RADON EMANOMETRY CASE STUDIES IN NAMIBIA : THE SPITZKOPPE AND TUMAS URANIUM DEPOSITS

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1 RADON EMANOMETRY CASE STUDIES IN NAMIBIA : THE SPITZKOPPE AND TUMAS URANIUM DEPOSITS Dr. B. Corner 1, Dr. V. Osiyuk 2, S. Lytvyniuk 2, A. Kuchmin 2, and D. Verran 3 1. Corner Geophysics Namibia, branko@iafrica.com.na 2. SWA Uranium Mines (Pty) Ltd., Namibia. 3. Remote Exploration Services (Pty)Ltd., South Africa/Namibia ABSTRACT The RadonX TM technique uses the principle of adsorption of radon, emanating from buried uranium mineralization, onto activated charcoal. The charcoal is contained within a cartridge which is fitted into the base of an inverted cup, and is buried in the ground for a period of approximately 10 days. The technique differs from alpha-sensitive radon detection systems in that it measures the gamma radiation arising from the daughter products of the adsorbed radon, namely 214 Bi and 214 Pb. Two case studies are presented, dealing with the detection of partially buried shallow uranium deposits, using the RadonX TM radon emanometry technique. The case studies are sited over the Spitzkoppe and Tumas deposits in the Central Zone of Namibia, comprising uranium mineralization contained in shallow calcretized palaeo-valley gravels buried under a thin cover of calcrete or gypcrete duricrust. The results show that both deposits have clear RadonX TM target anomaly signatures, in large part correlating with the higher grade areas. The technique is thus an effective and essential adjunct to gamma spectrometric surveys in surrounding areas of cover. Although these deposits are shallow, with a depth extent of up to ~20 m, case studies in other areas show an excellent depth of penetration, of 100m or more, under favourable permeability conditions. Key words: radon, emanometry, uranium, exploration 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 both of diurnal pressure variations and diurnal groundwater tidal variations. A high permeability of the cover strata is a prerequisite for a measurable radon flux at surface. Most instrumentation, used in radon-based uranium exploration, relies on alpha-particle 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 was initially developed by the SA Atomic Energy Board (Hambleton-Jones and Smit, 1980), and termed Radonon-Activated-Charcoal (ROAC). The RadonX TM radon emanometry technique is a refinement of ROAC through improvement of sensitivity and hence signal-tonoise ratio. Two case studies are presented, using RadonX TM, over the Spitzkoppe deposit, near Spitzkoppe mountain, and over the Tumas deposit along the Tumas drainage system, northeast and southeast of Swakopmund respectively. The deposits occur in the central Namib region of Namibia, with the mineralization being contained in shallow calcretized palaeo-valley gravels buried under a thin veneer of calcrete or gypcrete duricrust. Both past and the present studies have shown that the RadonX TM technique has proven to be highly effective through residual and transported surficial cover, with a depth of penetration of 100m or more under favourable permeability conditions. The case studies demonstrate that a well designed grid survey could accurately map the disposition and grade of shallow mineralization under a cover of permeable sediments. METHODOLOGY The field detector essentially comprises activated charcoal, contained within a cartridge fitted into the base of a plastic cup. 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 11 th SAGA Biennial Technical Meeting and Exhibition Short Paper

2 Radon emanometry: The Tumas and Rössing South U deposits, Namibia Corner, Sinclair and Verran 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 approximately 10 days, has been found to be adequate to obtain representative and repeatable results. On retrieval, the charcoal-filled cartridges are closed and the exact period of residence in the ground, as well as the exact 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 daughter products. 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 emphasis of all procedures, choice of activated charcoal, and equipment parameter design, was to optimize counting statistics so as to increase the 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 which limits its passage to surface and detection after cup retrieval. RESULTS The Spitzkoppe uranium deposit The uranium mineralization is hosted in a shallow, relatively flat lying calcretized palaeo-valley. Cover thickness varies from outcrop to 5 m over most of the known deposit, locally being up to 15 m thick. Figure 1 shows this schematically, overlaid onto an airborne uranium image which clearly outlines the outcrop extent of the ore zone. A number of boreholes were drilled by the General Mining and Finance Corp. (Pty) Ltd., in the mid-1970 s. Grade zonation from this historic drilling is shown in Figure 3. The RadonX TM data, shown both in Figures 2 and 3, clearly correlate well with the surface exposure of the ore zone, as well as with the highest grades, even considering the relatively coarse cup spacing of 200 x 500 m. RadonX TM anomalies indicate mineralization extensions away from the previously drilled area, under cover (Figure 3). The Tumas deposit The uranium mineralization is hosted in calcretized valley-fill sediments associated with the shallow (<20 m) Tumas palaeo-drainage channel. Cover varies from 0 to 10 m. Figure 4 shows the disposition of the mineralized palaeo-channel at shallow depth, mapped in a past program using dipole-dipole resistivity, overlaid onto an airborne uranium image. Outcrop manifestations of the ore are indicated by the isolated uranium anomalies. In Figures 5 and 6, the RadonX TM anomalies show an excellent correlation with the historic grade-thickness products. Note that only the historic boreholes, drilled exclusively into the palaeochannel, were available for comparison at the time of writing. CONCLUSIONS Both the Spitzkoppe and Tumas duricrust palaeovalley uranium occurrences have clear RadonX TM target anomaly signatures. Even considering the relatively coarse reconnaissance grids used, the RadonX TM anomalies correlate well with increases in grade. This is facilitated by the shallow extent of the ore zones. An excellent depth of penetration, of 100 m or more, has been demonstrated under favourable permeability conditions in a separate case study. Target resolution is inevitably improved with detailed grid surveys. Accurate mapping of uranium mineralization is facilitated, particularly if shallow, where loss, or displacement, of radon anomaly signatures due to impervious cover is less likely. The RadonX TM technique is an effective and essential adjunct to gamma spectrometric surveys, and provides a unique and important data layer in itself, in areas of cover. Sensitivity is significantly improved, in terms of anomaly amplitude, compared to a 30-day alphadetection survey conducted in the past over the Tumas deposit. ACKNOWLEDGMENTS The kind permission of SWA Uranium Mines (Pty) Ltd, and Reptile Uranium Namibia (Pty) Ltd to publish these results, is gratefully acknowledged REFERENCES Hambleton-Jones, B.B., and Smit, M.C.B., 1980, ROAC - A new dimension in radon prospecting: South African Atomic Energy Report, Per-48, pp th SAGA Biennial Technical Meeting and Exhibition Short Paper

3 Figure 1. Spitzkoppe calcrete valley-fill deposit. Airborne uranium image, showing historic borehole localities (dots), outcrop limit, and >5 m cover depth contours in the drilled area. (Courtesy SWA Uranium Mines (Pty) Ltd). Figure 2. Spitzkoppe duricrust valley-fill deposit. Panel of a RadonX TM image derived from a larger 200 x 500 m cup grid (dots). (Courtesy SWA Uranium Mines (Pty) Ltd).

4 Figure 3. Spitzkoppe duricrust valley-fill deposit. RadonX TM contours overlaid onto U grades in the area of historic drilling, underlain by the RadonX TM image. (Courtesy SWA Uranium Mines (Pty) Ltd). Figure 4. Tumas duricrust valley-fill example. Present-day drainage (open blue lines) and palaeo-channel, overlaid onto an airborne uranium image. (Courtesy Reptile Uranium Namibia (Pty) Ltd).

5 Figure 5. Tumas duricrust valley-fill example. RadonX TM image derived from a 200 x 500 m cup grid (dots). (Courtesy Reptile Uranium Namibia (Pty) Ltd). Figure 6. Tumas duricrust valley-fill example. RadonX TM contours overlaid onto an image of historic grade.thickness products (dots=boreholes). Recent infill drilling is not shown (Courtesy Reptile Uranium Namibia (Pty) Ltd).