Granite-related overpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry District, Australia

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 (≥20 m sec−1), approaching volcanic ejecta rates. An abrupt release of overpressured magmatic-hydrothermal fluid is suggested by the localization 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