Page 145 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 123

2.4.5 Epithermal ores

The term epithermal is given to deposits that form at shallow depths (surface to
about 2 km) in the Earth's crust over the temperature range 150 ëC to 300 ëC
(Berger and Eimon, 1983). Epithermal gold tends to have a more distinctive
primary provenance than mesothermal gold, occurring mostly in the volcanic
rocks and shallow depth intrusives of convergent tectonic settings. Two distinc-
tive styles of epithermal gold mineralisation are developed from fluids of
contrasting geochemistry. Each style has its own characteristic mineralogy and
wall rock alteration signature:
· low sulfidation systems are formed from reduced near-neutral pH fluids with
large meteoric water input
· high sulfidation systems form from oxidised acidic fluids generated in the
magmatic-hydrothermal system.
White and Hedenquist (1995b) list the characteristic forms of the two styles
in Table 2.10 to help distinguish between them in the field. The authors point to
the necessity of making this distinction because `although the two styles have
similar alteration mineralogies, the distribution of the alteration zones is differ-
ent and the economic potential is associated with different parts of the system'.
Alteration zoning, but only where the style is correctly identified, indicates the
prospective parts of a system. The authors in Table 2.11 give examples of
epithermal gold deposits. The ore minerals in epithermal gold-rich ores are
shown in Table 2.12 in order of frequency of occurrence. This table is based
upon compilation of mineral data from more than 130 epithermal deposits in the
south-west Pacific region (White, 1995a) and 47 deposits in North and Central
America (Buchanan, 1981).

Table 2.10 Examples of epithermal deposits (after White and Hedenquist,
1995b)
Low sulphidation High sulphidation

McLaughlin, California, USA Goldfield, Nevada, USA
Round Mountain, Nevada, USA Summerville, Colorado, USA
Hishikari, Japan Iwato, Kasuga and Akeshi, Japan
Emperor, Fiji La Coipa, Chile
Gold Cross, New Zealand El Indio, Chile
Waihi, New Zealand Pueblo Viejo, Dominican Republic
Lebong Tandai, Indonesia Chinkuashih, Taiwan
Kelian, Indoensia Rodalquilar, Spain
Porgera Zone VII, Papua New Guinea Lepanto, Philippines
Pajingo, Australia Lahoca, Hungary