Porphyry and Epithermal Systems -Science-driven Exploration Successes

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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
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 todays 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, Chile Discovered 1969, United
Nations 3,300 Mt _at_ 0.63 Cu, 0.016 Mo
Bajo de la Alumbrera, Argentina Discovered 1971,
United Nations 700 Mt _at_ 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 gt600 Moz silver
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Tops of porphyry copper deposits
Guinaoang porphyry copper-gold system,
Philippines Discovered 1983, RGC Exploration gt500
Mt _at_ 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, Chile Blind vein discoveries 1998-2007,
Meridian Gold 8.4 Mt _at_ 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 _at_ 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 _at_ 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
Circum-Pacific Region

• Parameters
• 37year 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)

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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