Volcano Sedimentary Origin of Archaean Banded Iron Formation of Nellore Schist Belt from Chundi-Malakonda-Ayyavaripalle Area, Prakasam District, Andhra Pradesh and its Petro-chemical Characterization

Banded iron formations (BIFs) are the most significant source of iron in the world. The present study addresses the petrographic and geochemical characterization of the BIF from Chundi-Malakonda-Ayyavaripalle area in the Nellore schist belt of Prakasam district, Andhra Pradesh, their genesis and iron enrichment processes. Six bands of banded magnetite quartzite (BMQ) of Archaean age occur in the area. Field investigations and petrography revealed that the studied BIF samples are hard, compact, weathered and composed of alternate layers of magnetite/martite and subordinate quartz, biotite and muscovite. The BIFs occur in three associations: volcanic (Band 1,3,4,5), sedimentary (Band 6) and volcano-sedimentary (Band 2). The geochemical composition of the whole rock reveals that iron and silica represent 78.48% to 91.74% of the average composition. The total iron (TFe) contents range from 16.25 to 61.80 wt % (average of 38.47 wt %) and is consistent with low-grade siliceous BIF by global standards. All BIFs have high SiO2/Al2O3, Fe/Ti, and low Al/(Al+Fe+Mn), which suggest a mixed hydrogenous and hydrothermal origins. Geochemical results for the BIFs lead to the conclusion that all the BIFs are of Algoma-type with Precambrian affinity and formed in small sloped or terraced silled basins in the back-arc areas with MORB affinity, surrounding an immature island arc. Depending on the degree of replacement and shape of the magnetite crystals, two textural types of magnetite (Magnetite-I and Magnetite-II) are observed. Ore formation occurs in three stages: Magnetite (protore), Martites and Martite-goethites.