HEAVY METALS DISTRIBUTION IN ROCK, SEDIMENTS, GROUNDWATER, MINE TAILINGS AND LEACHING EXPERIMENTS IN MINERAL DE POZOS HISTORICAL MINING SITE, CENTRAL MEXICO

The historical mining district of Mineral de Pozos is located some 280 km NW of Mexico city, at the northwestern edge of Guanajuato state. Silver and gold mineralization on quartz-rich epithermal veins was first discovered in the zone during the 1600´s, but the main period of exploitation (bonanza) was from 1880 to 1926. Exploitation in the district is estimated at 1,200,000 tons with average grades of 1.35 kg/ton Ag, and 8.5 g/ton Au, and some veins with 2.5 kg/ton Ag and 40 g/ton Au. The mine tailings were deposited along the main creek bed and then dispersed over a wide area. The concentrations of Cr, Co and Ni in rocks, sediments and mine waste material are below the average concentrations in the earth’s crust. But concentration of Cu, Zn, As, Cd and Pb in rock, sediments and mine waste material are above the averages in the earth’s crust. Concentration ranges in mine tailings are: As=109 to 306 mg/kg; Cd=1.5 to 2.1 mg/kg; Cu=95 to 140 mg/kg; Pb=85 to 145 mg/kg; and Zn=109 to 306 mg/kg. Concentrations of As and Pb in groundwater (As=0.011 to 0.090 mg/l; and Pb=0.025 to 0.035) are generally above the standards for drinking water.

Humid cell and leaching column experiments were conducted on creek sediments and mine tailings in order to assess the potential for heavy metal leaching and the relationship between mine tailings and As and Pb concentrations in groundwater. Results indicate that potential for leaching As is relatively low (leachates average less than 0.050 mg/l), with only one of the seven leaching columns with concentrations above 0.050 mg/l. Lead concentrations in the leaching experiments range from 0.001 to 0.180 mg/l with an average of 0.010 mg/l. On the other hand, Zn in leachates is relatively high (up to 80 mg/l). These results suggest that, given the relatively small amount of mine tailings dispersed (less than 1.2 million tons), the As and Pb concentrations in groundwater may be due to natural sources and processes in the mineralized area rather than derived from tailings. However, the high concentrations of Zn in tailings and leachates (up to 306 mg/kg and 80 mg/l, respectively) suggest that Zn could be present in a relatively soluble secondary phase (smithsonite?), but under the geochemical characteristics of the aquifer (near neutral pH and high Fe content), Zn in solution is controlled by adsorption onto Fe-oxy-hydroxides.