Behavior of major, trace, and rare earth elements in an atypical lateritic profile overlying micaceous quartzites, Centre Cameroon: imprint of the parent rock structure

Abstract

An atypical lateritic weathering profile overlying micaceous quartzites in central Cameroon was studied to understand its morphology and the behavior of major and trace elements (including rare earth elements; REE). Mineralogical analyses were performed by X-ray powder diffraction (XRPD). Major and trace element geochemistry were determined using X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The studied micaceous quartzites consist of 1 to 2 cm thick quartzitic platelets separated by millimeter to multi-millimeter beds of ferromagnesian minerals (biotite, hornblende, garnet and, epidote), muscovite, graphite, and magnetite. According to chemical index of alteration, mafic index of alteration, index of lateritization, the A – Fe – Mg – Ca – Na – K diagrams, and the Weathering index of granite, the intense hydrolysis of ferromagnesian beds resulted in the genesis of soil unit consisting of quartz, muscovite, illite, kaolinite, anatase, and hematite which occur within rotten quartzites or coarse saprolites (CS). Thus, the weathering profile presents the following macromorphological organization: micaceous quartzite/CS-soil/CS-soil/CS-soil/CS-soil/ CS-soil/nodular materials/loose stony clayey soil. The pH-Eh diagrams indicate that formation of these soils within the saprolitic zone is the result of differential weathering that occurred under acidic-reductant conditions, which enhanced the rapid weathering of ferromagnesian mineral beds. The main pedogenetic processes along the weathering profile are translocation of REE, the transformation of primary minerals into neoformed-clays and Fe-oxide-hydroxides formation (associated mainly with light REE losses), and the addition of organic matter in surface horizons (associated with an accumulation of transition metals, high field strength elements, and heavy REE).