Ore-forming Fluids Of The Hongshi Gold Deposit, Tianshui, Gansu Province

The Hongshi gold deposit is a newly discovered cryptoexplosive breccia type gold deposit in the western Qinling belt,and its geological characteristics are special in this area.A detailed study on the Hongshi gold deposit would provide new evidence for enrichment of the gold mineralization theory and further exporation of gold deposits in the western Qinling belt.Therefore,a detailed field investigation,fluid inclusions and stable isotope study has been carried out on the Hongshi gold deposit,with focus on an analysis of the ore-forming fluids and origin of the Hongshi gold deposit.The breccia in the Hongshi gold deposit is mainly composed of wall-rocks and quartz monzonite;the cement mainly include composed of quartz,pyrite,chalcopyrite,arsenopyrite,galena,sphalerite and nature gold.Based on mineral assemblage and cross-cutting relationship,the ore-forming processes of the Hongshi gold deposit can be divided into the barren sulfide quartz,quartz-pyrite,quartz-polymetallic,and carbonate stages.Gold was mainly precipitated in the quartz-polymetallic stage.The fluid inclusions in the Hongshi gold deposit are mainly daughter mineral-bearing,liquid-rich,gas-rich,and CO₂-bearing fluid inclusions.The barren sulfide quartz,quartz-pyrite,and quartz-polymetallic stages contain all types of the fluid inclusions.But in the carbonate stage,only liquid-rich fluid inclusions are present.Microthermometric data show that the homogenization temperatures of fluid inclusions,from barren sulfide quartz stage to carbonate stage,are 365.4,333.7,276.6,and 213.2?,respectively;the salinities are 15.64,10.46,8.28,2.78 wt%NaCl equiv.,respectively.Fluid inclusion assemblages and stable isotopic data indicate that the early stage fluids?e.g.,the barren sulfide quartz and quartz-pyrite stages?in the Hongshi deposit were magmatic in origin,whereas the late stage fluids?e.g.,quartz-polymetallic,and carbonate stages?were magmatic water mixing with minor meteoric water.Comprehensive analysis suggested that the Hongshi gold deposit is a magmatic hydrothermal gold deposit,and that fluid boiling occurred during fluid evolution of the Hongshi deposit,resulting in gold and sulfide precipitation.