Applied Geochemistry (v.16, #3)

The weathering products of the granitic rocks (which are poor in base cations) and their sediments in the Fláje basin, in an environment that is strongly influenced by SO2 and NOx emissions have been studied. High deposition of SO2 and consequent acidification of the environment causes further depletion of base cations, mobilisation of Al and modification of the weathering reactions resulting in changes of the geochemical features of recent sediments. Depletion of the bottom sediments of the Fláje basin in base cations and Al and their enrichment in Si, compared to the fresh stream sediments and eluvium, suggests a mineralogical modification of the weathering product due to prolonged contact with basin water. Depletion of the non-silicate phase of the sediment in base cations which are replaced with Al on the other hand appears to be more effective in eluvium and those sediments which are exposed directly to the effect of acid deposition, i.e. which are not below water level for the major part of the year. The water of the basin appears to be the main factor influencing the composition of the recent sediments of the basin in the acidified catchment.

On-site mercury analysis of soil at hazardous waste sites by Robert W Gerlach; Mae Sexauer Gustin; Jeanette M Van Emon (281-290).
Two field methods for Hg, immunoassay and anodic stripping voltammetry (ASV), that can provide onsite results for quick decisions at hazardous waste sites were evaluated. Each method was applied to samples from two Superfund sites that contain high levels of Hg; Sulphur Bank Mercury Mine site, Clear Lake, California, and Carson River Mercury site, Nevada. Two laboratory methods were used for comparison purposes; cold vapor atomic fluorescence spectrometry (CVAFS) and inductively coupled plasma-mass spectrometry (ICP-MS). The immunoassay was found to be accurate for high and low Hg concentrations compared to the 5 and 15 μg/g soil sample standards provided with it. Despite poor agreement between immunoassay and confirmatory analysis results at concentrations near the comparison standards, the immunoassay could be used as an effective screening method provided care is taken in identifying an operational screening level. The ASV method had an analytical range of 1–50 μg/g, with a CV of 15%. ASV results were comparable to CVAFS (CV=15%) and more precise than ICP-MS (CV=20%). The lower limit of quantitative results was 3 μg/g for field samples, and is attributed to uncertainty associated with sampling.

Dynamic changes in hydrogeochemical conditions caused by tunnel excavation at the Aspo Hard Rock Laboratory (HRL), Sweden by Yasunori Mahara; T Igarashi; T Hasegawa; K Miyakawa; Y Tanaka; K Kiho (291-315).
Hydraulic changes caused by tunneling at the Aspo Hard Rock Laboratory (HRL) in Sweden have been investigated over a period of 2a using different hydrochemical approaches, i.e. noble gas content, isotopic measurements and major ion concentrations. The dissolved noble gases (4He and Ne contents, and the ratio of 3He/4He, 40Ar/36Ar), stable isotopes, chemical concentrations of major ions, and 36Cl/Cl ratios, were determined in groundwater samples collected in the tunnel from borehole sections isolated by inflated packers. Groundwater was categorized into 3 groups based on 4He and Cl contents: undisturbed groundwater (i.e. prior to tunnel construction) with high 4He and Cl contents, groundwater that has been gradually changed by mixing with Baltic seawater and whose 4He and Cl contents have gradually increased with increasing depth, and groundwater that has been totally changed due to a rapid mixing of Baltic seawater and/or shallow groundwater and whose 4He and Cl contents are extremely low compared with other samples collected at the same surrounding depth. The oldest groundwater with a high salinity of more than 14,000 mg l−1 of Cl is estimated to be more than 1.8 Ma old. The groundwater residence time ranges from 0.9 to 900 Ka in the mixing-zone. Groundwater in the disturbed zone where rapid mixing has occurred is hard to date reliably and its primary hydrochemical character has already been lost.

Variations in the boron isotope composition of Coffea arabica beans by Michael E. Wieser; S.S. Iyer; H.R. Krouse; M.I. Cantagallo (317-322).
Significant B isotope abundance variations were found for a variety of Coffea arabica beans from a number of coffee-growing regions around the world. This may be attributed to the influence of local sources of B (including soil, water and fertilizer) each having a characteristic B isotope abundance ratio of its own. The results of this preliminary study indicate that B isotope abundance data can be used to study the biogeochemical cycle of B, an important micronutrient. In addition, the isotopic data can be employed in quality assurance programs of commercial coffee as the quality depends to a large extent on the genetic and geographic origin of the coffee.

Chemical and isotopic compositions are reported for water, and CO2 and noble gases in groundwater and soda springs from Bioko, Principé, São Tomé and Annobon, all islands located in the off-shore part of the Cameroon Volcanic Line in West Africa. The soda spring waters are of Ca–Mg–HCO3 type, with δD and δ18O values that range from −20 to −8‰ and −5.4 to −2.7‰ respectively, indicative of a meteoric origin. CO2 is the main gas species in the springs. δ13C–CO2 values vary from −2.8 to −5.0‰, overlapping the observed mantle C range (−3 to −8‰). CO2/3He ratios (3–9×109) suggest that most C (∼90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic 40Ar. 3He/4He ratios (3.0 to 10.1×10−6 or 2.1 to 7.2R a , where R a is the atmospheric ratio of 1.4×10−6) are much higher than those for typical crustal fluids (∼10−8) but lower than those expected for fluids derived from ‘high-3He/4He’ hotspots like Loihi and Iceland. This precludes significant contributions of such fluids in the source regions of the gases, and by inference, in the magmatism of these oceanic islands. Alternatively, approximately 90% of the He in São Tomé gases is inferred to be derived from a source similar to the MORB source. The 3He/4He ratio for the Bioko gas (6.6×10−6) may be derived from a source with a higher time integrated (U+Th)/3He ratio than the MORB source.

This study on the application of multi-element relationships in stream sediments to mineral exploration in the Walawe Ganga Basin presents one of the first of its kind in Sri Lanka. In order to determine the nature of these sediments, multi-element analysis of selected grain size (<63 μm, 63–125 μm, 125–177 μm and 177–250 μm) fractions was undertaken. The chemical composition of these fractions were compared with those of the upper continental crust.Some elements, notably Zr, Hf, Th, U, Ce and La, show very high enrichment factors (e.g. 54 for Zr) as compared to the upper crustal abundance. These are presumably associated with heavy minerals such as zircon, rutile and monazite found in abundance in the stream sediments of the Walawe Ganga Basin.Principal Component Analysis (PCA) of the geochemical data show that multi-element relationships could be effectively used to delineate target areas for mineral exploration. With the use of PCA, this study reveals that areas with associations of calc-silicate/marble and charnockitic rocks are probable source regions for mineral occurrences, particularly in the axial regions of anticlines.

Trace element and isotopic evidence for REE migration and fractionation in salts next to a basalt dyke by Marc Steinmann; Peter Stille; Kurt Mengel; Bernard Kiefel (351-361).
Neodymium and Sr isotopic compositions and the rare earth elements (REE) distribution patterns have been determined in salts adjacent to a basaltic dyke along 2 parallel horizontal profiles. The salts, originally consisting of carnallite (KMgCl3 · 6H2O), have been transformed during basalt intrusion mainly into halite (NaCl) and sylvite (KCl) by fluids saturated in NaCl. The Sr isotope data suggests that much more fluid penetrated the upper than the lower horizon. The Nd isotope data shows that in the upper horizon, where fluid flow was stronger, Nd is essentially derived from the basalt. In contrast, in the lower horizon a strong salt Nd component is present.The REE data document in both horizons is a strong depletion of Ce, Pr, Nd, Sm and Eu with increasing distance from the basalt. This depletion of the light rare earths (LREE) is stronger in the upper horizon where fluid flow was stronger. The authors suggest that this REE fractionation is more likely due to precipitation of LREE-enriched accessory minerals such as apatite, than to differential REE solubility caused by selective REE complexation. This finding is of interest for REE behaviour in brines in general, and for the behaviour of radioactive REE and actinides in a salt repository for high-level nuclear waste in particular.

The Nordåsvannet fjord in western Norway is a modern semi-enclosed basin suitable for studying sedimentary cycles as they occur under anoxic bottom conditions. It is characterized by strongly anoxic conditions in the water column and bottom sediments. Diagenetic pyrite formation occurs in the sediments, and syngenetic pyrite is formed in the lower water column. Organic matter burial in the fjord exceeds that of other environments with normal marine or upwelling conditions. This is due to the better preservation of organic matter. Organic matter composition appears to have changed over time with higher fractions of terrigenous organic matter being present in the most recent sediments. This may be a result of increased input of terrigenous organic matter, possibly due to sewage supply to the fjord over the last decades. Organic C and CaCO3 contents of the sediments do not appear to reflect a productivity signal. Calcium carbonate content is influenced by chemogenic calcite formation. Biogenic opal content appears to reflect a productivity signal, but different degrees of dissolution may obscure its clear recognition.

The concentrations of the lanthanide rare earth elements (REE) and Pt group elements (PGE) were measured in the Kupferschiefer from the Polish Zechstein Basin at, and in proximity to, the Rote Fäule near the Lubin Mining District. The Rote Fäule is a zone of post-depositional oxidation characterized by the presence of extensive amounts of Fe(III) oxides replacing syn-sedimentary framboidal pyrite. Outward from the Rote Fäule, the remainder of the Kupferschiefer is composed of Cu- and Pb/Zn-mineralized shale surrounding the Rote Fäule and a non-mineralized pyritic black shale in the central basin.The leading hypothesis explaining the high concentrations of PGE, and REE in the Kupferschiefer states that PGE, REE and the associated base metals were mobilized by oxidizing Cl brines which migrated outward from the Rote Fäule into the reduced Kupferschiefer. According to available thermodynamic data, PGE were in all likelihood present as chloro-complexes in these oxidizing brines, as geologically realistic concentrations of Pt, Pd and Au could be transported as chloro-complexes. The Eh of these brines decreased as they migrated further from the Rote Fäule and into the Kupferschiefer. Base metals and PGE were precipitated in the order of their decreased solubility in these brines. As a result, the concentrations of least soluble PGE (Pt) are highest in the Rote Fäule and in the transition zone adjacent to the Rote Fäule (e.g. [Pt]=202–537 ppb) while the concentrations of the more soluble metals in these brines (Ag, Cu, Pb, and Re) are highest in the reduced-mineralized Kupferschiefer. The sources of the PGE and REE are enigmatic. It is likely that the metals were derived either from the underlying Rotliegendes sandstones and volcanics, the Variscan basement rocks, or the Kupferschiefer shale whose metals were mobilized by saline, oxidizing fluids released during intra-continental rifting in the Triassic period.

The concentration variations of 16 trace elements were determined along the main stem of a medium-sized stream (catchment area=107 km2), which drains areas covered with acid sulphate soils developed on sulphide-bearing marine sediments. During high flows in autumn, there was a strong downstream increase in the concentrations of Al, Cd, Co, Cu, Mn, Ni, Se, U and Zn and a moderate increase in those of Cr and Tl, related to extensive leaching of the acid sulphate soils, which increase in abundance from the headwater towards the basin outlet. During high flow in early summer, the downstream increase in the concentrations of these elements was not as strong as in autumn, due to decreased amounts of available mobile element fractions in the acid sulphate soils. Under baseflow conditions, the runoff from areas with acid sulphate soils is low in comparison to that in areas covered with other soils/sediments, resulting in relatively small loads of trace elements throughout the stream. The concentration variations of As, Pb, Sb, Ti and V were unrelated to catchment cover and did not vary along the stream in a regular manner. These 5 elements are, therefore, in contrast to the others, not leached more abundantly from the acid sulphate soils than from other soils/sediments. Based on the identified hydrogeochemical features and controls, it is suggested that water-quality improvement measures should include methods that primarily aim at reducing the leaching of hazardous chemical elements in the source areas (acid sulphate soils).