Fluid inclusion and stable isotope (C–H-O-S) constraints on the genesis of the Heilongtan-Xiejiagou Au deposit, northern Hubei, China

Abstract

The Heilongtan-Xiejiagou Au deposit at Suizao (Suizhou-Zaoyang) area of Hubei province was recognized previously as an orogenic gold deposit by many researchers, in consistent with the fact that this area is dominated by extensive Early Cretaceous magmatism and associated magmatic hydrothermal deposits. In this study, we try to constrain the ore genetic type and formation mechanism of the Heilongtan-Xiejiagou Au deposit by studying the source of ore-forming fluids and materials. Three paragenetic mineralization stages were recognized, including (I) feldspar-quartz-sericite-pyrite veins, (II) quartz-sulfides veins, and (III) quartz-sulfides-carbonate veins. Combined Laser Raman analysis and microthermometry for fluid inclusions hosted in quartz and calcite, the fluid system of three hydrothermal stages is all composed of H2O-NaCl ± CO2 with medium–low temperatures and salinities. A gradually decreasing trend has been recorded for temperatures from 224 to 339 °C, through 182–301 °C and towards 138–249 °C, respectively, together with salinities changing from 9.9 to 15.6 wt% NaCl eq., through 6.3–12.7 wt% NaCl eq., to 2.9–8.1 wt% NaCl eq. from early to late stages. The calculated fluid pressures range from 52 to 185 MPa at stage I, and the stages II and III vary from 42 to 128 MPa and 19 to 96 MPa, respectively. The in-situ sulfur isotope data suggest that the sulfur sources were dominantly mixture of magma and surrounding metamorphic rocks, and the C–H-O isotope data imply the mineralizing fluids were originated dominantly from magmatic fluid and mixed with some meteoric water. By means of comparison with regional magmatism and mineralization ages, we consider that the fluids may have originated from the buried granite that also supplied some sulfur and particularly heat energy to drive the fluid circulation system to extract metallogenic materials (Au and other elements) also from country rocks, resulting in mineralization after fluid flow into the fractures and faults of the metamorphic rock strata. In consequence, we draw a conclusion that the Heilongtan-Xiejiagou Au deposit pertains to hydrothermal quartz vein-type deposit controlled by both of magmatic fluid and structure.