Características e Critérios da Exploração de Depósitos Orogenéticos de Ouro

Transcription

1 Características e Critérios da Exploração de Depósitos Orogenéticos de Ouro Richard Goldfarb, goldfarb@usgs.gov, SIMEXMIN 2012

2 Archean Lode Gold Deposits Greenstone Belts in granitoid-greenstone terranes Yilgarn, Zimbabwe, South Africa, Tanzanioa, central Brazil, Kolar, Superior Tholeiitic basalt, dolerite, ultramafics, granitoid, but also BIFs and carbonaceous seds in greenstones

3 Phanerozoic Lode Gold Deposits Accreted oceanic sequences; Continental margins Metasedimentary host rock terranes Mother Lode, Russian Far East, Tasman, Otago, Central Asia, Eastern Cordillera (S.A.), Alaska, BC

4 Mid 1980s Recognized as a Single Deposit Type Low- Sulfide Mesothermal Mother Lode Metamorphic Orogenic

5 Spatial/Temporal(±) Association with Granitoids, but NOT Genetic

6 Spatial/Temporal Association with VMS

7 KEY REGIONAL EXPLORATION TARGETS 1) Major structure 2) Metamorphic grade 3) Physical or chemical trap

8 Crustal-scale Deformation (Shear) Zones

9 Golden Mile (1,200 t) Gold Deposit=Focused flow of large fluid volume; controlled by rock permeability and thus crustal deformation

10 Spatial Association with Deep Crustal Faults/Terrane Boundaries (Juneau)

11 FOCUS OF ORE FLUIDS: VARIATIONS IN STRIKE OF CRUSTAL-SCALE FAULTS

12 GOLD-GREENSCHIST ASSOCIATION

13 Orogenic Gold Tonnages: Archean MM Settings (from R. Kerrich, 2009)

14 Ore Deposition POSTDATES Metamorphism of Host Rocks Late-kinematic Timing

15 CRITICAL STRUCTURAL CONTROLS 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: a) Large strength contrasts between units. b) Complex geometry c) Isolation of competent units in incompetent units.

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17 Chemical Trap=Homestake Fm BIF

18 Chemical vs. Physical Traps for Orogenic Au from D. Groves

19 Deformation Styles

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22 Gold Occurrences

23 Alaska-Juneau Deposit Paragenesis--Mafic Host Rock

24 Mineralogy of Giants

25 Sericitic Alteration

26 Listwanite

27 Carbonization of Wallrocks

28 Subika Alteration, South Ahafo district, Ghana from Enders et al., 2008

29 Liswanites in Pz metased terranes, Xinjiang

30 SULFIDATION from Hodgson, Gold 2000 Hodgson, Gold 2000

31 Arsenopyrite and/or pyrite haloes

32 Geochemistry of Alteration Whole rock data: CO 2, K 2 O, H 2 O, S, Au, As (± Ag, Bi, Sb, Mo) SiO 2, Na 2 O, (± MgO, P 2 O 5, Sr) REE, HFSE Bulk chemical changes at constant mass (lithological variations) Alteration Index (CO 2 + K 2 O)/(CO 2 + K 2 O + Na 2 O + Al 2 O 3 ) Bierlein & Crowe, 2000

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34 Gold: The Best Pathfinder?

35 from Groves, Ore Deposit Models

36 Colvine s observations that there is a P-T control on ore style in the Superior province

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38 HIGH TEMPERATURE OROGENIC GOLD Yilgarn (Southern Cross) Muruntau stockworks

39 Fluid Inclusions Ore Fluid Chemistry P-T of Ore Deposition Ore fluid Source?

40 Best Inclusions - Sulfide Pressure Shadows

41 Generally 5-20 mole % gas

42 CO 2 -rich Fluids orogenic IRGD Ridley, Gold 2000

43 Laser Analysis of Volatiles

44 Orogenic Gold Fluids Temperatures Pressure Fluid comp Salinity ~ o C >1-4 kb H 2 O-CO 2 + CH 4, N 2, H 2 S <5-8 NaCl eq

45 POSSIBLE FLUID SOURCE REGIONS ISOTOPIC STUDIES? from Ridley and Diamond, Gold 2000

46 d 34 S of Alaskan Orogenic Deposits

47 Large et al, 2010

48 Isotopically Heavy Fluids--Orogenic Au

49 Fluid Inclusion Waters-- Meaningful dd?

50 Mm Belts: Metals versus Grades P-P: 3-22 ppm As, ppb Sb, ppb Hg Sillimanite: ppm As, ppb Sb, <5 ppb Hg Same trends for B, Bi, W

51 Metal Depletions in Otago Schist, NZ from Pitcairn, 2006 Depletion of metals in schist relative to mm temps. Concentration of metals in deposits at higher crustal levels

52 SUPPORT FOR METAMORPHIC MODEL No Gold in High Grade MM Rocks Gas Chemistry of Ore Fluids dd and d 18 O of Ore Fluids Uniform d 18 O Values for Veins Change in As, Au, Bi, Hg and Sb with MM grade

53 GREENSCHIST-AMPHIBOLITE P-T

54 Phanerozoic Orogenic Gold Provinces

55 Precambrian Orogenic Gold Boring Billion