Geophysical exploration for gold ? a review

Abstract

It is almost impossible to get a diict geophysical response from gold because of the low grades in deposits, except when electromagnetic detectors are used for individual shallow nuggets. However, indirect geophysical indications may occur through association of gold with particular host rocks, marker beds or structures which are, for example, of unusual magnetization, density, electric polarization, or conductivity/resistivity. Useful markers may be magnetic dolerites, banded iron formations, shales with magnetite, conductive and/or polarizable pyrites or other sulphide (detectable by IP methods) and silicified zones more resistive than surroundingrocks Mapping of faults and shear zones, with which gold may be associated, is also valuable (e.g. by magnetic, EM surveys, etc). Magnetite depletion is characteristic of some deposits, resulting in zones of low anomalies; for example, ferromagnetic minerals in mafic volcanics are destroyed by carbonatization. Resistivity patterns may indicate altered rocks which contain mineralization, thus the use of VLF?EM methods in Canada and MMR in Australia. Geophysical methods have been important in the search for gold in the Witwatersrand Basin of South Africa (magnetic, gravity and even seismic reflection methods), various areas of Canada (e.g. the Abitibi greenstone belt; magnetic, EM and IP surveys), and to a smaller extent in Australia (e.g. Tennants Creek, NT and Water Tank Hill, WA). However, the value of geophysical methods should not be exaggerated as physical contrasts are often low.