31 / 2025-03-27 09:59:30

Reassessing salinity proxies in bauxite deposit: B/Ga ratio as a reliable indicator for paleosalinity of intensely weathering profiles

Bauxitization; Laterization; Sr/Ba ratio; B/Ga ratio; Weathering processes.

Theme 3: Earth-Life Co-evolution Through Time > Session 3a: Precambrian Earth and Life Co-evolution

Salinity-sensitive geochemical proxies have emerged as powerful tools for reconstructing paleosalinity. Indicators such as Sr/Ba ratios, B concentrations, and B/Ga ratios have been widely applied to fine-grained clastic rocks and mixed carbonate deposits. Recent studies have extended the use of these paleosalinity proxies to intensely weathered profiles, including bauxites, interpreting variations in B content and B/Ga ratios as indicators of freshwater-brine interactions during weathering. However, the reliability and underlying mechanisms of these proxies in such weathered systems remain unverified. In this study, we analyzed samples from modern and ancient (Carboniferous to Permian) bauxite profiles in South China to investigate the behavior of salinity-related elements (Sr, Ba, B, Ga) during bauxite formation and evaluate the applicability of these proxies in environments dominated by intense chemical weathering. Our mineralogical and geochemical assessments revealed that Sr/Ba ratios are unsuitable for paleosalinity reconstruction in weathering profiles due to Sr redistribution controlled by phosphate minerals (e.g., monazite, xenotime). For B and Ga, while boron exhibits high mobility during weathering, it is strongly adsorbed by clay minerals such as kaolinite and illite, whereas gallium stabilizes in Al-bearing phases during bauxitization. This distinct behavior enables B/Ga ratios to effectively record brine-influenced conditions during weathering. We demonstrate that B/Ga successfully identifies aqueous salinity regimes in South China’s bauxite deposits under intense weathering and propose a simplified response model elucidating its environmental discriminative capacity in bauxite metallogeny.

This study establishes B/Ga as a robust proxy for discriminating mineralization environments, providing a theoretical foundation for future bauxite prospecting. Our findings underscore the necessity of proxy validation in weathering-altered systems and offer a geochemical framework to enhance exploration efficiency in deeply weathered terrains.