Abstract
Bauxite-bearing laterite of Boknur-Navge plateau, Belgaum district, Karnataka, exhibits textural, mineralogical and geochemical features of both syngenetic and epigenetic origin. While the syngenetic features of the lateritic residuum developed during a major episode of residual weatheringof'the source rock (Deccan basalt), the epigenetic features originated during an episode of secondary alteration of the pre-existing lateritic material.
Residual weathering of basalt took place in three stages and culminated in the formation of a well-differentiated lateritic residuum consisting of vermicular laterite and massive (textureless and pisolitic) bauxite. Lateritic weathering, under the activity of slightly acidic meteoric waters, involved leaching of ∼90% SiO2, ∼60% Fe2O3 and almost all the K, Na, Ca and Mg from the weathered zone leading to the relative enrichment of Al. Part of the dissolved Si, Fe and AI of the circulating waters were subjected to local migration and reprecipitation, resulting in the segregation of AI-rich (bauxite) and Fe-Si-rich (laterite) units within the laterite profile.
Secondary products derived from the alteration of the bauxite and laterite show zonat variation in their mode of occurrence, textures, mineralogy and chemical composition. During the secondary alteration, the bauxite and laterite of the ∼10 m thick bauxite-bearing laterite zone were subjected to 1) silication and ferrification at the upper horizon, 2) silication at the intermediate horizon, and 3) minordesilication at the lower horizon. Among the three types of alterations, silication is widespread. The 'late-or post-bauxite' episode of alteration of the lateritic residuum was probably brought about by the action of highly acidic meteoric waters charged with Si.
