Characteristics of ore-forming fluids of the Shihu gold deposit, western Hebei

Abstract：Analysis of the compositions of gas and liquid phases in quartz inclusions suggests that the ore-forming fluids in the Shihu gold deposit are a K+-Na+-Ca2+-SO2－4-NO－3-Cl－system rich in organic volatiles such as CO2, H2O and CH4 and C2H6. Analysis of sulfur and carbon isotopes shows that sulfur and carbon are derived from the mantle, and hydrogen and oxygen isotope analysis shows that the ore-forming fluids are a mixture of magmatic hydrothermal fluids and meteoric water. The average of six homogenization temperature data is 310℃, indicating mesothermal mineralization, and 53 data of decrepitation temperatures mainly cluster at two ranges, namely, 320-350℃ and 360-380℃, which reveals that approximately two mineralization occurred. The ore-forming pressure was 20.3 MPa and the ore-forming depth was 0.75 km, with lgfo2=39.95, Eh=-0.42 to -0.64V and pH=5.94 to 6.44-all these suggest that the deposit was emplaced in a near-surface, relatively oxidized and weakly alkaline ore-forming environment. The vertical isothermal diagram of quartz inclusions at different levels in orebody 101-4 suggests that the ore-forming fluids moved from south to north and from deep to shallow, with the direction of the fluid movement coinciding with the pitch direction of the orebody, implying that there are very good ore prospects at depths of the southern end of the mining area.