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,

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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 SO2in 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 ofepithermal 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+carbonatesericite,

    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 ofdifferent 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.

    2

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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.

    2

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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).

    3

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

    alteration minerals.

    3

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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.

    3

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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.

    3

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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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