Secondary minerals in a calcareous environment: an example from Um Gheig Pb/Zn mine site, Eastern Desert, Egypt

Abstract

Alteration of abandoned mine sites and wastes generates variable secondary mineral phases that incorporate different toxic trace elements with a prospective threat to the neighboring ecosystems. The main focus of this study was to investigate the mineralogical and geochemical changes at neutral pH where dry condition prevails around oxidation-primary contacts interface and the surface in Um Gheig Pb/Zn mine, Eastern Desert, Egypt. The secondary minerals were determined by M4 Tornado μ-EDXRF, Raman microscope and scanning electron microscopy with energy-dispersive system. Two alteration zones were recognized depending on ion availability and the Eh/pH conditions. The first include anglesite as an initial phase that quickly transformed into a more stable cerussite and hydrocerussite. Mendipite formation was controlled by the availability of Cl− ions in the solution. Hemimorphite was formed after sphalerite in the pore spaces, depending on the accessibility of Si ions from silicates dissolution. Iron (oxy) hydroxides were formed in a later stage due to their restricted mobility in carbonates. The second zone includes gypsum and anhydrite formed at the surface of the mine wastes due to continuous evaporation in arid environments. These secondary mineral phases can undergo different mineral transformations depending on the prevailing conditions. The element release ratios in the mine surface zone compared to the capillary fringe zone reached 12.1, 2.8, 1.6, 0.17, 0.09 and 0.03 for Sr, Cr, Pb, Zn, Cu, and Ni in the mine surface zone compared to 5.86, 0.01, 0.05, 0.02, 0.07 and 0.01 in the capillary fringe zone. The findings from this investigation have important implications for the management and the control of elements mobility from secondary phases formed in mined areas.