The effect of filtration size on the geochemistry of groundwater samples from a massive sulfide deposit at the Bathurst Mining Camp, New Brunswick, Canada

In hydrogeochemical studies, samples are commonly filtered to limit the fraction of analyte that is adsorbed or structurally bound to suspended particles, ensuring that only the dissolved fraction is analysed, and thereby reducing analytical bias during measurement. The standard filter size that has been adopted is 0.45 μm; however, ultrafiltration can be used to remove colloidal particles two orders of magnitude smaller. In the following, we investigate the effect that standard filtration (0.45 μm) and ultrafiltration (0.004 μm) have on the hydrogeochemistry of groundwaters from a volcanogenic massive sulfide (VMS) deposit at the Bathurst Mining Camp, New Brunswick, Canada. Groundwater samples were collected from six monitoring wells at the Nigadoo Mine tailings facility, and major and trace geochemistry were determined using a combination of inductively coupled plasma optical emission spectrometry, inductively coupled plasma mass spectrometry and ion chromatography. Waters at the Nigadoo deposit are generally enriched in Ca and SO4, relative to other major cations and anions. Some element contents – including those associated with VMS deposits – differ depending on the filtration technique used (e.g. As, Fe, Pb, rare earth elements and yttrium (REY)), some are equally affected by both techniques (e.g. Cu, Ni, Zn), and some are unaffected by filtration (e.g. Ba, Ca, Mn, Cl−). Shale-normalized REY anomalies (CeSN/CeSN*, EuSN/EuSN* and YSN/HoSN) and overall patterns can differ greatly (e.g. changing the sign of the anomaly) depending on the filtration technique used. We observe previously undocumented and, at this time, unexplainable fractionation of Ho and Yb (non-redox sensitive REY, unaffected by the tetrad effect) in unfiltered waters from the Nigadoo deposit. Differences in groundwater geochemistry induced by filtration technique can result in false positive and negative anomalies during environmental and exploration projects and must therefore be carefully considered. At the Nigadoo site, oxidation of sulfide minerals can occur, resulting in the formation of relatively unstable oxide minerals. Away from the tailings, where carbonate minerals are scarce and can no longer act as a pH buffer, the unstable oxide minerals break down and release metals and metalloids into the surrounding environment. The filtration methods used in this study can provide insight into where the specific metals and metalloids are hosted and how they are likely to behave under different redox conditions. Because VMS deposit pathfinder elements are enriched in unfiltered water, and differ by degree of filtration, geochemical analysis of the filtride material may also make an effective exploration tool. Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issues