16 January 2002 Geophysics helps in the search for opals at Lightning Ridge

Opal at Lightning Ridge typically occurs at the interface between the electrically resistive Wallangulla Sandstone and the deeper conducive 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 NSW Department of Mineral Resources (DMR) 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 transient electromagnetic (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.

Current status In June 1998, Michael Leys and David Robson of the DMR presented a paper at the first Lightning Ridge Opal Symposium, proposing to trial modern geophysical surveys that were relatively simple to acquire and to interpret by a claim holder. Subsequent field surveys were undertaken at Lightning Ridge in December 2000, using the semi-automatic ABEM LUND resistivity profiling equipment, a GEONICS EM-31 instrument and a rapid sampling Offenhauser magnetometer.

After discussions with the Lightning Ridge Miners Association, the Crusty's prospects and the Grawin-Glengarry Prospect at Mulga Rush were selected for investigation. The Crusty's Prospect lies approximately 23km northwest of Lightning Ridge along the Old Goodooga Road while the Grawin-Glengarry Prospect at Mulga Rush lies approximately 40km southwest of Lightning Ridge.

*Opal bearing claystone and the overlying sandstone are readily detected with both resistivity and TEM surveys.
*Opal miners could operate the semi-automated resistivity equipment and associated modelling software.
*Near-surface magnetic noise effects can greatly affect a 50m x 50m prospect scale magnetic data coverage. However, there is some evidence that prospective local scale structures have been detected.
*The resistivity and electromagnetic data acquired indicate that an ultra-detailed airborne electromagnetic survey would probably detect the opal hosting clay layer and would thus greatly assist in the search for additional, opal-bearing environments.

Following these trials, a small airborne electromagnetic survey was conducted by the Exploration NSW program as a pilot survey over the Coocoran opal area at Lightning Ridge. A helicopter-borne Humming Bird electromagnetic system acquired 1,000 line kilometres of data over a 60 square kilometre area at line spacings of 50m x 100m. Preliminary results indicate a low resistivity over the current Coocoran opal field together with several other low resistivity zones that now warrant further investigation. To investigate these responses, the DMR is contracting conductivity-depth sections that will lead to a better understanding of the source depth of the low resistivity zone. The DMR released full results of the airborne electromagnetic survey at Lightning Ridge in November 2001.

This article includes extracts from a paper titled "Case History: the search for opal at Lightning Ridge" by MTC Leys M. Moore and D. Robson, published as an Extended Abstract, ASEG 2001 Conference.