Waters Associated with the Carbonation of Ultramafic Mine Tailings, Woodsreef Asbestos Mine, Australia

The Woodsreef Asbestos Deposit, New South Wales, Australia, is a chrysotile mineralisation hosted in the ultramafic rocks of the Great Serpentinite Belt, predominantly consisting of schistose and massive serpentinite, as well as partially serpentinised harzburgite.  Chrysotile has been extracted from the deposit intermittently between 1906 and 1983, producing 24.2 Mt of ultramafic tailings.  The tailings result from dry-grinding of chrysotile ore and are stored above ground on an area covering about 0.5 km².

Extensive carbonate crusts have formed on the tailings pile since the closure of the mine.  Natural weathering dissolves Mg-silicate minerals present in the tailings and precipitates Mg-carbonate minerals in the form of crusts and cements.  Isotopic signatures of the carbonate minerals (δ¹³C, δ¹⁸O, F¹⁴C) indicate that carbonate crusts consisting of hydromagnesite predominantly incorporate CO₂ of atmospheric origin.  Estimation of the carbonate content has shown that large amounts of CO₂ have been sequestered in the tailings at Woodsreef at rates significantly elevated above the background CO₂ uptake rate by chemical weathering of coherent silicate rocks.  There is potential to further enhance the rates of CO₂ sequestration by optimizing the tailings storage for this purpose.  Natural weathering of ultramafic tailings thus represents a viable option for low-energy, low-cost sequestration of CO₂, directly from the atmosphere.

Since the carbonation of mine tailings during weathering occurs in the aqueous phase additional information on the process can be unlocked by investigating the chemistry and isotopic composition of waters interacting with the tailings.  The isotopic composition of these waters also represents an intermediate step in the pathway of the sequestered carbon and can thus serves to better constrain isotopic fractionation during formation of hydrated Mg-carbonates in these settings.

In this contribution we consider the chemistry and isotopic signatures of natural waters that are associated with the carbonation of the tailings of the Woodsreef Asbestos Mine.  Measurements of pH, T, conductivity, cation content, δ²H, δ¹³C_DIC, δ¹⁸O and F¹⁴C of water samples are presented and used to discuss the interaction of these waters with the tailings material.