Competency contrast-driven extension veining and sequential orogenic Au-Sb mineralization: Insights from the Zaorendao deposit, China

The coupling between extension veining and fluid evolution in orogenic Au-Sb deposits remains poorly understood. This study presents an integrated structural, geochronological (sericite Ar-Ar, monazite U-Pb), fluid inclusion, and oxygen isotopic investigation of the Zaorendao Au-Sb deposit (China). The deposit is hosted within an Early Triassic quartz diorite stock that intrudes slate and contains two main ore types: extensional quartz-stibnite-sphalerite veins (Sb) and disseminated sulfide-hosted Au ores in the wall rocks. Structural analysis reveals that competency contrasts between a quartz diorite and slate localized tensile failure. Geochronology constrains this veining and mineralization to 231 Ma during regional NE-SW shortening. Four hydrothermal stages are identified. Stage 1 is characterized by elongated coarse quartz (Qz1) veins and disseminated Au mineralization, with chlorite geothermometry from the alteration halo of 300 ± 33 °C. Extension structure suggests high-pressure fluid under near lithostatic conditions drove continuous extension and fluid discharge into the wall rocks, leading to Au precipitation via fluid-rock interaction. Stages 2 and 3 involve microfracturing and healing of Qz1, forming quartz overgrowth rim (Qz2). Pseudo-secondary fluid inclusion assemblages (FIAs) in healed microfractures within Qz1 contain coexisting liquid-rich and vapor-rich inclusions and yield a mean homogenization temperature of 181 ± 10 °C, reflecting a significant drop in fluid pressure (from near lithostatic to near hydrostatic levels) and temperature (from ∼300 °C to ∼180 °C). Similar δ18O values of Qz1 (20.0‰) and Qz2 (19.4‰) support a colloidal silica model, wherein silica colloids of Qz2 formed during Stage 2 (∼300 °C) under fluid decompression and coagulated onto Qz1 surface during Stage 3 as fluid cooling. In Stage 4, stibnite and sphalerite fill the veins, with FIAs in sphalerite showing temperatures of ∼180 °C, indicating that decompression-induced fluid cooling triggers stibnite precipitation. This study demonstrates a continuous genetic sequence where extension veining dynamics directly governed fluid evolution (pressure release and cooling) and sequential Au-Sb precipitation, providing a fundamental framework for understanding similar orogenic systems.