Geology and fluid geochemistry of the Na-metasomatism U deposits in the Longshoushan uranium metallogenic belt, NW China: Constraints on the ore-forming process

摘要

The Longshoushan belt in Gansu Province is a very important uranium metallogenic belt in Northwest China and contains a series of different types (magmatic, Na-metasomatism, supergenetic) of uranium deposits and occurrences. Of all these mineralization types, the Na-metasomatism type is the most predominant with great potential, represented by two Na-metasomatism uranium deposits (Xinshuijing and Jiling). Different from the fluorite-, illite-, quartz-dominated alteration minerals in other hydrothermal uranium mineralization system, the Na-metasomatism type uranium deposits are characterized by the unique albite-dominated (up to 90 vol%) alteration mineral assemblage, which is referred to albitite in many literatures. All the uranium orebodies in both the Xinshuijing and Jiling deposits are hosted within the metasomatic albitites, and controlled by the subsidiary faults of the regional Malugou Fault. The mineralization process in both deposits can be divided into four stages, namely: the Na-metasomatism stage, U mineralization stage, post-mineralization stage and supergene mineralization stage. The Na-metasomatism stage is predominantly composed of albite (with hematite staining), with minor chlorite and anatase, while the U mineralization stage is characterized by the occurrence of uranium minerals (mostly pitchblende), intergrown with some tiny albite, calcite, chlorite, pyrite, apatite, REE-minerals and sometimes hydrothermal zircons. The mineral assemblages of the Na-metasomatism stage and U mineralization stage are too intimately related to each other and might be the products of the evolution from the same ore-forming fluids. The post-mineralization stage is mainly the calcite-quartz veinlets or occur as aggregate clusters, while the supergene mineralization stage includes mineral assemblage of secondary uranium mineralization-clay minerals (e.g. kaolinite)-limonite. Mineralogical studies and comparison of major and trace elements assay results between fresh biotite granite (predominate host rocks), albitite and uranium ores suggest that the ore-forming fluids resulting in the formation of the Na-metasomatism alteration are oxidized and rich in Na, Sc, V, Cr, Co, Ni, U, Th, while the ore-forming fluids of the U mineralization stage are relatively reduced and rich in heavy rare earth elements, U, Th, PO34-. Fluid inclusion and calcite C-O isotope studies indicate that the ore-forming fluids are meteoric water-dominated, which circulated at depth (similar to 8 km, by fluid inclusion geobarometer) and scavenged uranium from the host biotite granites and metamorphic rocks of the Longshoushan Group. The ore precipitation is mainly attributed to physicochemical changes (f(O2), temperature, fluid composition) from fluid-rock interaction and further temperature decrease of the ore-forming fluids, while fluid boiling and fluid mixing processes are limited. The proposed ore-forming process is not only important for the understanding of the ore genesis for the Na-metasomatism mineralization systems in Longshoushan, but also highlights the significance of albite alteration, which could be used as an important indicator during further uranium exploration.