Detrital gold in the Greymouth-Hokitika area has been won from: (a) gravel
of the Pliocene Old Man Group, (b) glacial outwash gravel of Middle and Late
Quaternary age, (c) raised shoreline deposits of Middle and Late Quaternary age,
(d) deposits resulting from the erosion of (a) to (c). The source of the gold
was probably widely dispersed quartz veins in great thicknesses of greywacke and
low-grade schist eroded from east of the Alpine Fault, with a contribution from
veins in Greenland Group greywacke in the south. At times of high interglacial
and post-glacial sea level, gold was concentrated along marine shorelines, and
subsequent progressive uplift has resulted in a set of sub-parallel beach leads
up to 100 m above present sea level. During glaciations, gold was concentrated
in outwash streams close to moraines, especially where ice forming one moraine
penetrated and destroyed an older one. Worked deposits in the Old Man Group may
also be associated with glaciation. Apart from workings that are essentially
extensions of past workings, significant new prospects may depend on inferences
as to where prospective deposits are completely obscured by younger
ones.