Ore-forming Fluids Of The Lingjinyikuang Gold Deposit, Xiaoqinling Area, West Henan Province, China

Xiaoqinling gold district, located in the southern margin of North China craton, is the China’s second largest gold district. The Lingjinyikuang ore deposit is one of representative auriferous quartze vein type gold deposits in the Xiaoqinling district. Detailed field investigation and fluid inclusion study have been carried out on the Lingjinyikuang gold deposit, with a focus on an analysis of origin and evelution of ore-forming fluids and the genesis of the deposit. The ore minerals mainly include native gold, pyrite, chachopyrite, sphalerite, and galena. The gangue minerals are primarily quartze and calcite. The ore-forming process can be divided into five stages: the quartz–K-feldspar(I), quartz-coarse pyrite(II), quartz- fine grained pyrite-native gold(III), quartz- polymetallic sulfide-native gold(IV), and quartz-carbonate(V) stages. Three types of fluid inclusions are present in the deposit: two-phase aqueous inclusions(type A), three-phase CO2-containing inclusions(type B), three-phase CO2-rich inclusions(type C). The stage I contains mainly type A inclusions; the inclusions display homogenization temperature of 306.4-405.4℃ and salinities of 3.55-12.73 wt.% NaCl equiv.. The stage II contains maily type C inclusions, which yield homogenization temperature of 249.3-329.8℃ and salinities of 9.44-12.16 wt.% NaCl equiv.. The stage III contains type B and C inclusions,which show homogenization temperature of 213.3-315.4℃ and salinities of 3.55-12.73 wt.% NaCl equiv.. The stage IV contains all types of inclusions, which yield homogenization temperature of 195.6-287.4 ℃ and salinities of 3.89-11.05 wt.% NaCl equiv.. In the stage V, only type A inclusions are present; these inclusions show homogenization temperature of 141.3-244.4 ℃ and salinities of 1.81-12.68 wt.% NaCl equiv.. H-O isotope data indicate that the ore-forming fluids were dominated by metamorphic water in the early stages, and that the metamorphic water gradually mixed with circulating meteoric water during the late stages. Fluid inclusion data and the results of previous experimental studies, indicate that gold in the Lingjinyikuang deposit was transported mainly as Au-S complexes. The precipitation of gold is probably induced by fluid immiscible and mixing. Based on the information discussed above, a model for the formation of Lingjinyikuang deposit can be established. The model can be described as follows: during the proceses of plate subduction and overthrust in Inosinian, the rocks of Taihua group had undergone extensive metamorphism, forming a large number of metamorphic fluids. During the Yanshanian, the tectonic setting of the Xiaoqingling district translated from compression to extension. This change induced the metamorphic fluids intrusion along the fracture zone, and finally filled in the fault of upper crust, forming the Lingjinyikuang auriferous quartze vein type gold deposit.