The Origin of the Iron Ore deposits of Donimalai area of Sandur Schist Belt, Karnataka State, India

Donimalai area is located in the SE portion of the Sandur Schist belt of Precambrian age. There are 6 ore deposits comprising 9 ore bodies having more than 150 million tonnes of high grade hematite ore in Donimalai area.

Banded Iron Formation (BIF) is the host rock for the iron ore deposits. Basaltic rocks, shales with tuff, dolerite and quartz veins are other important litho-units in this area. Principaly the BIF consists of alternative bands of iron oxides and chert belonging to oxide facies.

Hard laminated ore, medium hard laminated ore, soft laminated ore, friable powdery ore and blue dust are the major ore types with minor occurences of lateritic/limonitic ore and shally ore.

Leaching of silica has resulted in the origin of friable ore and blue dust. Leaching of silica followed by enrichment of iron oxide by the circulating solutions has given rise to hard laminated ore, medium hard laminated ore and soft laminated ore depending upon the degree of enrichment. Shaly ore occurs at the contact of deposit 1 with shale. It is formed due to the enrichment of shale by iron-rich solutions, Original material of shale has been replaced followed by enrichment of iron oxide giving rise to shaly ore.

Laterite ore/laterite occurs parallel to the surface of the ground with a thickness varying between 2-10 meters. They also occur along steeply dipping fault planes upto depths of 100-150 meters. parallel to the bedding. Geomorphic evolution of planation surface around 1000 mt. R.L. was responsible for the formation of flat surfaced laterites/lateritic ores. Activity of solutions along fault planes parallel to bedding has caused lateritisation/limonitisation at deeper lavels also.