Geochemistry and Genesis of Banded Iron Formation from Kudremukh Schist Belt, Karnataka Nucleus, India

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.