VISUAL STATISTICAL CLASSIFICATION OF As Fe-RICH PRODUCTS OF ALTERATION OF TAILINGS FROM THE ENGUIALES TUNSGTEN MINE, FRANCE

Abstract

We have studied the products of alteration of sulfides contained in the tailings of a former tungsten mine at Enguialès, Aveyron, France. Most of these products are poorly crystalline and contain high concentrations of arsenic. A preliminary classification based on visual observations was carried out, and five products were distinguished according to their color and texture. The data obtained by electron-microprobe analysis were submitted to different statistical approaches in order to discriminate among the various materials and to test the efficiency of the visual classification. The statistics involved rely on logratios of the chemical elements because the ratio of any two elements is independent of the perturbations caused by other elements, and remains unchanged whatever the choice of subcompositional space. The statistical discrimination of the groups indicates that log(Fe/As) and log(S/As) best express the chemical differences. Both of these logratios correspond to the system Fe-As-S. In this study, the two approaches, visual observations and statistics, are complementary, and one method without the other could give only partial information or misinterpretations. A hierarchical clustering applied to the whole dataset confirms the existence of five groups, but visual and statistical classifications differ. On one hand, by using the logratio method, different subgroups with the same color were identified. On the other hand, some groups with different colors correspond to the same mineral, which was highlighted by Raman microspectrometry. These observations (concerning As-rich jarosite, for example) are explained mainly by differences in composition.Keywords: mining environment, As-bearing products, iron oxyhydroxides, Raman microspectroscopy, compositional data, logratios, multivariate statistical analysis, Enguialès mine, France. SOMMAIRELes haldes d'une ancienne mine de tungstène à Enguialès, Aveyron, en France, présentent des produits d'altération de sulfures, pour l'essentiel faiblement cristallins, et contenant de fortes teneurs en arsenic. Ces produits d'altération ont été préalablement classés de manière visuelle, c'est-à-dire en fonction de leur couleur et de leur texture, et cinq groupes ont été ainsi définis. Les données d'analyses en microsonde électronique obtenues sur ces matériaux ont été soumises à différentes approches statistiques dans le but de les discriminer de manière plus précise, et de tester l'efficacité de la classification visuelle. Les méthodes mises en oeuvre utilisent le log des rapports entre éléments chimiques, car tout rapport de deux éléments est indépendant des perturbations causées par les autres éléments, et demeure inchangé quelle que soit la sous-composition considérée. L'analyse discriminante des groupes visuels a montré que ce sont log(Fe/As) et log(S/As) qui expriment le mieux les différences chimiques entre groupes. Ces deux logs de rapports correspondent au système Fe-As-S. Dans cette étude, nous montrons que les deux approches (visuelle et statistique) son...

Arsenic in drinking water: sources, occurrence and health effects (a review)

“…Arsenic(V) can indeed be substituted for sulphate or sorbed onto more or less crystalline Fe or Mn oxides (e.g. Foster et al 1998Foster et al , 2003Lenain and Courtin-Nomade 2003;Morin et al 2003). However, arsenic can also be in competition with other anions than sulphate, such as phosphates, especially on Fe(III) or Al(III) oxides surface (Wang and Mulligan 2006) and this can enhance As desorption.…”