Epithermal Deposits

What are the main commodities of epithermal deposits?

Au, Ag, and Cu

What are some common by-products of epithermal deposits?

Pb, Zn, As, Sb, Hg, Te

What do epithermal deposits look like/what are they?

Shallow veins and breccia ores of Au-Ag formed as a byproduct of volcanic activity

What is the temperature and depth conditions of epithermal ore formation?

300 C and within 1 mile of surface

What sorts of environments do epithermal deposits form in?

Convergent boundaries, subduction zones, calc-alkaline porphyry systems

What is the class of epithermal deposits based on?

Sulfidation state. Estimates/measures gas fugacity of sulfur in hydrothermal fluid

What is sulfidation state?

Measure of the ratio of sulfur to chalcophile

What are high sulfidation ores?

Pyrite, tennantite, chalcopyrite, pyrrhotite, and enargite

what is the ore texture of high-sulfidation ores?

Crystal and vugs

What are the gangue minerals in high-sulfidation ores?

Clays

What are low-sulfidation ores?

Arsenopyrite, chalcopyrite, pyrrhotite

What is the ore texture of low-sulfidation ores?

colloform and crustiform

What is the source of heat in epithermal deposits?

Magma

What are the sources of metals and water in epithermal deposits?

Magmatic fluid transported metals and water from magma chambers. Fluids react with wall rocks, cool, and mix with groundwater.

How are metals transported in solution?

H2S and HS- form aqueous complex with volatile elements

How are ores deposited?

By cooling, boiling, and mixing with cool groundwater. Oxidation maybe?

What is the bladed texture (common in epithermal deposits) from?

Boiling and pH increase

Why does pH rise during boiling?

Because of loss of CO2

What mineral replacement happens during pH increases?

Silica replacement of calcite

What is the geological setting of high-sulfidation deposits?

Around volcanic centers, particularly within calderas, 1-5 km depth.W

What are the host rocks of high-sulfidation ores?

Intensely altered, intermediate to felsic volcanic or high-level intrusive rocks (including lavas and pyroclastics).

What is a key subsurface feature of high-sulfidation deposits?

Lithocap. Highly silicified alteration zone (sub-horizontal).

What are the different zones of high-sulfidation deposits?

Central zone, advances argillic zone, argillic zone, and propylitic zone.

What is the central zone?

Porous, massive, and quartz rich. Contains vuggy silica (evidence of leaching).

What is the advanced argillic zone?

Alunite, Kaolinite group, and anhydrite

What is the argillic zone?

Kaolinite, smectite, illite

What is the propyllitic zone?

Calcite, sericite, epidote (unaltered)

What is a sulfosalt?

A sulfide mineral where metal is chemically bonded to a complex of sulfur-semimetal, rather than sulfur alone.

What is the acidity of fluids in high sulfidation ores?

Extremely acidic (pH of 1-2).

Low sulfidation structure

Several km away from volcanic structure. Bedded sequences of intermediate to felsic composition of lavas and pyroclastic rocks.

What is the main host of Au in low sulfidation deposits?

Native Au or electrum (Au-Ag alloy).

Vein textures (by increasing degree of oversaturation)

Zonal, Comb, Crustiform, Colloform, Mosaic

Expression of low sulfidation ore

Haloes of a few meters of argillic alteration, regional propyllitic alteration, and chalcedony/opal

What does boiling lead to in low sulfidation deposits?

Reduction of Au solubility