Applied Geochemistry (v.19, #3)

Light Rare Earth Elements enrichment in an acidic mine lake (Lusatia, Germany) by Elke Bozau; Marc Leblanc; Jean Luc Seidel; Hans-Joachim Stärk (261-271).
The distribution of Rare Earth Elements (REE) was investigated in the acidic waters (lake and groundwater) of a lignite mining district (Germany). The Fe- and SO4-rich lake water (pH 2.7) displays high REE contents (e.g. La∼70 μg/l, Ce∼160 μg/l) and an enrichment of light REE (LREE) in the NASC normalised pattern. Considering the hydrodynamic model and geochemical data, the lake water composition may be calculated as a mixture of inflowing Quaternary and mining dump groundwaters. The groundwater of the dump aquifer is LREE enriched. Nevertheless, the leachates of dump sediments generally have low REE contents and display flat NASC normalised patterns. However, geochemical differences and REE pattern in undisturbed lignite (LREE enriched pattern and low water soluble REE contents) and the weathered lignite of the dumps (flat REE pattern and high water soluble REE contents) suggest that lignite is probably the main REE source rock for the lake water.

Precambrian U ore deposits, such as Oklo, Cigar-Lake or Palmottu, constitute invaluable analogues of nuclear waste repositories that provide direct evidence of U mobility or sequestration, over very large time intervals in geological formations. In this study, pervasive millimeter thick calcite veins filling microfractures in gneiss and granite surrounding the Palmottu U ore (Finland) were analysed, as fingerprints of past fluid circulation through the site. Stable Pb isotopes and short halflife 234U-230Th isotopes, all products of the U decay chain, have been chosen to investigate long term (over a Ga) and recent (within the last 500 ka) U migrations, respectively. Lead, U and Th isotopes have been analysed by thermo-ionisation mass spectrometry on mg-size bulk carbonate samples, and ion probe analyses of Pb isotopes performed on microinclusions of pyrite, monazite and coffinite. A striking contrast was found between the preservation of a well defined Pb–Pb isochron (1925±70 Ma, MSDW=2.47, N=17) on the one hand, and fractionated 230Th/238U ratios (230Th/238U<1) on the other hand. This shows that although the veins were formed simultaneously to the ore itself, during the Fennoscandian orogeny, and behaved mainly as a closed system for several billion years, U migrations did occur in the last 0.5 Ma. The recent U mobility is probably related to changes in the groundwater circulation through the bedrock. Nowadays, the dispersion of the U is, however, restricted to within a few meters around the U ore. It is shown that complex processes of dissolution and re-adsorption (or re-crystallisation) took place within these tiny carbonate fracture fillings. Relatively unradiogenic 206Pb/204Pb ratios measured by SIMS in coffinite micro-inclusions demonstrate recent precipitation of this mineral. Coffinite being the main U-bearing phase in the carbonate fracture fillings, its crystallization probably played a major role in U redistribution within the fractures, and in the relatively restricted dispersion of U through the bedrock.

The application of ICP-MS methods to tephrochronological problems by Nicholas J.G. Pearce; John A. Westgate; William T. Perkins; Shari J. Preece (289-322).
The accurate recognition of tephra deposits is of great value to Earth scientists because they facilitate stratigraphic correlation. The most useful tephra deposits form from violent volcanic eruptions; they are isochronous and widespread. Most are dacitic and rhyolitic in composition, and can be difficult to identify unequivocally using major element chemistry alone. Distal tephras are typically thin and are prone to contamination and thus are awkward to analyse by bulk methods. Here, the authors review their previous work in the development of analytical techniques for the analysis of small volumes of glass separates from tephra deposits, both by solution nebulisation and by laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS), placing particular emphasis on the precision and accuracy of the various methods. In solution nebulisation ICP-MS, accurate data can be obtained from samples as small as 0.025 g. LA-ICP-MS methods are described for the analysis of small bulk samples and single glass shards as small as 40 μm in diameter. Accurate and reproducible analyses can be achieved by ICP-MS by both solution and laser ablation methods on homogeneous materials. Solution analyses are normally accurate to ±5% and have typical precisions (1 σ) of around ±4% for abundant trace elements (e.g. Zr, Rb) but this can deteriorate to about ±20% for rare elements in small samples (e.g. HREE in a 25 mg sample). Laser ablation methods are slightly less accurate (typically ±5–10%) and precision decreases from about ±3% at concentrations of a few hundred ppm, to about ±10% at 1 ppm and about ±30% at 0.05 ppm. An apparent lack of precision in the bulk analysis of small volumes of glass shards by LA-ICP-MS often represents within sample heterogeneity (and not analytical error), inter-shard variation becoming abundantly clear in some tephra deposits when individual glass shards are analysed. Single grain analysis on shards as small as 40 μm can provide an accurate analysis of the pure glass phase, which may not be achieved in solution or bulk sample LA-ICP-MS methods. Analyses affected by micro-phenocryst phases, such as feldspar or zircon can be easily removed following careful inspection of the data. Single shard LA-ICP-MS also allows any compositional variation within the parental magma to be defined.

In the present study the distribution of TBT between solid and water phase as a function of several parameters was determined. Two types of clay minerals (Na-montmorillonite SWy and kaolinite KGa) and quartz sand were used as sorbents in conventional batch experiments. Sorption coefficients (K d) followed the order montmorillonite (89 l/kg) > kaolinite (51 l/kg) > quartz (25 l/kg), while for sorption coefficients normalized to the surface area (Kd′) an opposite trend was observed, with the lowest value determined for montmorillonite (2.79 × 10−3 l/m2) and the highest for quartz sand (8.04 × 10−2 l/m2). The results demonstrate that numerous environmental parameters influence the adsorption process of TBT, such as solid/solution ratio, clay content and salinity. Another important factor governing TBT adsorption is pH, because it affects both the TBT species in the water phase as well as the surface properties of the mineral phase. The maximum of TBT adsorption onto clays was always around pH 6–7. According to the data, it is evident that the content of organic matter in the solid phase plays an important role on TBT adsorption, either as particulate organic matter (POM) or organic matter adsorbed to mineral particles (AOM). Experiments were carried out with well characterized organic matter and the results showed a linear increase of K d from 51 up to 2700 l/kg upon the addition of 5% of particulate organic matter to pure phased kaolinite. TBT adsorption onto mineral surfaces, which were previously enriched with adsorbed organic matter, was investigated at different pH. The present study points to the importance of identifying and characterizing sorbents and envrionmental conditions, in order to predict and model TBT distribution in natural systems.

Nitrate reduction by zero-valent iron under different pH regimes by Seunghee Choe; Howard M. Liljestrand; Jeehyeong Khim (335-342).
Nitrate in drinking water can pose a threat to human health. A study of the reduction of NO3 , by Fe0 is reported here. The anaerobic reduction of NO3 was carried out using Fe0 powder in unbuffered solutions from pH 2 to greater than 10. The initial pH of the solution was adjusted to 2, 3, or 4 by addition of HCl, H2SO4, or CH3COOH, because the Fe oxidation and NO3 reduction reactions consume acidity. Under the conditions of this study, NH3/NH4 + were the only N products. The formation of green rusts divided the NO3 reduction process into two phases. Green rusts formed around a pH of 6.5 and contributed to the stabilization of pH. With H2SO4, the available Fe surface area was limited, initially by the excessive accumulation of H2 gas at the interface, which inhibited NO3 reduction. The surface area normalized pseudo-first order reaction rates for NO3 reduction at pH>6.5 or after the formation of green rusts are consistent with those reported for buffered solutions.

Ion-exchange batch experiments were run on Cretaceous (Magothy aquifer) clay cores from a nearshore borehole and an inland borehole on Long Island, NY, to determine the origin of high SO4 2− concentrations in ground water. Desorption batch tests indicate that the amounts of SO4 2− released from the core samples are much greater (980–4700 μg/g of sediment) than the concentrations in ground-water samples. The locally high SO4 2− concentrations in pore water extracted from cores are consistent with the overall increase in SO4 2− concentrations in ground water along Magothy flow paths. Results of the sorption batch tests indicate that SO4 2− sorption onto clay is small but significant (40–120 μg/g of sediment) in the low-pH (<5) pore water of clays, and a significant part of the SO4 2− in Magothy pore water may result from the oxidation of FeS2 by dissolved Fe(III). The acidic conditions that result from FeS2 oxidation in acidic pore water should result in greater sorption of SO4 2− and other anions onto protonated surfaces than in neutral-pH pore water. Comparison of the amounts of Cl released from a clay core sample in desorption batch tests (4 μg/g of sediment) with the amounts of Cl sorbed to the same clay in sorption tests (3.7–5 μg/g) indicates that the high concentrations of Cl in pore water did not originate from connate seawater but were desorbed from sediment that was previously in contact with seawater. Furthermore, a hypothetical seawater transgression in the past is consistent with the observed pattern of sorbed cation complexes in the Magothy cores and could be a significant source of high SO4 2− concentrations in Magothy ground water.

Strontium isotope geochemistry of groundwater in the central part of the Dakota (Great Plains) aquifer, USA by David C. Gosselin; F. Edwin Harvey; Carol Frost; Randy Stotler; P. Allen Macfarlane (359-377).
The Dakota aquifer of the central and eastern Great Plains of the United States is an important source of water for municipal supplies, irrigation and industrial use. Although the regional flow system can be characterized generally as east to northeasterly from the Rocky Mountains towards the Missouri River, locally the flow systems are hydrologically complex. This study uses Sr isotopic data from groundwater and leached aquifer samples to document the complex subsystems within the Dakota aquifer in Nebraska and Kansas. The interaction of groundwater with the geologic material through which it flows has created spatial patterns in the isotopic measurements that are related to: long-term water–rock interaction, during which varying degrees of isotopic equilibrium between water and rock has been achieved; and the alteration of NaCl fluids by water-rock interaction. Specifically, Sr isotopic data distinguish brines from Kansas and western Nebraska from those in eastern Nebraska: the former are interpreted to reflect interaction with Permian rocks, whereas the latter record interaction with Pennsylvanian rocks. The Sr isotopic composition of groundwater from other parts of Nebraska and Kansas are a function of the dynamic interaction between groundwater and unlithified sediments (e.g., glacial till and loess), followed by interaction with oxidized and unoxidized sediments within the Dakota Formation. This study illustrates the power of combining Sr chemistry with more conventional geochemical data to obtain a more complete understanding of groundwater flow systems within regional aquifer systems where extensive monitoring networks do not exist.

The speciation of Hg is a critical determinant of its mobility, reactivity, and potential bioavailability in mine-impacted regions. Furthermore, Hg speciation in these complex natural systems is influenced by a number of physical, geological, and anthropogenic variables. In order to investigate the degree to which several of these variables may affect Hg speciation, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the Hg phases and relative proportions of these phases present in Hg-bearing wastes from selected mine-impacted regions in California and Nevada. The geological origin of Hg ore has a significant effect on Hg speciation in mine wastes. Specifically, samples collected from hot-spring Hg deposits were found to contain soluble Hg-chloride phases, while such phases were largely absent in samples from silica-carbonate Hg deposits; in both deposit types, however, Hg-sulfides in the form of cinnabar (HgS, hex.) and metacinnabar (HgS, cub.) dominate. Calcined wastes in which Hg ore was crushed and roasted in excess of 600 °C, contain high proportions of metacinnabar while the main Hg-containing phase in unroasted waste rock samples from the same mines is cinnabar. The calcining process is thought to promote the reconstructive phase transformation of cinnabar to metacinnabar, which typically occurs at 345 °C. The total Hg concentration in calcines is strongly correlated with particle size, with increases of nearly an order of magnitude in total Hg concentration between the 500–2000 μm and <45 μm size fractions (e.g., from 97–810 mg/kg Hg in calcines from the Sulphur Bank Mine, CA). The proportion of Hg-sulfides present also increased by 8–18% as particle size decreased over the same size range. This finding suggests that insoluble yet soft Hg-sulfides are subject to preferential mechanical weathering and become enriched in the fine-grained fraction, while soluble Hg phases are leached out more readily as particle size decreases. The speciation of Hg in mine wastes is similar to that in distributed sediments located downstream from the same waste piles, indicating that the transport of Hg from mine waste piles does not significantly impact Hg speciation. Hg LIII-EXAFS analysis of samples from Au mining regions, where elemental Hg(0) was introduced to aid in the Au recovery process, identified the presence of Hg-sulfides and schuetteite (Hg3O2SO4), which may have formed as a result of long-term Hg(0) burial in reducing high-sulfide sediments.

In the Orco Valley, inside the Gran Paradiso Massif, 3 main fault systems are present: (a) E–W striking faults dipping at 45–60° to the N, (b) high-angle NW–SE striking faults, and (c) high-angle NE–SW to NNE–SSW striking faults. The E–W striking faults and the interposed NW–SE-faults appear to represent a cogenetic structural association related to a larger scale transtensional shear zone, while the NE–SW faults are probably inherited by an older discontinuous deformation stage. Breccia bodies or veins, mostly consisting of carbonate (siderite–ankerite±calcite)+quartz with sulphide–Au mineralisation, recording a multistage mesothermal evolution, occur along both E–W and NW–SE fault systems. Three water types are recognised: type I, Ca2+–HCO3 waters, with minor SO4 2−; type II, (Ca2+/Na+)–HCO3 waters varying towards Na+–(HCO3 /Cl) waters; and type III, Mg2+–HCO3 waters. Type I and type III groundwaters are freshly recharged waters, only slightly exchanged with rocks. Type II includes waters which come into contact with carbonate fracture fillings, and geochemical modelling indicates that dissolution of carbonates along fractures is the main process controlling the groundwater chemistry. These waters evolve in a system open to uprising CO2, and their strongly negative δ13CCO2 suggests a substantial organic component in the CO2 discharge. In the past, CO2-bearing fluids were likely responsible for the formation of Fe-bearing carbonate fracture fillings. The persistence through time of the CO2 flux in the region has important implications for the reconstruction of the Alpine tectonic evolution and deep structure.

Impacts of pollutant loading, climate variability and site management on the surface water quality of a lowland raised bog, Thorne Moors, E. England, UK by Simon Bottrell; Jonathan Coulson; Michael Spence; Peter Roworth; Martin Novak; Linda Forbes (413-422).

Solidification/stabilisation technologies are attracting great interest from mining and energy industries alike, to solve their pressing waste disposal problems. “Geopolymers”, in particular, are becoming one of the more popular solidification/stabilisation methods since they can be applied to a variety of waste sources at low cost, yielding added-value products. However, the effect of Al source on the solidification/stabilisation of heavy metals within fly ash-based Geopolymers, has received little attention. This study examines the effect of variable Al source and alkali-activator on the final properties of fly ash-based Geopolymers as characterised by compressive strength testing, infrared and X-ray diffraction analyses. Leaching tests were performed to determine the efficiencies of Pb and Cu immobilisation, which were compared to the initial properties of the Al source (e.g. particle size, cation exchange capacity, total extractable cation concentration and suspension yield stress). It was observed that Pb was generally better immobilised than Cu. In addition, the total extractable cation concentration of the Al source greatly affected the efficiency of Pb immobilisation while the physical properties of the Al source (suspension yield stress and eventual compressive strength) determined the efficiencies of Cu immobilisation. For both metals, NaOH activation was the most favourable method for metal immobilisation, however, a clear mechanism of adsorption remains elusive.

A kinetic study of the oxidation of arsenopyrite in acidic solutions: implications for the environment by Yu Yunmei; Zhu Yongxuan; A.E. Williams-Jones; Gao Zhenmin; Li Dexian (435-444).
Arsenopyrite is an important component of many ore deposits and dissolves in the O2-rich, acidic surface waters that are commonly found in the vicinity of active mines, releasing As, Fe and S to the environment. However, despite the potentially serious effect of this pollution on the human and animal population, the rate at which such oxidation occurs is poorly known. Kinetic experiments were therefore conducted in a mixed flow reactor to investigate the oxidation of arsenopyrite in Fe2(SO4)3 solutions (pH=l.8) having a concentration of l×l0−2 to 1 ×l0−5 mol kg−1 at temperatures of 45, 35, 25 and 15 °C. The results of these experiments show that the rate of oxidation of arsenopyrite increases with increasing concentration of dissolved Fe2(SO4)3 and temperature. They also show that As released during the oxidation of arsenopyrite has the form As(III), and that the rate of conversion of As(III) to As(V) is relatively low, although it tends to increase with increasing concentration of dissolved Fe2(SO4)3 and temperature. In the presence of Cl, oxidation of arsenopyrite is accelerated, as is the conversion of As(III) to As(V). These findings indicate that exploitation of arsenopyrite-bearing ores will cause contamination of groundwaters by As at levels sufficient to have a major negative effect on the health of humans and animals.
Keywords: Arsenopyrite; AMD; Mine wastes; Mine drainage; Arsenate and arsenite;

A dataset of major ion composition of 246 samples from cold-water springs discharging from perched-water bodies at volcanic islands (Azores archipelago, Portugal) reveal waters with low mineralization, which evolve due to two main geochemical processes: (1) seawater spraying and (2) dissolution of primary minerals of volcanic rocks. As a result, water facies range from Na–Cl to Na–HCO3 type waters. The relationship between alkali, alkali–earth metals and HCO3 shows differences between waters discharging from perched-water bodies in basaltic rocks comparing to more evolved rocks of trachytic nature. The use of principal component analysis shows that water-rock interaction is limited, which is compatible with the geochemical observations and with the hydrogeological environment.

Occurrence of native selenium in Yutangba and its environmental implications by Jianming Zhu; Wei Zuo; Xiaobing Liang; Shehong Li; Baoshan Zheng (461-467).
Rare native Se has been discovered in Yutangba, Enshi City, Hubei Province, China, where a sudden incidence of Se poisoning occurred in the early 1960s. It was first found in a small area where native Se was produced on a large scale and was diverse in forms due to different formation mechanisms. Genetically, native Se can be divided into 3 categories: the primary native Se occurring in carbonaceous-siliceous rocks and tiny Se crystals formed in cracks of rocks during tectonic activities; micro-Se crystals formed in the weathering processes of Se-rich rocks; and larger Se crystals derived from natural burning of stone coal on the subsurface of abandoned stone coal spoils. The different forms of native Se found in Yutangba demonstrate that Se can be activated, transformed, remobilized and enriched at special sites such as in the unsaturated-subsurface zone or in the saturated zone. This discovery of native Se has not only resolved a long-standing controversial question on modes of occurrence of Se, which has puzzled many researchers in China, but has provided new possible evidence for the pathogenic explanation of the sudden prevalence of Se poisoning in Yutangba. The research results also have important implications for studying the mineralogy, ore geochemistry and environmental geochemistry of Se.