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METALLOGENIC MODELS AND GOLD EXPLORATION Laurence Robb Dept. of Earth Sciences, University of Oxford, UK
METALLOGENIC MODELS AND GOLD EXPLORATION
Laurence Robb
Dept. of Earth Sciences, University of Oxford, UK
RANDGOLD RESOURCES
ANNUAL REPORT 2015
THE GOLD CYCLE.....BOOM & BUST
....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
Wedepohl, 1978
GOLD ABUNDANCES........
Gold deposits 2-10 ppm
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’
....after F. Robert et al., 2007
Witwatersrand
Basin
Murantau
Twangiza
Damang
METALLOGENIC MODELS FOR GOLD
Murchison
Bin Yauri
Morila
Bushveld
granites
Burma
IOCG??
OROGENIC GOLD SYSTEMS
....after Goldfarb & Groves,
2015
‘CRUSTAL CONTINUUM’ MODEL
NOT WIDELY
PRESERVED
THE NORM
DIFFICULT TO FIND
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)
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
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
MORILA
tonalite
DAMANG
....but the Birimian is not only
about conventional orogenic gold....
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
White et al., 2010
DAMANG....detailed IR spectroscopic study utilized to identify and map
unusual rock-buffered alteration assemblages in amphibolite grade
White et al., 2010
White et al., 2012a,b
White et al., 2014
White et al., 2014
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
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
RUBBERVALE Fm – 2970 Ma
(Brandl et al., 1996)
VMS (Cu-Zn) MINERALIZATION IN RUBBERVALE
Fm. AND OROGENIC or INTRUSION RELATED Au
MINERALIZATION IN ANTIMONY LINE ARE COEVAL
INTRUSION-RELATED GOLD SYSTEMS
[IOCG]
....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?
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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BOSCHHOEK..... a high-level
Fe-Cu-Zn-Ag-Au system
INTRUSION-RELATED GOLD SYSTEMS
[REDUCED W-Bi-Te]
‘RIRGS’
Robert et al., 2007
UZBEKISTAN
Muruntau – 200 Moz
ALASKA:
Fort Knox – 8 Moz
Donlin Creek
DO THEY OCCUR ELSEWHERE ?
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
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)
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
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
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)
PALAEO-PLACER GOLD
SYSTEMS
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
BUT......
WHAT HAS BEEN LARGELY
OVERLOOKED IS THE
SCALE
NATURE
TIMING
OF GOLD REMOBILIZATION
IN THE HOST SEDIMENTS
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
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
THESE AQUEO-CARBONIC FLUIDS COULD HAVE TRANSPORTED GOLD TO A
VARIETY OF STRUCTURAL AND/OR CHEMICAL TRAP SITES.........
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’
