The Metalgenesis Of The Epithermal Deposit In Gandese Metallogenic Belt, Tibet, China

The extraordinary Gangdese metallogenic belt have been formed on Tibetplateau during intensive Inida-Eurasia collisional movement. Numerous researcheshave been focused on the porphyry copper deposits but little has concentrated atanother type magmatic related deposits with extremely economic values which is theepithermal deposit. Nongruri gold deposit was the first deposit with epithermalcharacters in minerals assemblages and forming environments. Based on detailedstudies on Nongruri gold deposit, We can recognize the similarities and differencesbetween deposits like Nongruri and classical epithermal deposits, and establish anepithermal model on the background of continental to continental collisional setting,which probably guide the regional epithermal deposits surveying.Oreshoots concentrate in Linbuzong formation with mineral assemblage ofnative gold, electrum, pyrite, arsenopyrite, stibnite and realgar et al., which formsAu-Ag-Sb-As elemental combination. Isotopic data indicate that sulfur comes frommagmatic origin and Pb from mantle mixing with magmatism under subduction zone.The ore bodies are mainly controlled by the North-South direction normal faultsystems. The⁴⁰Ar/³⁹Ar age implies the thermal activity concentrated between25-20Ma. SHRIMP U-Pb dating of igneous ziron document the sequence of igneous frommeso-coarse grained granite porphyry （67.7±0.6Ma，later Cretaceous） throughmeso-coarse grained moyite（66.6±0.7Ma，later Cretaceous）to meso-fine grainedmonzonitic granite porphyry（18.8±0.3Ma，Miocene）,with which combining thegeochemical data shows granite porphyry and moyite are the product of arcmagmatism of Gangdese belt, and adakitic affinity of monzonitic granite porphyry.Ore forming may be constrained at20-19Ma by the activity age of normal faultsystem and those intrusions, which indicates mineralization occurred at Miocene.LA-ICP-MS and EPMA data demonstrate that gold in this deposit comes out asform micro-and submicroscopic type. Microscopic ones mostly appear in inclusionand fracture, meanwhile, those submicroscopic type emerge as enclave, lattice and/orsolid solution gold in form, which forms also proved by gold content is consistentwith the elemental content of Cobalt, Nickel, Copper, Zinc, Arsenic, Silver and Lead.Ore-forming fluid has characters of low temperature（average of229.6℃） andlow salinity（average of7.0wt%NaCleqv） with high concentration CO₂, and reducinggas like lillte CH₄, N₂and so on which belongs to type of Ca²⁺-Na⁺-K⁺-Cl--SO₄^2-. Additionally, an average pressure of ore-forming system is247bar implying an depthof819meters below surface. H-O data suggests that fluid of ore-forming originedfrom monzonitic granite porphyry igneous and mixed meteoric and geothermal wateras migration outward along NS normal faults to form a mixing fluids. Eventually, themixed and later boiled ore fluid began to precipitate.Combination the field surveying and indoor research, we have constructed modelof Nongruri-type epithermal gold ore deposit. Basis on the regional data analyses, wesuggest that ore deposits like Nongruri epithermal deposit are coupling with Gangdeseporphyry copper belt in temporal and spatial distribution, and genesis of ore-forming,which points out the indicator for this type of deposits in Gangdese metallogenic beltbased on tectonics, igneous and hydrothermal alteration.