Petrography, Chemistry and Economic Potential of the Magnetite Ores of Pokphur Area, Nagaland

Abstract

Unlike other magnetite ores of India, the magnetite ore deposit of Pokphur area, Nagaland is unique because of its ophiolitic association, igneous origin and economic potentiality. The magnetite ores occur as sheet like bodies over the ultramafic to mafic cumulate sequence. These ores have been characterized with respect to their petrography and bulk chemistry in order to evaluate their economic potentiality. Ore petrographic studies on five bulk samples collected from different locations in the Pokphur magnetite body reveal that magnetite is the dominating ore-mineral constituting about 40 to 50 modal percent. The matrix materials are altered silicates of chloritic composition. Other iron minerals recorded are martite, hematite, maghemite, lepidocrocite and goethite. The magnetite crystals vary widely in their grain size (<10μm to >2mm) and are dispersed in the silicate matrix. Magnetite grains have suffered partial ‘martitization’ in grain boundaries and in fracture planes. Coexistence of chromite and magnetite is recorded where magnetite occurs in the inter-granular spaces of chromite. The replacement of chromite by magnetite is notable and this replacement also starts from grain boundaries and along fine cracks within chromite grains. Occasional presence of ilmenite and trace specks of sulphides is also noticed. Apart from the supergene oxides ( e g . g o e t h i t e , ma r t i t e , l e p i d o c r o c i t e e t c ) , two g e n e r a t i o n s o f p r ima r y ( i g n e o u s ) crystallization of iron oxide minerals are observed in the magnetite ores: an earlier generation represented by isolated crystals of magnetite and a subsequent (second) generation of maghemite that follows the flow pattern of the lava. Bulk chemistry of the ores have a composition of Fe (t) = 43-53%, Cr = 2.3-3.8%, Ni = 0.3-0.7%, and Co = 0.03-0.04%. TiO2 exceeding one percent is also recorded in one sample. The silica and alumina content together are quite high (19-28 wt. %). The compositions of the ore clearly indicate that these ores cannot be used to extract iron metal by conventional blast furnace route. However, the magnetite ore of this region is of strategic importance as it contains significant quantity of nickel, chromium and cobalt. The National Metallurgical Laboratory, Jamshedpur is exploring various smelting processes and the possibilities of preparing Ni-hard steel from these magnetite ores