The current spate of exploration activity in the Curnamona Province is providing good case histories to illustrate the problems associated with exploration in deeply covered areas.
The Curnamona Province is emerging as a new frontier for the discovery of world class gold/copper and lead/zinc/silver deposits. The province covers an area of some 60,000 sq kms and embraces the well known Broken Hill Block in NSW and the Olary Block in SA. It straddles the NSW/SA border and stretches from Mt Painter in the north-west to Broken Hill in the south-east, with the recently discovered Portia gold/copper prospect (Pasminco/Werrie) in the approximate centre, near Olary, SA. It is currently an arena for intense exploration activity, mostly by major companies.
Outside the Broken Hill Block and the southern part of the Olary block, the region is extensively covered by overburden, up to 500 metres thick in the central and northern parts. Until recently the depth of cover has inhibited effective exploration over large parts of the province. The key to the revival of interest and activity in the Curnamona Province has been the availability of good quality aeromagnetic and gravity data provided through the government funded Discovery 2000 (NSW DMR) the SA Exploration Intitiative (MESA) and the Broken Hill Initiative (AGSO) programmes. This data has assisted explorers to “peel back” the cover and identify prospective settings for mineral deposits and define drilling targets. The willingness of companies to drill deep holes and not be daunted by the extensive and deep cover is also a major factor.
Magnetic and gravity data are being successfully used to map prospective stratigraphy and to identify specific drilling targets. Targets include BHT style stratiform Pb-Zn-Ag deposits, Olympic Dam style breccia hosted Cu-Au-U deposits and Ernest Henry style stratabound Cu-Au deposits. These are large and high value deposits, such that explorers are encouraged to drill deep and expensive holes. Reliable interpretation of the geophysical information to determine optimum drill targets is of critical importance. At these depths, subtleties in the data become very important. Precise measurement of anomalies and careful modelling by experienced interpreters is essential. The cost of not doing this may be the total waste of a miss-targeted $100,000 drillhole, or overlooking a perfectly good target because it has been interpreted to be deeper than it really is. Other techniques such as gravity and electrical methods become less relevant at these depths.