铀的地球化学性质与成矿——以华南铀成矿省为例

铀是强不相容元素,随着岩浆演化而不断富集,在岩浆演化末期受结构氧增加影响进入独居石、磷钇矿等副矿物中。岩浆演化通常无法直接形成达到工业品位的铀矿床。铀是对氧逸度敏感的变价元素。在表生风化过程中岩体(层)中的铀被氧化为UO_2~(2+)而极易溶解进入水体中,并可在还原环境沉淀而富集成矿,氧化还原界面是找矿的理想选区。大气水可通过断裂构造系统进入一定深度,并受热源作用形成高氧逸度的热液而萃取出岩体(层)中的铀在还原位置沉淀富集形成矿床。新元古代氧化事件以及Marinoan冰期结束使得表生风化过程中更多的U进入水体;而寒武纪生命大爆发,易在沉积盆地底部形成还原环境,有利于U的沉淀富集。受上述三方面因素控制,在华南形成了广泛分布的富铀黑色页岩层,并被之后的沉积物覆盖,成为华南各型铀矿床的铀源层。印支期构造运动使部分富铀黑色页岩层发生部分熔融形成了富铀的S型花岗岩,该类岩石亦是之后铀成矿作用的铀源岩。燕山运动后期华南发生伸展构造背景下的岩浆热事件为以大气水为主的高氧逸度热液的形成并作用于铀源岩(层)提供了有利条件,促使华南各类型铀矿床开始在白垩纪集中形成。

The geochemical behavior of uranium and mineralization: South China uranium province as an example

Uranium is a strong incompatible element, which is continuously enriched with the evolution of magma. At the end of magma evolution, uranium is affected by the increase of structural oxygen and enters accessory minerals, such as monazite and xenotime. Magmatic evolution usually can''t lead to the formation of uranium deposits. Uranium is a variable valence element which is sensitive to oxygen fugacity. During supergene weathering process, uranium in rock is oxidized to UO₂²⁺ and easily dissolved into high oxygen fugacity fluid, which is then precipitated and enriched in reducing environments, favorable for mineralization. Influenced by the Neoproterozoic Oxidation Event and the end of Marinoan ice age, more U entered water during supergene weathering. The Cambrian Explosion resulted in reducing environment in sedimentary basins, forming black shale rich in organic matter, which was favorable to the precipitation and enrichment of U, responsible for the formation of Sinian and Lower Cambrian uranium-rich black shale in South China. The uranium-rich black shale was the main uranium source for uranium deposits in South China. The Indosinian tectonic movement caused partial melting of some uranium-rich black shale, forming S-type granites, which were also the uranium source for the subsequent uranium mineralization. The tectonic-magmatic thermal events in an extensional setting in South China during the Late Yanshanian Movement provided favorable conditions for the formation of high oxygen fugacity hydrothermal fluids, which resulted in the formation of various types of uranium deposits in South China in Cretaceous. High oxygen fugacity hydrothermal fluids mainly composed of meteoric water can migrate through fissure system and extract U from uranium source rocks by oxidation of U⁴⁺ to U⁶⁺. The reductive materials related to organic-rich black shale, basic dyke and volcanic degassing are responsible for the precipitation and mineralization of U.