Dynamic Arsenic Cycling in Scorodite-Bearing Hardpan Cements, Montague Gold Mines, Nova Scotia
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Hardpans, or cemented layers, form from precipitation and subsequent cementation of secondary minerals in mine tailings and can act as both physical and chemical barriers. During precipitation, metals in the tailings are sequestered, making hardpan a potentially viable method of natural attenuation. At Montague Gold Mines, Nova Scotia, tailings are partially cemented by the iron (Fe) arsenate mineral scorodite (FeAsO4•2H2O). Scorodite is known as a phase that can effectively limit aqueous arsenic (As) concentrations due to its relatively low solubility (<1 ppm, pH 5) and high As content (~30 wt.%). However, scorodite will not lower As concentrations from waters to below the Canadian drinking water guideline (0.010 ppm). To identify current field conditions influencing scorodite precipitation and dissolution and to better understand the mineralogical and chemical relationship between hardpan and tailings, coexisting waters and solids were sampled to provide information on tailings-water interactions. Hardpan cement compositions were found to include Fe arsenate and Fe oxyhydroxide in addition to scorodite. End-member pore water chemistry was identified based on pH and dissolved concentration extremes (e.g. pH 3.78, As(aq) 35.8 ppm) compared to most other samples (avg. pH 6.41, As(aq) 2.07 ppm). These end-member characteristics coincide with the most extensive and dispersed areas of hardpan. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity and dissolved As+5 and Fe+3 for scorodite precipitation. A proposed model of progressive arsenopyrite oxidation suggests localized As cycling involving scorodite is occurring but is dependent on sulfide persistence. Therefore, permanent As sequestration is not expected. Remediation efforts would have to consider the possibility of scorodite dissolution after complete sulfide consumption or as a consequence of applying certain technologies, such as a cover. Conversely, if scorodite stability were maintained, the hardpan could be considered as a component in remediating the tailings at Montague.