T. Campbell McCuaig - Centre for Exploration Targeting. FUTORES Noel White Symposium, Townsville, June 2013

Transcription

1 Multiscale structural controls on mineral systems T. Campbell McCuaig - Centre for Exploration Targeting FUTORES Noel White Symposium, Townsville, June 2013

2 The Problem Ga Ni and Au in the Yilgarn WA How to predict location and geometry of new mineral districts camps - oreshoots? Craton / district scale Deposit scale ves Au er et al ) 2km St. I (Mill Camp scale 400km Oreshoot scale 150km ig et al ) (McCua 100m New Ho olland Au (Henson n, 2008)

3 Fundamental premise: Architecture as a Multiscale Fluid (incl Magma) Delivery System

4 Two fundamental controls Whole-lithosphere architecture A new look at old lineaments Mineralisation as a consequence of self-organising critical systems A fluid-physics based approach

5 Take Away Messages Now recognise multiscale fluid-flow flow conduits as the primary element of ore-forming hydrothermal systems. Long-lived vertically-accretive structures are key pathways for metal transfer through the lithosphere. These are not the obvious structures at or above the level of mineralisation an important message for targeting. Do not need ACTIVE structures for oreshoots at deposit scale. Kinematics is of limited use for oreshoot targeting. Replace the historical i structure-centric t ti framework with a fluid physics-centric framework (SOCS). Key is favourable architecture in 3D acting as lightening rods for transient oreshoot forming events. Threshold barriers to mass and energy transfer are critical for the formation of highest quality deposits need to understand them and map by proxy in geoscience datasets.

6 Whole lithosphere architecture and mineral systems location through time 150km Links to Deposit Scale - Isotopic maps as proxies for terrane architecture through time - Explains distributions of Ni (red), Au (gold) McCuaig et al. (2010) Sm-Nd map from Champion and Cassidy (2007)

7 Ka am mb ba ald da St. Iv ve es 2700Ma - Fundamental architecture reactivated through time - Komatiites (and 2700 Ma Ma Ma (Miller et al. 2010) Ma Ma

8 Thrust architecture established ca. 42Ma ina Reactivated and mineralised ca. 9Ma An nta am mi Pe eru 1km 1km McCuaig (2003; courtesy of Antamina)

9 An nta am mina Pe eru 10km Controls stratigraphy uvariations Fundamental architecture reactivated over >150Ma 100km Main cross-orogenorogen control is cryptic, not marked by discrete structure at surface BASIN MARGIN (Love et al., 2004)

10 Vertical Accretive Growth History of Fundamental Lithospheric Flaws

11 Common Characteristics of Large Scale Ore-Controlling Structures Strike-extensive. Depth-extensive t (often lithospheric h i mantle) with relatively steep dips (as imaged in geophysics). Commonly yjuxtapose distinctly different basement domains. Multiply-reactivated (commonly with variable senses of movement) with a very long history movement) with a very long history. Vertically-accretive growth histories. These are not the obvious structures at or above the level of mineralisation an important message for targeting.

12 Anastomosing Near-Surface Pattern overlying Fundamental Structure at depth Sierra Foothills Gold Province, California. From Bierlein et al (2008)

13 Current paradigm for structural targeting Structure-deformation driven Assumes oreshoots are generated by dynamic syn-ore deformation. assumes knowledge of structural geometry and inferred syn-ore stress field can be used to predict zones of anomalous fluid flow. Assumes localised mineralised volumes are intrinsically a property of these structures. Therefore: Determine active structures. Understand kinematic history. Predict space problems on faults and hunt them, particularly at intersection with favourable lithology (chemistry and rheology).

14 Some limitations of structure-deformation approach to targeting Oreshoots not following kinematic model (common). Vertical stockwork pipes with no bounding shear zone/fault (common). Multiple hydrothermal events widely spaced in time using same pathway under different kinematic histories (common).

15 A new view on how fluid flow is organised through whole-lithosphere lithosphere architecture Key points: Mineralisation events are transient events (avalanches) within much longer barren deformation-magmatism-alteration events. Multiple repeated pulses Scale-invariant power law distribution of deposit sizes. Deposits display fractal spatial geometry. These are all attributes of self-organised critical systems

16 Model for Self-Organised System Energy Sink Energy Flux Release in transient Avalanches Potential Energy Gradient Threshold Barrier Self-Organized System Entropy (exported to environment as diffuse heat) Energy Source Energy Flux fed into system at a slow rate (Hronsky, 2011)

17 Breaching of a Threshold Barrier Electric Charges Accumulate Slowly Transient Rapid Breach of Threshold Barrier Threshold Barrier: Resistive Air Ground The Lightning Analogy for Ore-Forming Systems (Hronsky, 2012)

18 Ground Transient Rapid Breach of Threshold Barrier Threshold Barrier: Resistive Air Electric Charges Accumulate Slowly The Lightning Analogy for Ore-Forming Systems (Hronsky, 2012)

19 A General Model for Ore-forming SOC Systems Threshold Barrier (Can be a physical or geodynamic seal) Episodic focused energy and mass flux Fluid Sink Thermal Halo-produced d by entropy dumped into environment Transient Exit Conduit Fluid Reservoir Fluid (Energy) Source Slow persistent fluid flux (Hronsky, 2011)

20 Porphyry Cu Example Fluid Exit Conduit Fluid Reservoir From Sillitoe (2010)

21 Threshold Barrier can also be geodynamic Tosdal (2009) Similar effect by: Similar effect by: Any stress switch (moving through neutral stress state). A move from retreating to advancing arc (magma ponding).

22 A New Perspective: Ore Shoots as Fluid Exit Conduits Proposed that most ore deposits can be considered as forming in transient fluid-exit conduits, associated with the episodic rupture of over-pressured reservoirs at depth (Hronsky, 2011). Zones of extreme, transient crustal permeability. Conduits break their way up to the surface, taking the easiest path (preferring vertical pipes). Pathisdictatedbyrheologyof3Drockmass,NOT by active structures at deposit scale. Fluid-pulse l related stress changes large and often overwhelm ambient stress field. Multiple mineralisation events commonly exploit Multiple mineralisation events commonly exploit same architecture favourable under a variety of stress regimes! 22

23 Oreshoots as Exit Conduits El Teniente: A well documented d example of multiple, superimposed focused fluid exit events all using the same plumbing Vry et al (2010)

24 Stockwork Exit Conduits No bounding shear zones! 0.2m ~1m ~100m Stacked reverse vein arrays New Holland: (Henson, 2008)

25 Chonoliths are subhorizontal exit conduits hosting NiS Nebo Babel (Seat et al., 2008) Noril sk Camp (Source: Steve Beresford) Kabanga North Kabanga Main She a rzo ne Kabanga (Wolfgang Maier) Nkomati (Li et al, 2002)

26 Need to think of our ore-systems as a connected network from source to sink

27 Flat lode systems need steep feeders somewhere! Sunrise Dam example: Baker et al (2010) 27

28 Copy the Komatiite NiS Approach: Find the Conduit then find the Oreshoot F d d NiS l i t h f d For decades NiS geologists have focussed on locating high Mg magma conduits, THEN finding ore.

29 Take Away Messages Now recognise multiscale fluid-flow flow conduits as the primary element of ore-forming hydrothermal systems. Long-lived vertically-accretive structures are key pathways for metal transfer through the lithosphere. These are not the obvious structures at or above the level of mineralisation an important message for targeting. Do not need ACTIVE structures for oreshoots at deposit scale. Kinematics is of limited use for oreshoot targeting. Replace the historical i structure-centric t ti framework with a fluid physics-centric framework (SOCS). Key is favourable architecture in 3D acting as lightening rods for transient oreshoot forming events. Threshold barriers to mass and energy transfer are critical for the formation of highest quality deposits need to understand them and map by proxy in geoscience datasets.