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Emergence in crustal plumbing systems
Journal of Geochemical Exploration 101 (2009) 47
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Emergence in crustal plumbing systems Bruce E. Hobbs a,⁎, Alison Ord b, Chongbin Zhao c a b c
CSIRO Exploration and Mining, University of Western Australia CSIRO Exploration and Mining, Perth, Australia Computational Geosciences Research Centre, Central South University, Changsha, China
The formation of large orogenic gold deposits requires large volumes of fluid to be transported from deep in the crust and/or upper mantle to sites higher in the crust to mix and react with meteoric fluids and wall rocks. We explore two interconnected aspects of this process: (i) mechanical controls on the permeability distribution within fracture networks that allow the transport of these fluids and (ii) conditions for emergent behaviour to appear in such networks such that singularly large volumes of fluid may be transported at some sites whereas the network “shuts down” at other sites. The first issue involves the stress conditions and geometrical relations that need to exist in fracture networks in order that a near to hydrostatic fluid pressure gradient is maintained throughout the network thus allowing convective fluid systems to be generated and maintained throughout parts or all of the network. This depends on the existence, for a porous system, of a critical height that can maintain a hydrostatic pressure gradient; the system mechanically collapses so that the permeability becomes zero if this critical height is
⁎ Corresponding author. E-mail address: [email protected] (B.E. Hobbs). 0375-6742/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.gexplo.2008.12.019
exceeded. This enables the construction of “permeability maps” in stress-fracture orientation-fracture thickness space that delineate the conditions under which near hydrostatic gradients can be maintained in an otherwise lithostatically overpressured system. The second issue involves establishing the permeability scaling laws that emerge from a network of fractures comprised of pathways with contrasting permeabilities. This follows similar work in electrical and elastic structural systems. For some ranges of permeabilities the network is highly permeable whilst for other ranges the reverse holds. An intermediate range exists where the permeability scales in a fractal manner. These physical and geometrical aspects of fracture networks taken together enable conditions to be defined for the emergence of high permeability pathways capable of transporting large volumes of fluid from the lower to the upper parts of the crust and of generating mixing sites and temperature gradients conducive to generating giant ore deposits.
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Emergence in crustal plumbing systems Bruce E. Hobbs a,⁎, Alison Ord b, Chongbin Zhao c a b c
CSIRO Exploration and Mining, University of Western Australia CSIRO Exploration and Mining, Perth, Australia Computational Geosciences Research Centre, Central South University, Changsha, China
The formation of large orogenic gold deposits requires large volumes of fluid to be transported from deep in the crust and/or upper mantle to sites higher in the crust to mix and react with meteoric fluids and wall rocks. We explore two interconnected aspects of this process: (i) mechanical controls on the permeability distribution within fracture networks that allow the transport of these fluids and (ii) conditions for emergent behaviour to appear in such networks such that singularly large volumes of fluid may be transported at some sites whereas the network “shuts down” at other sites. The first issue involves the stress conditions and geometrical relations that need to exist in fracture networks in order that a near to hydrostatic fluid pressure gradient is maintained throughout the network thus allowing convective fluid systems to be generated and maintained throughout parts or all of the network. This depends on the existence, for a porous system, of a critical height that can maintain a hydrostatic pressure gradient; the system mechanically collapses so that the permeability becomes zero if this critical height is
⁎ Corresponding author. E-mail address: [email protected] (B.E. Hobbs). 0375-6742/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.gexplo.2008.12.019
exceeded. This enables the construction of “permeability maps” in stress-fracture orientation-fracture thickness space that delineate the conditions under which near hydrostatic gradients can be maintained in an otherwise lithostatically overpressured system. The second issue involves establishing the permeability scaling laws that emerge from a network of fractures comprised of pathways with contrasting permeabilities. This follows similar work in electrical and elastic structural systems. For some ranges of permeabilities the network is highly permeable whilst for other ranges the reverse holds. An intermediate range exists where the permeability scales in a fractal manner. These physical and geometrical aspects of fracture networks taken together enable conditions to be defined for the emergence of high permeability pathways capable of transporting large volumes of fluid from the lower to the upper parts of the crust and of generating mixing sites and temperature gradients conducive to generating giant ore deposits.