Geology And Ore-forming Fluid Geochemistry Of The Shangshanghe Segment Of The Laowan Gold Deposit,Tongbai County Of Henan Province

The Laowan gold deposit, located in Tongbai County, Henan Province, is one of the large orogenic gold deposits in the North Slope of the Tongbai-Dabie Mountain gold and silver metallogenic belt. Ore deposit geology and ore-forming fluids geochemistry studies have been carried out on this deposit in order to ascertain its fluid mineralization process. Based on veins cutting relation, ore texture and structure, mineral assemblage, and pyrite size and crystal form, fluid mineralization process of the Shangshanghe segment of the Laowan gold deposit is divided into four stages, i.e., stage Ⅰ of quartz with coarse grained euhedral pyrite, stage Ⅱ of quartz with fine grained subhedral-anhedron pyrite, stage Ⅲ of quartz with polymetallic sulfide, and stage Ⅳ of quartz with carbonate. H2O-CO2, pure CO2, and aqueous inclusions are observed under microscopy, with a few aqueous inclusions with daughter salt crystal. The H2O-CO2, pure CO2, and aqueous inclusions are always seen in the first, second, and third stages. Coexisting of the H2O-CO2 and the aqueous inclusions can be observed sometimes. Microthermometric data of fluid inclusions indicate that ore-forming fluids were NaCl- H2O-CO2 system with middle temperature, low salinity, and CO2 content. The fluids represented by the pure CO2 and aqueous inclusions were resulted from an immiscible process of NaCl- H2O-CO2 fluids represented by the H2O-CO2 inclusions. The three types of inclusions are captured at the same time during the host mineral precipitation. Homogenization temperature and salinity of fluid inclusions decrease from 303~379℃ and 4.07%~9.59%NaCl.eq in the first stage to 138~195℃ and 1.06%~2.74%NaCl.eq in the fourth stage, respectively. The first fluid immiscible process occurred in the first stage of the mineralization and the second fluid immiscible process occurred in the second and third stages of mineralization owing to CO2 effervescence of fluids. The values of δ18OH2O and δDH2O change from-78.1‰ ~-64.2‰ and 6.56‰ ~ 9.71‰ in the first stage through-79.5‰~-76.3‰ and 1.89‰~4.01‰ in the second stage to-72.6‰ and 0.08‰in the third stage, respectively. These isotopic data that the ore-forming fluids were a magmatic hydrothermal fluid in the first stage, a mixing fluid of magmatic hydrothermal fluid with a small amount of meteoric water in the second stage, and a mixing fluid of magmatic hydrothermal fluid with a large amount meteoric water in the third stage.