The
source and transport regions of fluidised (transported) breccias outcrop in the
Cloncurry Fe-oxide-Cu-Au district, providing insights into the origins of other
fluidised magmatic-hydrothermal breccias such as kimberlites. Discordant dikes
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. The brecciation initiated at depths >10 km and may have
resulted in surface breaching. We used analytical equations for particle
transport to estimate clast velocities (e' 20 ms-1), approaching
volcanic ejecta rates (up to 80 ms-1). The wide range of
CO2 fluid inclusion densities (up to 150 MPa), the localisation of
the base of the breccias in contact aureoles, and the scale and discordancy of
the bodies suggests the breccia transport process was triggered by an abrupt
release of overpressured fluid. At these depths, such extreme behaviour may have
been achieved by release of dissolved fluids from crystallising magma, in
combination with a strongly fractured and fluid-laden source, sitting under a
strong, low permeability barrier, and such parameters may be analogous to the
asthenosphere/lithosphere boundary in the case of kimberlite initiation. The
relationship of these breccias to the Ernest Henry iron-oxide-Cu-Au deposit
suggests they may have been either fertile or failed' orebody
feeders.