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Developing the Orogenic Gold Deposit Model: Insights from R&D for Exploration Success by Dave Lentz (UNB)

2m

Accretionary ophiolitic sequence (with quartz veins), basement Santorini, Greece

Orogenic Gold first used by Bohlke (1982)

INTRODUCTION PART I: Review Gold Deposit Settings • Historical Evolution of ideas • Description of Orogenic Au Systems • Enigmatic aspects of the metamorphogenic model PART II: Geothermal to Hydrothermal Evolution • Metamorphic Considerations to Thermal Evolution • Fluid Source (and Solubility Implications) PART III: Geodynamic Evolution • Accretionary Geodynamics (to collision) • Structural-Metamorphic Evolution & Settings • Implications for refining the metamorphogenic Orogenic Gold Model

PART I: Review Gold Deposit Settings Mineralization in forearc to back arc system Accretionary Wedge fore arc settings

Mitchell & Garson (1982)

OROGENIC GOLD: Magmatic to Metamorphic hydrothermal continuum

Groves et al. (1998)

Magmatic-dominated

Groves et al. (1998)

Metamorphic-dominated

Metamorphic dominated Setting

Greenschist

Kirkland Lake Dome Sigma/Giant-Con Hollinger-McIntyre

Brittle

Prehnitepumpellite

Juneau Belt Donlin Creek Ross Mine

Amphibolite

Granulite

Ductile

Ductile-Brittle Red Lake Eastmain/Lynn Lake Musselwhite Lake Lilois

Advective Heat Transfer

Me

tal

Zon

atio

n

Fluid Egress along Crustal-scale Shear Zone

Zone of deposition Low salinities (< 3 wt % NaCl, KCl, etc.) Source Region (or deeper)

Fyfe & Henley (1973) RETROGRESSION

PART II: Geothermal to Hydrothermal Evolution

Fluid movement Ethridge et al. (1983) • Fluid Flux >> normal i.e., high F/R • stable isotopic depletion • P-T changes in fluids • evolution of transient permeability

Wood & Walther (1986)

Ethridge et al. (1983)

Fluid Movement Brittle towards ductile transition Connected Permeability

No Connected Permeability (transient)

Gregory & Backus (1980)

Shallow crust– convection possible

METEORIC

?

METAMORPHIC Single-pass; pervasive

Single-pass, channelized/focused into fractures,faults, etc.

Single-pass; sub-horizontal flow constrained by layering

Wood & Walther (1986)

Channellized flow effects (Ductile zone) Fractures Fold hinges

Permeable layers

Contacts

Fault and shear zones

Orogenic Gold & Fault-Valve Model • Orogenic gold terminology originally used by Bohlke • Fault-valve model as described by Sibson • Related to faulting and seismic activity – Quartz veins are fossil remains of earthquakes

Surface

A

B A

A Orebody

a (positive)

b (negative)

B

B

Zone of enrichment depletion or complex dispersion of ore, gangue, and indicator elements A

B

Line of traverse or drill hole

Boyle (1982)

A

c (complex)

B

PART III: Geodynamic Evolution

SETTINGS • Precollision

• Subduction; • syn-arc genesis Subduction Duration KEY • syncollision

• subduction; • post-arc genesis Mitchell & Garson (1982)

D1/M1 (early) F1/F2 folds thrusts

D1 (late) F2 folds thrusts

D2/M2 (early) F3, F4

P-T-t path Considerations (convergent tectonics) • clockwise paths Reflecting burial to exumation (at various crustal levels)

• geothermal gradient evolution (P-T-t)

Mafic grid Spear (1995)

Nature’s Gold Factory

Kerrich (2000)

Large/Old Accretionary Wedge Precollisional

D1 (early) Evolution • Underplating • Uplift • Extension • Erosion

Early Stage

Later Stage

Structural Flow Pattern

Internal Heating • Radioactivity • Mechanical

Platt (1987)

Tarney et al. (1991)

(a)

Otago accretio nary prism, regional quartz veins, fluids replenished by subduction

(b)

Regional quartz vein s, northern New England

(c)

Up-T , pluton -driven flow, Au stralia, focu sed into metapelites

(d)

Regional quartz veins; Connecticut

(e)

Regional quartz veins in hot spot, New Hampshire

(f)

Individual quartz veins, Connecticut

(g)

Individual quartz veins, Scotland

(h)

Average ductile shear zone

Regional— Channelized

Conduits

Amphibolite facies (i)

Greenschist facies

Regional— dominantly pervasive

Regional Minumum for average anticlines (j)

Regiuonal metamorphism, Scotland (k) Theory (l)

Numerical models

Max (m) Min Barrovian metamorphism, New England

0

1

2

3

4

5

log10 (time-integrated flux) (m3 m-2)

6

Dehydration sequence

Low Geotherm

High Geotherm

Fyfe et al. (1978)

Subduction Zone Metamorphism (low T & high P) Ernst (1990)

OROGENIC GOLD: Crustal Continuum model

Hagemann & Cassidy (2000)

COMPLEX Polyphase deformation D1/M1 (fold/thrust belt) F1, F2 (10-20 Ma) 20 Ma

Exumation (< 5 Ma)! D2/M2 (open folding) regional to contact metamorphism (10-30 Ma)

10 Ma

e

m u x

n o i at 30 Ma 400 Ma 40 Ma

100 Ma (?)

Thermochronologic Constraints Beware: thermochronologic age gaps 2+ events maybe superimposed

D1 D2

UPLIFT PATH

UPLIFT PATH

Bleeker (2001)

Yellowknife Greenstone Belt

Exumation evidence: (Yellowknife Fault Zone) Molasse deposition, plus deformation

Yellowknife Greenstone Belt - Archean

MGS map (North & South Lynn Lake Belts) Burntwood & Sickle groups Paleoproterozoic

Exumation evidence: Molasse deposition, plus deformation

Lynn Lake Greenstone Belt - Paleoproterozoic

Northern NB (after van Staal 2003)

Silurian Weir Fm

Silurian Simpson’s Field Fm.

Southern NB (after McLeod et al. 1994)

Oak Bay conglomerates

Sawyer Brook FaultTaylor Brook Fault Mascarene Basin development basal Oak Bay Fm

Deformed Molasse • Low T, high P deformation (accretionary) • D1 (F1, F2) with M1 (Sanbagawa-type) followed by rapid exumation & erosion • Polymictic conglomerates with quartz cobbles common & local paleoplacer (fault-controlled valleys) then reburial (paleosurface marker) • Moderate P-T (Barrovian-type) • Late low P-high T (Abakuma/Buchan-type)

Becareful: petrographic evidence of exumation

may be lacking because of no retrogression; many misinterpret P-T-t paths and thermochronologic history by forgetting basic geologic constraints

Piezothermal Arrays

When do you get the water out?

Stuwe (1998)

P-T Path Considerations Spear (1995)

Subducting Slab – Accretionary Wedge Fluids Kodiak, Alaska

Vrolijk & Myers (1990)

Subducting Slab – Accretionary Wedge Geotherms Kodiak, Alaska

Subduction refrigeration

Vrolijk & Myers (1990)

Subducting Slab – Accretionary Wedge Geotherms

cold/old

Inverted geotherm f(slab T, t, rate) = subduction refrigeration

Peacock (1987)

Subducting Slab – Accretionary Wedge Geotherms

WEDGE

WEDGE Decollement

SLAB

SLAB

Peacock (1987)

Thrust-Related Reversed Geotherms

England & Thompson (1986)

Inverted geotherms : thermal re-equilibration process • low thermal conductivity • static model • no thermal heat advection Thermal rebound to a normal gradient (> 25 Ma) LATE HEATING & LATE DEHYDRATION & LOW T FLUIDS

LATE D2

England & Thompson (1986)

Brittle

Prehnitepumpellite

Amphibolite

Granulite

Ductile

Greenschist

OROGENIC GOLD Crustal Continuum Model

Shear Zone PT (t)

1 2

340C

3 4

Syn D1

240C

400C 480C 400C

5

Greenschist

340C

Amphibolite Greenschist

Cool Slab/Wedge fluids

Exumation rate ?

Inverted Geothermal Gradient

Paleosurface evolution

Late D2

Me

tal

Zon

atio

n

Fluid Egress along Crustal-scale Shear Zone

Fyfe & Henley (1973)

Accretionary Wedge-Arc interaction

Hagemann & Cassidy (2000)

Evolution to anomalous high T gradient

Rapid heat input (magmatic heat advection)

is key to driving rapid fluid expulsion (late collisional – D2) Abukuma type (high T-low P) metamorphic gradient preserved

Conclusions • Cold-Old slabs refrigerate the base of the accretionary structures & allow low-T hydration, then late dehydration as subcreted material is uplifted & heated by various mechanisms • The core of the wedges are hotter due to radioactive and frictional heating so the Inverted or Reversed geotherms typify Accretionary Wedge Systems • Low-T metamorphic dehydration reactions during subaccretion of hydrated crust produces isotopically light metamorphic volatiles (i.e. no need for meteoric fluids) that egress through the pile &, if focused, may produce gold veins

Conclusions • During early to late stage collision, late low T, isotopically light fluids are released at depth as a normal geothermal gradient is established, which helps explain the late lower T retrograde shear zones & silica abundances • Low temperature gold complexes (e.g. bisulphide) can dominate the fluid system (no need for chloride complexes) • Oxidized to reduced fluids with S, Sb, As, Hg, etc. like active accretionary systems, with CO2, CH4, etc. at moderate pH’s, but low salinities (< SW) as they are dominated by dehydration reactions.

Acknowledgements

• Funding from NSERC Discovery grants • Funding from NB DNR-Minerals • Funding from Manitoba Geological Survey • Funding from Yukon Geology Program • Funding from NSERC-CRD - with Freewest, Stratabound, First Narrows, Eagle Plains, Northern Freegold CIM Distinguished Lecture program is supported by; Canadian Mining and Metallurgical Foundation

Exumation evidence: Molasse deposition, plus deformation

Red Lake - Archean

Exumation evidence: Molasse deposition, plus deformation

Sioux Lookout - Archean

microlithon

microlithon

Microlithon-septum Fabric development (pressure solution)

foliation

Geochemical & Isotopic changes = MASS TRANSFER Lentz (1999)

Geochemical & Isotopic changes = MASS TRANSFER

Lentz (1999)

Lentz (1999)

Accretionary ophiolitic sequence (with quartz veins), basement Santorini, Greece

S1/2

S1/2

• Greenstone • Qtz veins • boudinaged veins • pressure solution • melange

Mafic spilite

Silica Solubility Considerations

Silica Solubility Considerations • prograde solubility • fluids moving down geothermal gradients • always saturated in silica/qtz = mass flux problem • problems of self sealing

Bebout & Barton (1989)

Normal Metamorphic Gradient

England & Thompson (1986)

Stable Isotope Systematics

Ridley & Diamond (2000)

Altered Basalt
18O = 14‰
18Ofluid = 8‰ (200oC)
18Ofluid = 14‰ (500oC)

Shale
18O = 20‰
18Ofluid= 14‰ (200oC)
18Ofluid = 20‰ (500oC)

Implications for gold solubility – bisulfide complexing