Numerical modelling of Numerical modelling of fluid and heat transport fluid and heat transport

during deformation in the during deformation in the late Archean Yilgarn late Archean Yilgarn

cratoncratonand its relevance to and its relevance to late orogenic gold late orogenic gold

mineralizationmineralizationPeter Sorjonen-Ward, Bruce Peter Sorjonen-Ward, Bruce

Hobbs,Hobbs,Alison Ord, Yanhua Zhang and Alison Ord, Yanhua Zhang and

Chongbin ZhaoChongbin Zhao

CSIRO Exploration and MiningCSIRO Exploration and MiningExploration Geodynamics Chapman Exploration Geodynamics Chapman

ConferemceConferemce

Numerical modelling applications Numerical modelling applications to orogenic gold mineralization in to orogenic gold mineralization in

the Yilgarnthe Yilgarn

Scope of presentationScope of presentation• Yilgarn architecture and boundary Yilgarn architecture and boundary

conditions conditions • Coupled fluid flow and deformationCoupled fluid flow and deformation• Coupled thermal and fluid flow modelsCoupled thermal and fluid flow models

Modelling here is addressing potential Modelling here is addressing potential viability of fluid pathways, not viability of fluid pathways, not constrained by mass balance or time constrained by mass balance or time considerationsconsiderations

Generating and sustaining Generating and sustaining a mineral system requiresa mineral system requires

• An architecture that enhances fluid flow An architecture that enhances fluid flow withwith– efficient fluid-rock interaction in the source

region– efficient focussing into depositional site

• Mechanisms for timely fluid production Mechanisms for timely fluid production • P-T conditions and fluid chemistries that P-T conditions and fluid chemistries that

optimize extraction and depositional optimize extraction and depositional efficiencyefficiency

Models for Yilgarn fluids and Models for Yilgarn fluids and goldgold

- provenance and pathways- provenance and pathways• Deposits formed across a range of metamorphic grades

over a similar time – crustal continuum model• Many deposits formed relatively late with respect to

metamorphic peak• Some areas, such as Coolgardie region have mineral

parageneses recording temperature gradients away from plutons (Witt-Knight-Mikucki model)

• Isotopic and geochemical alteration attributes suggest fluid derivation and prolonged interaction with radiogenically evolved regional scale crustal reservoirImplications:– Fluid flow across lateral as well as vertical temperature

gradients– Fluid production linked to deformation and thermal

evolution

Yilgarn geology and Yilgarn geology and magneticsmagnetics

Low-pass filtering by Paul Gow

Yilgarn structural Yilgarn structural domainsdomains

Sou

thern

Cro

ss P

rovin

ceS

ou

thern

Cro

ss P

rovin

ceEaste

rn G

old

field

s Pro

vin

ceEaste

rn G

old

field

s Pro

vin

ce

Magmas and fluids –Magmas and fluids –regional scaleregional scale

• Large scale magnetic anomalies relate Large scale magnetic anomalies relate to monzogranites emplaced to present to monzogranites emplaced to present level within 10 Ma of mineralizationlevel within 10 Ma of mineralization

• Dominant gold mineralizing fluids are Dominant gold mineralizing fluids are weakly reducing, weakly acidic and of weakly reducing, weakly acidic and of low salinity low salinity

• Evolved isotopic signatures suggest Evolved isotopic signatures suggest interaction with – though not interaction with – though not necessarily derivation from granitic necessarily derivation from granitic lower and middle crustlower and middle crust

South Polaris deposit in South Polaris deposit in Southern Cross ProvinceSouthern Cross Province

Gold deposited in equilbriumGold deposited in equilbriumwith diopside and K-feldsparwith diopside and K-feldspar

Racetrack deposit in Racetrack deposit in Ora banda domainOra banda domain

Sub-greenschist facies Sub-greenschist facies gold depositiongold deposition

Yilgarn mineralization broadly synchronousYilgarn mineralization broadly synchronous across range of metamorphic grades?across range of metamorphic grades?

Critical structural elements and Critical structural elements and requirements for the Yilgarnrequirements for the Yilgarn

– Structural studies of mineralized veins indicate compressive deformation during regional uplift and decompression

– Limited strain at site of deposition but coeval high strains at depth require major decoupling in middle crust – coincident with granitic sheets?

– Generation of large volume of fluids, within large lower crustal reservoir, relatively late, in order to satisfy geochemical mass balance and isotopic constraints

– Seismic data indicate reflectors of opposing dip, which suggest domains of tectonic wedging, backthrusting and “pop-up” structures

– Favourable architecture for formation of overpressured seals and rapid uplift of deeper crust

Models designed to Models designed to investigateinvestigate

1.1. Architectures and mechanisms that Architectures and mechanisms that promote both lateral and upwards fluid promote both lateral and upwards fluid flow during contractional deformationflow during contractional deformation

2.2. Potential for convective flow systemsPotential for convective flow systems

3.3. Thermal impact of plutons embedded Thermal impact of plutons embedded in regional metamorphic regimein regional metamorphic regime

4.4. Consequences for fluid flow and Consequences for fluid flow and mineralization patterns triggered by mineralization patterns triggered by fluid mixingfluid mixing

Models designed to Models designed to investigateinvestigate

1.1. Architectures and mechanisms that Architectures and mechanisms that promote both lateral and upwards promote both lateral and upwards fluid flow during contractional fluid flow during contractional deformationdeformation

2.2. Potential for convective flow systemsPotential for convective flow systems

3.3. Thermal impact of plutons embedded in Thermal impact of plutons embedded in regional metamorphic regimeregional metamorphic regime

4.4. Consequences for fluid flow and Consequences for fluid flow and mineralization patterns triggered by fluid mineralization patterns triggered by fluid mixingmixing

1.1. Architectures and mechanisms that Architectures and mechanisms that promote both lateral and upwards promote both lateral and upwards fluid flow during contractional fluid flow during contractional deformationdeformation

2.2. Potential for convective flow systemsPotential for convective flow systems

3.3. Thermal impact of plutons embedded in Thermal impact of plutons embedded in regional metamorphic regimeregional metamorphic regime

4.4. Consequences for fluid flow and Consequences for fluid flow and mineralization patterns triggered by fluid mineralization patterns triggered by fluid mixingmixing

Generating sufficient fluids Generating sufficient fluids in the right place at the right in the right place at the right

timetime• Granulitic lower crust inappropriate since already dehydrated?• Fluids from melting in lower crust sequestered again during

crystallization of hydrous phases (where not restitic)?• Fluids exsolved form crystallizing granites insufficient?• Local metamorphic devolatilization insufficient? • Rapidly formed accretionary prism could provide a more steady

supply of fluid, but in many cases mineralization is late and evidence for accretionary prism is lacking

• Orogenically derived meteoric fluids if downdraw is feasible (and isotopic characteristics are appropriate)

• Basinal fluids in submergent foreland basin or extending arc terrain (if salinity and isotopic attributes of mineralizing fluids is consistent)

• Is there material transfer of fluids from the mantle to the crust, or is the mantle role geodynamic, via buoyancy, rigidity and heat flow?

Intrusive sheets in Intrusive sheets in basal part of basal part of Karakoram BatholithKarakoram Batholith

Deformed Deformed amphibolites and amphibolites and intrusive sheets intrusive sheets at base of at base of Karakoram Karakoram BatholithBatholith

Lithostatically overpressured Lithostatically overpressured system – requires sustained system – requires sustained

fluid supplyfluid supply

Symmetry and asymmetrySymmetry and asymmetry

Interpreting the seismicInterpreting the seismic

W-directed middle crustal duplexes could W-directed middle crustal duplexes could represent:represent:

• Imbricated basement substrate, which implies Imbricated basement substrate, which implies foreland to west – difficult to understand given foreland to west – difficult to understand given higher grade and granite abundance in this higher grade and granite abundance in this regionregion

• Inherited seismic fabric from earlier event – Inherited seismic fabric from earlier event – unlikely given volume of melting and reworking unlikely given volume of melting and reworking at 2.7-2.6 Gaat 2.7-2.6 Ga

• Deformation controlling melt migration from Deformation controlling melt migration from source region to higher crustal levelssource region to higher crustal levels

Tectonic wedging Tectonic wedging architecturearchitecture

FLAC3D models coupling FLAC3D models coupling deformation and fluid flowdeformation and fluid flow

• Darcy fluid flow in porous rockDarcy fluid flow in porous rock• Mohr-Coulomb elastic-plastic Mohr-Coulomb elastic-plastic

rheologyrheology• No temperature dependanceNo temperature dependance• No time dependanceNo time dependance

Transfer of deformation Transfer of deformation within orogen within orogen

from thrust wedge to from thrust wedge to interiorinteriorThrusting

velocities

Incremental shear strain

low

high

Potential backthrust formation where shear strain is localizing

FLAC3D model of Yilgarn FLAC3D model of Yilgarn sectionsection

Why topographic elevation Why topographic elevation in the west?in the west?

• Pressures greater in west, Pressures greater in west, not merely higher not merely higher temperaturestemperatures

• Envisage that system is Envisage that system is about to collapse, about to collapse, removing relief and removing relief and exhuming higher grade exhuming higher grade rocks by extensional shear rocks by extensional shear along east-dipping along east-dipping Kunanalling and Ida faultsKunanalling and Ida faults

• Alternative modified Alternative modified model with no topographymodel with no topography

Simulating the generation of fluid Simulating the generation of fluid sources during contractional sources during contractional

deformationdeformation• Scenario 1: Fluid production Scenario 1: Fluid production

in lower crust through in lower crust through dehydration and partial dehydration and partial melting during crustal melting during crustal thickeningthickening

• Scenario 2: Fluid production Scenario 2: Fluid production through uplift and through uplift and decompression melting decompression melting during ongoing during ongoing compressivecompressive deformationdeformation

Fluid source beneath overthrust terrainFluid source beneath overthrust terrain

Fluid source beneath “Kalgoorlie region”Fluid source beneath “Kalgoorlie region”

No topography or fluid sourceNo topography or fluid source

Lateral flow less prominent, but oblique Lateral flow less prominent, but oblique flow in faultsflow in faults

and zones of deformation-induced and zones of deformation-induced dilatancy (brown)dilatancy (brown)

Fluid source Fluid source and and topographytopography

No fluid No fluid source or source or topographytopography

Deformation and fluid flow Deformation and fluid flow modelling modelling

- principal conclusions- principal conclusions• Hydraulic head due to topographic Hydraulic head due to topographic

elevation during contractional elevation during contractional deformation is critical to lateral fluid flowdeformation is critical to lateral fluid flow

• Precise depth and location of fluid source Precise depth and location of fluid source is less important though obviously critical is less important though obviously critical as potential reservoir supplyas potential reservoir supply

• Downwards fluid flow is possible during Downwards fluid flow is possible during compressive deformation given compressive deformation given appropriate fluid pressure gradientsappropriate fluid pressure gradients

Models designed to Models designed to investigateinvestigate

1.1. Architectures and mechanisms that Architectures and mechanisms that promote both lateral and upwards fluid promote both lateral and upwards fluid flow during contractional deformationflow during contractional deformation

2.2. Thermal evolution and potential for Thermal evolution and potential for convective flow systemsconvective flow systems

3.3. Thermal impact of plutons embedded Thermal impact of plutons embedded in regional metamorphic regimein regional metamorphic regime

4.4. Consequences for fluid flow and Consequences for fluid flow and mineralization patterns triggered by fluid mineralization patterns triggered by fluid mixingmixing

1.1. Architectures and mechanisms that Architectures and mechanisms that promote both lateral and upwards fluid promote both lateral and upwards fluid flow during contractional deformationflow during contractional deformation

2.2. Thermal evolution and potential for Thermal evolution and potential for convective flow systemsconvective flow systems

3.3. Thermal impact of plutons embedded Thermal impact of plutons embedded in regional metamorphic regimein regional metamorphic regime

4.4. Consequences for fluid flow and Consequences for fluid flow and mineralization patterns triggered by fluid mineralization patterns triggered by fluid mixingmixing

THERMAL PROCESSESTHERMAL PROCESSESMODELLED SO FARMODELLED SO FAR

• Conductive delay due to plume impact, Conductive delay due to plume impact, and critical temperature thresholds for and critical temperature thresholds for devolatilizing reactions in middle and devolatilizing reactions in middle and lower crustlower crust

• Full-crustal circulation to simulate Full-crustal circulation to simulate regional metamorphic pattern and Hall regional metamorphic pattern and Hall modelmodel

• Effect of smaller scale convective Effect of smaller scale convective processes and embedded plutons to processes and embedded plutons to simulate lateral fluid flow modelssimulate lateral fluid flow models

Rate of thermal evolution Rate of thermal evolution with respect to external with respect to external

factorsfactors• Conductive heat transfer from plume Conductive heat transfer from plume

impingementimpingement• Radiogenic heat productionRadiogenic heat production• Advection through magma emplacementAdvection through magma emplacement• Erosion plus uplift during thrusting leads Erosion plus uplift during thrusting leads

to higher geothermal gradient near to higher geothermal gradient near surface early during orogenesissurface early during orogenesis

• Sedimentation, burial and radiogenic heat Sedimentation, burial and radiogenic heat production lead to higher gradients in production lead to higher gradients in middle crust later during orogenesismiddle crust later during orogenesis

Conductive thermal evolution in the Yilgarn, as a Conductive thermal evolution in the Yilgarn, as a consequence of plume related to komatiites at consequence of plume related to komatiites at

2705 Ma, showing the time at which metamorphic 2705 Ma, showing the time at which metamorphic and melt generation thresholds are attained at and melt generation thresholds are attained at

particular crustal levels (granite data courtesy of particular crustal levels (granite data courtesy of L. Wyborn)L. Wyborn)

2705 Ma2705 Ma komatiiteskomatiites

2680 Ma2680 Ma dolerites dolerites and initial and initial

rifting rifting phase phase

recorded by recorded by Black FlagsBlack Flags

2660 Ma2660 Ma thermal thermal peak in peak in

lower and lower and middle crustmiddle crust

Rate of thermal evolution with Rate of thermal evolution with respect to deformationrespect to deformation

• Influence on geothermal gradientInfluence on geothermal gradient• Influence on rheology and deformation mechanismsInfluence on rheology and deformation mechanisms• Influence on timing of fluid production in hydrous Influence on timing of fluid production in hydrous

sequencessequences

Active Honshu arc compared to post-orogenic Active Honshu arc compared to post-orogenic Yilgarn architectureYilgarn architecture

EGF-01 Yilgarn profile

Model geometry for coupledModel geometry for coupledfluid flow, heat flow and fluid flow, heat flow and

fluid-fluid chemical reactionsfluid-fluid chemical reactions

Hydrostatic pressure gradientHydrostatic pressure gradient

Pressure gradient near lithostaticPressure gradient near lithostatic

Model results - some Model results - some caveatscaveats

• These models simulate fluid-fluid These models simulate fluid-fluid reactions, not fluid-wallrock reactions, not fluid-wallrock reactionsreactions

• Results are highly dependent on Results are highly dependent on permeabilities assigned to crustal permeabilities assigned to crustal units and structuresunits and structures

• Sensitive to (lack of) thermodynamic Sensitive to (lack of) thermodynamic constraints!constraints!

Lithostatic pore pressure Lithostatic pore pressure gradient with no plutons gradient with no plutons

activeactive

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines

m s2 -1X 1 0 -5

- 0 .4- 0 .200 .2

Blue = anticlockwise flowBlue = anticlockwise flow, red = clockwise flowred = clockwise flow

Hydrostatic pressure gradient – thermal effect of Hydrostatic pressure gradient – thermal effect of pluton locationpluton location

Blue = anticlockwise flowBlue = anticlockwise flow, red = clockwise flowred = clockwise flow

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines - P luto n P 3 active

m s2 -1x 1 0 -5

- 0 .200 .20 .40 .6

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P luto n P 4 active

m s2 -1x 1 0 -5

- 0 .100 .20 .40 .6 0 .10 .30 .5

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P lu to n P 1

m s2 -1x 1 0 -5

00 . 4 - 0 . 4 - 0 . 8 - 1 . 2 - 1 . 6

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P lu to n P 2 active

m s2 -1x 1 0 -5

- 0 .200 .20 .40 .60 .8 - 0 .4 - 0 .6 - 1 .0- 0 .8

Effect of pluton location on fluid flow patternsBlue = anticlockwise flowBlue = anticlockwise flow, red = clockwise flowred = clockwise flow

Pluton P2

Pluton P1

Pluton P3

Pluton P4

Effect of pluton location and Effect of pluton location and pressure gradient on convective pressure gradient on convective

streamline patternsstreamline patterns

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines

m s2 -1X 1 0 -5

- 0 .4- 0 .200 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines litho static with no pluto n

m s2 -1X 1 0 -5

- 0 .4- 0 .200 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines

m s2 -1X 1 0 -5

- 0 .4- 0 .200 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines litho static with no pluto n

m s2 -1X 1 0 -5

- 0 .4- 0 .200 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P lu to n P 2 active

m s2 -1x 1 0 -5

- 0 .200 .20 .40 .60 .8 - 0 .4 - 0 .6 - 1 .0- 0 .8

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P lu to n P 2 active

m s2 -1x 1 0 -5

- 0 .200 .20 .40 .60 .8 - 0 .4 - 0 .6 - 1 .0- 0 .8

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P lu to n P 1

m s2 -1x 1 0 -5

00 . 4 - 0 . 4 - 0 . 8 - 1 . 2 - 1 . 6

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines P lu to n P 1

m s2 -1x 1 0 -5

00 . 4 - 0 . 4 - 0 . 8 - 1 . 2 - 1 . 6

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines

m s2 -1x 1 0 -5

- 0 .21 .4 00 .20 .40 .60 .81 .01 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamline L itho static with P luto n P 1 actives

m s2 -1x 1 0 -5

- 0 .21 .4 00 .20 .40 .60 .81 .01 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines

m s2 -1x 1 0 -5

- 0 .21 .4 00 .20 .40 .60 .81 .01 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamline L itho static with P luto n P 1 actives

m s2 -1x 1 0 -5

- 0 .21 .4 00 .20 .40 .60 .81 .01 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines L itho satic w ith P luto n P 2 active

m s2 -1x 1 0 -5

- 0 .21 .4 00 .20 .40 .60 .81 .01 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

F lu id flo w streamlines L itho satic w ith P luto n P 2 active

m s2 -1x 1 0 -5

- 0 .21 .4 00 .20 .40 .60 .81 .01 .2

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 k m

F lu id flo w streamlines

m s2 - 1

- 0 .4- 0 .200 .20 .40 .60 .8

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 km

- 2 0 km

- 4 0 km

2 0 km

0 k m

F lu id flo w streamlines

m s2 - 1

- 0 .4- 0 .200 .20 .40 .60 .8

Ongoing evaluation of Ongoing evaluation of models models

against field-based against field-based constraintsconstraints• Isotopic and geochemical evidence for prograde

or retrograde alteration in specific shear zones, such as

- down-temperature alteration during upflow (K metasomatism)- up-temperature alteration during downflow (Na metasomatism)

• Compare P-T conditions from metamorphic assemblages with temperature distribution predicted by model convection

• Confirm presence of K-feldspar or muscovite or aluminosilicate stability in alteration assemblages predicted by pH distribution for models that couple fluid chemistry

Coupled thermal and fluid flow Coupled thermal and fluid flow modelsmodels

- principal conclusions- principal conclusions• Thermal effect of small plutons

emplaced ahead of a prograding metamorphic front can have a significant impact on the pattern and intensity of fluid transport and convection:

- at distances considerably greater than pluton diameter

- with focussing into adjacent more permeable layers

- promote lateral thermal gradients

Yilgarn numerical modelsYilgarn numerical models- principal conclusions- principal conclusions

• Indicate generic structural sites that are favourable for fluid mixing and gold precipitation

- footwall environments related to major shear zones, such as the Bardoc Shear

- at rheological boundaries within broad antiforms such as the Scotia-Kanowna and Goongarrie–Mount Pleasant Antiforms

• Indicate generic structural sites that are favourable for fluid mixing and gold precipitation

- footwall environments related to major shear zones, such as the Bardoc Shear

- at rheological boundaries within broad antiforms such as the Scotia-Kanowna and Goongarrie–Mount Pleasant Antiforms