Skarn Deposits

What is a Skarn Deposit?

A skarn deposit is a type of ore deposit that is either directly associated with skarn rock or formed by processes related to skarn formation.

Skarns are coarse-grained rocks formed by metasomatism process when carbonate-rich country rocks, typically limestone or dolomite which are altered by hot, ore bearing hydrothermal solution.

The fluids involved in skarn formation are often associated with the intrusion of igneous rocks, particularly granitic/syenite plutons, into the surrounding rocks. The intrusion heats and chemically active fluids react with the surrounding rocks, causing them to recrystallize and change their mineral composition. The fluids released from the cooling magma can also react with the carbonate rocks, contributing to the formation of skarn.

• Skarns can also form in other settings where hot fluids interact with carbonate rocks, like areas with active faults or hydrothermal systems.
• Few places, skarn can form by interaction between carbonate/alkali rich fluids with silicate rock, mainly associated with alkaline magmatism.
• The specific minerals found in a skarn depend on the original composition of the hydrothermal fluid and the host rock. Common skarn minerals include garnet, pyroxene, wollastonite, and amphibole.

Formation of Skarn Deposits

The formation of skarn deposits typically involves several stages:

1. Initial metamorphism: The intrusion of magma heats the surrounding rocks, leading to the formation of hornfels from shales and marble from limestone. During this stage, some skarn, called reaction skarn, can form along the contacts between different rock types, like shale and limestone. This initial stage is primarily characterized by the diffusion of elements within a relatively stagnant fluid.
2. Metasomatism: As the magma continues to crystallize, it releases hot, ore bearing hydrothermal fluids. These fluids interact with the surrounding rocks and lead to the formation of skarn.
   • Prograde stage: Early skarn formation involves the development of anhydrous minerals, meaning they don’t contain water in their crystal structure. This stage is characterized by high temperatures and often results in the formation of minerals like garnet and pyroxene.
   • Retrograde stage: As the system cools and fluids rich in water are introduced, retrograde alteration occurs. This involves the breakdown of earlier-formed minerals and the formation of hydrous minerals like amphibole, epidote, chlorite, and talc.
3. Ore deposition: The deposition of ore minerals often occurs during the later stages of skarn formation, particularly during the late retrograde alteration. This deposition is often controlled by changes in fluid temperature, pressure, and chemical composition as the fluids interact with the surrounding rocks. For example, the neutralization of acidic fluids by carbonate rocks can trigger the precipitation of sulfide minerals.

Factors Influencing Skarn Deposit Formation

Several factors play a role in where and how skarn deposits form:

• Composition of the intrusive rock: The type of magma that intrudes in the carbonate rocks and influences the composition of the resulting skarn and the types of ore minerals present. For example, copper skarns are often associated with calc-alkaline intrusions, while tin skarns are associated with more differentiated granitic intrusions.
• Depth of formation: The depth at which the intrusion and skarn formation occur affects the temperature, pressure, and availability of fluids, which in turn impact the mineral assemblages and ore grades. Deeper skarns, like tungsten and tin skarns, tend to be associated with higher temperatures and pressures, while shallower skarns, like copper and zinc-lead skarns, form under lower temperature and pressure conditions.
• Host rock characteristics: The type of carbonate rock, its purity, physical properties and its structural features, like fractures and faults, influence the extent and distribution of skarn formation and mineralization. Pure limestones, for example, have low permeability, which can limit fluid flow and skarn development. On the other hand, fractures and faults can act as pathways for fluids, enhancing skarn formation and mineralization.
• Fluid composition: The chemical composition of the hydrothermal fluids, including their salinity, pH, and metal content, is crucial in determining the type of skarn that forms and the types of ore minerals that are deposited. For example, fluids rich in iron can lead to the formation of magnetite skarns, while fluids rich in tungsten can result in tungsten skarns.

Key Features for Identifying Skarn Deposits

Geologists use a combination of characteristics to identify skarn deposits, including:

• Spatial association with intrusions: Skarn deposits are commonly found near or at the contact between igneous intrusions and carbonate rocks.
• Distinctive calc-silicate minerals: The presence of coarse-grained calc-silicate minerals like garnet, pyroxene, wollastonite, epidote and amphibole is a key indicator of skarn.
• Replacement textures: Skarn minerals often exhibit replacement textures, where they have grown in place of pre-existing carbonate minerals.
• Zoning patterns: Skarn deposits often display distinct zoning patterns, where the mineral assemblages and ore grades change systematically with distance from the intrusion or along fluid pathways. This zoning can be helpful in understanding the evolution of the hydrothermal system and in exploration for new orebodies.
• Fluid inclusion and stable isotope data: Analyzing fluid inclusions trapped within skarn minerals can provide information about the temperature, pressure, and composition of the fluids involved in skarn formation. Stable isotope analysis of minerals and fluids can also help to determine the source of the fluids and the processes involved in ore deposition.

Types of Skarn Deposits

Classification based on the dominant economic mineral(s):

Skarn deposits are classified based on the dominant economic metal(s) present. Here’s a breakdown:

• Iron Skarns: These skarns are characterized by magnetite as the main ore mineral. They are further subdivided into two types:
  • Calcic iron skarns: Typically found in oceanic island arc terranes, associated with gabbroic to dioritic stocks in basalt-andesite sequences. They are low in sulfide content and may contain minor amounts of copper, zinc, cobalt, gold, and sometimes nickel.
  • Magnesian iron skarns: Associated with granodiorite to quartz monzonite intrusions in continental margin settings.
• Copper Skarns: Predominantly found in calcic skarns associated with calc-alkaline granodiorite to quartz monzonite stocks in continental margin orogenic belts. They often exhibit a close spatial relationship with porphyry copper deposits.
• Molybdenum Skarns: Frequently occur in silty carbonate or calcareous clastic rocks associated with highly differentiated silicic intrusions, similar to those associated with porphyry molybdenum deposits. These skarns often contain other metals like tungsten, copper, bismuth, lead, zinc, tin, or uranium.
• Gold Skarns: These skarns have high gold grades and are enriched in arsenic, bismuth, and tellurium, but are deficient in base metals. They are characterized by reduced and iron-rich gangue minerals, particularly iron-rich pyroxene and lesser grandite garnet. The host rocks often have a higher clastic component.
• Tungsten Skarns: Closely associated with granitic intrusions, particularly those with a high degree of fractionation. The presence of fluorine in the hydrothermal fluids is crucial for scheelite precipitation. Based on mineralogy, they can be subdivided into:
  • Reduced skarns: Formed in carbonaceous or deep environments and characterized by iron-bearing assemblages (hedenbergitic pyroxene, almandine-rich garnet, biotite, hornblende), molybdenum-rich scheelite, and pyrrhotite.
  • Oxidized skarns: Formed in non-carbonaceous or hematitic host rocks or at shallower depths and characterized by iron-bearing assemblages (andraditic garnet, epidote), molybdenum-poor scheelite, and pyrite.
• Tin Skarns: Associated with “tin granites”, which are reduced and enriched in fluorine, rubidium, lithium, tin, beryllium, tungsten, and molybdenum. A distinctive feature is the greisen alteration stage characterized by fluorine-rich minerals like fluorite, topaz, tourmaline, muscovite, grunerite, and ilmenite.
• Zinc-Lead Skarns: Characterized by manganese- and iron-rich minerals, these skarns often occur along structural or lithologic contacts. They are generally found at a distance from the intrusive contact, with limited metamorphic aureoles, and exhibit high pyroxene to garnet ratios. They may form chimney or manto deposits as well.

There are two main types of skarns:

Classification based on the position of the skarn relative to the intrusion:

1. Exoskarns form on the outside of the intrusive body, at the contact between the intrusion and the carbonate rock.
2. Endoskarns: Skarns that form within the intrusive body itself, typically near the contact with the country rocks.

While these classifications provide a framework for understanding skarn deposits, it’s important to remember that there can be overlap between types, and variations within each type. The complex interplay of factors like host rock composition, fluid chemistry, temperature, and pressure contributes to the diversity of skarn deposits.

Here are the key features that distinguish skarn deposits from other types of mineral deposits:

• Association with Igneous Intrusions: Skarn deposits are intimately linked to igneous intrusions, typically granitic to dioritic in composition. The heat and hydrothermal fluids emanating from these intrusions drive the metasomatic processes that form skarns. This association is often, but not always, spatially evident, particularly in distal deposits formed farther away from the intrusion.
• Formation by Metasomatic Replacement of Carbonate Rocks: Skarn deposits are formed through the metasomatic replacement of carbonate rocks, primarily limestone and dolomite. This process involves the transfer of elements between the hydrothermal fluids and the carbonate host rocks, resulting in the formation of distinctive calc-silicate minerals. The preference for carbonate rocks stems from their high reactivity with the acidic hydrothermal fluids, leading to changes in fluid chemistry that trigger the precipitation of skarn minerals.
• Distinctive Calc-Silicate Mineralogy: Skarn deposits are characterized by the presence of a unique assemblage of calc-silicate minerals, including garnets (grossularite-andradite series) and pyroxenes (diopside-hedenbergite series), often in coarse-grained aggregates. These minerals are not typically found in other types of mineral deposits, making them diagnostic of the skarn environment. The specific composition of these minerals can vary depending on the type of skarn deposit, reflecting differences in the composition of the hydrothermal fluids and host rocks.
• Irregular Form and Control by Permeability: Skarn orebodies typically exhibit irregular shapes, often forming as lenses, pods, or veins that are controlled by the permeability of the host rocks. Structures like fractures, faults, and bedding planes act as conduits for hydrothermal fluids, influencing the localization and geometry of skarn development. The degree of replacement can vary significantly, resulting in massive orebodies or disseminated mineralization.
• Alteration and Mineralization Zoning: Skarn deposits commonly display zoning patterns in both alteration and mineralization. This zoning reflects changes in temperature, fluid composition, and the reactivity of the host rocks as the hydrothermal system evolves. The zoning pattern is typically centered on the intrusion, with proximal zones showing higher-temperature assemblages and distal zones exhibiting lower-temperature assemblages.
• Variety of Economic Metals: Skarn deposits are known to host a diverse range of economically valuable metals, including iron, copper, tungsten, tin, gold, zinc, and lead. This wide array of metal associations is a testament to the variability of skarn-forming processes and the influence of different magmatic and hydrothermal fluid compositions. Each metal type tends to have a characteristic skarn mineralogy and association with specific intrusion types.

In contrast to other types of mineral deposits, such as sedimentary or magmatic deposits, skarns are formed through a combination of magmatic, metamorphic, and hydrothermal processes, resulting in their unique characteristics. They are distinct from hydrothermal vein deposits in their close spatial and genetic relationship with igneous intrusions and their replacement-style mineralization in carbonate rocks. Additionally, their high-temperature formation and characteristic calc-silicate mineralogy set them apart from other types of metasomatic deposits, such as those formed at lower temperatures in sedimentary environments.

Key Skarn Minerals

Skarn deposits are characterized by a distinctive assemblage of calc-silicate minerals, including:

• Garnets: Common garnet species in skarns include andradite, grossularite, and almandine. The composition of the garnets can vary depending on the type of skarn deposit and its evolutionary stage. For example, gold skarns are often characterized by andraditic garnet, while copper skarns may contain more grossularite.
• Pyroxenes: Pyroxenes in skarns include diopside, hedenbergite, wollastonite, and johannsenite. Similar to garnets, the composition of pyroxenes can provide insights into the type of skarn and its formation conditions. Zinc-lead skarns, for example, often contain manganese-rich pyroxenes like johannsenite.
• Other Minerals: In addition to garnets and pyroxenes, skarns may contain a variety of other minerals, including:
  • Prograde skarn minerals: calcite, dolomite, quartz, vesuvianite, wollastonite.
  • Retrograde skarn minerals: hornblende, biotite, plagioclase, epidote, sphene, chlorite, actinolite, apatite.

The specific mineral assemblages and their spatial distribution within a skarn deposit can provide valuable clues about the formation processes and the potential for economic mineralization.

Economic Significance and Uses of Skarn Deposits

Skarn deposits are economically significant as a source of a wide range of metals. The most important metals obtained from skarn deposits include copper, tungsten, iron, tin, molybdenum, zinc-lead, and gold. Skarn deposits can also contain smaller quantities of uranium, silver, boron, fluorine, and rare-earth elements. The specific metals found in a skarn deposit are determined by the composition of the hydrothermal fluids and the host rocks involved in its formation.

The sources provide many specific examples of the economic importance of skarn deposits:

• Tungsten: Almost all Canadian tungsten production came from skarn deposits. This accounted for about 5% of the world’s annual tungsten production at one point.
• Iron: Skarn iron ore deposits were once an important source of iron, accounting for about 2% of global iron production. A notable example is the Cornwall deposit in Pennsylvania.
• Copper: Copper skarns may be the world’s most abundant skarn type. In some regions, such as the southwestern United States, the Ural Mountains, Kazakhstan, and the Lower Yangtze River area of China, skarn deposits are a major source of copper.
• Gold: Historically, gold skarn deposits have produced an estimated 1000 tonnes of gold, which is about 1% of total gold production up to 1975. The economic importance of gold skarn deposits is growing, especially in the Canadian Cordillera.

Overall, skarn deposits are valuable because they can be large and high-grade. This means they can be mined using open-pit or underground methods. Some skarn deposits are mined primarily for one metal, while others are mined for multiple metals. Skarn deposits are also scientifically important because they provide information about the geological processes and history of the region in which they are found.

Comparing Skarn and Hydrothermal Deposits

While all skarn deposits are hydrothermal, not all hydrothermal deposits are skarns.

• Hydrothermal deposits are a broad class of ore deposits formed by the precipitation of minerals from hot, aqueous fluids of various origins in diverse geological environments. These fluids can be meteoric, connate, metamorphic, or magmatic, and they often represent mixtures from multiple sources.
• Skarn deposits represent a subset of hydrothermal deposits characterized by their distinctive mineralogy and formation process. They form through the interaction of hot, mineral-rich fluids with carbonate rocks, typically limestone or dolomite, leading to the development of a metamorphic rock known as skarn.

Key Distinctions:

• Host Rock: Skarn deposits are specifically associated with carbonate rocks like limestone or dolomite. In contrast, hydrothermal deposits can occur in a wide variety of rock types, including volcanic rocks, sedimentary rocks, and igneous rocks.
• Mineralogy: Skarns are characterized by the presence of distinctive calcium-magnesium-iron-manganese-aluminum silicate minerals (calc-silicate minerals) formed during the metasomatic replacement of the carbonate host rock. Common skarn minerals include garnet, pyroxene, wollastonite, epidote, and amphibole. The specific minerals present depend on the composition of the hydrothermal fluids and the protolith. While some hydrothermal deposits share these minerals, many others do not. For instance, hydrothermal vein deposits are characterized by a simpler mineralogy, often dominated by quartz, carbonates, and sulfides.
• Formation Process: Skarn formation involves a more complex interplay of processes compared to some other hydrothermal deposits. It typically begins with contact metamorphism caused by the heat from a nearby igneous intrusion. This is followed by metasomatism, where the chemical composition of the host rock is altered by interaction with hydrothermal fluids emanating from the cooling intrusion or other sources. The metasomatic process is responsible for the development of the characteristic skarn mineralogy. In contrast, some hydrothermal deposits, like certain vein deposits, may form primarily through the infilling of fractures by mineral precipitates without significant metasomatic alteration of the host rock.
• Spatial Relationship to Intrusions: Skarn deposits are generally found in close proximity to igneous intrusions, often within a kilometer of the contact zone. While many hydrothermal deposits also form near intrusions, others, like orogenic gold deposits, may form far from any obvious igneous source.

Similarities:

• Role of Hydrothermal Fluids: Both skarn deposits and hydrothermal deposits form through the action of hot, aqueous fluids that transport and deposit minerals.
• Temperature: Both types of deposits form at elevated temperatures, although the specific temperature range can vary depending on the type of deposit and depth of formation.
• Economic Significance: Both skarn deposits and hydrothermal deposits can host a wide range of economically valuable metals, including copper, gold, tungsten, molybdenum, and zinc-lead.

Economic Importance of Skarn Deposits

• Skarn deposits are economically important because they can contain valuable metals like: Copper, Tungsten, Iron, Tin, Molybdenum, Zinc, Lead, Gold
• Skarns can also be a source of industrial minerals like: Graphite, Garnet, Talc, Wollastonite

Summary:

Skarn deposits are a specific type of hydrothermal deposit characterized by their formation in carbonate rocks through a combination of contact metamorphism and metasomatism. They exhibit distinctive calc-silicate mineralogy and typically occur near igneous intrusions. Hydrothermal deposits encompass a much broader range of ore deposits formed from hot, aqueous fluids in various geological settings.