滇西金满脉状铜矿床成矿年龄讨论

THE METALLOGENIC AGE OF JINMAN VEIN COPPER DEPOSIT,WESTERN YUNNAN

对滇西兰坪盆地内金满脉状铜矿床成矿主阶段的石英样品进行了40Ar 39Ar快中子活化测定,谱线特征为马鞍型,其坪年龄为(58 05±0 54)Ma,最小视年龄为(56 76±0 81)Ma,等时线年龄为(54 30±0 15)Ma,三者均十分接近,说明所测石英样品的年龄可靠。而石英的流体包裹体Rb Sr同位素分析数据因线性关系不太好,导致等时线年龄(68±76)Ma误差大,因而仅具参考意义。砂岩及矿化砂页岩中磷灰石的裂变径迹年龄为58 7～61 7Ma,反映了兰坪盆地内一期构造热演化事件的发生时间,其与40Ar 39Ar分析的年龄基本一致,说明金满脉状铜矿床的形成时代相当于喜马拉雅早期。结合滇西大区域内铜及其他矿种的成矿主要集中在喜马拉雅早期这一现象,认为滇西在此时期必定存在着诱发某种大规模成矿作用的关键因素,并在区域内统一制约着矿床的形成。

Quartz samples collected from the Jinman vein copper deposit in the Lanping Basin of western Yunnan were determined by 40Ar/39Ar fast neutron activation techniques,and the spectra are characterized as being saddleshaped. The samples obtained a plateau age of (5805±054) Ma,a minimum appearance age of (5676±081) Ma and an isochron age of (5430±015) Ma,the three ages being close to each other,indicating that the ages of the quartz samples so far determined are reliable. But the RbSr isochron age error is large because the linear relationship (r=0776 0) of the RbSr isotopic composition is not very close. So the RbSr isochron age has only reference significance. Apatite fission track ages (587-617 Ma) of the host rocks and ores represent the time at which a tectonically thermal event took place within the Lanping Basin,Yunnan Province. This age is also consistent with the 40Ar/39Ar age. So the authors suggest that the time of formation of copper ores is corresponding to Early Himalayan. In consideration of the fact that copper ore and othertype ores in the vast area of western Yunnan are concentrated mainly in the Early Himalayan period,it is concluded that there must be some indispensable key factors leading to metallogenesis on a large scale during Early Himalayan period in western Yunnan and also constraining in unison the formation of ore deposits there.