Opal Eludes the High Tech Probe
June 10, 2004
MINERS who search for opal looking for surface clues might feel some perverse pleasure to hear that the gemstone has defeated another high tech effort to find its presence.
The Department of Mineral Resources has returned to mapping as the most reliable method of finding opal, and will shortly release maps of the weathered sandstone formations which overlay opal clay in the Lightning Ridge area. It has abandoned aeromagnetic surveying (AMS) as it proved a dud at pinpointing opal clay. Yet AMS was one of the most powerful tools employed in their Discovery 2000 program and has kickstarted mineral investment in NSW to the tune of hundreds of millions of dollars. It is widely used to map seabed structures in the search for oil and gas, as well as land based mineral deposits. In 2000 the department tralled a ground-based seismic recorders, and while they did not penetrate more than a metre the results were "promising" , according to Dr John Watkins the department's chief geologist.
So in June 2001 as part of seismic survey between the Ridge and Broken Hill it was decided to map a small area of known opal fields. At first the results looked good, Dr Watkins said. But on closer inspection - particularly trying to match miner's drill logs with with strata identified in the survey - it was clear the system didn't work.
"Imagine a sponge cake with the cream the opal layer you're trying to map," he said. "Pour chocolate sauce over it - that's the first weathering event, then strawberry sauce for the second weathering. What turned up on the survey was not the cake and cream layers but the distribution of chocolate and strawberry sauce."
Or in science speak: "The different AEM responses shown in the data are largely a function of the different physical and chemical properties of the sedimentary rocks in the area," Dr Watson said. "These properties include the presence or absence of iron oxides, clay content and clay type, porosity and permeability and the presence or absence of silicification. The different AEM responses are therefore largely a result of the way the different rocks have responded to the process of weathering. In the Lightning Ridge area, there has been two weathering events that have affected the rocks - a Late Cretaceous to Early Tertiary event and a mid Tertiary event " Although opal formation is probably associated with the earlier event, the AEM survey could not differentiate the two events."
Source: The Black Opal Advocate