Overview:
Epithermal Deposits
Epithermal deposits are major global sources of gold and silver and a significant contributor to base metal production (Pb, Zn, Cu). The term "epithermal" is derived from the latin term for "shallow heat" which is appropriate considering epithermal deposits form in the shallow crustal environment in association with intrusive/volcanic activity. These deposits are typically classified as High, Intermediate, or Low-sulphidation based on the mineral assemblage and the pH/Eh of the mineralising fluids.
Economic elements associated with epithermal deposits include gold, silver, lead, zinc, copper, antimony, arsenic, and mercury. Minerals may include: native gold and silver, argentite, tennatite, enargite, gold tellurides, electrum, stibnite, and cinnabar. Minor elements include tellurium, copper, zinc, lead, and bismuth.
The video below from Andrew Jackson provides an excellent, overview of epithermal deposits.
The video on Epithermal has been provided by Andrew Jackson of Sprott Global Resource Investments Ltd.
Download (Epithermal-deposits.pdf)
Low Sulphidation Epithermal
Low sulphidation (LS) epithermal deposits are categorized as they represent the most distal parts of an intrusion - hydrothermal system. LS epithermal deposits form within 0.5 km - 2 km in depth at temperatures below 250 degrees C. Metals are deposited through the process of boiling fluids, fluid mixing, and vapor release. If these systems reach the surface, they form fumaroles, geysers, and other volcanic features made famous from locations like Rotorua, Yellowstone, and Pamukkale.
Vein textures in LS deposits indicate the depth of formation. The uppermost zones are typically metal-poor but indicate the presence of a LS system and the exploration potential at depth. This zone contains carbonate blades commonly replaced by silica with saccharoidal quartz texture, chalcedony, and agate. Sulphides may be present but are typically limited to marcasite and pyrite.
Lower in the LS system, in the boiling zone, is the high-grade zone where precious metals are deposited out of solution. Early deposition of quartz and other gangue minerals will seal fractures in the host rock resulting in over-pressurization of fluids. As the hydrothermal fluids boil they release and precipitate metal and associated elements from solution. The highest grade LS epithermal deposits require multiple influxes of fluids to repeat this process thus resulting in high-grade mineralization. Gangue minerals in the "pay zone" commonly include quartz, calcite, adularia, sericite (muscovite), and pyrite.
Below the boiling level at the deepest zone of an LS, base metal sulphides are precipitated. Common minerals include galena, sphalerite, chalcopyrite with associated gangue minerals of fluorite, pyrite, arsenopyrite, and pyrrhotite. These base metal veins may be massive and quartz-poor in some cases. Given the base metal zone is deeper in the system (~500m), confining pressures are higher thus stockwork and breccia development is typically poor or limited. When country rock are more porous or reactive units, this zone can result in significant base and precious metal grades.
(image courtesy of Exploration Alliance; www.explorationalliance.com)
Intermediate Sulphidation Epithermal
Intermediate sulphidation epithermal (IS) deposits, represent a middle member between the spectrum of low and high sulphidation deposits. Their genesis can be quite complex due to the involvement of fluids with meteoric and/or magmatic origin in their formation and evolution. Characteristic features of IS include the presence of coarser banding textures than LS but they includes the presence of alunite (KAl3(SO4)2(OH)6) similar to high sulphidation epithermal deposits.
High Sulphidation Epithermal
(image courtesy of Mirasol Resources, Ltd)
High sulphidation (HS) epithermal deposits form at depths less than 1 km and are spatially related to volcanic centers and diatremes. Mineralizing fluids are sourced from high-level degassing of magma and consist of hydrochloric acid, sulphur, and carbon dioxide. HS deposits contain a high metal to sulphur ratio within the sulphide minerals. Alteration is indicative of strongly acidic conditions resulting in the presence of clay minerals. HS systems are sourced from hydrothermal fluids undiluted from groundwater resulting in acidic alteration of country rocks and deposition of metals. Vuggy quartz in an indicative texture of HS deposits as early stage hot, acidic fluids alter host rocks by the alteration of feldspars to clays before entirely removing them resulting in the remaining vuggy quartz.
The surface alteration of HS deposits can be extensive thus used as exploration tools for vectoring. Some larger deposits have such extensive alteration zones they are easily observed using satellite imagery and remote sensing. HS deposits can represent the upper portions of porphyry systems such as Yanochoa, Peru and Pascua-Lama, Chile.
Key minerals and characteristics of HS epithermal deposits include the following:
Alternation zone: Aerially extensive and visually prominent due to acidic alteration of volcanics to bleached clays.
Alteration minerals: Alunite ( KAl3(SO4)2(OH)6), Jarosite, halloysite, kaolinite, dickite, diaspore, anatase, and prophyllite at deeper levels. Absence of carbonate minerals.
Sulphides: Commonly 10-90% of ore zone with dominate pyrite. Metal bearing minerals include enargite, luzonite, chalcocite, covellite, and bornite.
Global Distribution of Epithermal Deposits
Examples of Epithermal Deposits:
