Iron-Formation of the Chiknayakanhalli Greenstone Belt, Karnataka, India

Abstract

The iron-formation of the Chiknayakanhalli (CN Halli) greenstone belt constitutes the youngest lithological unit of the late Archaean supracrustal geosynclinal sequence (3 to 2.4 b.y. old) which unconformably overlies the older (3 b.y.) granitic gneisses. They are consistently located in the core parts of the synclinal fold structures and occur over-lying either carbonates or chlorite schists. There are four main sedimentary facies, viz., oxide - (OF), mixed oxide-silicate - (MOSF), mixed oxide.silicate·carbonate-(MOSCF), . and carbonate-facies-(CF) constituting the iron-formation, All the facies types are strongly banded on metre to micron scale and frequently show small to large scale fold structures of both depositional and tectonic origin. While the lighter bands account for 43 to 57 per cent of the rocks and contain either silica (chert to quartz) and lor silicate (grunerite), the dark bands are chiefly made up of hematite, magnetite and goethite in OF, magnetite in MOSF, siderite and magnetite in MOSCF and ankerite-ferroan ankerite in CP. Although some of hematite and goethite could be interpreted as secondary after magnetite, a good proportion of these and the bulk of magnetite are syngenetic sedimentary. Grunerite appears to have been derived by the metamorphism of the precursor minnesotaite. The striking simplicity in chemical composition and distinctly low minor and trace element contents .are in accordance with the formation of eN Halli Iron-Formation through the process of inorganic chemical deposition. The carbonates underlying the iron-formation show upward iron enrichment and the iron-formation is interpreted as terminal phases of chemical sedimentary carbonate deposition in an essentially 'quiet' and' closed' environment.