661铀矿床铅同位素组成与成矿物质来源探讨

Lead Isotope Compositions and Its Significance for Ore-forming Material of No. 661 Uranium Deposit,Zhejiang Province,China

661铀矿床位于浙江省境内,是赣杭铀成矿带东段重要的火山岩型铀矿床之一,矿体赋存于晚中生代磨石山群九里坪组流纹岩中。对采自该矿的与铀成矿相关的方解石进行了系统铅同位素测定。结果表明,该矿床不同阶段方解石具有一致的Pb同位素组成和较窄的变化范围,暗示成矿过程中铅可能来自于同一的且较为均一的铅源,少量晚期样品仅表现为206Pb/204Pb的增大,指示后期流体可能通过淋滤作用加入了部分早沉淀的铀。通过与基底陈蔡群变质岩和磨石山群火山岩铅同位素组成对比发现,矿石具有与磨石山群火山岩一致的铅同位素组成和变化趋势。在铅构造模式和铅同位素Δγ-Δβ成因图解中矿石和火山岩均显示出造山带铅或地壳与地幔混合的俯冲带岩浆作用铅特征,这与华东南地区的火山岩成因上主要由中下地壳熔融,并不同程度加入了地幔组分的结果一致。这些证据表明火山岩铅为该矿床的主要铅源。

No.661 uranium deposit,located at Zhejiang Province,is one of important volcanic rock-hosted uranium deposits in China.It is located eastern of Gan-Hang uranium belt,and its uranium mineralization occurs in the rhyolite of Jiuliping Formations.Lead isotopic composition of calcite from this deposit was systematically analyzed in this paper.The results showed that the lead isotopic compositions of calcite of different stages were similar and fall in a narrow range,indicating a uniform and homogenous origin of lead isotope.Ore-forming fluid may eluviate a little of uranium precipitated at early stages,resulting in an increase of 206Pb/204Pb in part of calcites of late stages.In comparison with that of metamorphite from Chencai Group and volcanic rocks from Moshishan Group,the lead isotopic composition of calcite and its variation tendency are the same as the volcanic rock but different from metamorphite.Based on diagrams of lead tectonic pattern and Δγ-Δβ genetic classification,almost all of lead isotopes from calcites and volcanic rocks fall into orogenic belt lead,which is consistent with the fact that the source material of volcanic rocks in Southern China originated mainly from partial melting of rocks in middle-lower crust and partly of the mantle origin.All these evidences showed that lead isotope composition of this deposit was derived from volcanic rocks.