Developing the Orogenic Gold Deposit Model: Insights from R&D for Exploration Success

Home , Accretionary wedge

"Accretionary Wedge Geodynamic Evolution, Metamorphic Equilibria, Metasomatic Processes, & GOLD” by Dave Lentz (UNB) Accretionary ophiolitic sequence (with quartz veins), basement Santorini, Greece 2m

Orogenic Gold first used by Bohlke (1982) Developing the Orogenic Gold Deposit Model: Insights from R&D for Exploration Success

by Dave Lentz (UNB) Accretionary ophiolitic sequence (with quartz veins), basement Santorini, Greece 2m

Orogenic Gold first used by Bohlke (1982) SPONSORS 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) How are Gold Systems Related to Crustal Growth?

From Goldfarb (2006)

Magmatic-dominated Metamorphic-dominated

Groves et al. (1998) Metamorphic,Metamorphic, Transitional,Transitional, andand MagmaticMagmatic GoldGold ModelsModels Poulsen (2000) Metamorphic dominated Setting Juneau Belt Prehnite- Donlin Creek pumpellite Ross Mine Kirkland Lake Dome

Brittle Sigma/Giant-Con Greenschist Hollinger-McIntyre Ductile-Brittle

Amphibolite Red Lake Eastmain/Lynn Lake Musselwhite

Granulite Ductile Lake Lilois Fluid Egress along Advective Crustal-scale Heat n Shear Zone o i t Transfer a n o Z

l a t e Zone of deposition M 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 10-15 km ?

METAMORPHIC Single-pass; Single-pass; Single-pass, sub-horizontal pervasive channelized/focused flow constrained into fractures,faults, etc. by layering Wood & Walther (1986) Channellized flow effects (Ductile zone)

Fractures Fold hinges

Contacts Permeable layers

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

AB content Element ABa (positive) Element content Element

AB b (negative) Orebody

Zone of enrichment depletion or complex dispersion of ore, gangue, and indicator elements Element content Element A B Lin e of tr a ver se or d rill ho le AB c (complex) Boyle (1982) 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) Otago Goldfields AuriferousAuriferous orogenicorogenic hydrothermalhydrothermal systemssystems inin MesozoicMesozoic schistschist Otago Goldfields

Waikaka Waitahuna From R. Goldfarb (2006) From Craw, 2005 Macraes Gold, New Zealand

From Craw (2005) Macraes Gold, New Zealand

From Craw (2005) MacraesMacraes -- OrogenicOrogenic GoldGold

Consistencies with orogenic model Inconsistencies (de Ronde et al., 2000): (Craw and co-workers): • >99% H2O, with light hydrogen isotopes • 1-2 wt% NaCl eq. • Major shear, jogs, brittle - ductile • BUT CH , N ; some clathrate and 4 wt% Greenschist facies 4 2 • NaCl eq. flincs 18 34 • Vein P-T, mineralogy, δ O, δ S From R. Goldfarb (2006)

Early Stage Large/Old Accretionary Wedge

Precollisional Later Stage D1 (early) Evolution • Underplating • Uplift • Extension • Erosion Structural Flow Pattern Internal Heating • Radioactivity • Mechanical Platt (1987) Tarney et al. (1991) (a) Otago accretionary prism, regional quartz veins, fluids replenished by subduction

(b) Regional quartz veins, northern New England

(c) Up-T , pluton-driven flow, Australia, focused into metapelites Regional— (d) Regional quartz veins; Connecticut Channelized (e) Regional quartz veins in hot spot, New Hampshire

(f) Individual quartz veins, Connecticut (g) Individual quartz veins, Scotland Conduits (h) Average ductile shear zone

Amphibolite facies (i) Greenschist facies

Regional Minumum for Regional— Barrovian metamorphism; Barrovian northern New England average anticlines dominantly (j) Regiuonal pervasive metamorphism, Scotland (k) Theory

(l) Numerical models

(m) Min Max Barrovian metamorphism, New England

0123456 3-2 log10 (time-integrated flux) (m m ) 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)!

ion D2/M2 (open folding) at 10 Ma m regional to contact exu 30 Ma metamorphism (10-30 Ma) 400 Ma 40 Ma 100 Ma (?) Thermochronologic Constraints

Beware: thermochronologic age gaps 2+ events maybe superimposed

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 Fault- Taylor Brook Fault

Mascarene Basin development basal Oak Bay Fm DeformedDeformed MolasseMolasse • 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 PiezothermalPiezothermal ArraysArrays

When do you get the water out? Stuwe (1998) PP--TT PathPath ConsiderationsConsiderations

Spear (1995) 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) SubductingSubducting SlabSlab –– AccretionaryAccretionary WedgeWedge FluidsFluids

Kodiak, Alaska

Vrolijk & Myers (1990) SubductingSubducting SlabSlab –– AccretionaryAccretionary WedgeWedge GeothermsGeotherms

Kodiak, Alaska

Subduction refrigeration Vrolijk & Myers (1990) SubductingSubducting SlabSlab –– AccretionaryAccretionary WedgeWedge GeothermsGeotherms

cold/old

Inverted geotherm f(slab T, t, rate) = subduction refrigeration Peacock (1987) Subducting Slab – Accretionary Wedge Geotherms

WEDGE WEDGE

Decollement

SLAB SLAB

Fertilization & Hydration Front Peacock (1987) Normal Metamorphic Gradient

England & Thompson (1986) ThrustThrust--RelatedRelated ReversedReversed GeothermsGeotherms

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) Cordillerian Geodynamics

After Hyndman (1990)

From R. Goldfarb (2006) After Hyndman and Lewis (1995) Prehnite- pumpellite Brittle Greenschist

Amphibolite OROGENIC GOLD

Granulite Ductile Crustal Continuum Model

Exumation rate ? rate Exumation

D c u e l i t r B / l t t i e

S Greenschist

R O Amphibolite Greenschist Cool fluids Slab/Wedge 240C 340C Paleosurface evolution 400C 480C 400C 340C

3 2 4 1 5 ) b k (

e r u s s e Pr Shear Zone PT (t) Inverted Geothermal Gradient Syn D1 Fluid Egress along Late Crustal-scale Shear Zone D2 n o i t a n o Z

l a t e M

Fyfe & Henley (1973) Accretionary Wedge-Arc interaction

Hagemann & Cassidy (2000) IsotopicallyIsotopically HeavyHeavy FluidsFluids----OrogenicOrogenic AuAu StableStable isotopicisotopic ConstraintsConstraints

From R. Goldfarb (2006) OxygenOxygen && HydrogenHydrogen IsotopesIsotopes ofof OreOre QuartzQuartz

From R. Goldfarb (2006) StableStable IsotopeIsotope SystematicsSystematics

Ridley & Diamond (2000) Altered Basalt δ18O = 14‰ 18 o δ Ofluid = 8‰ (200 C)

18 o δ Ofluid = 14‰ (500 C)

Shale δ18O = 20‰

18 o δ Ofluid= 14‰ (200 C)

18 o δ Ofluid = 20‰ (500 C) ImplicationsImplications forfor goldgold solubilitysolubility

Bisulfide-Gold complexing: favourable at low T’s & neutral pH’s 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 ConclusionsConclusions • 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 ConclusionsConclusions • 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

• Greenstone • Qtz veins • boudinaged veins S1/2 • pressure solution • melange

Mafic S1/2 spilite Click to return to menu