At Reefton, mesothermal gold deposits occur in the Ordovician Greenland
Group basement and bituminous coal deposits occur in overlying Eocene Brunner
Coal Measures. These different resource types occur in close proximity to one
another in the same topography, vegetation and climate. Both types of resources
have been mined for many years and as a result historic mining areas discharge
water with chemistry reflecting the mineralogy of the deposits. Coal measures
release natural acid rock drainage throughout the region due to oxidation of
pyrite in coal and associated sediments. Levels of acidity are increased in
mining areas, and pH near three is common. Gold mines contain pyrite,
arsenopyrite and stibnite. Discharge waters from these sites have pH near seven
because of the high acid neutralising capacity of the basement rocks. Both coal
and gold mining sites have iron oxyhydroxide precipitates in drainage waters.
These precipitates are more abundant at gold mining sites, and one deposit is up
to 5 m thick. Iron oxyhydroxide at coal mine sites contain Zn (up to 89 ppm), Ni
(up to 10 ppm), As (up to 163 ppm), Cr (up to 23 ppm), Pb (up to 16), and Cu (up
to 11 ppm). Jarosite group minerals have been identified in the coalmine iron
oxyhydroxide, although most of the material is amorphous. Gold mine iron
oxyhydroxide contains Zn (up to 353 ppm), Ni (up to 592 ppm), Cr (up to 17 ppm),
Cu (up to 72 ppm), Pb (up to 29 ppm), Sb (up to 230 ppm), and this material is
particularly high in As (up to 20 wt per cent). Most of the material is
amorphous but distinctive zones (mm scale) are As rich (up to 30 wtper
cent As oxide). Pharmacosiderite, K2Fe4(As04)3 (OH)5.6.3H20, is a secondary
mineral in one deposit. These iron oxyhydroxide deposits are useful indicators
for the metals mobilised into the environment by oxidation in the historic
workings.