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Detecting Deeper Deposits of Uranium

RadonX is a new radon emanometry tool thatís helping to delineate buried uranium mineralization

By Virginia Heffernan

Already among the world’s top uranium producers, Namibia has become a hot spot for uranium exploration after several recent discoveries and expansions in the southwestern African country.

The new wave of explorers is deploying a mix of tools to outline mineralization. But a new radon detection technique adapted from dated technology is proving to be particularly amenable to conditions in Namibia, where radon gas can migrate to surface from buried uranium deposits through permeable cover strata.

“The Namib dune sands are highly porous and permeable. We are seeing radon leaking though to surface from bedrock from depths of up to 120 metres, which is phenomenal given radon’s half-life and that it decays relatively quickly,” says Branko Corner, a geophysicist who helped develop RadonX, a radon emanometry tool based on the Radon-on-Activated Charcoal (ROAC) technique developed by South Africa’s Atomic Energy Board in the 1970s and marketed commercially by Cape Town-based Remote Exploration Services (Pty) Ltd (RES).
 
Radon gas associated with uranium mineralization has a half-life of 3.8 days. About 80% decays within 12 days, and 100% within 30 days.

Radon detection is a common uranium exploration tool for buried deposits that are too deep to be detected by radiometrics, but the RadonX technique is different because it measures the gamma radiation emanating from the daughter products of radon (214Bi and 214Pb), rather than the alpha radiation emanating from the radon itself. As a result, RadonX has proved to be more sensitive and thus better able to highlight anomalies than alpha-sensitive methods.

To measure the gamma radiation, charcoal-filled cartridges are fitted to inverted plastic cups and buried for a period of about 10 days. The radon gas migrates to the surface along with the ground air and is adsorbed onto the charcoal. An on-site gamma spectrometer then measures emissions from radon’s daughter products. A lead castle is used to reduce and correct for background effects.

Corner uses Geosoft Oasis montaj to plot, process and visualize his RadonX data, using the ID FFT Filter add-on to filter and compare profiles.

The technique proved highly effective in a case study conducted at the Rössing South deposit, where Australia-based Extract Resources has outlined enough uranium mineralization to support a 15 million tonne per year open pit operation about 15 km south of the  well-established Rössing uranium mine east of Swakopmund. Extract describes Rössing South as the highest grade granite-hosted uranium deposit in Namibia and potentially one of the largest uranium deposits in the world.

Previous alpha-based radon emanometry trials over the blind deposit had suggested that Rössing South was probably not amenable to detection.  But when RES offered to complete a detailed grid survey using RadonX, there was a strong correlation between the mineralization buried at about 80 m below surface and radon gamma emissions (Figure 1), even though the readings were patchy because of impervious cover in areas.

“The deeper the occurrence, the more devious the pathways to surface,” says Corner. “The radon may reach surface in patches, following fractures in the overlying duricrust or cover sediments. It’s not necessarily a perfect correlation with the mineralization, but does constitute a very clear target anomaly for follow-up drilling.”

A couple of years ago, Cheetah Minerals Exploration - a private company Corner is involved with - decided together with its joint venture partners to dust off the old ROAC radon detection system and try it out on their exploration license. After some tinkering to improve sensitivity, the technique proved to be so effective in detecting uranium mineralization during case study trials, that Corner and his colleagues at RES decided to market the refined technology as RadonX.

RadonX is a qualitative technique best suited to the early stages of exploration, both to identify potential uranium targets and to rule out barren ground. Regional exploration using a relatively coarse grid (e.g. 200 x 500 m) can and should be enhanced by in-filling at closer grid spacing if the regional work warrants follow-up. Because measurements are made in the field, decisions about whether to initiate follow-up and where to place the detailed grid can be made almost immediately.  
   
The survey cost is based on the number of plastic cups used to bury the charcoal-filled cartridges. The basic cost is R325 (about US$44) per cup, including all deployment, collection and measurement, data reduction, grid preparation and report costs.  A discount may be offered depending on the size of the grid, or the need for further surveys.

In the case of palaeo-valley hosted uranium deposits commonly found in Namibia, a radon survey that detects the presence of uranium mineralization can be combined with an electromagnetic survey that effectively delineates the palaeo-valley where the mineralization occurs.

At the Tumas deposit in central Namibia, for instance, uranium mineralization is hosted in calcretized valley-fill sediments associated with a shallow (<25 m) palaeo drainage channel. Figure 2 shows an excellent correlation between the RadonX contours and the mineralized palaeo-channel outlined by resistivity mapping, despite the reconnaissance nature of the 200 x 500 m grid.
 
The latest palaeo-valley deposit to reach production in Namibia is Paladin Energy’s Langer Heinrich, which has a targeted annual production of 2.6Mlb U3O8 and a minimum project life of 17 years.  The deposit was discovered in 1973 during a government-sponsored airborne radiometric survey. Mineralization is near-surface, 1-30m thick and 50m-1100m wide, depending on the width of the palaeo channel.

Now the race is on to find the next Langer Heinrich or Rossing South under the desert sand and calcrete that covers much of the county. Explorers are motivated by the favourable geology and recent exploration successes, a mining friendly regime in Namibia, and the positive outlook for uranium demand.  As of August, 2009, there are 413 new nuclear reactors planned or proposed worldwide, up 30% from the same time last year, according to Resource Capital Research’s most recent quarterly report.

Two more Namibian mines, Areva’s Trekkopje and Forsys Metal's Valencia, are set for commissioning by 2011 and there are several advanced exploration projects that have bulk mining potential.

At Rössing South, Extract Resources has outlined a resource of 108m pounds of uranium oxide grading 0.043% U308.  Southwest of Rössing, Bannerman Resources has outlined 127m pounds grading 0.02% U308. And Deep Yellow is drilling high grade  mineralization in a new skarn-type mineral environment at its Inca project nearby.

The next generation of discoveries could catapult Namibia into first place among global uranium producers. Corner and his RES colleagues hope to play a part in the country’s mining renaissance by using radon gas to detect deeper deposits that previous radiometric surveys may have missed.