Mineral discrimination and removal of inductive coupling with multifrequency IP

Abstract

Abstract In-situ complex resistivity measurements over the frequency range 10 (super -2) to 10 (super +5) Hz have been made on 26 North American massive sulfide, graphite, magnetite, pyrrhotite, and porphyry copper deposits. The results reveal significant differences between the spectral responses of massive sulfides and graphite and present encouragement for their differentiation in the field. There are also differences between the spectra of magnetite and nickeliferrous pyrrhotite mineralization, which may prove useful in attempting to distinguish between these two common IP sources in nickel sulfide exploration. Lastly, there are differences in the spectra typically arising from the economic mineralization and the barren pyrite halo in porphyry copper systems. It appears that all these differences arise mainly from mineral texture, since laboratory studies of different specific mineral-electrolyte interfaces show relatively small variations.All of the in-situ spectra may be described by one or two simple Cole-Cole relaxation models. Since the frequency dependence of these models is typically only about 0.25, and the frequency dependence of inductive electromagnetic coupling is near 1.0, it is possible to recognize and to remove automatically the effects of inductive coupling from IP spectra.The spectral response of small deposits or of deeply buried deposits varies from that of the homogeneous earth response, but these variations may be readily determined from the same 'dilution factor' B 2 = (delta lnrho a )/(delta lnrho 2 ) currently used to calculate apparent IP effects.