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Porphyry and Epithermal Systems - Science-driven Exploration Successes. Richard Sillitoe. Porphyry and epithermal deposits. Porphyry copper-molybdenum and copper-gold deposits are centred on shallow-level porphyry intrusions. Grasberg, Indonesia. Epithermal gold and silver - PowerPoint PPT Presentation
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Porphyry and Epithermal Systems -Science-driven Exploration Successes
Richard Sillitoe
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Porphyry and epithermal deposits
Porphyry copper-molybdenum and copper-gold deposits are centred on shallow-level porphyry intrusions
Epithermal gold and silver deposits are typically
hosted by volcanic rocks
Grasberg, Indonesia
Round Mountain, Nevada
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Porphyry and epithermal systems – the state of play at end of 1960s (1)
• Epithermal-hot spring linkage proposed (D.White), but relationship uncertain
• Types of epithermal deposits not appreciated (forgetting F.L.Ransome and W.Lindgren)
• Porphyry-epithermal connection unknown• Porphyry intrusion-volcano connection unknown• Plate tectonic setting and relationship of porphyry copper deposits to
subduction unrecognised (plate tectonics in its infancy)
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Porphyry and epithermal systems – the state of play at end of 1960s (2)
• Ages of copper and gold belts and provinces poorly defined (isotopic dating in its infancy)
• Zoning patterns of hydrothermal alteration in porphyry and epithermal deposits poorly appreciated
• Porphyry gold and gold-rich porphyry copper deposits undefined (notwithstanding Panguna)
• Bulk-tonnage epithermal gold deposits undefined (reflecting low gold price)
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The favourite buzz words for today’s press releases on porphyry and epithermal projects
HOT-SPRING SINTER
STEAM-HEATED ENVIRONMENT
PALEO-WATER TABLE
HIGH-SULPHIDATION SYSTEM
VUGGY QUARTZ
LOW-SULPHIDATION SYSTEM
POTASSIC ALTERATION
DIATREME-HOSTED
HYPOGENE COPPER ENRICHMENT
ADVANCED ARGILLIC LITHOCAP
Unknown to the exploration community at the end of 1960s
Active hot-spring sinter terrace,Champagne Pool, New Zealand
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Porphyry-epithermal relationships
Linkages between porphyry, high- and intermediate-sulphidation epithermal, skarn, carbonate-replacement, and Carlin-like environments now widely appreciated
The necessary information was supplied by worldwide exploration activities
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High-sulphidation - porphyry transition
• 1.5 – 2 km vertical interval represented from paleo-surface to porphyry deposit
• Vuggy quartz → quartz-alunite → quartz-pyrophyllite → quartz-sericite from top downwards
• Au-dominated → Cu-dominated from top downwards
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Shallow epithermal features
• High- and intermediate sulphidation deposits with andesitic-dacitic arc volcanism
• Low-sulphidation deposits with compositionally bimodal (basalt-rhyolite) volcanism in extensional (rift) settings
• Steam-heated environment, hot-spring sinter and paleo-water table silicification recognised above Au-Ag mineralization
- Again exploration supplied the data
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Deposit-scale features
Alteration zoning — recognising importance of potassic alteration for copper introduction in porphyry copper deposits
Los Pelambres, ChileDiscovered 1969, United Nations
3,300 Mt @ 0.63% Cu, 0.016% Mo
Bajo de la Alumbrera, ArgentinaDiscovered 1971, United Nations700 Mt @ 0.51% Cu, 0.66 g/t Au
Potassic alteration(beneath weathering zone)
Potassic alteration(weathered at surface)
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Deposit-scale features
Alteration zoning — recognising significance of alteration types in high-, intermediate- and low-sulphidation epithermal systems
La Coipa HS deposit, ChileDiscovered 1983, Amax
8.46 Moz Au Eq
Vuggy quartz in discovery outcrop: residue after leaching by highly acidic fluid
Pascua-Lama HS deposit, Chile-ArgentinaDiscovered 1989, Lac Minerals,
then Barrick Gold
Barren steam-heated alteration above 16 Moz gold and >600 Moz silver
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Tops of porphyry copper deposits
Guinaoang porphyry copper-gold system, PhilippinesDiscovered 1983, RGC Exploration>500 Mt @ 0.4% Cu, 0.4 g/t Au
Deposit concealed beneath advanced argillic lithocap containing high-
sulphidation mineralization
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Tops of low- and intermediate-sulphidation epithermal deposits
El Peñón, ChileBlind vein discoveries 1998-2007, Meridian Gold
8.4 Mt @ 14 g/t Au, 234 g/t AgFence drilling to intersect predicted favourable stratigraphic interval
Esquel, ArgentinaDiscovered 2000, Minera El Desquite (Brancote)3.8 Moz Au, 7 Moz AgRecognition that two-thirds of deposit is concealed beneath pre-mineral cover
Fruta del Norte, EcuadorDiscovered 2006, Aurelian
13.7 Moz Au, 22.4 Moz AgDrilling deep beneath a linear silicified zone containing anomalous arsenic and antimony
Quebrada Colorada
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Recognition of new mineralization styles
Disseminated mineralization in phreatomagmatic diatreme breccia
Soon led to discovery of nearby Lobo porphyry gold deposit by same j.v.Combined: 5.5 Moz Au
Wafi, Papua new GuineaDiscovered 1990, CRA Exploration
Hypogene copper enrichment due to high-sulphidation copper sulphides at base of lithocap overprinting porphyry copper-gold deposit
Montana Tunnels, Montana, USADiscovered 1983, Centennial Minerals61 mt @ 0.96 g/t Au, 12 g/t Ag, 0.67% Zn, 0.28% Pb
Marte porphyry gold deposit, ChileDiscovered 1982,
Anglo American-Cominco j.v.
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District-scale porphyry copper alignments and clusters (“trendology”)
Chuquicamata district, ChileOrogen-parallel alignment
New discoveries 2000-2006, Codelco
Detailed geology and scout RC drilling beneath alluvial cover
Oyu Tolgoi district, Mongolia Arc-transverse alignment
New discovery 2007, Ivanhoe Mines(supplied by I.Kavalieris)
Deep IP survey – on trend
• Comparable brownfield discoveries in Los Bronces alignment (Sulfatos, Anglo American) and Escondida cluster (Pampa Escondida, MEL), Chile
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Definition of porphyry copper belts and epochs
Isotopic dating has defined regional-scale belts and corresponding epochs in most porphyry copper provinces
Isotopic dating now routine selection tool in Andean copper province and elsewhere
Examples:• Gaby (Gabriela Mistral): 540Mt @
0.52% Cu Ox – prioritised during initial exploration because of 43 Ma age
• Other prospects – discarded because of 290-200 Ma ages
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Definition of epithermal belts and provinces
Examples:• Northern Nevada rift: 16-14 Ma• Patagonia: 160-150 Ma
Regional-scale belts and provinces become focus of exploration for specific epithermal deposit type
After Riley et al. (2001)
After John et al. (2000)
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Origin of metallogenic provinces
Tectono-magmatic processes or predisposition?
• Clustering of 10 Moz Au belts and isolated deposits of different types and ages• Suggestive of predisposition – metal preconcentration or other chemical parameter
(e.g. redox state)• Focus exploration on endowed arc segments, but usually well explored (exception
Colombian Andes)• Or define unrecognised gold-rich arc segments – but how?
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Key role of geology in porphyry and epithermal exploration
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1970 1980 1990 2000
Geological work
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1970 1980 1990 2000
Geochemistry
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1970 1980 1990 2000
Geophysics
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1970 1980 1990 2000
Drilling
Serendipity
Parameters
• 37–year history
• 81 deposits
• Mainly porphyry, epithermal, & sediment-hosted gold (minor VMS & orogenic gold)
Main conclusions
• Notwithstanding exploration changes, little overall evolution in discovery methodology (but see next slide)
• Geologic fieldwork: 90% of discoveries
– routine observation, mapping, & interpretation
– familiarity with deposit models (since 1980s)
• Geochemistry: 70% of discoveries
– stream sediment, soil, & rock chip
• Geophysics: 15% of discoveries (only 50% of programs)
– Ground IP & EM
• Drilling & serendipity: 12% of discoveries
• Remote sensing (satellite imagery, airborne scanners: 0%)
Circum-Pacific Region
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Discovery Year
1970 1980 1990 2000
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The future of porphyry and epithermal exploration
We need:• New geological concepts• Characterisation of distal manifestations of concealed and potentially
deep orebodies• New technological break-throughs• Properly qualified and motivated personnel to do the job
Last 40 years have brought great advances in the porphyry-epithermal environment; next 20 years must bring even greater advances if we are to satisfy growing demand for copper,
gold and silver and societal expectations in general – all within increasingly stringent environmental and community constraints
Rio Tinto, 2008