Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

R. M. k. Khan ¹, P. K. Govil ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

The Banded Iron Fonnation is interbedded with 2.9 Ga old mafic and acid volcanics. It mainly consists of magnetite and quartz; calcite is also present in some mesobands. Thin silicate rich layers with abundant actinolite, cummingtonite, grunerite, biotite, riebeckite and aegirine occur between the quartz and magnetite rich rhythmites. Mixed oxide. silicate and carbonate facies (MOSCF) BIFs are found which arc prcmctamorphic cherty and shaly types. The distribution of Al₂O₃ , CaO, MgO, K₂O, Na₂O, Ni, Cr, Co, Rb, Sr, Zr, Cr/MgO and REEs indicate a combination of hydrothenna1, volcanogenic and continental sources. The relationship between Cr/Zr, Zr/Cr+Co+Ni, ZeN and Nd/Yb reflect that certain fine silicate-rich bands between the microbands arc made up of volcanic ash, whereas others are made up of shales of continental source or a mixture of both. LA enrichment, depletion in total REE, positive Eu anomalies, Nd/Yb, La/Lu and LREE/HREE ratios in Fe-rhythmites indicate that Fe, Si and REEs of the BIF were supplied by hydrothennal and fumarolic activity accompanying submarine volcanism. The enrichment in total REE and obliteration of La signature is probably a consequence of mixing of non-chemogenic component and ambient sea water with the chemical precipitates. Absence of negative Ce anomalies is consistent with most of the Archaean BIFs of other localities. Deposition of BIF appears to have taken place below wave base and photic zone of the shallow shelf region. It is inferred that oxygen was generated at some other place in the basin. Ocean circulation and upwelling brought FeO and 0₂ together resulting in precipitation of iron along with fine grained argillites.

Keywords

Geochemistry, Banded Iron Formation, Kudremukh Schist Belt, Kamataka.

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R. M. K. Khan ¹, S. Nirmal Charan ¹, M. Arora ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

The iron-formations ofKudremukh mainly contain magnetite, recrystallized chert, gruneritecu mmingtonite (X_Mg 0.21 to 0.33), hornblende, bioti te along with minor amounts of ankerite (X_Mg O. 30 to 0.52), calcite (X_Mg 0.06 to 0.22), chamosite and pyri teo Chamosite, a rare relict mineral, was probably formed by reaction between high Al-clay and Fe. Magnetite was formed during metamorphism. Along with the precipitation of alternating layers rich in silica and iron, clayey material in the form of montmorillonite, illite and/or glauconite along with Fe-Mg carbonates were also deposited, which after metamorphism (430-520°C) gave rise to the present mineralogy.

Keywords

BIF, Mineral Chemistry, Kudremukh Schist Belt, Karnataka.

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