Delineation of a greenstone belt using aeromagnetics, Landsat and photogeology — A case study from the South Indian Shield

Intense aeromagnetic anomalies with amplitudes ranging from several thousands of nanotesla (nT) are observed over the Cuddapah-Kurnool group of sediments and also over pans of the outcropping Archaean basement in the Vijayapuri-Macherla region of the Proterozoic Cuddapah basin in South India. The anomaly zone has a NW-SE elongation and extends over a strike length of about 30 km with varying widths of 5 to 10 km. From the nature and amplitude of the magnetic anomalies it is inferred that the source of these intense magnetic anomalies could be banded iron formations (BIF's) that are commonly associated with greenstone belts of Archaean age. This inference is corroborated by Landsat, photogeological studies and field checks. Continuation of the magnetic anomalies further south and southeast suggests the possible continuation of the iron formations and the associated schistose rocks beneath the Kurnool sediments.

Results of in situ and laboratory susceptibility measurements agree with the observed variation in the amplitudes of the magnetic anomalies. NRM measurements and Koenigsberger ratios (Q_n) of the order of 10 to 30 suggest the presence of a large component of remanent magnetization in certain parts of the iron formations. The study has demonstrated the complementary role of different remote sensing tools in identification and delineation of greenstone belts of Archaean age.