Geochemistry and stable isotope composition of fresh alkaline porphyry copper tailings: Implications on sources and mobility of elements during transport and early stages of deposition

Prevention of acid mine drainage (AMD) in sulfide-containing tailings requires the identification of the geochemical processes and element pathways in the early stages of tailing deposition. However, analyses of recently deposited tailings in active tailings impoundments are scarce because mineralogical changes occur near the detection limits of many assays. This study shows that a detailed geochemical study which includes stable isotopes of water (delta H-2, delta O-18), dissolved sulfates (delta S-34, delta O-18) and hydrochernical parameter (pH, Eh, DOC, major and trace elements) from tailings samples taken at different depths in rainy and dry seasons allows the understanding of weathering (oxidation, dissolution, sorption, and desorption), water and element pathways, and mixing processes in active tailings impoundments. Fresh alkaline tailings (pH 9.2-10.2) from the Cu-Mo porphyry deposit in El Teniente, Chile had low carbonate (0.8-1.1 Wt-% CaCO3 equivalent) and sulfide concentrations (0.8-1.3 wt.%, mainly as pyrite). In the alkaline tailings water, Mo and Cu (up to 3.9 mg/L Mo and 0.016 mg/L Cu) were mobile as MoO42- and Cu (OH)(2)(0). During the flotation, tailings water reached equilibrium with gypsum (up to 738 mg/L Ca and 1765 mg/ L SO4). The delta S-34 VS. delta O-18 covariations of dissolved sulfate (2.3 to 4.5% delta S-34 and 4.1 to 6.0 % delta O-18) revealed the sulfate sources: the dissolution of primary sulfates (12.0 to 13.2%. delta S-34, 7.4 to 10.9%.delta O-18) and oxidation of primary sulfides (-6.7 to 1.7%. delta S-34). Sedimented tailings in the tailings impoundment can be divided into three layers with different water sources, element pathways, and geochemical processes. The deeper sediments (> 1 m depth) were infiltrated by catchment water, which partly replaced the original tailings water, especially during the winter season. This may have resulted in the change from alkaline to near-neutral pH and towards lower concentrations of most dissolved elements. The neutral pH and high DOC (up to 99.4 mg/L C) of the catchment water mobilized Cu (up to 0.25 mg/L) due to formation of organic Cu complexes; and Zn (up to 130 mg/L) due to dissolution of Zn oxides and desorption). At I m depth, tailings pore water obtained during the winter season was chemically and isotopically similar to fresh tailings water (pH 9.8-10.6, 26.7-35.5 mg/L Cl, 2.3-6.0 mg/L Mo). During the summer, a vadose zone evolved locally and temporarily up to 1.2 m depth. resulting in a higher concentration of dissolved solids in the pore water due to evaporation. During periodical new deposition of fresh tailings, the geochemistry of the surface layer was geochemically similar to fresh tailings. In periods without deposition, sulfide oxidation was suggested by decreasing pH (7.7-9.5), enrichment of MoO42- and SO42-, and changes in the isotopic composition of dissolved sulfates. Further enrichment for Na, K, Cl, SO4, Mg, Cu, and Mo (up to 23.8 mg/L Mo) resulted from capillary transport towards the surface followed by evaporation and the precipitation of highly soluble efflorescent salts (e.g., mirabilite, syngenite) at the tailing surface during summer. (C) 2008 Elsevier B.V. All rights reserved.

Low molecular weight carboxylic acids in oxidizing porphyry copper tailings

The distribution of low molecular weight carboxylic acids (LMWCA) was investigated in pore water profiles from two porphyry copper tailings impoundments in Chile (Piuquenes at La Andina and Cauquenes at El Teniente mine). The objectives of this study were (1) to determine the distribution of LMWCA, which are interpreted to be the metabolic byproducts of the autotroph microbial community in this low organic carbon system, and (2) to infer the potential role of these acids in cycling of Fe and other elements in the tailings impoundments. The speciation and mobility of iron, and potential for the release of H+ via hydrolysis of the ferric iron, are key factors in the formation of acid mine drainage in sulfidic mine wastes. In the low-pH oxidation zone of the Piuquenes tailings, Fe(III) is the dominant iron species and shows high mobility. LMWCA, which occur mainly between the oxidation front down to 300 cm below the tailings surface at both locations (e.g., max concentrations of 0.12 mmol/L formate, 0.17 mmol/L acetate, and 0.01 mmol/L pyruvate at Piuquenes and 0.14 mmol/L formate, 0.14 mmol/L acetate, and 0.006 mmol/L pyruvate at Cauquenes), are observed at the same location as high Fe concentrations (up to 71.2 mmol/L Fe(II) and 16.1 mmol/L Fe(III), respectively). In this zone, secondary Fe(111) hydroxides are depleted. Our data suggest that LMWCA may influence the mobility of iron in two ways. First, complexation of Fe(III), through formation of bidentate Fe(III)-LMWCA complexes (e.g., pyruvate, oxalate), may enhance the dissolution of Fe(III) (oxy)hydroxides or may prevent precipitation of Fe(III) (oxy)hydroxides. Soluble Fe(III) chelate complexes which may be mobilized downward and convert to Fe(II) by Fe(III) reducing bacteria. Second, monodentate LMWCA (e.g., acetate and formate) can be used by iron-reducing bacteria as electron donors (e.g., Acidophilum spp.), with ferric iron as the electron acceptor. These processes may, in part, explain the low abundances of secondary Fe(III) hydroxide precipitates below the oxidation front and the high concentrations of Fe(II) observed in the pore waters of some low-sulfide systems. The reduction of Fe(III) and the subsequent increase of iron mobility and potential acidity transfer (Fe(II) oxidation can result in the release of H+ in an oxic environment) should be taken in account in mine waste management strategies.

Geochemical evolution of active porphyry copper tailings impoundments : from alkaline deposition towards acidification

J. Smuda: Geochemical evolution of active porphyry copper tailings impoundments Thesis abstract Mine waste is the largest volume of materials handled in the world. The oxidation of sulfidic mine waste may result in the release of acid mine drainage (AMD) rich in heavy metals and arsenic to the environment, one of the major problems the mining industry is facing today. To control and reduce this environmental impact, it is crucial to identify the main geochemical and hydrological processes influencing contaminant liberation, transport, and retention. This thesis presents the results of a geochemical, mineralogical and stable isotope study (δ2H, δ18O, δ34S) from two active porphyry copper tailings impoundments in Mediterranean (Carén tailings impoundment, El Teniente mine, Central Chile) and hyper-arid climate (Talabre tailings impoundment, Chuquicamata, Northern Chile) from the deposition in alkaline environment (pH 10.5) towards acidification after several years of exposure. The major hydrological results were the identification of vertical contaminant and water transport in the uppermost, not water-saturated zone, triggered by capillary rise due to evaporation, and infiltration downwards due to new tailings deposition, and of horizontal transport in the groundwater zone. At the surface of the sedimented tailings, evaporation of pore water led to the precipitation of Na-Ca-Mg sulfates (e.g., gypsum, tenorite), in hyper-arid climate also halite. At the Carén tailings impoundment, renewed deposition in a 4-week interval inhibited a pH decrease below neutral values and the formation of an efflorescent salt crust. At the Talabre tailings impoundment, deposition breaks of several years resulted in the formation of acidic oxidation zones in the timeframe of less than 4 years. This process enabled the transport of liberated Cu, Zn, and Fe via capillary rise to the surface, where these metals precipitated as heavy-metal sulfates (e.g., devilline, krohnkite) and chlorides (eriochalcite, atacamite). Renewed depositing may dissolve efflorescent salts and transport liberated elements towards the groundwater zone. This zone was found to be highly dynamic due to infiltration and mixing with water from different sources, like groundwater, catchment water, and infiltration from superficial waters. There, Cu was found to be partially mobile due to complexation with Cl (in Cl-rich groundwater, Talabre) and dissolved organic matter (in zones with infiltration of catchment water rich in dissolved organic matter, Carén). A laboratory study on the isotopic fractionation of sulfur and oxygen of sulfate in different minerals groups (water-soluble sulfates, low- and high-crystalline Fe(III) oxyhydroxides) contributed to the use of stable isotopes as tracer of geochemical and transport processes for environmental studies. The results highlight that a detailed geochemical, stable isotope and mineralogical study permits the identification of contamination processes and pathways already during the deposition of mine tailings. This knowledge allows the early planning of adequate actions to reduce and control the environmental impact during tailings deposition and after the closing of the impoundment. J. Smuda: Geochemical evolution of active porphyry copper tailings impoundments Résumé de these Les déchets miniers constituent les plus grands volumes de matériel gérés dans le monde. L'oxydation des déchets miniers sulfuriques peut conduire à la libération de drainages miniers acides (DMA) riches en métaux et arsenic dans l'environnement, ce qui est l'un des principaux problèmes de l'industrie minière aujourd'hui. Pour contrôler et réduire ces impacts sur l'environnement, il est crucial d'identifier les principaux processus géochimiques et hydrologiques influençant la libération, le transport et la rétention des contaminants. Cette thèse présente les résultats d'une étude géochimique, minéralogique et des isotopes stables (δ2H, δ18O, δ34S) sur des déchets miniers de 2 sites de dépôt actifs en climat méditerranéen (Dépôt de déchets de Carén, mine de El Teniente, Centre du Chili) et en climat hyper-aride (Dépôt de déchets de Talabre, mine de Chuquicamata, Nord du Chili). L'objectif était d'étudier l'évolution des déchets de la déposition en milieu alcalin (pH = 10.5) vers l'acidification après plusieurs années d'exposition. Le principal résultat hydrologique a été l'identification de 2 types de transport : un transport vertical de l'eau et des contaminants dans la zone non saturée en surface, induit par la montée capillaire due à l'évaporation et par l'infiltration subséquente de la déposition de sédiments frais ; et un transport horizontal dans la zone des eaux souterraines. À la surface des déchets, l'évaporation de l'eau interstitielle conduit à la précipitation de sulfates de Na-Ca-Mg (ex. gypse, ténorite) et halite en climat hyper-aride. Dans le site de Carén, une nouvelle déposition de déchets frais à 4 semaines intervalle a empêché la baise du pH en deçà des valeurs neutres et la formation d'une croûte de sels efflorescentes en surface. Dans le site de Talabre, les fentes de dessiccation des dépôts ont entraîné la formation d'une zone d'oxydation à pH acide en moins de 4 ans. Ce processus a permis la libération et le transport par capillarité de Cu, Zn, Fe vers la surface, où ces éléments précipitent sous forme de sulfates de métaux lourds (ex., dévilline, krohnkite) de chlorures (ex. ériochalcite, atacamite). Une nouvelle déposition de sédiments frais pourrait dissoudre ces sels et les transporter vers la zone des eaux souterraines. Cette dernière zone était très dynamique en raison du mélange d'eaux provenant de différentes sources, comme les eaux souterraines, l'eau de captage et l'infiltration des eaux superficielles. Egalement dans cette zone, le cuivre était partiellement mobile à cause de la formation de complexe avec le chlore (dans les zone riche en Cl, Talabre) et avec la matière organique dissoute (dans les zones où s'infiltre l'eau de captage riche en matière organique, Carén). Une étude en laboratoire sur le fractionnement des isotopes stables de sulfure et d'oxygène des sulfates dans différents groupes de minéraux (sulfates hydrosolubles, sulfures de oxy-hydroxyde de Fe(III) faiblement ou fortement cristallins) a permis d'apporter une contribution à leur utilisation comme traceurs dans l'étude des processus géochimiques et de transport lors d'études environnementales. Les résultats montrent qu'une étude détaillée de la géochimie, des isotopes stables et de la minéralogie permet d'identifier les processus et les voies de contamination déjà pendant la période de dépôt des déchets miniers. Cette connaissance permet de planifier, dès le début de l'exploitation, des mesures adéquates pour réduire et contrôler l'impact sur l'environnement pendant la période de dépôts de déchets miniers et après la fermeture du site.

Microbial communities in a porphyry copper tailings impoundment and their impact on the geochemical dynamics of the mine waste

The distribution and diversity of acidophilic bacteria of a tailings impoundment at the La Andina copper mine, Chile, was examined. The tailings have low sulfide (1.7% pyrite equivalent) and carbonate (1.4% calcite equivalent) contents and are stratified into three distinct zones: a surface (0-70-80 cm) `oxidation zone' characterized by low-pH (2.5-4), a `neutralization zone' (70-80 to 300-400 cm) and an unaltered `primary zone' below 400 cm. A combined cultivation-dependent and biomolecular approach (terminal restriction enzyme fragment length polymorphism and 16S rRNA clone library analysis) was used to characterize the indigenous prokaryotic communities in the mine tailings. Total cell counts showed that the microbial biomass was greatest in the top 125 cm of the tailings. The largest numbers of bacteria (10(9) g(-1) dry weight of tailings) were found at the oxidation front (the junction between the oxidation and neutralization zones), where sulfide minerals and oxygen were both present. The dominant iron-/sulfur-oxidizing bacteria identified at the oxidation front included bacteria of the genus Leptospirillum (detected by molecular methods), and Gram-positive iron-oxidizing acidophiles related to Sulfobacillus (identified both by molecular and cultivation methods). Acidithiobacillus ferrooxidans was also detected, albeit in relatively small numbers. Heterotrophic acidophiles related to Acidobacterium capsulatum were found by molecular methods, while another Acidobacterium-like bacterium and an Acidiphilium sp. were isolated from oxidation zone samples. A conceptual model was developed, based on microbiological and geochemical data derived from the tailings, to account for the biogeochemical evolution of the Piuquenes tailings impoundment.

Development of hypogene and supergene alteration and copper mineralization patterns, Sar Cheshmeh porphyry copper deposits, Iran /

The formation of the Sar Cheshmeh porphyry Cu-Mo deposit is related to the culmination of calc-alkaline igneous activity in the Kerman region. The deposit comprises a suite of Late Cenozoic intrusive sub-volcanic and extrusive rocks emplaced into a folded series of Eocene andesitic lavas and pyroclastic sediments. The earliest stage of magmatism was emplacement of a large granodiorite stock about 29 m.y.b.p. This was followed by intrusion of two separate porphyritic bodies at 15 (Sar Cheshrneh porphyry) and 12 m.y.b.p. (Late porphyry) and a series of sub-volcanic dikes between 12 and 9 m.y.b.p. Magmatic activity terminated with multi-phase extrusion of a Pelean dacitic dome complex between 10 and 2.8 m.y.b.p. The country rocks and the earlier porphyritic intrusions are pervasively altered to biotite-rich potassium silicate (metasomatic and hydrothermal) sericite-clay, phyllic and chlorite-clay, argillic assemblages. These grade outwards to an extensive propylitic zone. Within the ore body, the later intra-. and post-mineral dikes only reach the propylitic grade. At least three different sets of quartz veins are present, including a sericite-chlorite-quartz set which locally retrogrades pervasive secondary biotite to sericite. In the hypogene zone, metasomatic and hydrothermal alteration is related to all stages of magmatism but copper mineralization and veining are restricted to a period of 15 to 9 m.y.b.p.related to the early intrusive phases. The copper mineralization and silicate alteration do not fit a simple annular ring model but have been greatly modified by, 1. The existence of an ititial, outer ring, of metasomatic alteration overprinted by an inner.ring of hydrothermal alteration and, 2. later extensive dilating effects of intra- and post-mineral dikes. The hydrothermal clay mineral assemblage in the hypogene zone is illite-chlorite-kaolinite-smectite (beidellite). Preliminary studies indicate that the amount of each of these clays varies vertically and that hydrothermal zonation of clay minerals is possible. However, these minerals alter to illite-kaolinite assemblages in the supergene sulfide zone and to more kaolinite-rich assemblages in the supergene leached zone. Hydrothermal biotite breaks down readily in the supergene zone and is not well preserved in surface outcrops. The distribution of copper minerals in the supergene sulfide enrichment zone is only partly related to rock type being more dependent on topography and the availability of fractures.

Geology of the Sand Creek porphyry molybdenum prospect /

The Sand Creek Prospect is located within the eastern exposed margin of the Coast Plutonic Complex. The occurrence is a plug and dyke porphyry molybdenum deposit. The rock types, listed in decreasing age: 1) metamorphlc schists and gneisses; 2) diorite suite rocks - diorite, quartz diorite, tonalite; 3) rocks of andesitic composition; 4) granodiorites, coarse porphyritic granodiorite, quartzfeldspar porphyry, feldspar porphyry; and 5) lamprophyre. Hydrothermal alteration is known to have resulted from emplacement of the hornblende-feldspar porphyry through to the quartz-feldspar porphyry. Molybdenum mineralization is chiefly associated with the quartz-feldspar porphyry. Ore mineralogy is dominated by pyrite with subordinate molybdenite, chalcopyrite, covelline, sphalerite, galena, scheelite, cassiterite and wolframite. Molybdenite exhibits a textural gradation outward from the quartz-feldspar porphyry. That is, disseminated rosettes and rosettes in quartz veins to fine-grained molybdenite in quartz veins and potassic altered fractures to fine-grained molybdenite paint or 6mears in the peripheral zones. The quartz-feldspar porphyry dykes were emplaced in an inhomogeneous stress field. The trend of dykes, faults and shear zones is 0^1° to 063° and dips between 58° NW and 86* SE. Joint Pole distribution reflects this fault orientation. These late deformatior maxima are probably superimposed upon annuli representing diapiric emplacement of the plutons. A model of emplacement involving two magmatic pulses is given in the following sequence: Diorite pulse (i) dioritequartz diorite, (ii) tonalites; granodiorite pulse (iii) hornblende-fildspar microporphyry, hornblende/biotite porphyry, (iv) coarse grained granodiorite, (v) quartz-feldspar porphyry, (vi) feldspar porphyry, and (vii) lamprophyre. The combination of plutonic and coarse porphyritic textures, extensive propylitic overprinting of potassic alteration assemblages suggests that the. prospect represents the lower reaches of a porphyry system.