Cadmium sources, mobility, and natural attenuation in contrasting environments (carbonate-rich and carbonate-poor) in the Capillitas polymetallic mineral deposit, NW Argentina

Abstract

Cadmium is one of the most toxic metals commonly released due to mining activities. However, its dynamic in contrasting mining environments has not been exhaustively studied. Through mineralogical and hydrochemical analyses, this study identifies Cd mineral sources and its mobility in surficial waters from two different mining environments (i.e. carbonate-rich and carbonate-poor mineral assemblages) within the polymetallic Capillitas deposit, in NW Argentina. Furthermore, a freshwater tributary stream without mining operations is analyzed as natural background. Results show that sphalerite weathering is the main process releasing Cd into surficial waters in both locations. Sphalerite is Cd-rich in the carbonate-poor rocks of the high-to intermediate-sulfidation environment. At this site, the mineral assemblage is poorly neutralizing and the metal dissolved concentrations increase downstream. The carbonate-rich rocks release Cd up to 1.25 mg L−1, which is higher than similar environments around the world. However, they rapidly decrease downstream to 0.0005 mg L−1 in < 3 km. The main processes that reduce Cd concentrations in the Capillitas hydrological basin are dilution with fresh waters, adsorption, precipitation, and co-precipitation with authigenic minerals such as efflorescent salts and Mn oxy-hydroxides.