Deciphering fluid origins in the Paleozoic Laoshankou Fe-Cu-Au deposit, East Junggar: Constraints from noble gases and halogens

To constrain the ore-fluid source(s) of the Laoshankou Fe-Cu-Au deposit (Junggar orogen, NW China), we analyzed the fluid inclusion (FI) noble gas (Ar, Kr and Xe) and halogen (Cl, Br and I) compositions in the hydrothermal epidote and quartz. Four hypogene alteration/mineralization stages, including (I) pre-ore Ca-silicate, (II) early-ore amphibole-epidote-magnetite, (III) late-ore pyrite-chalcopyrite, and (IV) post-ore hydrothermal veining, have been identified at Laoshankou. Stage II FIs have salinity of 15.7 wt.% (NaCl eq.), I/Cl molar ratios of 75 × 10⁻⁶–135 × 10⁻⁶, and Br/Cl molar ratios of 1.4 × 10⁻³–2.1 × 10⁻³. The moderately-high seawater-corrected Br*/I ratios (0.5–1.5) and low ⁴⁰Ar_E/Cl slope (~10⁻⁵) indicate the presence of sedimentary marine pore fluid, which was modified by seawater reacting with the Beitashan Fm. volcanic rocks. Stage III fluid is more saline than their stage II and IV counterparts, reaching up to 23.3 wt.% (NaCl+CaCl₂ eq.) close to halite saturation (~26 wt.%). The fluid has I/Cl ratios of 75 × 10⁻⁶–90 × 10⁻⁶ and Br/Cl ratios of 1.5 × 10⁻³–1.8 × 10⁻³. Considering the increasing ⁴⁰Ar_E/Cl trend toward bittern brine and the higher ³⁶Ar content than air-saturated water (ASW), a bittern fluid source is inferred from seawater evaporation, which was modified by interaction with organic-rich marine sedimentary rocks. Stage IV FIs have lower temperature (110–228 °C) and Br/Cl (0.90 × 10⁻³–1.2 × 10⁻³), but higher ³⁶Ar content than ASW, indicative of dissolved evaporite or halite input. Considering also the low δD_fluid (−114‰ to −144‰) and δ¹⁸O_fluid (2.1‰–3.5‰) values, meteoric water (with minor dissolved evaporites) likely dominated the stage IV fluid. The evaporites may have formed through continuous evaporation of the stage III surface-derived bittern. Involvement of non-magmatic fluids and different ore-fluid origins in stages II and III suggest that the ore-forming process was different from a typical magmatic-hydrothermal fluid-dominated skarn mineralization, which was previously proposed for Laoshankou. Our noble gas and halogen study at Laoshankou provide new insights on the fluid sources for the Paleozoic Fe−Cu (−Au) deposits in the Central Asian Orogenic Belt (CAOB), and our non-magmatic fluid source interpretation is consistent with the basin inversion setting for the mineralization.