Magma, Fluids And Gold Mineralization Of The Porphyry-epithermal Gold Deposits In The Yichunarea, Heilongjiang Province

The Yichun area of Heilongjiang province, located in the east part of the Central Asian-Mongolian orogenic belt, experienced multi-period magmatism during Mesozoic, and accompanied with large-scale mineralization. This dissertation focused on the representative deposits in the Yichun area(Gaosongshan, Dong’an and Tuanjiegou gold deposits), pinpointed the mineralization geological background, tectonomagmatic evolution and the characteristics of the ore-forming fluid, as well as defined the genesis type of the deposits.Gaosongshan and Dong’an gold deposits are hosted by vocanic rocks and alkali-feldspar granites of the Yanshanian. Ore bodys occurred as veins under controlled by faults. The main wall-rock alterations include silicification, pyritization, sericitization, adularization and fluoritization. The wall rocks of Tuanjiegou gold deposit are Yanshanian Putaogou granite porphyry and dacite of Ningyuancun Formation of. The ore bodys occurred primarily as vein and lenticular hosted within the prophyrite and the contact zone between prophyrite and schist of Heilongjiang Group. The alteration zones from the prophyrite outward are successively quartzification-sericitization zone, argillization zone and propylitization zone outward from the prophyrite.The Mesozoic magmatism in this area concentrated in early-middle Jurassic and early Cretaceous. Ore-host alkali-feldspar granite of Dong’an deposit is characterized by high fractionated I-type granite and its zircon U-Pb age is 183 Ma, which are product of the subduction of the Pacific plate during early-middle Jurassic. Putaogou granite porphyry(104 Ma) in Tuanjiegou deposit belongs to S-type granite and generated by the re-melting of crustal material. The vocanics of Banzifang Formation and Ningyuancun Formation in Yichun area, characterized by continental marginal vocanic rocks, are generated in an extensional tectonic environment related to subduction of the Pacific plate in Early Cretaceous. The magma source of Banzifang Formation vocanics is metasomatic lithospheric mantle. The zircon U-Pb ages of Ningyuancun Formation vocanics are 104-109 Ma, and they were generated by partial melting of the lower mafic crust.The research of fluid inclusion indicates that the ore-forming fluid of Gaosongshan and Dong’an gold deposit is nearly neutral fluid of medium-low temperature(180~280 ℃) and low salinity(<5.0 wt%), which mainly derived from meteoric water. The ore-forming fluid during early period of Tuanjiegou gold deposit is characterized by medium-high temperature(190~350 ℃) and medium-low and mddium-high salinity(0.4~5.4 wt%, 30.6~58.5 wt%), while the temperature and salinity apparently decreased during the late period due to mixing of meteoric water. The fluid boiling is the primary mechanism of the mineral enrichment and precipitation of these gold deposits.Dong’an, Gaosongshan and Tuanjiegou gold deposit are all hosted in vocanics-intrusive rocks of the Yanshanian Period. Occurrence features of ore bodys, ore composition, ore pattern and their fabric, wall rock alteration and characteristics of ore-forming fluid reveal that Dong’an and Gaosongshan gold deposits are low-sulfidation epithermal gold deposits, while Tuanjiegou gold deposit belongs to porphyry gold deposit. Gaosongshan, Dong’an and Tuanjiegou gold deposit are adjacent in space and have the same metallogenic epoch, constituting a porphyry-epithermal mineralization system related to the early Cretaceous magmatism. The recognition of this mineralization system has significance to the regional ore-prospecting and co-exploration in northeast China.