Potential lixiviation of trace elements in tailings from the mines La Asuncion and Las Torres in the Guanajuato Mining District, Mexico

Mining wastes from the abandoned mine La Asuncion, and the active mine Las Torres, in the Guanajuato Mining District were investigated. The first one belongs to the La Sierra vein system, and the second one to the Veta Madre vein system. The aim of this work was to evaluate the mobility of the trace elements zinc, copper, cadmium, selenium and arsenic from mine wastes. Tailings and rock waste samples were characterized by chemical and mineralogical methods. Column leaching experiments were running for five months. Mineralogically the tailings are formed mainly by quartz, feldspars and, in a lower proportion, calcite and pyrite: trace amounts of fluorite, aguilarite, galena, chalcopyrite and sphalerite are also present. Tailings from La Asuncion mine contain 180-330 mg/kg Zn 59-65 mg/kg Pb, 2.5 mg/kg Cd, 14-18 mg/kg 0; and 21-25 mg/kg As. Tailings from Las Torres mine contain 62 mg/kg Zn, 35 tailings Pb, 3.5 mg/kg Cd, 13 mg/kg Cr, and 36 mg/kg As. Mercury was not detected in any of the studied tailings. High selenium concentrations in tailings from La Asuncion mine (27-38 mg/kg) reflect the high sulfur selenides content reported for La Sierra vein system. Sulfide to carbonate ratio does not favor acid mine drainage production in most wastes. Column experiments resulted in low concentrations of metals: Zn(< 180 mu g/L), Cu(< 80 mu g/L), Sb(< 350 mu g/L), and Se(< 50 mu g/L) within the first 24 hours: much lower and stable concentrations were found after the 3(rd) week. Arsenic concentrations for all sites remain below the detection limit (2 mu g/L). These results indicate that the geochemical conditions do not allow leaching of metals. Selenium is present as aguilarite (Ag4SeS), which is stable under the physicochemical conditions measured in the field (pH= 7.92, Eh =270 mV). Besides, the occurrence of aguilarite as inclusion in pyrite grains difficults the dissolution of selenium.