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A detailed gravity survey comprising approximately 11 800 gravity stations has been carried out in a 33 000-km2 region of the southern Canadian shield between the Sudbury and Kirkland Lake mining camps in east-central Ontario. The purpose of the survey was to shed some light on the economic mineral potential of the less well-explored parts of this very important mining district of Canada.

This article describes an interpretation of gravity and aeromagnetic data in the Sudbury-Cobalt area, the aim of which has been to derive information of exploration interest for base and precious metals. In addition to using standard procedures such as vertical gradient mapping to increase the resolution of the Bouguer gravity data, a new analytical method called the “apparent-density map” has been introduced as a lithologic-mapping tool. The apparent-density algorithm is used to define rock-unit boundaries and to assign average densities to these units for the purpose of preparing a basement-lithology map. The results suggest that the mineral-rich Archean Abitibi greenstone belt, consisting mainly of mafic rocks, extends beneath the Lower Proterozoic sedimentary cover much farther than had previously been realized. The volumes, depth extent, and major stratigraphic subdivisions of Archean greenstones, together with a lithologic map of the Archean basement, all derived from the gravity and magnetic data, are of material benefit in outlining prospective regions for mineral exploration. Three-dimensional gravity models of the Round Lake Batholith and the adjacent steeply dipping mafic and ultramafic metavol-canics suggest a favorable environment for gold exploration. The gravity and magnetic maps have also been helpful in locating possible diabase feeder zones and in determining their volumes and depths—information that is important for outlining potential silver-cobalt prospects.

It is concluded that gravity and aeromagnetic surveys provide an informative, cost-effective approach to regional exploration for mineral-prospective zones in the Sudbury-Cobalt area, and that these surveys should be followed up by direct-search exploration strategies.

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