Bernhard Dold

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

Two flotation tailings sites (Ojancos and P. Cerda) from the Fe oxide Cu–Au Punta del Cobre belt, south of Copiapo, 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 CO3 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 FeS2, 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.

Sulfide oxidation and acid mine drainage formation within two active tailings impoundments in the Golden Quadrangle of the Apuseni Mountains, Romania

Sulfidic mine tailings have to be classified as one of the major source of hazardous materials leading to water contamination. This study highlights the processes leading to sulfide oxidation and acid mine drainage (AMD) formation in the active stage of two tailings impoundments located in the southern part of the Apuseni Mountains, in Romania, a well-known region for its long-term gold-silver and metal mining activity. Sampling was undertaken when both impoundments were still in operation in order to assess their actual stage of oxidation and long-term behavior in terms of the potential for acid mine drainage generation. Both tailings have high potential for AMD formation (2.5 and 3.7 wt.% of pyrite equivalent, respectively) with lesser amount of carbonates (5.6 and 3.6 wt.% of calcite equivalent) as neutralization potential (ABA=-55.6 and -85.1 tCaCO(3)/1000 t ) and showed clear signs of sulfide oxidation yet during operation. Sequential extraction results indicate a stronger enrichment and mobility of elements in the oxidized tailings: Fe as Fe(III) oxy-hydroxides and oxides (transformation from sulfide minerals, leaching in oxidation zone), Ca mainly in water soluble and exchangeable form where gypsum and calcite are dissolved and higher mobility of Cu for Ribita and Pb for Mialu. Two processes leading to the formation of mine drainage at this stage could be highlighted (1) a neutral Fe(II) plume forming in the impoundment with ferrihydrite precipitation at its outcrop and (2) acid mine drainage seeping in the unsaturated zone of the active dam, leading to the formation of schwertmannite at its outcrop.

Remediation of a marine shore tailings deposit and the importance of water-rock interaction on element cycling in the coastal aquifer.

We present the study of the geochemical processes associated with the first successful remediation of a marine shore tailings deposit in a coastal desert environment (Bahía de Ite, in the Atacama Desert of Peru). The remediation approach implemented a wetland on top of the oxidized tailings. The site is characterized by a high hydraulic gradient produced by agricultural irrigation on upstream gravel terraces that pushed river water (∼500 mg/L SO(4)) toward the sea and through the tailings deposit. The geochemical and isotopic (δ(2)H(water) and δ(18)O(water), δ(34)S(sulfate), δ(18)O(sulfate)) approach applied here revealed that evaporite horizons (anhydrite and halite) in the gravel terraces are the source of increased concentrations of SO(4), Cl, and Na up to ∼1500 mg/L in the springs at the base of the gravel terraces. Deeper groundwater interacting with underlying marine sequences increased the concentrations of SO(4), Cl, and Na up to 6000 mg/L and increased the alkalinity up to 923 mg/L CaCO(3) eq. in the coastal aquifer. These waters infiltrated into the tailings deposit at the shelf-tailings interface. Nonremediated tailings had a low-pH oxidation zone (pH 1-4) with significant accumulations of efflorescent salts (10-20 cm thick) at the surface because of upward capillary transport of metal cations in the arid climate. Remediated tailings were characterized by neutral pH and reducing conditions (pH ∼7, Eh ∼100 mV). As a result, most bivalent metals such as Cu, Zn, and Ni had very low concentrations (around 0.01 mg/L or below detection limit) because of reduction and sorption processes. In contrast, these reducing conditions increased the mobility of iron from two sources in this system: (1) The originally Fe(III)-rich oxidation zone, where Fe(III) was reduced during the remediation process and formed an Fe(II) plume, and (2) reductive dissolution of Fe(III) oxides present in the original shelf lithology formed an Fe-Mn plume at 10-m depth. These two Fe-rich plumes were pushed toward the shoreline where more oxidizing and higher pH conditions triggered the precipitation of Fe(III)hydroxide coatings on silicates. These coatings acted as a filter for the arsenic, which naturally infiltrated with the river water (∼500 μg/L As natural background) into the tailings deposit.

Temporal evolution of bacterial communities associated with the in situ wetland-based remediation of a marine shore porphyry copper tailings deposit

Mine tailings are a serious threat to the environment and public health. Remediation of these residues can be carried out effectively by the activation of specific microbial processes. This article presents detailed information about temporal changes in bacterial community composition during the remediation of a section of porphyry copper tailings deposited on the Bahía de Ite shoreline (Peru). An experimental remediation cell was flooded and transformed into a wetland in order to prevent oxidation processes, immobilizing metals. Initially, the top oxidation zone of the tailings deposit displayed a low pH (3.1) and high concentrations of metals, sulfate, and chloride, in a sandy grain size geological matrix. This habitat was dominated by sulfur- and iron-oxidizing bacteria, such as Leptospirillum spp., Acidithiobacillus spp., and Sulfobacillus spp., in a microbial community which structure resembled acid mine drainage environments. After wetland implementation, the cell was water-saturated, the acidity was consumed and metals dropped to a fraction of their initial respective concentrations. Bacterial communities analyzed by massive sequencing showed time-dependent changes both in composition and cell numbers. The final remediation stage was characterized by the highest bacterial diversity and evenness. Aside from classical sulfate reducers from the phyla δ-Proteobacteria and Firmicutes, community structure comprised taxa derived from very diverse habitats. The community was also characterized by an elevated proportion of rare phyla and unaffiliated sequences. Numerical ecology analysis confirmed that the temporal population evolution was driven by pH, redox, and K. Results of this study demonstrated the usefulness of a detailed follow-up of the remediation process, not only for the elucidation of the communities gradually switching from autotrophic, oxidizing to heterotrophic and reducing living conditions, but also for the long term management of the remediation wetlands.

GEOCHEMISTRY AND MINERALOGY OF THE QUIULACOCHA TAILINGS IMPOUNDMENT FROM THE POLYMETALLIC Zn-Pb-(Ag-Bi-Cu) DEPOSIT CERRO DE PASCO, PERU.

The Quiulacocha tailings cover 114 ha, comprising 79 Mt of tailings, which contain ~ 50 wt% pyrite. The tailings are located at 4340 m altitude in a tropical puna climate with about 1025 mm/a rainfall. The tailings are partially overlain by the Excelsior waste-rock dump, which contains about 26,400,000 m 3 of waste rocks that cover 94 ha and contain ~60 wt% of pyrite. In the Quiulacocha impoundment there are two different types of tailings recognized: 1.) Zn-Pb-rich tailings and 2.) Cu-rich tailings. During the sampling campaign, the Zn-Pb-rich part of Quiulacocha was not producing important excesses of acid mine drainage (AMD) from the oxidation zone, where pH is increased to near neutral values at 1 m depth. The underlying tailings were able to neutralize the acidity produced in the oxidation zone through sulfide oxidation by the underlying carbonates (dolomite and siderite). The main source of AMD in this mine-waste system is from the Excelsior waste-rock dump. Its acid seepage infiltrates into Quiulacocha forming a Fe-Zn-Pb plume with a pH 5.5 - 6.1 and containing up to 7440 mg/L Fe, 627 mg/L Zn, and 1.22 mg/L Pb. The plume was detected between 10 m to 13 m depth in the stratigraphy of Quiulacocha tailings. Additionally, the AMD seepage from the base of the Excelsior waste-rock dump is channeled on the tailings surface to the pond of Quiulacocha (pH 2.3), which covers Cu-rich tailings. Infiltration of this Fe(III)-rich AMD increases oxidation of tailings in the southwestern part of the impoundment and subsequently liberates As by enargite oxidation. Additionally, the AMD collected in the Quiulacocha pond was pumped into the active Ocroyoc tailings impoundment, where sulfide oxidation was strongly enhanced by the input of dissolved Fe(III). Therefore, a hydrological separation of the different mine-waste systems might be a first step to prevent further extension of the AMD problem.

EFFECTS ON ELEMENT MOBILITY BY THE CONSTRUCTION OF A WETLAND ON THE MARINE SHORE PORPHYRY COPPER TAILINGS DEPOSIT, BAHÍA DE ITE, PERU

The marine shore tailings deposit at the Bahía de Ite, Atacama desert, Southern Peru was studied in order to understand the biogeochemical processes resulting from the construction of a wetland on the oxidizing tailings. For this purpose, un-remediated and remediated parts of the tailings were studied by solid and aqueous geochemistry, mineralogy, and microbiology methods. Preliminary results show that the oxidizing tailings have a low-pH oxidation zone (pH 1 – 4) with a strong accumulation of efflorescent salts (10 – 20 cm thick) at the surface due to the upward capillary transport of metal cations in the arid climate (up to 800 mg/L Fe, 160 mg/L Cu, 15 mg/L Zn, 70 mg/L Mn, 0.2 mg/L Cd, 1.3 mg/L Co and 2.5 mg/L Ni). In contrast, these bivalent metals occur in very low concentrations (mainly under the detection limit) below the wetland due to the established near neutral pH and reducing conditions (~150 mV). The alkaline waters (pH 8) that infiltrated into the Bahía de Ite tailings deposit contained high natural background arsenic concentrations (~500 μg/L As). The preliminary data suggest that the infiltration of the wetland induced retention of the metal cations and the formation of a Fe(II) plume. The Fe(II) plume was pushed toward the sea due to the increased hydraulic pressure of the wetland, where, in contact with the more oxidizing (400 mV) and alkaline sea water, the precipitation of Fe(III) hydroxides was triggered in the area of the shore line. In the shoreline samples, the As concentrations in the pore water are below the detection limit, suggesting that the As is retained by the Fe(III) hydroxide. This conclusion was confirmed by sequential extraction data. Additional

GEOCHEMISTRY AND SECONDARY MINERALOGY AT THE SULFIDE-RICH EXCELSIOR WASTE ROCK DUMP FROM THE POLYMETALLIC Zn-Pb-(Ag-Bi-Cu) MINERALIZATION CERRO DE PASCO, PERU 1

The Excelsior waste rock dump of the polymetallic Zn-Pb-(Ag-Bi-Cu) Cerro de Pasco deposit, Peru is characterized by high acid-production potential due to its waste rock composition with 60 wt% pyrite and <5 wt% carbonates. The waste dump is locally rich in minerals that contain up to 12.7 wt% Pb, 4.9 wt% Zn, 1.1 wt% Cu, 6020 mg/kg As, and 381 mg/kg Cd. Acid mine drainage (AMD) at the base of the waste rock dump is rich in Fe, Zn, Pb, Cd and As. The object of this study is to determine the source of metals in AMD and if metal contamination was related to seasonal fluctuations of precipitation. Main secondary minerals are gypsum, hydronian / plumbo- / K-jarosites, goethite, schwertmannite, and efflorescent salts (Fe-, Zn-, Mg-, Mn-sulfates). Leach tests of mine waste material simulated rain events and demonstrated the high solubility of efflorescent salts and acid/metal liberation. Leachates have pH between 2.8 and 4.9 and contain a maximum of 447.1 mg/L Zn, 16.7 mg/L Cu, and 14.8 mg/L Cd. AMD appears to be controlled by precipitation of secondary efflorescent salts in dry seasons and re-dissolution in rainy seasons.

MINERALOGICAL, GEOCHEMICAL AND ISOTOPIC STUDY OF THE ACTIVE TAILINGS IMPOUNDMENT CARÉN FROM THE PORPHYRY COPPER DEPOSIT EL TENIENTE, CHILE 1

At the active tailings impoundment Caren (surface 22 km 2 ) 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 80g/L, Zn up to 180 g/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.

ELEMENT DISTRIBUTION IN WATER AND SEDIMENTS OF AN ACID MINE DRAINAGE DISCHARGE LAKE (pH ~1) OF THE Zn-Pb(-Ag-Bi-Cu) DEPOSIT, CERRO DE PASCO (PERU) 1

A study of an acidic lake (pH ~1) resulting from the deposition of copper leaching spoil waters and acid mine drainage of the Zn-Pb (-Ag-Bi-Cu) deposit Cerro de Pasco is presented. The data shows for the lake water concentration ranges and mean values of 1440-7180 (4330) mg/L Fe, 33.5-105 (87) mg/L Cu, 180-746 (493) mg/L Zn, 13-63 (48) mg/L As, 2.2-5.3 (2.9) mg/L Pb, 11205-42300 (29250) mg/L SO4 −2 and an acidity of 14480-21440 (16775) mg/L CaCO3. Lake water mean values did not differ significantly from values measured for discharge water. Within the lake water body most element concentrations increased with water depth. Infiltrated acidic waters dissolved the limestone bedrock under formation of argillaceous gypsum, overlying dissolving calcite. In the precipitating gel-like matter and upper parts of dissolved bedrock, heavy metals enriched in composition to the lake water. X-ray fluorescence data show for this kind of sediment concentration ranges and mean values of 0.76-12.38 (5,98) wt% Fe2O3, 51-3980 (1301) mg/kg Cu, 275-8034 (3822) mg/kg Zn, 43-5781 (898) mg/kg As, 21-1310 (261) mg/kg Pb, 0.7-11 (6.2) wt% S. Heavy metals are most likely fixed by organic matter (15 wt% TOC) in form of chelate complexes. The high amounts of organic carbon in the upper part of the sediments most likely results from the discharged spoil waters of the Cu extraction plant. Hydroxides formed during first steps of the infiltration of the bedrock dissolve during later steps of the lake water bedrock interaction.