Application of hydrogeochemistry to uranium exploration in the pine creek geosyncline, northern territory, Australia |
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| Authors: | AM Giblin AA Snelling |
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| Abstract: | In the Pine Creek Geosyncline, fast moving, annually recharged, low-salinity ground waters dissolve uranium- and magnesium-enriched gangue minerals from mineralized aquifer rocks. The level of dissolved uranium depends on prevailing pH, Eh, salinity and degree of adsorption, which limits its effectiveness as an exploration indicator. Near each known deposit, leaching of magnesium-enriched gangue minerals produces ground waters with very similar major-element concentration plots, the shape of which constitutes a mineralized aquifer “signature”. Gangue minerals also supply high levels of Mg2+ (expressed as NMg = Mg2+]/Ca2+ + Mg2+ + Na+ + K+] in milliequivalents per litre) to contained ground waters, NMg > 0.8 being common in ground waters from mineralized aquifers at each Pine Creek Geosyncline deposit. Data from Ranger One No. 3 ore body illustrates how progressive mixing of waters from mineralized and unmineralized aquifers causes graded reductions in NMg, which, when plotted onto a ground plan, delineate a hydrogeochemical aureole.High NMg (> 0.8) coincides with high uranium concentration (> 20 μg/l of U) in ground waters near Nabarlek and Ranger. Because pH-Eh conditions in aquifers at Jabiluka depress uranium solution, < 10 μg/l of U is present, although NMg values are generally > 0.8. To date NMg has always been < 0.8 in nonmineralized aquifer waters, whereas uranium may be > 50 μg/l in ground waters from felsic igneous aquifers, which can be identified as uneconomic by low (< 0.4) NMg, and by a fixed relationship between uranium and co-leached species such as F- and soluble salts.Measurements of pH, Eh, salinity, Fe(II), Ca, Mg, Na, K, Cl, SO4, total carbonate, phosphate, F-, Cu, Pb, Zn and U in waters from 48 percussion holes in and near the Koongarra ore bodies have been related to mineralogy recorded in drill logs. The composition of waters from 20 holes near and along strike from known mineralization, fitted the mineralized aquifer “signature”, had NMg > 0.8 and uranium up to 4100 μ/l. These data confirm the use in this region of NMg as a hydrogeochemical indicator of uranium mineralization; they also indicate additional zones of possible mineralization. |
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