Case History: the search for opal at Lightning Ridge

Abstract

Opal at Lightning Ridge typically occurs at the interface between the electrically resistive Wallangulla Sandstone and the deeper conductive Finch clay facies. The best quality opal is generally found beneath thicker portions of the Wallangulla Sandstone and associated with local jointing, faults and breccia zones. Electrical, electromagnetic, magnetic, induced polarisation and seismic reflection surveys have all been trialed by the New South Wales Department of Mineral Resources at Lightning Ridge. The electrical and electromagnetic methods appear to provide a means to define the conductive Finch clay facies (or ``opal level'), the overlying and moderately resistive Wallangulla Sandstone and the highly resistive surface gravel and silcrete (or ``shincracker') layers. Localised linear features appear to be well-defined in both TEM and high-definition ground magnetic data as electrically resistive and high frequency magnetic anomalies. Results indicate that an ultra-detailed airborne electromagnetic survey may assist in the future search for opal at Lightning Ridge.