Ore-forming fluid evolution and metal precipitation mechanism at Xierqu Fe–Cu deposit, East Tianshan (NW China): Integrated constraints from fluid inclusions and garnet geochemistry

The newly discovered Xierqu deposit signifies a Devonian Fe–Cu mineralization event, challenging the Carboniferous-dominated submarine volcanic-hosted Fe(Cu) metallogenic model in East Tianshan, NW China. Its ore-forming fluid evolution and metal precipitation mechanisms require clarification. Mineralization is spatially confined to skarn at the quartz diorite porphyry-Dananhu Formation limestone interface. Four ore-forming stages are identified. Fluid inclusion (FI) petrography reveals three assemblages: liquid-dominated (L-type), vapor-dominated (V-type), and multiphase with daughter minerals (S-type). The prograde skarn stage contains high-temperature (343–481°C), variable salinity L-type (6.3–14.2 wt% NaCl eqv.) and very high-salinity S-type (39.8–43.6 wt% NaCl eqv. at 449–491°C) FIs. The retrograde epidote stage shows L-type and V-type FIs with moderate temperatures (L-type: 304–351°C, 5.2–7.7 wt%; V-type: 285–353°C, 5.7–9.1 wt% NaCl eqv.). A significant cooling and dilution trend characterizes the quartz-sulfide stage, evidenced by primary L-type (Th 246–304°C; salinity 3.2–6.7 wt%) and V-type (Th 241–324°C; salinity 3.1–6.3 wt%) FIs in quartz. Calcite stage FIs (L-type: Th 132–201°C, salinity 1.1–2.6 wt%; V-type: Th 150–213°C, salinity 1.4–2.7 wt%) confirm substantial external fluid influx. Pronounced Fe/(Na + K) ratio variations from early high-salinity to retrograde medium–low-salinity fluids indicate significant Fe precipitation. The maximum Cu/(Na + K) ratio occurs near 470°C in prograde skarn; subsequent cooling to ∼ 250°C in the quartz-sulfide stage causes a dramatic ratio drop, triggering chalcopyrite precipitation. Combined microthermometry and Laser-ablation inductively coupled plasma-mass spectrometry (LA–ICP–MS) data reveal Fe precipitation resulted from destabilization of Fe-chloride complexes via hydrothermal cooling and boiling. Cu mineralization coincided with concurrent cooling and fluid mixing. High U contents and HREE-enriched patterns in Grt-1A/B dark zones indicate near-neutral pH and low fO₂ fluids. An expanded Y/Ho range and LREE enrichment in Grt-2 demonstrate mixing between evolved magmatic fluid and an external fluid with mildly acidic pH and elevated oxygen fugacity. This study advances understanding of Devonian Fe–Cu mineralization in the East Tianshan.