西藏工布江达县沙让斑岩钼矿床辉钼矿铼-锇同位素年龄及其地质意义

Re-Os Dating of Molybdenite from the Sharang Porphyry Molybdenum Deposit in Gongbo'gyamda County,Tibet and Its Geological Significance

沙让钼矿是念青唐古拉地区扎雪—亚贵拉成矿带的重要矿床,是西藏第一个达详查程度的独立钼矿,初步的勘查成果表明,钼资源量已经达到大型规模。为了查明念青唐古拉成矿带是否存在主碰撞期的大规模成矿作用,对沙让-亚贵拉-洞中拉矿集区沙让斑岩钼矿的7件辉钼矿样品进行了Re-Os同位素分析,辉钼矿187Re的含量22.75~46.66（μg/g）,187Os的含量为19.98~40.32（ng/g）,辉钼矿的模式年龄分布在51.57~52.69Ma的范围内,模式年龄较为一致,所获Re-Os等时线年龄为51±1.0Ma（MSWD=0.55）。该成矿年龄对应于冈底斯带主碰撞期岩浆岩底侵作用事件（介于47.0~52.5Ma之间（大约50Ma的始新世））,也与形成林子宗群帕那组43.93~53.52Ma的火山事件一致,由此表明,冈底斯构造岩浆带存在主碰撞期的大规模成矿,沙让花岗斑岩型钼矿形成于始新世早期,而分布于沙让花岗斑岩外4km范围内的矽卡岩型-热液脉型铅锌铜银（Mo）多金属矿可能也属该时期成矿的产物。

Located in the Zhaxue-Yaguila mining area, Nyainqentanglha Range, Tibet, the Sharang deposit is the first porphyry type molybdenum deposit, which has been detailed investigated. It is shown by preliminary survey that the reserves of molybdenum is very large. For the purpose of finding out whether there are mineralization movements during main Indian-Asian collision period （41 ~ 65Ma） in the Nyainqentanglha mineralization belt, seven molybdenite samples of Yaguila-Dongzhongla-Sharang mining area were selected to do the Re-Os dating. The 187Re and 187Os content are 22.75-46.66（μg/g）and 19. 98 040.32（ng/g） respectively, and the 187Re content of the molybdenite is high. The Re-Os dating yield a weighted average model age of 51.57 Ma to 52.69 Ma, which is in a narrow range, and an isoehrone age of 51±1.0Ma（MSWD----0.55）. The results indicate that there are large scale mineralization movements which is related to the underplating of magma （from 47.0Ma to 52.5Ma（Eocene（about 50Ma）） and the volcanic event（from 43.93Ma to 53.52 Ma） （leads to the forming of the linzizong group pana formation） during the main Indo-Asian collision period in the Gangdese belt. It is also proved that the porphyry type molybdenum deposit in the Sharang mining area formed during early Eocene, and the skarn-hydrothermal vein type Pb-Zn-Cu-Ag （Mo） deposit which is located about four kilometers away from the granite porphyry of Sharang deposit may also form at the same time.