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

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