Borehole Geophysics In Gold Exploration

Abstract

The Borehole Geophysics Section of the Geological Survey of Canada has acquired multiparameter borehole geophysical logs in several holes in gold deposits in Ontario. These measurements were made to document the geophysical signatures of the ore zones and host rocks, and to determine which physical properties may be used to outline zones favourable for gold mineralization. The geophysical logs acquired include: spectral gamma-ray, density, spectral gammagamma ratio (heavy element indicator), resistivity, induced polarization, self potential, magnetic susceptibility, temperature and temperature gradient. Although gold in economic quantities does not occur in high enough concentrations to be directly detected by geophysical methods, geophysics may be used to delineate certain lithologic units known to host the gold as well as structural features or alteration associated with gold mineralization. In many gold deposits, an increase in the concentration of potassium is associated with gold mineralization, due to sericite alteration. The potassium log (which is derived from the spectral gamma-ray log) can be used to outline zones of potassium enrichment. The resistivity and induced polarization logs also show characteristic responses to pyrite, which is frequently associated with gold mineralization. Increases in other elements, such as the barium enrichment associated with some of the Hemlo gold deposits, can also be detected by geophysical methods. Enrichment of barium, which is a ‘heavy’ element, is detectable by the spectral gamma-gamma method. In areas where gold is known to occur in certain lithologic units, the geophysical logs can be used to map lithology. The use of multiparameter borehole geophysics to map lithology and to detect alteration associated with gold mineralization is illustrated using examples from boreholes in the Kirkland Lake and Hemlo mining areas.