Lecture Notes: Epithermal Deposits by Andrew Jackson

Introduction

• Speaker: Andrew Jackson, Geologist with Global Resource Investments
• Purpose of Talk: Educate non-technical individuals on ore deposits, specifically epithermal deposits in this session.
• Series: Fifth talk in "Ore Deposits 101" series
• Objective: To introduce investors to the features and jargon of epithermal deposits.

Overview of Epithermal Deposits

• Definition: Form near the earth's surface within 1000 meters.
• Temperature & Pressure: 100-300°C with lower pressures allowing fluids to boil.
• Formation: Result from the cooling of hydrothermal fluids and concentration of metals.
• Geological Association: Often found near hot springs; closely associated with porphyry deposits.

Global Distribution and Economics

• Age: Most are young (tertiary or less than 50 million years old) due to erosion of older deposits.
• Location: Predominantly in subduction zones, especially around the Pacific Ring of Fire.
• Economic Importance: Significant source of global gold production, second to orogenic gold deposits.

Types of Epithermal Deposits

1. High Sulfidation:

   • Fluid Nature: Hot acidic fluids, undiluted by groundwater.
   • Characteristics: Leaching leaves a silica residue (vuggy silica).
   • Examples: Yanacocha (Peru), Pierina (Peru)
   • Mining Method: Generally open-pit due to disseminated nature.

2. Low Sulfidation:

   • Fluid Nature: Neutral pH, clear fluids.
   • Characteristics: Boiling of fluids causes gold deposition.
   • Examples: Hishikari Mine (Japan)
   • Mining Method: Typically underground, unless multiple veins.

3. Intermediate Sulfidation:

   • Fluid Nature: Partial interaction between magmatic fluids and groundwater.
   • Characteristics: Mixture of gold, silver, lead, and zinc.
   • Examples: Fruta del Norte (Ecuador), La Patera (Mexico)
   • Mining Method: Often underground, with some potential open-pit areas.

Exploration Techniques

• Initial Indicators: Volcanism and limited erosion areas.
• Remote Sensing: Multispectral analysis for surface alteration.
• Ground Exploration: Mapping and geochemical surveys.
• Geophysics: Used to detect subsurface structures.
• Drilling: Final stage to confirm mineralization.

Conclusion and Key Takeaways

• Associations: Form near or lateral to porphyry systems.
• Age: Mostly tertiary or younger.
• Types: Three main types with distinct features and mining methods.
• Economic Value: Critical for understanding and investing in mining operations.

• Next Talk Preview: Discussion on Carlin-type gold deposits.