西秦岭西成矿集区厂坝岩体的钼矿化特征及其Re-Os年龄

甘肃西成矿集区是西秦岭最重要的多金属成矿区之一,以铅锌矿床为主,其次发育有金、铜、钼等矿床(点)。厂坝黑云母二长花岗岩体位于西成矿集区中东部,岩体南缘接触带部位发育石英脉型钼矿,辉钼矿呈中粗粒半自形团块状产于粗粒石英脉内。Re-Os同位素年代学研究表明,其模式年龄范围为207.7±3.0~209.8±2.8Ma,加权平均年龄为208.9±1.1 Ma,等时线年龄为209±15 Ma,指示钼矿化发生于晚三叠世。辉钼矿的铼含量介于18.82×10~(-6)~21.97×10~(-6)之间,平均19.65×10~(-6),表明成矿物质主要为壳幔混合来源,可能以壳源为主。钼矿化与区域岩浆活动以及金、铅锌成矿作用时代相近,是印支期区域构造-岩浆-流体活动的产物。钼矿化年龄的厘定为区内找矿提供了新的思路。

Molybdenum mineralization characteristics and Re-Os age of the Changba pluton in the Xicheng ore concentration area, West Qinling Mountains

The Xicheng (Xihe-Chengxian) ore concentration area in Gansu Province is one of the most important polymetallic ore concentration areas in the West Qinling orogen. There are mainly lead-zinc deposits and subordinately gold, copper and molybdenite deposits (ore spots). The Changba biotite monzo-granite is located in the mid-east of the Xicheng ore concentration area, and the molybdenum-bearing coarse-grained quartz veins occur on its southern margin. The subhedral molybdenite occurs within the quartz veins in lumpy form. The Re-Os isotopic dating of 7 molybdenum samples yielded model ages ranging from 207.7±3.0 to 209.8±2.8 Ma, with the average model age of 208.9±1.1 Ma and the isochron age of 209±15 Ma, indicating that the Mo mineralization on the southern margin of the Changba pluton took place in the Late Triassic, slightly later than the formation of the Changba pluton. The low content of Re (18.82×10^-6~21.97×10^-6) shows that the material was mainly derived from the crust, with the mixture of a little mantle material. In combination with the ages (200~225 Ma) of the regional magmatism, it is held that gold, lead-zinc deposits and molybdenite mineralization on the south margin of the Changba pluton resulted from the tectonism, magmatism and fluid metallogenic event in the late Triassic period. The Re-Os dating of the molybdenite provides a new thinking for exploration of the Indosinian quartz vein type Mo deposits in the contact zone of the late Triassic granitoid plutons in the West Qinling orogen.