Epithermal Gold Deposit
Main Characteristics

Parameter	Low Sulphur deposit	High sulphur deposit
Ore minerals	Gold, pyrite, electrum, sphalerite, galena	Gold, chalcopyrite, pyrite, tellurides, covellite
Gangue minerals	Quartz, calcite, adularia, illite, carbonates	Quartz, alunite, barite, kaolinite, pyrophyllite
Textures	Veins, and open space filling drusy cavities, symmetrical banding and Colloform.	Wall rock replacement, breccias, veins
Deposit characteristics	Cavity veins and stockworks ore common	Disseminated ores and replacement ores common
Main metals	Au, Ag, Zn, Pb, and minor Cu, Sb, As, Hg, Se	Au, Ag, Cu, As, and minor Pb, Hg, Sb, Te, Mo, Bi

The genesis of epithermal deposits have been a subject of much debate over the years and are based on theories on the source for the mineralizing fluids. Some geologists believe that the epithermal deposits may have been formed entirely by the circulation of meteoric fluids, with no part played by a magmatic source (fig. 3.5). The area is situated in a U-shaped valley which has many geothermal springs; these have been used to demonstrate a relationship between hot springs and epithermal deposits. Studies of these springs mainly by the use of fluid inclusions and stable isotope studies has shown that the fluids are dilute, weakly alkaline chloride water which has undergone boiling, fluid mixing and oxidation at temperatures around 250^oC (LS). It can be said that in many areas the ore minerals are deposited by magmatic fluids with a certain amount of dilution by meteoric waters (HS).

Some geologists believe that a caldera setting is a requisite for the formation of an epithermal deposit. If there is an ore deposit in such a situation it is because there is a well developed fracture network for the hydrothermal fluids to localize mineralization. Evidence has been found in the United States on studies of Tertiary volcanoes, which has proved that the ores are 2-17 million years younger than the caldera forming volcanism. There is also no specific rock type in which an ore deposit may occur. In general rocks which sustain large open fracture systems over extended periods of time are likely targets for mineralization.

Fig. 3.6 convection of meteoric water in cooling volcanic rocks depositing Au beneath hot active springs.