扬子西南缘拉拉IOCG矿床Pb同位素特征及意义

Pb isotopic features of the Lala IOCG ore deposit on the southwestern margin of the Yangtze Block and their significance

扬子地块西南缘拉拉铁氧化物-铜-金(IOCG)矿床中,已公开报道的岩石及硫化物矿物Pb同位素为放射性成因Pb含量极高的异常Pb,不能用常规的Pb同位素等时线计算法及全球平均Pb同位素增长曲线解释其特征。结合Pb同位素演化基本原理,对矿床中的Pb同位素特征进行再解译。用校正后的岩石Pb同位素组成建立了Pb同位素的黑云母片岩增长曲线和钠长石变粒岩增长曲线,二者可作为扬子地块西南缘元古宙Pb铅同位素演化的参考。硫化物矿物的Pb同位素组成位于2条增长曲线之间,并分为线性的上、下2个群;上群拟合混合等时线年龄为1119Ma,代表变质热液成矿期的年龄;下群拟合混合等时线年龄为1605Ma,代表火山喷发—沉积成矿期年龄。上、下2个群的Pb同位素混合等时线年龄分别与矿床中的辉钼矿ReOs年龄及火山岩锆石U-Pb年龄基本一致,表明了利用放射性成因Pb进行测年的可行性与可靠性。黑云母片岩和钠长石变粒岩构成了拉拉IOCG矿床的金属成矿物质的端元储库,硫化物矿物中的金属成矿物质具有混合来源的特征。

Pb isotope data available concerning the ore sulfides and host rocks from the Lala IOCG ore deposit on the southwestern margin of the Yangtze Block are data of radiogenic anomalous Pb. Generalized global growth curves and multistage evolution models are not applicable to interpret their features. In this study, the data were reinterpreted according to the Pb isotopic evolution princi-ple. Two independent Pb growth curves were built for the biotite schist and albite granulite based on the corrected Pb isotope com-position of host rocks, which provide a new reference for the evolution of Pb isotope in the Proterozoic rocks on the southwestern margin of the Yangtze Block. All the Pb isotope compositions of ore sulfides plotted on or between the two curves form two distinc-tive linear clusters. The lower cluster yielded an age of 1605Ma,which represents the chalcopyrite and pyrite mineralization during the volcanic eruption-sedimentation stage, whereas the upper cluster yielded an age of 1119Ma, which represents the metamorphic mineralization stage. The age of the upper cluster and that of the lower cluster are consistent with the molybdnite Re-Os isochron age and zircon U-Pb age of volcanic rocks respectively, which suggests that the method of Pb isotopic dating is feasible and reliable. Biotite schist and albite granulite are metal fundamental reservoirs of the Lala IOCG ore deposit. The metal materials of ore sulfides are characterized by a mixed source.