By Leia Michele Toovey-Exclusive to Gold Investing News
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 a lode deposit, 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. 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 characteristics distinguish epithermal deposits. These deposits are found near the surface and mineralization occurs at a maximum depth of 1 km, but rarely deeper than 600 m. 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. 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.
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.
Epithermal gold deposits can be broken down into three main sub-types: high sulphidation ores, intermediate sulphidation ores and 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.
Miners and explorers with interests in epithermal gold deposits
An exploration and development company focused in Argentina and Chile. Mariana’s key discoveries include the Las Calandrias Gold-Silver Project (100 percent) and Sierra Blanca Silver-Gold Project (70 percent), in the prospective Deseado Massif, Santa Cruz Province. In Chile, major Cliffs Natural Resources Inc. (70 percent) is exploring along the Iron-Oxide-Copper-Gold (IOCG) Belt in joint venture with Mariana (30 percent). At Las Calandrias, scout drilling of the Calandria Sur prospect in 2009 intersected wide zones of epithermal gold-silver mineralization.
Cornerstone owns gold, silver, copper, nickel, rare earth elements, and uranium properties in Canada and Ecuador, and recently signed an agreement to acquire the La Fortuna copper-silver-gold project located in the Valparaíso Region of central Chile, approximately 100 km north of Santiago.
Bellhaven is a Canadian-based mineral exploration company focused on the discovery and advancement of gold and copper properties in Panama and Colombia. In Columbia, Bellhaven owns the La Mina project, and in Panama, the Pitaloza project.
Astral Mining Corporation is a junior exploration company focused on identifying high potential gold and silver deposits in North America. Currently, Astral has projects in British Columbia and Mexico.
Gold Resource Corporation is a mining company focused on production and development of select, high-grade gold and silver projects with low operating costs. The company wholly-owns five high-grade gold and silver properties in Mexico’s southern state of Oaxaca, including the flagship property The El Aguila Project, which recently commenced mill operations and initial production.