Gold is extracted by cyanidation
of a sulphide-rich concentrate at Macraes mine, and after cyanidation, the
concentrate is discharged to tailings impoundments. The concentrate tailings
were discharged to a dedicated concentrate impoundment from 1989 to 1993,
afterwhich they were blended with the flotation tailings. Since mid-1998,
concentrate tailings have been redirected to the dedicated concentrate
impoundment. The concentrate tailings contain up to 1 ppm gold, and will be
reprocessed in the future. However, it is a regulatory requirement that a
contingency plan be in place to rehabilitate the concentrate tailings should the
mine operation cease for any reason.
Initial predictions suggested
that the sulphide-rich tailings would oxidise and acidify, creating a potential
long term environmental problem. Investigations of the state of the tailings
after 4 years show that these predictions were overly conservative. Only minor
oxidation has occurred in the upper 2 metres of the tailings where the pH has
locally dropped from 10 (discharge condition) to between 7 and 8. Below 2
metres, the tailings are essentially unchanged from their deposition state. The
low rate of change in the tailings is due to their very low permeability which
in turn is a consequence of the fine grain size (ca. 15 m). Dewatering is
extremely slow, and incursion of oxygenated water is negligible. Calculations
suggest that water movement over 30 m will take at least 100 years and possibly
1000 years.
The contingency plan is to
capitalise on the very low permeability and keep the tailings from drying with a
semipermeable cap. A layer of limestone will be added to the top of the
tailings, beneath the cap, to enhance alkalinity of any percolating water. If
acidification does occur, contrary to expectations, any acid produced will be
neutralised by underlying calcite-bearing schist, schist in the concentrate
impoundment wall, and the flotation tailings immediately downstream. In
addition, dilution by alkaline ground and surface waters would further
ameliorate any acidic discharge.