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