| Abstract:The Xiaoqinling gold ore district is located in the southern part of the North China platform. The strata in the district are volcano-sedimentary sequence of Archean Taihua facies. There are a few Yanshanian granitic intrusions, named Huashan, wenyu and Niangniangshan from west to east. Gold deposits are found around the Wenyu intrusion and controlled by Dayueping-Jinloban anticline and its derivative structures. Most of the deposits are of quartz vein type, containing pyrite, chalcopyrite, gelena, sphalerite etc. Wall rocks show alterations such as sericitization, silicification, chloritization, pyritization and carbonatization, δOH2O and δDH2O Of inclusions in quartz are 9.18‰ and -63.2‰ for metamorphic rocks, 9.89‰ and -85.7‰ for granite, 9.89‰ and -77.7‰ for felsic vein rocks, and 11.04‰ and-78.5‰ for quartz veins, respectively. In mine water, δ18O=-9.57‰ and δD=-74‰. Calculations show that δ18O values of metamorphic rocks, granites, vein rocks and ore-forming solutions are 7.26‰, 7.00‰, 5.69‰ and 1.99‰, respectively.Nine inclusion samples have been studied. They are dominantly liquid inclusions, with a few gaseous inclusions and three-phase inclusions containing daughter crystals of NaCl and liquid CO2. The inclusions are usually 2-5 min size, and have rounded, roundish or irregular forms. Inclusions are usually aligned in certain direction. Homogenization temperatures of the inclusion are mostly in the range of 280-300℃. The pit and Eh values of inclusions are 6.02 and 98.28 for quartz in metamorphic rocks, 3.66 and 238.47 for quartz in granite, respectively. The pH and Eh values of quartz, sphalerite and calcite in the veins are 4.79, 5.95 and 6.22, and 171.55, 103.00 and 87.02, respectively. Gaseous phase of the inclusions is dominated by water vapor, with a little CO2, CH4, and CO. Ore fluids are acid to weak acid, poor in K+, Na+, and rich in Ca2+ and Mg2+; granitic solution is acid, poor in Ca2+ and Mg2+, and rich in K+ and Na+; metamorphic solution is weak acid to neutral, with content of K+ and Na+ between that of ore fluids and granitic solution, and content of Ca2+ and Mg2+ close to that of ore fluids and higher than that of granitic solution. It is thus concluded that the deposits are of mesothermal quartz vein type; ore fluids are a mixture of metamorphic water, granitic magmatic water and supergene water. After emplacement the granitic magma was fractionated, and gold was concentrated in postmagmatic solution, which was then mixed with gold remobilized in the strata, leading to the formation of gold deposits. |
