黔东北地区MVT型铅锌矿床闪锌矿Rb-Sr定年及其地质意义

贵州东北部是中国著名的MVT型铅锌成矿带,长期以来,由于缺少精确成矿同位素年代学数据,对该区矿床成矿类型及其成矿作用的争议较大。文章对铜仁市下寒武统清虚洞组中的卜口场铅锌矿床和中寒武统敖溪组中的大硐喇汞锌矿床进行了同位素年代学研究,获得了闪锌矿的Rb-Sr等时线年龄,分别为(349.6±9.1)Ma(MSWD=0.51,n=5)和(349.2±5.2)Ma(MSWD=0.031,n=4),两个矿床闪锌矿的共同Rb-Sr等时线年龄为(348.6±1.9)Ma(MSWD=0.46,n=9),初始值(87 Sr/86 Sr)i为0.7095。Rb-Sr同位素测年结果表明,黔东北地区不同碳酸盐地层中铅锌、汞矿床的成矿时代是一致的,均为后生矿床类型,其成矿物质可能主要来源于包括震旦系碳酸盐岩在内的下伏地层。区内铅锌、汞矿床在加里东运动后期至华力西运动早期,华夏板块与扬子板块后碰撞过程中,可能与保铜玉深大断裂(中元古代末期上扬子古陆与江南古陆结合带,即最重要的控矿因素)的继续活化、扭动、撕裂紧密相关。

Rb-Sr dating of sphalerites from MVT Pb-Zn deposits in northeastern Guizhou Province and its geological implications

Northeastern Guizhou is a famous MVT type lead-zinc metallogenic belt; nevertheless, due to the absence of accurate and credible metallogenic age data, the deposit type and mineralization in this belt has long been a topic of controversy. Rb-Sr ages of sphalerites from the Bukouchang Pb-Zn deposit in the Lower-Cambrian Qingxudong Formation and the Dadongla Hg-Zn deposit in the Middle-Cambrian Aoxi Formation are reported in this paper, with the isochron age being (349.6±9.1) Ma (MSWD=0.51,n=5) and (349.2±5.2) Ma (MSWD=0.031,n=4), respectively. The isochron age of all sphalerites is (348.6±1.9) (MSWD=0.46,n=9) with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7095. Rb-Sr dating result implies that the metallogenic ages of Pb-Zn-Hg deposits in deferent carbonate strata are nearly coeval, suggesting epigenetic deposit type. Metallogenic materials for these deposits might have been mainly derived from the underlying strata including the Sinian carbonate strata. Mineralization of Pb-Zn-Hg deposits in northeast Guizhou Province was probably caused by the reactivation, torsion and rift of the Baotongyu deep fracture, which was the late Mesoproterozoic suture line at the junction of the Upper Yangtze and the Jiangnan old land and also served as the most important ore control factor, resulting from the post-collision between the Yangtze and Cathaysian plates in late Caledonian to early Variscan period.