Study On Geodynamic Evolution Of A'nyemaqen-Bayan Har Orogenic Belt And Metallogenesis In Dachang Au, Sb Deposit

A'nyemaqen-Bayan Har orogenic belt is located on the south of South Eastern Kunlun fault in the Eastern Kunlun Mountain. The strata are composed of Pre-Carboniferous, Carboniferous-Permian, Triassic and Quaternary in the study area. Dominant Triassic Bayan Har group occurs in Bayan Har region, which belongs to carbonaceous turbidite series. Regional magmatic activities were strong, whose active stages focused in Caledon, Hercynian and Indosinian-Yanshanian in the study area. Yindosinian acid intrusive activities are strongest and related close to gold regional metallogenesis. There exist two pieces of NEE-striked structural belts, i.e. A'nyemaqen mélange belt and Bayan Har fault zone, which commenced in Hercynian. While orogenic activities almost happened in Indosinian. A series of large-scale shear zone developed in the edge of Bayan Har fault zone became regional channel way or ore hosting structures for orogenic gold deposits in the region. The magnetic field in Triassic Bayan Har sedimentary basin is characterized by quiet negative magnetic field. Several apparent magnetic anormaly belts, around which there are dense positive anormaly regions. The gravitational field in Bayan Har belt shows gravity low anormalies, which reflect there are thick Triassic flysch constructions in it. A'nyemaqen-Bayan Har region underwent a complicated geological evolutional history, which strongly affected whole Eastern Kunlun mountain. A'nyemaqen mélange belt was situated in the north margin of Paleo-tethys in early Permian. It was thought that there existed a deep-water basin, A'nyemaqen Ocean, in A'nyemaqen district in early Permian because of the discovery of abyssal radiolaria siliceous rock in early Permian. And it was considered that the A'nyemaqen Ocean close in the late stage of early Permian, because reliable late Permian strata didn't be found on the south of South Eastern Kunlun fault. During A'nyemaqen Ocean plate commenced to subduct northward, the total study area changed from a stretching orogenic system to a compressing one, while a plenty of volcanic rock erupted and magmatic rock intruded. When A'nyemaqen Ocean died out and closed ultimately, A'nyemaqen district transformed into an intracontinental stage. Up to late Indosinian, strong crust-mantle interaction happened in the area, and mantle-derived magmatic activities developed extensively to form a series of mafic-ultramafic intrusive bodies. There exist an ancient basement in Bayan Har orogenic belt, which belongs to a remnant of a paleo-continental block (South China plate?). During A'nyemaqen Ocean died out and closed, the Bayan Har block on the south of it was stretched,extended and resulted in the formation of Bayan Har Sea. Triassic Bayan Har sedimentary basin is an early active-basin-type of the middle-Tethys after the north Paleo-Tethys rift-type trench closed in late Permian. Triassic sedimentary basin formed in early Triassic, and it closed to become an orogenic belt till late stage of Triassic or early stage of Jurassic. It means that Indosinian movement affected this area from Early Triassic carbonate precipitation to late Triassic active sandy flysch construction. During this process, the north margin of Bayan Har Sea changed from convergence to divergence environment.Bayan Har Ocean plate subducted northward caused a plenty of volcanic eruption in the North Eastern Kunlun belt, which carried many clastic material to the south margin of the Bayan Har basin. Plate subduction induced earthquake and triggered continental clastic substance to form seafloor turbidite. The latest subduction belt subducted southwestward under the Triassic Bayan Har group sediments or volcanic arc belt. After late Triassic lateral compression and intracontinental convergence, Triassic Bayan Har sediments were concreted and subsequently folded. Strong crust-mantle interaction happened till the end of Triassic. Then lithosphere was detached, and mantle magma diapir became very apparent. The tectonic system changed from compression to extension system. It means that Indosinian was another important tectonic active epoch in the Eastern Kunlun mountain. Many mantle-derived material and energy was concerned with structural and magmatic activities and metallogenesis. Dachang Au, Sb deposit is located in Songpan-Ganzi Indosinian fold system in the third class tectonic units in north Bayan Har belt. Strata is really simple in the mine area, which consist of the early Permian Ma'erzheng formation in Buqingshan group and the sandstone, slate formation of middle Bayan Har group. The latter formation is the ore-hosting wall rock for Dachang Au, Sb deposit. This carbonaceous turbidite carries organic Carbon and abundant Au, so it is an Au-bearing source bed and supplies mineral matter for the late stage of hydrothermal metallogenesis. A NEE-striked fault is the most important ore-controlling structure in the mine area, which is a part of Gande-Maduo deep fault. 27 pieces of Au, Sb veins were recognized in the mine area, where the shape of ore bodies are simple, including similar bedded, banded and podiform. The ore types consist of major alteration-rock-type with minor quartz-vein-type.According to fluid inclusion analysis, the primary fluid inclusions can be divided into three types, i.e. a. gas-liquid phase fluid inclusions, i.e. NaCl-H20 type. b. CO2-bearing three phase fluid inclusions, i.e. C02-H2O-NaCl type. c. C02-enriched fluid inclusions. Based on analysis of fluid inclusions in quartz and calcite samples, the homogeneous temperatures mostly focus on 160~260 ℃, and average value is 217.6℃, which belongs to mesothermal deposit. The average salinity is 5.7 wt%NaCl in metallogenic fluid, and average density is 0.88g/cm3 in it. It means that the metallogenic fluid is of low salinity and low density fluid in Dachang Au, Sb deposit. The components of fluid are composed of major H2O, with minor gas phase, i.e. CO2, CO, H2, CH2, C2H2, CH4 etc. In a word, according to analysis of fluid inclusions, it was thought that the metallogenic hydrothermal fluid consist of dominant magmatic water in the early stage of metallogenesis, and major meteoric water with a few mixed fluid of construction water and deep derived magmatic water in the metallogenic stage. Based on analysis of metallogenic geodynamic settings, regional ore-controlling geological conditions and comparison of typical orogenic gold deposits, it was thought that Dachang Au, Sb deposit belongs to typical orogenic gold deposit. The metallogenic depth was 6km by calculation of relationship of pressure and depth. And it was thought that Dachang Au, Sb deposit belong to mesozonal orogenic gold deposit, with late epizonal orogenic Au (Sb) mineralization. It was concluded that the metallogenic model consists of follows: after late Triassic Bayan Har Ocean closed, there were strong tectonic and magmatic activities. Magmatic hydrothermal fluid inflowed along fault structure, and leached partial gold mineral matter from Bayan Har flysch sedimentary construction. At the same time, meteoric fluid convection happened, and Au mineral matter was...