1) Intro to Orogenic Gold(造山型金矿床)

But as early as 1981…..
Late 1980s—Finally Widely Recognized as a Single Coherent Deposit Type
• Low- Sulfide • Mesothermal • Mother Lode • Metamorphic • Orogenic
Late-kinematic Timing
Roberto Eleonore Deposit
Relative chronology: ore zones have been deformed and metamorphosed and some zones are now transformed into paragneiss (Dube et al, 2012)
Crustal-scale Deformation (Shear) Zones
Golden Mile (1,200 t)
Gold Deposit=Focused flow of large fluid volume; controlled by rock permeability and thus crustal deformation
SEISMICITY AND AU
Sibson et al, 1988
Fluid pressures and stress regimes enhance permeability (transient), and thus define flow pathways
Geophysics: Identification of Main Faults
Dennis et al., 2010
•Magnetotelluric surveys by Geoscience Victoria in Lachlan fb •Transient electromagnetic method for upper 100s meters •Major faults=high conductivity ( i.e., fracture density, graphite)
Favorable location within a structure
Ahafo Belt, Ghana--Faulted Contact
Enders et al., 2008
Regional Anticlines
• • • • Fold hinge zones Culminations—highest strat points Tight folds in slate belts (e.g., Ballarat) Open folds in sandstones (e.g., Telfer)
Spatial Association with Deep Crustal Faults/Terrane Boundaries (Juneau)
Barberton District
Gold mines in the structurally most complicated areas, near the main shear zones
DAMANG, GHANA—GOLD ALONG ANTICLINAL HINGES
White et al., 2010
ANTICLINAL vs SYNCLINAL ZONES
Production / Productivity of Greenstones
Anticlinal Zones
~ 2245 t Au or 240 kg Au/km2
Airborne EM in DRC Archean Greensone Belts
• Target common graphite-gold association • Plunging conductors id graphitic units
2nd and 3rd order faults: Shear hosted discrete veins and network veins in tension fractures
Phanerozoic Lode Gold Deposits
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Accreted oceanic sequences; Continental margins Metasedimentary host rock terranes Mother Lode, Russian Far East, Tasman, Otago, Central Asia, Eastern Cordillera (S.A.), Alaska, BC
Spatial/Temporal Association with VMS’
GOLD-VMS SPATIAL ASSOCIATION IN THE ABITIBI
Robert et al., 2005)
Spatial/Temporal(±) Association with Granitoids, but NOT Genetic
• Granites may be pregold, syn-gold, or post gold • Granites may be any composition
KEY REGIONAL EXPLORATION CRITERIA
1) Major structure 2) Metamorphic grade 3) Physical or chemical trap
PHYSICAL TRAPS FOR AU
• Late structural timing of gold is important constraint. • Geometry visible today is close to geometry during mineralization. • For selective failure of specific rock units need:
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Regional structural control
Bulawayo-Bubi greenstone belt, Zimbabwe: Gold mines are in the structurally most complicated parts of an region, near the main shear zones
GLOBAL GOLD ENDOWMENT—WHAT TO TARGET?
(& in situ resources as of December 2013)
ENDOWMENT=PAST PRODUCTION+RESERVES+RESOURCES TOTAL LODE=6573 Moz Au (205,000 t Au) EXCLUDES 2576 Moz Paleoplacer (Data from Lipson, 2014)
wk.baidu.com
Contrasting Features for Some Archean Deposits in High-Grade Terranes (see Phillips and Powell, 2009)
• Deposits such as Big Bell, Renco, Challenger, Hemlo do not fit the continuum model • In contrast to most orogenic Au, they have been metamorphosed and occur in upper amphibolite-granulite facies • Is there something different about Archean P-T-t systematics, such that some deposits form on a prograde mm path and/or are overprinted by a 2nd mm event?
OROGENIC GOLD DEPOSITS: GLOBAL DISTRIBUTION TO DEPOSIT MODELS TO EXPLORATION CONCEPTS
CUGB Workshop September 17-19, 2014 Richard J. Goldfarb, USGS & CUGB (goldfarb@usgs.gov) David I Groves, UWA (di_groves@hotmail.com)
EILU, 2012
FOCUS OF ORE FLUIDS: VARIATIONS IN STRIKE OF CRUSTAL-SCALE FAULTS
Major Jog on 2nd Order Fault Hosts Victory-Defiance
EILU, 2012
Regional Fault-Metamorphic Belt Intersections May Be Critical
Where were we 25 years ago?
USGS Lode Au Descriptive Models-1986
1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) Porphyry Cu-Au (Afton, Copper Mountain) Au-Ag-Te veins (New World, Cripple Ck) Hot Spring Au-Ag (Round Mountain, Sleeper) Creede-type epithermal (Creede, Pachuca) Comstock-type epithermal (Comstock, Republic) Sado-type epithermal (Hayden Hill) Epithermal quartz-alunite (El Indio, Goldfield) Carbonate-hosted Au-Ag (Carlin) Distal disseminated Ag-Au (Candelaria, Cove) Quartz pebble conglom Au-U (Wits) Olympic Dam-type Cu-U-Au Low sulfide gold-quartz veins (Bralorne, Mother Lode) Homestake-type Au (Lupin) Gold-antimony deposits (Reefton, Olympiada) Gold on flat faults (Picacho) Kuroko (Noranda-type) VMS(Horne, Dumagami) IRGS? Au skarn? Greenstone-hosted Au?
Synclinal Zones
~ 185 t Au or 35 kg Au/km2
Inherent to Barrovian Mm Belts
Spatial Association with Greenschist
(from R. Kerrich, 2009)
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Ore Deposition POSTDATES Metamorphism of Host Rocks
I) Orogenic gold introduction: geological & geochemical characteristics
Archean Lode Gold Deposits
(David will discuss these in detail)
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Greenstone Belts in granitoid-greenstone terranes Yilgarn, Zimbabwe, South Africa, Tanzania, central Brazil, Kolar, Superior Tholeiitic basalt, dolerite, ultramafics, granitoid, but also BIFs and carbonaceous seds in greenstones