Broad Geological features

The following are the broad geological domains of the Southern Region.

(i) Archaean Basement Rocks (>2500 Ma): The basement rocks  comprise of Archaean granulite facies containing quartzites, garnet-sillimanite gneiss, marble, calciphyres, amphibolites and charnockites in Tamil Nadu, Kerala, Andhra Pradesh,  Karnataka and Goa. The peninsular gneisses, which are mainly composed of migmatites and banded grey granites are fairly homogeneous and are exposed over large tracts in these states.

(ii) Late Archaean  Dharwar Supergroup rocks: They comprise piles of volcano-sedimentary sequence broadly divisible into a lower Bababudan and an upper Chitradurga Group. The Bababudan Group is characterized by platformal sediments with quartz pebble conglomerates, pebbly quartzite and fuchsite quartzite and banded magnetite quartzites which were followed by sub-aerial mafic volcanics. The Chitradurga Group is typified by geosynclinal sediments with subordinate volcanics and well developed linear tracts of limestone, manganese and associated iron formations. They occur mostly in Karnataka and parts of Andhra Pradesh.

(iii) Lower Proterozoic Closepet Granites: Granites of this age extend in a north-south direction as a 50 km wide narrow belt. This belt of younger potassic granite is believed to mark a major geo-suture separating two distinct crustal blocks of Archaean age. The  western block is characterized by a number of well developed low grade granite-greenstone belts with their iron-manganese ores and the eastern block is marked mainly by younger gneisses of granitic and granodioritic composition enclosing within them a number of narrow, linear bands of auriferous schist belts.

(iv) Middle to Upper Proterozoic Cuddapah Supergroup rocks and their equivalents such as Bhima Group & Kaladgi Group of rocks: These are the most significant geological domains of Indian geology. The Cuddapah basin comprises a sedimentary thickness of 12 km and  volcanic sequences in the form of sills and dykes, which are resting on the Archaean peninsular gneissic complex marked by a pronounced Eparchaean unconformity. This basin comprises of rocks of Cuddapah Supergroup which includes Kurnool Group.

The Kaladgi and Bhima basins occur in the northern parts and extend below the Deccan Traps. These basins comprise of rocks of clastic/ chemogenic origin.

(v) Deccan trap of  Mesozoic-Tertiary age: These formations spread over very small parts of the Region and overlie the northern extensions of the Dharwar craton. 

(vi) Younger basins (Mesozoic–Tertiary): These sediments include Gondwanas of Palar basin, Cretaceous rocks of Tiruchinapally, Cuddalore Sandstone, Warkala beds, Quilon beds etc.

(vii) Beach and Inland Placers : They are part of Quaternary group of rocks. The beach and inland placers of Tamil Nadu and Kerala host some of the richest deposits of ilmenite, monazite, rutile, garnet, zircon and sillimanite.    

Summary of Investigations : Important finds

A major part of radiometric surveys were targeted in locating palaeoplacer QPC type in Late Archaean Dharwars and other deposits in Proterozoic basins. Some of the uranium occurrences identified during the period are :

Walkunji :  It is located northeast of Mangalore in South Kanara district of Karnataka. The host rocks are oligomictic quartz pebble conglomerates resting on Archaean peninsular gneisses. It is a palaeoplacer QPC type of uranium mineralisation occurring intermittently over 2500 m strike length and within a vertical depth of 200m. The radioactive minerals identified include uraninite, brannerite and pitchblende.

Arbail :  It is located 53 km east of Karwar on Karwar-Hubli road in North Kanara district of Karnataka. The host rocks are quartz arenites resting over Archaean granites. It is also a palaeoplacer uranium deposit comparatively at younger stratigraphic horizon than  QPC type of Walkunji, occurring intermittently over 7 km strike length and within a vertical depth of 250m. The radioactive minerals identified include uraninite, uranothrorite, brannerite and thucolite. Gold upto 1.5 ppm is also reported in mineralized zone.

These occurrences are, however, economically not viable.

Tummalapalle - Rachakuntapalle : It is located 12 km south of Pulivendla in Cuddapah district of Andhra Pradesh. The host rocks are phosphatic, siliceous dolostones of Vempalle Formation of Cuddapah Supergroup  resting on Archaean granite gneisses.  It is a unique type of stratabound deposit hosted in carbonate rocks and occurs over 6600 m strike length and within a vertical depth of 275m. 

Gogi  :  It is located in Gulburga district of Karnataka. The host rocks are deformed limestones of Bhima Group and the basement granites. It is a structurally controlled vein type of deposit and occurs over 2000 m strike length and within a vertical depth of 180m. The investigation is in progress.

Besides the above, a number of uranium occurrences are located along the fractures in the basement rocks at Mulapalle, Sanipaya-T.Sundapalle, Madireddygaripalle in Cuddapah district, Andhra Pradesh. In addition to uranium, rare metal occurrences were located  associated with alkaline rocks of Tamil Nadu at Sevattur, Pakkanadu, Kullampatti, Narlapalli, Pungurthi in Salem district. Substantial concentrations of Nb-Ta were located at  Marlagalla, Mandya district and at Arehalli, Mundur from where recovery operations for columbite-tantalite have been completed.