Stable Hydrogen and Oxygen Isotope Composition of Waters from Mine Tailings in Different Climatic Environments

The stable isotope composition of waters (δ2H, δ18O) can
be used as a natural tracer of hydrologic processes in
systems affected by acid mine drainage. We investigated
the δ2H and δ18O values of pore waters from four oxidizing
sulfidic mine tailings impoundments in different climatic
regions of Chile (Piuquenes at La Andina with Alpine climate,
Cauquenes and Carén at El Teniente with Mediterranean
climate, and Talabre at the Chuquicamata deposit with
hyperarid climate). No clear relationship was found between
altitude and isotopic composition. The observed displacement
of the tailings pore waters from the local meteoric
water line toward higher δ18O values (by ∼ +2‰ δ18O
relative to δ2H) is partly due to water−rock interaction
processes, including hydration and O-isotope exchange with
sulfates and Fe(III) oxyhydroxides produced by pyrite
oxidation. In most tailings, from the saturated zone toward
the surface, isotopically different zones can be distinguished.
Zone I is characterized by an upward depletion of 2H
and 18O in the pore waters from the saturated zone and
the lowermost vadose zone, due to ascending diffused
isotopically light water triggered by the constant loss of
water vapor by evaporation at the surface. In zone II, the
capillary flow of a mix of vapor and liquid water causes
an evaporative isotopic enrichment in 2H and 18O. At the top
of the tailings in dry climate a zone III between the
capillary zone and the surface contains isotopically light
diffused and atmospheric water vapor. In temperate climates,
the upper part of the profile is affected by recent rainfall
and zone III may not differ isotopically from zone II.