Geophysical exploration for gold in deeply weathered terrains; two tropical cases

Abstract

Basic geophysical techniques were used to characterize two gold deposits that lie within tropical-to-subtropical climates. These two deposits are the Posse deposit, which lies within the Mara Rosa Greenstone Belt of central Brazil and the Syama Mine which lies within the Bago, Greenstone Belt of southeastern Mali. Both deposits were discovered by geo-chemical techniques but subsequent magnetic and induced polarization (IP) surveys were used to identify rock types and basic structures and contacts while VLF-EM was used to define more detailed structures associated with both ore deposits. The radiometric method was used at Syama to map the surface expression of sericitic alteration. The success of geophysics in both areas led to an extension of those surveys along strike. The results were used to assist in the ranking of geochemical anomalies. The desire for additional oxide reserves at Syama and the success of the ground geophysical surveys led to an airborne magnetic and radiometric survey over the mine. The results confirmed the usefulness of the two methods for mapping geology and structure in greenstone belts of West Africa. Additionally, the results were used to identify and rank prospective oxide reserve targets. Initial follow-up soil auguring was successful in identifying significant gold reserves but the success rate declined as the lesser-ranked targets were tested. Potassium concentrations unrelated to the subsurface geology in areas of transported laterite limited the success of the venture. Although both deposits are covered by a thick mantle of laterite, in neither case did this impede the ability of basic geophysical techniques to provide very useful information relating to lithologies, structure and alteration.