Epithermal Gold Deposits

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).

An epithermal gold deposit is one in which the gold mineralization occurs within 1 to 2 km of surface and is deposited from hot fluids. The fluids are estimated to range in temperature from less than 100C to about 300C and, during the formation of a deposit, can appear at the surface as hot springs, similar to those found in Yellowstone National Park (in northwestern Wyoming, southern Montana and eastern Idaho). The deposits are most often formed in areas of active volcanism around the margins of continents.

Epithermal gold mineralization can be formed from two types of chemically distinct fluids -- "low sulphidation" (LS) fluids, which are reduced and have a near-neutral pH (the measure of the concentration of hydrogen ions) and "high sulphidation" (HS) fluids, which are more oxidized and acidic. LS fluids are a mixture of rainwater that has percolated into the subsurface and magmatic water (derived from a molten rock source deeper in the earth) that has risen toward the surface.

Gold is carried in solution and, for LS waters, is deposited when the water approaches the surface and boils. HS fluids are mainly derived from a magmatic source and deposit gold near the surface when the solution cools or is diluted by mixing with rainwater. The gold in solution may come either directly from the magma source or it may be leached out of the host volcanic rocks as the fluids travel through them. In both LS and HS models, fluids travel toward the surface via fractures in the rock, and mineralization often occurs within these conduits. LS fluids usually form large cavity-filling veins, or a series of finer veins, called stockworks, that host the gold. The hotter, more acidic HS fluids penetrate farther into the host rock, creating mineralization that may include veins but which is mostly scattered throughout the rock. LS deposits can also contain economic quantities of silver, and minor amounts of lead, zinc and copper, whereas HS systems often produce economic quantities of copper and some silver. Other minerals associated with LS systems are quartz (including chalcedony), carbonate, pyrite, sphalerite and galena, whereas an HS system contains quartz, alunite, pyrite and copper sulphides such as enargite.

Geochemical exploration for these deposits can result in different chemical anomalies, depending on the type of mineralization involved. LS systems tend to be higher in zinc and lead, and lower in copper, with a high silver-to-gold ratio. HS systems can be higher in arsenic and copper with a lower silver-to-gold ratio.

Many countries have epithermal gold deposits, including Japan, Indonesia, Chile and the western U.S., each of which occupies a portion of the "Rim of Fire," the area of volcanism that rings the Pacific Ocean from Southeast Asia to western South America.

Epithermal gold is also found in BM 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 50-300oC 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 foundritish Columbia at the Baker mine, in the Toodoggone district, and near the Taseko River.

Many 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 50-300oC 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.