Published in MESA Journal 41 April 2006
I ran into this treatise while looking for maps of the Coober Pedy Opalfields and found it interesting. I’ve never heard of any Coober Pedy opals miners using this technique but nevertheless found it very interesting.
I think it may be of use for finding a suitable area for opal formation before actually doing any test drilling.
It was published by Jingping Zhe and Brian J Morris ZZ, Resistivity Imaging Pty Ltd 2 Geological Survey Branch, PIRSA .
A new high-resolution resistivity instrument, developed by Jingping Zhe, was tested within the Coober Pedy Precious Stones Field, 900 km NW of Adelaide, to determine its suitability for opal exploration.
The new technique records the electrical resistivity from the surface to a depth of investigation of around 35 m, and results from three test sites show that: there is a good correlation between resistivity and lithology, the technique appears to distinguish between areas of low and high opal prospectivity, sub-surface geological features important for opal exploration can be delineated.
The technique may provide a quick, cost-effective method for assessing the rough opal potential of an opal claim, as well as locating specific prospecting targets.
Resistivity is a popular geophysical method used in mineral and environmental exploration (Telford etal., 1990). With the development of
advanced electronics and data processing techniques, resistivity can work more efficiently and deliver more results than previously (Stummer, 2003).
The new resistivity acquisition system developed by ZZ Resistivity Imaging Pty Ltd is an example of this technological advancement and differs from traditional instruments in 3 significant ways:
- It acquires data from a continuous spread of electrodes, avoiding traditional acquisition pattern limitations. This enables collection of considerably more data and produces more accurate results.
- It acquires 26 sets of data simultaneously, compared to current instruments that acquire one set of data at a time. Thus the acquisition of data is more efficient and cost effective.
The final result is a true resistivity distribution map rather than the traditional apparent resistivity map or curve. This allows easier and more accurate interpretation.
The geology of the Coober Pedy Precious Stones Field has been described in detail by Robertson and Scott (1990). Opal is found in Early Cretaceous marine Bulldog Shale which, when fresh, is a dark grey, silty and sandy, smectite-rich claystone.
Intense weathering has produced a well developed weathering profile comprising up to 50 m of white to mauve, bleached, leached, porous, kaolinitic claystone, locally referred to as ‘sandstone’. Opal is found as veins in horizontal ‘levels’ or in steeply dipping verticals infilling fractures and joints in the bleached claystone down to 25 m below the surface and, generally, from 5 m above to 1–2 m below the transition to fresher, denser and darker claystone. Gypsum, alunite, iron oxides and tubules are commonly associated with the levels and verticals. The bleached claystone can be variably silicified to ‘blue ground”.
The Bulldog Shale is commonly overlain by 4–8 m of Tertiary to Quaternary Russo Beds of variable composition, but usually comprising:
- A lower gypsum unit of red-brown, gypsiferous clay-silt with angular to rounded claystone and silcrete rock fragments and gypcrete.
- A middle conglomerate unit of predominantly rounded silcrete cobbles in red-brown, sandy silt-clay matrix. Hard massive silcrete may be present.
- An upper carbonate unit of red-brown, calcareous, sandy clay-silt with rounded silcrete pebbles and platy calcrete development.
There’s much more to be found about opal prospecting on the internet at this location: www.aseg.org.au/sites/default/files/case_study.pdf . If you are interested you can read on.