Sulfide Oxidation Generating Acid-Mine Drainage in the Concordia Abandoned Mine, Salta, Argentina

KIRSCHBAUM, ALICIA; TONDA, ROMINA; VILLAGRÁN, PEDRO; ARNOSIO, MARCELO

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

Pyrite is an essential mineral of the metallic mineral deposits and is always classified as a waste mineral because of its null economic value. Hence, during the mining process, this mineral is accumulated in waste dumps and tailings. In the Puna region of Argentina there are several Pb, Ag, Zn abandoned mines, such as La Poma, Pan de Azúcar, La Concordia. This last one is located at the western portion of Salta province, 3770 m above sea level, where the climate is arid. The mine is located 15 km from San Antonio de Los Cobres village, and it was abandoned in 1986 without any planning for its closure. Nowadays this underground mine is water-filled and the groundwater is in contact with the sulfide minerals generating acid mine drainage (AMD). The Concordia river heading is located within the mine, where the water pH is low (3-4). However, until reaching the San Antonio River, the pH of the stream during its pathway changes to neutral values when the water from the Pompeya thermal system (pH 8) flows into the stream. Then, the river crosses the San Antonio town and finally flows into the Salinas Grandes closed basin. During the mining activity and after the concentration process, the dumping material was stored in four tailings. The estimated total volume of sediment in these tailings is 42000 m3. After the mine activity finished, the Concordia stream passed through the tailings transporting some of the sediments and metallic minerals that were deposited there. In the first tailing the pyrite concentration is between 0.9 and 2.2%. The sulfide oxidation mineral from the enriched pyrite level was investigated by means of microscopy equipped with an energy dispersive system for microanalysis. The analysis of the samples showed that the pyrite grains are limpid and some of them have a slight oxidation coating. Those grains have been exposed to weathering during the last twenty five years, thus indicating that the weathering process in arid climate produces a low oxidation level, and hence, pyrite grains represent a durable source of AMD.