UAV-based gamma-ray spectrometry for lithological mapping at the Sea Point Contact, South Africa

Unmanned Aerial Vehicle Gamma-Ray Spectrometry (UAV-GRS) revolutionises near-surface radiometric surveys with fine-scale spatial resolution and logistical flexibility. This study employed the Gamma Radiation Airborne Detection (GRAD) system, to quantify primordial radionuclides (⁴⁰K,²³⁸U,and ²³²Th) across the geologically complex Sea Point Contact, South Africa. Full-spectrum analysis (FSA) was used to the in situ gamma ray spectra to derive the radionuclide activity concentrations, followed by spatial interpolation to map lithological variability. Uranium (U) activity concentrations show a systematic increase from the northern granite (66.24 ± 24.7Bq/kg) through the metasedimentary rocks (82.01 ± 22.01 Bq/kg Bq/kg) and southern granite (92.65 ± 23.22 Bq/kg), reaching peak values in the microgranite (102 ± 21.76 Bq/kg). In contrast, thorium (Th) activities display more limited variability (39.53-53.47 Bq/kg), reflecting primary mineralogical control, while ⁴⁰K activities increase moderately toward the south (1287.1-1395 Bq/kg), consistent with progressive enrichment in K-bearing phases. Activity ratios provide further insight into magmatic evolution. Low Th/U ratios in the southern granite (0.57 ± 0.22) and microgranite (0.62 ± 0.26), indicate strong U enrichment and decoupling from Th and K during late-stage magmatic differentiation. Higher Th/U and Th/K ratios in the northern granite and Malmesbury metasedimentary rocks reflect more primitive compositions and greater retention of Th in refractory accessory minerals. Collectively, the radiometric data and activity ratios document a north-south progression in magmatic differentiation, culminating in the emplacement of a highly evolved, U-enriched microgranitic intrusion, and demonstrate the effectiveness of the GRAD system for resolving geochemical gradients across intrusive contacts at outcrop scale.