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Unlock your guides15.1 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
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