Abstract
We investigated geochemical processes controlling uranium release in neutral-pH (pH ≥ 6) rock drainage (NRD) at a prospective gold deposit hosted in granite, schist, and gneiss. Although uranium is not an economic target at this deposit, it is present in the host rock at a median abundance of 3.7 µg/g, i.e., above the average uranium content of the Earth’s crust. Field bin and column waste-rock weathering experiments using gneiss and schist mine waste rock produced circumneutral-pH (7.6 to 8.4) and high-alkalinity (41 to 499 mg/L as CaCO3) drainage, while granite produced drainage with lower pH (pH 4.7 to >8) and lower alkalinity (<10 to 210 mg/L as CaCO3). In all instances, U release was associated with calcium release and formation of weakly sorbing calcium-carbonato-uranyl aqueous complexes. This process accounted for the higher release of uranium from carbonate-bearing gneiss and schist than from granite despite the latter’s higher solid-phase uranium content. In addition, unweathered carbonate-bearing rocks having a higher sulfide-mineral content released more uranium than their oxidized counterparts because sulfuric acid produced during sulfide-mineral oxidation promoted dissolution of carbonate minerals, release of calcium, and formation of calcium-carbonato-uranyl aqueous complexes. Substantial uranium attenuation occurred during a sequencing experiment involving application of uranium-rich gneiss drainage into columns containing Fe-oxide rich schist. Geochemical modeling indicated that uranium attenuation in the sequencing experiment could be explained through surface complexation and that this process is highly sensitive to dissolved calcium concentrations and pCO2 under NRD conditions.
Highlights
Uranium contamination in water is a global concern due to this element’s chemical toxicity towards humans and other living organisms [1]
Uranium(VI) forms the uranyl aquo cation (UO2 2+ ), which is soluble at a wider pH range but can be attenuated via sorption onto clays, organic matter, and hydrous ferric oxides (HFO) [13,16,17,18,19,20,21,22]
Drill-core geochemical analyses indicate that rock at the Coffee deposit is generally non-acid generating (NAG) owing to excess of total inorganic carbon (TIC) over sulfur, which average 0.5 wt.% and 1 (Supplementary Figure S2)
Summary
Uranium contamination in water is a global concern due to this element’s chemical toxicity towards humans and other living organisms [1]. Uranium(aq) mobility is controlled by pH, redox conditions, aqueous complexation, interactions with mineral surfaces, and dissolution/precipitation of minerals [12,13,14]. Uranium(VI) forms the uranyl aquo cation (UO2 2+ ), which is soluble at a wider pH range but can be attenuated via sorption onto clays, organic matter, and hydrous ferric oxides (HFO) [13,16,17,18,19,20,21,22]. From pH 7 to pH 9 and in waters containing dissolved inorganic carbon (DIC), uranyl dicarbonate UO2 (CO3 )2 2− ] sorbs onto HFO via outer-sphere complexation [20]. Uranyl sorption is considerably weakened by formation of the calcium-carbonato-uranyl aqueous complexes Ca2 UO2 (CO3 ) and CaUO2 (CO3 )3 2−
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