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

摘要

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, CO_2 inclusions, CO_2-H_2O inclusions, hydrocarbon inclusions, and hydrocarbon-H_2O inclusions. The ore-forming fluids are characterized by a H_2O + CO_2 + CH_4 ± -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 H_2O + NaCl → H_2-O + NaCl + CO_2 + CH_4 ± N_2 → H_2O + NaCl ± CH_4 ± CO_2 with a slight decrease in homogenization temperature and salinity. The δ~(18)O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δD_(H2O) and calculated δ~(18)O_(H2O) values of the ore-forming fluids range from -56.9 to -116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ~(13)C_(PDB) and δ~(18)O_(SMOW) values of hydrothermal calcite change in the range of -9.1‰ to -0.5‰ and 11.1-23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fraction-ation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfldes 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 H_2O-NaCl-CO_2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.