Electrical and seismic responses of shallow, volcanogenic, massive sulphide ore deposits

Abstract

SP, resistivity/IP and seismic refraction responses of the Woodlawn Orebody and Mt.Bulga Deposit are examined and compared. Both exhibit similar responses produced mainly by uneconomic and disseminated sulphide mineralization and host rock features, demonstrating that the magnitude and character of electrical and seismic responses are not reliable indicators of size and economic sulphide content of volcanogenic sulphide ores.

SP, soil geochemistry and electrogeochemistry are found to be the most effective exploration methods followed by resistivity/IP and seismic refraction. The large SP responses over both ore zones are simulated using new methods which allowed the width and depth of oxidation to be computed.

Conventional and compensation array resistivity responses best define the deposits. Computer simulation shows that dipole- dipole and Unipole arrays are most useful. First order IP responses are large and similar, but the ore zones are not easily distinguished from polarizable host rocks. Second order responses, at Woodlawn, better define these lithologies and cross-plots of EM coupling removed first order parameters prove useful. The supergene and gossan zones are defined as sources of electrical anomalies and correlate with interpreted SP sources.

Seismic velocities of fresh Woodlawn ore samples indicate only small contrasts with host rocks. Refraction travel-time data are highly complex but host rocks are clearly distinguished by their seismic velocities. Both deposits appear as low velocity zones at the general bedrock level which are shallower and narrower than the electrical sources associated with the ore zones. Extensive model simulation shows that the Reciprocal interpretation method is most useful when compared to other time- term methods for refraction interpretation but has some limitations. Computer simulation shows the significance of non- critical refractions, diffractions and laterally hidden zones which define the lateral resolution of the refraction method.

The results of this study and the interpretative techniques

developed will assist the exploration for similar and deeper massive volcanogenic orebodies in comparable geological environments.