甘肃小柳沟钨钼矿床Re-Os、Ar-Ar同位素定年及其成矿意义

Molydenite Re-Os and Muscovite Ar-Ar Dating of Tungsten-Molybdenum Ore Deposit in Xiaoliugou and Its Implication for Ore-Forming Processes North Qilian Mountains,Gansu Province

对甘肃小柳沟钨钼矿床进行了辉钼矿Re—Os和白云母Ar-Ar同位素定年。2件辉钼矿样品获得的Re—Os模式年龄分别为（427±6）Ma和（428±6）Ma，平均（427．5±6）Ma；3件自云母Ar—Ar定年获得的Ar-Ar坪年龄分别为：（391．98±1．95）、（391．13±1．88）、（392．04±1．98）Ma，平均（391．7±1．9）Ma。上述年龄指示小柳沟钨钼矿化发生在428-392Ma之间。碰撞造山过程中地壳收缩增厚、深部岩石增温加压和变质脱水，特别是在构造体制由挤压向伸展转换阶段的构造减压，将进一步促使部分熔融过程及花岗岩浆的形成。含矿岩体从岩浆源区通过部分熔融作用获取成矿物质，在岩浆后续分异演化过程中，作为不相容元素的W、M0将倾向于在残余的熔体中进一步富集。随着岩浆上侵到浅部，因温度、压力的下降而使含水流体或挥发分饱和而发生出溶，成矿元素将优先进入溶液，并随溶液迁移到赋矿的断裂或裂隙中沉淀成矿。因此，碰撞造山过程为祁连造山带同期含矿岩体的形成和钨钼成矿提供了有利的动力学环境，而花岗岩浆上侵、结晶分异和流体析出，为W、Mo元素的最终迁移、富集成矿提供了热动力学条件。因为造山后伸展阶段更利于花岗岩浆上侵和流体的分异，所以此阶段形成的花岗岩与成矿关系更为密切。因此，祁连地区钨钼的进一步找矿工作，应对造山阶段尤其是造山作用后伸展阶段花岗岩给予高度重视。

The molydenite Re-Os and muscovite Ar-Ar dating of Xiaoliugou tungsten and molybdenum ore deposit were carried out in this research. The results showed that Re-Os model ages of two molydenite samples are （427±6）Ma and （428±6）Ma respectively, and the average age is （427.5±6）Ma. The Re-Os dating results indicate that the Xiaoliugou W and Mo mineralization were generated in Silurian period. The Ar-Ar plateau ages of three muscovite samples from the same location are （391.98±1.95）Ma, （391.13±1.88）Ma, and （392.04±1.98）Ma correspondingly, and the average age are （391.7±1.9）Ma. The Ar-Ar dating results reflect that the ore-forming processes had been continuing till early Devonian epoch. The W, Mo ore deposits from Xiaoliugou around Jinfosi granite pluton and related to granite bodies were the north Qilian collision orogenic product. The granite magma were formed by the material in deep partial melting inducted in crust contraction thickening, increasing temperature, pressure, and metamorphism dehydra- tion, especially tectonics decompression in course of tectonic regime transformation from compression to extension. Ore-forming material distributed in magma was gained by means of partial melting of source rock, furthermore, W and Mo as incompatible elements were enriched in residual melt following magma fractionation. Additionally, the rise of magma to shallow levels of the Earth＇s crust inevitably resulted in saturation with respect to an aqueous fluid or vapor phase, such as H20 fluid exsolved from silicate melt. The W, Mo elements were to preferentially partition themselves into fluid phase to silicate melt, so that ore-forming elements enriched into fluid and moved into faults or fissures to precipitate finally. So collision orogenic processes provided favorable geodynamic setting for generation of W, Mo ore deposits and related granites, and further attention was required on the collision orogenic grainites in ore-prospecting work in Qilian orogen.