The late Archaean greenstone belts of Karnataka are unique in having manganese formations with mineable ores. An excellent example of these Mn-bearing greenstone belts is the Sandur schist belt where Fe + Mn formations are confined to the basin shelf, clearly separated from the deeper-water manganese-free BIF that accumulated at the basin margin and flanking the marine basin. Sedimentary textures and structures including stromatolites show that Fe + Mn formations have been deposited on a shallow shelf within the photic zone and above wave base. Along with quartz Fe + Mn minerals found in arenites, argillites and carbonates include ripidolite, Mg-chlorite, sericite, Mn-siderite, kutnahorite, dolomite, ferroan dolomite, psilomelane, pyrolusite, cryptomelane, haematite and magnetite. The manganese and iron contents of these rocks range up to 25% and 46%, respectively. Al 2O 3, TiO 2, MgO, CaO, MnO 2, Fe 2O 3 (T) abundances and ratios show a wide variation. Ni, Cr, Co, Zr, V, Sc, Rb, Sr, U, Th, ΣREE, LREE/HREE, La, Ce and Eu anomalies and their binary relationships indicate that wherever the terrigenous component has increased, the concentration of elements of felsic parentage such as Zr and Hf has gone up. Elevated concentrations of Ni, Cr, Co and Sc are contributed by chlorite and other components characteristic of basic volcanic debris. The carbonates, where clastic input is minimal, are extremely depleted in REE and other trace elements. It is proposed that FeO, MnO and SiO 2 probably were added to Archaean ocean water at oceanic ridge vents. The compositional data suggest that the Fe + Mn formations of the Sandur schist belt were generated by chemical and clastic sedimentary processes on a shallow shelf that led into a deeper basin open on one side. In this environment, chemical and clastic sediments and organic matter were mixed and buried. During transgression, chemical precipitation took place at the sediment-water interface, whereas at the time of regression, these chemical sediments were buried by clastic sediments. Redox-potential differences between the watermasses on the shallower and deeper shelf, respectively, fractionated Fe and Mn. The Fe + Mn formations were deposited within the photic zone and above wave base and in a setting where the water column was oxygenated but organic material accumulated. By contrast, BIF was deposited below both wave base and photic zone in a relatively oxygen-deficient setting where formation of stable Mn-oxides was not possible. Fe + Mn formation and BIF provide an important constraint on greenstone tectonics. A shorter distance between Archaean oceanic ridge and shelf environments, namely the smaller plates model, appears to explain several features of Fe + Mn formations, BIFs and greenstone belts.