oreshoot targeting new paradigm
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A New Paradigm for UnderstandingOre-Shoots in Gold Deposits
Jon Hronsky
Centre for Exploration Targeting Seminar
13 April 2012 1
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Schematic Plan Section - Peters (1993)
The Ore-Shoot Concept :
A localised well-mineralised rock volume
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Ore Shoots:
A general and long knownempirical association with
heterogeneities:
What is the process
meaning?
Peters (1993)
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The oldest process model:
Norseman shear-link concept of the 1930s
Campbell (1990)
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Ore-Fluid Focusing:The Current Paradigm
Not commonly explicitly articulated
Recognises (correctly) that mineralised rock volumesrepresent sites of anomalous ore-fluid flux
Assumes these anomalous volumes represent localised
more dilatant rock volumes embedded within a surrounding,larger-scale, less focused fluid flow system
Assumes they are generated by dynamic syn-oredeformation
Based above assumptions, assumes knowledge of
structural geometry and inferred syn-ore stress field can beused to predict these anomalous dilational sites
Assumes localised mineralised volumes hosted bystructures such as faults and shear zones are intrinsically aproperty of these structures
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Ridley (1993)
The Current Paradigm
for Ore Fluid Focusing:Localised volume of higher fluid flux
embedded within a broader flow system
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Problem with the Current Paradigm:Lack of consistent predictive relationship
between structural geometry and ore
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Cracow Epithermal Gold Deposit
Mickelthwaite (2008)
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Ore-Fluid Focusing:The Current Paradigm
Not commonly explicitly articulated
Correctly recognises (correctly) that mineralised rockvolumes represent sites of anomalous ore-fluid flux
Assumes these anomalous volumes represent localised
more dilatant rock volumes embedded within a surrounding,larger-scale, less focused fluid flow system
Assumes they are generated by dynamic syn-oredeformation
Based above assumptions, assumes knowledge of
structural geometry and inferred syn-ore stress field can beused to predict these anomalous dilational sites
Assumes localised mineralised volumes hosted bystructures such as faults and shear zones are intrinsically aproperty of these structures
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WRONG!
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A New Perspective:Ore Shoots as Fluid Exit Conduits
Proposed that most ore deposits can be considered asforming in transient fluid-exit conduits, associated with theepisodic rupture of over-pressured reservoirs at depth(Hronsky, 2011)
Represent examples of self-organised critical systems
Provides the key to understanding ore-localisationprocesses
Explains failure of current paradigm for predictive structuraltargeting of ore-shoots in hydrothermal ore systems
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A General Model forOre-forming SOC Systems
11Fluid (Energy) Source
Fluid Reservoir
Fluid Sink
Transient Exit Conduit
Threshold Barrier(need not be a physical seal)
Thermal Halo-produced by entropy
dumped into environment
Episodic focused energy
and mass flux
Slow persistent fluid flux
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Electric Charges Accumulate Slowly
Threshold Barrier:
Resistive Air
Ground
Transient Rapid Breachof Threshold Barrier
The Lightning Analogy for Ore-Forming Systems
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Electric Charges Accumulate Slowly
Threshold Barrier:
Resistive Air
Ground
Transient Rapid Breach
of Threshold Barrier
The Lightning Analogy for Ore-Forming Systems
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Porphyry Cu Example
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From Sillitoe (2010)
Fluid Reservoir
Fluid Exit Conduit
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Fluid Exit Conduits
Rock volumes that have been conduits for large amounts offluid flux, usually over multiple cyclic events
Zones of extremecrustal permeability
Zones of localized intense fracturing
Sourced from overpressured reservoir zone at depth (may bepartly located within this zone)
Conduits nucleate at local heterogeneities in reservoir
They break their way up to the surface, taking the easiest
path May re-use existing structures or fracture previously intact
rock
Fluid-pulse related stress changes large and overwhelmambient stress field
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El Teniente:
A well documented example of multiple, superimposed focusedfluid exit events all using the same plumbing
Vry et al (2010)
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17Fluid Conduit Zones in Orogenic Gold deposits(A: Mother Lode, California (Goldfarb et al. 2005) B,C: Bendigo (Cox, 2005)
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Favona North Epithermal Vein, Waihi(Torckler 2008)
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~200mm
~100m
New Holland:
(Henson, 2008)
Examples of Fluid Exit Conduits
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100m
Section view
Fitzroy Fault and Au distribution (gold blobs):
Image from Gocad looking SW?
Strongly fault controlled
Image from: Carl Young
Kanowna Belle
Example(Henson, 2008)
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Ernest Henry IOCG deposit:
Pipe-like breccia zone
(Cleverley, 2008)
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The 1997 Umbria-Marche Earthquakeas a Modern Process model
Documented by Miller et al (2004)
Two ruptures of M 5.6-6.0 near the top of an evaporite unit, atabout 6km depth.
Evaporite unit known from deep (4.8km) petroleum drilling along
strike to contain over-pressured CO2 fluids. Initial ruptures followed by a 30 day sequence of thousands of
after shocks, including some > M 5.0.
Modeling indicates a strong correlation between the propagationof a fluid pressure pulse from this reservoir and the distributionof the after shock swarm.
Localised transient, post-seismic permeability associated withthis fluid pulse has been modeled as 4 x 10-11m2; 105 to 106 times> than background crustal permeability at that depth (6km)
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1997 Umbria Marche EQ: Cross-sectionMiller et al (2004)
Overpressured
Evaporite sequence
(known from deep drilling)
Main Shock
Rupture Site
Propagating Fluid pressure pulse:
(after-shock swarm)
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Fluids do not respond passively tostructure : They create their own Pipes!
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1997 Umbria-Marche EQ
Miller et al (2004)
Modeled Changes in Coulomb
Failure Stress post rupture
no correlation with aftershock
swarm
Modeled Changes in Pore
Fluid Pressure post rupture-
good correlation withaftershock swarm
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De Ronde et al (2011)
Brothers Volcano
Kermadec Arc:Another modern example of a
focused fluid exit conduit
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The New Paradigm
Now recognise fluid-flow conduits as the primary elementof ore-forming hydrothermal systems.
Replace the historical structure-centric framework with afluid-centric framework.
Important concept: fluid conduits may be much moreextensive features than the pre-existing structures whichhost them.
A single fluid conduit may move between host structuresas it propagates upward, depending on which pathwayprovides the path of least resistance.
Fluid flow conduits may commonly follow quite torturouspaths from source to sink, including right-angled bends inthree dimensions.
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Elbow Bend Example:Emperor Epithermal Au deposit, Fiji
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Ore-Fluid Flow
Cross-Section (Begg, 1996)
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Shatter Shear
Intersection Traceof the
HW Shear
Flat-plunging
Fluid flow pipe
Steep-plunging ore-shoot associated
with loci of steep fluid flow pipe
Plan-section (Begg, 1996)
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Lepanto-Far South East Elbow Bend Example
Hedenquist and Taran (2009)
Inferred Ore Fluid Path
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Copying the Komatiite NiS Approach:Find the Conduit then Find the Ore-shoot
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Flat lode systems need steep feeders somewhere!
Sunrise Dam example: Baker et al (2010)
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Controls on ConduitLocalisation
Key control is rheological structure of rock massabove reservoir how easy is it for the fluid pulse tofracture itself upwards?
Fault and shear zones effectively just another rocktype
Certain geometric patterns consistently favourableLightning Rod analogy
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Fluid-pressure driven conduit development commonly prefers pipe-like
volumes of more competent rock rather than pre-existing structures:
Structures are important because they establish the rock geometry
Source: David Groves
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(Syenite in red; Actinolite-magnetite-epidote-calcite alteration pipe (syenite-
related) bounded by dashed lines ; high-grade later gold lodes in green)
Wallaby(Miller, 2008)
Repetitive Self-Organisation
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Repetitive Self Organisationusing the Same Rock Volume:
Some Geometries mustbe Fundamentally Favourable!
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Three Key DepositionalProcesses
1. Pressure Reduction (fluid unmixing)
2. Surface substrate (ie fracture margin)character and availability
3. Fluid Mixing (at discharge site only!)
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Boiling Zone ControlEpithermal Gold
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Cross-section through the Honko and Sanjin ore bodies at Hishikari,
Japan (Faure et al, 2002; modified after Ibaraki and Suzuki, 1993). The
bonanza veins are depicted as thick black lines; they have a limited
vertical extent and are closely associated with the unconformity between
the underlying Shimanto sediments and the overlying Hishikari
Andesites.
Boiling Zone
Fluid Flow
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41Murakami et al (2009)
Pressure controls Gold Grade in Porphyry Deposits
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42Rusk et al (2008)
Section through the Butte Deposit:( ~3km of vertical exposure after restoration of Continental Fault offset)
Note correlation between base of mineralization and lower limit of brine inclusions
(ie evidence for phase separation as critical control on mineralization)
Base of Mineralization
Approx Paleo-depth:
6-9km
FLUID FLUX
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Barren Deep Core:
Single Phase Parental Fluids
Au-Cu Centre:
Low density vapour very saline brine
Peripheral Cu Zone:
Denser vapour less saline brine
Deep Periphery:
Single Phase Parental Fluids
Bingham Canyon Zonation in Cu/Au and Grade
and Relationship to Ore Fluid Physical Evolution
(Landtwing et al, In Press)
Interpreted Fluid Flow Conduits
Campbell (1990)
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Norseman:
Local Quartz-Au association within conduit
implies pressure is key process
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Clout (1989)
Long Section No.3 West Lode,Lake View Mine,
Golden Mile, Kalgoorlie
High-Grade Green-Leader
Au-Telluride Ore-shoot
Base of
Boiling Zone?
Interpreted main
fluid flow conduits
Low-grade spent-fluid zone
Multi-stage Fluid Unmixing may be Important
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The role of Surface Energy Effects andthe Substrate
Evidence:
Au grains are often closely associated with boundaries ofgangue sulphide grains
Au grains commonly at wall-rock margins to veins or
associated with thin laminae of wall-rocks within the vein High-grade Au-carbon association
Fine-scale (mm-cm) variability in Au grade associated withselective sulphide replacement of a magnetite bearing BIF
Adsorption:
process by which a liquid solute accumulates on the surfaceof a solid
Champagne Pool, CIP examples
Likely to be an important detailed control on high Au grades
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BIF-hosted Gold Deposit:Note fine scale pyrite replacement of magnetite and associated Au deposition
(photo from David Groves)
Au-ore
Barren
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Champagne Pool
Taupo Region, NZ
Several 100 ppm Au adsorpted
on to amorphous orange As-Sb-
S colloids from highly Auunder -saturated fluids
(Renders & Seward, 1989)
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Peters (1993)
Localised High-Grade Ore-Shoots within Ore-Fluid Conduits
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In practice, both
processes commonly
occur together
Campbell (1990)
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53Norseman Example: Pre-ore dyke geometry controls local ore-shootswithin conduit a very common control in Orogenic Au deposits
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Ore-Shoot Process Model:Mararoa Reef (Norseman) Case study
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First Order control: Fluid exit conduit
-intersection of (more brittle) mafic
stratigraphy and cross-cutting u/mafic
dyke swarm
?
?
Second Order control:
Base of Primary Depositional Zone?
Third Order control:
Local Dilational Zone
-local deflection of conduit-hosting
shear fracture, controlled by dyke
intersections
Campbell (1990)
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Practical Implications
Ore-shoots can be hosted by all pre-existing structuresnot just the latest syn-ore ones
The age of the host structure is not necessarily the ageof mineralisation
A conduit that hosts one localised high-grade ore-shoot volume is likely to host others
Important to separate out controls at conduit scale fromlocalised ore-shoots
Exploration should focus down the plunge trend of theconduit not the localised ore-shoot
Need to develop methods to map conduits where theyare not ore bodies (alteration, stable isotopes?)
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The key geological element for targeting is localisedrheological heterogeneity (in 3D)
If an ore-shoot stops at depth need to understand why -bottom of primary depositional zone or end of local high-
grade shoot? Shear zone associated conduits may host local zones of
dilatancy and hence much higher-grade shoots stockwork zones will not and therefore be more uniformin grade
Barren hydrothermal breccias may be low-pressure topsto ore systems
Boiling zones may preferentially occur beneath morecoherent cap-rocks
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END