Element cycling and secondary mineralogy in porphyry copper tailings as a function of climate, primary mineralogy, and mineral processing

Abstract

A comparative geochemical, mineralogical, and microbiological study of three mine tailings impoundments from the La Andina, El Teniente, and El Salvador porphyry copper deposits, Chile is presented. These tailings can be characterized as low-sulfide (1.7, 1.0, and 6.2 wt% pyrite equivalent, respectively) and low-carbonate containing (1.4, 0, and 0 wt% calcite equivalent, respectively). The main focus was on the mineralogical and geochemical changes at the interface between the oxidation zone and the primary zone in the sulfidic flotation tailings. The criteria used for selection of the tailings impoundments included knowledge of climate, flotation process, the absence of anthropogenic alteration (additional water or tailings input) after operations had ceased, and the knowledge that, at each site, all tailings had been derived from only one mine. In this way the influence of climate, flotation process, and ore mineralogy can be qualitatively studied. Two schematic models of element cycling in sulfide mine tailings controlled by climatic conditions are presented. The secondary minerals jarosite, schwertmannite (for the first time in mine tailings), a vermiculite-type mixed-layer mineral, as well as traces of goethite were determined in the oxidation zones by X-ray diffraction, differential X-ray diffraction, or microscopy. Seven-step sequential extractions and electron-microbeam analyses indicate that schwertmannite and jarosite play an important role in the retention of oxyanions (e.g. HMoO 4 −, H 2AsO 4 −, and SO 4 2−) in the low-pH oxidation zones. The bivalent cations (e.g. Cu 2+, Zn 2+, and Mn 2+) are leached from the oxidation zones in precipitation-dominated climates (e.g. La Andina). Below this zone, increasing pH controls the sorption of bivalent cations through adsorbents as secondary Mn(II) hydroxides, Fe(III) hydroxides, or clay minerals. Below the groundwater table, with increasingly reducing conditions, pH-controlled replacement processes take place, as shown by the alteration of chalcopyrite to covellite, leading to secondary Cu enrichments of potential economic interest. At El Teniente and El Salvador, in climates where evaporation exceeds precipitation, the water-flow direction changes to upwards migration via capillary forces. As a result mobilized elements are transferred to the top of the tailings under oxidizing conditions. Sulfide replacement processes are less important in arid climates. Supersaturation controls the precipitation of mainly water-soluble secondary sulfates (e.g. bonattite, chalcanthite) and strong enrichment at the top of the tailings. The presence of metals in water-soluble form at the top of the tailings could lead to a low cost recovering technique for low ore-grade material in evaporation-controlled climates. In the low-pH oxidation zone, due to their high ionic activity, certain mobile elements are found to substitute into secondary minerals. Examples are Al- for Fe substitution in jarosite, and substitution of Cu and Zn for K in biotite, resulting in a vermiculite-type mixed-layer mineral. At El Salvador the quantities of secondary ferric oxyhydroxide minerals are low in relation to the relatively high pyrite (6.2 wt%) content and the low-pH in the oxidation zone (paste-pH 2). The low oxidation activity obtained during microbiological tests and the presence of higher Mo concentrations (160–1000 ppm) than those of Andina and El Teniente (32–171 ppm), suggest that the sparseness of ferric oxyhydroxides is due, at least in part, to Mo poisoning of sulfide oxidizing bacteria as Thiobacillus ferrooxidans. The low-pH (2–3.5) of the El Salvador tailings is attributed to the slow inorganic oxidation of pyrite, the liberation of acidity by supergene jarosite, and the complete absence of carbonate neutralization potential.