相山矿田铀的中和还原成矿作用

THE NEUTRALIZING REDUCTION MINERALIZATION OF URANIUM IN THE XIANGSHAN URANIUM ORE FIELD

该文以相山矿田为例介绍了铀的中和还原成矿作用的概念和机理。现已证明铀矿床不仅可以形成在还原显著增加的氧化－还原过渡带，而且可以形成在没有还原剂增加或溶液Ｅｈ值下降的中和作用带。实际上，中和作用是大多数情况下导致铀还原沉淀的关键因素。中和还原成矿作用的思想为研究铀成矿机理提供了一条新的思路。

With case study of the Xiangshan uranium deposit, the present paper has illustrated the concept and mechanism of neutralizing reduction mineralization. Uranium has two main valences, i.e., Ⅳ and Ⅵ, with U(Ⅳ) tending to precipitate as uranium minerals characterized by low solubility, but U(Ⅵ) exhibiting strong mobility in solutions. Hence, studies of mechanism of uranium mineralization used to focus on the process of reduction and the reducing agents. However, a lot of hydrogeochemical data from uranium deposits show that the environments for the dissolution and migration of uranium vary from acid, neutral to alkali media, while the environments for mineralization are almost always limited to neutral media, that is, almost all uranium deposits are formed in a near neutral environment. When the reductive ability of uranic solution remains constant, the process of reduction and precipitation of uranium mainly caused by neutralization instead of by the increase in the reducing agents or the decrease in the Eh value of the fluid is termed neutralizing reduction precipitation, and the resulting mineralization is called neutralizing reduction mineralization. The Xiangshan uranium orefield was found in a Mesozoic volcanic basin in southeastern China, in which there exist soda metasomatic type and fluorite hydromica type uranium mineralizations. Fluid inclusion study shows that the hydrothermal solution for the soda metasomatic type mineralization was alkali in the pre ore satage (pH=8.86～9.24), and became near neutral(pH=6.66～7.21) in the ore forming stage. From the pre ore stage to the mineralization stage, the fluid for the fluorite hydromica type uranium deposits changed from weak acidic (pH≈5) to near neutral (pH=6.1～7.01). In summary, the geothermal solutions evolved from acidic or alkali to neutral during the process of uranium mineralization. The difference between the Eh value for water (Eh W) and the critical redox potential of uranium reduction precipitation (Eh U,C ) is termed relative redox potential between water and uranium (ΔEh W U ), which is a comprehensive geochemical parameter controlling the reduction and precipitation of uranium. If ΔEh W U >0, then uranium will migrate in solution; if ΔEh W U <0, uranium will be reduced and precipitated. Consequently, uranium deposits could be formed not only in the transitional zones with distinctly incresed reducing agents, but also in the neutralizing zones without increase in amounts of reducing agents or decrease in Eh of the solution. Neutralization is the key factor for the reduction and precipitation of uranium in most circumstances. The idea of neutralizing reduction mineralization furnishes a new way to the study of mechanism of uranium mineralization.