Relationship between regolith maturity and K/Th ratio: insights from the Bosumtwi impact crater

The Bosumtwi crater, a 10.5 km-diameter meteoritic impact structure formed 1.07 Ma ago, formed on a target dominated by Paleoproterozoic metasedimentary rocks (ca 2.1 Ga) and intrusive bodies covered by a thick lateritic regolith. Airborne radiometric data and field observations reveal that new lateritic profiles have developed on the crater ejecta, which consists primarily of brecciated metasedimentary rocks and granitoids. The objective of this study is to determine whether airborne radiometric data can discriminate between mature lateritic profiles outside the crater and its ejecta and more recent lateritic profiles that have developed on the crater rim and ejecta. This study demonstrates that the log10(K/Th) values are negatively correlated with the Chemical Alteration Index, and that the distributions of log10(K/Th) values of lateritic profiles inside and outside the ejecta are clearly distinct. A map of log10(K/Th) derived from airborne radiometric data allows spatial differentiation between mature and recent regolith profiles. This map also led to the discovery of an outer ejecta layer, extending a few kilometres beyond the distal ridge, that shares alteration characteristics similar to those of the inner ejecta layer. The Bosumtwi impact crater, previously recognised as a rampart crater analogous to those observed on Mars, appears to be comparable with double-layered-ejecta rampart craters. On Mars, this subtype is less common than ‘single-layered-ejecta rampart craters’ but is preferentially concentrated in topography lows likely associated with volatile-rich sedimentary deposits. From a methodological perspective, this study demonstrates that log10(K/Th) maps derived from airborne radiometric data offer a robust tool for exploration geology, particularly for revealing subtle variations in low-K regions (<1 wt%) and mapping variations in regolith development and surface rock alteration.