Differential stress regime as a control on economic antimony deposition in epizonal orogenic gold systems

Epizonal orogenic gold systems in shallow metamorphic belts commonly exhibit contrasting styles of antimony (Sb)-rich and Sb-poor mineralization, yet the controlling factors remain debated. Here, we integrate vein classifications with fluid inclusion microthermometry and oxygen isotope data from the giant Zaozigou Au-Sb deposit (West Qinling, China) to constrain the pressure-temperature (P-T) conditions and fluid evolution. Our results demonstrate that differential stress (σ1 − σ3) governs the spatial segregation of Sb-rich and Sb-poor mineralization through its control on failure modes and fluid processes. Two distinct orebody types are identified: (1) steeply dipping Au orebodies (60°−90°) with subeconomic Sb grades (<1 wt%), hosted in shear fractures and dominated by disseminated arsenopyrite and pyrite with invisible Au; and (2) gently dipping Au-Sb orebodies (19°−45°) with high Sb grades (average 5 wt%), characterized by native gold-bearing stibnite-quartz veins in extension fractures. Fluid inclusion data reveal that steeply dipping orebodies formed under stable P-T conditions (∼0.8 kbar, ∼250 °C), whereas gently dipping orebodies experienced significant decompression (∼1.1 kbar to ∼0.4 kbar) and cooling (∼250 °C to ∼190 °C). Our comprehensive analysis demonstrates that steeply dipping orebodies formed through shear failure of intact rock under high differential stress conditions (σ1 − σ3 ≥ 5.7T) and subsequent fault reactivation under decreased differential stress conditions related to seismic stress release, with gold precipitation dominated by fluid-rock interaction. In contrast, gently dipping orebodies developed in intact rock under low differential stress (σ1 − σ3 < 4T), where fluid immiscibility triggered H2S degassing and rapid Au-Sb deposition. The mechanical relationships observed at Zaozigou may provide new insights into the enrichment of Sb in shallow epizonal orogenic gold deposits. The systematic decrease of differential stress with reduced depth in brittle regimes creates conditions favorable for Sb mineralization at upper crustal levels, where lower stress environments promote extensional failure and the substantial pressure fluctuations required for efficient Sb deposition. This stress-based framework offers a potential explanation for the characteristic vertical zoning of Sb in orogenic systems, complementing existing genetic models.