the dynamic evolution of the geochemical regime in this system. The δ 34 S values of sulfides range from –9.8‰ to + 13.4‰, and equilibrium temperatures (280–420°C) indicate near-equilibrium sulfur isotope partitioning at the fluid scale between coexisting sulfides and sulfates. The calculated bulk sulfur isotope values (δ 34 S Σ = +9.9‰ to + 10.7‰) indicate a homogeneous, reduced sulfur reservoir, with negligible contributions from wall rocks. Gold was predominantly transported as Au(HS) 2 - complexes in near-neutral, moderately high-temperature fluids, where fluctuations in pressure and oxygen fugacity ( f O 2 ), as recorded by (i) systematic δ 34 S variations, (ii) coexistence of sulfate and sulfide minerals in the BSP area, and (iii) oscillatory As–Au zoning with coupled sulfur isotope variations in pyrite, were the primary drivers of its precipitation. Episodic fluid phase separation and redox oscillations destabilized Au–sulfur complexes, triggering the precipitation of native gold. Pyrite displays oscillatory As–Au zoning and variable δ 34 S values, indicative of local, transient non-equilibrium crystallization during rapid physicochemical fluctuations under cyclic perturbations. The positive correlation between increasing As and Au concentrations further indicates that As substitution enhances gold incorporation into pyrite via lattice modification and the destabilization of Au complexes. Notably, the precipitation of siderite and its subsequent replacement by pyrite reflect cyclic variations in redox conditions and pH, likely controlled by CO 2 and H 2 S concentrations. These shifts induce transient disequilibria in the fluid system, facilitating episodic pyrite growth and triggering gold deposition by destabilizing Au–HS complexes. Specifically, siderite precipitation induces local disequilibrium by modifying the redox state and pH of hydrothermal fluids, further facilitating pyrite regeneration and efficient gold accumulation. Collectively, these results demonstrate that the Daping gold deposit formed within a dynamic hydrothermal system characterized by episodic pressure fluctuations, redox oscillations, and fluid–rock interactions, all of which governed gold transport and precipitation. Integrating sulfur isotope systematics with pyrite geochemistry yields new insights into the mechanisms of gold enrichment in orogenic systems, providing a robust framework for exploration in analogous geological settings.