火山岩型铀矿床的基本特征、研究进展与展望

The characteristics,research progresses and prospects of volcanogenic uranium deposits

火山岩型铀矿床主要分布在环太平洋带、阿尔卑斯-喜马拉雅山带、俄罗斯西伯利亚地台南部,形成于白垩纪、古近纪和新近纪等三个时期。其形成过程主要包括铀从源岩中萃取出、热液流体的运输、以及物理化学环境的变化导致铀的沉淀。铀的源岩一般是中酸性-酸性岩石,通常呈碱性、准铝质至过铝质,富硅质、富钾、贫钙,富集U、F、Nb、Ta、Zr、Th和Rb等元素,而亏损Mg、Fe、Ti、P、Ba、Sr、La和Eu等元素;与之有关的火山岩和次火山岩多数显示出A型花岗岩特征。铀通常赋存于副矿物或火山玻璃中,因此,铀源岩发生蚀变、副矿物发生蜕晶化是成矿物质释放的关键。火山岩型铀矿床中铀主要以碳酸盐和氟络合物离子形式迁移,铀的迁移形式对其沉淀机制有着重要影响。成矿流体通常是大气降水和岩浆水的混合,成矿温度集中在100~350℃、盐度一般小于15%NaCleqv。铀沉淀机制主要包括还原作用、pH值升高和沸腾作用。火山岩型铀矿床常伴生Mo、Be、Ag、Cu、Pb、Zn、Hg和F等元素。未来火山岩型铀矿成因机制的研究应加强成矿物质来源、铀与伴生元素成因关系研究,这不仅有助于提高对火山岩型铀矿床成矿过程的认识,也可为勘查找矿工作提供理论帮助。

Volcanogenic uranium deposits are mainly distributed in the Circum-Pacific belt,Alpine-Himalayan belt,and Southern Siberian Platform of Russia,and they usually formed in three periods:Cretaceous,Paleogene,and Neogene.Formation of volcanogenic uranium deposits is dependent on extraction of uranium from felsic rocks,transport by hydrothermal solutions,and deposition induced by chemical or physical changes.The source rocks are generally acid and intermediate-acid,show alkaline and meta-aluminous to peraluminous characters,and are enriched in U,F,Nb,Ta,Zr,Th and Rb,and depleted in Mg,Fe,Ti,P,Ba,Sr,La and Eu.In addition,most volcanic and subvolcanic rocks show an A-type geochemical signature.Uranium is generally hosted in accessory minerals or volcanic glass.Therefore,alteration of source rocks and metamictization of accessory minerals are the key factors controlling U release.Uranium is chiefly transported as fluoride and carbonate complexes in volcanogenic uranium deposits.The ore-forming fluids are the mixing of meteoric and magmatic waters,with temperatures of 100~350℃ and low-middle salinity(<15%NaCleqv).Deposition mechanism of U mainly includes increasing pH,reduction,and boiling.Volcanogenic uranium deposits are usually enriched in Mo,Be,Ag,Cu,Pb,Zn,Hg and F.Magma rich in F may be an important factor in controlling the enrichment of these associated elements in volcanogenic uranium deposits.This paper suggests that it is necessary to further strengthen the research on the sources of ore-forming materials and the genetical links between uranium and associated elements,which may help to understand the process of the uranium mineralization and to support the ore exploration in the future.