15.1 Ore-Forming Processes and Mineral Deposits
4 min read•july 31, 2024
Ore-forming processes shape Earth's mineral wealth, creating diverse deposits through magmatic, hydrothermal, and sedimentary mechanisms. These processes concentrate valuable elements, forming economically significant accumulations that drive the mining industry and fuel technological advancement.
Understanding mineral deposit formation is crucial for economic mineralogy. By studying these processes, geologists can better predict where valuable resources might be found, guiding exploration efforts and informing sustainable resource management strategies.
Mineral deposit formation processes
Magmatic and hydrothermal processes
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- concentrate valuable minerals during magma crystallization
- separates minerals based on melting points
- forms distinct magma layers with different compositions
- transport and deposit minerals via hot, mineral-rich fluids
- Fluids dissolve metals from source rocks and precipitate them elsewhere
- Temperature, pressure, and chemical changes trigger
Sedimentary and metamorphic processes
- Sedimentary processes concentrate minerals through weathering and deposition
- Mechanical weathering breaks down rocks into mineral grains
- Chemical weathering alters mineral compositions
- Transportation and deposition sort minerals by density and size
- Metamorphic processes redistribute minerals under high temperature and pressure
- Recrystallization alters mineral structures and compositions
- Fluid migration concentrates certain elements in new locations
Surface and evaporative processes
- concentrates metals in upper portions of ore deposits
- Near-surface weathering and oxidation dissolve and reprecipitate metals
- Secondary enrichment zones form below the water table
- accumulates valuable minerals through selective removal
- Weathering removes soluble components, leaving behind resistant minerals
- Forms deposits like bauxite (aluminum ore) and nickel laterites
- precipitate minerals from saturated solutions
- Occurs in arid environments with high evaporation rates
- Forms deposits like salt (halite) and potash (potassium salts)
Mineral deposit classification
Magmatic and hydrothermal deposits
- Magmatic deposits categorized by formation mechanism
- form by direct crystallization from magma (chromite layers in mafic intrusions)
- crystallize from residual magmatic fluids (gemstone-bearing pegmatites)
- Hydrothermal deposits classified by formation temperature
- form at low temperatures (<200°C) (gold-silver veins)
- form at medium temperatures (200-300°C) ()
- form at high temperatures (300-500°C) (tungsten skarns)
Sedimentary and metamorphic deposits
- Sedimentary deposits divided by concentration mechanism
- form by mechanical concentration (gold nuggets in river gravels)
- precipitate from solution ()
- Metamorphic deposits include contact and regional metamorphic types
- form by contact metamorphism near intrusions (copper-gold skarns)
- Metamorphosed ore deposits are pre-existing deposits altered by metamorphism (metamorphosed volcanogenic massive sulfides)
Specialized deposit types
- Weathering-related deposits form near the Earth's surface
- develop in tropical climates (nickel laterites)
- cap sulfide deposits (iron oxide-rich cappings)
- Supergene enrichment zones concentrate metals below the water table
- Volcanic-associated massive sulfide (VMS) deposits form in submarine environments
- Characterized by stratiform sulfide lenses (copper-zinc-lead deposits)
- Associated with submarine volcanic activity
- are large, low-grade deposits in porphyritic intrusions
- Typically enriched in copper, molybdenum, or gold
- Formed by extensive and mineralization
Hydrothermal fluids in ore formation
Transport and deposition mechanisms
- efficiently transport metals and elements
- Dissolve and carry metals from source rocks to deposition sites
- Act as a concentrated solution of economically valuable elements
- Metal solubility in hydrothermal fluids influenced by various factors
- Temperature affects the amount of dissolved metals
- Pressure changes can trigger mineral precipitation
- pH influences the stability of metal complexes
- Complexing agents (chloride, sulfide ions) enhance metal solubility
Fluid-rock interactions and alteration
- enrich hydrothermal fluids
- Leach metals from surrounding rocks during circulation
- Alter the composition of both the fluid and the rock
- Hydrothermal alteration accompanies ore deposition
- Creates distinctive alteration halos around deposits
- Serves as an exploration guide for mineral prospecting
- Common alteration types include , , and
Hydrothermal system characteristics
- Hydrothermal systems operate on various scales
- Localized vein systems form in fractures and faults
- Large-scale circulation patterns develop in porphyry and epithermal environments
- Composition and origin of hydrothermal fluids influence deposit types
- Magmatic fluids often associated with high-temperature deposits
- Metamorphic fluids can form orogenic gold deposits
- Meteoric (rainwater) fluids contribute to low-temperature deposits
Geological settings for mineral deposits
Plate tectonic settings
- Convergent plate boundaries host various deposit types
- Porphyry deposits form in subduction-related magmatic arcs
- Epithermal deposits develop in volcanic arcs
- form in back-arc basins
- Extensional tectonic settings facilitate specific deposit formation
- Sediment-hosted stratiform copper deposits form in rift basins
- Some epithermal gold-silver deposits occur in extensional volcanic terrains
Cratonic and oceanic environments
- Stable conducive to certain deposit types
- Banded iron formations form in ancient sedimentary basins
- occur at basement-cover contacts
- Diamond-bearing kimberlites intrude stable continental crust
- host submarine hydrothermal deposits
- Volcanogenic massive sulfide deposits form at mid-ocean ridges
- Seafloor massive sulfide deposits develop at hydrothermal vents
Orogenic belts and sedimentary basins
- Orogenic belts favorable for various deposit types
- Orogenic gold deposits form during mountain-building events
- Skarn deposits develop where intrusions contact carbonate rocks
- Some occur in mafic-ultramafic intrusions
- Sedimentary basins important for
- Sediment-hosted lead-zinc deposits form in carbonate sequences
- Evaporite deposits accumulate in restricted basins
- Coal deposits develop from buried plant material in swamps and deltas
Weathering environments
- Tropical to subtropical climates conducive to specific deposit types
- Lateritic nickel deposits form by intense weathering of ultramafic rocks
- Bauxite deposits develop from aluminum-rich parent rocks
- Supergene enrichment creates high-grade zones in copper deposits
Key Terms to Review (33)
Banded Iron Formations: Banded iron formations (BIFs) are sedimentary rocks consisting of alternating layers of iron-rich minerals and silica, primarily formed in the Precambrian era. These formations are crucial as they provide evidence of early Earth’s oxygenation process and the role of microbial life in precipitating iron oxides from seawater, linking them directly to ore-forming processes and mineral deposits.
Chemical Sedimentary Deposits: Chemical sedimentary deposits are formed from the precipitation of minerals from water, often as a result of evaporation or chemical reactions. These deposits play a crucial role in understanding the processes that lead to ore formation and mineral deposits, as they can host valuable minerals and can indicate past environmental conditions.
Chloritization: Chloritization is a process in mineralogy where primary minerals, especially biotite and hornblende, are altered to form chlorite, a greenish mineral belonging to the phyllosilicate group. This alteration typically occurs under specific temperature and pressure conditions during the metamorphic processes or hydrothermal alterations, affecting the overall mineral composition of rocks and influencing their economic potential.
Cratonic Environments: Cratonic environments refer to the stable interior portions of continental crust that have remained relatively undisturbed by tectonic activity for long periods. These areas often feature ancient rock formations and are characterized by their flat or gently undulating terrain, which influences the geological processes that lead to the formation of mineral deposits and ore resources.
Epithermal Deposits: Epithermal deposits are mineral deposits formed from hydrothermal processes at relatively shallow depths, typically within a range of 200 to 1,500 meters below the Earth's surface. These deposits are often associated with volcanic activity and can contain valuable metals like gold, silver, and copper, making them significant in ore-forming processes. The formation conditions, including temperature and pressure, lead to distinct mineral assemblages and textures that can be crucial for identifying and exploiting these resources.
Evaporative Processes: Evaporative processes refer to the natural phenomenon where water evaporates from a surface, leading to the concentration of dissolved minerals in the remaining solution. This process plays a crucial role in the formation of certain mineral deposits, especially in arid environments where evaporation rates exceed precipitation. Through evaporation, minerals such as halite, gypsum, and various salts can crystallize out of solution, contributing to significant ore-forming processes.
Fluid-rock interactions: Fluid-rock interactions refer to the processes that occur when fluids (like water, gas, or molten rock) move through and interact with solid rocks. This interaction is crucial in forming mineral deposits, influencing the chemistry of rocks, and playing a vital role in the processes that lead to ore formation.
Fractional crystallization: Fractional crystallization is a geological process where different minerals crystallize from a cooling magma at different temperatures, leading to the separation of minerals based on their unique crystallization points. This process is significant because it helps explain the formation of diverse igneous rock types and contributes to the development of mineral deposits by concentrating valuable elements as the molten rock cools and solidifies.
Gossan formations: Gossan formations are weathered, oxidized surface deposits that occur above mineral ore bodies, primarily rich in iron and other metals. These formations often serve as indicators of underlying mineralization, as they form from the alteration of sulfide minerals due to exposure to oxygen and water. The study of gossans is essential for locating valuable mineral deposits, as their presence often suggests the potential for economically viable ore below.
Hydrothermal alteration: Hydrothermal alteration refers to the chemical and mineralogical changes that occur in rocks due to the interaction with hot, aqueous fluids, typically associated with magmatic activity. This process often leads to the formation of new minerals, particularly hydroxides, and significantly impacts the properties and economic potential of the affected rocks, especially in relation to mineral deposits and their structures.
Hydrothermal fluids: Hydrothermal fluids are hot, mineral-rich water solutions that circulate through rocks in the Earth's crust, often at significant depths. These fluids play a crucial role in the formation of mineral deposits and ore bodies by transporting dissolved minerals and elements that precipitate as the fluids cool or interact with other geological materials. Their movement and interaction with surrounding rocks can lead to the concentration of valuable metals and minerals, making them key players in ore-forming processes.
Hydrothermal processes: Hydrothermal processes involve the movement and interaction of hot, mineral-rich water within the Earth's crust, leading to the formation of various mineral deposits. These processes play a crucial role in the concentration of valuable minerals and the creation of distinct geological features. They are essential for understanding how certain minerals form, especially native elements and halides, as well as ore deposits and gemstones.
Hypothermal deposits: Hypothermal deposits are mineral accumulations formed from hydrothermal fluids at high temperatures and pressures, typically at depths of 1-3 kilometers within the Earth's crust. These deposits often contain valuable metals like gold, silver, copper, and lead, which are precipitated as the hot fluids cool and react with surrounding rocks. The formation process is crucial for understanding various ore-forming processes that contribute to the development of significant mineral deposits.
Lateritic Deposits: Lateritic deposits are a type of mineral deposit formed through the weathering of rocks in tropical and subtropical environments, leading to the accumulation of minerals like iron and aluminum oxides. These deposits are often rich in metals and are significant sources of bauxite, which is the primary ore for aluminum production. The processes involved in their formation are closely linked to both chemical weathering and leaching, resulting in distinctive soil profiles that can be economically valuable.
Liquid immiscibility: Liquid immiscibility refers to the phenomenon where two or more liquids do not mix or blend together, resulting in distinct, separate layers. This characteristic is crucial in the formation of certain mineral deposits as it influences how different elements and compounds segregate within a magma or solution, leading to the concentration of economically valuable minerals in specific zones.
Magmatic processes: Magmatic processes refer to the series of geological events that occur during the formation and crystallization of magma, leading to the creation of igneous rocks and minerals. These processes include the generation of magma through melting of pre-existing rocks, the movement and cooling of this magma within the Earth's crust, and the subsequent solidification into various mineral forms. Understanding magmatic processes is crucial for recognizing how native element minerals form and how ore deposits are generated.
Magmatic sulfide deposits: Magmatic sulfide deposits are mineral accumulations formed from the cooling and crystallization of magma, which results in the concentration of metal sulfides, such as nickel, copper, and platinum group elements. These deposits typically occur in specific geological environments associated with mafic to ultramafic igneous rocks, where sulfide minerals crystallize early and settle to the bottom of a magma chamber, creating economically important resources. Their formation is closely linked to the processes that influence magma differentiation and the behavior of sulfur in magmas.
Mesothermal deposits: Mesothermal deposits are mineral deposits formed under moderate temperature and pressure conditions, typically found in the middle crust of the Earth. These deposits often occur in tectonically active regions and are associated with hydrothermal systems, where hot fluids circulate through rock formations, allowing for the deposition of valuable minerals such as gold, silver, and copper.
Mineral deposition: Mineral deposition is the process by which dissolved minerals precipitate from a solution and accumulate to form solid mineral deposits. This phenomenon is crucial in forming ore deposits, where valuable minerals concentrate in specific geological settings, leading to their economic extraction.
Oceanic spreading centers: Oceanic spreading centers are underwater mountain ranges formed by the upwelling of magma at divergent tectonic plate boundaries, where two oceanic plates move apart. These centers play a crucial role in the process of seafloor spreading, which contributes to the formation of new oceanic crust and the recycling of materials within the Earth's lithosphere. The geological activity associated with these centers can lead to the creation of hydrothermal vents and is significant for ore-forming processes.
Orogenic gold deposits: Orogenic gold deposits are a type of mineral deposit characterized by gold that is concentrated during mountain-building events, known as orogenies. These deposits typically form in or near tectonically active regions where geological processes create the right conditions for the precipitation of gold from hydrothermal fluids. The association with metamorphic rocks and the influence of structural features like faults and shear zones are critical in determining where these deposits can be found.
Orthomagmatic deposits: Orthomagmatic deposits are mineral accumulations formed directly from the crystallization of a molten rock, or magma, which cools and solidifies within the Earth's crust. These deposits typically contain valuable metals such as nickel, copper, and platinum-group elements and are formed as the magma undergoes differentiation, leading to the concentration of these minerals in certain zones.
Pegmatitic deposits: Pegmatitic deposits are exceptionally coarse-grained igneous rocks formed from the crystallization of magma that cools slowly in the Earth's crust. These deposits are characterized by their large crystals, often several centimeters to meters in size, and can contain rare minerals and elements that are not typically found in other types of igneous rocks.
Placer deposits: Placer deposits are concentrations of valuable minerals or metals that have accumulated in riverbeds, beaches, or other sedimentary environments due to the physical processes of erosion and sedimentation. These deposits are typically formed from the weathering and transportation of primary mineral sources, leading to the accumulation of heavier particles like gold, platinum, and gemstones, which become concentrated as lighter materials are washed away.
Porphyry Deposits: Porphyry deposits are large, low-grade ore bodies that contain significant amounts of copper, gold, and other metals, typically formed from hydrothermal processes associated with igneous intrusions. These deposits are characterized by their disseminated mineralization, often found within a stockwork of veins in a granitic or volcanic rock environment. The connection between porphyry deposits and mineral formation highlights how specific geological settings and processes can lead to the accumulation of economically important minerals.
Residual concentration: Residual concentration refers to the process where minerals become concentrated in a specific area after the more mobile elements have been leached away by weathering and other geological processes. This term is crucial for understanding how certain valuable minerals can accumulate in deposits that are economically viable for extraction. Essentially, it explains how certain elements remain behind in a concentrated form while others are removed, leading to the formation of mineral deposits that can be mined.
Sediment-hosted deposits: Sediment-hosted deposits are mineral deposits formed within sedimentary rocks, typically through the accumulation and alteration of minerals over time. These deposits often contain valuable metals and resources, such as copper, lead, and zinc, and are important for understanding ore-forming processes and the distribution of mineral resources in sedimentary environments.
Sericitization: Sericitization is the process by which feldspar minerals are altered into sericite, a fine-grained white mica, usually due to hydrothermal alterations. This transformation often occurs in mineral deposits, particularly in environments influenced by high-temperature and high-pressure conditions. Sericitization can indicate the presence of valuable ore deposits as it often occurs alongside other mineralization processes, providing important clues about the geological history and economic potential of an area.
Silicification: Silicification is the process where silica (SiO₂) replaces other minerals in rocks, typically resulting in the formation of siliceous minerals. This transformation can lead to significant changes in the physical and chemical properties of the rock, making silicification a crucial process in the formation of certain mineral deposits. It is commonly associated with the processes that create ore deposits, where silica plays a vital role in the concentration and preservation of valuable minerals.
Skarn Deposits: Skarn deposits are metamorphic mineral deposits that form through the interaction of hydrothermal fluids with carbonate rocks, resulting in the alteration of the original rock and the creation of valuable minerals. These deposits typically occur near intrusive igneous bodies, where heat and chemically reactive fluids lead to the transformation of surrounding limestone or dolostone into economically important minerals like garnet, pyroxene, and magnetite.
Supergene enrichment: Supergene enrichment is a geological process that enhances the concentration of certain minerals within ore deposits, typically occurring near the surface due to weathering and chemical alterations. This process often results in the formation of valuable secondary minerals that are more economically viable to extract. Supergene enrichment is particularly important in the context of copper, gold, and silver ores, where original sulfide minerals are transformed into oxides or other soluble forms that can precipitate out in concentrated deposits.
Unconformity-Related Uranium Deposits: Unconformity-related uranium deposits are mineral deposits formed in proximity to unconformities, which are surfaces that represent a significant gap in the geological record. These deposits typically occur where older rocks are overlain by younger sedimentary layers, creating a setting where uranium can be concentrated through processes such as hydrothermal activity, oxidation, and leaching. The unique conditions at unconformities allow for the migration of uranium-bearing fluids, making them prime locations for significant uranium ore accumulation.
Volcanic-associated massive sulfide deposits: Volcanic-associated massive sulfide deposits are mineral deposits formed by the interaction of seawater with volcanic materials on the ocean floor, resulting in the precipitation of sulfide minerals. These deposits typically contain valuable metals such as copper, lead, and zinc and are often associated with hydrothermal systems linked to volcanic activity. They are significant in understanding ore-forming processes and contribute to the global supply of metal resources.