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Enabling Technologies
Modelling Philosophy
1
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
2
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
A. Gradient inhydraulicpotential
B. Permeability
C. Solubilitysensitivity to P, T, C
D. Spatialgradient of P, T, C
E. Time (duration)
Key Parameter
is reflected in
ExplorationMineral System
scale-dependent translation
5 Questions1. Geodynamics2. Architecture3. Fluid
reservoirs4. Flow drivers &
pathways5. Deposition
Terrain Selection
Area Selection
Drill Targeting
What is a numerical model?
Numerical modeling is simply taking the equations that governa system and using them to simulate the changes in a systemwith math.
Always based on several assumptions
Create the most accurate model with the simplest equations
3
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Why use numerical models?What factors control the formation of ore deposits
Test hypotheses derived from field observations
Explore the effects of changing rock properties and/or boundaryConditions
e.g. permeability or mechanical strengthe.g. orientation of the far-field principal stresses
understand interactions between geological processes
determine the response of the system to any given parameter(e.g. deformation, fluid flow, heat transfer)
Provide quantitative results
Identify critical geological processes
Translate all of the qualitative research understanding intoquantitative spatial prediction
4
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Coupled Models
5
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Coupled Models
Proxies for earth processes – not replication
No code provides a proxy for all processes
Many codes evolved for different purposes (i.e. not for understanding earth processes)
Some basic parameters/relationships not well understood
All 3D models are boxes - boundary conditions critical
Formation of ore systems involves many complex interactionstime/length scale ≠ geological scale
So we need to couple codes which model different processes together
6
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Coupled Models
FLAC3Dfully coupled deformation and fluid flowcan handle permeability and pore pressure changes produced by deformation-induced dilatancy
7
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Coupled Models
FastFlohandles chemical reactions (equilibrium or kineticallycontrolled), mineral dissolution and precipitation, solute transport by advection and diffusion (with heat flow)
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
A
B
C
Deformation
Many ore deposits are structurally controlled
Create/destroy permeability
Pathways to fluids
Barriers/traps to fluids
Constitutive models: Elastic, plastic, viscous and combinationsof these
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Modelling Deformation
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Displacement vectors
Magnitudes of displacement vectors
Modelling Deformation
Simple Shearing of an Elastic Mohr-Coulomb Material.
If the material has non-associative constitutive behaviour, ie., the stress vector is not parallel to the plastic strain rate increment, then shear zones form spontaneously at yield (strain-hardening both+&-).
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Contours of plastic strain-rate.
Fluid Flow
Darcy flow (laminar flow through porous media) is the mostrelevant processes (not on all scales!)
Darcy flow is driven by hydraulic head (i.e. fluid flows downenergy potential gradients
Permeability in many cases is the key property to determinewhere fluid flows in a hydrothermal system
Permeability is a material property, but can by modified bydeformation (destruction and creation of pore space) andchemical reaction (dissolution / precipitation)
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Darcy’s Law
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
( )gPkq fρµ−∇−=
Fluid flux (m3/m2/s) = velocity x porosity
Permeability (m2)
Pressure gradient (Pa/m)
Fluid density (kg/m3)
Gravity (m/s2)Fluid viscosity (Pa s)
Flux = Rate x Driving Force
Fluid FLowWhat controls the driving force?
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Deformation– Dilation– Compaction
Fluid production/consumption– Dehydration reactions– Cooling magma
Topography
Fluid density– Temperature– Pressure– Chemical composition (salinity)
( )gPkq fρµ−∇−=
( )gPkq fρµ−∇−=
Modelling Fluid Flow
15
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Potential for mixing meteoric/basinal fluids with metamorphic fluids
Contours of shear strain + particle tracks
Downflow (due to extension)
Upflow (due to devol.)
Fluid moves towards shear zone – on edge of granite dome
Heat Transfer
Geologically relevant are only conduction and advection
Conduction is a diffusive process: ‘‘heat will flow from hot tocold’’
Advection is a transport process, whereby heat is transportedby a solid, fluid or gas
Advection is faster than diffusion
Normal geothermal gradient generally too small for convection(requires local heat source or high permeability)
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Heat Transfer
Conduction
Diffusive process: “heat will flow from hot to cold”
Fourier’s law:
Geotherms are conductive
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Heat Transfer
Advection
Heat is transported by a solid, fluid or gas
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Heat Transfer
Convection
Density-driven flow
Density varies with P, T, and chemistry
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
HEAT( )gPkq fρµ
−∇−=
Modelling Convection
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Transport
Molecular Diffusion is solute transport down a concentration gradient in chemical potential or concentrationSolute flux is directly proportional to the concentration gradient
Advection describes transport of solutes by movement of medium (e.g. fluid)
Dispersion describes the effect of an inhomogeneous flow field(heterogenity of porous media)
21
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Reaction
KINETIC MODELS Mineral reactions fall into three groups:
(1) reaction rates may be so slow relative to the time period of interest that the reaction can be ignored altogether
(2) those in which the rates are fast enough to maintain equilibrium
(3) the remaining reactions. Only those require a kinetic description
22
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Modelling Convection
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Gold concentration
(g/tonne)
5 Processes – basic relationship
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predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
dtcccp
pcT
TcPgA
rr
r
eee ... ⎟⎟⎠
⎞⎜⎜⎝
⎛∇
∂∂
+∇∂∂
+∇∂∂
∇= ∑∫ µκρ
Amount of material deposited at a particular point
Fluid viscosity
permeability
Fluid densitygravity
gradient in non-hydrostatic pressure
equilibrium solubility sensitivities with respect to T, P and c of other species
spatial gradients in T, p and cr
the time the system operates for
Current research issues
Accurate simulation of fracture porosity generation and fluidflow
Thermodynamic data for chemical modelling
Effective simulation of magma intrusion and wall rock effects
Geologically realistic consistency of models across all scales
Full coupling
Appropriate methodology
Appropriate constitutive model
25
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Efficiency gains
Computation time
Parameter space exploration (inversion process)
Stability
Visualisation and interpretation
User interaction
26
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Use in economic geology
In exploration, we are ONLY interested in prediction
So we observe patterns associated with known mineralisationand seek to find copies of them
NUMERICAL METHODS HELP TO UNDERSTAND PROCESSINTERACTION, AND TO MAKE PREDICTIONS ABOUT PHYSICALPROCESSES INTERACTING WITH COMPLEX GEOMETRIES
27
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
predictive mineral discovery*Cooperative Research Centre
A legacy for mineral exploration science
Convection cells and temperature contours
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modelingLaverton model concept
Laverton model geometry in FLAC3D
Laverton model faults
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Conclusion 2Conclusion 2
Under the effect of thrust loading from west, dilation is localized along the Bardoc shear and the lower part of the Ida fault, forming the dominant fluid transport channel.
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Geometry of the modelGeometry of the model
Greenstones
Ida FaultBardoc Shear
140 km
53 k
m
Moho
ZuleikaFaultUpper Crust
Lower Crust
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Model 1Model 1
• stratigraphic units + faults• no thrust loading• horizontal shortening
deformation
Dilation - Volumetric strain
Darcy Fluid Flow Vectors
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Conclusion 1Conclusion 1
The Ida fault is the major channel for fluids to migrate upwards, but the Bardoc shear is also important.
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Model 2Model 2
• stratigraphic units + faults• thrust loading
(the west margin)• horizontal shortening
deformation
Dilation - Volumetric strain
Darcy Fluid Flow Vectors
Darcy Fluid Flow
Vectors
(zoom-in view)
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Model architectureModel architecture
Granite – old and cold
Greenstone
Conglomerate
Shale (turbidites)
Fault
(late)
30 km
15 km
Present-day erosion?
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Rock propertiesRock properties
Medium
MediumHigh
LowVery
high Permeability
Strong
WeakModerate
Weak
Very wea
k
Strength (cohesion, friction
angle, tensile strength)
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Results 1: Regional Results 1: Regional devolatilisationdevolatilisation
Heat from the base: T = 500 °C
Initialise with steady-state geotherm (30 °C/km at surface)
T = 25 °C
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Results 1: Regional Results 1: Regional devolatilisationdevolatilisation
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Results 2: Local Results 2: Local devolatilisationdevolatilisation
Initialise with steady-state geotherm (30 °C/km at surface)
T = 25 °C
Late (low-Ca) granites: Initialised at 750 °C
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Fluid flow directions Fluid flow directions after 1 after 1 MyrMyr
Regional heating: Still dehydrating, upward flow (max. 3 x 10-10 m3/m2/s)
Local heating: Cooling and retrogression, downward flow (max. 0.9 x 10-10 m3/m2/s)
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Integrated fluid flux Integrated fluid flux after 1 after 1 MyrMyr
Regional heating
Max. = 1.04 x 104 m3/m2
Local heating
Max. = 3.49 x 103 m3/m2
1 Enabling Technologies1 Enabling Technologies Numerical modelingNumerical modeling
Results 3: Results 3: Regional Regional devolatilisationdevolatilisation + extension+ extension
Heat from the base: T = 500 °C
Initialise with steady-state geotherm (30 °C/km at surface)
T = 25 °C
Extend at strain rate ~8 x 10-15 s-1
Wallaby
Wallaby
Conceptual model
Model Conditions• Burial depth of around 8km• Pore fluid pressure initialised at around 0.7 x lithostatic(~ 140 Mpa)
• Insitu boundary stress applied at a ratio ofσ1 / σ2 = 1.2
σ3 / σ2 = 0.8
σ1 = 240 MPaσ2 = 200 MPaσ3 = 160 MPa
Software – UDEC and FLAC 3d Comparisons
1 Enabling Technologies 1 Enabling Technologies Mechanical modelingMechanical modeling
DeformationDeformation• Deformation of rock can create permeability (in solid rock) and destroy permeability (by enhancing compaction of unconsolidated sediment)
• Faults in mineral deposits often act as pathways to fluids, faults in hydrocarbon deposits often act as barriers to fluids
• The relation between stress and strain is described by a constitutive model
• Constitutive models: Elastic, plastic, viscous and combinations of these • Many ore deposits are structurally controlled• Important structures such as faults, jogs, and bends provide structural traps
• Focussing of metal laden fluids is an important issue in developing an ore deposit
1 Enabling Technologies 1 Enabling Technologies Mechanical modelingMechanical modeling
DeformationDeformation• Elastic materials are characterized by reversible deformations upon unloading.
The stress-strain laws are linear and path-independent.
e.g. Hooke’s law
Stress = constant * strain
stress
strain
1 Enabling Technologies 1 Enabling Technologies Mechanical modelingMechanical modeling
DeformationDeformationPlastic models potentially involve some degree of permanent, path-dependent deformations
non-linear stress-strain relations
Different models may be characterized by their yield function, hardening/softening functions and flow rule.
stress
strainStress = f(plastic strain)
1 Enabling Technologies 1 Enabling Technologies Mechanical modelingMechanical modeling
MohrMohr--CoulombCoulombRepresentative forupper crustal rocks
ValidationPhysical versus numerical experiments.
Volumetric strain versus axial
Axial stress versus axial strain strain
GA 02/000
1 Enabling Technologies 1 Enabling Technologies Mechanical modelingMechanical modeling
Fluid flowFluid flowDarcy Fluid Flow
Vectors
(zoom-in view)