Massive sulphide ZnPbCuAg deposits in the Halls Peak district of north-eastern NSW are of
the submarine volcanogenic type. They occur as a number of small lenses enclosed
in shale and epiclastic rocks, in association with felsic volcanics of early
Permian age. The deposits crop out in steep terrain near the eastern edge of the
New England plateau. Local relief of 700 m and
slopes up to 50, coupled with an average annual rainfall of about 1000 mm has
led to local severe land sliding in previously mined areas. Mining from the
1890s to the 1960s has also resulted in unconstrained drainage of heavy
metal-bearing acid waters and physical dispersion of finer grained metalliferous
sediments into the Chandler River. Waters emanating from the main
landslide area, from underground workings and draining scattered unconfined
stockpiles of massive sulphide ore have pH values of 3 to 4 and contain elevated
contents of Zn, Cu, Pb, Cd and Fe. Chemical mobility of heavy metals into
streams occurs under low pH conditions with Zn exhibiting the greatest
dispersion. Increasing pH conditions downstream cause precipitation of Fe phases
and coprecipitation of heavy metals. Physical dispersion of secondary
metal-bearing minerals into the surrounding creeks results in wt% levels of Cu,
Pb and Zn in the stream sediments thereby exceeding the regional background
values by one to three orders of magnitude. Areas disturbed by mining are
characterised by lack of vegetation, biomagnification of heavy metals into algae
and lower plants, and partial recolonisation by metal-tolerant grass species
(Snow Grass, Poa sieberana; Blady Grass, Lomandra longifolia).
Rehabilitation measures will have to include landslide stabilisation,
impervious sealing or removal of sulphidic materials, import of topsoil and
revegetation by metal-tolerant, slow-growing native grass and shrub
species.