Peeking under the covers

Australia is peeling back the layer of overburden that obscures most of the continent to find ore deposits previously hidden to exploration geologists.

At the end of the millennium, facing rapid deterioration in exploration spending and a dearth of “easy” near-surface targets, Australia launched an ambitious campaign to develop techniques that could penetrate both the overburden and the underlying bedrock to pinpoint new, hidden sources of wealth.

The research program came wrapped in its own media-friendly vernacular, boasting a new generation of “geological weaponry” that could overcome the “tyranny of depth” to turn a continent hidden by cover into a virtual “glass earth.”

Bold language perhaps, and yet there are signs the multi-million dollar, multi-disciplinary effort is paying off. Work to date has led to practical techniques to identify and discriminate geochemical anomalies in overburden, and to a better appreciation of related chemical processes, particularly with respect to supergene enrichment of gold. Other techniques are looking through the cover into the bedrock below.

“Applying the concepts, knowledge and methods from research activities has contributed to the discovery of new gold deposits,” says Dennis Gee, CEO of the Co-operative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME), which is heavily involved in the research. He says his group’s regolith studies helped uncover about half the new gold deposits discovered in Australia recently and cites Newcrest’s Ghost Crab, PacMin Mining’s Carouse Dam and Goldfields’ Aphrodite deposits as examples.

But with exploration expenditures at a 20-year low of less than A$700 million per year, according to the Australian Bureau of Statistics, researchers are under renewed pressure to convince mining companies to keep looking. Seeing through layers of overburden more than 25 metres thick or transported from elsewhere remains a major challenge.

“The successful exploration practice of the 1990s, of drilling structural targets from high-resolution aeromagnetics supported by geochemical anomalies, is largely exhausted,” says Gee. “[An] integrated approach, which essentially arrives at a 3-D model of the regolith, has the potential to reduce the size and cost of drilling programs considerably.”

Regolith, derived from the Greek words rhegos (blanket) and lithos (stone) is a general term for the layer of unconsolidated material, whether residual or transported, that covers bedrock. It can be up to hundreds of metres thick. Understanding the structure and geochemistry of this heterogeneous layer is vital to future exploration in Australia and other potentially mineral-rich countries obscured by cover.

Integration of geophysical and geochemical techniques may provide the key. Airborne electomagnetics, the only remote-sensing technique capable of producing a three-dimensional image of the subsurface, has the potential to provide high-power, low-noise data acquisition to penetrate the regolith and detect the transition to bedrock.

Along the Yarlbrinda shear zone in the Gawler Craton, for instance, detailed drilling of several “juicy” targets, or coincident geophysical and geochemical anomalies, detected only small patches of gold. A subsequent aerial electromagnetic (TEMPEST) survey that mapped the topography of the bedrock revealed why. The anomalies correlated with gold dispersed along a paleochannel, not with a major point source. In other words, the roughly A$4million spent on drilling could have been saved by running a A$150,000 aerial EM survey.

The “supreme” challenge, Gee says, is to look for ore under layers of overburden that have been transported from elsewhere. Because the overburden is not a product of in situ weathering, any indications of concealed mineral wealth are subtle at best. To overcome this and other obstacles, the CRC LME has been granted another A$35 million for continued study to 2008. The organization will maintain a strong focus on mineral exploration, though some of the money will be directed toward environmental and land-use issues.

The exploration research will involve 3-D mapping of the regolith, including mineralogy, petrography, chemistry, and physical and hydrological properties. The work will also attempt to identify the mechanisms that cause metal migration through transported cover.

Gee hopes the results will help mining companies identify signatures of gold and base metal dispersion in the aqueous environment, particularly in saline paleochannels. He envisions rapid development of 3D images using new logging techniques to reinterpret large drillhole databases.

A separate but related effort is the CSIRO’s aptly named “Glass Earth” project. While LEME focuses on the 3D modeling within the regolith, Glass Earth will look through the regolith to map the basement geology right down to the practical limit of exploration. The project uses a suite of technologies designed to render the continent transparent, including geophysics, airborne chemical mapping, hydrogeology, geochemistry, and modeling of chemical, fluid and heat flows.

The ultimate goal will be to produce a virtual map of the top 1,000 metres of the continent that will help mining companies target areas with the greatest mineral potential.

The most recent effort, according to a CSIRO media report, is to “throw the combined arsenal of modern geological weaponry” at a 500-km stretch of the province of Victoria, in the southeastern corner of the country. The project, called the Victorian Geotraverse, will “peel back the surface layers to disclose a hidden landscape of undiscovered gold and mineral wealth, buried hills, valleys and fossil streams, extinct volcanoes and lurking timebombs like salinity.”

The traverse will cut through the known goldfields in Victoria, which have produced about 2,500 tonnes of gold, and attempt to see what lies under the sediments that obscure bedrock to the north.

“The hills that contain Victoria’s gold-bearing quartz are 400 million years old,” says project leader Prof. Neil Phillips of the CSIRO. “That enormous rock mass then tilted. From the south and centre of the state, it plunges beneath the flat surface of the Murray plain, and we have every reason to believe the gold it contains extends there too. Perhaps two-thirds of it still lies underground, beyond our sight or powers of detection until now.”

Those words demonstrate Australia’s commitment to tackling a rapidly depleting mineral resource with innovation. Stay tuned to see if the effort pays off in the next generation of Olympic Dams, Mt. Isas and Broken Hills.

— The author is a Toronto-based freelance writer specializing in mining and the environment.