Geological Characteristics And Genesis Of Xicha Gold-silver Deposit In Jilin Province

Xicha gold-silver deposit is located in the Jiao-Liao-Ji orogenic belt in the east of the North China Craton,with the Pacific plate in the east and the Siberian plate in the north.The study area was restricted by the tectonic evolution of Liao-Ji ocean in Proterozoic,and was affected by the closure of the Paleo-Asian Ocean and the superposition and transformation of the Pacific Ocean tectonic domain in Mesozoic Cenozoic.It had a complex tectonic framework and mineralization.The strata exposed in the Xicha gold-silver mining area are the Paleoproterozoic Huangchagou Formation and Mayihe Formation,Jian Group.The Huochagou Formation is a set of metamorphic rock series characterized by graphite,while the Mayihe Formation is characterized by boron,and the two strata are in parallel unconformity.Influenced by the Tanlu fault zone,a series of NNE-trending fault were formed in the mining area,which controlled the morphology and distribution of orebodies.Multi-stage magmatic intrusions are developed in this area,and the lithology is mainly diorite,porphyritic granite and syenite porphyry,among which syenite porphyry is closely related to the deposit.The ore body is mainly composed of 6 gold and 4 silver ore bodies.The ore types mainly consists of three types:quartz vein type,alteration rock type and a small amount of cryptoexplosive breccia type.The ore minerals mainly include silver,electrum,arsenopyrite and pyrite.Gangue minerals mainly include quartz and carbonate rocks.The ore texture is mainly anhedral-hypidiomorphic textures and metasomatic texture,and the structure is mainly vein-like and veinlet constructed banded structure.The main alteration of country rock includes silicification,chloritization and carbonation.The metllogenic epoch can be divided into hydrothermal mineralization and surface mineralization,in which the hydrothermal metllogenic epoch can be further divided into three stages:quartz-pyrite stage,quartz-polymetallic sulfide stage and quartz-carbonate stage.In addition,minor band-like mineralization of pyrite and chalcopyrite can be seen in the stratum of Huangchagou Formation in the mining area.The temperature test results of fluid inclusion from the three-stage quartz show that the temperature of the main ore-forming stage(quartz-polymetallic sulphide stage)is mainly in the range of 220??250?,the salinity is 9-16wt%Na Cl,and the density is 0.84?1.0 g/cm³,which is generally a medium temperature,low salinity and low density fluid.The ore-forming pressure is 19.6–34.6 MPa and metallogenic depth is1.96–3.46 km.The results of laser Raman experiment show that the main gas components of ore-forming fluids are H₂O and CH₄,and C-O isotopes indicate that the ore-forming fluids are mainly derived from magmatic hydrothermal fluid,with the addition of atmospheric water in the later stage.LA-ICP-MS analysis of quartz shows that the elements increased significantly in the main ore-forming stage,such as Au,Ag,Cu,Pb,Zn,accompanied by Sb and Bi,indicating the unloading of a large number of ore-forming materials in the main ore-forming stage.The sulfur content of quartz in the three stages is almost unchanged,which indicates that there may be telluride and bismuth minerals in the main ore-forming stage besides sulfide.The comprehensive analysis of fluid inclusion,quartz LA-ICP-MS and C-O isotopes indicates that the ore-forming fluid is dominated by magmatic fluid.In the main ore-forming stage,the minerals were unloaded and deposited on a large scale,and atmospheric water was added in the late evolution of the fluid.Sulfur isotope shows that the?³⁴S average is 5.45‰,which is in the range of magmatic sulfur source,suggest that ore-forming materials mainly originated from deep magma.Based on the study of the age and genesis of granulites and syenite porphyry in the Xicha gold-silver mining area,it is believed that the mineralization of the Huangchagou Formation was formed at 2157Ma,and its genesis is related to the strong submarine volcanic eruption caused by the subduction of the Liaoji Ocean.The formation time of Xicha gold-silver deposit should be slightly later than the emplacement time of syenite porphyry(121Ma),which is in the tectonic system transition period(from extension to compression)caused by the large angle turning of the Palaeo-Pacific plate.The deposit type belongs to the medium-temperature hydrothermal vein gold deposit controlled by the structure.