Used
to describe gold deposits found on or just
below the surface close to ancient vents or volcanoes, formed at low temperature
and pressure.
An epithermal deposit is deposited
at shallow depth. The gold has become concentrated as mineral-rich hydrothermal
fluids pass through the heated rocks of a volcanic system. When the mineralization
cools on the surface it often forms very rich orebodies;
almost a golden cap sitting on top of a volcano. This has often undergone weathering
and the gold is spread over the surrounding area, although grades may still be
spectacular. The El Indio mine in Chile and the Hishikari mine in southern Japan,
both of epithermal origin, have produced grades
of over 100 grams per tonne.
Epithermal deposits are located
particularly in the broad arc of volcanic rocks swinging round the Pacific Basin
from Chile to Fiji (where the Emperor mine has been worked since 1932) to New
Zealand (the Waihi mine), on to touch eastern Australia, then Papua New Guinea,
Indonesia, the Philippines and finally Japan. The exploitation of these epithermal
deposits in the Pacific provides the gold mining industry with one of its biggest
technical challenges as the areas of mineralization are sometimes still active
and in ‘hot rocks’. The best are in Papua New Guinea where Bougainville (now closed),
Ok-Tedi, Porgera, Lihir and Misima are all of epithermal origin.