Geochemical Controls on Uranium Release from Neutral-pH Rock Drainage Produced by Weathering of Granite, Gneiss, and Schist

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.

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking 234U-235U-238U, Groundwater Ages, and Hydrogeochemistry

“…62,63 Three weathering facies are used to operationally define rocks: "oxide" facies rocks have >95% of surfaces showing visible oxidation and the presence of limonite/hematite stains of cubic vugs indicative of pyrite oxidation; "transition" facies rocks have 5−95% of surfaces with visible oxidation while fresh surfaces are also present; and "fresh" facies rocks show unoxidized pyrite, weak to absent Fe-oxides, and sample oxidation (<5%) that does not extend beyond fractures. 38 Approximately 62% of the ground at the site is underlain by permafrost, which controls hydrogeology and hydrology. Permafrost is aspect-and elevation-dependent, with the thickest instances measuring ∼165 m and occurring at high elevation along north-facing aspects.…”

“…2 Weathering experiments using locally sourced mine waste-rocks indicate that granite has the highest U abundance, yet schist and gneiss produce higher dissolved U release rates because their carbonate content promotes CaUC formation and uranyl desorption. 38…”

“…A pre-existing geochemical data set of 490 rock drill-core samples analyzed during geological exploration between 2014 and 2017 at the Coffee deposit provides information on U distribution in the deposit. 38 In the present study, 14 of those samples, along with a selection of 19 new drill-core samples collected in 2019, were analyzed using more detailed geochemical and mineralogical techniques to assess solid-phase U geochemistry. Sample pulps were stored indoor in a temperature-controlled facility prior to reanalysis in this study.…”

“…Rock weathering experiments at the site have also previously showed that granite produces lower U release rates than gneiss and schist. 38 Although granite rock on average has a higher U content than gneiss and schist, it has minimal carbonate which accounts for lower CaUC formation and therefore lower dissolved U release. 38 This lithological control results in U concentration maxima spatially overlapping with gneiss and schist geology (Figure 3).…”

“…90−92 This reaction network, in particular the resulting DIC and Ca concentrations at circumneutral pH, is conducive to complexation of U as CaUC species that inhibit U sorption. 28,36,38 This hydrogeochemistry is characteristic of gneiss and schist, which host the main orebody in carbonate and sulfide veins and their weathered derivatives. 62 Indeed, speciation modeling of groundwater indicates that CaUC complexes account for 94−99% of dissolved U in schist and gneiss bedrock groundwater (Supplementary Table S3).…”