METALLOGENIC MODELS AND GOLD EXPLORATION€¦ · Tongon 4.27 Iduapriem 3.5 Obuasi 33.43 Siguiri...
Transcript of METALLOGENIC MODELS AND GOLD EXPLORATION€¦ · Tongon 4.27 Iduapriem 3.5 Obuasi 33.43 Siguiri...
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METALLOGENIC MODELS AND GOLD EXPLORATION
Laurence Robb
Dept. of Earth Sciences, University of Oxford, UK
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RANDGOLD RESOURCES
ANNUAL REPORT 2015
THE GOLD CYCLE.....BOOM & BUST
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....and WHY GOLD ?
• because is still offers such good
returns on exploration investment....
• and although it is rare, it occurs in
many different ore deposit types
• .....but represents a challenge to
find .
....after Robert et al., 2007 and Groves et al., 2016
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Wedepohl, 1978
GOLD ABUNDANCES........
Gold deposits 2-10 ppm
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METALLOGENIC
MODELS ARE
GREAT......BUT YOU DO
NOT WANT TO LOSE
YOUR HEAD OVER
THEM.....
THE NEXT MAJOR
GOLD DISCOVERIES
WILL BE MADE BY
GEOLOGISTS WHO
‘THINK OUTSIDE THE
BOX’
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....after F. Robert et al., 2007
Witwatersrand
Basin
Murantau
Twangiza
Damang
METALLOGENIC MODELS FOR GOLD
Murchison
Bin Yauri
Morila
Bushveld
granites
Burma
IOCG??
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OROGENIC GOLD SYSTEMS
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....after Goldfarb & Groves,
2015
‘CRUSTAL CONTINUUM’ MODEL
NOT WIDELY
PRESERVED
THE NORM
DIFFICULT TO FIND
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TYPICALLY.......
• ORE SYSTEMS ASSOCIATED WITH
STEEPLY DIPPING DUCTILE
SHEARS (early reverse thrusting;
later strike-slip)
• HOST ROCKS AT VARIOUS MET.
GRADES, ORE DEPOSITION ON THE
RETROGRADE P-T-t PATH
Saunders et al., 2014
• ORES ASSOCIATED WITH QTZ-CARB
VEINS; ALTERATION YIELDS ENRICHMENTS
IN K, Rb, Na, As, W, Te
• FLUIDS - MIXED H20-CO2, LOW SALINITY via
METAMORPHIC DEVOLATILIZATION
• ORE PRECIPITATION RELATED TO
PRESSURE FLUCTUATIONS & CO2 UNMIXING
OR FLUID-ROCK REACTION (pH/Eh shifts)
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The Birimian of West Africa
Mine / Deposit Million oz Tarkwa 18.9
Damang 3.8
Morila 5.8
Loulo 10.94
Tongon 4.27
Iduapriem 3.5
Obuasi 33.43
Siguiri 4.95
Sabodala 3.51
Yatela 0.34
Bibiani 7.32
Syama 7.8
Sadiola ~ 8.0
Lefa 3.87
Ahafo - Including Subika 15.4
Ahafo 9.7
Subika 6.4
Akyem 7.6
Bogoso/Prestea 14.91
OROGENIC
GOLD........
WEST AFRICA PRODUCES ca. 250tpa FROM A RESOURCE BASE >165Moz Au
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OBUASI.....a classic shear zone
hosted orogenic Au deposit
...over 8km long and 1.5km deep
...has produced over 20Moz Au and
still has 33Moz remaining
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MORILA
tonalite
DAMANG
....but the Birimian is not only
about conventional orogenic gold....
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DAMANG....
• Au mineralization is hosted in palaeoplacer
style Tarkwaian qtz-pebble conglomerates and
quartzite
• Au mineralization is also hosted in and
alongside sub-horizontal qtz-pyrite veins that
introduce gold into the quartzites and overlying
phyllites as well as mafic intrusions
White et al., 2010
White et al., 2012a,b
White et al., 2014
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White et al., 2010
DAMANG....detailed IR spectroscopic study utilized to identify and map
unusual rock-buffered alteration assemblages in amphibolite grade
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White et al., 2010
White et al., 2012a,b
White et al., 2014
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White et al., 2014
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HAMMOND ET AL., 2011
RANDGOLD RESOURCES
ANNUAL REPORT 2015
MORILA....
amphibolite grade,
flat
lying structures and
a tabular ore-body
that does not crop
out on the surface
......alteration
assemblage
indistinct from the
country rock
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Jaguin et al., 2012,2013
MURCHISON GREENSTONE BELT,
NE MPUMULANGA PROVINCE, RSA
MGB HOSTS 2 DISTINCT STYLES OF
MINERALIZATION:-
•Orogenic style Au-Sb - ANTIMONY LINE
•VMS style Cu-Zn – COPPER-ZINC LINE
Cu-Zn LINE
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RUBBERVALE Fm – 2970 Ma
(Brandl et al., 1996)
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VMS (Cu-Zn) MINERALIZATION IN RUBBERVALE
Fm. AND OROGENIC or INTRUSION RELATED Au
MINERALIZATION IN ANTIMONY LINE ARE COEVAL
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INTRUSION-RELATED GOLD SYSTEMS
[IOCG]
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....after GROVES et al., 2016
IOCG....must be the most ‘confused’ class of mineral deposit types, without a
common link in terms of tectonic setting, emplacement level or age connotation....
OR IS IT THAT THEY ARE ALL RELATED TO A SPECIFIC FERTILE MAGMA TYPE?
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BUSHVELD COMPLEX....one of the great metallotects of the World (PGE-
Cr-Fe/Ti/V-Cu-Ni) ....also has an overlying granitic phase that is the host
to numerous polymetallic IOCG like deposits (Fe-Cu-Ag/Au-U)
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24
BOSCHHOEK..... a high-level
Fe-Cu-Zn-Ag-Au system
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INTRUSION-RELATED GOLD SYSTEMS
[REDUCED W-Bi-Te]
‘RIRGS’
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Robert et al., 2007
UZBEKISTAN
Muruntau – 200 Moz
ALASKA:
Fort Knox – 8 Moz
Donlin Creek
DO THEY OCCUR ELSEWHERE ?
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Koneev et al., 2005;
IGCP PROJECT 486
Morelli et al., 2007
MURUNTAU..... RIRGS
The association of Au with
Bi-W-Te etc points to a very
specific magma source
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TWANGIZA-NAMOYA GOLD BELT, DRC
(Kivu Provinvce)
• Kibaran orogenic belt (1300 - 900 Ma)
• Sn, W, Ta-Nb, Au-Bi mineralization
associated with the 975 Ma G4 S-type granites
• Mineralization at Twangiza may also be
related to younger intrusions at 750 Ma
• Au mineralization hosted in folded
Mesoproterozoic sediments and porphyry sills
TECHNICAL REPORT –
PHASE 1 TWANGIZA GOLD
PROJECT DRC, 2011
(BANRO)
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BURMA
• Potentially one of the most
prospective unexplored
mineral jurisdiction on the
planet
• Hugely rich in a variety of
commodities
Au, Pb-Zn, Sn-W, Cu-Ag,
• Opening up of country
prompted re-interest
• However limited recent
geological work
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EARLY- MIDDLE PERMIAN
LATE PERMIAN 260 MA
LATE TRIASSIC - EARLY JURASSIC 230-200 MA
LATE CRETACEOUS 70 MA
Sibumasu
Palaeo-Tethys Back-ArcBasin
Shelf Carbonates
SUKHOTHAI
ARC
Palaeo-Tethys
Palaeo-Tethys Sediments
Neo-Tethys
SUKHOTHAI
ARC
Sibumasu
Sibumasu
Gondwana Indochina
India
Indochina
Indochina
Indochina
Nan Suture
Chiang Rai Line
PALAEO-TETHYS SUTURE
SUKHOTHAIARC
MAINRANGE
PROVINCE
MAINRANGE
PROVINCE
Shelf Carbonates
W E
Nan Suture
Chiang Rai Line
PALAEO-TETHYS SUTURE
Back-arccollapse
I-Type S-Type
WUNTHO-
POPA
ARC
MMM
BELT
S-Type
SUKHOTHAIARC
Two major orogenic
events:
- Closure of Palaeo-
Tethys leading to
Indosinian Orogeny
-Closure of Neo-Tethys
leading to Himalayan
Orogeny
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The geology and metallogeny of Burma is dominated by the presence of 2 magmatic arcs:-
• Wuntho-Popa Arc (I-type, oxidized, Cu-Ag)
• Mogok-Mandalay-Mergui Belt (S-type, reduced, W-Sn, Au)
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PALAEO-PLACER GOLD
SYSTEMS
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Phillips,2013
Phillips,2013
WITWATERSRAND....
•Over 30% of all the gold ever
mined in the history of mankind
(>55 000 t) has come from the
Wits Basin....
• Does the future of the Wits
Basin lie in re-examining the
prevailing metallogenic models?
magnetics
…octohedral gold crystal in
quartz vein, Rose Deep
Basal Reef…
gold and pyrite along foresets
Lawrie Minter
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BUT......
WHAT HAS BEEN LARGELY
OVERLOOKED IS THE
SCALE
NATURE
TIMING
OF GOLD REMOBILIZATION
IN THE HOST SEDIMENTS
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ONE FLUID TYPE THAT HAS BEEN OVERLOOKED IN TERMS
OF ITS METAL-CARRYING CAPACITY AND EXTENSIVE FLUID
PATHWAYS IS AQUEO-CARBONIC AND ASSOCIATED WITH
HYDROCARBON FORMATION.......
Drennan and
Robb, 2006
Oberthur,
2013
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PIXE micro-maps of bitumen nodules suggest that the aqueo-carbonic fluids contained
organo-metallic complexes capable of transporting significant quantities of Au, U, Th,
As, Fe and Cu
Drennan and
Robb, 2006
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THESE AQUEO-CARBONIC FLUIDS COULD HAVE TRANSPORTED GOLD TO A
VARIETY OF STRUCTURAL AND/OR CHEMICAL TRAP SITES.........
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METALLOGENIC MODELS & GOLD
EXPLORATION
MODELS....very useful but need to be applied
imaginatively in exploration
Conventional exploration models still very
applicable, but new discoveries might require
looking at things differently..................................
• intrusion-related gold in orogenic belts
• understand IOCG systems better
• late, extensional, flat-lying structures
• relationship between exhalative and orogenic
processes
• redox trap sites in palaeo-placer deposits
...........WHERE ARE THE NEW GOLD TARGETS?
• Palaeoproterozoic of West Africa - Borborema
• Central Asian Orogenic Belt
• Tethyan Orogenic Belt
• Russian ‘far east’