Epithermal Gold Deposits

M any of the world's most famous gold deposits are related to subaerial volcanic activity, these are known as epithermal gold deposits. They commonly occur in island arcs and continental arcs associated with subduction. Epithermal deposits are classified as products of hydrothermal fluids which have a specific depth range. The deposits are found near the surface and mineralization occurs at a maximum depth of 1 km but it rarely exceeds a depth of 600 m. It is also considered temperatures to range from 50-300^oC under conditions of moderate pressure.

Most of the ore is found in veins. They tend to be irregular branching fissures, vesicle fillings, stockworks, breccia pipes and disseminations. The most common form of emplacement is open space fillings; these include cockscomb textures, crustifications, drusy cavities and symmetrical banding. Colloform textures are also found, these are typical of a shallow volcanic environment which indicate low temperatures and the free circulation of hydrothermal fluids. Evidence for repeated mineralization is evident; this includes re-brecciation and multistage banding.

Ore minerals are usually fine grained but have coarse grained well crystallized overgrowths of gangue minerals. The ore assemblages include sulfantimonides, gold and silver tellurides, stibnite, cinnabar, native mercury, electrum, native gold, native silver, selenides and to a lesser extent galena, sphalerite and chalcopyrite. Typical gangue minerals found are quartz, calcite, fluorite, barite and pyrite. Dolomite, hematite, chlorite, rhodonite are sometimes found

Fig. 3.4 Gold deposition of epithermal Au. Mineralizing fluids ascend from hot igneous intrusion, then mix and interact with meteoric water. Epithermal gold is deposited in the boiling zone (potential bonanza deposit). Alteration of country rocks by invading hot fluids (advanced argillic alteration). Gold can also be deposited in a hot spring environment (mineralizing fluids reach the surface and cool, depositing ore) and in a placer environment (erosion of an ore body, heavy metals redeposited in sedimentary environment).