What Types of Rocks Contain Gold

Gold, a precious metal of significant economic and cultural value, occurs in diverse geological environments across the Earth's crust. Gold-bearing rocks are primarily associated with specific geological formations and mineralization processes. Understanding the lithological and structural characteristics of gold-bearing rocks is crucial for exploration geologists, mining engineers, and researchers in economic geology. 

Gold typically forms deep within the Earth under high pressure and temperature conditions. It's often associated with hydrothermal activity, where hot fluids circulate through rocks, dissolving and transporting gold and other minerals. As these fluids cool or undergo chemical changes, they deposit the gold in cracks, faults, or pore spaces in rocks.

Gold-Bearing Rock Types

Quartz Veins

Gold in quartz vein from Colorado, USA

Quartz veins form when mineral-rich hydrothermal fluids move through cracks in the Earth's crust, depositing quartz and other minerals. This process occurs under high-pressure and high-temperature conditions, often near geological fault lines. While quartz veins can form in various rock types, they are most commonly associated with igneous and metamorphic rocks.

Gold Association: Gold frequently precipitates from mineral-rich solutions along with quartz, resulting in gold-bearing quartz veins. These veins are the most common host for lode gold deposits. The gold within quartz veins can be:

• Visible to the naked eye (known as "free gold")
• Microscopic
• Present as small particles, flakes, or larger nuggets
• Found in a matrix with other sulfide minerals such as pyrite

Mining Method: Miners often extract gold from quartz veins by crushing the rock and then using gravity or chemical methods to extract the gold.

Example Locations: The Mother Lode region in California and the Goldfields of Western Australia are famous for their gold-bearing quartz veins.

Granite 

Microscopic gold inclusions within quartz, granite alteration zones.

Granite is a coarse-grained igneous rock composed mainly of quartz, feldspar, and mica. It forms from the slow crystallization of magma beneath the Earth's surface in large bodies called plutons.

Gold Association in Granite:

Origin: Gold presence in granite is often linked to the late stages of magmatic differentiation and hydrothermal processes.

Process:

• As magma cools and solidifies to form granite, residual fluids enriched in volatile elements and metals (including gold) can be expelled.
• These hydrothermal fluids migrate through cracks and fissures in the rock.
• Gold, along with other minerals, precipitates from these fluids, forming quartz-gold veins.

Occurrence:

• Gold in granite typically appears as microscopic inclusions within quartz.
• Sometimes, it can be found as visible flakes.
• The presence of gold is often associated with hydrothermal activity, where hot, mineral-rich water interacts with the rock.

Example Locations: The Witwatersrand Basin in South Africa contains gold-bearing conglomerates within granite formations.

Rhyolite

Gold in altered rhyolite from the Tertiary of Colorado, USA.

Rhyolite is a fine-grained, high-silica igneous rock composed mainly of quartz and feldspar. It forms from the rapid cooling of high-silica magma.

Gold Association: Rhyolite-hosted gold deposits are often associated with epithermal systems, where gold is deposited from hydrothermal fluids at shallow depths. These deposits can be high-grade and economically significant.

Formation Process: In epithermal systems, gold-bearing hydrothermal fluids ascend through fractures in rhyolite, depositing gold along with other minerals like silver and quartz. The rapid cooling of these fluids near the surface leads to the formation of rich gold veins.

Example: The Sleeper Rhyolite in Nevada is an example of a gold deposit hosted in rhyolite. 

Schist

Gold in schist from Homestake Formation; Homestake Mine, Black Hills, South Dakota

Schist is a metamorphic rock with a foliated texture, characterized by a layered appearance due to the alignment of mineral grains under pressure. It often originates from mudstone or shale.

Gold Presence in Schist:

Occurrence:

• Gold can be found in quartz veins within schist.
• It may also be disseminated throughout the rock.
• Gold is often associated with sulfide minerals such as pyrite and arsenopyrite.

Formation Process:

• During regional metamorphism, hydrothermal fluids can remobilize gold from pre-existing mineralized zones.
• These fluids deposit gold in fractures and veins within the schist.
• The gold is typically fine-grained and disseminated throughout the quartz and schist matrix.
• This process can create economically viable gold deposits.

Mining Method: Extracting gold from schist involves crushing the rock to release the gold particles, followed by processing using gravity or chemical methods.

Example Locations: The Mother Lode in California and the Otago Schist in New Zealand are known for their gold-bearing schist formations.

Greenstone

Greenstone

Greenstone, a metamorphosed volcanic rock enriched with iron and magnesium, forms in greenstone belts—geological formations dating back billions of years. These belts undergo metamorphism and deformation, creating extensive networks of fractures and shear zones. These features serve as pathways for hydrothermal fluids, which extract gold from surrounding rocks and deposit it in quartz veins or along shear zones.

Gold in greenstone belts typically originates from ancient hydrothermal systems that infiltrated the rock during or after its formation. It is commonly found in quartz veins or as fine particles within the rock itself. Regions rich in greenstone belts, like the Canadian Shield and the Yilgarn Craton in Australia, are significant sources of gold.

Example Locations: The greenstone belts of the Canadian Shield, such as the Abitibi Greenstone Belt, are famous for their gold deposits

Basalt

Basalt is a fine-grained volcanic rock that forms from the rapid cooling of basaltic lava at or near the Earth's surface. It is rich in iron and magnesium.

Gold Presence in Basalt:

Occurrence:

• Gold can be found in basalt flows, where it may be trapped within the matrix of the rock.
• The gold is often very fine and requires extensive processing to extract.

Formation Process:

• Gold deposits in basalt are often associated with volcanic activity.
• Hydrothermal fluids circulating through the basalt can concentrate and deposit gold.
• Volcanic massive sulfide (VMS) deposits or epithermal systems, which form on the seafloor near mid-ocean ridges or volcanic arcs, can also contain significant gold concentrations.

Example Locations: The Deccan Traps in India and the Columbia River Basalt Group in the United States have regions where gold is found in basalt.

Conglomerate

Gold in Conglomerates

Conglomerates are sedimentary rocks composed of rounded clasts or pebbles cemented together by finer-grained material. They often form in high-energy environments like riverbeds or beaches, where currents are strong enough to transport and round larger particles.

Gold Presence in Conglomerates:

Occurrence:

• Gold in conglomerates is often found in the form of nuggets or grains.
• Conglomerates can host substantial gold deposits, particularly in ancient river beds where nuggets were concentrated by water action.

Formation Process:

• Gold in conglomerates typically derives from the weathering and erosion of gold-rich source rocks.
• Water transports the gold and deposits it along with other sediments in a high-energy environment.
• Over time, these sediments become compacted and cemented to form gold-bearing conglomerates.

Mining Method: Mining conglomerate gold typically involves extracting and processing large volumes of rock to recover the gold particles.

The most famous example of gold-bearing conglomerates is the Witwatersrand Basin in South Africa, which has produced a significant portion of the world's gold.

Tellurium Gold Ore 

Roasted Gold Ore, Tellurium gold ore Photo: Ron Wolf

Tellurium gold ore refers to gold-bearing ores that contain significant amounts of tellurium in the form of telluride minerals. Tellurides are minerals composed of tellurium combined with other elements such as gold, silver, or other metals. These minerals are often found in association with quartz veins and other hydrothermal deposits.

Gold in tellurium ores is primarily found in telluride minerals such as calaverite (AuTe₂), sylvanite (Ag,Au)Te₂, petzite (Ag₃AuTe₂), and krennerite (AuTe₂ to Au₃AgTe₈). These minerals typically contain both gold and silver, and their association with tellurium makes them distinctive from other types of gold ores.

Tellurium gold ores are formed through hydrothermal processes where hot fluids containing dissolved metals migrate through cracks and fissures in the Earth's crust. As these fluids cool and react with surrounding rocks, they precipitate minerals that contain gold and tellurium.

Examples of locations known for tellurium-associated gold ores include:

• Cripple Creek, Colorado, USA: Known for its rich telluride ores including calaverite and sylvanite.
• Boulder County, Colorado, USA: Historical mining areas have produced tellurium gold ores.
• Kalgoorlie, Australia: Some deposits in this region contain tellurium gold minerals.

Gold Formation and Concentration

Primary gold deposits form through hydrothermal processes, where hot fluids dissolve gold and other minerals from surrounding rocks.

These fluids move through fractures and porous zones in rocks, depositing gold as conditions change (e.g., temperature drops, pressure decreases).

Tectonic activity, magmatism, and metamorphism can all play roles in generating these gold-bearing fluids.

Secondary gold deposits:

• Weathering and erosion of primary gold deposits can lead to the formation of placer deposits.
• Gold, being dense and resistant to chemical weathering, can accumulate in stream sediments, beach sands, and glacial deposits.

Gold Extraction Methods

• Placer Mining: Involves panning, sluicing, and dredging to separate gold from sediment in placer deposits.
• Hard Rock Mining: Requires drilling, blasting, and crushing of ore to extract gold from quartz veins and other hard rock sources.
• Hydraulic Mining: Uses high-pressure water jets to dislodge gold-bearing material in placer deposits.
• Cyanidation: A chemical process used to extract gold from low-grade ore by dissolving it in a cyanide solution.

Identifying Gold-Bearing Rocks

Gold Exploration Techniques

Geologists use various methods to locate gold deposits, including geochemical sampling, geophysical surveys, and remote sensing.

Understanding the geological context and identifying favorable host rocks is crucial for successful gold exploration.

This overview covers the main types of rocks associated with gold mineralization and the processes involved in gold formation. The specific characteristics of gold deposits can vary significantly depending on the geological setting and formation history.

Geological Indicators for Gold

Veins and Lodes: Gold often forms in quartz veins and lodes. Look for visible veins of quartz, especially those with metallic luster.

Alteration Zones: Areas of hydrothermal alteration, where rocks have been chemically altered by hot fluids, are often gold-rich.

Sulfide Minerals: Pyrite (fool's gold), arsenopyrite, and chalcopyrite are often found with gold. These minerals can be indicators of nearby gold deposits.

Geological Maps: Studying geological maps and reports can help identify regions with known gold deposits.

How to recognize a rock containing gold?

Color and Streak: Gold has a distinct yellow color and leaves a yellow streak when rubbed against unglazed porcelain.

Density: Gold is very dense (19.3 g/cm³), so gold-bearing rocks will be noticeably heavier than non-gold-bearing rocks of the same size.

Hardness: Gold is relatively soft, with a hardness of 2.5-3 on the Mohs scale. It can be scratched by a steel file.

Luster: Gold has a metallic luster and is opaque.

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