Information in this presentation and some formats for the mineral summary

charts have been extracted from The Alteration Atlas (Thompson and

Thompson, 1996) and the SpecMIN™ software program.

1

Epithermal gold deposits occur largely in volcano-plutonic

arcs (island arcs as well as continental arcs) associated with

subduction zones, with ages similar to those of volcanism.

The deposits form at shallow depth, <1.5 km, and are hosted

mainly by volcanic rocks.

2

Schematic model of a volcanic-related hydrothermal system

(based on T. Leach diagrams).

3

Although 3 types of epithermal deposits can be

distinguished, the two most common end-member styles of

epithermal gold deposits are high sulfidation (HS) and low

sulfidation (LS).

The two deposit styles form from fluids of distinctly different

chemical composition in contrasting volcanic environment.

• The ore of HS deposits is hosted by leached silicic rock

associated with acidic fluids generated in the volcanic-

hydrothermal environment.

• In contrast, the fluid responsible for formation of LS ore

veins is similar to waters tapped by drilling beneath hot

springs into geothermal systems, waters that are reduced

and neutral-pH.

4

This models represents the type of fossil hydrothermal

systems responsible for HS ore deposits (Wolhetz and

Heiken, 1992):

• wiggly arrows represent rising sulfur-rich

magmatic gases;

• these gases condense and oxidize to form the acid fluids

responsible for leaching and argillic alteration of rocks

within the volcano and at the surface.

From Taylor (2007):

Acid-sulphate (high-sulphidation) type alteration fluids form

by the dissolution of large amounts of magmatic SO2 in high-

temperature hydrothermal systems, and also by reaction of

host rocks with steam-heated meteoric waters acidified by

oxidation of H2S (probably of magmatic origin: e.g., Rye et

al., 1992; Bethke et al., 2005), or by dissolution of CO2.

5

This models represents the type of fossil hydrothermal

systems responsible for LS ore deposits (Wolhetz and

Heiken, 1992):

• Characterized by adularia-sericite alteration and alkali-

chloride waters that have a neutral pH.

From Taylor (2007):

Altered rocks in low-sulphidation deposits generally comprise

two mineralogical zones: (1) inner zone of silicification

(replacement of wall rocks by quartz or chalcedonic silica);

and (2) outer zone of potassic -sericitic (phyllic) alteration

(quartz+K-feldspar and/or sericite, or sericite and illite-

smectite).

• Chlorite and carbonate are present in many deposits.

• Argillic alteration (kaolinite and smectite) is common.

6

Summary of characteristics of low and high sulfidation

systems.

7

Worldwide distribution of selected epithermal deposits

(Taylor, 2007).

8

Many hydrothermal minerals are stable over limited

temperature and/or pH ranges.

Mapping the distribution of alteration minerals in areas of

epithermal prospects may allow the thermal and

geochemical zonation to be reconstructed, leading to a

model of the hydrology of the extinct hydrothermal system.

Alteration minerals are also crucial to distinguish the style of

deposit, LS or HS.

9

From Taylor (2007):

In both high-sulphidation and low-sulphidation deposit

subtypes, hydrothermal alteration mineral assemblages are

commonly regularly zoned about vein- or breccia-filled fluid

conduits

• However they may be less regularly zoned in near-

surface environments, or where permeable rocks have

been replaced.

Characteristic alteration mineral assemblages in both deposit

subtypes can give way to propylitically altered rocks

containing quartz+chlorite+albite+carbonate±sericite,

epidote, and pyrite. The distribution and formation of the

earlier formed propylitic mineral assemblages generally

bears no obvious direct relationship to ore-related alteration

mineral assemblages.

10

A list of epithermal alteration minerals that can be identified

using reflectance spectroscopy is shown here.

11

The pH and temperature conditions of alteration can be

deduced based on mineral assemblages.

12

Another diagram showing the temperature stability of various

alteration minerals found in the epithermal environment.

13

Note: No scale is given because the widths of alteration

zones range from centimeters to tens of meters outward from

the vein (Wolhetz and Heiken, 1992).

14

15

VIS-NIR-SWIR plots showing some common propylitic

alteration minerals.

16

Chlorite is a very common alteration mineral and can occur

in a range of different alteration zones and deposit types.

This chart shows how chlorite can occur in a range of

different settings.

17

18

Advanced argillic alteration minerals are generally easy to

identify by SWIR features.

19

Alunite is a common constituent of advanced argillic

alteration.

Characteristic features are listed.

20

Alunite can occur in a range of different settings.

Distinguishing between the type of alunite present can help

determine the type of system and relative location.

21

22

Characteristics of dickite.

23

Characteristic of pyrophyllite.

24

Pyrophyllite can occur in several different environments.

25

Characteristics of diaspore.

26

27

Characteristics of zunyite.

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29

Weathered outcrops of steam-heated alteration are often

characterized by resistant quartz ± alunite 'ledges' and

extensive flanking bleached, clay-altered zones with

supergene alunite, jarosite and other limonite minerals

(Panteleyev, 1996).

30

VIS-NIR-SWIR features of common steam-heated argillic

alteration minerals.

31

32

This assemblage occurs as wallrock alteration around veins

and replacement zones in permeable lithologies.

Alteration may show a change in aluminum content and

temperature change away from vein in a progression from

illite � illite/smectite� montmorillonite.

33

Carbonates can be important in these systems (usually only

in LS environments) and may reflect condensation of CO2

from deeper boiling zones.

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35

Oxidation and/or weathering of sulfide-bearing epithermal

deposits can result in the formation of significant secondary

iron (± metal) species.

The three most common iron oxide/sulfate minerals are

shown here – in the VIS/NIR region.

In the VIS/NIR region the minerals goethite (hydroxide) and

hematite, (Fe-oxide) are commonly associated with jarosite

and have interference with its spectral features

Jarosite is rarely found in the pure end member state and is

usually mixed with goethite, as they are both products of the

same supergene cycles.

36

VIS-NIR features of common Fe oxides and sulfates are

shown.

37