Abandoned mine slags analysis by EPMA WDS x-ray mapping

The mining activity on the Iberian Pyritic Belt (Portugal and Spain) started before Phoenician times, became particularly intense during the Roman occupation of the Iberian Peninsula (for gold) and after the industrial revolution (for gold, copper, zinc, lead and sulphur). The commonest ore of this region is a massive polymetalic sulphide accumulation, where pyrite (FeS2) is the main mineral, with variable concentrations of chalcopyrite (CuFeS2), sphalerite (ZnS), galena (PbS), arsenopyrite (FeAsS2), other sulphides and sulfosalts which include minor elements like Mn, Co, Ni, Se, Cd, Sb, Te, Hg and Bi. Some of the main and minor elements of these ores are hazardous and the drainage basins of pollutant source areas often induce health concerns in the resident population. Electron microprobe study followed previous optical and XRD analysis of the slags. The study focused on the identification of phases how sulphide and metallic phases are distributed within the material and infer about leachable elements during weathering. Electron microprobe X-Ray maps show evidences of different behavior between the elements: Ca and Zn are completely leached; iron is retained in oxyhydroxides, lead and arsenic precipitate as sulphates. Electron microprobe studies are essential to understand complex materials as earth materials. Nevertheless, care is required to a correct interpretation of data and most quantitative compositional data are not trustworthy. Introduction Abandoned mine slags are non-stable waste products of various compositions and the source of important environment contaminations [1], [2]. The problem is particularly serious at the abandoned mine of S. Domingos (southern Portugal) where the acid mine drainage is very intense. Moreover, since the main exploration purpose was the extraction of sulphur, large piles of metals-enriched slag were left “in-situ” for at least forty years. The soil contamination is so strong that several square kilometres exhibit a complete depletion of vegetation resembling a desert-area or display small clusters of metal hyper-accumulator plant species (Figure 1). It is indeed a serious problem, which may be the first cause also of water stream contaminations. Fig1: Foto taken near S.Domingos abandoned mine The contamination overcame to human population and a comparison study of scalp hair analysis [3] related higher levels of As, Cu and Zn in downstream (Santana de Cambas) local population than those living a upstream the contamination source (Corte do Pinto). The neoformation materials control the metals that are or are not transported by surface waters because some heavy metals can be scavenged by them [2]. The capability of these phases to retain pollutant metals in acidic environments of abandoned mines is being studied, but transferability is not direct to other climatic conditions and availability of components. However, to establish a link between pollutant metal and retainer mineral, microanalytical techniques such as the electron microprobe analyzer, with adequate capability to detect phases and minor elements are required, although the complex phases encountered. Electron microprobe is a powerful technique that is used to study this type of materials. An approach consists of analyzing the several primary and secondary phases through quantitative analysis. However the phases are complex (Fig. 2a and 2b) and good uniform polishing is hard to achieve. In this work we focus in electron backscattered images and X-Ray map imaging in order to understand what happens during weathering and where elements are distributed. Figure 2aSecondary electron image of a detail of the weathered product Figure 2b: Backscattered electron image of the glass showing its interface with weathered product. Sub micrometric spherules are of Iron, Copper, Zinc and Lead sulphides.