Ore-forming processes of orogenic gold deposits in the northwest Fengtai Basin, South Qinling Orogen, central China: Constraints from pyrite texture and geochemistry

The genesis of orogenic gold deposits (OGDs), and in particular the source of their metals and fluids (crustal vs. sub-crustal), remains a matter of debate. The South Qinling Orogen, central China, hosts significant Triassic OGDs spatially associated with Devonian sedimentary basins containing SEDEX Pb-Zn mineralization, providing a natural laboratory for testing competing genetic models. This study investigates the Matigou and Pangjiahe OGDs (a total of > 60 t Au) in the Fengtai Basin based on detailed petrography combined with in-situ LA-ICP-MS trace element and LA-MC-ICP-MS sulfur isotope analyses of pyrite. Five pyrite generations were identified: syn -sedimentary Py0 in phyllites (fine-grained, framboidal and tubular in morphology; highly variable δ 34 S from − 35.7‰ to + 14.9‰; elevated background Au and trace-element contents); diagenetic Py1 in phyllites (coarser and euhedral; narrower δ 34 S from − 5.1‰ to + 9.4‰); metamorphic-hydrothermal Py2 in phyllites (euhedral; base metal inclusion-bearing; δ 34 S similar to Py1); magmatic-hydrothermal Py3 in granite porphyry (δ 34 S = +2‰ to + 5‰); and main-ore-stage Py4 hosted by both phyllites and granite porphyry (zoned, with elevated Au and As contents; δ 34 S = +8‰ to + 13‰). The Py4 δ 34 S signature is distinctly heavier and more tightly clustered than that of any pre-ore pyrite generation at the deposit site, requiring derivation of ore-stage sulfur from an isotopically homogeneous reservoir external to the immediate host horizon, rather than in-situ remobilization of local sedimentary, metamorphic, or magmatic pyrite. Taken together with regional structural, S-isotopic, and geochemical constraints, this signature is inconsistent with a sub-crustal (mantle) source and instead indicates a crustal, basin-internal reservoir. We propose that the ore-forming fluids were generated during Triassic metamorphism through devolatilization of, and metal leaching from, the underlying Devonian strata previously enriched by SEDEX activity. Precipitation of native gold and auriferous Py4 most likely occurred during the syn -orogenic tectonic transition through fault-valve cycling. This study highlights the importance of pre-existing, basin-hosted mineralization as a key metal source for subsequent orogenic gold systems.