What Types Of Rocks Found Gold ‣ GEOLOGY HUB

Gold is typically found in the following types of rocks:

1. Quartz Veins

Description: Quartz veins are among the most common host rocks for gold. These are veins or seams of quartz that form when mineral-rich water flows through cracks in the earth’s crust and cools. As the quartz solidifies, it traps gold and other minerals.
Gold Association: Gold can often be found as free-milling gold within these veins or as microscopic particles within the quartz.

Gold is found in quartz veins through a combination of geological processes involving hydrothermal fluids, which transport and deposit gold and other minerals. Here’s a detailed explanation of how gold ends up in quartz veins:

1. Formation of Hydrothermal Fluids

Source of Fluids: The process begins deep within the Earth, often in areas of volcanic or tectonic activity. Heat from magma or deep-seated geothermal activity causes water to become superheated, turning it into hydrothermal fluids. These fluids can originate from magma chambers, the mantle, or even deeply buried sedimentary layers.
Fluid Composition: Hydrothermal fluids are typically rich in dissolved minerals, including silica (the main component of quartz), gold, silver, and various metal sulfides (like pyrite).

2. Movement of Hydrothermal Fluids

Pathways: As the hydrothermal fluids are heated, they become buoyant and start moving upwards through cracks and fractures in the surrounding rocks. These pathways are often created by tectonic forces, such as faulting and folding, which create spaces for the fluids to flow.
Flowing through Rock Layers: The fluids flow through porous rocks and faults, picking up and dissolving more minerals from the surrounding rocks, including gold. This process can take place over millions of years, allowing significant amounts of minerals to be transported.

3. Deposition of Quartz and Gold

Changes in Conditions: As the hydrothermal fluids move upwards, they encounter cooler temperatures and lower pressures. These changes cause the minerals dissolved in the fluids to precipitate out of solution.
Quartz Deposition: Quartz, being a common and stable mineral at these conditions, precipitates out first, forming quartz veins. The silica in the fluids crystallizes, filling the cracks and spaces in the rock with solid quartz.
Gold Deposition: Gold, which is often carried in the fluids as a complex with sulfur or other elements, precipitates when the conditions are right. This typically occurs when there is a drop in temperature or pressure, a change in pH, or a chemical reaction with the surrounding rocks. Gold can precipitate out of solution and get trapped within the forming quartz veins.

4. Concentration of Gold

Vein Formation: Over time, the continued flow of hydrothermal fluids and the repeated precipitation of minerals can build up thick quartz veins. These veins can contain varying amounts of gold, often as fine particles or larger nuggets, dispersed within the quartz.
Gold Particles: Gold can occur as free gold (visible to the naked eye) or as microscopic particles. It often appears as small grains or inclusions within the quartz, sometimes along with other minerals like pyrite (fool’s gold).

5. Associated Minerals

Pyrite and Other Sulfides: Gold in quartz veins is often associated with pyrite and other sulfide minerals. These minerals can provide clues to prospectors about the presence of gold, as pyrite commonly occurs with gold in these hydrothermal settings.
Gangue Minerals: Other minerals, such as calcite, galena, and sphalerite, may also be found in association with gold-bearing quartz veins, depending on the specific geochemical environment.

Summary of the Process

Heat Source: Geological heat generates hydrothermal fluids.
Fluid Movement: Fluids move upwards through cracks, carrying dissolved minerals.
Quartz Formation: Quartz precipitates as conditions change, forming veins.
Gold Deposition: Gold precipitates out, often within the quartz matrix.
Concentration: Repeated cycles of fluid flow and precipitation concentrate gold in the veins.

Examples of Gold-Quartz Veins

Sandstone: Gold can be found in sandstone, a type of sedimentary rock composed mainly of sand-sized particles. In some cases, gold is deposited within the sandstone during sedimentation, often associated with quartz grains or other minerals.
Conglomerate: Conglomerate rocks, which consist of rounded pebbles and boulders cemented together, can also host gold. The gold particles are typically deposited along with the gravel and pebbles during the formation of the conglomerate. This can occur in river channels, alluvial fans, or beach environments.
Example: The Witwatersrand Basin in South Africa is a prime example of gold hosted in conglomerate rocks. The basin contains ancient, gold-rich conglomerates, which are the source of much of the world’s gold.

Mother Lode: The famous Mother Lode region in California is known for its rich gold-quartz veins, which were a major source of gold during the California Gold Rush.
Australian Goldfields: In regions like Victoria and Western Australia, gold is often found in quartz veins within metamorphosed sedimentary rocks.
Canadian Shield: The Abitibi Greenstone Belt in Canada hosts numerous gold-quartz vein deposits.

Understanding these processes helps geologists and prospectors identify and extract gold from quartz veins, which remain one of the primary sources of gold ore around the world.

2. Igneous Rocks

Types:
- Basalt: Sometimes contains gold, especially when it is part of a hydrothermal system.
- Granite: Gold can be found in association with granitic intrusions.
Gold Association: Gold is typically disseminated through these rocks and may form in association with other minerals such as pyrite.

Gold can be found in igneous rocks through processes related to the cooling and solidification of magma. These processes allow gold to concentrate in specific types of igneous rocks, often associated with certain geological settings and mineral compositions. Here’s how gold can be found in igneous rocks:

1. Magmatic Differentiation and Concentration

Magma Formation: Gold-bearing magma originates deep within the Earth, often in the mantle or lower crust. It is rich in various elements, including gold, which is carried in small amounts within the molten rock.
Crystallization Process: As magma cools, it begins to crystallize. Minerals with higher melting points crystallize first, while others remain in the melt. This process, known as magmatic differentiation, can concentrate certain elements, including gold, into specific parts of the magma.
Gold Enrichment: Gold has a tendency to remain in the liquid phase of the magma for longer periods. This means that as the magma continues to cool and differentiate, gold becomes increasingly concentrated in the residual melt. Eventually, this gold-rich magma may solidify to form igneous rocks that contain higher concentrations of gold.

2. Hydrothermal Activity Associated with Igneous Rocks

Intrusive Igneous Rocks: When magma cools slowly beneath the Earth’s surface, it forms intrusive igneous rocks, such as granite. These rocks can host gold if hydrothermal fluids are present. These fluids, generated by the cooling magma, can move through the surrounding rocks, dissolving and transporting gold.
Vein Formation: As these hydrothermal fluids move through fractures and cracks in the igneous rock, they can precipitate gold and other minerals, forming gold-bearing veins. This is common in settings where granite or other intrusive rocks are present.

3. Porphyry Deposits

Definition: Porphyry deposits are large, disseminated systems where gold, copper, and other metals are spread throughout the rock. They are typically associated with large, intrusive igneous bodies, like granitic plutons.
Formation: Porphyry deposits form when metal-rich fluids exsolved from cooling magma infiltrate the surrounding rocks, depositing metals like gold and copper in a network of veins and fractures. The gold is often found alongside other minerals like pyrite, chalcopyrite, and molybdenite.
Examples: Many of the world’s largest gold deposits are associated with porphyry systems, such as those found in the Andes Mountains of South America and the southwestern United States (e.g., Bingham Canyon, Utah).

4. Volcanogenic Massive Sulfide (VMS) Deposits

Description: VMS deposits are formed at divergent tectonic plate boundaries where volcanic activity is prevalent. These deposits occur when hydrothermal fluids are released from the seafloor, rapidly cooling and precipitating sulfide minerals.
Gold Association: Gold can be found in VMS deposits, often associated with sulfide minerals such as pyrite and chalcopyrite. These deposits form in volcanic settings, where igneous activity is high.
Example Locations: Examples include the Kidd Creek deposit in Ontario, Canada, and the Flin Flon belt in Manitoba, Canada.

5. Skarn Deposits

Formation: Skarn deposits form where igneous intrusions come into contact with carbonate rocks (like limestone). The heat and fluids from the intrusion cause chemical reactions, producing a variety of minerals.
Gold Presence: Gold can be concentrated in skarn deposits as a result of the interaction between magmatic fluids and the carbonate host rock. These deposits often contain gold along with other metals like copper, iron, and tungsten.
Examples: Notable skarn deposits include those in the Rocky Mountains and parts of Central Asia.

Description: While not a type of rock, placer deposits are accumulations of valuable minerals formed by gravity separation during sedimentary processes. These deposits are often found in riverbeds, streams, and coastal areas.
Gold Association: Gold is concentrated in these deposits as it is weathered out of rocks and carried by water. It can be found as nuggets, flakes, or dust.

6. Plutonic Gold Deposits

Description: These are gold deposits that form within large intrusive igneous bodies known as plutons (e.g., granite or diorite).
Formation: Gold can be concentrated in plutons through a combination of magmatic and hydrothermal processes. The cooling pluton can generate hydrothermal fluids that transport and deposit gold in the surrounding rock.
Example: The gold-rich granitic intrusions in the Sierra Nevada Mountains of California are an example of plutonic gold deposits.

7. Basaltic and Ultramafic Rocks

Gold Association: Although less common, gold can also be found in basaltic and ultramafic rocks, particularly if these rocks have been altered by hydrothermal fluids. In these cases, gold may be associated with other valuable minerals such as nickel and platinum.
Example: In regions like the Norilsk-Talnakh area in Russia, gold is found associated with massive sulfide deposits in ultramafic and mafic rocks.

Summary

Magmatic Differentiation: Gold concentrates in the residual melt, enriching certain igneous rocks.
Hydrothermal Fluids: Gold is transported and deposited by fluids emanating from cooling igneous bodies.
Porphyry and Skarn Deposits: Large systems where gold is associated with other metals, forming economically significant deposits.
VMS and Plutonic Deposits: Gold is found with sulfide minerals in volcanic and large plutonic settings.

Gold in igneous rocks is the result of both primary magmatic processes and secondary hydrothermal activities. These processes make igneous rocks a significant source of gold, contributing to many of the world’s largest gold deposits.

Hydrothermal Fluids: Metamorphic processes often produce hydrothermal fluids that are rich in silica and other elements. These fluids can migrate through the rock, depositing minerals as they go.
Quartz Vein Formation: As these fluids move and cool, they can precipitate quartz, forming quartz veins within the host metamorphic rock. Gold can be deposited along with the quartz, either as free gold (visible gold particles) or as microscopic particles within the quartz.

3. Metamorphic Rocks

Types:
- Greenstone Belts: These are zones of metamorphosed volcanic and sedimentary rocks. They are particularly known for hosting large gold deposits.
- Schists: Metamorphic rocks, especially those containing significant amounts of quartz, can also host gold.
Gold Association: Gold is often found in quartz veins within these metamorphic rocks, formed by hydrothermal activity.

Gold is found in metamorphic rocks through processes that involve the transformation of existing rocks under high temperatures and pressures. These geological conditions can lead to the remobilization and concentration of gold. Here’s how gold can be found in metamorphic rocks:

1. Metamorphism and Gold Remobilization

Metamorphism: Metamorphic rocks form from the alteration of pre-existing rocks (igneous, sedimentary, or older metamorphic rocks) under high temperatures and pressures. This process can occur deep within the Earth’s crust, often in regions of tectonic activity such as mountain-building zones (orogenies).
Gold Remobilization: During metamorphism, fluids are generated due to the breakdown of minerals and release of volatiles. These fluids can dissolve and mobilize gold present in the original rocks. As the fluids migrate through fractures and permeable zones, they can transport gold over long distances.

2. Formation of Quartz Veins

3. Greenstone Belts

Description: Greenstone belts are regions of metamorphosed volcanic and sedimentary rocks, often found in ancient cratons. They are called “greenstone” because of the green minerals (chlorite, actinolite) that are commonly found in them.
Gold Association: Greenstone belts are some of the most prolific sources of gold in the world. Gold is typically found in quartz veins within the greenstone, or disseminated within the rock. The gold in greenstone belts is thought to have been originally deposited by hydrothermal fluids during the metamorphism of volcanic and sedimentary rocks.
Example Locations: Significant greenstone belts include the Yilgarn Craton in Western Australia, the Abitibi Greenstone Belt in Canada, and the Barberton Greenstone Belt in South Africa.

4. Orogenic Gold Deposits

Definition: Orogenic gold deposits, also known as mesothermal gold deposits, are formed during mountain-building processes (orogenies). They are associated with regional metamorphism and deformation.
Formation Process: Orogenic gold deposits form when deep crustal fluids, generated during metamorphism, ascend through fault zones and fractures. These fluids are often under high pressure and temperature, and they can carry gold and other metals. When the fluids encounter cooler rocks or changes in pressure, gold precipitates out of the fluid.
Typical Rock Types: These deposits are commonly found in metamorphosed volcanic and sedimentary rocks, often within shear zones and faults. The gold is typically hosted in quartz veins that are parallel to the regional structural trends.

5. Contact Metamorphism and Skarn Deposits

Contact Metamorphism: This type of metamorphism occurs when magma intrudes into cooler surrounding rocks, causing localized heating and chemical reactions. The area around the intrusion experiences changes in mineral composition and texture.
Skarn Formation: Skarn deposits are formed at the contact between intrusive igneous rocks and carbonate sedimentary rocks (like limestone). The heat and fluids from the intrusion cause chemical reactions that form new minerals, including gold-bearing minerals.
Gold Association: Gold can be found in skarn deposits, often associated with other metals such as copper, iron, and tungsten. The gold is typically disseminated throughout the skarn, sometimes in association with sulfide minerals.

6. Amphibolite and Schist Hosts

Amphibolite: A metamorphic rock that forms from basalt or other mafic rocks. Amphibolite can host gold, especially when it is intersected by quartz veins that have transported gold from other sources.
Schist: Schist is a medium-grade metamorphic rock characterized by platy minerals. Gold can be found in quartz veins within schist, especially in regions that have undergone significant deformation.

7. Banded Iron Formations (BIF) and Gold

Description: BIFs are sedimentary rocks that have been metamorphosed and are composed of alternating layers of iron-rich minerals and silica. They are ancient rocks, often dating back to the Precambrian era.
Gold Association: In some regions, gold is found in association with BIFs, either within the iron-rich layers or in quartz veins that cut through the formations. The gold is believed to have been introduced during metamorphic or hydrothermal processes.

Summary of Gold Formation in Metamorphic Rocks

Metamorphic Processes: Heat and pressure lead to the release of fluids that can dissolve and transport gold.
Fluid Movement: These fluids move through fractures, faults, and permeable zones, depositing gold as they cool or react with the surrounding rocks.
Quartz Veins: Common hosts for gold in metamorphic settings, often formed by hydrothermal fluids.
Orogenic Deposits: Gold is associated with mountain-building processes and is found in shear zones and faults.
Skarn and Contact Metamorphism: Localized metamorphism around igneous intrusions can concentrate gold.

Examples of Gold-Bearing Metamorphic Regions

Witwatersrand Basin: South Africa’s Witwatersrand Basin is a famous example, hosting ancient sedimentary rocks that have undergone metamorphism, resulting in gold-rich conglomerates.
Nevada’s Carlin Trend: The Carlin-type gold deposits in Nevada are associated with metamorphosed sedimentary rocks and are known for their fine-grained, disseminated gold.

These geological processes explain why many of the world’s richest gold mines are located in regions where metamorphic rocks are present, making them significant sources of gold.

4. Sedimentary Rocks

Gold can be found in sedimentary rocks through several geological processes, primarily involving the concentration and deposition of gold particles within these rocks. Sedimentary rocks are formed from the accumulation of sediments, which can include minerals, organic matter, and other materials. Gold can be incorporated into these rocks through the following mechanisms:

1. Placer Deposits

Definition: Placer deposits are accumulations of valuable minerals, such as gold, that form in sedimentary environments. These deposits occur when gold, which is resistant to weathering and erosion, is transported and concentrated by natural forces.
Formation Process:
Weathering: Gold-bearing rocks, such as quartz veins in metamorphic or igneous rocks, undergo weathering and erosion, breaking down into smaller particles. The gold particles are released into the environment.
Transportation: These gold particles are carried away by streams, rivers, and other moving water bodies. Due to its high density, gold tends to settle quickly and accumulate in areas where the water velocity decreases, such as riverbeds, alluvial plains, and along the inside bends of rivers.
Deposition: Over time, repeated cycles of transportation and deposition lead to the formation of placer deposits, where gold accumulates in sediments. These deposits can be found in modern riverbeds, ancient river channels (paleoplacers), and along beaches.
Examples: Famous placer deposits include those found during the California Gold Rush, the Klondike Gold Rush in Canada, and the Witwatersrand Basin in South Africa.

2. Gold in Sandstone and Conglomerate

3. Gold in Shale and Siltstone

Shale: Shale is a fine-grained sedimentary rock that can contain trace amounts of gold, often associated with organic matter. During the formation of shale, fine particles of gold can become trapped along with clay minerals and organic material.
Siltstone: Similar to shale, siltstone can also contain small amounts of gold, particularly in areas where fine-grained sediments are deposited in quiet water environments like lakes, swamps, or deep marine settings.

4. Carlin-Type Deposits

Definition: Carlin-type gold deposits are a unique type of sedimentary rock-hosted gold deposits. They are characterized by very fine-grained, disseminated gold particles that are not visible to the naked eye.
Formation Process:
Hydrothermal Fluids: Carlin-type deposits form when low-temperature hydrothermal fluids, rich in gold, migrate through sedimentary rocks, such as limestone, dolostone, or siltstone. These fluids deposit gold within the rock, often associated with the replacement of minerals by silica or carbonate minerals.
Decarbonatization: The gold is often precipitated when the acidic hydrothermal fluids react with carbonate rocks, causing the pH to change and gold to precipitate out.
Ore Minerals: In addition to gold, these deposits often contain arsenic, mercury, and thallium.
Examples: The Carlin Trend in Nevada, USA, is one of the most famous regions for Carlin-type deposits, making Nevada one of the largest gold producers in the world.

5. Gold in Black Shale

Black Shale: Some black shales, which are rich in organic material, can contain significant amounts of gold. The organic matter within the shale can adsorb gold from seawater or hydrothermal fluids, concentrating it within the rock.
Examples: Black shale-hosted gold deposits are found in regions such as the Carlin Trend in Nevada, and some black shales in China and Russia are known to contain gold.

6. Sedimentary Exhalative (SEDEX) Deposits

Formation: SEDEX deposits form from hydrothermal fluids that exhale onto the seafloor, depositing sulfides and other minerals. Although SEDEX deposits are primarily known for zinc and lead, gold can also be associated with these deposits.
Gold Association: Gold in SEDEX deposits is usually found in fine-grained disseminated form, often within sulfide minerals like pyrite. The gold is precipitated from the fluids along with the sulfides.

7. Paleoplacer Deposits

Definition: Paleoplacer deposits are ancient placer deposits that have been lithified (turned into rock) and buried over geological time. These deposits formed in much the same way as modern placers but are now preserved in the geological record.
Examples: The Witwatersrand Basin is an example of a paleoplacer deposit, where ancient river systems deposited gold-bearing sediments, which were later buried and turned into rock. This basin has produced a significant portion of the world’s gold.

Summary of Gold Formation in Sedimentary Rocks

Placer and Paleoplacer Deposits: Gold is concentrated by mechanical processes, forming deposits in riverbeds, beaches, and ancient sedimentary environments.
Sedimentary Rock Hosts: Gold can be found in sandstone, conglomerate, shale, and siltstone, often associated with quartz grains, organic matter, or fine-grained sediments.
Carlin-Type and Black Shale Deposits: Gold is deposited by hydrothermal fluids in fine-grained, disseminated form, often associated with replacement or alteration of the host rock.
SEDEX Deposits: Gold can be found in sulfide-rich deposits formed by hydrothermal fluids on the seafloor.

Examples of Gold in Sedimentary Rocks

Witwatersrand Basin: The world’s largest goldfield, hosting gold in ancient conglomerate rocks.
Carlin Trend: A major gold-producing region in Nevada, known for Carlin-type gold deposits in sedimentary rocks.
Alaska Placer Deposits: Rich gold deposits found in river gravels and sands, sourced from the erosion of gold-bearing rocks.

These processes illustrate how sedimentary rocks can serve as significant hosts for gold, making them important targets for gold exploration and mining.

5. Placer Deposits

Gold is found in placer deposits through natural processes of erosion, transportation, and sedimentation. Placer deposits are accumulations of valuable minerals that form in sedimentary environments, primarily through the mechanical sorting of materials by water. Here’s a detailed look at how gold is found in placer deposits:

1. Weathering and Erosion

Source Rocks: Gold originates from primary sources such as gold-bearing quartz veins in igneous or metamorphic rocks. These source rocks undergo weathering and erosion due to natural forces such as wind, water, and ice.
Gold Release: During weathering, gold particles break free from the host rock and become liberated. Because gold is highly resistant to weathering, it often survives in its native form.

2. Transportation

Mechanism: Eroded gold particles are transported by streams, rivers, and other water bodies. As water flows over and through sediment, it carries the gold particles along with other sediments.
Sorting by Density: Gold has a high density (about 19.3 times that of water), which makes it prone to settle quickly compared to lighter minerals. As the gold-laden sediment is transported, the lighter particles are carried farther while the denser gold particles settle out in areas where the water velocity decreases.

3. Deposition

Placer Deposit Formation: Gold particles accumulate in specific areas where the velocity of the transporting water decreases, allowing them to settle. These areas include:
Inside Bends of Rivers: Slower water flow on the inside curves of rivers causes gold to accumulate.
Riverbeds and Alluvial Fans: Gold settles in riverbeds and alluvial fans where sediment is deposited.
Beach Deposits: Gold can also accumulate in beach sands where wave action concentrates heavy minerals.
Concentration Zones: Gold can become concentrated in certain zones of placer deposits, such as:
Pay Streaks: Specific layers or zones within the sediment where gold is more abundant.
Pockets: Localized areas with higher concentrations of gold.

4. Types of Placer Deposits

Eluvial Deposits: These are placer deposits that form near the source of the gold. They are often found on hill slopes or in the immediate vicinity of the source rock.
Alluvial Deposits: Formed in riverbeds and floodplains as the gold is transported and deposited by flowing water.
Beach Deposits: Formed where gold is concentrated in coastal sands due to wave action.
Tidal Flat Deposits: Formed in areas influenced by tidal actions, where gold is concentrated in sediment deposited by tidal flows.

5. Gold Recovery

Panning: Traditional method using a pan to wash sediment and separate gold based on its density. Gold settles to the bottom of the pan.
Sluicing: Uses a sluice box to wash sediment through riffles that trap gold particles. Water flows through the box, carrying lighter materials away while gold is trapped in the riffles.
Dredging: Involves using dredges to scoop up and process large volumes of sediment from riverbeds or other water bodies, separating gold from other materials.

6. Examples of Placer Deposits

California Gold Rush: Significant placer deposits were discovered in California riverbeds and foothills, leading to a major gold rush.
Klondike Gold Rush: Placer deposits in the Klondike River in Canada attracted prospectors and miners during the late 19th century.
Witwatersrand Basin: Although primarily known for its lode deposits, ancient placer deposits in the basin contributed to its rich gold production.

Summary of Gold in Placer Deposits

Weathering: Gold is released from primary source rocks through weathering.
Transportation: Gold particles are carried by water and sorted by density.
Deposition: Gold accumulates in areas where water velocity decreases, forming placer deposits.
Types: Includes eluvial, alluvial, beach, and tidal flat deposits.
Recovery Methods: Traditional panning, sluicing, and dredging are used to extract gold from placer deposits.

Placer deposits are a significant source of gold and have historically been crucial for gold prospecting and mining. Understanding the formation and distribution of these deposits helps in locating and extracting gold efficiently.