Research On Metallization Of Pusangguo Polymetallic Copper Deposit In Tibet, China

The Pusangguo polymetallic copper deposit is a unique deposit identified by high grades of ores and valuable abundances of elements as a giant metallic deposit in Nanlinmu basin, China. Since previous studies of the Pusangguo deposit are deficient and unable to understand its mineralization, this study has been focused on the mineralization to guide the exploration and understanding of the Pusangguo deposit. The Pusangguo deposit is tectonically located at one end of the famous Gangdise metallogenic belt, characterized by complex structures and frequent magma activities which might provide positive minerogenetic conditions. Based on precious studies and recent ore exploration works of the Pusangguo deposit, this study presents U-Pb age dating of zircon, chemical analyses of major, minor and REE elements in intrusive rocks, microthermometric analyses of homogenization temperatures and salinities of fluid inclusions and analyses of isotopic composition of H, O, S and Pb in samples. Compared with regional geology settings and the mineralization of other deposits in the region, this study presents the discussion including the evolution of magma affected on the deposit and a relationship between diagenesis and mineralization, as well as temporal and spatial evolutions of ore-forming fluids and couplings of physicochemical ore-forming conditions and positions of ore bodies. Works and conclusions of this study could be as follows:1. This study has finished detail studies on geological features of the Pusangguo deposit. Occurrences, shapes and scales of ore bodies have been controlled by skarn, showing lamellar, resemble lamellar and lenticular shapes of ore bodies occurring in skarn and tuffaceous hornfels in Cretacic Takena stratum. Ores in the deposit have been identified as many types of shapes, such as massive, disseminated, stringer, stripped, granular and earthy types. The ores have also been identified many textures as idiomorphic-subhedral-xenomorphic granular texture, separation textures of solid solutions, poikilitic texture, border ring texture, sharing ring texture metasomatic relict texture, dense disseminated texture and crush texture, etc. There are 3 stages occurring in the deposit, including skarn, hydrothermal mineralization and supergene stages. The skarn stage has been divided into earlier and later sub stages, and the hydrothermal mineralization stage has been divided into Fe-Cu sulfides sub stage and Pb-Zn-Cu sulfides sub stage. Since the stage of hydrothermal mineralization is the last mineralization stage of the deposit, there are 4 sub stages of the mineralization summarized in this study.2. This study has finished systematic studies on petrology of igneous rocks and features of their petrogeochemistry. Intermediate-acid intrusive rocks, identified as granodiorites, dioritic porphyrite and granodiorite-porphyry, are belonged to volcanic-arc and crust-mantle type granitoid in alkaline series with high contents of potassium and calcium. They are formed in the environment during the transition and subduction of the structure system. Basic intrusive rocks in the deposit are identified as gabbro which is belonged into alkaline series as basalt and andesite rocks formed in the island arc or orogenic belt around the plate margin.3. This study has propoesed a preliminary estimation of the source of ores. The mineralization of Cu and Zn has been controlled by intermediate-acid intrusive rocks in the deposit, while the mineralization of Pb has been controlled by the dioritic porphyrite. Sulfur isotope is shown as that from magma, indicating it is from upper mantle. The composition of lead isotope has indicated that ore-forming fluids are sourced from the upper crust, or mixed with materials from lower crust. Studies of fluid inclusions have showed that there are three stages of the mineralization. The earlier skarn stage is characterized by high temperature and salinity in fluids, whereas the later skarn stage is characterized by intermediate-high temperature and salinity in that. The ore-forming fluids are characterized by intermediate-high temperature and intermediate-low salinity, and they might be affected by the boiling.4. This study has concluded that the Pusangguo deposit is caused by magma activities with multiple stages. Since the U-Pb age dating in zircon form granodiorite have showed a result of 15.81±0.18 Ma, the magma intrusion in the deposit might be active in Miocene Himalayan Langhian period. Thus, those rocks are major diagenetic rocks, and they have an intimate relationship with the mineralization which has occurred in later stages than that of granodiorite. Ore-bearing magma from joint area of crust and mantle ascend along extensional structures, resulting that ore-forming elements of Cu, Pb, Zn, Ag and Co are repeatedly enriched in the “copper-enriched fluids” which are the ore-forming fluids of resemble lamelleted and lenticular ores with high ore grades found in skarn.5. This study has indicated the Pusangguo deposit as the skarn type. Based on systematic studies of ore-forming conditions and processes, this study has built a metallogenic model and summarized features of the mineralization of the Pusangguo deposit, and it is expected to provide a theoretical basis for the ore exploration in Nanmulin Basin.