The electrochemical mechanism of sulfide self-potentials

Abstract Self-potentials associated with a sulfide ore body result from the ohmic potential drop within the country rocks. The electric current is produced by separate but simultaneous reduction of oxidizing agents near the surface and oxidation of reducing agents at depth. The ore does not participate directly in either reaction, but serves as a conductor to transfer the electrons from the reducing agents to the oxidizing agents. The possibility for the above reactions to occur depends upon differences in oxidation potential of ground waters at different depths. In the zone of weathering, the oxidation potential is controlled by the reduction mechanism of oxygen, and ranges in value from 0.2 to 0.7 volt (on the hydrogen scale). If the ore tends to oxidize at some lower potential, then the latter is the available one. In the zone beneath the water table, the potential is probably controlled by the oxidation-reduction equilibria of iron-rich minerals, and ranges in value from 0 to -0.3 volt. The available potential is independent of ore type. The maximum potential difference available to produce natural currents is estimated at: graphite 0.8, pyrite 0.7, covellite 0.6, chalcocite 0.5, galena 0.3 volt. Self-potentials will be large if the ore body (1) is composed of minerals difficult to oxidize, (2) has low electrical resistance (physical continuity together with low resistivity), (3) extends vertically across the water table, and (4) exists close to the surface.