An Overview of Epithermal Gold Deposits

Epithermal gold deposits are a type of lode deposit containing economic concentrations of gold, silver and in some cases base metals.

epithermal gold deposits

Epithermal gold deposits are a type of lode deposit that contain economic concentrations of gold, silver and in some cases base metals including copper, lead and zinc. Gold is the principal commodity of epithermal deposits, and can be found as native gold, or alloyed with silver.

As lode deposits, epithermal deposits are characterized as having minerals either disseminated through the ore-body, or contained in a network of veins. Beyond epithermal deposits, other classic examples of lode deposits include Greenstone Belts, VMS and Intrusive Related Gold types.

Epithermal deposits are distinctive from low-grade bulk tonnage deposits such as porphyries in that they are typically high-grade, small size deposits. A few other characteristics distinguish epithermal deposits.

For example, these deposits are often found near the surface. Mineralization occurs at a maximum depth of 1 kilometer, but rarely runs deeper than 600 meters. Due to their shallow depth, it can also be noted that epithermal gold deposits form under moderate crustal temperatures of 50-300oC, and under medium pressure. These deposits commonly occur in island arcs and continental arcs associated with subduction. However, they can also be found in shallow marine environments and associated with hot springs.

Furthermore, due to their shallow-depth location, epithermal gold deposits are more susceptible to erosion; accordingly, these deposits represent a high-grade, easily mineable source of gold.

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Formation of epithermal gold deposits

There is considerable debate on the origin of the hydrothermal fluid that leads to the formation of epithermal deposits. While some geologists claim the hydrothermal fluid is created by magma, others believe that the epithermal deposits may have been formed entirely by the circulation of meteoric (sub-surface water) fluids, with no relation to any magmatic source.

In epithermal gold deposits, gold is deposited as mineralizing fluids ascend from a hot igneous intrusion, then mix and interact with the sub-surface water. Epithermal deposits form primarily by replacement, which occurs when hydrothermal fluids, created by some crustal pressure- rise, cool and then deposit minerals in the available cracks and faults in the host rock. As a result, in epithermal systems, most of the ore is found in veins. Gold can also be deposited in a hot spring environment where mineralizing fluids reach the surface and cool, depositing ore.

Types of epithermal gold deposits

Epithermal gold deposits can be broken down into three main sub-types:

  • High sulphidation ores
  • Intermediate sulphidation ores
  • Low sulphidation ores.

Each of these subtypes has its own characteristic alteration mineral assemblages, occurrences, textures, and, in some cases, characteristic suites mineral assemblages.

Silver rations vary widely among deposits—and even within a given deposit. Typically, silver to gold ratios tend to be higher in low-sulphidation subtype deposits than in high-sulphidation subtype deposits.

Hot spring type deposits generally host high precious metal to base metal ratios. Nearly any rock type, even metamorphic rocks, may host epithermal gold deposits. However, they are most commonly found in igneous and sedimentary rocks. Typically, epithermal deposits are younger than their enclosing rocks, except in the cases where deposits form in active volcanic settings and hot springs.

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This article was originally published on Gold Investing News on March 21, 2011. 

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Comments
  • Aung Aung Z.

    I would like to study more detailed for geochemistry of gold in epithermineralization

    Reply
  • Aung Aung Z.

    I would like to read more about behavior of gold in epithermal system.

    Reply
  • Kenneth S.

    In its attempts to generalize and be succinct, this article is imprecise and makes several misleading statements, such as:
    1) Epithermal deposits are primarily gold: Not true, there are numerous examples of silver-dominant epithermal deposits with minor amounts of gold relative to the silver content including Tonapah, Nevada, USA; Tayoltita, Durango, Mexico; Pachuca, Hidalgo, Mexico; Fresnillo, Durango, Mexico; Potosi, Bolivia; Los Gatos, Chihuahua, Mexico; Escobal, Guatemala; Dukat, Far East, Russia; etc. In general, I place the dividing line between gold-dominant and silver-dominant systems a a silver:gold ratio of 40. The crustal abundance Ag:Au ratio is about 50. Ag:Au ratios range from 1000.
    2) Epithermal deposits, by definition, are near surface deposits, generally forming at depths of < 1 km as stated but are not characerized by replacement but by open-space filling textures though replacement also occurs. The open-space-filling textures reflect their location in brittle rock above the ductile-brittle transition and formation at near hydrostatic pressure which presume open communication of the fluid to the paleosurface at time of formation. Consequently, the hydrothermal fluids often boil and this is one mechanism of gold-silver mineral deposition.,
    3) Temperatures of ore deposition are not as low as 50 degrees Centigrade. The top of the ore is typically characterized by temperatures of 180-200C and the bottom of the precious metal zone by temperatures of 260-300C. Some hot spring gold deposits in which deposits occur immediately below sinters, such as McLaughlin in Nevada, gold deposition may have occurred at temperatures as low as 110C.
    4) Epithermal deposits can be very large. Examples of large tonnage epithermal deposits include Round Mountain, Nevada, USA; Lahir, Papua New Guinea; Pascua-Lama, Chile; Velardena, Argentina; Cripple Creek, Colorado, USA.
    4) All hydrothermal mineral deposits are, with the rare exception of mineralized sinters, younger than the host rock, though when they occur in association with volcanic rocks, the volcanic rocks are often only slightly older.
    5) When significant gold in present in silver-rich epithermal systems. veins typically zone downward from a near surface gold-rich zone to intermdiate depth silver-rich zone then to deeper base metal zone. The Silverton District in Colorado is an excellent example of vertical zonation as is the San Sebastian mine, Durango, Mexico. Vertical zonation is not always the case. Lateral zonation also occurs as at La Cienega, Durango, Mexico.
    6) Low sulfidation deposits are not typically silver-rich relative to intermediate or high sulfidation epithermal deposits. McLaughlin, California, Knob Hill, Republic, Washinton, USA; Cripple Creek, Colorado, USA; Round Mountain, Nevada, USA; El Oro, Mexico, Mexico; Bullfrog, Nevada, USA; Kupol, Far East, Russia; and Lahir, PNG are all low sulfidation deposits with low Ag:Au ratios. At Round Mountain Ag:Au ratios in the core of the deposit are as low as 2 and on the periphery of the deposit, are as high as 20. The Gold Hill deposit, just to the north of Round Mountain 3 miles or so is unrelated to the Round Mountain deposit and is a silver-dominant system.

    Reply

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