Composition of ore-forming fluids in the Dayingezhuang gold deposit of the Jiaodong Peninsula, China

The Dayingezhuang gold deposit, located in the central section of the Zhaoping Fault Zone, northwestern Jiaodong Peninsula, within the phyllic zone in the footwall of main fault. Wall-rock alteration is intense and has a variety of types that include potash feldspathization, silication, sericitization, pyritization, chloritization and carbonatation. Amongst these alterations, silication, sericitization and pyritization are considered more closely related with the gold mineralization. Based on the cutting relations of veins and paragenesis of ore minerals, we determined three major mineralization stages: gold-quartz-pyrite, gold (silver)-quartz-polymetallic sulfides, quartz-calcite-pyrite. Employing the decrepitation thermometry to the ore quartz, we identified three concentrated ranges; 330 similar to 510 degrees C, 240 similar to 330 degrees C and 240 similar to 330 degrees C, which are corresponding with each of the mineralization stage above. The ore-forming fluids in the Dayingezhuang gold deposit are characterized by medium temperature and rich in CO2 content, contain a small amount of volatile gases, such as CH4, C2H6 and H2S. The scheme of NaCl-H2O-CO2 and the high level content of C2H6 existed in all of mineralization stages indicate that the ore-forming fluids mostly are the metamorphic water. Even though the gas-liquid content from different mineralization stages are similar with each other, we still observed certain rules along the ore-forming evolution. The trend of increasing IV, content indicates that the ore-forming fluids system switch to an open system in the late stage, and atmospheric water began to take part in the ore-forming fluids. The high quantity of H2S in the early gold mineralization indicates that the gold possibly was migrated as Au-S complex. The climbing ratios of Cl-/SO42- and Na+/K+ show that the ore-forming system convert from the CO2-H2O-K2SO4 into CO2-H2O-NaCl system with the evolving process. The concentration of Na+, Cl-, K+ and SO42- decreased from early stage to later stage indicates that the salinity of ore-fluid declined. The higher ratio of H2O/CO2 in the later stage represents that the fluid boiling may occurred in the middle stage. Overall, we think that a variety of fluid processes, including change of fluid inclusion types and fluid boiling, have been suggested for gold precipitation.