长坑-富湾金、银矿床铅同位素组成特征及其意义

Characteristics and significance of lead isotopes of ores from the Changkeng-Fuwan gold-silver deposit, Guangdong Province

测定的金、银矿石矿物铅同位素组成表明 ,矿石铅同位素组成变化幅度较大 ,在单阶段增长曲线图上均呈线性分布 ,金矿石拟合直线与 μ=8.64的普通铅单阶段增长曲线相切 ,说明金矿石铅为普通铅和放射成因铅两阶段混合而成的异常铅 ;单阶段演化方程计算结果显示 ,金矿石混合铅的正常端元在 1 0 4 8.1 Ma前已从它的放射性母体中 (源区 )分离出来 ,铅来源的最大可能是云开地区老于 1 0 4 8.1 Ma的中元古代地层 ;按两阶段异常铅演化方程 ,给定假设的含异常铅矿物结晶时间的可能范围 t2 =2 2 7～ 2 3 .3 Ma,计算得到对应的 t1 =1 2 2 5 .9～ 1 3 5 7.1 Ma,也表明异常铅来源于中元古界 ,属两阶段演化的产物。银矿石铅同位素组成部分落在金矿石的铅异常直线上 ,说明银矿石异常铅是富 2 0 7Pb的端元铅与金矿石的两阶段异常铅的不同比例混合铅 ,为三阶段演化的异常铅。铅同位素构造模式图反映金矿石铅的壳源性质 。

Lead isotope components of ore minerals from the Changkeng-Fuwan gold-silver deposit vary largely and distribute linearly on the single stage evolution curve of lead isotopes. The fitting line of lead isotopes of gold ores is tangent to the single stage evolution curve with μ=8.64 of common lead, indicating that the gold ore lead is two-stage abnormal lead resulted from common lead mixing with radiogenic lead. Calculating with single stage lead evolution formula shows that the normal part of the mixing lead of gold ores might have separated before 1 048.1 Ma from its source area (Meso-Proterozoic strata of western Guangdong and eastern Guangxi). If the possible crystallization abnormal lead-bearing in the minerals happened between 227 to 23.3 Ma(t 1), the lead source age (t 1) was calculated with using two-stage anomalous lead evolution formula to be 1225.9 to 1357.1Ma, also indicating that the abnormal lead of gold ores might come from Meso-Proterozoic strata and belong to two-stage evolution lead. Some spots of abnormal lead of silver ores distribute on the evolution curve of gold ore lead, showing that the lead of silver ores was resulted from mixing of the 207Pb-rich lead with the two-stage abnormal lead of gold ores by different ratios and belonged to three-stage evolution abnormal lead. Plotting of lead isotopes of gold and silver ores on the diagrams of 207Pb/ 204Pb and 208Pb/ 204Pb vs. 206Pb/ 204Pb evolution curves show that gold ore lead might come from Crust source area and silver ore lead from crust-mantle mixing source area.