Ore-forming process of the Huijiabao gold district, southwestern Guizhou Province, China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO2 inclusions, CO2-H2O inclusions, hydrocarbon inclusions, and hydrocarbon-H2O inclusions. The ore-forming fluids are characterized by a H2O + CO2 + CH4 +/- -N-2 system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H2O + NaCl -> H2O + NaCl + CO2 + CH4 +/- N-2 -> H2O + NaCl +/- CH4 +/- CO2 with a slight decrease in homogenization temperature and salinity. The delta O-18 values of the main-stage quartz vary from 15.2 parts per thousand to 24.1 parts per thousand, while the delta D-H2O and calculated delta O-18(H2O) values of the ore-forming fluids range from -56.9 to -116.3 parts per thousand and from 2.12 parts per thousand, to 12.7 parts per thousand, respectively. The delta C-13(PDB) and delta O-18(SMOW) values of hydrothermal calcite change in the range of -9.1 parts per thousand to -0.5 parts per thousand and 11.1-23.2 parts per thousand, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H2O-NaCl-CO2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold. (C) 2014 Elsevier Ltd. All rights reserved.