Peter Sorjonen-Ward, Bruce Hobbs, Alison Ord, Yanhua Zhang and Chongbin Zhao

1. Numerical modelling of fluid and heat transport during deformation in the late Archean Yilgarn cratonand its relevance to late orogenic gold mineralization Peter Sorjonen-Ward, Bruce Hobbs, Alison Ord, Yanhua Zhang and Chongbin Zhao CSIRO Exploration and Mining Exploration Geodynamics Chapman Conferemce

2. Numerical modelling applications to orogenic gold mineralization in the YilgarnScope of presentation • Yilgarn architecture and boundary conditions • Coupled fluid flow and deformation • Coupled thermal and fluid flow models Modelling here is addressing potential viability of fluid pathways, not constrained by mass balance or time considerations

3. Generating and sustaining a mineral system requires • An architecture that enhances fluid flow with • efficient fluid-rock interaction in the source region • efficient focussing into depositional site • Mechanisms for timely fluid production • P-T conditions and fluid chemistries that optimize extraction and depositional efficiency

4. Models for Yilgarn fluids and gold- 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 reservoir Implications: • Fluid flow across lateral as well as vertical temperature gradients • Fluid production linked to deformation and thermal evolution

5. Yilgarn geology and magnetics Low-pass filtering by Paul Gow

6. Yilgarn structural domains Eastern Goldfields Province Southern Cross Province

7. Magmas and fluids –regional scale • Large scale magnetic anomalies relate to monzogranites emplaced to present level within 10 Ma of mineralization • Dominant gold mineralizing fluids are weakly reducing, weakly acidic and of low salinity • Evolved isotopic signatures suggest interaction with – though not necessarily derivation from granitic lower and middle crust

8. Yilgarn mineralization broadly synchronous across range of metamorphic grades? South Polaris deposit in Southern Cross Province Gold deposited in equilbrium with diopside and K-feldspar Racetrack deposit in Ora banda domain Sub-greenschist facies gold deposition

9. Critical structural elements and requirements 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

10. Models designed to investigate • Architectures and mechanisms that promote both lateral and upwards fluid flow during contractional deformation • Potential for convective flow systems • Thermal impact of plutons embedded in regional metamorphic regime • Consequences for fluid flow and mineralization patterns triggered by fluid mixing

11. Models designed to investigate • Architectures and mechanisms that promote both lateral and upwards fluid flow during contractional deformation • Potential for convective flow systems • Thermal impact of plutons embedded in regional metamorphic regime • Consequences for fluid flow and mineralization patterns triggered by fluid mixing

12. Generating sufficient fluids in the right place at the right time • 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?

13. Intrusive sheets in basal part of Karakoram Batholith Deformed amphibolites and intrusive sheets at base of Karakoram Batholith

14. Lithostatically overpressured system – requires sustained fluid supply

15. Symmetry and asymmetry

16. Interpreting the seismic W-directed middle crustal duplexes could represent: • Imbricated basement substrate, which implies foreland to west – difficult to understand given higher grade and granite abundance in this region • Inherited seismic fabric from earlier event – unlikely given volume of melting and reworking at 2.7-2.6 Ga • Deformation controlling melt migration from source region to higher crustal levels

17. Tectonic wedging architecture

18. FLAC3D models coupling deformation and fluid flow • Darcy fluid flow in porous rock • Mohr-Coulomb elastic-plastic rheology • No temperature dependance • No time dependance

19. Transfer of deformation within orogen from thrust wedge to interior Thrusting velocities Incrementalshear strain low Potential backthrust formation where shear strain is localizing high

20. FLAC3D model of Yilgarn section Why topographic elevation in the west? • Pressures greater in west, not merely higher temperatures • Envisage that system is about to collapse, removing relief and exhuming higher grade rocks by extensional shear along east-dipping Kunanalling and Ida faults • Alternative modified model with no topography

21. Simulating the generation of fluid sources during contractional deformation • Scenario 1: Fluid production in lower crust through dehydration and partial melting during crustal thickening • Scenario 2: Fluid production through uplift and decompression melting during ongoing compressive deformation

22. Fluid source beneath overthrust terrain

23. Fluid source beneath “Kalgoorlie region”

24. No topography or fluid sourceLateral flow less prominent, but oblique flow in faults and zones of deformation-induced dilatancy (brown)

25. Fluid source and topography No fluid source or topography

26. Deformation and fluid flow modelling - principal conclusions • Hydraulic head due to topographic elevation during contractional deformation is critical to lateral fluid flow • Precise depth and location of fluid source is less important though obviously critical as potential reservoir supply • Downwards fluid flow is possible during compressive deformation given appropriate fluid pressure gradients

27. Models designed to investigate • Architectures and mechanisms that promote both lateral and upwards fluid flow during contractional deformation • Thermal evolution and potential for convective flow systems • Thermal impact of plutons embedded in regional metamorphic regime • Consequences for fluid flow and mineralization patterns triggered by fluid mixing

28. THERMAL PROCESSESMODELLED SO FAR • Conductive delay due to plume impact, and critical temperature thresholds for devolatilizing reactions in middle and lower crust • Full-crustal circulation to simulate regional metamorphic pattern and Hall model • Effect of smaller scale convective processes and embedded plutons to simulate lateral fluid flow models

29. Rate of thermal evolution with respect to external factors • Conductive heat transfer from plume impingement • Radiogenic heat production • Advection through magma emplacement • Erosion plus uplift during thrusting leads to higher geothermal gradient near surface early during orogenesis • Sedimentation, burial and radiogenic heat production lead to higher gradients in middle crust later during orogenesis

30. 2660 Mathermal peak in lower and middle crust Conductive thermal evolution in the Yilgarn, as a consequence of plume related to komatiites at 2705 Ma, showing the time at which metamorphic and melt generation thresholds are attained at particular crustal levels (granite data courtesy of L. Wyborn) 2680 Madolerites and initial rifting phase recorded by Black Flags 2705 Makomatiites

31. Rate of thermal evolution with respect to deformation • Influence on geothermal gradient • Influence on rheology and deformation mechanisms • Influence on timing of fluid production in hydrous sequences EGF-01 Yilgarn profile Active Honshu arc compared to post-orogenic Yilgarn architecture

32. Model geometry for coupledfluid flow, heat flow and fluid-fluid chemical reactions

33. Hydrostatic pressure gradient

34. Pressure gradient near lithostatic

35. Model results - some caveats • These models simulate fluid-fluid reactions, not fluid-wallrock reactions • Results are highly dependent on permeabilities assigned to crustal units and structures • Sensitive to (lack of) thermodynamic constraints!

36. Lithostatic pore pressure gradient with no plutons active Blue = anticlockwise flow, red = clockwise flow

37. Hydrostatic pressure gradient – thermal effect of pluton locationBlue = anticlockwise flow, red = clockwise flow

38. Effect of pluton location on fluid flow patternsBlue = anticlockwise flow, red = clockwise flow Pluton P3 Pluton P1 Pluton P4 Pluton P2

39. Effect of pluton location and pressure gradient on convective streamline patterns

40. Ongoing evaluation of models against field-based constraints • 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

41. Coupled thermal and fluid flow models- 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

42. Yilgarn numerical models- 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