Coromandel Peninsula gold mineralisation has an epithermal style which
involves extensive alteration of host rocks and addition of pyrite over large
volumes of rock. These altered rocks therefore have high potential for
generating acid rock drainage. Natural acid groundwater discharges occur in
altered rocks, but not in propylitically altered rock. There is little calcite
available in mineralised rocks to neutralise this acid. In contrast, the
mesothermal veins of the Otago Schist cause little alteration of the host rock,
and that alteration has similar mineralogy to the host rock. Calcite is a common
mineral in host and mineralised zones, and sulphides are generally subordinate.
Hence acidification of Otago rocks is unlikely. This lack of acidification in
Otago facilitates arsenic mobility in solution, up to hundreds of ppm. Arsenic
is insoluble under acidic conditions in the Coromandel. Base metals are soluble
under acidic conditions of the Coromandel, but they are immobilised by
adsorption on to the widespread clay minerals where the solutions are
neutralised. Base metals are generally immobile in Otago, but when they are
mobilised, adsorption is less effectiveas an attenuation mechanism because clay
minerals are rare.