The Northwest-Southeast Trending Granite-Gneiss-Migmatite Belt of the Precambrian Basement of Madagascar: Probable Signature of the Collisional Event in the Central-East Part of the East Africa Orogen

Abstract

Gold occurs in various quantities mainly in the central and southern parts of Sri Lanka. Although gold is recovered mainly from river sediments at present, it has been mined in the past also from a concordant quartz reef in central Sri Lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern Sri Lanka. The sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins, minor pegmatites and sulphide mineralizations mainly of pyrite, pyrrhotite and chalcopyrite. The mineralization of gold is exclusively in the Highland Complex rocks of Sri Lanka which represent Archaean-Late Proterozoic metamorphic complexes.In Madagascar gold is present in the Precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks, concordant quartz reefs and more recent discordant quartz veins. The concordant quartz veins and diffused gold are related to both Archaean and Proterozoic rock associations and the discordant veins which have been identified as Permo-Triassic are found mainly in the north within the contact zone of Proterozoic rocks and the Phanerozoic cover. Gold is also recovered form ancient and recent alluvial accumulations and lateritic earth. The mineralization of vein deposits is mainly gold-quartz and gold-sulphide associations, the sulphides being galena, chalcopyrite and pyrite.In south India, gold occurs in Archaean metamorphic units comprising of granite-greenstone complexes in the Kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic Wynad fields. In both cases gold occurs in gold-quartz veins which occupy fissures and shears within the Archaean metavolcanics. Gold also occurs in the Attapadi area of Kerala both in sediments and in primary epigenetic gold deposits which are hosted by Precambrian amphibolites and granitic gneisses. The Attapadi lode veins are predominantly structurally controlled and are closely associated with the Proterozoic Bhavani shear zone. Gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins, gold quartz carbonate veins and gold quartz sulphide veins. The sulphide minerals are mainly galena, pyrite, chalcopyrite and arsenopyrite.The rocks in the southern Madagascar, southern India and Sri Lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the Pan-African. The emplacement of gold mineralization mainly in Madagascar and south India can be correlated in the Archaean. The gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in Sri Lanka, as a Pan-African mineralizing event. It is also evident that CO2 rich fluids from a deep seated source has continued from Archaean and were still active during the Proterozoic being responsible for the Pan-African event of gold deposition particularly in the younger structures in these countries.