HR: 09:00h
AN: MR21A-05 INVITED    [Abstracts]
TI: Quantifying Fabric Anisotropy in Breccias: Insights Into Brecciation in a Mineralizing Environment
AU: * Blenkinsop, T G
EM: Thomas.Blenkinsop@jcu.edu.au
AF: School of Earth and Environmental Sciences, James Cook University, Douglas Campus, Townsville, QLD 4811, Australia
AU: Oliver, N
EM: Nick.Oliver@jcu.edu.au
AF: School of Earth and Environmental Sciences, James Cook University, Douglas Campus, Townsville, QLD 4811, Australia
AU: Cihan, M
EM: Mustafa.Cihan@jcu.edu.au
AF: School of Earth and Environmental Sciences, James Cook University, Douglas Campus, Townsville, QLD 4811, Australia
AB: Breccias are commonly regarded as having random fabrics, but clast alignment can cause significant fabric anisotropy. Orientation and shape are two complementary aspects of clast anisotropy. Standard circular statistics can be used to characterize orientation anisotropy, such as the direction and length of the mean resultant vector (with standard error and confidence intervals), and the concentration parameter, kappa. The Rayleigh test can be applied to establish statistical significance. Shape anisotropy can be characterized by the aspect ratio. However, none of these quantities fully describe anisotropy. The axial ratio of the finite shape matrix, a concept borrowed from strain analysis, accounts for both anisotropy components. A review of brecciation mechanisms suggests that anisotropy may be imparted by: 1) preferred fracturing directions 2) fabrics in rocks prior to brecciation 3) laminar flow during transport 4) preferred orientation during deposition. Anisotropy may be reduced by turbulent flow during transport. The orientation anisotropy and the final shape matrix are the most sensitive discriminants among breccias from the Proterozoic Mount Isa inlier, where breccias are prominent hosts in several ore deposits. These parameters reveal fabrics that can not be discerned without measurement. Breccias that have considerable clast transport distances have low anisotropies and no preferred orientations, consistent with random fabrics that were formed by fluidization. Such a fluidized breccia hosts the Cu-Au deposit at Ernest Henry mine, and regional examples contain infill sulphides. In situ breccias, on the other hand, have orientation and total anisotropies that are inherited from the fragmentation process, and regional examples are not mineralized. Clast roughness in the Mount Isa breccias varies very little, but particle size distribution, clast/(matrix+infill) ratios, and circularity, are additional useful parameters to characterize breccias and understand their genesis.
UR: http://www.jcu.edu.au/ees/staff/academic/JCUDEV_008366.html
DE: 5104 Fracture and flow
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8012 High strain deformation zones
DE: 8025 Mesoscopic fabrics
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting