Mode of occurrence and characteristics of Mn-ore bodies in iron ore group of rocks, north Orissa, India and its significance in resource evaluation

Abstract

Several meso-scale manganese ore bodies, scattered within Jone's horse-shoe shaped synclinorium, in Bonai-Keonjhar region of north Orissa are well known in the mineral map of India. Different grades of manganese ores are being exploited from this region by various agencies over a few decades. However, deceptive nature of ore bodies and complexity in control of mineralisation greatly confuse the exploration geologists for evaluation of these resources. In a recent study, the authors have classified Mn-ore bodies of this region into three broad categories such as stratiform, stratabound (-replacement) and lateritoid types based on mode of occurrence and their other chemical characteristics. Mn-ore bands occur in close association with BIF and iron ores. Volcaniclastic shale in large geographic extension encloses these ore bodies. In the stratiform category of ore bodies (BMnF, analogous of BIF), manganese and shale bands, in variables thickness, alternate with each other and extend to a great depth. Such ore bodies generally constitute marginal to low-grade ores, are characterised by low Mn/Fe ratio (similar to 2) and have relatively lower abundance of trace (1500 to 2500 ppm) and relatively higher REE constituents. The stratabound-replacement types of ore bodies are of intra-stratal nature, occurring within tuffaceous shale. These are mostly shear-controlled ore bodies extending along a zone of certain width. Increase in average Mn/Fe ratio (similar to 6) and trace content (5000 to 8500 ppm) by 5 to 2.5 order of magnitude respectively or more above stratiform category are characteristic of these deposits. The lateritoid ore bodies have limited depth persistency. Such deposits are usually very low in Mn/Fe ratio (<1), trace (<2000 ppm) and REE contents. Different methods of exploration techniques are suggested for various categories of Mn-ore bodies. In this context, the above findings would be the database for the exploration geologists to evaluate the potential of newer/existing Mn-ore resources in this part of north Orissa.