Geology Lesson: Hydrothermal Deposits

Introduction

• Focus on hydrothermal deposits, specifically gold deposits in hydrothermal veins.
• Location Highlight: Yellowstone Park, known for its hydrothermal activity.

Hydrothermal Concept

• Definition: "Hydrothermal" means hot water.
• Mechanism: Hot water boils up from deep within the Earth, carrying minerals.
• Key Example: Old Faithful Geyser in Yellowstone.

Boiling Water and Pressure

• Water boils at 100°C at sea level.
• Boiling point decreases with elevation and increases with depth due to pressure.
• Deep Earth Conditions:
  • Extreme pressure raises boiling point significantly.
  • Creates superheated fluid capable of dissolving minerals like glass, quartz, iron, and even gold.

Mineral Dissolution and Precipitation

• Superheated fluids dissolve minerals under high pressure and temperature.
• Ascent and Pressure Reduction:
  • Fluids rise through cracks; pressure decreases.
  • Minerals start precipitating out at different pressures.

Key Minerals in Hydrothermal Deposits

• Common Minerals: Quartz, iron, and gold.
• Pressure and Deposition:
  • Similar pressure for quartz, iron, and gold deposition.
  • Variations based on sulfur content affect iron deposition:
    • High sulfur: pyrites form.
    • Low sulfur/high oxygen: oxides (e.g., magnetite).

Gold Precipitation

• Gold does not react with sulfur.
• Forms of Gold:
  • Embedded in metal sulfides.
  • As native gold blobs or pieces.

Additional Mineral Deposits

• Other minerals may also be present and precipitate at various points.
• Calcium Salts: Often the last to precipitate, sometimes at the surface.
• Surface Activity:
  • Boiling water at surface can hold some iron.
  • Presence of red bacterial colonies might mimic iron deposits.

Geysers and Surface Phenomena

• Formation:
  • Hot fluids reach the surface, boil away, creating hot springs/geysers.
  • Geysers occur due to a cap of water building pressure before eruption.
• Old Faithful: Classic example of geyser activity.

Surface Accessibility of Hydrothermal Deposits

• Exposure:
  • Other geological activities push deposits towards the surface.
  • Erosion and mountain building expose these mineral-rich veins.

Considerations and Variables

• Numerous factors affect hydrothermal systems: acidity, pH, temperature, sulfur, and oxygen levels.
• Summary:
  • Hot waters dissolve minerals deep in Earth.
  • Minerals precipitate out to form hydrothermal veins.
  • Remaining fluids can form surface geothermal features.

Conclusion

• Overview of how hydrothermal gold deposits form and reach the surface.
• Encouragement for further learning and exploration of geology.