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

Abstract

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 CO 2 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.

The noble gas systematics of late-orogenic H2O–CO2 fluids, Mt Isa, Australia

“…3. These data and fluid inclusion types (Table 1) are similar to those reported by other workers in the region (De Jong and Williams, 1995;Oliver, 1995;Rotherham, 1997;Baker, 1998;Xu, 2000;Perring et al, 2000;Oliver et al, 2006;Baker et al, 2008). However, for the current study, we deliberately retain a general classification of fluid inclusion types that emphasizes only the broadest differences in salinity and XCO 2 (cf.…”

“…A similar range of fluid pressure has been estimated for Cloncurry District Na-Ca alteration systems previously ( Fig. 1; Oliver et al, 2006;Bertelli and Baker, 2010).…”

“…If this CO 2 expansion was accommodated by surface venting during widespread hydrothermal brecciation (Mark et al, 2006b;Oliver et al, 2006;Bertelli and Baker, 2010), the pressure drop could have occurred catastrophically within a short period of time. If the minimum documented pressures of $0.8-1 kbar approximate hydrostatic pressure, and the overlying hydrothermal fluids had an average density of $1 g cm À3 , the maximum mineralisation depth is suggested as $8-10 km.…”

Transport efficiency and dynamics of hydraulic fracture networks

“…For example, [17] and [18] estimated flow velocities of 0.01-0.1 m/s from the size of grains that were carried up by the fluid. Using similar arguments, [24] derived flow velocities in excess of 5 m/s in a fluidized breccia with m-sized clasts from the Cloncurry District, Australia.…”

“…Such "mobile hydrofractures" thus propagate together with their contained fluid and can reach velocities in the order of m/s [16,22,23]. Possible fluid sources which may generate the necessary fluid pressure for hydraulic brecciation and hydraulic fracturing are fluid released by igneous intrusions [24], fluid release due to decompression [25,26] or mineral dehydration reactions [27]. Dehydration of the mineral biotite appears to be the main fluid source in case of Hidden Valley [4].…”