Epithermal Deposits: Ore Deposits 101 Lecture Notes

Introduction

• Speaker: Andrew Jackson, Geologist at Global Resource Investments
• Purpose: Help non-technical people understand ore deposits.
• Series Overview: Focus on main types of deposits and jargon used in press releases by exploration and mining companies.
• Talk Focus: Epithermal deposits (5th in the series).

What are Epithermal Deposits?

• Form close to the earth's surface (within 1000m).
• Temperatures: 100-300 degrees Celsius.
• Formation observed today in hot springs (e.g., Yellowstone, Rotorua).
• Concentration process: Metals are concentrated by melting and moving through the crust, where valueless minerals are removed.

Geological Context

• Relation to Porphyry Deposits:
  • Epithermal deposits are genetically and spatially associated with porphyry deposits.
  • Example: Far Southeast and Lepanto deposits in the Philippines.
• Age of Deposits:
  • Majority are relatively young (tertiary or less than 50 million years).
  • Preservation issues due to erosion in mountainous areas.

Global Distribution

• Commonly found in active subduction zones (e.g., the Ring of Fire).
• Some older deposits exist (e.g., Appalachians) which have survived erosion.

Importance of Epithermal Deposits

• Major source of gold (12% of annual global production).
• Second largest producers after porphyry deposits.

Types of Epithermal Deposits

1. High Sulfidation Epithermal Deposits:

   • Form in geothermal systems with undiluted acidic fluids.
   • Characterized by significant alteration and mineralization.
   • Example: Barrick's Pueblo Viejo in the Dominican Republic.

2. Low Sulfidation Epithermal Deposits:

   • Result from fluid boiling due to dropping confining pressure.
   • Usually well-banded, crystal clear fluids with high silica content.
   • Example: Nevada's Carlin Trend.

3. Intermediate Sulfidation Epithermal Deposits:

   • Form with some dilution by groundwater, between high and low sulfidation types.
   • Characterized by both gold and significant silver deposits.
   • Example: Kinross's Fruta del Norte in Ecuador.

Formation Process

• High Sulfidation:

  • Initial leaching and alteration followed by mineralization.
  • Alteration indicators: Ruggie quartz, white clays.
  • Mining typically involves open pit.

• Low Sulfidation:

  • Gold deposition occurs when fluids boil and precipitate gold.
  • Boiling zones critical for mineralization.
  • Often mined from underground, particularly in narrow veins.

• Intermediate Sulfidation:

  • Mineralization controlled by boiling, with base metal deposition at greater depths.
  • May involve both steep veins and flat layers.

Exploration Techniques

• Focus on areas with felsic to intermediate volcanism.
• Use of multispectral analysis can indicate alteration zones.
• Ground exploration techniques include mapping and geochemistry to target drill testing.

Key Takeaways

• Epithermal deposits form above or laterally to porphyry systems.
• Three main types (high, low, intermediate sulfidation) with distinct characteristics.
• Importance in gold production and economic viability.

Conclusion

• Next talk: Column-type gold deposits.