Applied Geochemistry (v.15, #8)

Diagenesis may be evaluated quantitatively by using petrographic observations and results of paleohydrologic reconstructions in combination with geochemical reaction path model calculations. The authors have applied a reaction path method by simulating diagenesis in the Eocene Wilcox sandstones in the Gulf of Mexico basin to evaluate the effects of variable Pco2, fluid composition, amount of rock reaction and burial history. The results show that increases in Pco2 cause the amount of carbonate phases to increase, instead of creating secondary porosity, and closed system reactions with a chemically evolved pore fluid cause a reduction in the amount of carbonate phases, thereby preserving primary porosity. Diagenesis resulting from increased rock reaction per pore volume is characterized by a dominance of Fe-free mineral phases, and albite forms in simulations at temperatures above 100°C with neutral pH evolved fluids. The results approximate petrographic observations of previous workers on the Wilcox with only a few exceptions. Continued simulations using different fluid compositions and organic acid anions may increase the capability to reproduce observed paragenetic sequences.

The pore space of deep saline aquifers in the Alberta (sedimentary) Basin offers a significant volume for waste storage by “hydrodynamic trapping”. Furthermore, given the slow regional fluid flow in these deep saline aquifers, ample time exists for waste-water/rock chemical reactions to take place. A geochemical computer model (PATHARC) was used to compute the interaction of industrial waste streams comprising CO2, H2SO4 and H2S with the minerals in typical carbonate and sandstone aquifers from the Alberta Basin. The results support the idea that these acids can be neutralized by such reactions and that new mineral products are formed, such as calcite, siderite, anhydrite/gypsum and pyrrhotite, thereby trapping the CO3, SO4 and S ions that are formed when the acid gases dissolve in the formation water. Siliciclastic aquifers appear to be a better host for “mineral trapping” than carbonate aquifers, especially with regard to CO2. Carbonate aquifers may be more prone to leakage due to high CO2 pressures generated by reaction with H2SO4 and H2S. Even though permeability decreases are expected due to this “mineral trapping”, they can be partially controlled so that plugging of the aquifer does not occur.

The Rabbit Lake U mine in-pit tailings management facility (TMF) is located in northern Saskatchewan, Canada. The tailings body is approximately 425 m long×300 m wide and 91 m thick at its centre. An investigation of the TMF was performed to collect tailings samples from depth to quantify the distribution of As and Ni in the tailings with respect to ore type and assess the distribution of As and Ni with respect to tailings mineralogy. The tailings body consists of alternating layers of ice, frozen tailings and unfrozen tailings which varied in texture from a slurry to a firm silty sand. The tailings solids are predominately composed of quartz (16–36%), calcium sulphate (0.3–54%) and illite (3 and 14%). Arsenic and Ni concentrations in the tailings show similar patterns with depth which were strongly related to historical changes in As and Ni concentrations in the mill feed. Mineralogy of the ore bodies indicated that As and Ni in the mill feed occurred primarily as 1:1 molar ratio arsenides such as niccolite and gersdorffite. Arsenic and Ni concentrations in the tailings were also measured at a near 1:1 molar ratio. Mill process records showed that an average of 71% of the As in the mill feed was solubilized during leaching. SEM analysis suggested that solubilized As is precipitated as Ca2+, Fe3+ and Ni2+ arsenates during the neutralization process. Mill records indicated that 17,000 tonnes of As were discharged to the TMF of which approximately 88% was as arsenates and 12% as primary arsenides.

The principal Al-bearing components of two surface quadrats in the central Te Kopia geothermal field are the atmosphere, substrate (∼10 wt% Al in ignimbrite, clay and protosoils, 0.3–0.6 Al wt% in sinter), vegetation (4–5 g Al/m2) and waters (1–4 μg/g Al in semi-permanent acid surface waters, 6–9 μg/g in acid pools, 10–14 μg/g in post-rain, ephemeral streams and pools). About 0.7 g/ha/a of Al is received from the atmosphere. Water transports Al in and out of each quadrat and distributes it between the different components. During initial alteration of the parent ignimbrite by alkali chloride water in the deep reservoir, Al either remained within the quadrat boundaries or transfers out were balanced by contemporaneous gains. Subsequently, alteration by acid sulfate fluids redistributes elements into new mineral assemblages but again with no net movement of Al in or out of either quadrat. The latest, surface alteration event involves interaction of all the previously and variously altered rocks by steam, gases and steam condensate. A primary product of this process is transient, hydrated, Al-rich, water-soluble sulfate efflorescences whose persistence indicates a steady flux of Al at the surface. The magnitude of this flux depends on available moisture and the activities of H+, SiO4− 4, SO2− 4 and K+ such that variations in the rate of discharge of Al alone may be used to detect changes in surface conditions as may result from exploitation of a geothermal field.

4He concentrations in excess of the solubility equilibrium with the atmosphere by up to two to three orders of magnitude are observed in the Carrizo Aquifer in Texas, the Ojo Alamo and Nacimiento aquifers in the San Juan Basin, New Mexico, and the Auob Sandstone Aquifer in Namibia. A simple 4He accumulation model is applied to explain these excess 4He concentrations in terms of both in situ production and a crustal flux across the bottom layer of the aquifer. Results from the model simulations suggest variability in the 4He fluxes, ranging from 6×10−6 cm3 STP cm−2 yr−1 for the Auob Sandstone Aquifer to 3.6×10−7 cm3 STP cm−2 yr−1 for the Carrizo aquifer. For the Ojo Alamo and Nacimiento aquifers an intermediate value of 3×10−6 cm3 STP cm−2 yr−1 was estimated. The contribution of in-situ produced 4He to the measured concentrations was also estimated. This contribution is negligible for the Auob Sandstone Aquifer as compared with both the concentrations measured at the top and bottom of the aquifer for most of the pathway. In the Carrizo aquifer, in-situ produced 4He contributes 27.5% and 15.4%, to the total 4He observed at the top and bottom of the aquifer, respectively. For both aquifers of the San Juan Basin in-situ production almost entirely dominates the 4He concentrations at the top of the aquifer for most of the pathway. In contrast, the internal production is negligible as compared with the measured concentrations at the bottom of these aquifers, reaching, at most, 1.1%. The model simulations require an exponential decrease in the horizontal velocity of the water with increasing recharge distance to reproduce the distribution of 4He in these aquifers. For the Auob Sandstone Aquifer the highest range in the velocity values is obtained (25 to 0.4 m yr−1). The simulations for the Carrizo aquifer and both aquifers located in the San Juan Basin require velocities varying from 4 to 0.1 m yr−1, and from 2 to 0.3 m yr−1, respectively. For each aquifer, average permeability values were also estimated. They are generally in agreement with results obtained from pumping tests, hydrodynamic modeling and previous 14C measurements. On the basis of the results obtained by calibrating the model with the measured 4He concentrations, the mean water residence times were estimated. They agree reasonably well with 14C ages. When applied as chronologies for noble gas temperatures in the same aquifers, the calculated 4He ages allow the identification of three different climate periods similar to those previously identified using 14C ages: the Holocene period (0–10 Ka BP), the Last Glacial Maximum (≈18 Ka BP), and the preceeding period (30–150 Ka BP).

The Juan de Fuca Ridge is a hydrothermally active, sediment covered, spreading ridge situated a few hundred kilometres off the west coast of North America in the northeastern Pacific Ocean. Sediments from seven sites drilled during the Ocean Drilling Program (ODP) Legs 139 and 168 were analyzed for total hydrolyzable amino acids (THAA), individual amino acid distributions, total organic C (TOC) and total N (TN) contents. The aim was to evaluate the effects of hydrothermal stress on the decomposition and transformation of sedimentary amino acids. Hydrolyzable amino acids account for up to 3.3% of the total organic C content and up to 12% of the total N content of the upper sediments. The total amounts of amino acids decrease significantly with depth in all drilled holes. This trend is particularly pronounced in holes with a thermal gradient of around 0.6°C/m or higher. The most abundant amino acids in shallow sediments are glycine, alanine, lysine, glutamic acid, valine and histidine. The changes in amino acid distributions in low temperature holes are characterized by increased relative abundances of non-protein β-alanine and γ-aminobutyric acid. In high temperature holes the amino acid compositions are characterized by high abundances of glycine, alanine, serine, ornithine and histidine at depth. D/L ratios of samples with amino acid distributions similar to those found in acid hydrolysates of kerogen, indicate that racemization rates of amino acids bound by condensation reactions may be diminished.

The impact of climatic and vegetation conditions over the past 15 Ka on the chemical composition and 14C dating of groundwater from the Franconian Albvorland aquifer system is discussed. Seven groundwaters over a flow distance of 25.5 km are investigated. Groundwater dating is made by 14C of dissolved inorganic carbon (DIC) and aqueous fulvic acid as well as 18O. 14C dating via fulvic acid gives groundwater ages consistent with climatic and vegetation records and variations in the groundwater composition. No correction for geochemical processes is required, since under these geochemical conditions fulvic acid remains stable over this time period and flow-distance. On the other hand, 14C dating via DIC requires correction of the 14C value due to perturbation by different geochemical processes. Up to a groundwater flow distance of approximately 17 km and an age of about 10 Ka, the 14C dating by DIC shows considerable dependence on the 14C-correction model applied. Beyond this groundwater age, 14C-DIC dating results in an overestimation by two to three 14C half-lives (T 1/2=5730 a). This deviation may result from different groundwater recharge conditions at the end of the past glaciation and geochemical processes acting on DIC that cannot be adequately characterized. The present study has implications for humic substance mediated transport of pollutants in natural aquatic systems over long time periods.

Ca6[Al(OH)6]2(CrO4)3·26H2O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca6[Al(OH)6]2(CrO4)3·26H2O⇌6Ca2++2Al(OH) 4+3CrO2− 4+4OH+26H2O varies with pH unless a CaCrO4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca2++CrO2− 4=CaCrO4(aq) was obtained by minimizing the variance in the IAP for Ca6[Al(OH)6]2(CrO4)3·26H2O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K SP) of Ca6[Al(OH)6]2(CrO4)3·26H2O at 25°C is −41.46±0.30. The temperature dependence of the log K SP is log K SP=AB/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG 0 r,298 and ΔH 0 r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol−1. the values of ΔC 0 P,r,298 and ΔS 0 r,298 are −1506±140 and −534±83 J mol−1 K−1. Using these values and published standard state partial molal quantities for constituent ions, ΔG 0 f,298=−15,131±19 kJ mol−1, ΔH 0 f,298=−17,330±8.6 kJ mol−1, ΔS 0 298=2.19±0.10 kJ mol−1 K−1, and ΔC 0 Pf,298=2.12±0.53 kJ mol−1 K−1, were calculated.

Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California by Kaye S Savage; Tracy N Tingle; Peggy A O’Day; Glenn A Waychunas; Dennis K Bird (1219-1244).
Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain ∼20–1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the fine-grained (<63 μm) fraction of these Fe-rich weathering products.Individual pyrite grains in albite-chlorite schists of the Clio mine tailings contain an average of 1.2 wt.% As. Pyrite grains are coarsely zoned, with local As concentrations ranging from ∼0 to 5 wt.%. Electron microprobe, transmission electron microscope, and extended X-ray absorption fine-structure spectroscopy (EXAFS) analyses indicate that As substitutes for S in pyrite and is not present as inclusions of arsenopyrite or other As-bearing phases. Comparison with simulated EXAFS spectra demonstrates that As atoms are locally clustered in the pyrite lattice and that the unit cell of arsenian pyrite is expanded by ∼2.6% relative to pure pyrite. During weathering, clustered substitution of As into pyrite may be responsible for accelerating oxidation, hydrolysis, and dissolution of arsenian pyrite relative to pure pyrite in weathered tailings. Arsenic K-edge EXAFS analysis of the fine-grained Fe-rich weathering products are consistent with corner-sharing between As(V) tetrahedra and Fe(III)-octahedra. Determinations of nearest-neighbor distances and atomic identities, generated from least-squares fitting algorithms to spectral data, indicate that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for SO4 in jarosite. Erosional transport of As-bearing goethite and jarosite to Don Pedro Reservoir increases the potential for As mobility and bioavailability by desorption or dissolution. Both the substrate minerals and dissolved As species are expected to respond to seasonal changes in lake chemistry caused by thermal stratification and turnover within the monomictic Don Pedro Reservoir. Arsenic is predicted to be most bioavailable and toxic in the reservoir’s summer hypolimnion.