Acknowledgments. Rick Sibson for the Fault valve concept - the seminal work in this area

Transcription

1 Self-Organised Critical Systems and Ore Formation: The Key to Spatially Predictive Targeting? Jon Hronsky October

2 Acknowledgments David Groves, for teaching me and many others how to think about ore deposits and almost a quarter of century of inspiration and support Rick Sibson for the Fault valve concept - the seminal work in this area 2

3 Every Science Needs an Organising Framework Otherwise it is Just a Collection of Observations SEA LEVEL "Trap" controlled by fluid-rock interactions/p-t "Seal" controls pathway Migration pathway - rock interactions INCR. TEMPERATURE WITH DEPTH? Timing of movement? So urce for metals & brines 3

4 Basic Physics of Ore Formation Mass Sink Concentrated Metal Deposit Advective Mass flux (= Energy Flow) Diffuse Metal Source Region 4

5 Complex Systems Non-equilibrium, energy-flow systems which spontaneously organize themselves so complex patterns emerge These complex patterns are not predictable from studying individual components Only exist as long as they are open to energy flow 5

6 Belousov-Zhabotinsky Reaction Hurricane Benard Cells Examples of Spontaneously Generation of Order 6

7 Schneider & Sagan (2005) Self Organization as a response to potential energy gradients: Benard Convection Example 7

8 Schneider & Sagan (2005) More Efficient Energy Dissipation via Self Organization: The top bottle drains in 11 seconds using the Tornado in a Bottle connector to initiate a vortex - compared with six minutes without. 8

9 SOC Systems Bak, Tang & Weisenfeld (1987) first described an important category of complex systems; Self- Organized Critical (SOC) Systems Primary descriptive characteristics : energy release events show a scale-invariant power-law behaviour spatial manifestation shows fractal geometry Seismogenic crust considered classic example of this phenomena in nature 9

10 The Gutenberg-Richter Law for Earthquakes 10

11 Dynamic Characteristics of SOC Systems -1 Driving energy flux added slowly A local threshold exists which provides an impediment to energy flow through the system Critical for self-organization System comprises many interacting components and system dynamics are dependent on these complex interactions Energy release events occur as rapid, transient events termed avalanches 11

12 Dynamic Characteristics of SOC Systems -2 Avalanches exhibit scale-invariant, power-law behaviour over a large range of scales SOC systems exhibit extremal dynamics: avalanches will predominately occur where the net threshold barrier is smallest: Local heterogeneity is very important! As long as the system continues to be driven, it maintains itself globally in a stationary state (energy flux in balanced by energy flux out, over time) 12

13 Simple Model for a Self-Organized System Energy Sink Energy Flux Reduced Exergy Released in transient Avalanches Potential Energy Gradient Threshold Barrier Self-Organized System Entropy (exported to environment as diffuse heat) Energy Flux fed into system at a slow rate Energy Source 13

14 Ore Formation: The Primary Hypothesis Many (if not most) Ore-forming systems are examples of Self Organized Critical systems In these systems, energy flux occurs primarily as advective fluid flux (fluid here includes magma) The fluids involved in ore-formation have a continuum with general crustal fluids; the key to ore-formation is the dynamic processes that concentrate their mass flux Chemical processes of ore-deposition influence the nature and location of mineralization within the system but are a second-order effect Self-Organized Critical behaviour is essential for oreformation because it is the only viable physical mechanism in the crust to produce the concentrated mass fluxes required for ore-formation 14

15 Lines of Evidence-1 Power-law size frequency distributions Fractal spatial distributions Evidence for transient, multiple pulses of intense fluid flow Strong association of many deposits with pipe-like conduit zones of extreme permeability and fluid/magma flow 15

16 Lines of Evidence-2 Common association with obvious threshold barriers to flow: Antiformal seals Local zones of compressional deformation Faults poorly oriented for reactivation (eg Sibson fault-valves ) Strong association with localized zones of structural heterogeneity (ie zones of easiest failure) Evidence for continuum of compositions between ore-fluids and background crustal fluids Evidence for ineffectiveness of convection in most geologically relevant situations 16

17 Different Provinces have Different Size-FrequencyDistributions Source: Nick Hayward (Written Comm; 1999)

18 Fractal Spatial Distribution of Mineral Deposits in Basin and Range, W. USA 18 (Carlson, 1991) Random Distributions Modeled Fractal distribution Observed deposit distribution

19 Evidence for Multiple Transient Fluid Flux Events A fundamental characteristic of all significant ore-systems is that they represent multiple dynamic episodes Multiple overprinting generations of veins and brecciation Thermal constraints on basin-hosted base-metal deposits indicates they must form by episodic pulses of concentrated fluid flow (eg Cathles & Smith, 1983) 19

20 Multicyclic Veining in Orogenic Gold deposits (A: Mother Lode, California (Goldfarb et al. 2005) B,C: Bendigo (Cox, 2005) 20

21 Multicyclic Mineral Deposition in MVT deposits Cathles (2005) 21

22 Extreme Permeability and Fluid Flux in Conduits Many ore deposits are clearly associated with pipe-like volumes of very high fluid/magma flux, clearly emplaced very dynamically These must link an overpressured fluid reservoir at depth to some fluid sink These conduits represent extreme physical processes, quite different from background fluid flow processes ; eg Chonoliths Immediately post a seismic event, some fault-related damage zones are associated with transient extreme fluid permeability and fluid flow. Miller et al (2004) : localised transient, post-seismic permeability of 4 x m 2 ; 10 5 to 10 6 times > than background crustal permeability at that depth (6km) 22

23 Magmatic Examples of Fluid Exit Conduits Examples of Chonoliths from the Noril sk Camp Zen ko and Czamanske

24 Babel-Nebo (Hronsky, 2003) Chonoliths -- more examples Nkomati (Li et al, 2002) Kabanga North Kabanga Main Shearzone Kabanga (Wolfgang Maier) 24

25 Hydrothermal Examples of Fluid Exit Conduits ~200mm ~100m New Holland: Example of host-rock rheology controlling the geometry of a conduit (Henson, 2008)

26 Section view Fitzroy Fault and Au distribution (gold blobs): Image from Gocad looking SW? Strongly fault controlled Kanowna Belle Example (Henson, 2008) 100m Image from: Carl Young

27 Ernest Henry IOCG deposit: Pipe-like breccia zone (Cleverley, 2008)

28 Barriers to Flow: Grasberg Example Strongly Compressional Setting Grasberg ~10 km basement Rift basalts and volcanics Source: Kevin Hill

29 Ore-Forming Fluids are part of a Crustal Continuum Cl (ppm) Chemical data from a wide range of crustal fluids: Red boxes = from Ore deposits (Barnicoat, 2008 modified after Yardley, 2005) 29

30 Average Crustal Permeability generally too small for Convection Minimum permeability for convection (geotherm = 30 /km) Average permeability versus depth: Field-based estimates (Manning & Ingebritsen 1999) Lab values (Shmonov et al. 2003) (From Sheldon, 2008)

31 A General Model for Ore-forming SOC Systems Fluid Sink Threshold Barrier (need not be a physical seal) Episodic focused energy and mass flux Thermal Halo-produced by entropy dumped into environment Transient Exit Conduit Fluid Reservoir Slow persistent fluid flux Fluid (Energy) Source 31

32 Modern Example: NE Honshu Slide from Sibson (2009); Data from Hasegawa et al. (2005) 32

33 Targeting Implications 33

34 1. ANTIFORMAL SEAL Need a Localised Threshold Barrier ( Throttle ) and Paleo- Overpressured Reservoir Relatively impermeable unit (eg shale) Overpressured fluid reservoir Fluid source 2. BASEMENT HORST / BASEMENT MARGIN SEAL Relatively impermeable sequence Aquifer Unit Density driven downward fluid flow Relatively impermeable basement 3. INTRUSION CARAPACE SEAL Carapace of solidifying intrusion

35 Ore Location within the System has implications for Ore geometry and targeting 2A. Discharge Zone-Hosted Surface Sink Surface - (boundary with hydrosphere or atmosphere) Discharge zone Fluid Sink Ore Forming Ore Forming Fluid feeder conduit 2B. Discharge Zone-Hosted Sub-surface Sink Strata sink Discharge zone Fluid Source Ore Forming 1. Conduit-Hosted Fluid feeder conduit 35

36 Fault Geometry and Ore-fluid Focusing : The Existing Paradigm Source: Montpellier University Website 36

37 But no consistent structural relationship between fault veining and mineralisation! Cracow Epithermal Gold Deposit Mickelthwaite (2008) 37

38 Fluids do not respond passively to structure : They create their own Pipes! Modeled Changes in Coulomb Failure Stress post rupture no correlation with aftershock swarm Modeled Changes in Pore Fluid Pressure post rupturegood correlation with aftershock swarm 1997 Umbria-Marche EQ Miller et al (2004) 38

39 Conduit-focused rather than Structure-focused Targeting Perspective 39

40 Repetitive Self-Organisation using the Same Rock Volume: Need to Understand Why! 40

41 The Temporal Dimension to Targeting 41

42 Example: Tampakan District, Mindanao Red area = amount of convergence partitioned into intra-plate shortening during subduction reversal. From Rohrlach (2002)

43 Conclusion: The Evolution of Perspective in Economic Geology Early 1900s Ore specimencentric Host Rock - centric Structure-centric 21 st Century Fluid Flux-centric 43