Characterization and stable isotopic fingerprinting of mine seepage in hyperarid environments: An example of the Namib Lead & Zinc mine, Namibia

Abstract

Mine drainage at the Namib Lead & Zinc mine in the hyperarid environment of the Namib Desert close to Swakopmund, Namibia, has been investigated using mineralogical, hydrochemical, and isotopic methods. The principal ore minerals are galena and sphalerite. Mine drainage is neutral due to the reactions with the marble gangue rock. Mine water seepage is of the Na-Ca-Cl-SO₄ type and sources of dissolved ions are likely halite and gypsum in the unsaturated zone above the mine. Concentrations of dissolved metals are relatively low, and the principal attenuation mechanism of metals is probably their adsorption on ferric minerals because equilibrium of the seepage water with secondary minerals that host the metals is not attained. Based on strongly enriched δ²H (up to 13.27 ‰) and δ¹⁸O (up to 4.5 ‰) values, seepage water originates from advective fog and is strongly evaporated after the fog deposition. The high δ¹³C(DIC) values indicate equilibrium with carbonates and CO₂ de-gassing. The δ³⁴S(SO₄) values are enriched in shallower depths, probably because of pedogenic gypsum dissolution and then the δ³⁴S values decrease with depth, probably due to the increasing input of sulfur from sulfides. Mine drainage at the Namib Lead & Zinc mine does not represent any risk for the environment due to its neutral character and relatively low seepage water volumes caused by high degree of aridity.