Comparative Study Of The Genesis Of The Ore-bearing Porphyry And Barren Porphyry In The Xiongmei Copper Deposit,tibet

Xiongmei Copper Deposit locating in Xiongmei,Shenzha County, is a newly discovered porphyry copper deposit in recent years which belongs to the middle segment of Bangong-Nujiang metallogenic belt and locates in about 500 km away from Duolong ore concentration in the west。The meaning of this deposit lies in that it breaks the limitation of prospecting just concentrating on the point in Duolong ore concentration area of the western part of Bangonghu-Nujiang metallogenic belt for years. At the same time, it makes this metallogenic belt own the real meaning of belt, vastly broadened the promising prospecting areas.This article makes preliminary classification the alteration zonation of the surface of Xiongmei copper deposit basing on field geological works and interi or works.This article finds two groups of porphyries by using LA-ICP-MS zirc on U-Pb dating to porphyry body in Xiongmei copper deposit. One group of p orphyry is ore-bearing porphyry and its age is 106.7±0.48Ma(MSWD=0.92);the other group of porphyry is barren porphyry but its three ages parts are 121.8±2.3Ma(MSWD=0.32),122.8±2.1Ma(MSWD=1.16),121.5±2.5Ma(MSWD=0.54).Although the two groups of porphyries are all granodiorite-porphyry, the barren porphyries have more potassium feldspar and less intensity of mineralization than the ore-bearing porphyries.The petrogeochemical analysis results indicate that the two groups of porphyries are enriched in large ion lithophile elements(LILE)Rb, Ba, Th, U, K, Pb and depleted in high field strength elements(HFSE)Nb, Ta, Ti, and shows essential characteristics of post-collisional magmatic rocks, but there are some obvious differences in magma sources and petrogenesis.Ore-bearing porphyry and barren porphyry all belong to strongly peraluminous S-type granitoids, but the sources of ore-bearing porphyry is composed of greywacke and the sources of barren porphyry is based on argillaceous rock.In the magmatic differentiation process, ore-bearing porphyries are controlled by the fractional crystallization of plagioclase and potash feldspar, but the barren porphyries are controlled by the fractional crystallization of potash feldspar and biotite.The closure of the Tethys oceanic basin in the Bangong-Nujiang belt happened in the Early Cretaceous Period about 140Ma~130Ma, and the age of the ore-bearing porphyry in Xiongmei copper deposit is 106.7±0.48 Ma. So the tectonic environment for mineralization in Xiongmei copper deposit is in the post-collision of orogenic stage. In the post-collision, the greywacke and argillaceous rock melted basing on the underplating of mantle-derived magmas, formatting the ore-bearing porphyry and the barren porphyry of the Xiongmei copper deposit.