RADON EMANOMETRY IN URANIUM EXPLORATION USING ACTIVATED CHARCOAL : NAMIBIAN CASE STUDIES. Dr. B. Corner*, H. Sinclair** and D.

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1 RADON EMANOMETRY IN URANIUM EXPLORATION USING ACTIVATED CHARCOAL : NAMIBIAN CASE STUDIES Dr. B. Corner*, H. Sinclair** and D. Verran** Remote Exploration Services (Pty) Ltd * P. O. Box 2055 ** P.O Box 38668, Swakopmund Pinelands 7430 Namibia Cape Town branko@iafrica.com.na South Africa INTRODUCTION Radon emanometry is based on the ability of radon ( 222 Rn), a gaseous daughter product originating from buried uranium, to migrate to surface together with the ground air. This is facilitated through the pumping action of diurnal pressure variations and a high permeability of the cover strata. Most instrumentation, used in radon-based uranium exploration, relies on alphaparticle detection. However, detection has also been achieved through the measurement of the gamma emission from radon s daughter products, 214 Bi and 214 Pb, following adsorption of the radon onto activated charcoal. This technique, initially developed by the SA Atomic Energy Board (Hambleton-Jones and Smit, 1980), was termed Radon-on-Activated-Charcoal (ROAC). The present study discusses the implementation and refinement of the ROAC technique. The presented case study data show that the currently developed system, RadonX TM, has proven to be highly effective through both residual and transported surficial cover, with a depth of penetration of 100m or more under favourable permeability conditions. METHODOLOGY The field detector essentially comprises activated charcoal, contained within a cartridge fitted into the base of a plastic cup (Figure 1). The inverted cup is buried in a shallow hole, ~40cm deep, so as to minimise solar heating of the charcoal. A plastic sheet is placed over the cup if necessary, prior to filling the hole, to reduce moisture penetration from above. The location is recorded with a GPS receiver and is marked with a survey flag for ease of later recovery. Integration of the daily radon flux, over a period of 10 days, was found to be adequate to obtain representative and repeatable results. On retrieval the charcoal-filled cartridges are closed and the period of residence in the ground, as well as the period from retrieval to measurement, is recorded. The data are normalized to 10 days for all cartridges. All readings are corrected for variations in the time elapsed between retrieval and measurement. A gamma spectrometer is used to measure the gamma radiation arising from the 214 Bi and 214 Pb. Measurements are made on site, in areas of low background, immediately after retrieval. Background effects are nevertheless further reduced through the use of a lead castle. The

2 emphasis of all procedures, choice of activated charcoal, and equipment parameter design, was to optimize counting statistics so as to increase signal-to-noise ratio. Radon ( 222 Rn) arising from uranium has a half-life of days, and will decay almost completely in roughly 10 times this period, i.e. ~38 days. This provides sufficient time for 222 Rn to migrate to surface, to be adsorbed onto the charcoal, and to remain adsorbed for long enough after retrieval to be measured (through its daughter products 214 Bi and 214 Pb). A major benefit of the RadonX TM technique is that thoron ( 220 Rn) arising from thorium that might be contained in the bedrock, is not measured due to its short half-life of 55 seconds. CASE STUDIES Basement and duricrust case-study data are presented from two areas in the Namib Desert of Namibia, host to numerous uranium deposits and occurrences. Basement example A profile is shown from an area which has potential for uraniferous granites within the basement, comprising rocks of the Damara Sequence (Figure 2). The area is covered by aeolian sands and an underlying sequence of older aeolian sandstones of the Tsondab Formation. Time domain electromagnetic (TDEM) soundings showed that the Tsondab sandstones have a low resistivity and hence a significant porosity. The RadonX TM anomalies are seen to be symmetrical about an anticline, interpreted from aeromagnetic data. RadonX TM is thus readily responding to geology, and also indicates potential mineralization, through the relatively thick cover of sediments. A depth of penetration analysis was facilitated by using the TDEM soundings. Figure 3 shows a plot of RadonX TM gamma counts versus depth to bedrock, interpreted from the TDEM soundings. RadonX TM anomalies are clearly evident at depths even in excess of 100m. Permeability, associated with either primary or secondary porosity, is the key to providing the pathways for the radon and ground air to migrate to surface. Duricrust example The uranium mineralization of interest in this example is hosted in calcretized valley-fill sediments associated with a palaeo-drainage channel. Figure 4 shows the disposition of the mineralized palaeo-channel at depth, mapped in a past program using dipole-dipole resistivity, overlaid onto an airborne uranium image. Figures 5 and 6 show the RadonX TM results, and an excellent correlation of radon anomalies with the palaeo-channel. CONCLUSIONS The RadonX TM technique has proven to be a rapid, effective and essential adjunct to gamma spectrometric surveys in view of the limited penetration of the latter. Important conclusions are: Sensitivity is significantly improved, in terms of anomaly amplitude, compared to a 30-day alpha-detection survey conducted previously in the duricrust area. Resolution is improved with detailed grid surveys, and accurate mapping of uranium mineralisation is facilitated. Displacement of anomalies may occur if cover sequences are non-permeable, and radon migration is confined to fractures and faults.

3 An excellent depth of penetration, of 100m or more, is evident under favourable permeability conditions. ACKNOWLEDGMENTS Permission to publish the results by Reptile Uranium Namibia (Pty) Ltd, Pitchstone Exploration Ltd., and Manica Minerals Ltd., is gratefully acknowledged. REFERENCE Hambleton-Jones, B.B., and Smit, M.C.B., 1980, ROAC - A new dimension in radon prospecting: South African Atomic Energy Report, Per-48, pp24. Figure 1. A RadonX TM cup buried in a shallow hole. Figure 2. Basement example. RadonX TM counts, and bedrock profile derived from TDEM soundings. The schematic geology is interpreted from aeromagnetic data.

4 Figure 3. Basement area, with aeolian cover. RadonX TM counts versus depth to bedrock, showing significant responses at depths in excess of 100m. Figure 4. Duricrust example. Present-day drainage (open blue lines) and palaeo-channel, overlaid onto an airborne uranium image.

5 Figure 5. Duricrust example. RadonX TM results from a grid with 200m stations along 500m lines (dots). Figure 6. Duricrust example. RadonX TM contours overlaid onto mineralized palaeochannel, showing good correlation.