Finding sphalerite at Broken Hill with drillhole magnetometric resistivity

Abstract

Lode horizons north and south of the Broken Hill, New South Wales orebody contain a number of zones rich in sphalerite and poor in galena. One example is Pasminco Mining's new opencut Potosi Mine, which averages 8.5% Zn and 2% Pb with little other sulphide. The mineralisation at the Potosi Mine has been well defined by applied potential and induced polarisation (IP) surveys, but different techniques are required for deeper, downplunge exploration. Drillhole electromagnetics (DHEM) is routinely used but may miss those zones where sphalerite is almost the only sulphide. It was therefore decided to try a series of drillhole magnetometric resistivity (DHMMR) surveys, since this method responds to contrasting, rather than absolute, conductivity. The results were very successful with the known sulphides (and some previously unsuspected zones) responding at downhole depths of more than 600 m. The method has detected mineralisation at distances greater than 150 m from the drillhole and has given clear responses to sulphides where DHEM has failed. The method has less resolving capability than DHEM, although it has proven to be more definitive where sulphide lenses occur both above and below the drillhole. The results also suggest that the magnetic field, which responds to changes in resistivity, has a much larger search radius than the phase angle, which measures the IP effect. DHMMR is expected to play a wider role in the Broken Hill district and elsewhere, in the search for sphalerite-rich orebodies.