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

Abstract

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 15.6wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220°C and salinities of 1.1–7.4wt.% 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 1.2–12.4wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~12km.Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined 40Ar/39Ar isotopic plateau ages of 197±2 and 194±2Ma, and 39Ar/36Ar-40Ar/36Ar normal isochrones of 198±2 and 199±2Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198Ma) 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 200Ma.