The nature of "quartz eyes" hosted by dykes associated with Au-Bi-As-Cu, Mo-Cu, and base-metal-Au-Ag mineral occurrences in the mountain freegold region (Dawson Range), Yukon, Canada

Thierry Bineli Betsi, David R. Lentz

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    14 Citations (Scopus)

    Abstract

    Various origins have been assigned to rounded to subrounded and elliptical quartz megacrysts ("quartz eyes") in dyke rocks associated with mineral deposits/occurrences worldwide. An exact interpretation of their nature is likely to tightly constrain the petrogenesis of the host rocks, and by association may be critical in evaluating genetic models for spatially associated ore minerals. ChromaSEM-CL imaging and electron probe microanalysis (EPMA) of "quartz eyes" within porphyry dykes associated with Au-Bi-Cu-As, Mo-Cu, and base-metal-Au-Ag mineral occurrences across the Northern Freegold Resources (NFR) property in the Dawson Range of Yukon Territory, Canada, reveals that the cathodoluminescence (CL) response of quartz is a function of its trace-element abundance(s). Bright blue luminescent growth zones are in most cases richer in Fe (up to 8839 ppm) and Ti (up to 229 ppm) relative to CL dark growth zones, with up to 41 times lower concentration of these elements. Assuming aTiO2 = 1, the Ti-poor dull cores consistently recorded lower temperatures (mostly < 600 °C) compared to Ti-rich brighter blue rims (up to 860 °C). This suggests either overgrowth on xenocrystic cores or an increase in crystallization temperature. The temperature rise likely reflects magma mixing, and is therefore consistent with the phenocryst/phenoclasts having formed in a magma chamber rather than by secondary processes. Also, the great variability in composition and temperature of crystallization and/or reequilibration of brighter blue growth zones of two quartz crystals (660 °C and 855 °C) from a single sample suggests that multiple episodes of magma mixing and incremental growth of parental magma chambers occurred. Some "quartz eyes" are overprinted by variably oriented, bifurcating, and anastomosing fluid migration trails ("splatter and cobweb textures") of red to reddish-brown CL quartz that is in most cases of low-temperature origin, and trace-elements poor, thus implying interaction of deuteric fluids with quartz phenocrysts/phenoclasts. The presence of "quartz eye" crystals with broken and angular blue cores, overgrown by oscillatory-zoned rims in which the zoning pattern does not correspond with the crystal boundaries, further suggests that some quartz crystals had been explosively fragmented (phenocrysts) and are now hosted in a recrystallized tuffisitic groundmass. The volatile exsolution that likely accompanied both magma mixing and decompression (as suggested by dendritic quartz, fine-grained recrystallized tuffisitic groundmass, and corroded quartz grains) was probably an important process that could have favoured the ore formation.

    Original languageEnglish
    Pages (from-to)347-368
    Number of pages22
    JournalJournal of Geosciences
    Volume55
    Issue number4
    DOIs
    Publication statusPublished - 2010

    Fingerprint

    mountain region
    base metal
    quartz
    mineral
    cathodoluminescence
    crystal
    magma
    magma chamber
    crystallization
    trace element
    phenocryst
    fluid
    temperature
    ore mineral
    exsolution
    mineral deposit
    decompression
    petrogenesis
    porphyry
    electron probe analysis

    All Science Journal Classification (ASJC) codes

    • Earth and Planetary Sciences(all)

    Cite this

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    title = "The nature of {"}quartz eyes{"} hosted by dykes associated with Au-Bi-As-Cu, Mo-Cu, and base-metal-Au-Ag mineral occurrences in the mountain freegold region (Dawson Range), Yukon, Canada",
    abstract = "Various origins have been assigned to rounded to subrounded and elliptical quartz megacrysts ({"}quartz eyes{"}) in dyke rocks associated with mineral deposits/occurrences worldwide. An exact interpretation of their nature is likely to tightly constrain the petrogenesis of the host rocks, and by association may be critical in evaluating genetic models for spatially associated ore minerals. ChromaSEM-CL imaging and electron probe microanalysis (EPMA) of {"}quartz eyes{"} within porphyry dykes associated with Au-Bi-Cu-As, Mo-Cu, and base-metal-Au-Ag mineral occurrences across the Northern Freegold Resources (NFR) property in the Dawson Range of Yukon Territory, Canada, reveals that the cathodoluminescence (CL) response of quartz is a function of its trace-element abundance(s). Bright blue luminescent growth zones are in most cases richer in Fe (up to 8839 ppm) and Ti (up to 229 ppm) relative to CL dark growth zones, with up to 41 times lower concentration of these elements. Assuming aTiO2 = 1, the Ti-poor dull cores consistently recorded lower temperatures (mostly < 600 °C) compared to Ti-rich brighter blue rims (up to 860 °C). This suggests either overgrowth on xenocrystic cores or an increase in crystallization temperature. The temperature rise likely reflects magma mixing, and is therefore consistent with the phenocryst/phenoclasts having formed in a magma chamber rather than by secondary processes. Also, the great variability in composition and temperature of crystallization and/or reequilibration of brighter blue growth zones of two quartz crystals (660 °C and 855 °C) from a single sample suggests that multiple episodes of magma mixing and incremental growth of parental magma chambers occurred. Some {"}quartz eyes{"} are overprinted by variably oriented, bifurcating, and anastomosing fluid migration trails ({"}splatter and cobweb textures{"}) of red to reddish-brown CL quartz that is in most cases of low-temperature origin, and trace-elements poor, thus implying interaction of deuteric fluids with quartz phenocrysts/phenoclasts. The presence of {"}quartz eye{"} crystals with broken and angular blue cores, overgrown by oscillatory-zoned rims in which the zoning pattern does not correspond with the crystal boundaries, further suggests that some quartz crystals had been explosively fragmented (phenocrysts) and are now hosted in a recrystallized tuffisitic groundmass. The volatile exsolution that likely accompanied both magma mixing and decompression (as suggested by dendritic quartz, fine-grained recrystallized tuffisitic groundmass, and corroded quartz grains) was probably an important process that could have favoured the ore formation.",
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    T1 - The nature of "quartz eyes" hosted by dykes associated with Au-Bi-As-Cu, Mo-Cu, and base-metal-Au-Ag mineral occurrences in the mountain freegold region (Dawson Range), Yukon, Canada

    AU - Betsi, Thierry Bineli

    AU - Lentz, David R.

    PY - 2010

    Y1 - 2010

    N2 - Various origins have been assigned to rounded to subrounded and elliptical quartz megacrysts ("quartz eyes") in dyke rocks associated with mineral deposits/occurrences worldwide. An exact interpretation of their nature is likely to tightly constrain the petrogenesis of the host rocks, and by association may be critical in evaluating genetic models for spatially associated ore minerals. ChromaSEM-CL imaging and electron probe microanalysis (EPMA) of "quartz eyes" within porphyry dykes associated with Au-Bi-Cu-As, Mo-Cu, and base-metal-Au-Ag mineral occurrences across the Northern Freegold Resources (NFR) property in the Dawson Range of Yukon Territory, Canada, reveals that the cathodoluminescence (CL) response of quartz is a function of its trace-element abundance(s). Bright blue luminescent growth zones are in most cases richer in Fe (up to 8839 ppm) and Ti (up to 229 ppm) relative to CL dark growth zones, with up to 41 times lower concentration of these elements. Assuming aTiO2 = 1, the Ti-poor dull cores consistently recorded lower temperatures (mostly < 600 °C) compared to Ti-rich brighter blue rims (up to 860 °C). This suggests either overgrowth on xenocrystic cores or an increase in crystallization temperature. The temperature rise likely reflects magma mixing, and is therefore consistent with the phenocryst/phenoclasts having formed in a magma chamber rather than by secondary processes. Also, the great variability in composition and temperature of crystallization and/or reequilibration of brighter blue growth zones of two quartz crystals (660 °C and 855 °C) from a single sample suggests that multiple episodes of magma mixing and incremental growth of parental magma chambers occurred. Some "quartz eyes" are overprinted by variably oriented, bifurcating, and anastomosing fluid migration trails ("splatter and cobweb textures") of red to reddish-brown CL quartz that is in most cases of low-temperature origin, and trace-elements poor, thus implying interaction of deuteric fluids with quartz phenocrysts/phenoclasts. The presence of "quartz eye" crystals with broken and angular blue cores, overgrown by oscillatory-zoned rims in which the zoning pattern does not correspond with the crystal boundaries, further suggests that some quartz crystals had been explosively fragmented (phenocrysts) and are now hosted in a recrystallized tuffisitic groundmass. The volatile exsolution that likely accompanied both magma mixing and decompression (as suggested by dendritic quartz, fine-grained recrystallized tuffisitic groundmass, and corroded quartz grains) was probably an important process that could have favoured the ore formation.

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