- Email: [email protected]
Application of synchrotron-based micro-analysis to mine waste mineralogy
Goldschmidt Conference Abstracts 2006
Mineralogy and bioaccessibility of arsenic-bearing secondary phases in gold mine tailings 1
1
A289
Application of synchrotron-based micro-analysis to mine waste mineralogy H.E. JAMIESON, S.R. WALKER, C.F. ANDRADE
2
H.E. JAMIESON , M.C. CORRIVEAU , M.B. PARSONS , I. KOCH3, K.J. REIMER3
Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ont., Canada K7N 3L6 ([email protected])
1
Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ont., Canada K7N 3L6 (jamieson@ geol.queensu.ca) 2 Geological Survey of Canada-Atlantic, Dartmouth, NS, Canada 3 Royal Military College, Kingston, Ont., Canada High concentrations of arsenic in windblown and vehicleraised dust from abandoned gold mine tailings in Nova Scotia, Canada pose a potential health risk for local residents who use these areas for recreational activities. The exposure involves inhalation of dust, as well as oral ingestion of particles. It is critical to understand the nature of the arsenic-hosting mineral phases in dust particles in order to evaluate the human health risk. The tailings consist dominantly of quartz, muscovite, Fe-rich clinochlore, and albite. Arsenic concentration in the samples collected from three sites varies from 400 to 28,600 ppm in the bulk samples and 3000 to 10,500 ppm in the <38 lm fraction. This significantly exceeds 12 ppm, the soil quality guideline of the Canadian Council of Ministers of the Environment. The arsenic was originally in the form of arsenopyrite, but vigorous oxidation has destroyed most of the sulfide in near-surface samples. Scorodite (FeAsO4 Æ 2H2O) was found to be the dominant secondary phase in some samples. At the microscopic scale, scorodite cements silicate grains and in the field it forms hardpans which, at some sites, have been pulverized by vehicle activity. In other samples, arsenic is hosted in iron oxyhydroxides with variable As2O5 and CaO concentrations (up to 30 wt.% and 8 wt.%, respectively). In order to evaluate how much of the arsenic is bioaccessible, the samples were subjected to an in vitro two-part extraction method designed to mimic the human digestion system. The results indicate that the percent of bioaccessible arsenic in the scorodite-rich samples is significantly lower than in the samples where arsenic is hosted by iron oxyhydroxides and Ca–Fe arsenates. However, because the total arsenic in the scorodite-rich sample is much higher, the actual concentration of arsenic that is bioaccessible is also higher. Recent recognition that expanding residential developments are increasing the chances of human exposure to arsenic-bearing tailings at many abandoned sites in Nova Scotia has led to the formation of a government-managed advisory committee as well as an interdisciplinary research group. Residents in several areas have been advised to avoid contact with gold mine tailings. doi:10.1016/j.gca.2006.06.586
Synchrotron-based micro-analysis has proven to be a powerful tool that can be used to identify mineralogical hosts of arsenic in contaminated soils and sediments. This is important for understanding whether arsenic will remain sequestered in association with the solid phase or released as environmental conditions change. We have conducted grain-by-grain microXRD and As K-edge microXANES in gold mine waste from a large site near Yellowknife in northern Canada. Arsenic is present at two to five orders of magnitude above the Canadian government soil guideline of 12 ppm. The arsenic was originally present in sulfide form (as arsenopyrite (FeAsS) and arsenic-bearing pyrite (FeS2)) and introduced naturally as part of the ore-forming process. The ore was roasted in order to liberate gold. This transformed most of the arsenopyrite and much of the pyrite to arsenic-bearing iron oxides. Our synchrotron work has shown that these roaster oxides are nanocrystalline composite grains of maghemite and hematite that contain up to 7 wt% As as a mixture of AsV and AsIII. The trivalent arsenic species persist in the roaster oxides even where they have been exposed to the atmosphere for over 60 years. We have also determined that the solid arsenic-bearing phases are vulnerable to reductive dissolution when buried in the oxygen-poor environment of lake sediments. doi:10.1016/j.gca.2006.06.587
Mineralogy and bioaccessibility of arsenic-bearing secondary phases in gold mine tailings 1
1
A289
Application of synchrotron-based micro-analysis to mine waste mineralogy H.E. JAMIESON, S.R. WALKER, C.F. ANDRADE
2
H.E. JAMIESON , M.C. CORRIVEAU , M.B. PARSONS , I. KOCH3, K.J. REIMER3
Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ont., Canada K7N 3L6 ([email protected])
1
Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ont., Canada K7N 3L6 (jamieson@ geol.queensu.ca) 2 Geological Survey of Canada-Atlantic, Dartmouth, NS, Canada 3 Royal Military College, Kingston, Ont., Canada High concentrations of arsenic in windblown and vehicleraised dust from abandoned gold mine tailings in Nova Scotia, Canada pose a potential health risk for local residents who use these areas for recreational activities. The exposure involves inhalation of dust, as well as oral ingestion of particles. It is critical to understand the nature of the arsenic-hosting mineral phases in dust particles in order to evaluate the human health risk. The tailings consist dominantly of quartz, muscovite, Fe-rich clinochlore, and albite. Arsenic concentration in the samples collected from three sites varies from 400 to 28,600 ppm in the bulk samples and 3000 to 10,500 ppm in the <38 lm fraction. This significantly exceeds 12 ppm, the soil quality guideline of the Canadian Council of Ministers of the Environment. The arsenic was originally in the form of arsenopyrite, but vigorous oxidation has destroyed most of the sulfide in near-surface samples. Scorodite (FeAsO4 Æ 2H2O) was found to be the dominant secondary phase in some samples. At the microscopic scale, scorodite cements silicate grains and in the field it forms hardpans which, at some sites, have been pulverized by vehicle activity. In other samples, arsenic is hosted in iron oxyhydroxides with variable As2O5 and CaO concentrations (up to 30 wt.% and 8 wt.%, respectively). In order to evaluate how much of the arsenic is bioaccessible, the samples were subjected to an in vitro two-part extraction method designed to mimic the human digestion system. The results indicate that the percent of bioaccessible arsenic in the scorodite-rich samples is significantly lower than in the samples where arsenic is hosted by iron oxyhydroxides and Ca–Fe arsenates. However, because the total arsenic in the scorodite-rich sample is much higher, the actual concentration of arsenic that is bioaccessible is also higher. Recent recognition that expanding residential developments are increasing the chances of human exposure to arsenic-bearing tailings at many abandoned sites in Nova Scotia has led to the formation of a government-managed advisory committee as well as an interdisciplinary research group. Residents in several areas have been advised to avoid contact with gold mine tailings. doi:10.1016/j.gca.2006.06.586
Synchrotron-based micro-analysis has proven to be a powerful tool that can be used to identify mineralogical hosts of arsenic in contaminated soils and sediments. This is important for understanding whether arsenic will remain sequestered in association with the solid phase or released as environmental conditions change. We have conducted grain-by-grain microXRD and As K-edge microXANES in gold mine waste from a large site near Yellowknife in northern Canada. Arsenic is present at two to five orders of magnitude above the Canadian government soil guideline of 12 ppm. The arsenic was originally present in sulfide form (as arsenopyrite (FeAsS) and arsenic-bearing pyrite (FeS2)) and introduced naturally as part of the ore-forming process. The ore was roasted in order to liberate gold. This transformed most of the arsenopyrite and much of the pyrite to arsenic-bearing iron oxides. Our synchrotron work has shown that these roaster oxides are nanocrystalline composite grains of maghemite and hematite that contain up to 7 wt% As as a mixture of AsV and AsIII. The trivalent arsenic species persist in the roaster oxides even where they have been exposed to the atmosphere for over 60 years. We have also determined that the solid arsenic-bearing phases are vulnerable to reductive dissolution when buried in the oxygen-poor environment of lake sediments. doi:10.1016/j.gca.2006.06.587