西藏曲水县鸡公村石英脉型钼矿床成矿时代约束

Geochronological constraints on Jigongcun quartz-vein type molybdenum deposit in Quxu County,Tibet

西藏鸡公村钼矿床位于冈底斯铜钼多金属成矿带的中部,是一个产于曲水岩体花岗闪长岩内部、受脆性逆断裂控制的石英脉型钼矿床。为查明该矿床的成矿时代,对5件辉钼矿样品进行了Re-Os同位素分析,辉钼矿w(187Re)为886·1～1063μg/g,w(187Os)为331～395ng/g,辉钼矿的模式年龄较为一致,分布在22·29～22·81Ma,所获Re-Os等时线年龄为(21·8±6·2)Ma(MSWD=2·2),该年龄对应于印度大陆与亚洲大陆碰撞后的陆内后碰撞造山向走滑伸展转换的时间,早于冈底斯斑岩型铜钼多金属主要成矿期(17～12Ma)。由此表明,在冈底斯成矿带陆内后碰撞造山向伸展转换的过渡环境下,可能存在着集中成矿事件(30～20Ma),它可能与传统的冈底斯斑岩成矿事件(17～12Ma)一起构成了一个在时间上连续的成矿序列。鸡公村辉钼矿较高的铼〔w(Re)为1410×10-6～1691×10-6〕,高于中国其他地区的钼(铜)矿床,暗示其成矿物质可能主要来源于拆沉的下地壳被地幔加热后再熔融的深源(下地壳-上地幔)岩浆。

The Jigongcun molybenum deposit, located in the middle of the Gangdese metallogenic belt in Tibet, is a quartz-vein type molybenum deposit hosted in Qushui granodiorite batholith, with ore bodies controlled by the brittle reverse fault. In order to define the ore-forming time of Jigongcun molybenum deposit, the authors select- ed five molybdenite samples for Re-Os isotope analysis. The 187Re and 87Os values of molybdenite vary from 886.1μg/g to 1063 μg/g and 331 ng/g to 395 ng/g, respectively, yielding five narrowly distributed Re-Os model ages ranging from 22.29 Ma to 22.81 Ma and an isochrone age of （21.8 ± 6.2） Ma （MSWD = 2.2）. The plotted isochrone age corresponds to the transitional period of the Gangdese orogenic belt from the post-Indi- a-Eurasia collision to strike-slip extension, earlier than the Gangdise porphyry Cu-Mo polymetallic metallogenic time （17--12 Ma）. This study indicates that, during the transitional period of the Gangdise orogenic belt from continental collision to strike-slip extension, there might have existed intensive metallogenetic events （30- 20 Ma）, which could constitute a continuous mineralization sequence （only at that time） together with the later porphyry Cu-Mo polymetallic metailogenic epoch （17--12 Ma）. The Re values of five molybdenites vary from 1410× 10-6 to 1691 × 10 6, much higher than the values of Mo-Cu deposits in China. This indicates that the metals of the Jigongcun molybdenum deposit might upper mantle） , which experienced remeltin~ from have had a deep-seated magma source （the lower crust to the the upper mantle） , which experienced remelting from the delaminated lower crust heated by the mantle.