A.J. Wall | Geology Reports

USGS2011

Ultramafic-derived arsenic in a fractured bedrock aquifer

In the fractured bedrock aquifer of northern Vermont, USA, As concentrations in groundwater range from <1 to 327??g/L (<13-4360nm/L) and these elevated occurrences have a general spatial association with ultramafic rock bodies. The ultramafic rocks in this region are comprised mainly of serpentinites and talc-magnesite rocks with average As concentration of 93ppm and a range from 1 to 1105ppm. By comparison, the other main lithologies in the study area are depleted in As relative to the ultramafics: the average As concentration in metabasaltic rocks is 4.1ppm with a range of <1-69ppm, and mean As concentration in meta-sedimentary phyllites and schists is 22ppm with a range of <1-190ppm. In the ultramafic rocks, As is correlated with Sb and light rare earth elements, indicating that As was introduced to the ultramafic rocks during metasomatism by fluids derived from the subducting slab. Evidence from sequential chemical extraction, X-ray diffraction (XRD) and stoichiometric analysis indicates that the majority of the As is located in antigorite and magnesite (MgCO3) with lesser amounts in magnetite (Fe3O4). Hydrochemistry of monitoring wells drilled into fractured ultramafic rock in a groundwater recharge area with no anthropogenic As source reveals above background As (2-9??g/L) and an Mg-HCO3 hydrochemical signature that reflects dissolution of antigorite and magnesite, confirming that As in groundwater can be derived from ultramafic rock dissolution. Arsenic mobility in groundwater affected by ultramafic rock dissolution may be enhanced by alkaline pH values and relatively high HCO3- concentrations. ?? 2011 Elsevier Ltd.

Applied Geochemistry↗

USGS2009

Copper isotope fractionation in acid mine drainage

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ 65 Cu values (based on 65 Cu/ 63 Cu) of enargite (δ 65 Cu = −0.01 ± 0.10‰; 2 σ ) and chalcopyrite (δ 65 Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ 65 Cu < 1‰). These mineral samples show lower δ 65 Cu values than stream waters (1.38‰ ⩽ δ 65 Cu ⩽ 1.69‰). The average isotopic fractionation (Δ aq-min = δ 65 Cu aq − δ 65 Cu min , where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in 65 Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite ( Δ aq-min o = - 0.57 ± 0.14 ‰ ">‰ Δaq-mino=-0.57±0.14‰ , where min o refers to the starting mineral) and no apparent fractionation for enargite ( Δ aq-min o = 0.14 ± 0.14 ‰ ">‰ Δaq-mino=0.14±0.14‰ ). Abiotic fractionation is attributed to preferential oxidation of 65 Cu + at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of 65 Cu aq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ 65 Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.

Geochimica et Cosmochimica Acta↗