Characteristics Of Geology And Geochemistry Of Gongpoquan Porphyry Copper Deposit, Beishan,Gansu Province, And Its Mineralization

The Gongpoquan copper deposit, is a typical porphyry copper deposit, located in the Gongpoquan island arc zone of Beishan copper metallogenic belt in Gansu—Mongolia province. Even though the predecessors have acquired a lot of valuable data and information about Gongpoquan copper deposit, there is lack of systematic geological and geochemical study. This paper is based on lots of field investigation, combined with the laboratory research and the main conclusions are as follows:1. The main porphyry types are granodiorite porphyry and dacite porphyry which belong to the high-K calc-alkaline series and have the same spider diagram of primitive mantle normalized trace elements and Chondrite-normalized REE patterns. The granodiorite porphyry and dacite porphyry enrich LILF, such as Ba, Rb, Pb etc; and relatively impoverish HFSE, such as Nb, Ta etc.2. The study of fluid inclusions in the ore-bearing quartz vein shows there are four types of fluid inclusions, pure vapor inclusions (Ⅰ-type), pure aqueous inclusions (II-type), nature aqueous-rich inclusions (Ⅲa-type) and secondary aqueous-rich inclusions (Ⅲb-type). The fluid inclusions have homogenization temperature values range from 250℃ to 500℃, the salinity values range ftom 2.0% to 6.0%, and the density values range from 0.75 to 1.Og/cm3, which show that the ore-forming fluid is medin-high temperature, low salinity and low density. Laser-Raman microspectroscopy analyses reveal that the liquid phase is composed of H2O, and the vapour phase are mainly H2, N2, CH4.3. Analysis of hydrogen, oxygen and sulfur isotope indicate that the emplacement of porphyry provided elements such as Cu and S. The porphyry became mobilized and generated ore-forming fluid through differentiation, the decompressing boiling of which resulted in separation of phases while upward migrating, and then Cu precipitated. Therefore, the decompressing boiling might be mainly responsible for the precipitation of mental in fluid.