贵州三穗龙湾铀矿硅质岩LA-ICP-MS锆石U-Pb年代学和地球化学研究

Zircon U-Pb Age and Geochemistry of Siliceous Rocks from the Longwan Uranium Deposit in Sansui County,Guizhou Province

龙湾铀矿位于黔中‒湘西北成矿带,为贵州省第一个勘查发现的大型碳硅泥岩型铀矿床,其赋矿围岩主要为碳质泥岩与硅质岩。本文对龙湾铀矿硅质岩开展了LA-ICP-MS锆石U-Pb年代学和地球化学等方面研究。结果显示:①硅质岩的Al/(Al+Fe+Mn)值为0.16~0.41(平均0.25);Fe/Ti值为33~126(平均73);MnO/TiO_(2)值介于0.16~0.74之间;MgO/Al_(2)O_(3)为0.67~4.21(平均1.78);指示其形成于大陆与海洋的高盐度海水沉积,并具有典型热水沉积的特点;②硅质岩的LaN/YbN值为1.27~11.69(平均4.99),暗示为大陆边缘与深海盆地硅质岩;LaN/CeN值为1.72~2.99,δCe为0.41~0.74,表明硅质岩的沉积环境为开阔盆地与大陆边缘之间;V/(V+Ni)值为0.88~0.98(平均0.93),可知其应属静海环境;U/Th值为3.58~34.4(平均15.1),表明本区硅质岩为热水沉积岩;③硅质岩中的锆石具明显的振荡环带,绝大多数Th/U值>0.1,稀土元素总量(ΣREE)较低,亏损LREE富集HREE,具有Ce正异常、Eu负异常,均表明其为岩浆成因。48颗锆石U-Pb年龄主要集中在中元古代、晚元古代、埃迪卡拉纪‒寒武纪,表明这3期构造岩浆活动活跃,为区内最主要的构造活动时期;④硅质岩Th含量为5.30×10^(-6)~9.09×10^(-6)(平均7.42×10^(-6));U含量为27.0×10^(-6)~193×10^(-6)(平均109×10^(-6));U/Th值3.58~34.4(平均15.1);为富U沉积建造,为后期铀矿成矿提供了良好的物质来源。

The Longwan uranium deposit is located in the central part of the Guizhou-Northwest Hunan metallogenic belt.The deposit is mainly hosted in carbonaceous mudstone and siliceous rocks,and is the first large-scale carbonaceous-siliceous-argillaceous rock type uranium deposit prospected in Guizhou province.The siliceous host rocks have Al/(Al+Fe+Mn)ranging from 0.16 to 0.41 with an average of 0.25,Fe/Ti ranging from 33 to 126 with an average of 73,MnO/TiO_(2) from 0.16 to 0.74,and MgO/Al_(2)O_(3) from 0.67 to 4.21,with an average of 1.78.It shows that the rocks were formed in the continental and oceanic high-salinity seawater deposits with the characteristic of typical hot water deposition.The LaN/YbN ratios of the siliceous rocks range from 1.27 to 11.7 with an average of 4.99,indicating a continental margin and deep sea basin.The siliceous rocks have LaN/CeN ranging from 1.72 to 2.99 andδCe from 0.41 to 0.74,indicating a sedimentary environment of open basin and continental margin.The V/(V+Ni)ratios(0.88 to 0.98,with an average value of 0.93)of the siliceous rocks indicate a static sea environment.The larger than unit U/Th values(3.58 to 34.4 with an average of 15.1)of the siliceous rocks indicate a hydrothermal sedimentary origin.The zircon grains in the siliceous rocks have obvious oscillating zoning,with Th/U ratios greater than 0.1,positive Ce anomalies,negative Eu anomalies,LREE depletion,HREE enrichment,and lowΣREE,indicating that the zircon grains in the siliceous rocks are of magmatic origin.The U-Pb ages of zircon grains from the siliceous rocks are mainly concentrated in Mesoproterozoic,Late Proterozoic,Ediacaran-Cambrian,indicating that three periods of tectonic activities in the area.The siliceous rocks are U-rich,which have high U contents ranging from 27.0×10^(-6) to 193×10^(-6),with an average of 109×10^(-6),Th contents of 5.30×10^(-6) to 9.09×10^(-6),with an average of 7.42×10^(-6),and U/Th ratios from 3.58 to 34.4,with an average value of 15.1,and may provide uranium for the later period U mineralization.