湖南沅水流域白垩红层碎屑锆石及其对金刚石找矿的指示  

Detrital Zircon of the Cretaceous Redbed in the Yuanjiang Basin of Hunan and its Implication for Placer Diamond Exploration

作　　者：杨瀚 丘志力 邓小芹[1,2,3] 马瑛 李子云 YANG Han;QIU Zhili;DENG Xiaoqin;MA Ying;LI Ziyun(School of Earth Sciences and Engineering,Sun Yat-sen University,Zhuhai 519082,Guangdong,China;Guangdong Provincial Key Laboratory of Geological Process and Mineral Resource Survey,Guangzhou 510275,Guangdong,China;Guangdong Provincial Key Laboratory of Geodynamics and Geohazards,Guangzhou 510275,Guangdong,China;No.413 Geological Brigade,Hunan Bureau of Geology and Mineral Exploration and Development,Changde 415000,Hunan,China)

机构地区：[1]中山大学地球科学与工程学院,广东珠海519082 [2]广东省地球动力作用与地质灾害重点实验室,广东广州510275 [3]广东省地质过程与矿产资源探查重点实验室,广东广州510275 [4]湖南省地质矿产勘查开发局四一三队,湖南常德415000

出　　处：《大地构造与成矿学》2020年第6期1160-1171,共12页Geotectonica et Metallogenia

基　　金：国家自然基金项目(41473030、41272086、41703028)资助。

摘　　要：湖南沅水流域的白垩系陆相红层是发现金刚石及其指示矿物的重要层位。金刚石和锆石共同参与沉积过程,研究白垩系红层中的碎屑锆石对追踪和揭示砂矿金刚石的来源及其沉积历史具有重要意义。本文采用LA-ICP-MS技术对3组来自湖南沅水流域白垩系红层的共240颗碎屑锆石样品进行了测试分析。结果显示,3组样品的锆石U-Pb年龄频谱特征接近扬子型(年龄集中于~800 Ma、~2000 Ma和~2500 Ma),表明锆石来源于扬子克拉通内部,结合沅水流域部分金刚石近源搬运的特点,指示砂矿金刚石源区也可能来自克拉通内部,甚至是湖南境内。本次研究还发现其中2颗锆石的微量元素地球化学特征属于钾镁煌斑岩型,其U-Pb年龄分别为788±11 Ma和869±13 Ma,暗示研究区在新元古代中期曾发育过钾镁煌斑岩。由于目前已发现的中生代钾镁煌斑岩中只含有极少量的微粒金刚石,不可能是砂矿大金刚石的补给源,因此尚未发现的元古代钾镁煌斑岩可能才是金刚石的携带者。另外,3组样品中80%以上的锆石年龄为元古代和太古代,指示研究区大部分的岩浆构造热事件主要发生在前寒武纪。南华系江口组是湖南地区发现砂矿金刚石的最古老地层,与下伏板溪群呈假整合接触。因此,其下伏的、未出露的板溪群中存在的钾镁煌斑岩可能是进一步寻找金刚石原生矿的重要目标层位。The Cretaceous redbed in the Yuanjiang Basin of Hunan province contains abundant placer diamond and associated indicator minerals.Detrital zircon might have been transported and deposited along with diamond,and thus possibly share same source and sink,therefore,the detrital zircon from the Cretaceous redbed can be used to trace the origin of the placer diamond.In this paper,240 detrital zircon grains from three Cretaceous redbed samples of the Yuanjiang Basin were analyzed by using LA-ICP-MS.The results show that most of the zircon grains exhibit U-Pb age distribution patterns similar to that of the Yangtze Block,with age peaks of ca.800 Ma,ca.2000 Ma and ca.2500 Ma,which suggests that the detrital zircon came from the Yangtze Block.Considering the characteristics of short-distance transportation of the placer diamond,the placer diamond may also originate from the Yangtze Block,or more specifically within Hunan province.In addition,two zircon grains with geochemical characteristics of typical lamproite origin yielded U-Pb ages of 788±11 Ma and 869±13 Ma,respectively.Because the known Mesozoic lamproites in the study area contain only very low content of microdiamond and hence can be excluded as the primary source for the macro Hunan placer diamond,so the undiscovered Neoproterozoic lamproites are likely the major source of the placer diamond.Besides,more than 80%of the detrital zircon grains have U-Pb ages of Proterozoic and Archean periods,implying that most of the magmatic events occurred in the Precambrian.The Jiangkou Formation,which is unconformably overlain the Banxi Group,is the oldest diamond-bearing strata in Hunan province,therefore,the Neoproterozoic lamproites buried in the underlying Banxi Group are the most promising targets for further exploration of primary diamond ore.

关 键 词：白垩系红层 碎屑锆石 砂矿金刚石 找矿 湖南沅水流域 

分 类 号：P597[天文地球—地球化学] P595[天文地球—地质学]