GEOPHYSICAL CATEGORIZATION OF THE PRECAMBRIAN IN SOUTH INDIA*

Abstract

AbstractThe ground follow‐up of a magnetic and radiometric air survey had to cope with the usual dilemma to check a great number of anomalies within a short time. A limited aggregate of magnetic anomalies, expected to correspond to magnetite quartzites was statistically selected for ground identification by this method: the ratios length/width times amplitude were listed for all coherent contours and the calculation of the standard deviation per unit area resulted in different key‐numbers for a new lithological “Salem Unit” and for the charnockitic or gneissic environment. The ground work thus directed and reduced by 85% yielded a substantial potential of iron ore. This was supported by by abundant determinations of the magnetic susceptibilities, confirming the sources of anomalies and revealing the amenabilities of iron ores for the magnetic separation process.The lines of truncation of anomalies were found to represent a system of local and regional faults and shear zones, which segregated the area into different tectonic blocks. These sutures have also provided the ways of intrusion for alkaline and basic magmae in the style of a “Rift” structure, housing several carbonatites and impregnations of metal sulphides.The airborne radiometrics obtained many uranium indications by gamma ray spectrometry. However, they led only to disseminated uranium‐silicates, associated with syenites, granites and pegmatites, each emanating a characteristic photon energy spectrum. But in general the radiation of thorium prevails, marking northern Madras as a “Thorium Province”.