Geochemical signatures of polygenetic origin of a banded iron formation (BIF) of the Archaean Sandur greenstone belt (schist belt) Karnataka nucleus, India
Manikyamba, C.; Balaram, V.; Naqvi, S.M.
Precambrian Research 61(1-2): 137-164
ISSN/ISBN: 0301-9268 DOI: 10.1016/0301-9268(93)90061-6
Chert, ferruginous cherts, cherty banded iron formations (BIF), shaly BIF and shales are found interbedded at the eastern part of the Sandur greenstone belt. Cherts and shales are two end members in which deposition of Fe (sub 2) O (sub 3) and Al (sub 2) O (sub 3) in variable proportions has given rise to observed large-scale variation in major, trace and REE abundances. Iron, silica and REE have precipitated from the cold marine water which was enriched in these components by hydrothermal solutions from MOR vents. However, some minor amounts of Fe, Si and REE appear to have been brought in as dissolved load by rivers. Scatter in K, Mg, Ti, Cr, Ni, Zr, Hf, V, Sc, Y, Rb, Sr, Nb and Ta abundances and ratios indicate that the clastic components have both volcanoclastic and terrigenous sources. Large-scale variation in sumREE, moderate to pronounced La enrichment, positive Eu and negative Ce anomalies are characteristic of the majority of the samples of CBIF and SBIF. REE pattern shapes of CBIF are similar to hydrothermal solutions from EPR or Rs vents. Mixing of the FeO+SiO (sub 2) rich hydrothermal solutions with ambient sea water and clastic input has resulted in an observed large variation in LREE/HREE ratio. However, total population of the samples shows the simultaneous increase of LREE and HREE from cherts to shales through CBIF and SBIF, and obliteration of hydrothermal signature in La content and the magnitude of the positive Eu anomalies. Negative Ce anomalies appear to be the result of a reaction with photosynthetic oxygen produced by bacteria which have developed stromatolites in the underlying formation. Hydrothermal and fluvial solutions provided Si, Fe and trace elements including REE to the cold marine waters of the seas. These hydrothermal solutions were emplaced in the relatively deeper part of the basin having a reducing and neutral to alkaline environment; and due to the thermal and chemical gradient convected towards the shoreline where photosynthesis was producing O (sub 2) . Here, on the stable shelf region below the wavebase and photic zone FeO and Ce (super 3+) of these solutions were oxidized and mixed with clastic material of divergent origin. The Ce (super 4+) was precipitated in the varves like SBIF and shales. Near the shoreline the environment was intermittently oxidizing but acidic most of the time and thus the precipitation of silica took place continuously, whereas precipitation of iron occurred intermittently due to the intermittent availability of oxygen or FeO or both. Our observations suggest that the BIFs of the Sandur belt are a product of hybridity between the hydrothermal emplacement of Si and Fe and divergent clastic sediments to ambient cold ocean water. The precipitation of Fe (sub 2) O (sub 3) was biogenically mediated; a model combining these processes explains most of the feature of BIFs of the Sandur schist belt.