小秦岭车仓峪钼矿成因研究:辉钼矿Re-Os年龄及黄铁矿微量元素制约

Genesis of the Checangyu molybdenum deposit in the Xiaoqinling district:Constraints from the Re-Os dating of molybdenite and in situ trace element analysis of pyrite

车仓峪钼矿位于小秦岭中生代娘娘山花岗岩体与太华群基底的接触带内,矿体发育在由一组X节理控制的石英脉内。本文对车仓峪钼矿开展了辉钼矿Re-Os定年以及黄铁矿微量元素LA-ICP-MS原位分析工作。所得辉钼矿Re-Os模式年龄为133.8±4.3Ma和132.7±2.2Ma,表明车仓峪钼矿的成矿年龄为早白垩世,与娘娘山岩体的成岩年龄一致。辉钼矿样品的Re含量较低,为83×10-6和86×10-6,指示了成矿物质可能来源于I型花岗岩——娘娘山花岗岩。对与辉钼矿共生的黄铁矿进行LA-ICP-MS微量元素原位分析,发现黄铁矿中Ni含量低（4.5×10-6～76.1×10-6,平均17.4×10-6）,表明其来源应该是酸性岩,也就是其围岩娘娘山花岗岩。综合辉钼矿定年以及黄铁矿微量元素分析结果,车仓峪钼矿应该是早白垩世娘娘山花岗岩侵位时,由岩浆冷凝分异出的成矿流体充填和交代围岩形成。同时,小秦岭燕山期Mo矿化并不伴随Au矿化,结合已有资料,从侧面说明燕山期可能并不是小秦岭金的主成矿期。

The Checangyu Mo deposit is located in the contact zone between the Niangniangshan granite and the Taihua Group basement in the Xiaoqinling gold province. Mo-bearing quartz veins are controlled by a group of X joint. In this study,we carried out a research including Re-Os geochronology of molybdenite and in situ trace element concentration measurements of pyrite in the Checangyu deposit. Molybdenite Re-Os dating yields model ages of 133. 8 ± 4. 3Ma and 132. 7 ± 2. 2Ma,which represent the Mo mineralization age. These ages are almost simultaneous with the emplacement of the Niangniangshan pluton. The Re contents of molybdenite are 83 × 10- 6and 86 × 10- 6,indicating ore-forming materials from I type Niangniangshan granite. Nickel contents of pyrite coexisting with molybdenite are pretty low（ 4. 5 × 10- 6～ 76. 1 × 10- 6,17. 4 × 10- 6in average）,indicating a felsic affinity oreforming material source. As the country rock,the Niangniangshan pluton is suggested to be the source of ore-forming materials.Therefore,geochemical data indicate that the Checangyu Mo deposit formed during the emplacement of the Niangniangshan pluton in the Early Cretaceous and the ore-bearing fluid was derived from the magma. No gold mineralization coexisting with Mo in this deposit may imply that the Yanshanian period might not be the main gold mineralization age in the Xiaoqinling district.