Epithermal gold
-types of mineralizing fluids

There are two types of mineralizing fluids which are responsible for forming epithermal gold deposits, they have a different chemical composition and are from contrasting volcanic environments (Fig. 3.4). The first is a low sulphidation (LS) fluid which is reduced and has a near neutral pH. It is a mixture of magmatic and meteoric waters. The geochemical composition is similar to waters found when drilling hot springs in geothermal systems. Gold is carried in solution and is precipitated out in veins near the surface at the zone of boiling. Some of the features which are characteristic of this type of setting are Colloform and brecciated quartz which is cemented by adularia and bladed calcite.

The second type of mineralizing fluid is a high sulphidation (HS) fluid. This is an oxidized and acidic fluid which dominantly originates from a magmatic source. It deposits gold near the surface when the fluid cools or is diluted by meteoric waters. The gold in this type of deposit is hosted by leached silicic rock from acidic fluids which are generated in a volcanic hydrothermal environment

Fig. 3.5 Different epithermal gold assemblages related to fluid type and location.

With both LS and HS deposits, the mineralizing fluids ascend to the surface via fracture networks. Mineralization often occurs within these conduits. LS fluids usually deposit ore in large cavity filling veins, or in a series of finer veins known as stockworks. The more acidic HS fluids penetrate farther into the surrounding country rocks, which deposit ore in veins which may be scattered throughout the host rock.