Geological Sciences

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Internal thermal boundary layer stability in phase transition modulated convection
(AGU, 1997-02) Butler, S.L.; Peltier, W.R.
The stability of a horizontal thermal boundary layer embedded within a very viscous fluid is investigated using the formalism of linear stability analysis. Thin thermal boundary layers in deep fluid regions and in the absence of phase transition and dynamical effects are thereby shown to be unstable at extremely long wavelengths. The stability of the internal thermal boundary layer which may exist at 660 km depth in the Earth's mantle as a consequence of the dynamical influence of the endothermic phase transition from γ spinel to a mixture of perovskite and magnesiowüstite, recently discussed in some detail by Solheim and Peltier [1994a], is investigated in order to better understand the “avalanche effect” observed in this and similar nonlinear, time dependent simulations of the mantle convection process. It is demonstrated that if the stability problem is treated as purely thermal, then the boundary layer is predicted to be extremely unstable and the presence of the 660‐km endothermic phase transition at middepth within the boundary layer is further destabilizing. When the kinematic effect of flow convergence onto the boundary layer and phase transition region is active, however, it is shown that the layer may be strongly stabilized. In the regime of physically realistic velocity convergence, the critical Rayleigh number is predicted to lie in the range suggested by the numerical simulations of Solheim and Peltier [1994a]. A threshold value of the magnitude of the Clapeyron slope of the endothermic phase transition for a given velocity convergence is also shown to exist, beyond which the fastest‐growing mode of instability changes from avalanche type to layered type.
Recommended nomenclature of epidote-group minerals
(GeoScience World, 2006) Armbruster, Thomas; Bonazzi, Paola; Akasaka, Mashide; Bermanec, Vladimir; Chopin, Christian; Giere, Reto; Heuss-Aßbichler, Soraya; Liebscher, Axel; Menchetti, Silvio; Pan, Yuanming; Pasero, Marco
Epidote-group minerals are monoclinic in symmetry and have topology consistent with space group P21/m and the general formula A2M3[T2O7][TO4](O,F)(OH,O). Zoisite is an orthorhombic polymorph of clinozoisite Ca2Al3[Si2O7][SiO4]O(OH) and is thus not considered a member of the epidote-group. Epidote-group minerals are divided into three subgroups. (1) Members of the clinozoisite subgroup are derived from the mineral clinozoisite Ca2Al3[Si2O7][SiO4]O(OH) by homovalent substitutions only. The key cation- and anion-sites are A1 = M2+, A2 = M2+, M1 = M3+, M2 = M3+, M3 = M3+, O4 = O2-, O10 = (OH)-. In other words, the dominant valence as listed above must be maintained. (2) Members of the allanite subgroup are REE-rich minerals typified by the eponymous mineral “allanite”. This subgroup may be derived from clinozoisite by homovalent substitutions and one coupled heterovalent substitution of the type A2(REE)3+ + M3M2+ → A2Ca2+ + M3M3+. Thus the valences on the key sites are: A1 = M2+, A2 = M3+, M1 = M3+, M2 = M3+, M3 = M2+, O4 = O2-, O10 = (OH)-. (3) Members of the dollaseite subgroup are REE-rich minerals typified by the eponymous mineral “dollaseite”. This subgroup may be derived from clinozoisite by homovalent substitutions and two coupled heterovalent substitutions of the type A2(REE)3+ + M3M2+ → A2Ca2+ + M3M3+ and M1M2+ + O4F-→ M1M3+ + O4O-2. Thus the valences on the key sites are: A1 = M2+, A2 = M3+, M1 = M2+, M2 = M3+, M3 = M2+, O4 = F-, O10 = (OH)-. The key cation-sites M3 and A1 (and, in principle, M2) determine the root name. In both clinozoisite and allanite subgroups no prefix is added to the root name if M1 = Al. The prefixes ferri, mangani, chromo, and vanado indicate dominant Fe3+, Mn3+, Cr3+, and V3+ on M1, respectively. In the dollaseite subgroup no prefix is added to the root name if M1 = Mg. Otherwise a proper prefix must be attached; the prefixes ferro and mangano indicate dominant Fe2+ and Mn2+ at M1, respectively. The dominant cation on A2 (other than Ca) is treated according to the Extended Levinson suffix designation. This simple nomenclature requires renaming of the following approved species: Niigataite (old) = clinozoisite-(Sr) (new), hancockite (old) = epidote-(Pb) (new), tweddillite (old) = manganipiemontite-(Sr) (new). Minor modifications are necessary for the following species: Strontiopiemontite (old) = piemontite-( Sr) (new), androsite-(La) (old) = manganiandrosite-(La) (new). Before a mineral name can be assigned, the proper subgroup has to be determined. The determination of a proper subgroup is made by the dominating valence at M3, M1, and A2 expressed as M2+ and or M3+, not by a single, dominant ion (i.e., Fe2+, or Mg, or Al). In addition, the dominant valence on O4: X- or X2- must be ascertained. [M2+]A2 > 0.50, [M3+]M3 > 0.50 → clinozoisite subgroup, [M3++ M4+]A2 > 0.50, [M2+]M3 > 0.50 → allanite subgroup, {[M2+]M3+M1 – [M3++ M4+]A2 } > 0.50 and [X-]O4 > 0.5 → dollaseite subgroup. Coupled heterovalent substitutions in epidote-group minerals require a special application of the so-called 50 % rule in solid-solution series. (1) Clinozoisite subgroup: The dominant trivalent cation on M3 determines the name, whereas the A2 cation appearing in the suffix has to be selected from among the divalent cations. (2) Allanite and dollaseite subgroups: For the sites involved in the charge compensation of a heterovalent substitution in A2 and O4 (i.e. M3 in the allanite subgroup; M3 and M1 in the dollaseite subgroup), identification of the relevant end-member formula must take into account the dominant divalent charge-compensating octahedral cation (M2+) and not the dominant cation in these sites. Formal guidelines and examples are provided in order to determine a mineral “working name” from electron-microprobe analytical data.
Widespread Archean basement beneath the Yangtze craton
(GeoScience World, 2006) Zheng, Jianping; Griffin, William L; O’Reilly, Suzanne Y.; Zhang, Ming; Pearson, Norman; Pan, Yuanming
The age distribution of the crust is a fundamental parameter in modeling continental evolution and the rate of crustal accretion through Earth’s history, but this is usually estimated from surface exposures. The exposed Yangtze craton in eastern China consists mainly of Proterozoic rocks with rare Archean outcrops. However, the U-Pb ages and Hf isotope systematics of xenocrystic zircons brought to the surface in lamproite diatremes from three Proterozoic outcrop areas of the craton suggest the widespread presence of unexposed Archean basement, with zircon age populations of 2900–2800 Ma and 2600– 2500 Ma and Hf model ages of 2.6 to ca. 3.5 Ga or older. The zircons also record thermal events reworked on the craton ca. 2020 Ma (remelting of older crust) and 1000–850 Ma (addition of juvenile mantle material). The observation of deep crust significantly older than the upper crust will require revision of models for the rates of crustal generation through time.
Acidic microenvironments in waste rock characterized by neutral drainage: Bacteria-mineral interactions at sulfide surfaces
(MDPI, 2014-03-21) Dockrey, John W.; Lindsay, Matthew; Mayer, K. Ulrich; Beckie, Roger D.; Norlund, Kelsey L. I.; Warren, Lesley; Southam, Gordon
Microbial populations and microbe-mineral interactions were examined in waste rock characterized by neutral rock drainage (NRD). Samples of three primary sulfide-bearing waste rock types (i.e., marble-hornfels, intrusive, exoskarn) were collected from field-scale experiments at the Antamina Cu–Zn–Mo mine, Peru. Microbial communities within all samples were dominated by neutrophilic thiosulfate oxidizing bacteria. However, acidophilic iron and sulfur oxidizers were present within intrusive waste rock characterized by bulk circumneutral pH drainage. The extensive development of microbially colonized porous Fe(III) (oxy)hydroxide and Fe(III) (oxy)hydroxysulfate precipitates was observed at sulfide-mineral surfaces during examination by field emission-scanning electron microscopy-energy dispersive X-ray spectroscopy (FE-SEM-EDS). Linear combination fitting of bulk extended X-ray absorption fine structure (EXAFS) spectra for these precipitates indicated they were composed of schwertmannite [Fe8O8(OH)6–4.5(SO4)1–1.75], lepidocrocite [γ-FeO(OH)] and K-jarosite [KFe3(OH)6(SO4)2]. The presence of schwertmannite and K-jarosite is indicative of the development of localized acidic microenvironments at sulfide-mineral surfaces. Extensive bacterial colonization of this porous layer and pitting of underlying sulfide-mineral surfaces suggests that acidic microenvironments can play an important role in sulfide-mineral oxidation under bulk circumneutral pH conditions. These findings have important implications for water quality management in NRD settings.
Geochemical and mineralogical aspects of sulfide mine tailings
(Elsevier, 2015-01-30) Lindsay, Matthew
Tailings generated during processing of sulfide ores represent a substantial risk to water resources. The oxidation of sulfide minerals within tailings deposits can generate low-quality water containing elevated concentrations of SO4, Fe, and associated metal(loid)s. Acid generated during the oxidation of pyrite [FeS2], pyrrhotite [Fe(1-x)S] and other sulfide minerals is neutralized to varying degrees by the dissolution of carbonate, (oxy)hydroxide, and silicate minerals. The extent of acid neutralization and, therefore, pore-water pH is a principal control on the mobility of sulfide-oxidation products within tailings deposits. Metals including Fe(III), Cu, Zn, and Ni often occur at high concentrations and exhibit greater mobility at low pH characteristic of acid mine drainage (AMD). In contrast, (hydr)oxyanion-forming elements including As, Sb, Se, and Mo commonly exhibit greater mobility at circumneutral pH associated with neutral mine drainage (NMD). These differences in mobility largely result from the pH-dependence of mineral precipitation-dissolution and sorption-desorption reactions. Cemented layers of secondary (oxy)hydroxide and (hydroxy)sulfate minerals, referred to as hardpans, may promote attenuation of sulfide-mineral oxidation products within and below the oxidation zone. Hardpans may also limit oxygen ingress and pore-water migration within sulfide tailings deposits. Reduction-oxidation (redox) processes are another important control on metal(loid) mobility within sulfide tailings deposits. Reductive dissolution or transformation of secondary (oxy)hydroxide phases can enhance Fe, Mn, and As mobility within sulfide tailings. Production of H2S via microbial sulfate reduction may promote attenuation of sulfide-oxidation products, including Fe, Zn, Ni, and Tl, via metal-sulfide precipitation. Understanding the dynamics of these interrelated geochemical and mineralogical processes is critical for anticipating and managing water quality associated with sulfide mine tailings.
Long-term mineralogical and geochemical evolution of sulfide mine tailings under a shallow water cover
(Elsevier, 2015-06-01) Moncur, Michael; Ptacek, Carol; Lindsay, Matthew; Blowes, David; Jambor, John
The long-term influence of a shallow water cover limiting sulfide-mineral oxidation was examined in tailings deposited near the end of operation in 1951 of the former Sherritt-Gordon Zn-Cu mine (Sherridon, Manitoba, Canada). Surface-water, pore-water and core samples were collected in 2001 and 2009 from above and within tailings deposited into a natural lake. Mineralogical and geochemical characterization focused on two contrasting areas of this deposit: (i) sub-aerial tailings with the water table positioned at a depth of approximately 50 cm; and (ii) sub-aqueous tailings stored under a 100 cm water cover. Mineralogical analyses of the sub-aerial tailings showed a zone of extensive sulfide-mineral alteration extending 40 cm below the tailings surface. Moderate alteration was observed at depths ranging from 40–60 cm and was limited to depths > 60 cm. In contrast, sulfide-mineral alteration within the submerged tailings was confined to a < 6 cm thick zone located immediately below the water-tailings interface. This narrow zone exhibited minimal sulfide-mineral alteration relative to the sub-aerial tailings. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy showed results that were consistent with the mineralogical investigation. Pore-water within the upper 40 cm of the sub-aerial tailings was characterized by low pH (1.9-4.2), depleted alkalinity, and elevated SO4 and metal concentrations. Most-probable number (MPN) enumerations revealed abundant populations of acidophilic sulfur-oxidizing bacteria within these tailings. Conversely, pore-water in the sub-aqueous tailings was characterized by near-neutral pH, moderate alkalinity, and relatively low concentrations of dissolved SO4 and metals. These tailings exhibited signs of dissimilatory sulfate reduction (DSR) including elevated populations of sulfate reducing bacteria (SRB), elevated pore-water H2S concentrations, and strong δ34S-SO4 and δ13C-DIC fractionation. Additionally, mineralogical investigation revealed the presence of secondary coatings on primary sulfide minerals, which may serve as a control on metal mobility within the sub-aqueous tailings. Results from this study provide critical long-term information on the viability of sub-aqueous tailings disposal as a long-term approach for managing sulfide-mineral oxidation.
Initial geochemical characteristics of fluid fine tailings in an oil sands end pit lake
(Elsevier, 2016-06-15) Dompierre, Kathryn; Lindsay, Matthew; Cruz-Hernández, Pablo; Halferdahl, Geoffrey
Geochemical characteristics of fluid fine tailings (FFT) were examined in Base Mine Lake (BML), which is the first full-scale demonstration oil sands end pit lake (EPL) in northern Alberta, Canada. Approximately 186 M m3 of FFT was deposited between 1994 and 2012, before BML was established on December 31, 2012. Bulk FFT samples (n = 588) were collected in July and August 2013 at various depths at 15 sampling sites. Temperature, solids content, electrical conductivity (EC), pH, Eh and alkalinity were measured for all samples. Detailed geochemical analyses were performed on a subset of samples (n = 284). Pore-water pH decreased with depth by approximately 0.5 within the upper 10 m of the FFT. Major pore-water constituents included Na (880 ± 96 mg L−1) and Cl (560 ± 95 mg L-1); Ca (19 ± 4.1 mg L-1), Mg (11 ± 2.0 mg L-1), K (16 ± 2.3 mg L-1) and NH3 (9.9 ± 4.7 mg L−1) were consistently observed. Iron and Mn concentrations were low within FFT pore water, whereas SO4 concentrations decreased sharply across the FFT-water interface. Geochemical modeling indicated that FeS(s) precipitation was favoured under SO4-reducing conditions. Pore water was also under-saturated with respect to gypsum [CaSO4·2H2O], and near saturation with respect to calcite [CaCO3], dolomite [CaMg(CO3)2] and siderite [FeCO3]. X-ray diffraction (XRD) suggested that carbonate-mineral dissolution largely depleted calcite and dolomite. X-ray absorption near edge structure (XANES) spectroscopy revealed the presence of FeS(s), pyrite [FeS2], and siderite. Carbonate-mineral dissolution and secondary mineral precipitation have likely contributed to FFT dewatering and settlement. However, the long-term importance of these processes within EPLs remains unknown. These results provide a reference for assessing the long-term geochemical evolution of oil sands EPLs, and offer insight into the chemistry of pore water released from FFT to the overlying water cover.
Trace element-mineral associations in modern and ancient iron terraces in acid drainage environments
(Elsevier, 2016-07-31) Cruz-Hernandez, Pablo; Pérez-López, Rafael; Parviainen, Annika; Lindsay, Matthew B. J.; Nieto, José M.
Iron-rich sediments commonly cover riverbeds that have been affected by acid drainage associated with sulfide-mineral oxidation. Freshly-formed precipitates correspond to poorly-crystalline oxyhydroxysulfates that recrystallize over time. This study examined the distribution and mineral association of trace elements (e.g., As, Cu, Zn) in modern and ancient (~ 6 Ma) Fe terraces in the Tinto river basin, Spain. The mineral composition of the terraces was determined by Raman μ-spectroscopy. Chemical digestions, electron probe microanalyses, and synchrotron-based μ-X-ray fluorescence mapping were used to examine As, Cu, and Zn distribution and corresponding mineral associations. Fresh precipitates at modern terrace surfaces were dominated by schwertmannite, which contained high As, Cu, Mn, and Zn concentrations. However, schwertmannite transforms into goethite over days to weeks in the deeper part of the current terraces and into hematite over centuries. Affinity for trace elements was generally highest for schwertmannite and lowest for hematite, which suggests that their retention by Fe terraces decreases during mineral transformation. Hence, schwertmannite acts as temporary sink for contaminants, which are again released over long time periods. These findings should be considered for management and treatment of possible water resources affected by acid mine drainage.
Geochemical characteristics of oil sands fluid petroleum coke
(Elsevier, 2016-11-30) Nesbitt, Jake A.; Lindsay, Matthew B. J.; chen, ning
The geochemical characteristics of fluid petroleum coke from an oil sands mine in the Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada were investigated. Continuous core samples were collected to 8 m below surface at several locations (n = 12) from three coke deposits at an active oil sands mine. Bulk elemental analyses revealed the coke composition was dominated by C (84.2 ± 2.3 wt. %) and S (6.99 ± 0.26 wt. %). Silicon (9210 ± 3000 mg kg−1), Al (5980 ± 1200 mg kg−1), Fe (4760 ± 1200 mg kg−1), and Ti (1380 ± 430 mg kg−1) were present in lesser amounts. Vanadium (1280 ± 120 mg kg−1) and Ni (230 ± 80 mg kg−1) occurred at the highest concentrations among potentially-hazardous minor and trace elements. Sequential extractions revealed potential for release of these elements under field-relevant conditions. Synchrotron powder X-ray diffraction revealed the presence of Si and Ti oxides, organically-complexed V and hydrated Ni sulfate, and provided information about the aromatic carbon matrix. X-ray absorption near edge structure (XANES) spectroscopy at the V and Ni K-edges revealed that these metals were largely hosted in porphyrins and similar organic complexes throughout coke grains. Slight variations among V and Ni K-edge spectra were largely attributed to slight variations in local coordination of V(IV) and Ni(II) within these organic compounds. However, linear combination fits were improved by including reference spectra for phases with octahedrally-coordinated V(III) and Ni(II). Sulfur and Fe K-edge XANES confirmed that thiophenic coordination and pyritic-ilmenitic coordination are predominant, respectively. These results provide new information on the geochemical and mineralogical composition of oil sands fluid petroleum coke and improve understanding of potential controls on associated water chemistry.
Vanadium geochemistry of oil sands fluid petroleum coke
(American Chemical Society, 2017-02-09) Nesbitt, Jake A.; Lindsay, Matthew
Vanadium has previously been linked to elevated toxicity of leachates derived from oil sands petroleum coke. However, geochemical controls on V mobility within coke deposits remain poorly constrained. Detailed examinations of pore-water and solid-phase V geochemistry were, therefore, performed on oil sands fluid petroleum coke deposits in Alberta, Canada. Sample collection focused on both active and reclaimed deposits, which contained more than 3 × 10⁷ m³ of fluid petroleum coke. Dissolved V concentrations were highest (up to 3.0 mg/Lˉ¹) immediately below the water table, but decreased rapidly with increasing depth. This trend corresponded to a transition from mildly acidic (pH 6 – 7) and oxic conditions to mildly alkaline (pH 7 – 8.5) and anoxic conditions. Scanning electron microscopy (SEM), electron microprobe analysis (EMPA) and micro-X-ray fluorescence (µXRF) mapping revealed coke particles exhibited an internal structure characterized by successive concentric layers. The outer margins of these layers were characterized by elevated V, Fe, Si, and Al concentrations, indicating the presence of inorganic phases. Micro-X-ray absorption near-edge structure (μXANES) spectroscopy revealed that V speciation was dominated by V(IV) porphyrins except at outer margins of layers, where octahedrally-coordinated V(III) was a major component. Minor to trace V(V) was also detected within fluid petroleum coke particles.
Dissolved selenium(VI) removal by zero-valent iron under oxic conditions: Influence of sulfate and nitrate
(American Chemical Society, 2017-04-17) Das, Soumya; Lindsay, Matthew; Essilfie-Dughan, Joseph; Hendry, M. James
Dissolved Se(VI) removal by three commercially-available zero-valent irons (ZVIs) was examined in oxic batch experiments under circumneutral pH conditions in the presence and absence of NO3- and SO42-. Environmentally relevant Se(VI) (1 mg/L), NO3- ([NO3-N] = 15 mg/L) and SO42- (1800 mg/L) were employed to simulate mining-impacted waters. Ninety percent Se(VI) removal was achieved within 4 to 8 h in the absence of SO42- and NO3-. Similar Se(VI) removal was observed after 10 to 32 h in the presence of NO3-. Dissolved Se(VI) removal rates exhibited the greatest decrease in the presence of SO42-; 90% Se(VI) removal was measured after 50 to 191 h for SO42- and after 150 to 194 h for SO42- plus NO3- depending on the ZVI tested. Despite differences in removal rates among batches and ZVI materials, Se(VI) removal consistently followed first-order reaction kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analyses of reacted solids showed that Fe(0) present in ZVI undergoes oxidation to magnetite [Fe3O4], wüstite [FeO], lepidocrocite [γ-FeOOH], and goethite [α-FeOOH] over time. X-ray absorption near edge structure spectroscopy indicated that Se(VI) was reduced to Se(IV) and Se(0) during removal. These results demonstrate that ZVI can be effectively used control Se(VI) concentrations in mining impacted waters.
Chemical mass transport between fluid fine tailings and the overlying water cover of an oil sands end pit lake
(American Geophysical Union, 2017-05-17) Dompierre, Kathryn A.; Barbour, S. Lee; North, Rebecca L.; Carey, Sean K.; Lindsay, Matthew
Fluid fine tailings (FFT) are a principal by-product of the bitumen extraction process at oil sands mines. Base Mine Lake (BML)—the first full-scale demonstration oil sands end pit lake (EPL)—contains approximately 1.9 3 108 m^3 of FFT stored under a water cover within a decommissioned mine pit. Chemical mass transfer from the FFT to the water cover can occur via two key processes: (1) advection-dispersion driven by tailings settlement; and (2) FFT disturbance due to fluid movement in the water cover. Dissolved chloride (Cl) was used to evaluate the water cover mass balance and to track mass transport within the underlying FFT based on field sampling and numerical modeling. Results indicated that FFT was the dominant Cl source to the water cover and that the FFT is exhibiting a transient advection-dispersion mass transport regime with intermittent disturbance near the FFT-water interface. The advective pore water flux was estimated by the mass balance to be 0.002 m^3 m^-2 d^-1, which represents 0.73 m of FFT settlement per year. However, the FFT pore water Cl concentrations and corresponding mass transport simulations indicated that advection rates and disturbance depths vary between sample locations. The disturbance depth was estimated to vary with location between 0.75 and 0.95 m. This investigation provides valuable insight for assessing the geochemical evolution of the water cover and performance of EPLs as an oil sands reclamation strategy.
Nickel geochemistry of oil sands fluid petroleum coke deposits, Alberta, Canada
(Canadian Science Publishing, 2018-04-30) Nesbitt, Jake A.; Robertson, Jared M.; Swerhone, Lawrence A.; Lindsay, Matthew
Nickel (Ni) leaching from oil sands petroleum coke can have toxicological effects on aquatic organisms. However, geochemical controls on Ni release, transport and attenuation within coke deposits remains limited. We examined the geochemistry of fluid coke and associated pore-waters from two deposits at an oil sands mine near Fort McMurray, Alberta, Canada. Synchrotron-based micro-X-ray fluorescence (µXRF) and micro-X-ray absorption near edge structure (µXANES) spectroscopy show that Ni(II)-porphyrin complexes dominate, but inorganic phases including Ni(II)-sulfide and Ni(II)-oxide comprise a minor component of fluid coke. Sequential chemical extractions suggested that sorption-desorption reactions may influence Ni mobility within fluid coke deposits. Although only a small proportion of total Ni (< 4%) is susceptible to leaching under environmentally-relevant concentrations, dissolved Ni concentrations (n = 65) range from 2 to 120 μg/L (median 7.8 μg/L) within the two deposits and generally decrease with depth below the water table. Pore-water Ni concentrations are negatively correlated with pH, but not with dissolved sulfate, bicarbonate, or chloride. Overall, our findings suggest that pore-water pH and sorption-desorption reactions are principal controls on dissolved Ni concentrations within oil sands fluid petroleum coke deposits.
The persistence of brines in sedimentary basins
(American Geophysical Union, 2018-05-08) Ferguson, Grant; McIntosh, Jennifer C.; Grasby, Stephen E.; Hendry, M. Jim; Jasechko, Scott; Lindsay, Matthew; Luijendijk, Elco
Brines are commonly found at depth in sedimentary basins. Many of these brines are known to be connate waters that have persisted since the early Paleozoic Era. Yet questions remain about their distribution and mechanisms for retention at depth in the Earth’s crust. Here we demonstrate that there is insufficient topography to drive these dense fluids from the bottom of deep sedimentary basins. Our assessment based on driving force ratio indicates that sedimentary basins with driving force ratio > 1 contain connate waters and frequently host large evaporite deposits. These stagnant conditions appear to be relatively stable over geological time and insensitive to factors such as glaciations, erosion, compaction, and hydrocarbon generation.
Sodium transport and attenuation in soil cover materials for oil sands mine reclamation
(Elsevier, 2018-11-03) Vessey, Colton; Lindsay, Matthew; Barbour, S. Lee
Reclamation soil covers are used in oil sands mine closure to support vegetative growth over tailings. Geochemical processes within these covers may impact solute transport during upward migration of oil sands process-affected water (OSPW) from the underlying tailings. In this study, we examined the geochemical processes controlling Na transport and attenuation within the peat and clay-till cover soils at Sandhill Fen in northern Alberta, Canada. We analyzed soil core samples collected along transects of this 54-ha pilot-scale oil sands mine reclamation wetland. The geochemical (Na, Ca, Mg, K, Cl, SO4, HCO3) and isotopic (δ2H, δ18O) compositions of extracted pore water were analyzed statistically to identify OSPW and fresh surface water within the cover. Depth-dependent trends in pore water sodium concentrations were not apparent, suggesting that the soil cover had been fully flushed by a mixture of OSPW and fresh surface water used to flood the fen. Relative concentrations of Na, Ca and Mg were used to define the extent of cation exchange within the clay cover. Complementary laboratory column experiments showed that cation exchange removed up to 50% of dissolved Na as the first pore volume of simulated OSPW passed through the peat and till. However, Na attenuation by these materials declined rapidly and was limited after 4 (peat) to 7 (till) pore volumes of OSPW flushing. Reactive transport modeling confirmed that cation exchange was the dominant control on Na attenuation and corresponding Ca and Mg release within the till and peat columns. Mineral precipitation-dissolution reactions also influenced dissolved Ca and Mg concentrations and, therefore, indirectly impacted Na attenuation. Overall, this study helps constrain the geochemical processes controlling Na transport and attenuation in oil sands reclamation soil covers exposed to OSPW, and indicates that the attenuation of Na from OSPW by these covers is short-lived.
Aqueous- and solid-phase molybdenum geochemistry of oil sands fluid petroleum coke deposits, Alberta, Canada
(Elsevier, 2018-11-12) Robertson, Jared M.; Nesbitt, Jake A.; Lindsay, Matthew B. J.
Fluid petroleum coke generated at oil sands operations in the Athabasca Oil Sands Region of northern Alberta, Canada, contains elevated concentrations of molybdenum (Mo) and other metals including nickel (Ni) and vanadium (V). Solid-phase Mo concentrations in fluid petroleum coke are typically 10 to 100 times lower than V and Ni, yet dissolved Mo concentrations in associated pore waters are often comparable with these metals. We collected pore water and solids from fluid petroleum coke deposits in the AOSR to examine geochemical controls on Mo mobility. Dissolved Mo concentrations increased with depth below the water table, reaching maxima of 1.4 to 2.2 mg L-1, within a mixing zone between slightly acidic and oxic meteoric water and mildly alkaline and anoxic oil sands process-affected water (OSPW). Dissolved Mo concentrations decreased slightly with depth below the mixing zone. X-ray absorption spectroscopy revealed that Mo(VI) and Mo(IV) species were present in coke solids. The Mo(VI) occurred as tetrahedrally coordinated MoO42- adsorbed via inner- and outer-sphere complexation, and was coordinated in an environment similar to Fe-(hydr)oxide surface complexes. The OSPW likely promoted desorption of outer-sphere Mo(VI) complexes, resulting in higher dissolved Mo concentrations in the mixing zone. The principal Mo(IV) species was MoS2, which originated as a catalyst added upstream of the fluid coking process. Although MoS2 is likely stable under anoxic conditions below the mixing zone, oxidative weathering in the presence of meteoric water may promote long-term Mo release.
Influence of As(V) on precipitation and transformation of schwertmannite in acid mine drainage-impacted waters
(Schweizerbart science publishers, 2018-12-07) Cruz-Hernandez, Pablo; Carrero, Sergio; Pérez-Lópeza, Rafael; Fernandez-Martinez, Alejandro; Lindsay, Matthew B. J.; Dejoie, Catherine; Nieto, José M.
Iron-rich sediments commonly cover riverbeds affected by acid mine drainage (AMD). Initial precipitates are often dominated by schwertmannite, which has an exceptionally high capacity to sequester As and other toxic elements. This poorly crystalline Fe oxyhydroxysulfate rapidly recrystallizes to goethite; however, the influence of trace elements on ageing rates and products is poorly understood. This study examined the influence of As(V) concentrations on the kinetics of schwertmannite precipitation and transformation. Schwertmannite was synthesized in the presence of various initial dissolved As concentrations (i.e., 0–2 mM) and subsequently aged at 40, 60 or 85 °C for 1 h to 300 d. The initial As concentration had a profound impact on schwertmannite precipitation and transformation. Schwertmannite precipitation was inhibited at higher initial As concentrations in favor of pseudo-amorphous Fe-hydroxyarsenate formation. Schwertmannite transformation to goethite was accompanied by sulfate release and, over longer time, As release. Pair distribution function (PDF) analysis of high-energy X-ray diffraction (HEXD) patterns revealed that increasing initial As concentration produced structural defects in associated precipitates. Schwertmannite precipitation exerts an important control on As mobility in AMD-impacted waters; however, this study has demonstrated that the long-term stability of schwertmannite and associated precipitates should be considered when designing AMD remediation strategies and AMD treatment systems.
Mineralogy and geochemistry of oil sands froth treatment tailings: Implications for acid generation and metal(loid) release
(Elsevier, 2019-02-04) Lindsay, Matthew B. J.; Vessey, Colton; Robertson, Jared M.
Froth treatment tailings (FTT) are one of three principal tailings streams generated during bitumen extraction at oil sands mines in northern Alberta, Canada. Unlike the coarse tailings and fluid fine tailings, FTT are enriched in sulfide-minerals content and exhibit the potential for acid generation and metal(loid) leaching. However, the mineralogical and geochemical characteristics of this sulfide-bearing tailings stream remain poorly constrained. We examined samples of fresh FTT (n = 3) and partially-weathered FTT collected from a sub-aerial beach deposit (n = 15). X-ray diffraction revealed that weathering-resistant silicates, phyllosilicates, and oxides dominated (85 ± 7.3 wt. %) the FTT mineral assemblage, while sulfides (6.2 ± 3.6 wt. %) and carbonates (8.9 ± 4.3 wt. %) were relatively minor phases. Pyrite [FeS2] was the principal sulfide in all samples, while minor amounts of marcasite [FeS2] occurred only in beach samples. Sulfide mineral textures were highly variable and included euhedral to subhedral pyrite crystals, discrete and clustered pyrite framboids, and marcasite replacements of pyrite framboids. Siderite [FeCO3] accounted for 55 to 90 % of all carbonates, while dolomite [CaMg(CO3)2], calcite [CaCO3] and ankerite [Ca(Fe,Mg,Mn)(CO3)2] accounted for the remainder. Statistical analysis of bulk geochemical compositions suggested that environmentally-relevant metal(loid)s, including As, Cu, Co, Fe, Mn, Ni, Pb and Zn, were likely associated with sulfides, carbonates and, to a lesser extent, phyllosilicates. Electron probe microanalyses revealed a wide range of As, Cu, Co, Mn, Ni and Zn concentrations in pyrite, with As and Cu concentrations elevated in framboids. Rare earth elements (REEs), Th and U also occurred at elevated concentrations and statistical analyses suggest they are associated with zircon and, potentially, monazite and xenotime. Static acid-base accounting (ABA) tests indicated that all FTT samples are potentially acid generating. Our study describes the mineralogical and geochemical characteristics of oil sands FTT, and indicates that oxidative weathering has the potential to generate acidic drainage containing elevated dissolved concentrations of several metal(loid)s.
Tracing molybdenum attenuation in mining environments using molybdenum stable isotopes
(American Chemical Society, 2019-04-18) Skierszkan, Elliott K.; Robertson, Jared M.; Lindsay, Matthew; Stockwell, Justin S.; Dockrey, John W.; Das, Soumya; Weis, Dominique; Beckie, Roger D.; Mayer, K. Ulrich
Molybdenum contamination is a concern in mining regions worldwide. Better understanding of processes controlling Mo mobility in mine wastes is critical for assessing potential impacts and developing water-quality management strategies associated to this element. Here, we used Mo stable isotope (δ98/95Mo) analyses to investigate geochemical controls on Mo mobility within a tailings management facility (TMF) featuring oxic and anoxic environments. These isotopic analyses were integrated with X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and aqueous chemical data. Dissolved Mo concentrations were inversely correlated with δ98/95Mo values such that enrichment of heavy Mo isotopes in solution reflected attenuation processes. Inner-sphere complexation of Mo(VI) with ferrihydrite was the primary driver of Mo removal and was accompanied by a circa 1 ‰ isotope fractionation. Limited Mo attenuation and isotope fractionation was observed in Fe(II)- and Mo-rich anoxic TMF seepage, while attenuation and isotope fractionation were greatest during discharge and oxidation of this seepage after discharge into a pond where Fe-(oxyhydr)oxide precipitation promoted Mo sorption. Overall, this study highlights the role of sorption onto Fe-(oxyhydr)oxides in attenuating Mo in oxic environments, a process which can be traced by Mo isotope analyses.
Selenate removal by zero-valent iron under anoxic conditions: effects of nitrate and sulfate
(Springer Nature, 2019-08-16) Das, Soumya; Lindsay, Matthew; Hendry, M. Jim
Batch experiments were conducted to examine aqueous Se(VI) removal by zero-valent iron (ZVI) under anoxic conditions in the presence and absence of NO3− and SO42−. Initial concentrations for Se(VI), SO4, and NO3–N of 5 mg L−1, 1800 mg L−1, and 13 mg L−1, respectively, were employed to mimic mine waters. In the control experiment, 90% Se(VI) removal occurred within 1.5 h without SO42− and NO3− (B1). This removal threshold was reached after 3 h with NO3− added (B3) and after 33 h with SO42− added (B2). Removal reached 90% after 42 h with both SO42− and NO3− added (B4). Modeled Se(VI) removal rates consistently followed first-order kinetics and revealed that the presence of SO42− and, to a lesser extent, NO3− inhibited Se(VI) removal. Increases in pH and Fe coupled with decreasing Eh are consistent with ZVI corrosion under anoxic conditions. Transmission electron microscopy, Raman spectroscopy, and X-ray diffraction revealed magnetite [Fe3O4] and lepidocrocite [γ-FeOOH] formed at ZVI surfaces during the experiments. X-ray absorption near edge structure spectroscopy indicated that Se(VI) was predominantly reduced to Se(0) (70–80%), but Se(IV) (10–13%) and Se(-II) (2–13%) were also detected at reacted ZVI surfaces. Overall, the results show that although SO42− and NO3− present in mine wastes can reduce reaction rates, Se(VI) removal by ZVI under anoxic conditions is associated with extensive reduction to insoluble Se(0).

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