L3 - Hydrothermal deposits and processes Flashcards
What is hydrothermalism?
Hydrothermalism involves hot aqueous fluids that transmit thermal energy and concentrate elements due to circulation in the crust
What properties of water make it effective in hydrothermal systems?
- Dissolves ions due to dipole moment.
- Presence of ligands like Cl⁻, HS⁻, CO₃²⁻, SO₄²⁻ for metal cations.
- High surface tension for wetting mineral surfaces.
- High heat conductivity
What happens to water as it rises in the crust?
As hydrostatic pressure decreases near the surface, water approaches the boiling curve and begins to boil
What are the compositions of seawater-derived aqueous fluids?
- Mild salinity.
- Na⁺, K⁺, Mg²⁺, Cl⁻, SO₄²⁻.
- Oxygenated
What are the compositions of meteoric water-derived aqueous fluids?
- Low salinity.
- Ca²⁺, Mg²⁺, Si²⁺, HCO₃⁻, SO₄²⁻.
- Oxygenated.
How do aqueous fluids interact with minerals?
- Water surrounds mineral grains and fills porosity due to high surface tension.
- Exists as interlayer water between silicate structures or as structural water within minerals
What is connate water?
Water trapped during sediment deposition that escapes during compaction or lithification.
What happens to water during sediment lithification?
- Initial sediment contains 50-60% water.
- Compaction reduces water content to 10-20%.
- At high temperatures (>200°C), structural water and surface water are released
What are metamorphic fluids?
Fluids released as minerals adjust to higher pressure and temperature conditions, typically containing:
- Low salinity, low sulfur.
- CO₂, CH₄.
What are magmatic-hydrothermal fluids?
Fluids exsolved from melts, typically with:
- High salinity.
- Cl⁻, HS⁻, CO₃²⁻, SO₄²⁻
What is a disseminated hydrothermal deposit?
Rocks with fine-grained ore minerals scattered throughout the rock
What is a vein deposit?
Mineral precipitation along a vein surface
What is a stockwork deposit?
A rock containing a complex system of veins or fractures filled with minerals.
What are gangue minerals?
Invaluable materials that do not contain ore minerals.
What are shoot and lode deposits?
- Shoot: A concentrated area of ore within a lode.
- Lode: A deposit of ore in a rock formation
What mechanisms control metal precipitation in hydrothermal systems?
- Changes in temperature and pressure.
- Boiling.
- Fluid mixing.
- pH and redox changes.
What other precipitation mechanisms exist in hydrothermal systems?
- Adsorption on mineral surfaces.
- Biological influence.
- Cementation of pore spaces or replacement of pre-existing minerals.
What are hydrocarbon fluids?
Aqueous fluids containing organic compounds and reduced hydrocarbon molecules (C-H).
How do redox reactions contribute to ore formation?
Redox reactions enable dissolution, transport, and reprecipitation of elements, often driven by microbial activity.
What is the role of evaporation in sedimentary processes?
Evaporation causes salts dissolved in water to precipitate, forming evaporite deposits.
What is weathering, and what are its effects?
Weathering involves:
- Breakdown of primary rocks/minerals.
- Transport of soluble ions.
- Formation of secondary minerals.
- Accumulation of less soluble residuals like Si and Al.
What are the key weathering processes?
- Mineral dissolution.
- Mineral hydrolysis (e.g., feldspar to clay).
- Oxidation
What is sediment transport, and what forces affect particles?
Transport involves movement of particles by water, controlled by:
- Gravity and friction resisting movement.
- Hydraulic lift and drag promoting movement
What is critical flow velocity in sediment transport?
The minimum velocity required to entrain a particle
