GENESIS OF GOLD MINERALIZATION IN ATTAPPADI, SOUTHERN GRANULITE TERRAIN, INDIA

Abstract

In India more than 99 percentage of the total primary gold is from a vast expanse of over 4,0000 sq.km area covering major part of Karnataka, Andhra Predesh, Tamil Nadu and Kerala. Therefore, it is imperative to fully assess the gold potential of the gold fertile land of southern India. Traditionally, high grade metamorphic settings have been regarded as unfavorable for hosting gold deposits. As a result, exploration as well as research has been biased towards amphibolite facies and lower grade metamorphic settings in Precambrian terrains. However, now it is becoming increasingly clear that gold deposits do occur in granulite facies terrains and these areas are unexplored. This is an attempt to look at the evolutionary history of gold deposits in Southern Granulite Terrain using geochemical, fluid inclusion study and taking into consideration of the existing knowledge from published structural and geochronological studies. Metavolcanics and metasedimentary rocks designated as 'Attappadi supracrustals' occur as enclaves and remnants within the gneisses exposed in the crustal scale Bhavani shear zone. The assemblage of rock types such as metapyroxinite, talc-tremoliteactinolite schist, amphibolites, Banded Iron Formations (BIF), sillimanite/kyanite bearing quartzite andfuchsite quartzite in Attappadi represents a greenstone belt setup. Geochemical studies of metavolcanics show that the Attappadi greenstone belt consists of Fe-rich tholeiites. This metavolcanics (tholeiitic composition) intercalated Algoma type BIF. The chondrite-normalized REE patterns of low K-tholeiite of Attappdi exhibit enrichments in LREE with respect to HREE. The possible reasons for overall enrichment of LREE reflect the composition of an enriched source EMORB or related to metasomatic enrichment in a hydrothermal system. The BIFs show LREE enrichment xii with a striking positive Eu anomaly. Possible source materials of positive Eu anomaly of BIF have been attributed to a hydrothermal activity in oceanic environment. Considering various criteria like lithological assemblages, geological setting and geochemistry it is proposed that at least a major part of the volcanism of the Attappadi supracrustal sequence must have evolved in a spreading center tectonic setting. The protoliths of gneisses in Attappadi are of monzodioritic in composition, they are intruded into the Attappadi supracrustals during the melting of lower crust in a convergent tectonic setting and crustal thickening possibly associated with a subduction related processes, during the late stage of greenstone belt deformation have provided a favorable geochemical environment for gold mineralization under the conditions of deformation and metamorphism. The mineralized zones typically occur within or in the vicinity of regional, crustal-scale deformation zones with a brittle to ductile type of deformation.The veins are concentrated in a 20-to 30-km wide 30-km long, linear NE-SW trending zone. The regional-scale sigmoidal pattern of the vein arrays strongly suggests that this broad zone acted as a regional scale shear zone. Gold is intimately associated with sulfide minerals, including pyrite, pyrrhotite chalcopyrite and galena in quartz veins. One or possibly two mineralizing events appear to have deposited gold in Attappadi greenstone belt. The first stage gave rise to a mineral assemblage consisting of simultaneous pyrite and gold deposition and followed by a late stage deposition of chalcopyrite and galena filling microfractures in quartz. Fe-rich tholeiite possibly under different physical conditions must have provided required sulfur and gold to hydrothermal fluid in Attappadi greenstone belt xiii Fluid inclusion study of gold-quartz veins in Attappadi area provide good evidence of fluid chemistry, depositional environment, and origin of mineralizing fluids in this deformed terrain. The mineralizing fluids have relatively low salinity (3- 6wt%NaCl eq.), consistent density ofC02 (0.6-0.7 g/cm3) and H2-0-C02rich. During the late stage of Attappadi greenstone belt deformation and metamorphism, the circulating hydrothermal fluids (H20-C02 fluid) were responsible for the breakdown of ferromagnesian minerals and release of silica which along with gold from the tholeiite rocks formed the quartz veins within the shear zones. This is correlated to the higher gold content available in the mafic rocks and also corroborated by the spatial proximity of the auriferous quartz veins to them. Alternatively magmatic fluid can also be considered as possible source for vein-type gold deposit in Attappadi, mainly because of the widespread distribution ofgranitic intrusive in the supracrustals. However, less saline and H20-C02 fluids in inclusions present in quartz veins have relatively consistent composition throughout Attappadi region suggesting a regional uniform, homogeneous fluid source related to metamorphism. The combination of the fluid inclusion and other data suggest a pressure-temperature range of ore formation of the order of 250-300°C and 2.5 kb. The close association of gold with sulfide minerals within quartz veins indicates that gold was transported as bi sulfide complexes. The phase separation due to the lowering of lithostatic pressure during regional upliftment caused fluid immiscibility which has been proposed as the principle mechanism for gold deposition in Attappadi greenstone belt. Summing up the source of fluids, the Attappadi greenstone belt constitutes orogenic gold deposits that formed by metamorphic fluids from accretionary processes and generated by prograde metamorphism and thermal re-equilibration of subducted volcano-sedimentary terrains