exploration and its environmental condition at Kothagiri
Adathurai is located about 5Km to the SW of Kothagiri (11� 25�00� 76� 51�00�) in the Eastern parts of the Nilgiris, falling in the Survey of India toposheet No: 58A/15. The Nilgiri hills massif, a tilted and uplifted segment of the late Archaean crust is made up of garnet and hypersthene-bearing enderbitic to charnockitic rocks with layers and bodies of mafic pyroxene-plagioclase rocks with or without garnet and enclaves of pyroxenitic rocks. Granulite facies metamorphism occurred about 2.5Ga ago and closely followed the emplacement of the igneous protoliths. The crustal segment evidently represents an early Proterozoic addition to the Archaean Dharwar craton in the north, and the separating Moyar shear zone a major suture zone. The Bhavani shear zone to the south is regarded as reworked Nilgiri-type crust. High grade metamorphism in the Bhavani and Moyar shear zones and the adjacent Dharwar craton is coeval with the granulite facies event in the Nilgiri hill massif. Estimates of metamorphic conditions in the Nilgiri hills and adjacent shear zones indicate temperatures between 700-850o C and pressures of 7 to 10 kb. Since gold-bearing quartz veins are localized within this shear zone, it is inferred that the gold metallogeny in Nilgiris is post-Archean and most probably of late Proterozoic age, co-relatable with late Proterozoic incipient charnockite formation and carbonate metasomatism. The Nilgiris covering an area of 2, in the Nilgiri district composed mostly of charnockites of the Archaean age. The area exposes an interbanded sequence of charnockite and Sathyamangalam group of rocks, their migmatitic derivatives namely garnetiferous quartzo feldspathic gneiss and fissile biotite, hornblende, biotite gneiss( Bhavani gneiss), and older intrusive complex of garnetiferous gabbro with pyroxenite and younger basic intrusives of basaltic, doleritic and gabbroic composition. Indicating a series of antiforms and synforms exposes detached outcrops, marked by the general strike in the direction of N10OW-S10OE to N40OE- S40OW, with dips ranging from 30O-50O towards S80OW-N50OW there. In all, three faces of deformation (F1, F2 and F3) and the corresponding structural manifestations could be deciphered in the area. A number of NNW-SSE trending transverse step faults and two major sets of steeply dipping joints viz, NNW dipping easterly, and ENE-WSW to E-W with dips on either side and also NNE-SSW joints with dip towards west are found Quartz veins with thickness of 0.85-1.90m and dipping at 28Oto 42O towards N80OW is emplaced in Charnockite. Hydrothermal alterations represented by chloritization, sericitization and carbonatization are widespread in the area, affecting almost all the litho types. Carbonatization occurs as finely disseminated ankerite grains disposed along the foliation plane, which suggests that carbonatization took place syn-tectonically within the shear-zones. Sericitization is characterized by the formation of white mica, particularly in zones of high strain. Gold in Nilgiris is restricted to primary gold associated with quartz veins and sulfides within quartz veins and Placer gold deposits within the stream gravel. The secondary and placer gold are thought to have been derived from the primary gold itself after processes like mobilization, chemical purification and re-concentration. Primary quartz veins containing gold and sulfides mainly occupy the mesoscopic shears and faults and cris-crossing fractures and also with in the hydrothermal alteration zones between the quartz vein and country rock associated with the major shear zones. A detailed geochemical study on the soil has been carried out and heavy metal concentration has been studied. At present the heavy metal concentration is well with in the standard limits in the surrounding areas. The gold mineralization and other heavy metals such as Cu and Hg are restricted to the mineralized zone. Hence at present there is no environmental hazard due to gold mineralization. However, commercial mining may increase the heavy metal concentration in this area in future.