Hydrothermal Fluid Evolution Associated with Gold Mineralization at the Wenyu Mine, Xiaoqinling District, China

Abstract: The Wenyu mesothermal gold deposit is located in the Xiaoqinling district about 1000 km southwest of Beijing in central China. It occurs in the Late Archean to Early Proterozoic metamorphosed volcanic and sedimentary rocks. Three distinct stages of veins have been identified: (I) gold‐poor quartz–pyrite veins, (II) gold‐rich sulfide–quartz veins, and (III) gold‐poor carbonate–quartz veins. Stage II can be subdivided into IIa and IIb. Gold typically occurs as fracture‐fillings associated with chalcopyrite and galena. Fluid inclusions were examined in quartz samples from veins of both stage I and II. Three types of fluid inclusions are identified: CO₂–H₂O, CO₂–rich, and aqueous inclusions. The first two types are of primary in origin. The last type occurs in two ways: coexisting with CO₂–H₂O and CO₂–rich inclusions and thus primary in origin; and occurring along late healed fractures and hence secondary in origin. CO₂–H₂O inclusions display progressively decreasing Th and increasing Thco₂, from the highest Th (311–408C) and lowest Thco₂ (average 18C) in stage I quartz through middle Th (284–358C) and ThCO₂(average 25C) in stage IIa quartz to the lowest Th (275–314C) and highest ThCO₂ (average 28C) in stage IIb quartz, indicating an evolving H₂O–CO₂–NaCl fluid system. CO₂–rich and primary aqueous inclusions show consistent ThCO₂ or Th with their coexistent CO₂–H₂O inclusions. Whereas the secondary aqueous inclusions in stage I and IIa quartz have almost the same Th and salinity as the primary aqueous inclusions in stage IIb quartz. Comparing with CO₂–H₂O inclusions, these non–CO₂, low salinity aqueous inclusions may come from different origin, most probably meteoric water. Unlike in both stage I and IIa quartz, fluid inclusions in stage IIb do not show evidence of fluid immiscibility. The fact that most of gold is associated with stage IIa and IIb veins and not with veins of stage I which is the main stage of vein formation suggests that gold deposition occurs at the later stage of fluid immiscibility. The continuing phase separation led to the deposition of large amounts of gold at the Wenyu mine.