Y2: 3D Geological Models of the Eastern Goldfields ...Y2 project team % =

pmd CRC

‘‘Reduced Discovery Risk through Improved Targeting’Reduced Discovery Risk through Improved Targeting’

Y2: 3D Geological Y2: 3D Geological Models of the Eastern Models of the Eastern Goldfields Province, Goldfields Province,

Yilgarn CratonYilgarn Craton

T. FominB. GroenewaldB. GolebyP. HensonM. Nicoll

B. BellR. Blewett T. BrennanK. CassidyD. Champion

Y2 project team % = <4 ft :

pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery

pmd CRC5 questions??What is the geodynamic setting and PT

history of the system?What is the architecture of the system?What are the fluids, their source and/or

reservoirs?What are the fluid flow drivers and

pathways?What are the metal transport and

deposition processes?


pmd CRC• Introduction• Deformation history

(current paradigm)• The “main” event D2• Role of “late basins” in

Eastern Yilgarn geodynamics

• Evidence for switching tectonic mode

• Alternative processes for switching tectonic mode

• Orogenic surge in the Eastern Yilgarn

• Conclusions

Talk OutlineTalk Outline


pmd CRCCurrent EYC deformation frameworkCurrent EYC deformation framework

Greenstones GranitesGreenstone depositionDe early extension early granites (2685-75)

1. D1 N-S compression Late basins post-D1 pre-D2 porphyry (2675)

2. D2 (E-W compression) syn-D2 granite (ca. 2660)MAIN EVENT

post-D2 granite (> 2660)3. D3 (ENE compression)

extensional collapse late-tectonic granite (< ca. 2660)

4. D4 (E-W compression) post-tectonic granite (2620-2600)

Source: Archibald et al., 1981; Swager, 1989, Witt & Swager, 1989;

Williams & Whitaker, 1992; Hammond & Nesbit, 1992; Swager et al., 1997


pmd CRC

Feb 2004 – 17th AGC Hobart

N

Most would have described the ‘fabric’ as ‘S2’However, it is a product of at least 3 ‘D2’ ‘events’

Dyke analogous to late basins as both markers of episodic deformation


pmd CRCDe1 historyDe1 history ~2700 Ma

Swager & Griffin, 1990



Major thickness changes in ‘Upper Basalt’

After Swager & Griffin, 1990



Lower plate?Upper plate?

Williams & Whitaker, 1992

Krapež et al., 2000Major 2765 Ma melting event, rapidly exhumed


pmd CRCD1 historyD1 history


pmd CRCIs D1 visible in NIs D1 visible in N--S lines ?S lines ?

Y4 line Ora Banda looking east

Y1 line looking west through Kanowna


pmd CRC‘‘D2’ the ‘main’ eventD2’ the ‘main’ event

• Main compressional event (~E-W) NNW-trending folds and thrusts

• Peak LP-HT metamorphism

• Associated granites

• Thought to overprint late basinsNY1 seismic lineNext slide


pmd CRCSeismic reflection profile (central EYC)Seismic reflection profile (central EYC)

East Yilgarn is a thick- and thin-skinned fold-thrust belt during the ‘D2’ event 01AGSNY1

384 km in length48 km in depth


pmd CRCLate basinsLate basins

• Elongate NNW-trending basins adjacent to essentially linearsections of major faults

• Last component of greenstones, major unconformity (~2665-2650? Ma)

Geoscience Australiafrom Krapež et al., 2000


pmd CRCLate basinsLate basins

from Krapež, 1997

• Fluvial and/or deep marine with axial derivation

• Fining up sequences, atypical of foreland basins

KurrawangBasin log

Geoscience Australia


pmd CRCOra Banda

Swager and Griffin, 1990

F2a Late basins and ‘D2’ in the EYCLate basins and ‘D2’ in the EYC

Kurrawang syncline

10 km

Blewett et al., in press PCR

Late basin folded (E-W compression) and u/coverlies prefolded (E-W compression) sequence

F2a

Next slide

F2b

F2b


pmd CRCLate basins and ‘D2’ in the EGPLate basins and ‘D2’ in the EGP

Pig Well (Late basin)

W ELate basin folded (E-W compression) and u/c overlies

prefolded (E-W compression) sequence Blewett et al., in press


pmd CRCWhat are the greenstones What are the greenstones saying?saying?

• Episodicity and tectonic mode switching

• Diachroneity?• Wangkathaa

OrogenyModified from Krapez et al., 2000

Consider unconformity-bound sequences

W E

Tim

e

D2aD2ED2b

Blewett et al., in Press PCR


pmd CRC

TimeTime--space evolution space evolution

26502651

2652

2653

2654

2655

26562657 2657

2658 2658

2659 2659

2660 2660

2661 26612662 2662

2663 2663

2664 2664

2665 2665

2666 2666

2667 2667

2668 2668

2669 2669

2670 2670

Ora Banda

Kilkenny

Laverton

Ida

Faul

tD2a

D2E

D2b

D3

De

rc e asi ng ra te o f displa ce ment

Ac tive thrusting

Ac tive thrust// extension

D2E

D ??2E

D2 b

D ??2 b

D2 b

D2a

D2 a

D ??2a

D2E

?

Ma

Ma

Late basin

Granite

Detrital ageIntrusive age

ThrustNormal Fault

Celia

Fau

lt

Ock

erbu

rry F

ault

Geoscience Australia

• Episodicity and tectonic mode switching

• Diachroneity?• Wangkathaa

Orogeny


pmd CRCSetting the sceneGreenstone & Black Flags

Middle crust (gneiss)

Lower crust

Upper crust

GraniteSurface0 Km

6 Km Base of GreenstoneBase of upper crust12 Km

30 Km

40 Km

Base of middle crust

Moho – base of lower crust

500 km

Initial state:

• Post D1 and DE (Formation of Black Flag sequence) and no D1 geometry imposed

• No topography

• Model describes D2 processes (D2A, D2E, D2B) beginning with approximately east-west compression and west-directed thrusting in a foreland / orogen setting.

• Causes could be subduction, slab roll-back, post-orogenic collapse, underplating.


pmd CRC

At time t – progression of D2

Detachment

Lower crust?

D2A D2E D2B

3km

MOHO

D2 is progressive rather than episodic

Fluids pond under detachment

Extruded wedge

New basin forms through wedge extrusion onto foreland

Older basin becomes deformed as D2Bapproaches

Remnant of oldest basin, post - D2B

New thrusts forming

Resultant thickened crust

New granites??West East


pmd CRCD2A: initial D2 compressive phase – building up orogen ahead of middle crust shortening.

T1: thrusting of upper crust (top ~10km) initiated.

Incipient thrust


Active thrustOlder thrust

EW

Greenstone

Granite

Detachment

Progression of D2 is from here west (not back east)

Foreland basin?

σ1σ3

0 Myrs




T2: thrusting progresses westward, with new shallow dipping thrusts formed. Older thrusts are steepened as new thrusts form.


EW

To what extent does the upper crustal thickening affect the lower crust?

What about middle crust thickening?

σ1σ3





EW

T3: thrusting progresses westward. Older thrusts are steepened as new thrusts form and the wedge starts to destabilise.

Mid-crustal shortening & thickening

How many thrust wedges required? Failure angle / pressure?

σ1σ3


pmd CRCD2A: initial D2 compressive phase – building up orogen ahead of middle crust shortening. What part does

erosion play?



EW

T4: Thrust wedge becomes very unstable as new frontal thrust forms.

100km

12km σ1σ3

Faults too steep!!!


pmd CRCD2E: Extension through extrusion of frontal wedge



EW

T5: Frontal wedge extruded as orogen collapses. Extrusion at front causes normal reactivation on some thrust faults and forms a basin near the front of the wedge.

2.5Myrs

σ3σ1


pmd CRCD2B : Second phase of compression



EW

T6: Mid-crustal shortening approaches newly formed basin causing compression and uplift of the region. Basin fault reactivated as thrust fault.

σ3σ1


pmd CRCPost D2B : Regional uplift and preservation of basin

T7: Compressed basin is uplifted with the rest of the upper crust and somewhat eroded. The remnant of the basin is preserved today.


EW



pmd CRCD2BD2ED2A

Driving force –mid crustal shortening

W

N

E

S150km, 5Myrs


pmd CRCD2E

D2BD2A


150km, 5Myrs


pmd CRCBasin

150km, 5Myrs

D2A

D2BD2E



pmd CRCD2B

150km, 5Myrs

Deformed part of basin

D2ED2BD2A


Growth of basin


pmd CRCD2xD2x


pmd CRCD2xD2x

N

N

Regional ‘D2’

Paddington, Ora Banda and Bardoc refolded folds

After Weinberg pers com


pmd CRCDomical detachmentDomical detachmentES

N

N W

D1/D2 +/- D2x type II fold interference of detachment, or D2 imbricate stacks?


pmd CRCFluid focussing in the EGPFluid focussing in the EGP

Domical shape of ‘detachment’ under Kalgoorlie and Kanowna (view vertical downwards)

Kalgoorlie

KB

Surface trace of breaching

fault



Domical shape of ‘detachment’ Kanowna with alteration ‘bleeding’ from breaching fault



Spoon-shaped geometry of Lefroy Shear (brown) with Kalgoorlie in a funnel position


pmd CRCThe detachment as a seismic The detachment as a seismic brittlebrittle--ductile transition?ductile transition?

Magistrale, 2002



6 km

• Did the detachment initiate at ~10-12 km depth during D2a, analogous to a seismogenic brittle ductile transition?

~6 km depth + 7 km erosion –10 km?3 km late basin = 10 km



• Does the detachment represent a fluid interface (Regenauer-Lieb pers comm), or a fluid pondinginterface (Drummond pers comm)?

• Either way, it would be a zone of weakness


pmd CRCIs the detachment a 10 km Is the detachment a 10 km palaeodepthpalaeodepth indicator?indicator?

D1?D2?D3?10 km

D1 D2a D2b+

Late

bas

ins

3km

+ e

rosio

n

Simpler FlatterYounger

~ 10 km


pmd CRCAn example of an important switchAn example of an important switch

Carlin gold Emperor seamount change


pmd CRCSwitching: are the NWSwitching: are the NW--SE compression SE compression events reflecting NEevents reflecting NE--SW extension?SW extension?


pmd CRCImplications for mineral systemsImplications for mineral systems

Late Late orogenicorogenicAuAu

mineralisationmineralisation


pmd CRCImplications for mineral systemsImplications for mineral systemsKalgoorlie lode Au events (Bateman et al., 2002)

Fimiston lodes >2655 Ma

Mt Charlotte – 2630 Ma

Oroya Shoot ~ 2652 Ma

• Are these switches predictive?• Have we mis-assigned Au to the ‘wrong’ event?

Switching ‘points’ in palaeostress


pmd CRC


pmd CRC

pmd CRC


pmd CRC

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