Like other lode deposits, the minerals in epithermal deposits can be disseminated throughout the orebody, though more often they are contained in a network of veins. Aside from epithermal deposits, classic examples of lode deposits include VMS deposits and Carlin-type deposits.
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. Perhaps most notably, they are found at shallow depths, meaning that they are susceptible to erosion and can be easily mined.
Gold is often the dominant metal in epithermal deposits, and for that reason epithermal gold deposits are a key point of interest when discussing epithermal deposits. Read on for a brief overview of epithermal gold deposits, from how they form to what the different types are. These deposits can be very desirable to mining companies, and it’s worth being aware of what makes them so alluring.
Formation of epithermal gold deposits
As mentioned, epithermal gold deposits are often found near the surface of the Earth. In fact, mineralization generally occurs at a maximum depth of 1.5 kilometers, and rarely runs deeper than 600 meters — perfect for mining companies that want to get metal out of the ground easily.
Due to their shallow depth, epithermal gold deposits form under moderate crustal temperatures of less than 300 degrees Celsius, and under medium pressure. These deposits commonly occur in island, volcanic and continental arcs associated with subduction. However, they can also be found in shallow marine environments and in association with hot springs.
But how exactly do epithermal gold deposits form? Ground water is key to the process — when it comes into contact with hot molten rock deep underground, the silicate minerals in the rock are dissolved, as are metals in the rock, including gold, silver, lead and zinc. This ground water is also known as hydrothermal fluid, and as it moves it enters open spaces, such as cracks, joints and faults in the rock. Eventually the hydrothermal fluid cools, creating veins, and the resulting material contains the dissolved metals.
Nearly any rock type may host epithermal gold deposits. However, they are most commonly found in igneous and sedimentary rocks. Typically, epithermal gold deposits are younger than their enclosing rocks, except in cases where deposits form in active volcanic settings and hot springs.
Today most epithermal gold deposits are mined using open-cut pit technologies, though in the past underground techniques were used. As already noted, veins in these deposits tend not to extend far underground, but are often high grade. The Nevada-based Comstock deposit and Cripple Creek deposit in Colorado are examples of epithermal bonanzas.
Types of epithermal gold deposits
Epithermal gold deposits can be broken down into three main subtypes. Each of these subtypes has its own characteristic alteration mineral assemblages, occurrences, textures and, in some cases, characteristic mineral assemblage suites. They subtypes are as follows:
- High sufidation ores
- Intermediate sulfidation ores
- Low sulfidation ores
As explained above, gold is often the dominant metal in epithermal deposits, but that is not always the case. Sometimes these deposits hold more silver than gold — indeed, metals ratios vary widely among deposits, and even within a given deposit. Typically, silver-to-gold ratios tend to be higher in low-sulfidation deposits than in high-sulfidation deposits.
All three deposit subtypes form under similar circumstances; however, intermediate- and high-sulfidation deposits form at greater depths. Unsurprisingly, high-sulfide deposits tend to be richer in sulfides, and may contain pyrite and enargite in addition to gold and silver. These higher-sulfide epithermal deposits that tend to be linked to porphyry deposits.
This is an updated version of an article originally published by the Investing News Network in 2011.
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Securities Disclosure: I, Priscila Barrera, hold no direct investment interest in any company mentioned in this article.