Oreshoot Targeting New Paradigm

7/27/2019 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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Oreshoot Targeting New Paradigm

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