Alteration of As-bearing phases in a small watershed located on a high grade arsenic-geochemical anomaly (French Massif Central)

Abstract

International audienceAt a watershed scale, sediments and soil weathering exerts a control on solid and dissolved transport of trace elements in surface waters and it can be considered as a source of pollution. The studied subwatershed (1.5 km2) was located on an As geochemical anomaly. The studied soil profile showed a significant decrease of As content from 1500 mg.kg-1 in the 135-165 cm deepest soil layer to 385 mg.kg-1 in the upper 0-5 cm soil layer. Directly in the stream, suspended matter and the <63 μm fraction of bed sediments had As concentrations greater than 400 mg.kg-1. In all these solid fractions, the main representative As-bearing phases were determined at two different observation scales: bulk analyses using X-ray absorption structure spectroscopy (XAS) and microanalyses using scanning electron microscope (SEM) and associated electron probe microanalyses (EPMA), as well as micro-Raman spectroscopy and synchrotron-based micro-scanning X-ray diffraction (μSXRD) characterization. Three main As-bearing phases were identified: (i) arsenates (mostly pharmacosiderite), the most concentrated phases As in both the coherent weathered bedrock and the 135-165 cm soil layer but not observed in the river solid fraction, (ii) Fe-oxyhydroxides with in situ As content up to 15.4 wt.% in the deepest soil layer, and (iii) aluminosilicates, the least concentrated As carriers. The mineralogical evolution of As-bearing phases in the soil profile, coupled with the decrease of bulk As content, may be related to pedogenesis processes, suggesting an evolution of arsenates into As-rich Fe-oxyhydroxides. Therefore, weathering and mineralogical evolution of these As-rich phases may release As to surface waters

Mapping arsenopyrite alteration in a quartz vein-hosted gold deposit using microbeam analytical techniques

“…Scorodite (FeAsO 4 •2H 2 O) was also recorded in the quartz vein during this study. Its Raman spectrum displays strong peaks at 801 and 888 cm -1 (Bossy et al, 2010;Das and Hendry, 2011). Finally, Fig.…”

Arsenic bioaccessibility in a gold mining area: a health risk assessment for children

“…Elevated As concentrations have been found in the environment (Girouard and Zagury 2009;Nathanail et al 2005;Palumbo-Roe et al 2005Pouschat and Zagury 2006;Rieuwerts et al 2006;Sarkar et al 2007), which may pose a threat to human health. This is especially true for tailings around gold mining areas (Borba et al 2003;Bossy et al 2010;Chung et al 2005;Juhasz et al 2007), with the highest concentration of total As reaching 31% (Meunier et al 2010).…”

Fate of arsenic-bearing phases during the suspended transport in a gold mining district (Isle river Basin, France)

“…Studied surface waters did not appear oversaturated for any type of arsenates during the studied period (dissolved As ranging from 7 to 35 μg/L; Grosbois et al, 2009). Therefore, they probably formed upstream in mining sites, in weathered bedrock and corresponding alterites and/or wetlands as described in several studies ( [Juillot et al, 1999] , [Drahota and Filippi, 2009] and [Bossy et al, 2010] ) but they were barely transported by SPM.…”

“…up to 15.2 wt.% and 6.0 wt.%, respectively and they showed no measurable P, low Fe and Mn, with concentrations lower than 3.0 wt.% and 0.3 wt.%, respectively. Although Fe-arsenates (pharmacosiderite group, (Ba, Na, K)Fe 4 (AsO 4 ) 3 (OH) 3 .6H 2 O)) were commonly observed in weathered bedrock and in the corresponding saprolite of the Upper Isle River soil (Bossy et al, 2010), any of SPM arsenate minerals did not correspond to these Fe-arsenate chemical composition. Two of these grains could be Ca-arsenates with typical Ca/As molar ratios (1.25-4.0; Zhu et al, 2006) which could form with lime addition in fields and in small lakes to increase water pH.…”

“…The natural mean geochemical background in arsenic, determined in non-mining influenced areas, is ~ 120 mg/kg in soils (Chery and Gateau, 1998), ~ 70 mg/kg in bed sediments (Grosbois et al, 2007) and ~ 10 μg/L in the < 0.45 μm aqueous fraction (Grosbois et al, 2009). Therefore, in the solid fraction (bed sediments and soils), the most enriched trace-element is arsenic in comparison to the regional geochemical background ( [Grosbois et al, 2007] and [Bossy et al, 2010] ).…”