Mineralogy and Geochemistry of Clay Deposit of Gadag Greenstone Belt, Dharwar Craton, India:Implications for Residual Deposition

The clay deposit occurs near Majjur-Varvi area, which is a large syncline, called as the Majjur syncline, bounded to the NE and SW by steep thrust faults, namely Sortur and Suganhalli discontinuities of the well-known Gadag greenstone belt, Dharwar Craton in southern India. The lowest part of the stratigraphy of this area comprises schistose metabasalts, followed by felsic metavolcanics, conglomerates and greywackes. The clay bodies occur within the felsic metavolcanic rocks. The X-Ray diffraction and Infra-Red studies indicate that kaolinite is the dominant clay mineral, with subordinate illite. Quartz and plagioclase are the associated minerals. The clays are characterized by higher contents of SiO₂, CaO, Na₂O, K₂O, MgO and lower contents of Al₂O₃ compared to kaolinite mineral, which could be due to the presence of quartz, plagioclase and illite with these clays. Clays occur as non-linear horizontal bodies which do not support the hydrothermal process for their formation. While the cross sections in the quarries resemble shale. The observed gradation from top to bottom is soil, clays and a weathered zone with fragments of precursor rock, which essentially indicates in-situ development of the weathered or residual profile. Plagioclase was altered to clay while quartz was dissolved into solution and carried away from the site of weathering. Kaolinite clays in the study area occur as residual clay bodies that were formed by in-situ chemical weathering of felsic metavolcanics under tropical to sub-tropical humid and acidic conditions.