BIOMINERALIZATION AND ZN ISOTOPE FRACTIONATION IN A MINING-AFFECTED STREAM IN SW SARDINIA, ITALY

In southwestern Sardinia, Italy, the Rio Naracauli drains a catchment that includes several abandoned mines. The drainage from the mines and associated waste rocks has led to extreme concentrations of Zn, but because of the near-neutral pH, concentrations of other metals remain low. In the upper 3 km of the Rio Naracauli, two different Zn minerals are forming in separate biomineralization processes. In the upper 600 meters of the stream, hydrozincite {Zn₅(CO₃)₂(OH)₆} forms, with isotopically heavier Zn preferentially incorporated in the hydrozincite. The fractionation factor, Δ_hdz-water, is defined as approximately equal to δ_hdz – δ_water for coexisting water-solid pairs. For the hydrozincite, Δ_hdz-water was determined from 10 water-solid pairs to be +0.35 ‰ ± 0.05 ‰ (1 σ). In the reach from approximately 2300 to 3000 m downstream from the headwater spring, an amorphous Zn-silicate precipitates from the water, with Δ_{Zn silicate-water} of +0.51‰, again with isotopically heavier Zn in the solid. In the case of the Zn silicate, the uncertainty of the Δ value is greater, as only 3 water-solid pairs were analyzed. In both cases, observations by SEM and other microscopic techniques demonstrate an active role of bacterial surfaces in the nucleation and precipitation process. These textural observations, combined with the isotopic data, are consistent with an active extracellular biomineralization process in which the bacterial surface provides a favorable microenvironment for nucleation and crystal growth. For the hydrozincite-forming reach, saturation index (SI) values for hydrozincite in the bulk solution are typically in the range of +3 to 5. For the Zn-silicate, it is not possible to calculate SI values since the solubility of the solid is unknown, but Ion Activity Product (IAP) values are almost invariant through the Zn-silicate forming reach of the stream, suggesting a steady-state or near-equilibrium process.