Abstract The aim of geophysical surveys is to obtain information on subsurface geology. While execution of surveys using specific techniques may differ in detail, it will almost invariably consist of three steps: surveying, data processing, and data interpretation. A successful survey will yield more information on the geological target—its existence, location, shape, size, etc. New information is obtained by interpreting geophysical data. The success of a survey depends to a large extent on decisions made before the survey initiation. An exploration geophysicist working for a mining company is often asked the following question: Can we use geophysics in prospecting for this particular commodity? If yes, what techniques should we use and how do we specify survey parameter. Decisions that are usually based on experience often cannot be justified scientifically. The proper approach would be to carry out test surveys to investigate the physical properties of the target and other bodies that might interfere with its response. In recent years, exploration geophysics has progressed beyond target finding to mapping subsurface geology. Analyzing the sequence of geophysical survey steps as shown in Figure 1, the main flow (surveying, processing, interpretation) and the associated areas of research can be identified. To make an intelligent decision on the use of a technique, the geophysicist should have at least a rudimentary knowledge of the physical properties of the target and the surrounding media the response of which might interfere with target identification. Most physical property studies have been done in the laboratory on samples collected in the field. While this approach may be satisfactory for some geophysical methods (gravity, magnetics), it is not for others. Electrical properties of earth materials vary substantially (by several orders of magnitude) depending on whether they are measured in situ or in a laboratory. It is virtually impossible to simulate real conditions in the laboratory. An attempt can be made to recompose the original water content, but microinhomogeneities typical of many geological environments (e.g., rock fractures and their frequency and variation with depth) cannot be duplicated.

Abstract The Noranda camp in the southern Abitibi greenstone belt comprises over 20 volcanogenic massive sulfide deposits hosted by volcanic rocks of the 2704–2695 Ma Blake River Group. Decades of research and exploration have provided a firm understanding of the characteristics of these deposits as well as the geological controls on deposit location. Observations made on the deposits of the Noranda camp significantly contributed to the syngenetic model of massive sulfide formation and shaped the current understanding of ancient and modern sea-floor hydrothermal systems. The Horne and Quemont deposits, which are the largest deposits in the Noranda camp, are hosted by 2702 Ma felsic volcanic successions dominated by volcaniclastic rocks. The massive sulfide ores of these deposits largely formed through processes of subseafloor infiltration and replacement of the highly permeable wall rocks. Laterally extensive hydrothermal alteration halos dominated by chlorite and sericite surround the replacement ores. The Horne deposit formed in an extensional setting in a graben bounded by synvolcanic faults. Rapid extension accompanying deposit formation resulted in the upwelling of mantle-derived mafic melts and the emplacement of a thick package of mafic rocks in the stratigraphic hanging wall of the deposit. Most of the massive sulfide deposits in the Noranda camp are hosted by a 2700–2698 Ma bimodal volcanic succession that formed in a large volcanic subsidence structure to the north. The ~2,000-m-thick lava flow-dominated volcanic package is floored by the large, multiphase, synvolcanic Flavrian pluton. The deposits in this part of the Noranda camp are small (<5 million tonnes) and primarily formed as sulfide mounds on the ancient sea floor. Synvolcanic structures provided cross-stratal permeability for the hydrothermal fluids and controlled the location of volcanic vents. Thin tuffaceous units mark the sea-floor positions hosting the massive sulfide mounds within the flow-dominated volcanic succession. The concordant massive sulfide lenses overlie discordant alteration pipes composed of chlorite- and sericite-altered rocks. Contact metamorphism associated with the emplacement of the ~2690 Ma Lac Dufault pluton converted the hydrothermal alteration pipes into cordierite-anthophyllite assemblages. Recent brownfields exploration successes have demonstrated that massive sulfide discoveries are still possible in one of Canada’s most mature mining camp through three-dimensional geological modeling performed at the camp scale. Geologic target generation through computer modeling has reversed the general trend of progressively deeper exploration with time in the Noranda camp. Deep exploration currently focuses on the reevaluation of a previously uneconomic low-grade ore zone at the Horne deposit.