东秦岭地区钼矿床的铼－锇同位素年龄及其意义

Re-Os ISOTOPE AGES OF MOLYBDENUM DEPOSITS IN EAST QINLING AND THEIR SIGNIFICANCE

金属矿床的成矿年龄多年来一直是矿床地质工作者关注的问题之一。中国地质科学院岩矿测试技术研究所建立的铼－锇同位素定年实验室，为研究钼矿床的形成时代提供了新的手段。因辉钼矿是最主要的含铼矿物之一，且基本上不含普通锇，其中所含￣（１８７）Ｏｓ是由￣（１８７）Ｒｅ衰变而来，故放射成因￣（１８７）Ｏｓ的积累是矿物年龄及铼含量的函数，本文采用同位素稀释－等离子体质谱法，测定了东秦岭钼矿带中几个不同类型的大型钼矿床的Ｒｅ－Ｏｓ年龄：黄龙铺碳酸岩脉型钼（铅）矿氏的Ｒｅ－Ｏｓ年龄为印支期，其余的斑岩型钼矿床和斑岩－矽卡岩型钼（钨）矿床的Ｒｅ－Ｏｓ年龄均为燕山期。

The East Qinling molybdenum ore belt lies on the southern margin of the North China platform and is one of the areas in China where importantmolybdenum deposits are concentrated.The belt includes porphyry type moly-bdenum deposits( Jinduicheng and Shijiawan in Shaanxi Province) and por-phyry-skarn molybdenum-tungsten deposit(Nannihu-Sandaozhuang in HenanProvince) related to Mesozoic granites,as well as carbonatite vein-type moly-bdenum-(lead)-uranium deposits(Huanglongpu and Huayangchuan in ShaanxiProvince) not related to Mesozoic granites. The ages of these ore deposits are generally represented by ages of related rock bodies determined by Rb-Sr isochron or K-Ar method.However,these ore-bearing rock bodies were com-monly subjected to differer degress of hydrothermal alteration and hence the Rb-Sr and K-Ar systems are not as satisfactory as closed ones,rendering the rock-forming ages determined by the above methods unreliable. It is well known that molybdenite contains essentially no common osmium and ￣(187)Re is radioactive and yields stable ￣(187)Os through β decay. Thereforeaccumulation of radiogenetic ￣(187)Os is a function of ageand Re content of molybdenite. In this paper,￣(187)Re and ￣(187)Os contents of molybdenite from thesemolybdenum deposits are determined by Inductively Coupled Plasma MassSpectrometry(ICP-MS), with the Re-Os ages calculated.The analytical errorsof Re and os are around±3%(2σ),and the errors of Re-Os ages are between3%and 4%(2σ).The results show that the apparent Re-Os ages of the Jinduicheng andShijiawan deposits are l29 ×l0￣6·139 ×l0￣6 a and l38 ×l0￣6a, respectively. Theapparent age of the Nannihu-Sandao-Zhuang molybdenum-tungsten deposit is 146 ×l0￣6·156 ×10￣6 a with an isochron age of 147 ×l0￣6a(n=5), and the Hua-nglongpu Mo-(Pb) deposit is 222 ×l0￣6·230 ×l0￣6 a with an isochron age of 221 ×10￣6 a(n=5).The Re-Os dating method not only provides directly reliableore-forming ages for these deposits,but also demonstrates that there exist twometallogenic epochs of molybdenum deposits in the East Qinling Mountains,i.e. Indosinian and Yanshanian periods. The result is of importance in outli-ning the temporal and spacial distribution of molybdenum deposits and theevolution of molybdenum mineralization in this region.