ANTIMONY MOBILITY AND SPECIATION IN RECENTLY DEPOSITED MINE WASTE RESULTING FROM STIBNITE MINING AT BEAVER BROOK, NEWFOUNDLAND

The speciation of antimony (Sb) was determined in waters and tailings at Beaver Brook antimony deposit in order to understand its mobility during initial stages of stibnite (Sb₂S₃) decomposition. The most abundant Sb host in studied tailings is stibnite. Stibnite dissolves when exposed to near-surface conditions, Sb(III) is released to the pore water and oxidizes to Sb(V). While Sb is released into the drainage water, it is simultaneously incorporated in to the mineral structures of secondary minerals with different stabilities. We have measured concentrations of total Sb in tailings pore water up to 26.4 mg/L and only 0.9% of the total is Sb(III). In all surface water samples, Sb is mostly present in its fully oxidized form (98.9 - 100% of total Sb). The secondary minerals containing Sb include Sb-Fe oxides where Sb is an important part of their structure with a wide range of Fe/Sb ratios. Sb concentration can reach 37.8 percent by weight, making these secondary phases important Sb hosts in surface layers of tailings. Fe oxides enriched in Sb such as goethite where Sb (average of 3.9 wt%) is adsorbed or incorporated are common, but account for a small fraction of the total solid phase Sb. The elevated Mg concentrations in tailing ponds and pore water promote the precipitation of brandholzite (Mg[Sb(OH)₆]₂·6H₂O) during dry periods, which is easily dissolved during rainy periods. Brandholzite dissolution combined with stibnite dissolution may contribute dissolved Sb, whereas Sb hosted by Sb-Fe hydroxides are considered more stable in pH neutral, surface environment.