Indicator and Pathfinder Minerals in Gold Exploration

Indicator and pathfinder minerals are used in mineral exploration to effectively narrow down the search area for many different types of ore deposits.

When found as grains within sediments and/or rocks, these minerals give clues as to the possible presence and location of specific types of mineralization. They are commonly used in the search for many types of ore deposits, including gold, copper, nickel, tungsten, diamond and platinum-group metals deposits.

In indicator mineral-based exploration programs, sediments and rocks are either geochemically tested or visually inspected for clues about the distance and direction of their source. When it comes to visual inspection, rocks are examined for certain grain morphologies and surface textures in order to obtain information about transport distance and bedrock source.

When pathfinder or indicator minerals are found, explorers will use a number of techniques to gain information on where a gold deposit might be.

Gold grain morphology

In gold exploration, a specific technique called gold grain morphology is employed. Gold grain morphology makes use of the fact that all rocks undergo physical and chemical changes through a process called weathering. Weathering may eventually lead to erosion, which occurs when a rock is worn down enough that it breaks off from its “host” rock and is transported to a new location.

Every mineral has a unique resistance to this process. For example, gold’s resistance to physical weathering processes is low. On the other hand, chemically inert gold is resistant to chemical weathering. Therefore, while gold grains are physically altered, and often transported to new locations, the chemical nature of gold does not change.

Gold grains themselves can provide insight into the bedrock location of a valuable gold deposit. In fact, gold grain abundance and grain characteristics have been applied systematically for the past 35 years in the search for sources using size, shape and chemical compositions of rocks with gold. In short, gold grain morphology uses the weathering of the metal as a means to classify gold grains and narrow down their distance from source.

Gold grain classification

In 1991, R.N.W. DiLabio devised a classification scheme in which gold grains are classified into three categories: pristine, modified and reshaped. For pristine gold grains, the primary shapes and surface textures of the grains are preserved. In modified gold grains, some primary surface textures are preserved, however, some edges and protrusions have been damaged during transport. For reshaped gold grains, the primary surface textures are destroyed and the original grain shape is no longer discernible.

By taking multiple samples of sediment, and classifying the gold grains according to the above scheme, the search area for a source of gold in an area can be narrowed down.

Gold grain inclusions

Gold grains can also be examined for the presence of other minerals.

Inclusions of other minerals within gold grains can provide insight into the ore and gangue minerals present in the bedrock source of the gold. This can provide exploration geologists with clues as to the deposit type and associated mineralization.

For example, enargite inclusions may indicate a high sulfidation epithermal source, while argentite inclusions may indicate mesothermal gold.

While inclusions can provide valuable insights into deposits types and even their possible location, inclusions are often easily eroded from the gold through transport. Thus, they are often only useful when found in close proximity to the bedrock source.

Composition studies

Like gold grain morphology, composition studies involve gathering and examining mineral samples to obtain information on transport directions and distances.  Compositional studies identify different populations within/between samples and potential bedrock sources. Silver, platinum, palladium, copper, mercury, lead, iron, and tellurium can all be examined for clues leading to gold deposits.

Geochemistry and pathfinders

Explorers will often use geochemistry to test minerals for clues and patterns that may lead to gold — the process depends on identifying analyzing pathfinder minerals.

Pathfinder minerals are minerals that often formed in close association with gold — their presence, even in seemingly gold-barren rock, opens up the possibility of locating a nearby gold deposit. Examples of pathfinder minerals that are geochemical indicators for gold include: silver, copper, lead, zinc, cobalt, nickel, arsenic, antimony, tellurium, selenium and mercury. Pathfinders are particularly important due to the fact that gold is so rare — locating it in a sample is very unlikely.

Exploring for gold is a challenging task that requires many different approaches for success. Using satellite systems and remote sensing as well as mineral clues can help junior miners find resource deposits without having to heavily invest in land or financially risky operations beforehand.

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This article was originally published on Gold Investing News in 2011.

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