A comparative study of the ore-forming fluids of the typical gold-antimony deposits along Middle Xuefeng arc structure belt

The Gutaishan and Longwangjiang quartz vein type gold-antimony deposits, located in Xinhua and Xupu County, respectively, are two representative ones in the middle of Xuefeng arc structure belt. The Gutaishan gold-antimony deposit occurs in the external contact zone of Baimashan composite granite mass, while Longwangjiang gold-antimony deposit is far from Baimashan composite granite mass. Based on the study of fluid inclusions and hydrogen and oxygen isotopic geochemistry, this paper discusses the characteristics and origin of ore-forming fluids so as to provide an insight into the ore genesis. Petrographic observation and microthemometric measurements of fluid inclusions show that ore-forming stage quartz veins in the Gutaishan gold-antimony deposit mainly contain liquid-vapor two-phase aqueous inclusions and CO₂-H₂O three-phase inclusions. The homogeneous temperature and salinity of primary fluid inclusions are in the range of 199 ~298 ℃ and 2.07 ~11.46%NaCl_eqv. The ore-forming stage quartz veins of the Longwangjiang gold-antimony deposit mainly contain liquid-vapor two-phase aqueous inclusions, with the homogeneous temperature from 164 ℃ to 238 ℃ and salinity from 1.40% NaCl_eqv to 8.41% NaCl_eqv. Hydrogen and oxygen isotopic geochemistry shows that the ore-forming fluids of the Gutaishan gold-antimony deposit was composed of the mixture of magmatic water and metamorphic water, and the Longwangjiang gold-antimony deposit was mainly derived from metamorphic water. Based on geological and fluid inclusion characteristics, it is suggested that fluid immiscibility may have been the main mechanism for metal precipitation in the Gutaishan gold-antimony deposit. The mechanism of the ore-forming process in the Longwangjiang deposit was probably caused by the change of temperature and/or pressure, or the addition (mixing) of metamorphic water.