The Veladero district in the Rio Turbio Basin (San Juan, Argentina) consists of at least eight alteration zones within an area of 14 km2. It contains several large bulk-tonnage Au-Ag-deposits in altered, silicified volcanic rocks. The oxidized zone extends to considerable depth. The Veladero orebody occurs as a 750 m x 3000 m tabular body between 4050 and 4350 m elevation. It is characterized by multiple stages of brecciation, alteration, and precious metal mineralization. The water of Rio Turbio has a low pH and shows enhanced concentrations of As and Cu which exceed maximum contaminant levels in drinking waters. X-ray diffraction of mineral substrates in that area sampled in the dry valleys around Rio Turbio shows the association quartz-plagioclase-montmorillonite-illite-kaolinite-gypsum-jarosite indicating derivation from an epithermal acid sulfate alteration zone. The rare presence of carbonates in these substrates which would not form in equilibrium with jarosite indicates mechanical mixing of sediment from different sources. Weathering which proceeds slowly due to arid conditions leads to the reaction:2 jarosite –> 3 goethite + K2SO4 + 3 H2SO4. Thus, mainly jarosite formed in epithermal alteration zones can be considered as driving force for acid rock drainage rather than sulphide minerals. A method to quantify the mineral components contributing to acid production and consumption by major element analysis is demonstrated. Acid base accounting using these data and the reaction above reveals a maximum acid production of 74 kg H2SO4/t rock. Decomposition of jarosite bearing substrates is investigated at ambient temperature in 0.01 n NaOH. Such conditions are well outside the stability field of jarosite in terms of Eh-pH. An acid forming reaction leading to pH as low as 3.66 is observed after three days. However, acid formation is 2 to 100 times lower than acid forming potentials calculated from compositional data. This is due to incomplete reaction. Further analyses are currently in progress. Jarosite bearing mineral substrates are characterized by elevated concentrations of Cu (up to 1209 µg/g) and As (up to 461 µg/g). Chromium, Pb and Zn exceed values of 300, 100 and 300 µg/g, respectively. Heavy metals released from decomposing jarosite as well as associated minerals and – due to low pH – from other mineral surfaces causes the high heavy metal concentrations in the river water. This is explored on the basis of extraction experiments. Entisol soils of the San Juan province reaching thicknesses up to 40 cm on the slopes of the valleys – studied for comparison – have moderate acid neutralisation potentials mainly < 30 kg H2SO4/ t soil, which is in accordance with experimentally determined acid neutralisation potentials.

Sindern, S., Arroqui Langer, A., Warnsloh, J.M. and Cardus, A. (2008) Assessment of acid-rock-drainage from epithermal Au-Ag mineralisations, Rio Turbio, San Juan, Argentina. SDGG, 60, 295.