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.