The fate of arsenic in low to medium temperature hydrothermal systems

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Arsenic is a toxic element present in most shallow fluid systems, and sometimes in very high concentrations which pose a considerable health risk. The shallow geological and geochemical processes involving arsenic are relatively well investigated and will not be the topic of the present proposal. However, they are typically related to pre-enrichment of arsenic in higher temperature, hydrothermal systems from which the element arsenic is precipitated in various oxidation states such as native arsenic, arsenides, sulfarsenides and sulfides (and, rarely, as arsenites or arsenates). At temperatures below about 250°C, native arsenic, realgar, orpiment and Co-Ni-Fe arsenides play an important role. Their assemblages, their conditions of formation, their thermodynamic phase relations, the composition of the solutions they crystallize from and the geological/geochemical processes responsible for their precipitation (cooling, fluid mixing, fluid-rock interaction) will be investigated in this project in a well known region (Schwarzwald, SW Germany) involving a perfect, large sample set from different stratigraphic levels (and, hence formed at various temperatures) in order to understand a higher temperature part of the arsenic cycle and its connection to the low-temperature, shallow proccesses. This work will involve mineral chemical, fluid inclusion and paragenetic analyses as well as thermodynamic calculations.