Quaternary travertines of the Middle Pecora Valley (Tuscany, Italy) contain up to 257 ppm arsenic. Such a content is environmentally relevant, but low enough to make the exact chemical speciation of ...arsenic difficult by applying conventional investigation techniques. The task was addressed by use of the Electron Spin Echo (ESE) spectroscopy, taking advantage from the modulation by the arsenic nucleus of the decay spectrum of the paramagnetic ion Mn(II), occurring as replacement of Ca in the calcite lattice. Interpretation of the spectra suggests that arsenic occurs in the calcite lattice in the position of C, through the substitution CO
2−
3
⇔
AsO
3−
3. This mechanism of arsenic incorporation by calcite may be an effective limit of arsenic mobility under conditions where immobilization through sorption by iron and/or manganese oxyhydroxides is not operating.
The Department of Earth Sciences of University of Florence has conducted over the past ten years, numerous studies about the distribution of arsenic and heavy metals in mineralized areas of Tuscany, ...particularly in the Pecora basin. The area hosts several polymetallic ore bodies and a pyrite ore deposit. The studies have identified several geochemical anomalies (As, Cu, Pb, Zn…) both in the areas which host the ore bodies and in the coastal plain (Scarlino Plain). To increase the knowledge concerning the distribution of As and heavy metals in other Tuscan coastal plains, research is under way in the alluvial plains of the Bruna, Cornia and Alma rivers. The preliminary analysis have focused on soils and stream sediments, to better understand the correlations between the downstream transport of rivers and the soils. We have made physic-chemical analysis, particle size analysis, mineralogical analysis for X-ray powder diffraction, chemical analysis for the determination of major element (X-ray Fluorescence) and for the determination of 35 minor elements and traces (AAS and ICP).Preliminary data show high concentrations of several elements (As, Zn, Co…). The concentrations of these elements in soils and stream bed sediments are not always consistent; in particular we have found higher concentrations in soils than in stream bed sediments in Cornia Plain, while the opposite happens in the Bruna basin. Therefore the natural processes of rocks weathering does not seem to have affected uniformly. The distribution of As and heavy metals in soils and stream bed sediments of the all three basins of interest are still under investigation.
The barite-pyrite-(Pb-Zn-Ag) deposit of Pollone is located in the southernmost tip of the Apuane Alps metamorphic core complex, and is hosted by a siliciclastic formation of pre-Norian age. The ...southern sector of the deposit mainly consists of stratiform, supposedly syngenetic, barite-pyrite orebodies, whereas the northern area is characterized by a barite-pyrite-(Pb-Zn-Ag) vein system. Vein geometry in the northern area is controlled by a shear zone, developed during the greenschist facies metamorphism which affected the Apuane Alps core complex between 27 and 8 Ma, that was responsible for fluid focusing and vein emplacement. At Pollone, arsenopyrite and chlorite geothermometers show broadly comparable results, and suggest local metamorphic peak temperatures between 320 and 350°C. Phengite geobarometry indicates minimum pressures of about 3.5 kbar. Fluid inclusion data and mineral equilibria suggest that the mineralizing fluids were initially hotter than the country rocks (about 450°C at 3.5-4.0 kbar). Rocks in direct contact with the orebodies are depleted in Rb and enriched in Sr in comparison to similar rocks elsewhere in the area. This is attributed to the presence of Rb-poor muscovite and Sr-rich barite. Rb-depleted muscovites suggest mineral-fluid interaction in a rock reservoir characterized by a different (modal) mineralogical composition than the Pollone host rocks. The progressive decrease of Sr in barite with increasing distance from the orebodies may be explained with a temperature decrease along the infiltration paths of mineralizing fluids (i.e., from the vein into the wall rocks). The similar O-isotope composition of quartz from veins and host rocks is explained with the overall homogeneous O-isotope composition of the Alpi Apuane basement rocks. This indicates a limited interaction between mineralizing fluids and the rocks exposed at Pollone. Remobilization of syngenetic orebodies was conceivably of minor importance in the production of metamorphogenec veins. Fluid cooling along a major tectonic lineament is thought to be responsible for barite deposition.PUBLICATION ABSTRACT
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