Epithermal deposits are types of hydrothermal mineral deposits formed from hot, mineral-rich fluids circulating near the Earth’s surface. They’re important for gold and silver mining because they can contain high concentrations of these precious metals. Epithermal deposits are generally classified into two main types: high-sulfidation (HS) and low-sulfidation (LS). Both types form in volcanic settings, but they differ significantly in how they develop, the minerals they contain, and the processes that shape them.
High-Sulfidation Epithermal Deposits
High-sulfidation epithermal systems form when acidic, sulfur-rich fluids rise from deeper magmatic sources. These fluids react strongly with the surrounding rocks, altering them chemically and depositing various minerals. High-sulfidation deposits often contain minerals like enargite, pyrite, covellite, and native sulfur, which tend to form in acidic conditions.
One characteristic of high-sulfidation deposits is the intense alteration they produce, called advanced argillic alteration, which typically includes minerals like alunite, kaolinite, and quartz. This alteration is highly destructive to the surrounding rocks, creating a “lithocap” or cap of altered rock that sits above the mineralized zone.
Key Features of High-Sulfidation Deposits:
- Source: Magmatic fluids with high acidity and sulfur content.
- Common Minerals: Enargite, covellite, pyrite, native sulfur.
- Alteration Style: Advanced argillic (alunite, kaolinite).
- Formation Depth: Shallow but directly linked to deeper magmatic activity.
Example Locations:
- Yanacocha, Peru: One of the world’s largest gold mines.
- La Coipa, Chile: Known for both gold and silver production.
Low-Sulfidation Epithermal Deposits
Low-sulfidation epithermal systems, by contrast, form from neutral to mildly acidic fluids that come from deeper crustal levels but are generally less connected to magma sources. Instead, they tend to form where meteoric (surface) water mixes with these deep hydrothermal fluids, cooling them and causing mineral precipitation. Low-sulfidation systems commonly contain precious metals like gold and silver alongside quartz veins and adularia (a type of potassium feldspar).
In low-sulfidation deposits, the alteration around veins is less intense than in high-sulfidation settings, and it usually takes the form of adularia-sericite alteration, which includes quartz and sericite. The deposition occurs in fractures, forming quartz veins that can extend for hundreds of meters, hosting high concentrations of gold and silver.
Key Features of Low-Sulfidation Deposits:
- Source: Deep crustal fluids mixed with meteoric water.
- Common Minerals: Quartz, adularia, gold, silver.
- Alteration Style: Adularia-sericite, less intense than high-sulfidation.
- Formation Depth: Shallow, often closer to the surface than high-sulfidation.
Example Locations:
- Hishikari, Japan: Known for exceptionally high-grade gold veins.
- Round Mountain, Nevada, USA: A major gold-producing low-sulfidation deposit.
Key Differences Between High-Sulfidation and Low-Sulfidation Deposits
- Fluid Chemistry: High-sulfidation systems involve acidic, sulfur-rich fluids, while low-sulfidation systems involve neutral to mildly acidic fluids that mix with meteoric water.
- Alteration: High-sulfidation deposits cause intense rock alteration with minerals like alunite and kaolinite, whereas low-sulfidation deposits show adularia-sericite alteration.
- Mineral Content: High-sulfidation deposits commonly contain sulfide minerals (like enargite), while low-sulfidation deposits are rich in quartz veins with native gold and silver.
- Deposit Style: High-sulfidation deposits often create broad zones of mineralization near volcanic centers, while low-sulfidation deposits tend to form narrow veins that can be mined selectively.
References
Sillitoe, R.H. (1993). Epithermal Models: Genetic Types, Geometrical Controls, and Shallow Features. Geological Association of Canada, Special Paper 40.
White, N.C., & Hedenquist, J.W. (1995). Epithermal Gold Deposits: Styles, Characteristics, and Exploration. Society of Economic Geologists Newsletter.
