Page 146 - Handbook of Gold Exploration and Evaluation

124 Handbook of gold exploration and evaluation

Table 2.11 Characteristic forms of low sulphidation and high sulphidation
alteration assemblages (after White and Hedenquist, 1995b)

Low sulphidation High sulphidation

(Adularia-sericite) (Acid sulphate)
Open-space veins dominant Veins subordinate, locally dominant
Disseminate ore mostly minor Disseminated ore dominant
Replacement of minor ore Replacement ore common
Stockwork ore common Stockwork ore minor

Low-sulphidation ores
Mineralising fluids in low sulphidation gold systems are typically dominated by
deeply circulating meteoric waters which have absorbed acid magmatic gases
(CO 2 , SO 2 , HCl). Such fluids equilibrate with their host rocks in a low
sulphidation, epithermal gold environment, thus becoming reduced and develop-
ing a near-neutral pH (Giggenbach, 1991). The ore association is produced by
the near-neutral pH solutions boiling at shallow depths, and generating CO 2 and
H 2 S-rich vapours. Condensation of these vapours in the vadose zone above the
water table, leads to the formation of steam heated acid-sulphate waters with
increasing pH. Eventually, the loss of H 2 S from the solution causes the solubility
of gold to decrease, thus leading to its precipitation (Henley et al., 1984).
Topography is of major importance. Major low sulphidation deposits formed
in shallow conduits in low relief areas are likely to be distributed above a feeder

Table 2.12 Ore minerals in epithermal ores (after White and Hedenquist, 1995b and
Buchanan, 1981)

Low sulphidation High sulphidation
Pyrite ubiquitous (abundant) ubiquitous (abundant)
Sphalerite common (variable) common (very minor)
Galena common (variable) common (very minor)
Chalcopyrite common (very minor) common (minor)
Enargite-Luzonite rare (very minor) ubiquitous (variable)
Tennantite-tetrahedrite common (very minor) common (variable)
Covellite uncommon (very minor) common (minor)
Stibnite uncommon (very minor) rare (very minor)
Orpiment rare (very minor) rare (very minor)
Realgar rare (very minor) rare (very minor)
Arsenopyrite common (minor) rare (very minor)
Cinnabar uncommon (minor) rare (very minor)
Electrum common (variable) uncommon (very minor)
Native gold common (very minor) common (minor)
Tellurides-selenides common (very minor) uncommon (variable)