Mineralogical, Geochemical and Isotopic Study of the Active Tailings Impoundment Carén from the Porphyry Copper Deposit El Teniente, Chile

At the active tailings impoundment Carén (surface 22 km) from the porphyry copper mine El Teniente, Chile, initial steps of primary mineral alteration and geochemical changes after deposition were studied. The tailings contained up to 3 vol% sulfides (mainly pyrite). At five points in the tailings impoundment, water samples up to a depth of 4 m were taken from piezometer. Preliminary data showed that the water, which was entering the tailings impoundment as an alkaline (pH 9.21) tailings slurry, changed to neutral (pH 7.41) conditions at the discharge of clear water from a decantation lake into a natural river system. The discharged clear water contained Cu up to 80μg/L, Zn up to 180 μg/L, Mg up to 30 mg/L, Mo up to 0.35 mg/L and sulfate (up to 1400 mg/L) indicating the liberation of metals from the very early stage of sulfide oxidation. Initial water and mineralogical data indicated a 2-layer structure of the tailings: (1) the first layer (0 m -1 m depth) was dominated by the alkaline pH of the fresh tailings but with lower pH at the surface and higher pH at 1 m depth. Incipient pyrite alteration was observed at the surface already two weeks after deposition; pyrite grains also in the youngest layers showed small oxidation rims. This process seemed to lower the pH at the surface of the tailings impoundment. New tailings deposition renewed in a 4-weeks rhythm the alkaline conditions at the surface. (2) In the second layer (1 m to 4 m depth) the pH decreased down to near-neutral environment, possibly due to neutralisation of the bases by the tailings. ______________________ 1 Poster paper presented at the 7 International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Institut de minéralogie et de géochemie, University of Lausanne, CH-1015 Lausanne, Switzerland