Scientific detective work has revealed that billions of tons of copper ore are missing from the world's largest copper mine.
A fault cutting through the giant Chuquicamata copper mine in Chile suggests that part of the rich deposit may have been displaced millions of years ago by tectonic movement along the fault.
A new dating method, uranium-helium thermochronology, has shed some light on this mystery, including the possibility that another copper deposit could be buried on the eastern side of the fault.
"We determined the direction and amount of the vertical motion along the fault using this new technique", says the team leader, Dr Brent McInnes, CSIRO Exploration and Mining.
"Our studies show that the missing western portion of the ore body has moved upwards by about half a kilometre from the present surface".
"We presume that it has since been removed by erosion. However, the results also imply that another copper deposit could be buried on the eastern side of the fault and this technique can help company geologists predict how deep they will have to drill to intersect it."
The new dating method was applied to rock samples collected from either side of the fault. "Differences in the sample ages indicated that the rocks on each side of the fault cooled at different rates, and therefore one side of the fault moved upwards relative to the other", says Dr McInnes.
The new technology will be used in the search for ore bodies buried by fault displacement in the vicinity of known mineral deposits, such as the giant Cadia mine near Orange in New South Wales. It can also be used to determine the history of earthquake motion along faults such as those that have affected Newcastle.
The powerful applications and relative simplicity of analysis prompted CSIRO to build its own uranium-helium dating facility, which opened earlier this year.
The results of the Chuquicamata study were published recently in the scientific journal Economic Geology (volume 94, pages 937-948).
The project was jointly funded by CODELCO, CSIRO, the US National Science Foundation, the Australian Department of Industry, Science and Resources and the Australian Geodynamics Cooperative Research Centre.
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Materials provided by CSIRO Australia. Note: Content may be edited for style and length.
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