Broadband electromagnetic measurements over a massive sulfide prospect

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Abstract
This paper presents the results of measurements of the tilt angle and ellipticity of the polarization ellipse of the magnetic field, made at 14 frequencies in the range 10.5 Hz to 86 kHz, over a massive sulfide prospect in the foothills copper belt of California. As is typical of this class of deposit, the massive sulfides grade laterally into the disseminated sulfides of a tuff unit. The host rock resistivities are in the range 80 to 500 ο-m. The interpretation of the results is carried out using crude dike models in free space, although an attempt is made to account for the conductive host rock by examining the electric and magnetic fields generated by magnetic dipole sources over a homogeneous half‐space. The utility of broadband multifrequency electromagnetic (EM) measurements over a complex earth is shown clearly by the results over the prospect. The response of the massive sulfides, disseminated sulfides, and the host rock can all be distinguished when the tilt angle and ellipticity values are contoured in frequency‐distance space. A comparison is made of the results obtained using vertical‐loop, rotating vertical‐loop, and horizontal‐loop transmitter configurations. It is found at this mineral prospect that (1) the results from the vertical‐loop transmitter are easier to interpret, in terms of simple models, than those produced by the horizontal‐loop transmitter, (2) although the rotating vertical‐loop transmitter gives a larger response from the massive sulfides, the results show no more fine structure in the massive sulfides than that shown by the response from the vertical‐loop transmitter; and (3) moving the vertical‐loop transmitter does not give significantly more information about the geology.

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