lecture n3: thermal modeling conduction versus advection … · 2020. 8. 31. · geologically...
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Enabling Technologies
Thermal Modelling
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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
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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Heat Transfer
Convection
Density-driven flow
Density varies with P, T, and chemistry
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HEAT( )gPkq fρµ
−∇−=
Convection in geological systems
Critical Rayleigh number
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KTgkHc
Ra pf
µαρ ∆
=2
Permeability
Height of systemTemperature difference
Convection occurs when Ra > critical valueRa critical = 4p2 ~ 39.5 (fixed T boundaries)
Heat Transfer
Is thermal convectionpossible?
Average crustal permeabilitiestoo low for convection
Convection may occur wherepermeability is abnormally high(e.g. faults), or wheregeothermal gradient isabnormally high (e.g. intrusions)
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Modelling Convection
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Modelling Convection
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Convection cells and temperature contours
Modelling Convection
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Gold concentration
(g/tonne)
Heat Transfer
Intrusions and hydrothermal ore depositsHeat from intrusions drives convection
Collect (“scavenge”) metals fromwide source areaDeposit metals in the narrow,upwelling regionBut flux is rather small
Heat drives dehydration reactions in thewall rocksCooling magma releases volatiles (fluids)
Dehydration and magmatic fluids areprobably more important for generatingore deposits.
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Sebastian Geiger (http://n.ethz.ch/student/sgeiger/)
Heat Transfer
Precipitation and dissolution due to convection
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CR ∇⋅≈ qx gradient in solubilityFluid fluxReaction rate =
… due to gradients in T, P, rock composition etc.
Reactions within thermal gradients
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Thermal Rock Properties
Specific heat capacity (c)
Thermal conductivity (λ)
Thermal diffusivity (κ)
Thermal capacity (ρ c)
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Thermodynamics of Folding
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Thermo-dynamic Simulation
Manuscript Geology (in press) available
Folded meta-quartz vein in Challenger Deposit: courtesy of Andy Tomkins (Monash)
Thermodynamics of Folding
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Thermodynamics of Folding
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Coupled thermal-mechanical models
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Coupled thermal-mechanical models
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Temperature development with thrusting
Coupled thermal-mechanical models
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Shear strain
Displacement vectors
Pink: tensile-failed areas
Thermal Modelling
CAN BE USED TO UNDERSTAND THE HEAT DISTRIBUTION IN THE LITHOSPHERE, AND BY INFERENCE ASPECTS OF ITS MECHANICAL AND HYDRAULIC ARCHITECTURE.
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A legacy for mineral exploration science
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