Laterization of basalts and sandstone associated with the enrichment of Al, Ga and Sc in the Bolaven Plateau, southern Laos

Abstract

This article reports geochemical behavior and enrichment of Al, Ga and Sc by laterization of basalts and sandstone in the Bolaven Plateau, southern Laos. The Bolaven Plateau consists of Neogene – Quaternary basalts and underlying Cretaceous sedimentary rocks. Laterites derived from the basalts and sedimentary rocks are developed on these parent rocks. With increasing the degree of laterization, loss of mobile elements such as Si, alkali elements and alkaline earth elements lead to the enrichment of immobile elements of Al, Ga and Sc relative to the parent rocks. Accordingly, Al2O3 contents range from 10 to 15 % in the basalts, from 16 to 30 % in the saprolites, from 17 to 48 % in the basaltic laterites and from 15 to 47 % in the sedimentary laterite. Ga contents range from 6 to 20 ppm in the basalts, from 9 to 46 ppm in the saprolites, from 24 to 83 ppm in the basaltic laterites and from 22 to 68 ppm in the sedimentary laterites. Sc contents range from 14 to 23 ppm in the basalts, from 14 to 45 ppm in the saprolites, from 14 to 69 ppm in the basaltic laterites and from 13 to 84 ppm in the sedimentary laterites. No signifiant difference is recognized in the enrichment of these elements between the basaltic laterites and sedimentary laterites. A similar geochemical behavior and positive linear correlation are recognized between Ga and Al and between Sc and Fe, suggesting that Ga and Sc exist by replacing Al and Fe in the laterites, respectively. Ga/Al ratios range widely but Ga is almost no depleted or slightly enriched relative to Al with increasing the degree of laterization. Sc/Fe ratios indicate that Sc may be slightly depleted relative to Fe by strong laterization.