Tectonics and mineralization in Sri Lanka

VITANAGE, P. W., 1985: Tectonics and mineralization in Sri Lanka. Bull. Geol. Soc. Finland 57, Part 1—2, 157—168. Over nine-tenths of Sri Lanka is underlain by Precambrian metasedimentary rocks, granitic gneisses and migmatites. The Highland Series (granulite facies), made up of typical metasedimentary rocks — quartzites, marble, white granulites and granulitic gneisses — along with charnockites and migmatites, occupies the central parts of the Island. The more argillaceous members of this series in the southwest are called the Southwest Group, with garnet—sillimanite—graphitegneisses, garnetiferous schistose rocks, marble, wollastonite—scapolite rocks and more basic charnockites. The rocks of the Highland Series are rich in mineral deposits. The younger Vijayan Series (almandine—amphibolite facies), consisting of microcline hornblende migmatitic gneisses, hornblende gneisses and calc-gneisses, occupies the northeast and east. The eastern Vijayan rocks in the northwest are predominantly pink-feldspar hornblende granites and granitic gneisses. These rocks are poor in mineral resources. The tectonic and structural pattern of the two Series differs considerably. The rocks of the Highland Series have been thrust into a series of anticlinal and synclinal structures, locally overturned and sheared, elongated domes and basins. Some of the commercially exploited minerals, e.g. graphite, are associated with the anticlinal structures, foliation joints and local lineaments in these structures. The rocks of the Vijayan Series, with their scarcity of economic minerals, show a marked contrast to the tectonic patterns of the Highland Series. The Vijayan rocks often exhibit irregular, albeit commonly oriented, E—W trends with steep foliation. Circular dome structures resembling igneous intrusions occur locally. These domes may prove to be potential sites for mineralization. Both these Series as well as the younger sediments — Jurassic deposits in the northwest and Miocene rocks in the north-northwest coastal belt and south-southeast — are intersected by a series of deep and shallow lineaments of Pre-Gondwana and Post-Gondwana times. Deposits of basemetals, copper and magnetite towards the northeast and along the western lowlands seem to be closely associated with some of these thrust or graben lineaments. The potential importance of these lineaments for basemetals, oil and gas and the tectonic control of the graphite deposits are discussed in the present paper.