Geochemistry of Stream Sediments and its Relation with Bedrock Geology in Parts of Sindhudurg District, Maharashtra

Geological mapping (1,25,000) and stream sediment sampling (1 km x 1 km grid) were carried out simultaneously in parts of Sindhudurg district, Maharashtra during the year 2001-02. The area is occupied by gneisses, granites, and low-grade supracrustals of possible Late Archaean age, Neo-Proterozoic Badami sediments, Deccan Trap basalts and extensive laterites.

This paper deals with the spatial distribution patterns of selected major oxides (Fe₂O₃, Al₂O₃, P₂O₅, MnO and MgO) and trace elements (Co, Ni, Cr, Zr, Sr, Rb and Ba) in stream sediments in relation to the underlying rock outcrops. The study area has neither Large-Scale anthropogenic activities nor any reported mineral occurrence. In such terrains, the study reveals that the spatial elemental distribution in stream sediments is mainly guided by the bedrock geology and chemistry. A gabbroic body in the central part of the area is reflected remarkably well in the high value stream-sediment contours of Co, Ni&Cr and matches fairly well with elevated values of Fe₂O₃, MgO and MnO Stream sediment samples over younger granites are rich in Zr, Ba and Sr. The elemental concentrations in stream sediments over gneissic rocks show moderately high values of Ba, Sr, Zr, Al₂O₃ and low values of remaining elements.

Cluster analysis of the data set reveals three groups within the elements discussed above, viz Group I (Cr-Co-Ni- Mg-Mn), Group II (Ba-Sr-Zr-Rb) and Group III (Al-Fe-P) Group-I elements are concentrated and depleted in ultramaficmafic rocks and acid rocks respectively while Group II elements are depleted in mafic rocks but ennched in acid and alkaline rocks Group-Ill elements show minor affinity towards both acid and basic rocks.

Therefore, in a terrain occupied by vaned rock units, stream sediment sampling is required to be dense enough to represent the changing lithology. A gneissic terrain requires a much denser sampling programme compared to an area occupied by Deccan Traps.