GENETIC MINERALOGY AND PROSPECTING POTENTIALITY OF GRANITOID OF NIMU-QUSHUI REGION IN TIBET

The area of Nimu-Qushui in Tibetan Plateau lies in Tethyan-Himalaya metallogenic belt. It has been noticed that the medium-acid magmatic rocks have close relation with some Au or Cu ore deposits and mineralized spots in this area. The major types of granitoid rocks are granodiorite and quartz monzodiorite, and the enclave is mainly monzodiorite. The rock-forming minerals include K-feldspar, plagioclase, amphibole, biotite, quartz, magnetite, titanite, apatite, epidote, etc.

The major element analyses of whole rocks, rare earth element (REE) analyses, trace element analyses, electron microprobe (EMPA) and X-ray analyses have been done. By these analyses and comparison with the granite in other Au or Cu belts in the world, the characteristics of the granite in this area are concluded as follows: (1) it belongs to magnetite-I type granite and has crust-mantle mixed characteristics; (2) it was formed by crystallization differentiation of melting magma and multiphase varied intrusion; (3) the degree of differentiation evolvement is not high; (4) the granite and its enclaves are congenetic; (5) the range of rock-forming temperature is 500°C-700°C, the range of rock-forming pressure is 2.50 Gpa-4.35 Gpa, and the range of rock-forming oxygen fugacity (lgƒO₂) is -15.35—-14.00; and (6) most granite bodies formed before the collision of plates, and few formed after the collision of plates.

Compared with the granitoid from Jiaodong Guojiangling in mineralogy, petrology, and geochemistry, following conclusions are drawn: (1) The minerals in the granitoid in Nimu-Qushui crystallized in the environment of oxidization; (2) the granitoid has a characteristic of volatile-rich components and high content of alkali; (3) the rock-forming and accessory minerals have a characteristic of I-type granite; and (4) the granite shows a characteristic in favor of Au and Cu ore formation, at least indirectly.