The Alteration Epithermal
Transcript of The Alteration Epithermal
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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.
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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.
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Schematic model of a volcanic-related hydrothermal system
(based on T. Leach diagrams).
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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 hotsprings into geothermal systems, waters that are reduced
and neutral-pH.
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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.
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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.
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Summary of characteristics of low and high sulfidation
systems.
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Worldwide distribution of selected epithermal deposits
(Taylor, 2007).
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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.
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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 theearlier formed propylitic mineral assemblages generally
bears no obvious direct relationship to ore-related alteration
mineral assemblages.
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A list of epithermal alteration minerals that can be identified
using reflectance spectroscopy is shown here.
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The pH and temperature conditions of alteration can be
deduced based on mineral assemblages.
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Another diagram showing the temperature stability of various
alteration minerals found in the epithermal environment.
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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).
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VIS-NIR-SWIR plots showing some common propylitic
alteration minerals.
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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.
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Advanced argillic alteration minerals are generally easy to
identify by SWIR features.
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Alunite is a common constituent of advanced argillic
alteration.
Characteristic features are listed.
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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.
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Characteristics of dickite.
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Characteristic of pyrophyllite.
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Pyrophyllite can occur in several different environments.
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Characteristics of diaspore.
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Characteristics of zunyite.
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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).
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VIS-NIR-SWIR features of common steam-heated argillic
alteration minerals.
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This assemblage occurs as wallrock alteration around veins
and replacement zones in permeable lithologies.
Alteration may show a change in aluminum content andtemperature change away from vein in a progression from
illite illite/smectite montmorillonite.
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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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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.
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VIS-NIR features of common Fe oxides and sulfates are
shown.
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