Geophysics helps in the search for opals at Lightning Ridge

摘要

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 polarization and seismic reflection surveys have all been trailed 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. Localized 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.