L3 - Hydrothermal deposits and processes

Question 1

What is hydrothermalism?

Answer 1

Hydrothermalism involves hot aqueous fluids that transmit thermal energy and concentrate elements due to circulation in the crust

Question 2

What properties of water make it effective in hydrothermal systems?

Answer 2

• 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

Question 3

What happens to water as it rises in the crust?

Answer 3

As hydrostatic pressure decreases near the surface, water approaches the boiling curve and begins to boil

Question 4

What are the compositions of seawater-derived aqueous fluids?

Answer 4

• Mild salinity.
• Na⁺, K⁺, Mg²⁺, Cl⁻, SO₄²⁻.
• Oxygenated

Question 5

What are the compositions of meteoric water-derived aqueous fluids?

Answer 5

• Low salinity.
• Ca²⁺, Mg²⁺, Si²⁺, HCO₃⁻, SO₄²⁻.
• Oxygenated.

Question 6

How do aqueous fluids interact with minerals?

Answer 6

• 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

Question 7

What is connate water?

Answer 7

Water trapped during sediment deposition that escapes during compaction or lithification.

Question 8

What happens to water during sediment lithification?

Answer 8

• 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

Question 9

What are metamorphic fluids?

Answer 9

Fluids released as minerals adjust to higher pressure and temperature conditions, typically containing:
- Low salinity, low sulfur.
- CO₂, CH₄.

Question 10

What are magmatic-hydrothermal fluids?

Answer 10

Fluids exsolved from melts, typically with:

• High salinity.
• Cl⁻, HS⁻, CO₃²⁻, SO₄²⁻

Question 11

What is a disseminated hydrothermal deposit?

Answer 11

Rocks with fine-grained ore minerals scattered throughout the rock

Question 12

What is a vein deposit?

Answer 12

Mineral precipitation along a vein surface

Question 13

What is a stockwork deposit?

Answer 13

A rock containing a complex system of veins or fractures filled with minerals.

Question 14

What are gangue minerals?

Answer 14

Invaluable materials that do not contain ore minerals.

Question 15

What are shoot and lode deposits?

Answer 15

• Shoot: A concentrated area of ore within a lode.
• Lode: A deposit of ore in a rock formation

Question 16

What mechanisms control metal precipitation in hydrothermal systems?

Answer 16

• Changes in temperature and pressure.
• Boiling.
• Fluid mixing.
• pH and redox changes.

Question 17

What other precipitation mechanisms exist in hydrothermal systems?

Answer 17

• Adsorption on mineral surfaces.
• Biological influence.
• Cementation of pore spaces or replacement of pre-existing minerals.

Question 18

What are hydrocarbon fluids?

Answer 18

Aqueous fluids containing organic compounds and reduced hydrocarbon molecules (C-H).

Question 19

How do redox reactions contribute to ore formation?

Answer 19

Redox reactions enable dissolution, transport, and reprecipitation of elements, often driven by microbial activity.

Question 20

What is the role of evaporation in sedimentary processes?

Answer 20

Evaporation causes salts dissolved in water to precipitate, forming evaporite deposits.

Question 21

What is weathering, and what are its effects?

Answer 21

Weathering involves:

• Breakdown of primary rocks/minerals.
• Transport of soluble ions.
• Formation of secondary minerals.
• Accumulation of less soluble residuals like Si and Al.

Question 22

What are the key weathering processes?

Answer 22

• Mineral dissolution.
• Mineral hydrolysis (e.g., feldspar to clay).
• Oxidation

Question 23

What is sediment transport, and what forces affect particles?

Answer 23

Transport involves movement of particles by water, controlled by:

• Gravity and friction resisting movement.
• Hydraulic lift and drag promoting movement

Question 24

What is critical flow velocity in sediment transport?

Answer 24

The minimum velocity required to entrain a particle

Question 25

How does Stokes’ Law describe sediment settling?

Answer 25

Settling velocity increases with:

Larger particle diameter.

Denser particles

Question 26

What are examples of mineral densities?

Answer 26

Quartz: 2.65 g/cm³.
Feldspar: 2.55 g/cm³.
Micas: 2.83 g/cm³.
Heavy minerals: >3.00 g/cm³.
Gold: 19 g/cm³.

Question 27

What are the types of deposits formed by sedimentary processes?

Answer 27

• Placer deposits (e.g., gold, uranium).
• Evaporite minerals (e.g., fertilizer salts).
• Weathering residuals (e.g., bauxite).
• Organic matter alteration (e.g., hydrocarbons).