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

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Developing the Orogenic Gold Deposit Model: Insights from R&D for Exploration Success "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 Hg 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 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 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 Paleosurface Inverted evolution Geothermal 1 e n o Gradient Z r a e 240C h S Shear ) 2 b k ( e Zone 340C l t t i e r r E B 3 / PT (t) u R Greenschist e l s 400C O i t s c e u 480C D Pr 4 400C Amphibolite Syn D1 5 340C Greenschist Exumation rate ? Cool Slab/Wedge fluids Fluid Egress along Crustal-scale Shear Zone Metal Zonation Late D2 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
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