Origin of alternate amphibole and quartz rich bands in amphibole bearing quartzite from North Khetri Copper Belt, Rajasthan, India

Abstract:

North Khetri Copper Belt (NKCB), located in the northern part of Western Indian shield, is one of most important copper repositories in India. Mineralization is hosted by quartzite containing abundant amphibole in the form of alternation of amphibole ± albite and quartz ± albite bands. In a succession near Chandmari mine of NKCB, this thick banded unit is laterally extended over few hundreds of meters. Band thickness varies from few millimetres to tens of centimetres. Such banding (commonly appearing as gneissic banding) of amphibole ± albite is unlikely to be generated in a quartzo-feldspathic rock metamorphosed at lower amphibolite facies. Amphibole, under microscope, shows pleochroism in shades of green. Their grains are inclusion free and do not show any orientation. From major oxide analysis, their average composition is found to be Ca₂Mg_2.5Fe²⁺_1.5Fe³⁺_0.5Al_1.5Si₇O₂₂(OH)₂. Forming Fe-Mg rich amphibole in a quartzo-feldspathic unit is again challenging its metamorphic origin. Also, amphibole of same composition found in the form of veins within the host rock. All these observations inferring that the amphibole is probably of hydrothermal origin.

Albite in host rock can supply Si, Al and O. If a fluid containing Fe, Mg and Ca intrudes into the host rock, the amphibole of desirable composition may form. However, such alternating banding of amphibole ± albite and quartz ± albite is enigmatic. It is possible that the banded nature is inherited from an existing banded rock with different mineralogy and mineral solubility, e.g. alternation of impure/siliceous carbonate and siliciclastic rocks. Presence of carbonate, mainly dolomite, has been reported early and also observed in field. When a hydrothermal fluid, containing some Fe, had invaded this unit, it reacted with carbonate units and replaced the carbonate by amphibole. This will result in amphibole of expected composition and will also mimic the inherited banded nature in original succession. Reaction will form new carbonate minerals which are also common in this area in the form of ankerite/dolomite veins. The probable reaction will be

[(1.5+x)Fe²⁺ + 0.5Fe³⁺ + 2OH^-]_fluid + [(3.5-x)Mg²⁺ +3Ca²⁺+ 2CO₃^2-]_cb + [7Si⁴⁺ + 2Al³⁺ + 22O^2-]_{qz-ab unit} = [Ca₂Mg_2.5Fe²⁺_1.5Fe³⁺_0.5Al_1.5Si₇O₂₂(OH)₂]_amp + [Ca(Mg_1-x, Fe²⁺_x)(CO₃)₂]_{ank/dol veins}