Topography formation and bauxitization in a granulite terrain from the Eastern Ghats Belt, India – exploring the control of weathering

Abstract:

Undulating topography with high hillocks of khondalite and charnockite within the low lying quartzo-feldspathic gneiss and augen gneiss is the characteristic of granulite terrains where bauxite is believed to form by chemical weathering process, as a capping on a flat-topped khondalite hill. As the “East Coast Bauxite” deposit develops in the Eastern Ghats Belt (EGB) of India, an area under the EGB is selected to investigate the relation between weathering and granulite topography in the light of bauxitization. Petrographic studies reveal that the charnockite and QFG show very little alteration, with XRD and FTIR analyses indicating only minor montmorillonite formation in these rocks, suggesting their resistance to chemical weathering. On the other hand, khondalites show substantial alteration to clay minerals, with the proportion of clay minerals increasing with the height of the hillock and stabilize bauxite at hill tops. At lower elevations, kaolinite, montmorillonite and diaspore are formed in weathered khondalites; with increasing topographic height, the minerals illite, goethite, diaspore and gibbsite are the major components. Geochemical modelling of khondalite weathering using the software Geochemist’s Workbench (GWB) indicates that khondalites can be predicted to weather extensively under open system condition which is most likely to prevail at high elevations and hill tops, where rain water flow down-slope and cannot be retained in the system. These results indicate bauxite formation is directly related to the chemical weathering of the khondalite hills and explains the reason of highly weathered khondalite hill tops. Extensive weathering in open system condition defines that khondalite hills are present before the initiation of bauxitization process. Occurrence of highly elevated khondalite hills with high hillocks of more resistive charnockites within low lying but more resistance QFG indicates chemical weathering is not the main process to shape the granulite terrain topography. Mechanical weathering or neo-tectonic activity may the reason for the present undulating topography of different granulite terrains.