Ore-forming Fluids Of The Fancha Gold Deposit, Lingbao, Henan Province

The Fancha gold deposit is one of the representative auriferous quartz vein type gold deposits in the Xiaoqingling gold district.The ore-forming fluid is one of the key controlling factors for the formation of the auriferous quartz vein type gold deposit,and it is also the key content of ore deposit research.In this paper,the geological characteristics and fluid inclusions of the Fancha deposit have been studied in detail,with the aim to reveal the origin and evolution of the fluid and the genesis of the deposit.Field investigation and petrographic observation show that ore-bodies in the Fancha deposit are preferentially hosted in the amphibolites of the Taihua Group and controlled by brittle fracture.Hydrothermal alteration in the Fancha deposit includes silicification,potash feldspathization,sericitization,pyritization and carbonatization.Mineral assemblage,cross-cutting and replacement relationships indicate that the ore-forming process can be divided into the quartz-K-feldspar(I),quartz-coarse pyrite(II),quartz-fine pyrite(III),quartz-polymetallic sulfide(IV)and carbonate stages(V).Gold is mainly precipitated in the III and IV stages.Three types of fluid inclusions have been identified in the Fancha deposit,base on their characteristics at room temperature: three-phase CO2-rich,three-phase CO2-bearing and two-phase aqueous inclusions.The fluid inclusions show the modes of homogenization temperatures of 363.3?,325.5?,273?,258.62? and salinities of 14.53,9.59,11.61,8.03 wt% NaCl.equiv.,for II,III,IV,and V stages,respectively.The ore-forming fluid in the Fancha deposit belongs to the CO2-H2O-NaCl-(±CH4)fluid system with the middle-low temperature and middle-low salinity.Hydrogen and oxygen isotope data indicate that the ore-forming fluids were dominated by metamorphic fluid in the early stage,and that the metamorphic fluid gradually mixed with circulating meteoric water during the late stages.Petrographic observation and microthermometry suggest that fluid mixing and immiscibility have been happened obviously in the main metallogenic stage.Fluid mixing and fluid immiscibility probably lead to gold deposition in the Fancha deposit.