Applied Geochemistry (v.15, #3)

The conditions for mineral alteration and formation damage during CO2 treatment of Tensleep sandstone reservoirs in northern Wyoming, USA, were examined through core-flooding laboratory experiments carried out under simulated reservoir conditions (80°C and 166 bars). Subsurface cores from the Tensleep sandstone, which were cemented by dolomite and anhydrite, and synthetic brines were used. The brines used were (Ca, Mg, Na)SO4–NaCl solution (9.69 g/l total dissolved solids) for Run 1 and a 0.25 mol/l NaCl solution for Run 2. The solution used in Run 1 was saturated with respect to anhydrite at run conditions, which is characteristic of Tensleep Formation waters.Three major reactions took place during flooding, including (1) dissolution of dolomite, (2) alteration of K-feldspar to form kaolinite, and (3) precipitation (in Run 1) or dissolution (in Run 2) of anhydrite. All sample solutions remained undersaturated with respect to carbonates. The permeability of all the cores (except one used in Run 2) decreased during the experiments despite the dissolution of authigenic cement. Kaolinite crystal growth occurring in pore throats likely reduced the permeability.Application of the experimental results to reservoirs in the Tensleep Formation indicates that an injection solution will obtain saturation with respect to dolomite (and anhydrite) in the immediate vicinity of the injection well. The injection of NaCl-type water, which can be obtained from other formations, causes a greater increase in porosity than the injection of Tensleep Formation waters because of the dissolution of both dolomite and anhydrite cements.

Modelling of modern mercury vapor transport in an ancient hydrothermal system: environmental and geochemical implications by A. Navarro-Flores; J. Martı́nez-Frı́as; X. Font; M. Viladevall (281-294).
Mercury has been used in some mineralizing hydrothermal systems as a significant pathfinder of Au and other elements. In addition, its geochemical behaviour reflects some characteristics related to the transport mechanism of metal-bearing fluids. In this work the authors analyse the geochemical aspects and the environmental impact of Hg mobilization from the old mining area of Valle del Azogue (VA), from smelting slags, mining waste and the partially exploited mineralization that remains a few metres underground. The main pathway of Hg is the transport of vapor from different foci, therefore a mathematical simulation of the process of Hg vapor transport was carried out in the Valle del Azogue (VA) mineral deposit. This is one of the two sites in Spain (the other being the famous district of Almaden), where Hg vapor emissions have been cited. The results obtained by means of mathematical simulation were compared with both the real Hg concentration values detected in the field (upper part of the mining area), and with the geochemical distribution patterns of Au and other elements related to the mineralization. A close relationship between the simulated Hg vapor distribution and soil gas concentrations was identified. It is proposed that the transport of Hg emissions from this old mining area could be the cause of present contamination of marine sediments in the area.
Keywords: Mercury vapor; Soil contamination; Numerical modelling; Valle del Azogue; SE Iberia;

Making diagenesis obey thermodynamics and kinetics: the case of quartz cementation in sandstones from offshore mid-Norway by Eric H. Oelkers; Per Arne Bjørkum; Olav Walderhaug; Paul H. Nadeau; William M. Murphy (295-309).
Calculation of the quantity and distribution of quartz cement as a function of time and temperature/depth in quartzose sandstones is performed using a coupled dissolution/diffusional–transport/precipitation model. This model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and/or clay grains at stylolite interfaces within the sandstones, and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible. Integration of the coupled mass transfer/transport equations over geologically relevant time frames is performed using the quasi-stationary state approximation. Results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data, are in close agreement with both the overall porosity and the distribution of quartz cement in the Middle Jurassic Garn Formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data. When quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants, the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0.008 cm−1 to 0.34 cm−1. In either case, these reactive surface areas are ∼2 to 4 orders of magnitude lower than that computed using geometric models.

Barium/Sr and Ca/Sr ratios have been used to model the relative importance of different sources of stream water. Major and trace element concentrations together with 87Sr/86Sr ratios were measured in precipitation, soil water, groundwater and stream water in a small (9.4 km2) catchment in northern Sweden. The study catchment is drained by a first order stream and mainly covered with podzolized Quaternary till of granitic composition. It is underlain by a 1.8 Ga granite. A model with mixing equations used in an iterative mode was developed in order to separate the stream water into 3 subsurface components: soil water, shallow groundwater, and deep groundwater. Contributions from precipitation are thus not included in the model. This source may be significant for the stream water generation, but it does not interfere with the calculations of the relative contributions from the subsurface components. The results show that the deep groundwater constitutes between 5 and 20% of the subsurface water discharge into the stream water. The highest values of the deep groundwater fraction occur during base flow. Soil water dominates during snowmelt seasons, whereas during base flow it is the least important fraction. Soil water accounts for 10–100% of the subsurface water discharge into the stream water. Shallow groundwater accounts for up to 80% of the subsurface water discharge with the lowest values at peak discharge during snowmelt seasons and the highest values during base flow. The validity of the model was tested by comparing the measured 87Sr/86Sr ratios in the stream water with the 87Sr/86Sr ratios predicted by the model. There was a systematic difference between the measured and modelled 87Sr/86Sr ratios which suggests that the fraction of soil water is overestimated by the model, especially during spring flood. As a consequence of this overestimation of soil water the amount of shallow groundwater is probably underestimated during this period. However, it is concluded that the differences between measured and predicted values are relatively small, and that element ratios are potentially effective tracers for different subsurface water flowpaths in catchments.

The calcite/portlandite phase boundary is an important chemical buffer in environmental and engineering applications where cementitious material is used. For a given pH or alkali concentration, the solution composition at this phase boundary is uniquely defined. Most chemical equilibria computer models predict a marked increase in the solubility of calcite with increasing pH along this phase boundary. This has implications in radioactive waste management because of the potential mobility of 14C-carbonate in high pH water systems. In this study, the effect of KOH concentration on the solution composition of calcite/portlandite saturated solutions was determined experimentally. Solid/water equilibration was approached from both undersaturation and supersaturation conditions and X-ray diffraction analysis was performed on all equilibrated solids. The experimental measurements of calcite solubility confirm the prediction of a substantive increase in carbonate concentration at high pH. Dissolved inorganic C (DIC) increases steadily with KOH concentration. At 5 molal KOH, the DIC concentration is 0.4 molal, which corresponds to 10 l of dissolved CO2 (measured at 25°C and 100 KPa) for each liter of solution. The triple point in the K2O–CaO–CO2–H2O system, where calcite and portlandite are joined by the precipitation of a third K-containing phase, was also delineated in a separate experiment. The third solid was identified to be buetschliite — a rare and highly-soluble polymorph of K2Ca(CO3)2. The triple or invariant point occurs somewhere between 8 and 10 molal KOH and the dissolved carbonate concentration here is equivalent to over 50 l CO2 gas per liter of solution.

Native gold in mineral precipitates from high-temperature volcanic gases of Colima volcano, Mexico by Yuri A. Taran; Alain Bernard; Juan-Carlos Gavilanes; Fatima Africano (337-346).
Trigonal and pentagonal shaped plates and prism, and octahedra of Au crystals 3–40 μm in dimension were found on the inner wall of a silica tube inserted into a 800°C fumarolic vent of Colima volcano, Mexico. Gold precipitates from the high-temperature and highly oxidized volcanic vapor (a mixture of magmatic gas with more than 90% of air) cover a narrow temperature range of 550–600°C, and occur in association with V-rich Na–K-sulfates. The Au concentration in the volcanic gas condensate is between 0.1 and 0.5 μg/kg. Using thermodynamic data for Au(c), Au(g), AuH(g) and AuS(g), open- and closed-system cooling of a simplified volcanic gas has been modeled with the following characteristics of volcanic gas+air mixture: P=1 bar, f SO 2 =0.01 bar; f SO 2 fixed by Fe2O3–Fe3O4 or Cu2O–CuO pairs for open-system cooling, and f SO 2 =0.1 bar for closed-system cooling. Volcanic vapor released from the shallow magma body transports Au as AuH(g) and AuS(g). According to calculations, after mixing with air, AuS(g) and AuH(g) oxidize to Au(g), and the temperature of the Au deposition depends only on the initial total concentration of Au species in the vapor. The temperature range of 550–600°C for Au precipitation at a high f SO 2 corresponds to a very low initial Au concentration, about 1 ng/kg in the volcanic gas condensate. This is at least two orders of magnitude lower than the observed Au content in the Colima gas, indicating the presence of other volatile gold species, e.g. AuCl x , Au(OH) x etc., or Au precipitation under non-equilibrium conditions from a volcanic gas+air mixture with coexisting free H2 and O2.

An X-ray absorption spectroscopy study of the coprecipitation of Tc and Re with mackinawite (FeS) by M.J Wharton; B Atkins; J.M Charnocka b ; F.R Livens; R.A.D Pattrick; D Collison (347-354).
X-ray absorption spectroscopy (XAS) has been used to define the local chemical environments of Tc and its proposed chemical analogue Re in precipitates of mackinawite (tetragonal FeS) and its oxidation products. Pertechnetate, [TcO4], is reduced to Tc(IV) on coprecipitation with FeS, while Tc(IV) undergoes no redox change. A TcS2-like phase is formed in both cases. On oxidation of the host lattice, Tc remains in oxidation state IV and forms a phase similar to TcO2. Perrhenate, [ReO4], behaves in a similar way to [TcO4] on coprecipitation although there is evidence that Re–S–Fe phases form. On reoxidation, the host FeS forms goethite and the Re may occupy channels in the goethite structure. Re(IV) is anomalous in that it forms a ReO2−like phase on coprecipitation with FeS, perhaps due to hydrolysis before precipitation of the FeS. These results suggest that the Tc/Re analogy is only partially sound and the elements are not identical. The failure of Tc held in FeS to reform [TcO4] on reoxidation of the FeS suggests that its mobility may be substantially reduced by natural anoxic environments and coprecipitation might be considered as a method of reducing Tc waste discharges.

Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift by Bernd R.T. Simoneit; Tarek A.T. Aboul-Kassim; J.J. Tiercelin (355-368).
Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3–4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a <200°C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales.

Surface water and peat in the northern Everglades have very low natural concentrations of U and are therefore sensitive to the addition of small amounts of U from anthropogenic sources such as fertilizer. Peat samples collected along a nutrient gradient in the northern Everglades have unusually high concentrations of U (>1 μg/g, dry basis) and also have a distinctive 234U/238U activity ratio (AR). AR values for U-enriched peat fall in the narrow range of AR values for commercial phosphate fertilizer (1.00±0.05). In contrast, AR values for low-U peat from background sites exceed 1.05. The spatial distribution of anomalous U concentration, and of fertilizer-like AR values in peat, parallel a previously documented pattern of P enrichment. These results strongly suggest that some of the U in nutrient-impacted peatlands is fertilizer-derived. Agricultural drainage water sampled in the northern Everglades has high concentrations of dissolved U (0.3–2.4 μg/l) compared to surface water from background sites (<0.1 μg/l). Measured AR values in drainage water (0.949–0.990) are also permissive of a fertilizer origin for the U and are different from AR values in surface water or peat at background sites (AR>1.05). Synoptic sampling of surface water along drainage canals indicate that Lake Okeechobee, and some drainage from agricultural fields, are sources of dissolved U, whereas wetlands farther downstream act as sinks for U. Historically cultivated agricultural soil has only a marginally elevated (+0.2 μg/g) average concentration of U compared to nearby uncultivated soil and incorporates only 20% of the U from an aqueous solution that was slurried with the soil. In contrast, a similar experiment with fresh Everglades peat indicated uptake of 90% of the added U. These experiments support the proposed removal of U from agricultural fields and concentration of U in downstream peatlands. The methodology of this study can be used to describe the behavior of fertilizer-derived U in other low-U environments.

The distribution of Se in cultivated topsoils, grains, human hair and drinking water has been studied in 15 villages from a Keshan disease area of the People's Republic of China, villages being classified into 3 groups according to the Keshan disease incidence in the local population. In grain, hair and water the total Se follows expected trends; i.e. the highest concentrations are found in the villages where there is no incidence of Keshan disease. However, the soils from the high-incidence Keshan disease villages have the highest total Se content, an apparent contradiction, as Keshan disease is a response to a Se deficient environment. Soil analyses suggest that the organic content of the soils is a major factor in controlling the availability of Se and it is the high-incidence Keshan disease villages that have the most organic-rich soils. Although higher in total Se, the organic-rich soils have little bioavailable Se resulting in a Se deficient food chain. Soil pH is also seen to be a related factor in restricting the availability of Se and all the grain samples collected on soils with a pH <7.6 had a total Se content of less than 10% of the total soil Se. In an environment that can be classified as Se deficient small changes in the soil organic content and pH can have a critical affect on the Keshan disease status of a village.