Explosive release of gases and rocks from the top of molten rock bodies (magma)

Gases dissolved in molten rock (“magma”) leave the magma as it crystallizes to form solid, igneous rocks underground. Near Cloncurry in western Queensland, Australia, there is evidence in ancient rocks that this process occurred extremely rapidly, which is highly unusual. Pipe-like bodies of fragmental rock, cemented by small rock pieces and minerals precipitated from these gases, are found extending out from the tops of crystallized igneous rocks. We calculated ‘fluidization’ velocities of particle and gas transport in these pipes to be > 10m/s, approaching the velocities sometimes observed in volcanic eruptions. These processes are important to explain some of the features of copper-gold deposits in the region and elsewhere in the world.

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    Data record related to this publication Explosive release of gases and rocks from the top of molten rock bodies (magma)
    Data Publication title Explosive release of gases and rocks from the top of molten rock bodies (magma)
  • Description

    Gases dissolved in molten rock (“magma”) leave the magma as it crystallizes to form solid, igneous rocks underground. Near Cloncurry in western Queensland, Australia, there is evidence in ancient rocks that this process occurred extremely rapidly, which is highly unusual. Pipe-like bodies of fragmental rock, cemented by small rock pieces and minerals precipitated from these gases, are found extending out from the tops of crystallized igneous rocks. We calculated ‘fluidization’ velocities of particle and gas transport in these pipes to be > 10m/s, approaching the velocities sometimes observed in volcanic eruptions. These processes are important to explain some of the features of copper-gold deposits in the region and elsewhere in the world.

  • Other Descriptors
    • Descriptor

      The source and transport regions of fluidized (transported) breccias outcrop in the Cloncurry Fe-oxide-Cu-Au district. Discordant dykes and pipes with rounded clasts of metasedimentary calc-silicate rocks and minor felsic and mafic intrusions extend several kilometres upwards and outwards from the contact aureole of the 1530 Ma Williams Batholith into overlying schists and amphibolites. We used analytical equations for particle transport to estimate clast velocities (≥ 20m s-1), approaching volcanic ejecta rates. An abrupt release of overpressured magmatic-hydrothermal fluid is suggested by the localisation of the base of the breccias in intensely veined contact aureoles (at around 10 km, constrained by mineral equilibria), incorporation of juvenile magmatic clasts, the scale and discordancy of the bodies, and the wide range of pressure variation (up to 150 MPa) inferred from CO2 fluid inclusion densities and related decrepitation textures. The abundance of clasts derived from depth, rather than from the adjacent wallrocks, suggests that the pressure in the pipes was sufficient to restrict the inwards spalling of fragments from breccia walls; that is, the breccias were explosive rather than implosive, and some may have vented to the surface. At these depths, such extreme behaviour may have been achieved by release of dissolved fluids from crystallizing magma, in combination with a strongly fractured and fluid-laden carapace, sitting under a strong, low permeability barrier. The relationship of these breccias to the Ernest Henry iron-oxide-Cu-Au deposit suggests they may have been sources of fluids or mechanical energy for ore genesis, or alternately provided permeable pathways for later ore fluids.

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    • Descriptor
    • Descriptor type
  • Data type dataset
  • Keywords
    • fluidization
    • overpressure
    • iron oxide-copper-gold
  • Funding source
  • Research grant(s)/Scheme name(s)
    • 12697 - Fluid mixing in hydrothermal systems
  • Research themes
    Industries and Economies in the Tropics
    FoR Codes (*)
    • 040307 - Ore Deposit Petrology (040307)
    • 040312 - Structural Geology (040312)
    • 010207 - Theoretical and Applied Mechanics (010207)
    SEO Codes
    • 840102 - Copper Ore Exploration (840102)
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    Temporal (time) coverage
  • Start Date 2002/01/31
  • End Date 2006/07/31
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    Spatial (location) coverage
  • Locations
    • Roxmere cattle station, approx. 30km south of Cloncurry in NW Queensland
    Data Locations

    Type Location Notes
    Physical Location Dr Nick Oliver School of Earth and Environmental Sciences Earth & Environmental Sciences Building 34 Douglas Campus, Townsville James Cook University, QLD 4811
    Attachment Granite-related overpressure and volatile release in the mid-crust.zip
    The Data Manager is: Nick Oliver
    College or Centre
    Access conditions Open: free access under license
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    Citation Oliver, Nick; Blenkinsop, Tom; Ridd, Peter (2011): Explosive release of gases and rocks from the top of molten rock bodies (magma). James Cook University.