English

Abstract

From 1926 until 1975, the Potrerillos-El Salvador mining district (porphyry copper deposits) sent most of its flotation tailings in suspension through the El Salado River directly to the sea at the Chanaral Bay, Atacama desert, Chile. Over 220 Mt of tailings, containing between 1-2 wt% pyrite, have been dumped into the bay, resulting in a displacement of the shore line over one kilometer further out to the sea. The tailings are estimated to be 10-15 m thick, covering an area of 4.5 km. The hydrology of the tailings system is controlled by a groundwater infiltration of highly saline water (up to 60 g/L Cl and 30 g/L Na) and the tidal cycle. Data suggests that low- saline water (up to 0.9 mg/L Cl and 0.8 mg/L Na), possibly from the municipally water supply or an unknown source, infiltrated into the tailings system. Oxidation resulted in a 70 to 188 cm thick low-pH (0.8 - 4) oxidation zone at the top, which is characterized by jarosite and unidentified orange-brown Fe(III) hydroxides as well as a vermiculite-type mixed-layer mineral as principal secondary mineral assemblage. The oxidation zone is underlain by a neutral primary zone. Mineralogical and hydrogeochemical data indicated, that the bivalent metal cations as Cu, Ni and Zn are liberated in the oxidation zone (up to 2265 mg/L, 18.1 mg/L and 20.3 mg/L, respectively). They are mobile under acidic conditions and are transported towards the tailings surface via capillary processes. These processes led to metal enrichment at the tailings surface (e.g. up to 2.4% Cu) in form of secondary chlorites or sulfates (dominated by eriochalcite (CuCl2*H20) and halite). These secondary minerals are mainly water-soluble and exposed to eolian transport towards the village of Chanaral. Data from sequential extraction show that Cu is present in the beach area (between 1000 and 2000 mg/kg) mainly in the exchangeable (adsorbed) fraction. This suggests that when the acid metal-rich solutions from the oxidation zone are in contact with the marine water during the tidal cycle, bivalent cations became adsorbed and their transport in the seawater seems mainly to be associated with particles in suspension. The transport direction seems to be laterally toward the sea during low tide. But also at neutral pH conditions in the lower part of the primary zone (3-7 m depth), Cu and Zn mobility is observed (up to 19 mg/L and 12 mg/L, respectively) associated with the highly saline waters, most probably as chloro complexes. In contrast to the bivalent cations, oxyanions as As and Mo are below detection limit in the pore water of the low-pH oxidation zone due to adsorption to secondary Fe(III) hydroxides, shown by sequential extractions. Below, in the saturated neutral zone, high concentrations of As and Mo were encountered with up to 608 and 10266 µg/L, respectively. The residual concentrations of As in the tailings at the surface, which are exposed to eolian transport, are between 14 and 61 mg/kg mainly associated to the Fe(III) hydroxide fractions. Two element flow directions (towards the tailings surface via capillarity and towards the sea) and two element groups with different geochemical behavior (cations such as Cu, Zn, Ni and oxyanions such as As and Mo) can be differentiated. It can be postulated, that the sea is mainly affected by As, Mo, Cu, Zn contamination by seepage form tailings and the El Salado River, while the population of Chanaral is mainly exposed to high concentrations of Cu and minor Ni and Zn via eolian transport in form of water-soluble secondary mineral particles.