Applied Geochemistry (v.15, #1)

Calculated isochores, determined from microthermometry and laser Raman microprobe analysis, for CH4 inclusions hosted in quartz grains from 3 different ore types in the Dugald River Zn–Pb–Ag deposit yield fluid trapping P–T conditions of 300–350°C and 0.8–2.3 kbar for post-depositional metamorphic conditions of sulfide mineralization. The consistent relationship between fluid composition and metamorphic grade of the host rocks suggests that CH4 has been formed by reaction of graphite with aqueous fluids during metamorphic retrogression. Even though fluids responsible for mineralization are generally similar to those for barren quartz veins in chemistry and microthermometry, the substantial contents of CH4 and CO2 in the Dugald River lode inclusions contrast sharply with CO2 alone in barren vein inclusions. The presence of CH4-rich inclusions in the mineralization at Dugald River and other similar deposits, and their absence in the barren veins in a regional context, can be regarded as a possible exploration guide for defining target areas hosting mineralization of this style.

The carboxylate (formate, acetate, propionate and oxalate) and common inorganic anions (F, Cl and SO2− 4) compositions for aqueous fluid inclusion leachates from 17 mineral samples collected from various deposits have been determined using ion chromatography in conjunction with microthermometric measurements on the fluid inclusions of their host minerals. The minerals, quartz, fluorite, barite, beryl and a few `ore' minerals (wolframite, pyrite and galena), came from hydrothermal vein-type deposits in felsic igneous rocks or Archean metamorphic rocks. The results indicate that short-chain carboxylates are common components in hydrothermal fluids and can be present in considerable amounts. Formic acid (as formate) is the dominant species over other carboxylic acids. The present study raises new questions about the origin and geochemical significance of carboxylates in hydrothermal ore-forming processes.

`Batch experiments' (25:1 v:w) were used to test the effects of pH upon the release of SO−2 4 from ferric precipitates formed in acid mine drainage (AMD) in southeastern Kentucky. Analytical grade CaO [`quicklime'], Ca(OH)2 [hydrated lime] and CaCO3 [referred to as `limestone'] were used as alkalinity generating agents and were mixed with ferric precipitates composed of amorphous iron oxyhydroxides, jarosite and goethite. Regression analyses indicated that aqueous SO−2 4 concentrations increased linearly with pH between values of 2.5 and 9.0. At pH=7, 30–45% of the oxalate soluble SO−2 4 was released from the ferric precipitates. The 3 types of alkaline neutralizing agents released similar, but not identical, quantities of SO−2 4 at a given pH. The results of these experiments suggest that AMD remediation schemes should consider raising the pH of waste streams only to values necessary to precipitate and/or sorb metals, in order to minimize SO−2 4 desorption. Iron oxyhydroxides that precipitate within AMD often sorb high concentrations of SO−2 4; a significant percentage of which will be released in streams draining carbonate bedrock.

A comparative assessment of two reactive-transport models, PHREEQC and HYDROGEOCHEM (HGC), was done to determine the suitability of each for simulating the movement of acidic contamination in alluvium. For simulations that accounted for aqueous complexation, precipitation and dissolution, the breakthrough and rinseout curves generated by each model were similar. The differences in simulated equilibrium concentrations between models were minor and were related to (1) different units in model output, (2) different activity coefficients, and (3) ionic-strength calculations. When adsorption processes were added to the models, the rinseout pH simulated by PHREEQC using the diffuse double-layer adsorption model rose to a pH of 6 after pore volume 15, about 1 pore volume later than the pH simulated by HGC using the constant-capacitance model.In PHREEQC simulation of a laboratory column experiment, the inability of the model to match measured outflow concentrations of selected constituents was related to the evident lack of local geochemical equilibrium in the column. The difference in timing and size of measured and simulated breakthrough of selected constituents indicated that the redox and adsorption reactions in the column occurred slowly when compared with the modeled reactions. MINTEQA2 and PHREEQC simulations of the column experiment indicated that the number of surface sites that took part in adsorption reactions was less than that estimated from the measured concentration of Fe hydroxide in the alluvium.

Geochemistry of the formation waters in the Po plain (Northern Italy): an overview. by Anna Conti; Elisa Sacchi; Marta Chiarle; Giovanni Martinelli; Giovanni Maria Zuppi (51-65).
The Po Valley brines represent the base level of the Quaternary aquifer located in a thick clay-sands sedimentary sequence. Geochemistry indicates that these are marine waters, evaporated to the stage of gypsum precipitation and trapped at the bottom of the basin in the late Messinian. Most of the groundwater samples collected from different springs and wells in the plain result from a mixture of these Na–Cl brines and shallow groundwaters laterally recharged by the Alpine and Apennine chains.Natural outflows of brackish waters are associated with major tectonic features. Mud volcanoes, located in the eastern sector of the Po plain, are constantly monitored as sudden chemical changes are significant precursors of seismic activity. In the western sector, calcite-filled veins isotopically record different degrees of water-rock interaction. These are outcropping fossil conduits, where mixing between shallow groundwaters and deep seated brines has occurred. The temporal continuity of the hydrological circuits allows the reconstruction of past and present groundwater circulation patterns.This paper summarises and integrates the geochemical data produced over many years in order to obtain a regional picture of brine origins and the natural mechanisms of groundwater flow.

Discharge is the dominant control on the TDS load of the Bow River; TDS varies inversely with discharge. Although discharge is the dominant control on concentration, the sources of ions in the river are atmospheric deposition and water/rock interaction. Atmospheric loading can be a significant source of some ions in the pristine headwaters of the river (e.g., 50% of K, 17% of SO4, 16% of Cl). In terms of water/rock interaction, the input of ions to the river is largely controlled by dissolution of carbonate and evaporite minerals.The chemical denudation rate for the Bow River at Banff is 678 kg/ha/a, or 1.50×108 kg of rock that is removed as dissolved load each year, in the low range for an alpine carbonate basin. An additional 11 kg/ha/a are removed as suspended load. A rock volume of 5.45×104 m3 is carried by the Bow River from Banff National Park each year.

Stepwise high temperature supercritical fluid extraction (HT–SFE) has been suggested as a tool to study the speciation of hydrocarbons in geological samples. Hydrocarbons extracted at the lower temperatures (e.g., 50°C) are presumed to be part of the freely extractable fraction, while those recovered at the high temperatures (e.g., 300 and 350°C) are those `trapped' within the macromolecular organic matrix and are therefore, resistant to desorption. The latter are released from the matrix after this undergoes thermally induced structural rearrangements. However, the question still remains if and to what extend, pyrolysis of the organic matrix can contribute to this fraction. This study shows, based on the characteristics of the sample matrix of two different shale samples subject to HT–SFE, that pyrolytic contributions at elevated extraction temperatures are only minor under the experimental conditions used, and that thermally induced structural changes in the macromolecular organic matrix are only partially irreversible.

Abiological formation of formic acid on rocks in nature by Kiyohisa Ohta; Hirosi Ogawa; Takayuki Mizuno (91-95).
The formation of formic acid by ants is well known since the discovery by S. Fisher in 1670. In general, it has been believed that formic acid is not formed abiologically in nature. To evaluate the possibility of abiological formation of formic acid on silicate rock, a photochemical reduction of CO2 using silicate rock powders (amphibolite, gneiss, granite, granodiorite, quartzdiorite, shale and zeolite) suspended in water was investigated under sunlight at ambient temperature and pressure.It was found from the study that 4.6×10−6 g formic acid/m2 is formed daily on wetting-silicate rocks exposed to sunlight in nature. The range of the expected global production rate is between 2.5 and 3000 kt/a or more. Therefore, formic acid formed on wet-rocks might play an important role in the weathering of rocks.

Characterization of groundwater humic substances: influence of sedimentary organic carbon by R Artinger; G Buckau; S Geyer; P Fritz; M Wolf; J.I Kim (97-116).
A total of 35 groundwaters from 4 different aquifer systems in Germany are investigated for their physico-chemical properties, dissolved organic C (DOC) and humic and fulvic acids. Humic substances are isolated and characterized for their elemental composition, UV/Vis and fluorescence spectroscopic properties, size distribution by gel permeation chromatography (GPC) and 14C content. For isolation of sufficient quantities of humic substances a mobile sampling system is developed based on a combination of reverse osmosis (RO) and XAD–8 adsorption chromatography. One of the aquifer systems (Gorleben) covers a wide range of hydrogeochemical conditions, whereas the other 3 aquifer systems (Munich, Franconian Albvorland and Fuhrberg) have less diverse properties. One specific feature of the Gorleben aquifer system is the presence of a very high DOC, which, in contrast to other aquifer systems, contains considerable amounts of aquatic humic acid. This is attributed to the release of aquatic humic substances originating from sedimentary organic C (SOC) that is abundant in Gorleben sediments. The results show that aquatic humic substances from different aquifer systems have dissimilar properties which differ from one another. Systematic differences are found among humic substances from different regions of the Gorleben aquifer system. Such differences are considered to be caused by the mixing of humic substances from the SOC. However, exact quantification of such mixing appears difficult because overlapping effects of different geochemical processes feigning a dissolution of SOC cannot be excluded.

Selenium deficiency (Keshan Disease) and toxicity diseases in humans occur within 20 km of each other in Enshi District in China and have been linked to environmental levels of Se. Low concentrations of Se are associated with Jurassic siltstones and sandstones, whereas high concentrations occur in areas underlain by Permian carbonaceous strata. Although these broad relationships between Se in the environment and the human population have been established previously, not all villages underlain by the carbonaceous strata suffer Se toxicity problems and the precise controls on Se distribution and availability have not been quantified. In the present study, soil, grain, drinking water and human hair samples are examined to determine the controls on Se availability in 3 Se environments in Enshi District. Five low-Se and Keshan Disease villages, 5 high-Se and no toxicity villages and 5 high-Se and toxicity villages were selected for the study. Results show that the majority of samples in the low-Se villages are deficient or marginal in Se, and that Se availability to plants is inhibited by adsorption onto organic matter and Fe oxyhydroxides in soil. Therefore, remediation strategies involving the application of Se fertiliser direct to the soil may not increase plant Se levels as expected. In the high-Se villages, localised lithological variations result in considerable ranges in Se concentrations in all sample types. Deficient and excessive levels of Se are recorded in samples from the same village. Selenium bioavailability in the high-Se toxicity villages is controlled by the total soil Se concentration and pH. A greater proportion of the Se is plant available in villages where the carbonaceous strata are interbedded with limestone. Villagers should be advised to avoid planting crops in these areas if possible.