Typomorphic Characterization and Geological Significance of Megacrystalline Uraninite in the Haita Area, Kangdian Region, Southwestern China

Megacrystalline uraninite within Neoproterozoic migmatites in the Haita area of the Kangdian region of China provides a unique condition for the investigation of uraninite typomorphism under high-temperature conditions. The present study represents the first systematic characterization of the typomorphic signatures and genetic significance of megacrystalline uraninite via optical microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XRS), and electron probe microanalysis (EPMA). The results show that uranium mineralization occurs as euhedral megacrystalline uraninite (black grains ≤ 10 mm) hosted in quartz veins, exhibiting frequent rhombic dodecahedral and subordinate cubic–octahedral morphologies. The paragenetic assemblage is quartz–uraninite–titanite–apatite–molybdenite. The investigated uraninite is characterized by elevated unit-cell parameters and a reduced oxygen index, with complex chemical compositions enriched in ThO2 and Y2O3. These typomorphic characteristics indicate crystallization under high-temperature reducing conditions with gradual cooling. Post-crystallization tectonic fragmentation and uplift-facilitated oxidation occur, generating secondary uranium minerals with concentric color zonation (orange–red to yellow–green halos). Mineralization was jointly controlled by migmatization and late-stage tectonism, with the breakup of the Rodinia supercontinent serving as the key driver of fluid mobilization and ore deposition. The data materialized in the present study improve our knowledge about uranium mineralization during continental breakup events.