Numerical models of fluid pathways in
extension-related mineral systems
1
Oliver, N.H.S., 1 McLellan J.G., 2 Hobbs, B.E., 1 Cleverley, J.S., 2 Ord, A. & 1 Feltrin, L.
1 Economic Geology Research Unit and Predictive Mineral Discovery Cooperative Research Centre,
School of Earth Sciences, James Cook University, Townsville 4811, Australia
2 CSIRO Exploration and Mining and Predictive Mineral Discovery Cooperative Research Centre, P.O. Box 1130, Bentley 6102, Australia
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Coupled Deformation, Thermal and
Fluid Flow Numerical Models
• Aims:
a) Examine the relationship between deformation,
fluid flow and heat transfer
b) Relate the initial work to a more focussed
scenario e.g. Basin/Sediment interfaces
c) Use outcomes to apply to a Mt Isa style model
d) Examine the role of large granite intrusions
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The ability to model all three processes together
allows one to evaluate the relative importance of
potentially competing processes which may have
been responsible for certain types of ore genesis
Fluid circulation within low permeability basement
has been proposed to occur beneath many
sediment hosted mineral deposits and usually
associated with upwards flow being responsible
for ore deposition.
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Conceptual Models:
• What happens to fluid when basins are extended?
• Can fluids scavenge metals from low permeability
basement material then return to basin sediments for
ore deposition?
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Pore pressure MPa
10 20
a)
2
Depth
km 4
30 40 50
έ = 1.3x10-10
ψ = 4°
6
8
b)
2
έ = 3.1x10-12
ψ = 4°
Depth
km 4
6
8
c)
2
Depth
km 4
έ = 1.6x10-15
ψ = 4°
6
8
d)
2
Depth
km 4
έ = 3.1x10-12
ψ = 1°
6
8
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Strain
dependent
relationship,
however subhydrostatic
pore-pressure
gradients are
mechanically
feasible during
basin extension
Isa Superbasin (Mt Isa Group)
Calvert Superbasin
Granite
faults
• Mt Isa style
setting
Extension results in
downflow within
more permeable
faults
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Temperature and fluid flow
in a Mt Isa style setting
(reduction of permeability
in the cover <2e diff)
a) Stable convection
cells
b) Extension destroys
convection cells and
fluids are focussed
toward the centre of
the model and
dilational zones (fluid
mixing)
c) Temperature gradient
to a) showing broad
cooling of cover
sequences
d) Perturbation of the
isotherms within
faults
Not good for SEDEX-style
deposits as fluids are
drawn downwards
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c-e
a) Widespread downflow
as a result of extension
– no convection cells
can be established
b) Deformation ceased
and convection cells
begin to form
(downflow in faults
dominate direction of
cells)
c) Convection collapses
as a result of the TBL
(oscillatory over Ma)
d),e) Temp distribution for
a)b) generally cooling
(even though broad
distribution of
unfocussed upflow)
Not good for SEDEX-style
deposits
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e-c
a) Widespread upflow as a
result of overpressure
b) Deformation ceased,
convection cells begin in
the cover sequences
drawing fluid into faults and
upward to potential
exhalation sites
c) Cell collapse following deep
convection (oscillatory
nature- TBL)
d) Streamlines showing
convection cells in the
cover sequences with
upward plumes
Ideal conditions for SEDEX-style
deposits
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c-e ovp
a) Temp distribution with
elevation of isotherms
especially around faults
b) Extreme attenuation of
isotherms near the surface
corresponding with large
convection in basement
(ideal conditions for
scouring metals at high
temperature then deposition
at surface)
c) Thermal decay following
convection cell collapse
Ideal conditions for SEDEX-style
deposits
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c-e ovp
Intrusion influence
From a conduction
solution
Local scale convection
around the margins of
the intrusion
Drawing cooler fluid
down around the edges
of the intrusion, from
cover sequences above
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Mt Isa SEDEX style deposits
• Broad view of the tectono-thermal evolution of the Isa terrain
Transition from…
Active rifting – uplift and erosion – blanketing by sag phase shales
and related SEDEX (or diagenetic) mineralisation indicates that
extensional strain rates decayed with time
Here we attempted to simulate this scenario with concurrent
extension, heating and then ceasing the deformation.
The best scenario for SEDEX style deposits is from the
overpressured model, which allows a better circulation of
convection cells and fluid movement as well as temperature
gradients suitable for ore deposition.
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Unconformity-related U and Olympic Dam style Fe-oxide
Cu Au deposits
Models with initial heating and fluid flow established large convection
cells which are effectively destroyed by extension at geologically
reasonable strain rates (10-14s-1)
Surface fluids are then driven downwards meeting remnants of the
decaying convection deep in the system.
This simulation provides a possible solution for mixing of near
surface and deeply derived fluids in:
a) Unconformity-related U deposits
b) Olympic-Dam style Fe-oxide Cu-Au deposits
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Future work
•
•
•
•
Investigate the tectono-thermal evolution of the Isa Inlier
Test conduction v’s convection models (large scale)
Test crustal thinning / thickening (large scale)
Conduction and convection around intrusives with
deformation
• Apply to several geometric scenarios
• Eventually test fully coupled models in 3d (still slow)
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In Conclusion:
Coupled deformation, fluid flow and heat
transfer models have introduced the
complexity of “to convect or not to convect”
Results show that deformation can destroy stable
convection cells and simple scenario testing
can point to conditions more favourable for
certain deposit types
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