Study On Geological Characteristics And Genesis Of Polymetallic Metallogenic Belt On The Periphery Of The Jinfosi Pluton In The Western Part Of The North Qilian Mountains

Jinfosi pluton is located in western part of the North Qilian Mountains, and a tungsten and molybdenum polymetallic metallogenic belt has been found, which is situated in the contact zone of Jinfosi pluton, and which is composed of some mineralization points, such as Dadaokou, Qingdaban, Ganbahenao wolfram-molybdenum deposits, Suotonggou lead-zinc deposit and Songmugou copper deposit and so on, all of those demonstrated that the region has a good geological conditions to form tungsten and molybdenum deposits. So there are important theoretical and practical significance to study the metallogenic geological conditions and mechanism of the metallogenic belt systematicly, and which can be helpful to find the W-Mo polymetallic deposits relate to the Jinfosi pluton.In this paper, based on systematic analysis to geological data of the study area, and synthetic studies on two typical deposits (Ganbahenao and the Qingdaban W-Mo deposits) by applying multiple disciplines and methods such as petrography, mineralogy, deposit theory, geochemistry, fluid inclusion, and isotope geology, the metallogenic geological conditions and mechanism of the polymetallic metallogenic belt have been discussed in the paper, and the metallogenic model has been established too. Through the above studies, the preliminary forecasting to the field resource potential has been done, and meanwhile, the prospects and direction of the prospecting area has been put forward, too.Polymetallic metallogenic belt on the periphery of the Jinfosi pluton mainly exists in the contract zone of Jinfosi granite. Because of the affect of regional deep and major fault zone, the individual ore body mainly occurs in the subsidiary faults, for instance the north-west, north-east and near east-west subsidiary faults in the pluton contact zone. Through the observation of the ore body and the research of the mineral assemblage characteristic, the mineralizing function can be divided into 2 mineralizing periods:hydrothermal and supergene stage, and the hydrothermal stage included silicates, polymetallic sulphides and carbonates three mineralization stages.Fluid inclusion in mineralization phase are primary inclusions, no secondary and pseudosecondar inclusions. They have variety shapes, including long strip, elliptic, rhombic, rectangular, trapezoidal, triangular, and various irregular shapes; their sizes change little, most sizes range from 2 to 20μm; The type of inclusions are simple, including liquid-rich phase, vapor-rich phase, single liquid phase, as well as inclusions containing daughter minerals. Thermodynamic parameters of fluid inclusions indicate that:the temperature of the main stage of mineralization is 200-300℃, the salinity is 5.7-8.7wt%, the density is 0.998-1.018g/cm3, and the corresponding depth and pressure of ore-forming are 17.28-27.56MPa and 0.64-1.02Km, which indicate that ore-forming fluid of the study area has a low-medium temperature, low salinity, low density characteristics, so the genetic type of the deposit is a epithermal deposits with low-moderate temperature.The studies of the composition and C-H-O isotopes of Ore-forming fluid shows that:δDv-SMOW values of ore-forming fluids are between-67--91‰, the relevant hydrothermal water'sδ18OH2O values are-1.21-4.04%o, and theδ13CV-PDB‰values range from-6%o to-4.2%o. The ore-forming fluids belonged to K+-Na+-SO4+-Cl-system, the gas phase constituent in fluid is mainly based on H2O, and a small amount of CO2 and N2 is in the next place. The above studies indicate that although ore-forming fluids come of magmatic, but with the process of mineralization, a large number of precipitation interlarded the fluid in the ore-forming period, led the oxygen isotopes shows the homothetic "drift" feature, so in the main mineralization stage, the ore-forming fluid was a mixed hydrotherm, which were magmatic hydrothermal fluids and meteoric water, and the fluid immiscibility action was the main reason of mineral precipitation and enrichment.The averageδ34S value is about 4.7‰, the value is most similar to the granitic magma's values (5%o-15%o), which suggests that the close relationship between Jinfosi granite and mineralization. At the same time, the geochemical characteristics of minerals and rocks show that they have a similar characteristic that the trace elements character of granite and mineral, as well as the mineral's REE distribution curves also consistent with Jinfosi granite's, all of the evidence illuminates that the intrusive granitic magma and remelting of the crystalline basement (Beidahe Group complex) provided an important source of ore forming materials for the formation of polymetallic deposits.Based on the above studies, which suggest that polymetallic deposits in the polymetallic metallogenic belt, formed in the periphery of Jinfosi pluton, western part of North Qilian, belong to mesothermal-epithermal post-magmatic hydrothermal type deposit, between mineralization and the intrusion of Jinfosi pluton, the relationship is so close, ore-forming hydrothermal solution is mixed hydrothermal of post-magmatic hydrothermal and atmospheric precipitation.