Structure-controlled Uranium + REE mineralization in low temperature basinal brine hydrothermal system at the contact of Kaladgi Basin and Peninsular Gneissic Complex, South India

The Kaladgi Basin, an E–W trending intracratonic basin in the northern part of the Dharwar Craton, preserves favourable structural and stratigraphic conditions for sandstone-hosted and unconformity-related U–REE mineralization. In the study area, the Neoproterozoic Cave Temple Arenite (CTA) of the Badami Group unconformably overlies deformed Mesoproterozoic rocks of the Bagalkot Group. The crystalline basement of the Kaladgi Supergroup comprises Meso- to Neoarchaean Peninsular Gneiss and the Chitradurga Greenstone Belt. This association of cratonic basement, schist belt, and basin-margin fault and fold systems provides an excellent structural framework for hydrothermal fluid circulation and mineralization.

Detailed thematic mapping at 1:25,000 scale in the Ramdurg–Suriban sector reveals that NNW–SSE–oriented Dharwarian stress generated a series of anticlines and synclines involving the Saundatti Quartzite, Malaprabha Phyllite, and Yaragatti Argillite, as constrained by conjugate fracture analysis and S–C fabric development. An E–W trending tectonic fault defines the contact between the Peninsular Gneissic Complex and Saundatti Quartzite, with comparable faulted contacts also developed within the Bagalkot Group. Intense faulting resulted in silicification, chalcedonic brecciation, and pervasive hydrothermal alteration along these contact zones. Transverse normal faults with associated brecciation accommodate strain related to the main E–W structure and indicate episodic reactivation of the basin architecture.

Fusion ICP–MS analysis of 20 bedrock samples collected proximal to these fault zones shows U₂₃₈ concentrations exceeding twice the threshold values of National Geochemical Mapping (NGCM) stream sediment sample. Uranium enrichment is spatially associated with Malaprabha Phyllite, first-cycle CTA, and silicified banded hematite quartzite veins of the Hiriyur Formation. Chondrite-normalized (La/Yb)_n versus (Eu/Eu*)_n systematics indicates a dominantly low-temperature basinal brine hydrothermal system characterized by low (La/Yb)_n <25 and negative Eu anomalies. Redox-sensitive (Ce/Ce*)_n versus (Eu/Eu*)_n plots further indicate reducing fluid conditions. In contrast, quartz–chlorite veins developed within sheared Malaprabha Phyllite and younger dolerite record comparatively higher-temperature fluids, marked by Eu²⁺ mobilization ((Eu/Eu*)_n > 0.8) and negative Ce anomalies. These results suggest that reactivated, structure-controlled tectonites acted as effective fluid pathways, with the TTG-dominated Peninsular Gneissic Complex serving as a likely uranium source and contributing to localized U–REE mineralization along the basin margin.