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Porphyry and Epithermal Systems - Science-driven Exploration Successes

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

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Porphyry and Epithermal Systems - Science-driven Exploration Successes

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  1. Porphyry and Epithermal Systems -Science-driven Exploration Successes Richard Sillitoe

  2. 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

  3. 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)

  4. 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)

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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)

  10. 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

  11. 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 high-sulphidation mineralization

  12. 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

  13. 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

  14. District-scale porphyry copper alignments and clusters (“trendology”) • Comparable brownfield discoveries in Los Bronces alignment (Sulfatos, Anglo American) and Escondida cluster (Pampa Escondida, MEL), Chile 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

  15. Definition of porphyry copper belts and epochs Isotopic dating has defined regional-scale belts and corresponding epochs in most porphyry copper provinces 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 Isotopic dating now routine selection tool in Andean copper province and elsewhere

  16. Definition of epithermal belts and provinces Regional-scale belts and provinces become focus of exploration for specific epithermal deposit type Examples: • Northern Nevada rift: 16-14 Ma • Patagonia: 160-150 Ma After Riley et al. (2001) After John et al. (2000)

  17. 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?

  18. Key role of geology in porphyry and epithermal exploration 68 s 6 t i s 16 o 67 p e 15 66 24 47 31 d 4 f o 36 23 46 65 14 30 r e 59 b 64 53 35 39 50 13 19 22 45 29 9 2 m u 58 61 72 74 77 79 63 70 18 38 49 12 52 55 21 26 28 34 42 44 2 8 4 N 10 11 17 20 32 33 37 40 41 43 48 51 54 56 57 60 62 69 71 73 75 76 78 7 25 27 80 81 5 6 3 1 Geological work 1970 2000 1980 1990 6 Discovery Year 4 2 2000 1980 1990 1970 Geochemistry 6 4 2 2000 1980 1990 1970 Geophysics 6 4 2 2000 1980 1990 1970 Serendipity 6 Drilling 4 2 2000 1980 1990 1970 Circum-Pacific Region 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%)

  19. 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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