A magnetic survey of banded iron formations in Migori segment of the Nyanza greenstone belt

Abstract

Migori segment lies within the greenstone complexes of Western Kenya. It forms about a third of the portion of the Nyanza greenstone belt to the south of the Winam Rift. The segment is located just north of the Kenya - Tanzania border. The greenstones represent a metamorphosed Archaean volcano - sedimentary succession, comprising metabasalts and diorities, andesites, greywackes, banded iron formation, shales and mudstone.

Ground geophysical surveys have been conducted in Macalder, Masara, Mukuru and Nyabisawa prospect areas, located in Migori District, to determine the geological setting of the banded iron formations, which are widely known to host gold. The total magnetic field intensity was measured with a proton precession magnetometer with an accuracy of + 0.1 nT. The vertical magnetic field intensity was measured with a flux-gate magnetometer with an accuracy of + 3.5 nT. Measurements were done along profiles. The measured fields were corrected for both the diurnal and geomagnetic variations and the residual anomaly isolated from the regional. Quantitative interpretation was performed on the reduced total field magnetic data utilizing current state - of - the art computer processing techniques. This interpretation involved both direct interpretation and forward modeling along five selected profiles. The principle basis of the interpretation was a rigorous analytical procedure termed MAGCALC and WINMAG, computer based direct interpretation techniques which analyses each measured magnetic anomaly along every survey traverse in terms of the horizontal location, depth from the surface, dip, width, magnetization and relative magnetic susceptibility of its causative source. Modeling was employed to augment the interpretation.

Interpretation involved fitting a calculated model of a thick sheet assuming uniform magnetization to the observed data by computer iterations. The causative bodies are interpreted to have prismatic shapes suggesting them to be dykes. The quantitative analysis of the observed magnetic anomalies reveals that the average depth to the causative magnetic bodies lies between 1.5m to 5m. The depth extent of these causative bodies lies in a range of 5m to 10m for four profiles. However modelling results for one profile showed a depth extent of 256 m, indicating the possibilities of deeply seated bodies in proximity to the existing shallow ones. The banded iron formations were found to be discontinuous, but the probability for new mineralised areas in Migori gold belt is high. The models obtained are considered reasonable as they match well with the geology of the area and with other geophysical studies that have been carried out in the research area.