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1st Coompana core in over 30 years

PACE Copper drilling retrieves first Nullarbor Plain bedrock core since 1982.

Figure 1 Location of Coompana Drilling Project drillholes shown over new Coompana gravity image (colour) overlain by a 1VD TMI image from the new Coompana Magnetic Survey showing the broadly interpreted geophysical domains.
Figure 1 Location of Coompana Drilling Project drillholes shown over new Coompana gravity image (colour) overlain by a 1VD TMI image from the new Coompana Magnetic Survey showing the broadly interpreted geophysical domains.

The Nullarbor Plain is one of Australia’s most iconic landscapes. A vast treeless veneer of Nullarbor Limestone overlies and obscures any trace of the geology beneath, meaning the crystalline bedrock of the Far West of South Australia (known as the Coompana Province) is largely a geological enigma.

The first real glimpse beneath the Nullarbor Plain came after the South Australian Department of Mines and Energy, together with the then Bureau of Mineral Resources, acquired regional 1 mile spaced total magnetic intensity (TMI) data over the area during the 1970s. This identified interesting magnetic anomalies that, for a time during the 1980s, led to a number of exploration programs investigating the mineral potential of the Nullarbor basement rocks. The last basement intersecting drillhole to be cored in the region (one of only two diamond holes) was CD1 drilled in late 1982 by Shell Company of Australia. Due to the logistical and technical challenges involved with drilling in the area, the potential for thick cover and the lack of geological data, no further mineral exploration has been undertaken in the Coompana Province since the early 1980s. However, the questions remained: How does the geology beneath the Nullarbor fit with the surrounding cratonic blocks; what is the cause of the remanent strongly magnetised anomalies; just how thick is the cover; and what is the mineral potential of the area?

The Coompana Drilling Project is a more than $3 million collaborative program between the Geological Survey of South Australia and Geoscience Australia, funded through the Commonwealth Exploring for the Future program. The drilling contract was awarded to South Australia based Boart Longyear Australia Pty Ltd via an open competitive tender process.

The primary objective of the drilling project was to retrieve new rock samples from the Coompana Province to test the various geophysical domains interpreted from the recently acquired Coompana Magnetic and Radiometric Survey and the PACE Copper Coompana Gravity Survey.

Drilling commenced in early April 2017 with hole CDP001. At the time of publishing, the eighth and final hole (CDP008) will have been completed bringing the drilling phase of the program to a close (Fig. 1). At completion we will have retrieved more than 1,500 m of new oriented HQ3 diamond core samples of the crystalline basement from beneath the Nullarbor Plain. As well as the core samples themselves, new downhole gamma and temperature logs will help constrain the stratigraphy of the overlying sediments and the thermal structure of the Coompana crust.

The first results from the drilling program, the preliminary field-data report for CDP001, is now available (Report Book 2017/00022). This report includes the drillhole completion data, field stratigraphic and lithological logs, basement unit descriptions, photos and associated downhole geophysical data. It also includes the results and a comparison between the various pre-drilling cover thickness geophysical techniques employed to determine the thickness and characteristics of the overlying sediments. Reports for holes CDP002 to CDP008 will follow in due course.

The completion of the drilling phase does not represent the end of the Coompana Drilling Project. The 1,500 m of core will now be scanned using the HyLoggerTM instrument and potentially the Minalyze XRF scanning instrument housed at the South Australia Drill Core Reference Library. Structural, geochemical, mineralogical and petrophysical analysis will be undertaken to completely characterise the core samples, with the results used to synthesise a new tectonic and geological model of the evolution of the Coompana Province and a re-assessment of the mineral potential of the region.

Preliminary results from the Drilling Project will be presented at this year’s Geological Survey of South Australia Discovery Day on 7 December at the Adelaide Convention Centre. A selection of the core will also be on display. The final results will then be released at a workshop and core viewing to be held in the first half of 2018.

– Rian Dutch

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