Mineralogical and chemical characteristics of ferromanganese ores from Sandur, Karnataka, India

Abstract

Ferromanganese oxide ores in Sandur, Karnataka, India, consist of the manganese minerals: cryptomelane, romanechite, pyrolusite, lithiophorite and manganite and the iron minerals: magnetite, hematite and goethite. A mixed Mn-Fe-Al phase is invariably present. Quartz, kaolinite, gibbsite and rutile are the minor associates in the ore. The manganese minerals mostly show colloform, mammillary, vein filled, fibrous and other replacement textures. The magnetite mostly occurs as idiomorphic crystals and is the earliest formed phase in the paragenetic sequence.
     The Fe-Mn rich lithologies have high Mn/Fe ratio (av. 2) and are low in trace element and REE content. These are also characterised by : (i) high Al₂O₃ (av. 7.44%), (ii) low phosphorus, (iii) preponderance of K₂O over Na₂O (iv) comparable level of CaO and MgO, (v) average of 0.1% Li₂O and (vi) ubiquitous BaO, markedly concentrated in romanechite.
     Major, trace and REE distribution patterns in the ferromanganese oxide ores and their discriminatory plots provide evidences, though equivocal, do not indicate a single genetic process. A hydrothermal component for the manganiferous lithologies deposited contemporaneously with the detrital material is suggested. The mineralisation is of stratabound replacement type, where the ore acquired all its trace and REE signatures from the surrounding argillite, subsequently subjected to residual concentration.