Petrographical and geochemical study of the Eboundja nepheline syenite–derived regolith (Southwest Cameroon): evidence of changes at mineral scale during tropical weathering

Weathering in tropical environments leads to significant changes in the chemistry and texture of primary rocks detectable at both macroscopic and microscopic scales in the resulting weathered materials. This study examines the petrography and geochemistry of nepheline syenite and its derived regolith in Southwest Cameroon, focusing on the mineral-scale responses to tropical weathering. The nepheline syenite protolith consists of microcline, nepheline, plagioclase, amphibole, zircon, magnetite, ilmenite, allanite, pyrochlore, and biotite. Key weathering mechanisms include dissolution, oxidation, chemical replacement, and microfracturing. These processes interact such that all primary minerals, except zircon, are transformed into secondary minerals in the pedolith. Due to its chemical stability, zircon is primarily affected by physical weathering processes such as micro-fracturing and abrasion. Microfracturing plays a crucial role in accelerating weathering intensity in the early stage. Notably, it facilitates the alteration of pyrochlore, the principal primary Nb-bearing mineral, from Ca-Na-F-rich to Ca-Na-F-poor and Sr-rich variants. In the weathered materials, Ti- and Fe-Ti-oxides (ilmenite and leucoxene) are identified as the major Nb carriers, potentially hosting the Nb released from complete dissolution of pyrochlore grains. Niobium is further enriched in weathered ilmenite through oxidative weathering. The humid, high water activity condition also causes direct transformation of biotite and chlorite to kaolinite, gibbsite and goethite. Overall, this study highlights the complex mineralogical and geochemical process, potentially facilitating Nb mineralization, during tropical weathering.