Fluid inclusion geochemistry and ⁴⁰Ar/³⁹Ar geochronology constraints on the genesis of the Jianchaling Au deposit, China

The giant Jianchaling gold deposit is located in the Shaanxi Province, China. The mineralization is hosted by WNW-trending faults in the Mianxian-Lueyang-Yangpingguan (MLY) area. The mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins. These stages, from early to late, are characterized by the mineral assemblage of: (1) quartz – coarse-grained pyrite – pyrrhotite – pentlandite – dolomite; (2) quartz – pyrite – gold – sphalerite – galena – carbonate – arsenopyrite – fuchsite; and (3) dolomite – calcite – quartz – fine-grained pyrite – realgar – orpiment. Three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements, including pure CO₂ and/or CH₄ (PC-type), NaCl-H₂O (W-type), and NaCl-CO₂-H₂O (C-type) fluid inclusions. These fluid inclusion types are present in quartz from the Stage 1 and 2 assemblages, whereas the Stage 3 quartz only contains W-type fluid inclusions. The Stage 2 assemblage is associated with the mineralization at the Jianchaling deposit. Fluid inclusions of Stage 1 quartz homogenize mainly between 250° and 360 °C, with salinities up to 15.6 wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220 °C and salinities of 1.1–7.4 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO₂-rich, probably metamorphic hydrothermal to CO₂-poor, meteoric fluid. All three types of fluid inclusions can be observed in the Stage 2 quartz, suggesting that this heterogeneous association was trapped from a boiling fluid system. These inclusions homogenized at temperatures of 200°–250 °C and salinities of 1.2–12.4 wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354 MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~ 12 km. Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined ⁴⁰Ar/³⁹Ar isotopic plateau ages of 197 ± 2 and 194 ± 2 Ma, and ³⁹Ar/³⁶Ar-⁴⁰Ar/³⁶Ar normal isochrones of 198 ± 2 and 199 ± 2 Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198 Ma) in age. We propose that Jianchaling is an orogenic gold deposit, and formed during continental collision related to the northward subduction of the Mian-Lue oceanic plate during the Early Jurassic. We also conclude that the beginning of the continental collision between the Yangtze and the North China Cratons took place around 200 Ma.