Genesis and evolution of the ore deposits in the early Precambrian greenstone belts and adjacent high grade metamorphic terrains of Peninsular India — A study in similarity and contrast

Abstract

Karnataka and the adjacent areas of Tamil Nadu and Andhra Pradesh in Peninsular India are characterized by sublinear ‘greenstone belts’ set in a tonalite-trondhjemite dominated gneissic terrain (mainly 3400-3000 Ma) skirted by a high grade metamorphic (amphibolite-granulite facies) belt in the south and the east. The greenstone belts are divided into the sedimentary-volcanic Sargur-Holenarsipur-Nuggihalli type (> 3000 Ma), the volcanic-dominated Kolar-Ramagiri-Hutti type (⩾ 3000 Ma) and the sediment-dominated Dharwar type (3000-2600 Ma), arranged in order of increasing areal coverage. The granulite facies metamorphism of the Peninsular Gneiss and associated supracrustals took place 2500–2600 Ma ago. The first type of greenstone belt is poor in ore content. These belts contain a few lean base metal deposits, a few chromite deposits and Ti- and V-bearing magnetite deposits in places. Locally banded magnetite deposits are also present. The second type of belt is known for its Au content. The Dharwar type of belt, of course, contains rich deposits of Fe and Mn and rare small deposits of Cu. Ore deposition in the high grade metamorphic terrain is also poor as in the Sargur type of greenstone belt. Whatever metallizations occur there are confined to the supracrustal enclaves. Differential metallization in the different segments is explained in terms of the differential development of the crust, the atmosphere and the biosphere.