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.
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.
Schematic model of a volcanic-related hydrothermal system (based on T. Leach diagrams).
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.
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.
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).
Summary of characteristics of low and high sulfidation systems.
Worldwide distribution of selected epithermal deposits (Taylor, 2007).
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.
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.
A list of epithermal alteration minerals that can be identified using reflectance spectroscopy is shown here.
The pH and temperature conditions of alteration can be deduced based on mineral assemblages.
Another diagram showing the temperature stability of various alteration minerals found in the epithermal environment.
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).
VIS-NIR-SWIR plots showing some common propylitic alteration minerals.
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.
Advanced argillic alteration minerals are generally easy to identify by SWIR features.
Alunite is a common constituent of advanced argillic alteration.
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.
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).
VIS-NIR-SWIR features of common steam-heated argillic alteration minerals.
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.
Carbonates can be important in these systems (usually only in LS environments) and may reflect condensation of CO2 from deeper boiling zones.
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
VIS-NIR features of common Fe oxides and sulfates are shown.
