Heavy Metal Distribution in Rocks, Sediments, Mine Tailings, Leaching Experiments, and Groundwater from the Mineral de Pozos Historical Mining Site, North-Central Mexico

Abstract

Silver and gold mineralization in epithermal veins was discovered during the 1600s in the Mineral de Pozos area, north-central Mexico. The main period of mining was between 1880 and 1926. Exploitation in the district is estimated at 1,200,000 tons of ore, with average grades of 1.35 kg/ton Ag, and 8.5 g/ton Au. Tailings were deposited (less than 1.2 million tons) along the main creek in the area, under semi-desert conditions, and widely dispersed by fluvial water during rainy seasons. Concentrations of Cr, Co, and Ni in rocks, sediments, and tailings are below average concentrations in the Earth's crust, but Cu, Zn, As, Cd, and Pb concentrations are above crustal averages. Concentrations of As and Pb in groundwater (As = 0.011 to 0.090 mg/l; and Pb = 0.025 to 0.035 mg/l) generally exceed the World Health Organization (WHO) standards for drinking water. Results from humidity cell and leaching column experiments indicate that the potential for leaching As is relatively low (average <0.050 mg/l); Pb concentrations in the experiments range from 0.001 to 0.180 mg/l (average of 0.010 mg/l), and Zn is relatively concentrated (up to 80 mg/l). These results suggest that, given the relatively small amount of mine tailings, the As and Pb concentrations in groundwater may be mostly derived from natural sources rather than from mine waste. The high concentration of Zn in tailings and leachates (up to 306 mg/kg and 80 mg/l, respectively), suggest that zinc could be leaching from the tailings; however, under geochemical conditions of the shallow aquifer (near neutral pH, oxidizing conditions, high Fe content), Zn in solution is mostly controlled by adsorption onto Fe-oxyhydroxides.