Quantitative interpretation of three-dimensional mise-a-la-masse surveys.

In the mise-a-la-masse method, conducting bodies are energised using current electrodes in mineralized outcrops or borehole intersections, and the resulting potential distribution mapped and used to indicate qualitatively the form of the conductor. In this thesis a quantitative three dimensional integral equation technique implemented on the Vax 8600 computer at Leicester University is used to develop interpretation schemes and carry out detailed modelling of mineralization in two field areas. The interpretation technique was evaluated by comparing theoretical data derived for bodies of simple shape - spheres, discs and slabs. It is shown that for a thin slab or disk, the potential gradient in a borehole at the intersection of the ore hosting the current electrode is inversely proportional to the area of the upper side of the body. This is also shown to be true for a spherical body, but only if the borehole in which measurements are taken intersects the centre of the sphere. The mise-a-la-masse method was employed at two field sites where borehole and other geophysical data were available. At Gairloch, NW Scotland the data allowed a three-dimensional model of the complex structure of the mineralization to be developed. In San Telmo, in the Spanish Pyrite Belt, an estimate of the size of a large mineralized lens was made by the use of the interpretation scheme and later by modelling. This result was used in planning further boreholes by the company in charge of the site. Downhole resistivity and IP were carried out together with a comprehensive suite of geophysical surveys over the outcropping mineralization at Gairloch. These are evaluated and contribute to the final interpretation of the area. The results from parametric model studies show that the use of the method can provide fast, in-field determination of the approximate size of an intersected conducting body with only one available drillhole. The implementation of the numerical method for modelling the data has also allowed more detailed interpretation of deep conductors.