Applied Geochemistry (v.17, #2)

Sulfide precipitation in the context of carbonate-hosted base metal deposits has been previously explained by numerous processes including SO4 reduction in the presence of hydrocarbons. This model has been suggested for numerous deposits although clear criteria to support the model have not been systematically provided. Numerous oil-inclusions are encompassed by fibrous calcite crystals in finely laminated Carboniferous limestone at the base of the Windsor Group, a unit that hosts numerous base metal occurrences in Nova Scotia, particularly the hydrocarbon-rich Jubilee Pb–Zn deposit in Cape Breton Island. Oil from two inclusion-rich samples from this deposit have been characterised by gas chromatography, gas chromatography–mass spectrometry, gas chromatography–isotope ratio mass spectrometry and bulk stable C isotopes. As established in the authors' former publications, the Jubilee deposit is a clear metallogenic case for which the ground preparation and mineralisation stages involved reduction of SO4 by hydrocarbons. Here, the question of potential sources of these hydrocarbons is addressed. It is postulated that the hydrocarbons that were trapped in the sulfide-related calcites at the deposit, correlate with previously characterised oil seeps, and it is demonstrated that their source is not the marine host-carbonates but stratigraphically deeper lacustrine formations of the Horton Group.

The δ 34S isotope signature of aqueous SO4 2− in the main river Arno and principal tributaries was investigated along with major ion chemistry in order to check the pollution effects in the water network of a heavily industrialized and densely urbanized catchment extending in Tuscany over an area of 8228 km2 from the Apennine Ridge to the Tyrrhenian Sea. Major towns include Florence and Pisa. Analyses are presented for 29 water samples from the Arno river and 36 water samples from the tributaries, all collected in 1996–1997 during low flow hydrologic conditions. Samples from the Arno are distributed from source to mouth and those from tributaries upstream from the confluence points. The Arno river solutes consist mainly of Ca2+ and HCO3 , but the proportions of Na+, Cl and SO4 2− increase downstream, in keeping with the basically anthropogenic origin of the latter. The δ 34S(SO4 2−) signature also increases downstream, with the most depleted values in the source zone. The seawater ingression into the river is documented by distinctly higher chemical and isotopic parameters at about 13 km from the mouth. In general, the tributaries behave similarly to the Arno for both their chemical and isotopic compositions, with increasing δ 34S, Na+, Cl and SO4 2− downstream. The δ 34S of SO4 2− in natural inputs is estimated to range approximately between −15 and +4‰ from oxidation of pyrite disseminated in country rocks, and between +15 and +23‰ from dissolution of evaporites, whereas the δ 34S of anthropogenic SO4 2− should be variable within the range 1–8‰.

Mimicking geochemical processes to solve environmental problems was implemented in dealing with waste acidic jarosite and alkaline coal fly ash. By placing these two chemically different materials adjacent to one another, a self-sealing layer was formed at the interface between both wastes, isolating and immobilizing chemical constituents in the process. A series of leaching experiments were performed on each material separately to study the release behavior of the principal constituents. Radiotracer experiments were conducted to explore diffusion and reaction of constituents such as Fe3+ in a combined jarosite/fly ash system. A model has been developed to simulate the coupled processes of diffusion and precipitation taking into account porosity change due to pore filling by precipitates. The formation of a self-sealing isolation layer in a hypothetical jarosite/fly ash disposal site was modelled. Leaching results indicate that the release of elements from jarosite is much larger than that from fly ash, and that the highly pH dependent release of Fe, Al, and Zn was controlled by the solubility of their hydroxides. Leaching results also suggest that precipitation reactions can be expected to occur at the interface between jarosite and alkaline coal fly ash where a large pH gradient exists. Radiotracer experiments showed that accumulation of constituents occurred at the interface. Modeled Fe3+ profiles in layered jarosite/fly ash were well validated by experiments. Modeling results also showed that with the accumulation of constituents at the interface, a new layer with low porosity was formed. Application of this model suggests that there is a potential use to form a self-sealing layer in jarosite/fly ash co-disposal sites.

Heavy metal distribution in recent sediments of the Tietê-Pinheiros river system in São Paulo state, Brazil by Ivone S da Silva; Gilberto Abate; Jaim Lichtig; Jorge C Masini (105-116).
The concentrations and possible chemical associations of Al, Fe, Mn, Ca, Cu, Pb, Cd, Zn, Ni and Cr in sediments of the Tietê-Pinheiros river system in São Paulo state, Brazil, were studied using a 3-step sequential extraction protocol recommended by the Standards, Measurements and Testing Programme (SM&T, formerly BCR). A single extraction with 0.1 mol l−1 HCl was applied in parallel to anoxic and air-dried samples. The river system crosses the metropolitan area of São Paulo (MASP), which houses a population of nearly 17 million people, and receives a large load of industrial and domestic wastes. Samples were collected from reservoirs in the surroundings of MASP, named Billings, Pirapora and Rasgão, and from the Barra Bonita reservoir, that is located in the Tietê river, 270 km downstream from São Paulo city. The distribution of metals indicates the recent pollution characteristics for samples from the Billings, Pirapora and Rasgão reservoirs. In these sediments the metals are associated to a large degree with reactive forms such as sulphides and carbonates, or adsorbed to amorphous oxyhydroxides of Fe and Mn. In samples from Barra Bonita, heavy metals are mainly associated with the residual fraction, suggesting that their concentrations are controlled significantly by transport processes with fine particles as carriers from diffuse pollution sources.