Ore-inspiring Ore-inspiring structuresstructures

- - some numerical modelling some numerical modelling perspectives on orogenic perspectives on orogenic

architectures favourable for architectures favourable for formation and preservation formation and preservation

of mineral depositsof mineral deposits

Peter Sorjonen-Ward, Paul GowPeter Sorjonen-Ward, Paul Gow11, ,

Phaedra UptonPhaedra Upton22 Yanhua Zhang Yanhua Zhang

CSIRO Exploration and MiningCSIRO Exploration and Mining

www.dem.csiro.auwww.dem.csiro.auCurrent addresses Current addresses 11 pagow@mim.com.aupagow@mim.com.au22 phaedra.upton@stonebow.otago.ac.nz phaedra.upton@stonebow.otago.ac.nz

Purpose of presentationPurpose of presentation

• Consider orogenic architecture that Consider orogenic architecture that favours both formation and preservation favours both formation and preservation of depositsof deposits

• Review concept through coupled Review concept through coupled numerical models of deformation and numerical models of deformation and flow based on flow based on – Archean Yilgarn craton– Modern PNG collisional zone

• Smaller scale aspects not discussed here!Smaller scale aspects not discussed here!

Do mineral systems Do mineral systems represent this?represent this?

Or do they change with scale Or do they change with scale like this?like this?

Butterflies by M C Escher, 1950

Requirements for the Requirements for the formation and preservation formation and preservation

of ore depositsof ore deposits• Critical architectures that efficiently transport and Critical architectures that efficiently transport and

focus mineralizing fluidsfocus mineralizing fluids• Faults as episodic channels or seals – feedback Faults as episodic channels or seals – feedback

betweenbetween– strain softening or hardening – rupture, dilation and precipitation of minerals

• Pervasive versus partitioned flow and access to rockPervasive versus partitioned flow and access to rock• Geodynamic settings that favour preservation of Geodynamic settings that favour preservation of

depositsdeposits– Porphyry and epithermal systems dominant in young

mountainous terrain– Late-orogenic lode gold deposits in greenschist facies and

higher grade terrain, from Archean to Cainozoic

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

timetime “structural control of ore deposits only takes place on faults

that were active at the time that the hydrothermal system was active” Mike Etheridge, 2000

Hence, active coupling between fluids and deformation

• In some terrains where architecture is potentially favourable, fluid production is ill-timed with respect to thermal peak

• In some terrains, architecture is inappropriate – faults do not form connected network for accessing fluids

• In some terrains, fluid supply is the limiting factor networks

Additional paradox of high fluid flux with relatively low strain

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

timetimeWhat processes and sources generate an adequate fluid supply?What processes and sources generate an adequate fluid supply?

– Granulitic lower crust inappropriate since already dehydrated– Fluids exsolved during crystallization of volatile-rich granites – Local metamorphic devolatilization – Rapidly formed accretionary prism could provide a more

steady supply of fluid, but in many cases mineralization is late

– Orogenically derived meteoric fluids if downdraw is feasible– Basinal fluids in submergent foreland basin or extending arc

terrain – if salinity of mineralizing fluids is consistent– Mantle degassing – does permeability structure of lower crust

permit sufficient fluid transfer?

• Thermomechanical modelling at Thermomechanical modelling at orogenic scale well advanced orogenic scale well advanced

• FLAC3D coupling of deformation FLAC3D coupling of deformation and fluid flowand fluid flow– Darcy fluid flow in porous rock– Mohr-Coulomb elastic-plastic rheology– Feedback between fluid pressure and

rock failure– No temperature dependance– No time dependance

Modelling orogenic Modelling orogenic architecrturearchitecrture

Mechanisms for enabling fluid flow Mechanisms for enabling fluid flow throughthrough

low permeability environments low permeability environments Lithostatically overpressured system – Lithostatically overpressured system –

requires sustained fluid supplyrequires sustained fluid supply

Critical orogenic Critical orogenic architecture for generating architecture for generating

ideal depositional sitesideal depositional sites• Dilational jogs in strike-slip systems are commonly Dilational jogs in strike-slip systems are commonly

invoked, based on earthquake research invoked, based on earthquake research – Regional analysis often suggests this, but detailed

studies often show more complex features

• Importance of thrust-related subhorizontal systemsImportance of thrust-related subhorizontal systems– Yilgarn, PNG, central Asia (Muruntau)

• Interaction between thrusts and reactivated Interaction between thrusts and reactivated transfer structures also considered importanttransfer structures also considered important

• Need to compromise between flow network that Need to compromise between flow network that maximizes fluid-rock or fluid-fluid interaction, maximizes fluid-rock or fluid-fluid interaction, without resulting in dispersion rather than focussed without resulting in dispersion rather than focussed depositiondeposition

Regional impression

Left-stepping sinistral dilational jog

Local environmentBack rotation within

contractional oblique-slip duplex

Pampalo deposit, Finland

0m

3000m

-3000m

Fubilan MonzonitePorphyry

Parrots BeakThrust+

+++

++

Ieru Fm

Darai Fm

Deposits in Deposits in hanging-wall hanging-wall of thrust systems:of thrust systems:Porphyry Cu/Au Porphyry Cu/Au deposits deposits in PNG fold beltin PNG fold belt

Grasberg Deposit (plan view) Grasberg Deposit (plan view) from Widodo et al.,1999from Widodo et al.,1999

Ok Tedi Deposit (cross-section)Ok Tedi Deposit (cross-section)from Mason (1994)from Mason (1994)

From Mason (1994)

GRASBERGIntrusiveCom plex

Kucing L iarM ineralisation

+ +

+

++

+

+Idenberg #1 Fau lt

Idenberg #2 Fault

1000m

Fluid flow in thrust terrain Fluid flow in thrust terrain controlled by hydraulic head, controlled by hydraulic head, deformation and permeabilitydeformation and permeability

Homogeneous permeability Highly permeable thrust

Fluid sources related to melting and metamorphism

High permeability in basal thrust and footwall stratigraphy

Some regional numerical models Some regional numerical models relating to mineralization during relating to mineralization during

convergenceconvergence• Interaction between thrusts and oblique Interaction between thrusts and oblique

convergence in PNGconvergence in PNG– Correlation between mineralization, uplift rate

and reactivation potential of transfer faults

• Divergent compressive structures in Divergent compressive structures in YilgarnYilgarn– Promoting lateral fluid flow and variable uplift

to maximize potential for thermal and pressure gradients and mixing of diverse fluids

PNG tectonic settingPNG tectonic setting

0 200

kilometres

400

2 º S

approximate limit of Coral Sea sp reading

Australian

Plate

Caroline Plate

Solomon SeaPlate

Bismarck SeaPlate

Mussau

Trench

Ok Tedi

Frieda

Grasberg

Porgera

Manus TrenchNew Guinea Trench

Fly Platform

Mobile Belt

Thrust Belt

Fold and

Accreted Arc Terrane

Indo-Austra lianP late moving n orth

Pacific Plate moving west

2º S

148º E133º E

PNG FLAC3D model PNG FLAC3D model geometrygeometry

indentorm obile beltextensional feature

strongplatform weak platform

fold beltarc-n

ormal

transfe

r

Mobile BeltFold BeltFlyPlatform

PortM oresbyKilom etres

0 250 500

Architectural elements•Terranes of different strength•Contrast in platform strength•Arc-normal inherited transfer faults•Shelf-edge extensional fault

Dynamic Elements•Oblique sinistral collision

•Convergence angle at15º and 45º

•1-2% shortening1000 km

600 km

c)

Modelling volumetric strainModelling volumetric strainCollision Obliquity: 45°

Collision Obliquity: 15°

0

0.5

1.0 %

- Greater volumetric strain at higher collision angles

- associated with vertical extension

- most pronounced where weak structures cut fold belt

Contours of vertical displacement dark = higher values

45°

15°

m ax zd = 2e3

m ax zd = 5e2

W eak/Strong A ustra lian C rust

45°

15°

m ax zd = 3 .5e3

m ax zd = 1 .4e2

W eak E xtensiona l fea ture

45°

15°

m ax zd = 3 .5e3

m ax zd = 1 .4e2

W eak E xtensiona l Feature and W eak A rc-N orm al Structures

45°

15°

m ax zd = 2e3

m ax zd = 5e2

W eak A rc-N orm al Structures

(a) (b)

(c) (d)

Effect of varying Effect of varying strength of strength of crustal units and crustal units and transfer faultstransfer faults

indentorm obile beltextensional feature

strongplatform weak platform

fold beltarc-n

ormal

transfe

r

Crustal uplift rates in PNG Crustal uplift rates in PNG collision zonecollision zone

Modelling vertical Modelling vertical displacementdisplacement

Modelling vertical Modelling vertical displacementdisplacement

0

500

1000

1500

2000

2500

3000

3500

4000

4500

300 350 400 450 500 550 600 650 700 750 800

distance (km)

Ele

vati

on

(m

)

PNG Elevation

Irian Jaya Elevation

south north

0

500

1000

1500

2000

2500

3000

3500

4000

4500

300 350 400 450 500 550 600 650 700 750 800

distance (km)

Ele

vati

on

(m

)

PNG Elevation

Irian Jaya Elevation

south north

Reactivated extensional structure adds peak

Current day topography

Greater uplift against strong Australian crust

10- 1 5

10- 1 8

10- 1 7

10- 1 3

Mesh

Initial IntrinsicPermeability (m )2

CumulativeVolumetricStrain

CumulativeFluid Flux

Southeast Northwest

fluid fluxmaxima

High volume strain at the northern end of the fold belt

(a)

(b)

(c)

(d)

Incipient development of “pop-up” in uplifted region

indentorm obile beltextensional feature

strongplatform weak platform

fold beltarc-n

ormal

transfe

r

Fold belt Indentor

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

Some regional numerical models Some regional numerical models relating to mineralization during relating to mineralization during

convergenceconvergence• Interaction between thrusts and oblique Interaction between thrusts and oblique

convergence in PNGconvergence in PNG– Correlation between mineralization, uplift rate

and reactivation potential of transfer faults

• Divergent compressive structures in Divergent compressive structures in YilgarnYilgarn– Promoting lateral fluid flow and variable uplift

to maximize potential for thermal and pressure gradients and mixing of diverse fluids

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

Symmetry and asymmetrySymmetry and asymmetry

Tectonic wedging Tectonic wedging architecturearchitecture

• Allows uplift with preservation of sealAllows uplift with preservation of seal• Lateral variations in thermal structureLateral variations in thermal structure• Lateral fluid flowLateral fluid flow• Role of footwall rheologyRole of footwall rheology

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

Fluid focussing in tectonic wedgesFluid focussing in tectonic wedges

Bardoc shearnot dilatingat depth

Fluid source beneath “Kalgoorlie region”Fluid source beneath “Kalgoorlie region”- Bardoc shear still not active conduit- Bardoc shear still not active conduit

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

Yilgarn 2D FIDAP thermal convective chemical model

Precipitation of Au (blue)

Dissolution regions (red)

Maximum precipitation rate: 10.6 ppm per million years

Geometry and permeability Geometry and permeability structures control temperature structures control temperature distributions and fluid mixing distributions and fluid mixing which in turn control the which in turn control the locations of gold precipitationlocations of gold precipitation

Yilgarn numerical modelsYilgarn numerical models- principal conclusions”on tectonic - principal conclusions”on tectonic

wedgingwedging

• 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

General implications of General implications of tectonic wedging architecture tectonic wedging architecture

- Potential to create fault-bounded domains of Potential to create fault-bounded domains of differential uplift and overpressuring differential uplift and overpressuring beneath relatively impermeable unitsbeneath relatively impermeable units

- Generates opportunities for mixing of Generates opportunities for mixing of separate fluids or destabilization through separate fluids or destabilization through rapid changes in pressure and temperature rapid changes in pressure and temperature

- May also contribute to the formation and May also contribute to the formation and preservation of greenschist facies deposits, preservation of greenschist facies deposits, in contrast to the lower long term in contrast to the lower long term preservation potential for deposits formed in preservation potential for deposits formed in elevated foreland fold and thrust belts.elevated foreland fold and thrust belts.

General implications and General implications and speculationspeculation

• Reinforces the dynamic feedback between deformation, Reinforces the dynamic feedback between deformation, magmatism and fluid production and migrationmagmatism and fluid production and migration

• Requires that magmatic and metamorphic fluid generation Requires that magmatic and metamorphic fluid generation is precisely timed with respect to deformation is precisely timed with respect to deformation

• Alternative fluid – and possibly heat - sources required if Alternative fluid – and possibly heat - sources required if lower crust is already anhydrouslower crust is already anhydrous

• Importance of post-collisional subsidence and waning Importance of post-collisional subsidence and waning volcanism volcanism – Skellefte district, Sweden– Tasmanian Cambrian – Yilgarn

• Need to study orogenic systems to identify wedging Need to study orogenic systems to identify wedging architectures, potentially througharchitectures, potentially through– Early deformation and polarity reversal– Facies changes recording subsidence during

compression

Implications for (future) PNG Implications for (future) PNG mineralizationmineralization

• What will prevent loss of deposits formed at high What will prevent loss of deposits formed at high crustal levels in areas of rapid uplift?crustal levels in areas of rapid uplift?

• Could deposits also be forming at depth equivalent Could deposits also be forming at depth equivalent to greenschist or amphibolite facies?to greenschist or amphibolite facies?

• If this is the case, then would greenschist facies gold If this is the case, then would greenschist facies gold deposits be exhumed within sinistral strike-slip deposits be exhumed within sinistral strike-slip systems orthogonal to recent granite-related systems orthogonal to recent granite-related transfer trend?transfer trend?

• Changes in convergence vector expressed asChanges in convergence vector expressed as– variations in uplift rate and hence lateral variations in

metamorphic grade – systematic changes in simple shear kinematic component

in deeper, orogen-parallel ductile shear zones

Future shear Future shear zone-hosted gold zone-hosted gold deposits to be deposits to be exhumed asexhumed asPNG fold belt is PNG fold belt is translated translated westwards?westwards?

low

high

Incremental shear strain

Potential Potential backthrust backthrust formation where formation where shear strain is shear strain is localizinglocalizing

Orogenic processes, Orogenic processes, mineralization and preservation mineralization and preservation

potentialpotential• Rifting and subsidence of arc maybe criticalRifting and subsidence of arc maybe critical• PNG deposits related to rapid uplift of elevated PNG deposits related to rapid uplift of elevated

terrain during ongoing plate convergence driven terrain during ongoing plate convergence driven uplift of elevated terrainuplift of elevated terrain

• Tectonic wedgingTectonic wedging– Provides potential for seal and lateral

gradients in fluid pressure and supply– Potential for preservation compared to

mineral systems formed in elevated terrain, if isostatic and thermal history appropriate

– Local extensional domains but essentially compressive yet with decompression

Unfavourable orogenic Unfavourable orogenic architectures?architectures?

• Orthogonal collision with aborted subduction of buoyant Orthogonal collision with aborted subduction of buoyant cratonic forelandcratonic foreland

• Rapid isostatic response and limited magmatismRapid isostatic response and limited magmatism• Examples include:Examples include:

– Helvetic – Penninic nappes in Alpine system – Caledonian in Norway– 1.93-1.90 Ga stage of Svecofennian Orogeny

• ““Steady-state” orthogonal subduction beneath Steady-state” orthogonal subduction beneath continental margincontinental margin

• Examples include:Examples include:– Cretaceous Shimanto accretionary complex, despite

sediment supply and postulated ridge subduction as anomalous thermal source

The endThe end

Yes, it really isYes, it really is

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