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Understanding Epithermal Ore Deposits

Aug 22, 2024

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.