Oceana Gold (NZ) Ltd's, Macraes Gold Operation currently
has two tailings impoundments, the larger of which covers approximately 75 ha.
When an impoundment is temporarily out of use, the surface dries and dust can be
blown from the surface to the surrounding area. At present water spraying is
used to stabilise the surface, but this is not ideal as Macraes mine can
experience water shortages during the summer. This paper examines an alternative
method of surface stabilisation using vegetation as a ground cover. The tailings
have a low natural fertility, so it is important to determine what will actually
grow in the tailings, and whether amendments are necessary. Also important is
the effect of the amendments on arsenic mobility in the tailings as the schist
rock at Macraes has naturally elevated levels of arsenic. Three plants species,
rye corn, barley, and lupin, were grown in metre-deep lysimeters at Macraes mine
site. The plants were grown in unaltered tailings, and tailings with one of two
amendments added: Bioboost (an organic fertiliser containing biosolids), and
superphosphate fertiliser. The leachate from each lysimeter was collected and
analysed monthly to determine the extent of arsenic mobility. In addition,
laboratory experiments were set up to determine arsenic mobility in tailings
with amendments at the centimetre scale.

All plants grew best with the fertiliser amendment, with
rye corn growing the best overall. Plants growing in unaltered tailings either
barely survived (rye corn, barley) or died (lupin). Plants in the Bioboost
amended tailings grew only slightly better than in the unamended tailings.
Laboratory experiments indicate that the addition of phosphate fertiliser
greatly increases arsenic leaching at the centimetre scale when compared to
unamended tailings. The addition of Bioboost only marginally increases arsenic
leaching, compared to unamended tailings. This trend has not been repeated in
the lysimeter trials on the metre scale over four months. Current results
suggest that of the plant types trialed and amendments used, rye corn with
superphosphate is the most practical short-term tailings stabilisation method at
Macraes.