Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwaters associated with Zn-Pb massive sulphide deposits

Ground and surface waters collected from two undisturbed Zn-Pb massive sulp hide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NE, Canada were analysed for the rare e arth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters ar e broadly similar within and between deposit areas, although there are spat ial variations reflecting differences in pH and redox conditions. Surface w aters are characterised by strong negative Ce anomalies ([Ce/Ce*](NASC) as low as 0.08), produced by oxidation of Ce3+ :to Ce4+ and preferential remov al of Ce4+ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting g enerally more reducing conditions in the subsurface environment and indicat ing that Ce oxidation is a rapid process in the surface waters. Deeper grou ndwaters at the Halfmile Lake deposit are characterised by REE patterns tha t are similar to the host lithologies, whereas most groundwaters at the Res tigouche deposit have LREE-depleted patterns compared to NASC, Halfmile Lak e deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have u nusual patterns which. reflect either the adsorption of light REE onto coll oids and fracture-zone minerals and/or precipitation of REE-phosphate miner als. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters i ndicating that the REE are removed from solution by adsorption to Fe and Mn -oxyhydroxides in the order middle REE greater than or equal to light REE > heavy REE. (C) 2000 Elsevier Science Ltd. All rights reserved.