A mineralogical and geochemical study of element mobility in sulfide mine tailings of Fe oxide Cu–Au deposits from the Punta del Cobre belt, northern Chile

Abstract

Two flotation tailings sites (Ojancos and P. Cerda) from the Fe oxide Cu–Au Punta del Cobre belt, south of Copiapó, Atacama desert, northern Chile, are geochemically (largely using sequential extractions) and mineralogically compared. Main ore minerals are pyrite, magnetite and/or hematite and chalcopyrite. Gangue is dominantly calcite with minor quartz. The host silicate assemblage is largely controlled by hydrothermal alteration and consists of variable amounts of the following minerals: K-feldspar±Ca-amphibole±biotite±sericite±chlorite±tourmaline±epidote±quartz. In this study, both the Ojancos and the P. Cerda tailings were deposited in valley dam impoundments and when they were filled, new tailings were deposited upstream. As a result, high quantities of seepage migrated downstream into the older tailings impoundment. At Ojancos, the recent upstream tailings have excess of acid potential (7.1 wt.% calcite and 3.5 wt.% pyrite), whereas the older downstream tailings are characterized by alternations of several meter-thick intervals with high neutralizing potential (about 40 wt.% calcite and 2 wt.% pyrite) and intervals with high acid potential (about 3 wt.% calcite and 4 wt.% pyrite). Acid mine drainage (AMD) with the precipitation of schwertmannite (pH 3.15) and chalcoalumite (pH 4.9) flows out at the interface between the uphill and downstream tailings. Strong downstream element transport is taking place and contributes to the formation of the cementation zone (mainly gypsum, ferrihydrite and goethite, and locally jarosite) in the older downstream impoundment. The cementation zone (pH=4) shows strong enrichment of heavy metals (e.g., up to 6800 ppm Cu, 680 ppm Zn, 1100 ppm As), mainly adsorbed and as secondary sulfides (e.g., covellite). In contrast, at the P. Cerda, tailings impoundment carbonates are homogeneously distributed and the overall neutralization potential exceeds the acid potential (average of about 10 wt.% calcite and up to 2.5 wt.% pyrite). The up to 5-m thick oxidation zones (paste pH=6.9–8.3) at P. Cerda are characterized by interlayering of coarser dark gray unoxidized layers with fine-grained, Fe(III) hydroxide-rich, ochre to red-brown colored horizons. The hyperarid climate dries out first the coarse, sulfide-rich horizons of the tailings and limits so the oxidation, which is restricted to the fine-grained, due to their higher moisture retention capacity. However, results indicate that during operation an important element transfer from the younger upstream tailings to the older downstream tailings impoundment took place, possibly by sorptive transport at ferric polymers or colloids in the form of neutral mine drainage (NMD). This would explain the metal enrichments in the cementation zone, which are mainly associated to the exchangeable fraction and not as secondary sulfides. This results, in both cases (in Ojancos mainly as AMD and in P. Cerda mainly as NMD), in Fe(III) input as ferric cation, as ferric polymer, or CO 3 complexes to the downstream impoundment. This constitutes a very effective acid potential transfer to the older downstream material because oxidation via input of external Fe(III) produces 16 mol of protons per mol FeS 2, i.e., eight times more than via oxidation with oxygen. In addition, the created acidity favors dissolution of the abundant Fe oxides magnetite and hematite of this ore deposit type providing so additional Fe(III) for sulfide oxidation.