Role of syn-sedimentary biogenic and epigenetic hydrothermal uranium enrichment in the formation of sediment-hosted uranium mineralization: evidence from the Neoproterozoic Badami Group, Southern India

This study investigates sedimentary and hydrothermal uranium (U) mineralization in the Neoproterozoic Cave-Temple Arenite Member of the Kerur Formation, Badami Group, Karnataka, within the South Indian Shield (SIS). The studied succession comprises three fluvio-alluvial depositional cycles, with the middle cycle recording evidence of marine sedimentation. Uranium enrichment occurs across the fluvial sediments of the 1^st and 2^nd cycles, as well as the marine sediments of the 2^nd cycle. The higher grade of mineralization, however, is restricted to the lowermost fluvio-alluvial segment of the 1^st cycle, where significant hydrothermal enrichment is evident.

Three distinct modes of U-mineralization have been identified. The first one (the primary mode) represents syn-depositional, microbially induced sedimentary uranium accumulation within organic matter (OM)-rich clay laminae of marine shales, siltstones, and fine sandstones of the 2^nd cycle. This mode is characterized by microcrystalline uranium-(calcium)-phosphosilicate (UPS) phases, often intermingled with uranium titanates, and is associated with microcrystalline sedimentary pyrites, often clustered as framboidal aggregates, and kerogenous OM. Provenance analyses and petrography of U-bearing sedimentary rocks suggest U-sourcing from the Archean-Palaeoproterozoic granitic and felsic basement rock suites of the Dharwar Craton. A positive correlation (r = +0.8, ρ < 0.01; n = 26) is observed between bulk rock OM content (TOC%) and uranium enrichment in the primary mode. Micro-RAMAN spectroscopy confirms the association of UPS phases and sedimentary pyrites with OM-rich matrices and clay-rich organic laminae, while the carbon and sulfur isotopic analyses of the bulk reinforce the biogenicity of the host sediment. The enrichment of redox-sensitive trace elements like V, Mo, Cu, Co, Ni, and As within the clay- and OM-rich sedimentary rocks further indicates the presence of active redox cycling along with biogeochemical and paleoproductivity processes during the syn-to-meta-depositional phases.

The remaining two modes correspond to secondary, post-depositional hydrothermal uranium enrichment within the fluvial sandstones of the 1^st cycle and the fluvio-marine sedimentary rocks of the 2^nd cycle. These modes manifest as uranium phosphosilicate and uranium silicate phases, associated with hydrothermal pyrites in fractures, micro-veinlets, or intergranular patches. Unlike the primary, syn-sedimentary mode, the hydrothermal mineralization does not show any distinct correlation between uranium concentration and TOC%. Isocon mass balance further suggests that uranium, iron, and high field strength elements (HFSEs) were mobilized from these sediments hosting the primary mode, likely facilitated by organometallic ligands, such as siderophores associated with OM, during the epigenetic hydrothermal process under oxygenated hydrothermal conditions. Mobilized uranium was subsequently trapped by pyrites in hydrothermal fractures, forming the secondary modes of U-mineralization.

This dual mechanism highlights an initial microbially mediated, OM-induced uranium accumulation, acting as a vast, low-grade source for later hydrothermal remobilization and enrichment. The findings emphasize the interplay of depositional environments, microbial activity, and hydrothermal remobilization of biogenic accumulation in uranium mineralization, with implications for sediment-hosted uranium exploration.