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
deposits are typically
hosted by volcanic rocks
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 intermediatesulphidation 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 (basaltrhyolite) volcanism in extensional (rift) settings
• Steam-heated environment,
hot-spring sinter and paleowater 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, Chile
Discovered 1969, United Nations
3,300 Mt @ 0.63% Cu, 0.016% Mo
Bajo de la Alumbrera, Argentina
Discovered 1971, United Nations
700 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, Chile
Discovered 1983, Amax
8.46 Moz Au Eq
Vuggy quartz in discovery outcrop: residue
after leaching by highly acidic fluid
Pascua-Lama HS deposit, Chile-Argentina
Discovered 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, Philippines
Discovered 1983, RGC Exploration
>500 Mt @ 0.4% Cu, 0.4 g/t Au
Deposit concealed beneath advanced
argillic lithocap containing highsulphidation mineralization
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Tops of low- and intermediate-sulphidation
epithermal deposits
El Peñón, Chile
Blind vein discoveries 1998-2007, Meridian Gold
8.4 Mt @ 14 g/t Au, 234 g/t Ag
Fence drilling to intersect predicted favourable stratigraphic interval
Quebrada Colorada
Esquel, Argentina
Discovered 2000, Minera El Desquite (Brancote)
3.8 Moz Au, 7 Moz Ag
Recognition that two-thirds of deposit is concealed beneath
pre-mineral cover
Fruta del Norte, Ecuador
Discovered 2006, Aurelian
13.7 Moz Au, 22.4 Moz Ag
Drilling deep beneath a linear silicified zone
containing anomalous arsenic and antimony
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Recognition of new mineralization styles
Montana Tunnels, Montana, USA
Discovered 1983, Centennial Minerals
61 mt @ 0.96 g/t Au, 12 g/t Ag, 0.67% Zn,
0.28% Pb
Disseminated mineralization in
phreatomagmatic diatreme breccia
Marte porphyry gold deposit, Chile
Discovered 1982,
Anglo American-Cominco j.v.
Soon led to discovery of
nearby Lobo porphyry gold
deposit by same j.v.
Combined: 5.5 Moz Au
Wafi, Papua new Guinea
Discovered 1990, CRA Exploration
Hypogene copper enrichment due to
high-sulphidation copper sulphides at
base of lithocap overprinting porphyry
copper-gold deposit
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District-scale porphyry copper alignments
and clusters (“trendology”)
Detailed geology and scout RC
drilling beneath alluvial cover
Chuquicamata district, Chile
Orogen-parallel alignment
New discoveries 2000-2006, Codelco
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
Regional-scale belts and provinces become focus of exploration for
specific epithermal deposit type
After Riley et al. (2001)
After John et al. (2000)
Examples:
• Northern Nevada rift: 16-14 Ma
• Patagonia: 160-150 Ma
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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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13 19 22
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35 39
45 50 53
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12 18 21 26 28
34 38
42 44 49 52 55
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58 61 63 70 72
7 10 11 17 20 25 27 32 33 37 40 41 43 48 51 54 56 57 60 62 69 71
1970
Circum-Pacific Region
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15
1980
74
1990
Discovery Year
2000
1990
2000
Geological work
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2
1970
6
1980
Geochemistry
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2
1970
6
1980
1990
2000
1980
1990
2000
1980
1990
2000
Geophysics
4
2
1970
6
4
77 79
73 75 76 78
Serendipity
Drilling
80 81
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%)
2
1970
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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
Rio Tinto, 2008
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
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