Migration of selected elements of environmental concern from unaltered pyrite-rich mineralizations to Fe-rich alteration crusts

Abstract

This study reports the results of simultaneous micro X-ray diffraction (μ-XRD) and micro X-ray fluorescence (μ-XRF) analyses, performed on a partially altered sulphide mineralization fragment sampled within the main waste-rock dump of the Fe–Cu sulphide Libiola Mine (northern Apennines, Italy) at ID18F beamline (ESRF, Grenoble), The study site is characterized by active acid mine drainage processes that, other than acid generation, determine the mobilization of metals and metalloids of environmental concern and other harmful elements that can be concentrated in waters and soils. The Fe-oxyhydroxides that form extensively as the main secondary products of sulphide oxidation processes are the key minerals in controlling the fate of these mobile elements, being able to effectively scavenge most of them through structural incorporation, adsorption, and co-precipitation mechanisms. When they occur as direct replacement of sulphide mineralization, or as precipitates rimming sulphides or filling porosity, they make it possible to reconstruct the mineralogical and chemical evolution of the system, spatially and temporally, and to understand the mechanism of migration and the fate of the toxic elements involved in the whole process. In this study we were able to follow, with micron-scale resolution, the spatial distribution of the chemical elements released during the sulphide- and gangue-mineral alteration and to evaluate their relationships with the newly forming Fe-oxyhydroxides. In particular, the mobility of Ni, Cu, Zn, As, Se, and Mo appears to be controlled by the goethite genesis which is able to take up these elements through several mechanisms such as structural incorporation (Ni), coprecipitation, and/or adsorption.