Ionic aureoles, overlying or contiguous to massive sulphide occurrences, are postulated to have been emplaced as a result of natural galvanic forces. The occurrences of these galvanic forces, most commonly referred to as the self-potential or spontaneous polarization phenomena, are well documented, but ionic migration in response to these electrical forces remained to be proven conclusively.
The purpose of this study was to attempt to document ionic migration attributable solely to electrogeochemical transport. Pleistocene glaciolacustrine sediments rich in clay-size particles was the geological substrate chosen to minimize migration processes other than those of electrogeochemical transport.
Laboratory experiments which simulated a field situation and utilized the radioisotope 65Zn as a tracer, revealed a readily detectable diffusion of Zn2+ into glaciolacustrine clay; the distance of penetration of the Zn2+ being significantly increased by the application of a DC electrical potential, similar in magnitude to a natural self-potential. The rate of diffusion was greatest around the margins of the cathodic electrode located at the base of the clay layers; the areas shown to have the greatest current density. The resulting pattern of Zn penetration into the clay, when viewed in a vertical section, showed a double peaked increase in Zn concentration on either side, and a distinct decrease in Zn penetration directly above this cathode. Extrapolation of the average diffusion coefficient found for Zn2+ (D0 = 6.3 × 10-9cm s-1) to other ions commonly associated with base metal deposits, revealed that only H+ could be expected to move through appreciable thicknesses (>20 m) of clay in the 8000 years since sediment deposition.
The hypothesis that H+ would diffuse through varved clay, and that the resulting pattern would be predictably modified by a self-potential field generated by an oxidizing sulphide body, was tested over the Magusi River volcanogenic massive sulphide deposit near Noranda, Quebec. The organic soil horizon (H) and top of the clay soil layer (C) were sampled and analyzed for conductivity, pH, and other major and minor elements. The mineral horizon revealed distinct changes in conductivity, pH, Ca, Mg, and Fe above the contacts of the hanging wall and footwall of the sulphide horizon, thus producing a double-peaked “rabbit-ear” anomalous pattern. The H soil horizon, when corrections for variable “total” element and carbon concentrations were applied, showed clear anomalies in Fe and other pH sensitive elements. A ratio of EDTA Fe/TOTAL Fe to total organic carbon revealed the best “rabbit-ear” anomaly above the sulphide horizon. The use of organic horizon geochemistry as a method of evaluating EM conductors overlain by water-saturated lacustrine sediments is recommended.