Borehole Geophysics In Gold Exploration

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