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Guangming Li, Chengdu Institute of Geology and Mineral Resources (China)
Bo Liu, Chengdu Institute of Geology and Mineral Resources (China)
Wenjun Qu, Chinese Academy of Geosciences (China)
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Recently, the Gangdese belt has been confirmed to be the most potential porphyry-type and the skarn-type polymetallic deposits copper ore belts in southern Tibet. In this belt more than ten middle to large scale porphyry copper deposits had been explorated, am0ng of them, the Qulong porphyry copper deposit is biggest one with copper resources of 706Mt.Besides theses, numerous skarn-type polymetallic deposits hosted in Jurassic-Cretaceous sedimentary rocks, such as the Jiama, Zhibula, Kenu, Chongmuda, have been found in the same regions. For a long time, the geneses for these skarn-type deposits are controversial. The most popular point of view is that theses skarn-type deposits were formed during an exhalation-sedimentation process in Jurassic period, but others believed they were related to the calc-alkaline magmatic activities in post collision. In nearly years, the mineralization ages of porphyry copper deposits in Gangdese belt had been well studied with the ages of 14.4 Ma to 17.6Ma by Re-Os ages dating. In this paper the authors choose the molybdenite samples from Jiama and Zhibula deposits, two representatives of skarn-type polymetallic deposits in Gangdese belt, located about 80km and 100km away from Lhasa, with the Cu+Pb resources of 1Mt and 0.5Mt respectively. Newly Re-Os ages had been gained, an isochron age of 15.18±0.98 Ma from 7 samples of molybdenite for the Jiama deposit and an isochron age of 16.90±0.98 Ma from 5 samples of molybdenite for the Zhibula deposit. The mineralization ages of these two skarn-type deposits are consistent with the mineralization ages of the nearby porphyry copper-molybdenum deposits such as the Qulong, and Tinggong deposits in Gangdese belt. The study results in this paper suggested that the Jiama and Zhibula polymetallic deposits are also the products of activities of the post collision magma activities in Gangdese belt. The skarn-type polymetallic deposits and the porphyry copper-molybdenum deposits were formed by the same post collision magmatic-hydrothermal ore-forming system at different depths and in different wall rocks. Following the collision between Eurasian and Indian Plates, the Gangdese belt was in an extensional geotectonic regime, and granitic magmas derived from the partial melts of the upper mantle or the thickened lower crust ascended along the regional fault systems to the surface, and skarn type and porphyry type mineralation occurred in some favorable sites. Porphyry Cu-Mo deposits usually coexist with skarn-type Cu-Pb-Zn deposits. It is very important to find theses skarn deposits in future exploration in the region.
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