Trace element and stable isotope data from a flowstone in a natural cave of the mining district of SW Sardinia (Italy): evidence for Zn²⁺-induced aragonite precipitation in comparatively wet climatic conditions

A speleothem from Crovassa Azzurra, a mine cave in SW Sardinia (Italy), has been analysed for mineralogy, minor and trace elements and stable isotopes. It is composed of layers of primary calcite and aragonite, with a region of secondary calcite. The primary carbonate is strikingly rich in Zn and Pb, presumably as the result of transport in solution from overlying Pb-Zn deposits. Immediately below the transition between calcite and aragonite, concentrations of Zn, Cd and P increase. At the transition between aragonite and Pb-rich aragonite, concentrations of Pb and P increase. Stable isotopes indicate an evolution toward more humid periods for these two transitions, conversely to what is normally observed in calcite-aragonite speleothems. The combined observation of increase in P and metals derived from oxidation of sulphides and the variation of isotopic composition of aragonite and calcite suggests that in this mine cave aragonite was deposited with increasing flowrate and thus more humid conditions. In addition, the effect of Zn2+ or Pb2+ in inhibiting precipitation of calcite appears to have been more important than that of Mg2+.