ABSTRACT

Uraninite is commonly smaller than 1 mm and widespread in felsic or alkaline igneous rocks as accessory minerals. However, abundant coarse-grained euhedral uraninites, up to 1 cm in size, are intergrown with sphene, apatite, zircon, and some other accessory minerals in a quartz vein from the Haita area of the Kangdian region, China. Although previous studies have examined uraninite in detail, the genesis of the uraninite-rich quartz vein remains controversial. In this study, we investigate the relationships among euhedral single-particle apatite (ApA), aggregated apatite (ApB), and uraninite in a uraninite-rich quartz vein. TESCAN integrated mineral analysis, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry were used to investigate the nature of the ore-forming fluids of U mineralization. The two types of apatites have similar major element compositions and rare earth element patterns, whereas trace elements, such as Mn, U, and Cu, exhibit different characteristics, indicating a two-stage evolution of the ore-forming fluids. In addition, both types of apatite are fluorapatite, indicating high F concentrations in the ore-forming fluid, which favors U mobility. Changes in the concentrations of multivalent elements (such as Mn, Eu, and Ce) in the two types of apatite indicate that a slow increase in the oxygen fugacity promotes megacrystalline uraninite formation. A significant increase in the U concentrations from ∼46.56 ppm in the ApA to ∼7,169.12 ppm in the ApB likely represent an enrichment of U in the fluid. This study provides a new geochemical basis for understanding the mechanism of megacrystalline uraninite formation.