Mineralogy and geochemistry of the copper-dominated neutral mine drainage at the Cu deposit Ľubietová-Podlipa (Slovakia)

Abstract

The neutral mine drainage system at Ľubietová in Slovakia is overwhelmingly dominated by copper (17 ppm in the most concentrated aqueous sample) and sulfate and almost devoid of iron (0.7 ppb in the most concentrated aqueous sample). This mine drainage is generated by oxidative dissolution of chalcopyrite (CuFeS2), as confirmed by sulfur isotopes, and its neutral nature is maintained by buffering via the primary carbonates. The rich oxidation zone, composed of abundant libethenite (Cu2(PO4)(OH)), pseudomalachite (Cu5(PO4)2(OH)4), and malachite (Cu2(CO3)(OH)2), appears to play only a passive and negligible role in the mine drainage generation. The copper- and sulfate-laden water is discharged through the lowermost adits in the ore field; inside the adits, blue gel covers the stream and converts slowly to crystalline langite (Cu4(SO4)(OH)6·2H2O). X-ray absorption spectroscopy on Cu K edge showed that the local structure inside the gel is similar to that in langite. Calculation of saturation indices showed supersaturation with respect to sulfates (langite, brochantite (Cu4(SO4)(OH)6)), malachite, and copper arsenates and phosphates. Arsenates and phosphates were not found and do not form, probably because of kinetic hindrance. The solubility product of langite was estimated as log Ksp = 17.73. Outside of the adits, transmission electron microscopy showed redox cycling of copper via the formation of nanocrystals of native copper on rock-forming sheet silicates. The copper concentrations are naturally attenuated, being reduced by a factor of 100 once the mine drainage reaches the Hutná river, about 1 km away from the discharge site. Sulfur isotopes document that no copper sulfides are precipitating; copper must be removed from the contaminated water in the form of sulfates or carbonates. The natural attenuation could be responsible for the enrichment of hyporheic sediments with copper whose mineralogy and geochemistry is unknown but its size is estimated to several metric tonnes of copper.