西秦岭降扎地区金、铀矿床年代学对比研究

西秦岭降扎地区寒武、志留系中的金、铀矿床,是我国相当重要的碳硅泥岩型矿床。根据金矿床中岩石和矿石异常铅模式年龄、氩-氩同位素年龄和钾-氩同位素年龄等,获得了一批有益的年代学信息。金矿床的各种同位素年龄数据主要有2组:(1)242-186Ma;(2)137-47Ma.结合区域地质发展史、成矿热液脉与岩脉的穿切关系以及矿床产出的其它宏观特征等,确定金矿床的成矿时代为137-47Ma,与区域铀矿床形成的主要时代(117-55Ma)相比,二者成矿时间大体相同。

COMPARATIVE CHRONOLOGICAL STUDY OF THE GOLD AND URANIUM DEPOSITS IN JIANGZHA, WESTERN QINLING MOUNTAINS, CHINA

The gold and uranium deposits in Jiangzha,western Qinling Mountains,are a very important silicalite-carbonate-argillite type of deposits.The gold deposits consist of the La’erma and Qiongmo ore deposit and the Yaxiang ore occurrence.They hosted the Cambrian system composed of black siliceous rock,slate and carbonate.The uranium deposits include a lot of deposits such as 510,511,512,513,710,403,104 and so on. They exist in the Silurian system.It is unsolved whether the mineralization of gold was simultanous with that of uranium.Using the methods of anomalous lead model, ⁴⁰Ar-³⁹Ar age spectrum,K-Ar isotope,Pb-Pb and U-Pb isochronical age,and electron paramagnetic resonance dating,the authors obtained a lot of useful information. In the gold deposits,lead isotopic compositions vary in a very wide range with rock lead isotopic values of ²⁰⁶Pb/²⁰⁴Pb=17.692-36.697, ²⁰⁷Pb/²⁰⁴Pb=15.480-16.775,and ²⁰⁸Pb/²⁰⁴Pb=36.644-39.019,and ore lead isotopic values ²⁰⁶Pb/²⁰⁴Pb=16.532-37.266, ²⁰⁷Pb/²⁰⁴Pb=15.257-16.870, and ²⁰⁸Pb/²⁰⁴Pb=36.703-39.638.They are of mixed type dominated by radiogenic lead.In general,the research of anomalous lead model can effectively trace the history of gold mineralization.So the age obtained on instantanous-growth model stands for the epoch of gold mineralization. ⁴⁰Ar-³⁹Ar step heating dating demonstrates that the Au-bearing quartz from La’erma gold deposit form saddl-shaped age spectrum of excess ⁴⁰Ar. Studies show that the minimum age of saddle-shaped spectra is 47±20Ma, which is close to its crystallizing age.The K-Ar isotopic ages of sericite, dickite and dacite porphyry vein vary from 242Ma to 127Ma.Therefore, the authors conclude that there are mainly two-group ages:(1)242-186Ma;(2)137-47Ma.According to the developmental history of regional geology,the crosscutting relationship between dykes and hydrothermal metallogenic veins and the other macroscopic geological features,the authors thought that the first group is the epoch of regional metamorphism and tectonic-magmatic activity,and second group is the epoch of gold metallization.Namely the main metallogenic epoch of the gold deposits occarred at about 137-47Ma.In the uranium deposits,based on methods of Pb-Pb,U-Pb isochron and electron paramagnetic resonance dating,the authors and the other researchers determined that the epoch of uranium mineralization is 117-55Ma,which is very close to the main metallogenic epoch of the gold deposits (137-47Ma). Obviously,these dates suggest that the main metallogenic epoch of the gold deposits is the same as or similar to that of the uranium deposits. Both of them were formed in the late Yanshanian period.