The principle of the mise‐á‐la‐masse method is to earth one current electrode of a pair in a conducting mineral show (in a borehole, in an outcrop etc.) and measure the resulting distribution of electric potentials. The distribution will, to some extent, reflect the geometry of the ore mass of which the mineral show forms a part.
In a mise‐á‐la‐masse survey of a lead‐zinc deposit in Central Sweden electric potentials were measured on the surface of the ground as well as in some 25 drillholes, in either case with earthings (successively) in three different parts of the irregular ore deposit. Besides this, measurements were made in drillholes with earthings in two further drillholes.
Geologic correlation between the drillholes is difficult in the present case on account of the irregular geometry of the ore deposit. However, the mise‐á‐la‐masse measurements clearly showed the dip and the pitch of the ore body, established connections between the different ore widths encountered in the various holes, and yielded information about the shape of the ore mass.
In particular, the survey showed that the ore lenses must be crescent‐shaped rather than tabular, and the dip was indicated to be westerly, instead of easterly as originally presumed.
Three‐dimensional models of equipotential surfaces were constructed from the observed drillhole and surface potentials (using transparent plastic sheets and thin copper wire) and these helped to elucidate the mass geometry further.
The surface and underground potential data collected in the present case should be of nterest to geophysicists working on analytic continuation problems.
The electrical resistivity of 60 pyrite, 31 chalcopyrite, 42 pyrrhotite, 8 arsenopyrite and liillingite, 6 cobaltite, 15 galena, 13 zincblende, 26 haematite, 46 magnetite, 16various manganese minerals, 23 complex ores and 7 graphitic shale samples (mostly from Swedish localities) was measured by the four point method. The method and the precautions needed in its application are briefly discussed. The results are presented in the form of a table which also gives other relevant data on the samples (e.g. percentage of ore in a given sample). The results are believed to be of interest to geophysicists engaged in prospecting for ore by electrical methods.The following can be mentioned among the main conclusions. The electrical resistivity of ore samples varies "locally" on a single sample often by factors of 10-100 but usually within about f 30% and it often varies by much greater amounts (factors of IOO-10000) from one sample to another. The possible causes of such variation are mentioned. The resistivity of pyrite, haematite and magnetite ore samples does not show any significant correlation with the ore content for the samples investigated. There is furthermore no correlation between the standard deviation of the..resistivity on a single sample on the one hand and sample resistivity or ore content on the other hand. In the case of chalcopyrite and pyrrhotite the observations suggested the following relations : For chalcopyrite : log,, (resistivity in ohmcm) = (6.2&4.2)/(vol. y' CuFeS,) -(r.3gfo.38) For pyrrhotite : log,, (rasistivity in ohmcm) = (73 &Ig) / (vol. yO FeS) -(3.26&0.31)The paper concludes by giving the approximate limits within which the electrical resistivity of the various ores investigated appreas to lie. The feasibility of detecting these ores by electrical operations is briefly discussed.
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