Gold Quartz Vein, AcupanEPITHERMAL GOLD DEPOSITS A LECTURE ON Antamok Mine, 2005 OUTLINE • INTRODUCTION – Why Is Epithermal Deposit Important? – Historical Perspective – What is an Epithermal Deposit? – Classes of Epithermal Deposits • CHARACTERISTICS OF EPITHERMAL DEPOSITS • KEY PROCESSES IN THE FORMATION OF EPITHERMAL DEPOSITS • HIGH SULFIDATION DEPOSITS • INTERMEDIATE AND LOW SULFIDATION DEPOSITS • GEOTHERMAL WATERS, STEAM HEATED ZONES, WATER TABLE MOVEMENT • EPITHERMAL VEIN TEXTURES • EXPLORATION IMPLICATIONS Quartz - truscottite Vein, Lebong Donok, 2007 WHY IS EPITHERMAL DEPOSIT IMPORTANT? Saunders 2010 Bonanza Gold From Sleeper Deposit, Nevada BECAUSE OF GOLD! Gold Price Today Sept 17, 2010: US$1,273.30 per troy ounce! Average grade of primary gold discoveries and gold produced over time. Mine production data for 1950 to 2000 from Mudd (2000) and 2000 to 2010 production data from Fellows (2010). What does this means? The world needs more exploration geologists now and in the next decade! THE FUTURE OF GEOLOGY & MINING STUDENTS IS BRIGHT! Source: McKeith, Schodde & Baltis, SEG Newsletter April 2010 •Peak Au discoveries – 1988 •Steady decrease from 1990- 2010 •Discoveries in late1980s >200M oz Au/yr •Discoveries in 2010 estimated <80M oz Au •Cost of exploration is increasing from: $3/oz – 1950 -1960 $20/oz – 1980s $40/oz -2000s Newly discovered gold resources (including by-product gold) compared to world gold production (with and without South Africa). Scenarios presented cover a range of possible resource conversion factors (40 to 100 percent). Gold from Lebong Tandai Drillcore, 2007 Epithermal Gold Deposits A very important style of gold deposit • Can be very big: •Lihir, PNG 170 Mt @ 3.5 g/t Au (595 t or 19.13 Moz Au) •Porgera, PNG 85 Mt @ 5.8 g/t Au, 33 g/t Ag (493 t or 15.85 Moz Au & 2,805 t Ag) • Can be very rich: •Cripple Creek, USA 630 t Au in veins grading 15 to 30 g/t •Hishikari, Japan 264 t Au, Honko veins 70 g/t Au, 49 g/t Ag White, 2009 Steam rises from the active hydrothermal system at Ladolam gold mine on Lihir Island, Papua New Guinea. Ladolam contains ~600 tonnes of gold and could have formed in 55,000 years. Photo by K. Brown and S. Simmons. LIHIR, PNG PORGERA, PNG Porgera is a very big gold and copper deposit in the highlands of PNG. It is both an open pit and underground operation by Barrick Gold Corporation, the largest gold mining company in the world. Hishikari gold mine’s Keisen No. 3 vein (left). The gold grade at Hishikari is 10X the average of worldwide deposits and contains 264 tonnes Au (8.5 Moz Au). The Hishikari mine (above) is located in northern Kagoshima Prefecture. The hot water seeping into mineshafts is supplied to the nearby hot spring spa. Photo credits: National Institute of Advanced Industrial Science and Technology (AIST) and Sumitomo Metal Mining Co., Ltd. HISHIKARI MINE, JAPAN Relative Amounts of Gold 50% Witwatersrand 12% Epithermal 10% Porphyry (+ intrusion hosted) 12% Sediment hosted (incl. 4% “Carlin”) 9% Greenstone lode (mesothermal or “orogenic”) 7% Other (Fe Fm, VHMS, etc.) Arribas, 2000 HISTORICAL PERSPECTIVE • Mining of gold dates back to prehistoric time (placer). • Gold ore mined before 2500 BC. • Gold discovered in California, 1848. • The word epithermal was defined by W. Lindgren in 1922 & 1933. Early underground mining De Re Metallica, Agricola 1556 What is an Epithermal Deposit? “Epithermal” was derived from two Greek words epi – above and therme - hot springs. It refers to deposits formed at low temperature and shallow depths . The term Epithermal was first defined by Lindgren (1922, 1933) based on observations of -mineralogy of ores and alteration -textures of ores and alteration and inferences about -temperature of deposition -depth of formation Quartz-amythest vein, Lebong Tandai Quartz-amythest vein cut by white quartz vein, Lebong Tandai What is an Epithermal Deposit? Epithermal deposits are recognizable by their •Characteristic minerals and textures •Characteristic hydrothermal alteration mineralogy and zoning These characteristics aided by fluid inclusion data indicate that epithermal deposits •Formed at low temperatures (100° to 320°C, typically 160° to 270° Celsius) •Developed at shallow crustal levels (<1 km, typically 50 to 700 m depth below the water table) Classification of epithermal deposits depends on multiple features. Hedinquist et al., 2002; N. White, 2009 Schematic section showing the depositional environment and crustal depth of the main gold systems. Modified from Poulsen et al. (2000) and Robert (2004) What is an Epithermal Deposit? • Epithermal deposits include a wide range of deposit styles – they are not all the same! • The different deposit classes are not fully characterized nor fully understood. • Not all epithermal deposits contain gold – some are dominated by other metals, notably Ag, Zn, Pb, Cu, Sn. • Some are closely related to intrusions, some are not. The related intrusions need not be porphyry copper-related intrusions. • Many different terms have been used to classify epithermal deposits – terminology is very confused! History of Nomenclature for Epithermal Deposits (Sillitoe and Hedinquist, 2003) Goldfield type Ransome (1907) Alunitic kaolinic gold veins Sericitic zinc-silver veins Gold-silver-adularia veins Fluoritic tellurium-adularia gold veins Emmons (1918) Gold-alunite deposits Argentite-gold quartz veins Argentite veins Base metal veins Gold quartz veins in rhyolite Gold telluride veins Gold selenide veins Lindgren (1933) Secondary quartzite Fedorov (1903); Nakovnik (1933) Acid Alkaline Sillitoe (1977) Epithermal Buchanan (1981) Enargite-gold Ashley (1982) Hot-spring type Giles and Nelson (1982) High Sulfur Low sulfur Bonham (1986, 1988) Acid sulfate Adularia-sericite Hayba et al. (1985) Heald et al. (1987) High sulfidation Low sulfidation Hedinquist (1987) Alunite-kaolinite Adularia-sericite Berger and Henley (1989) Type 1 adularia-sericite Type 2 adularia-sericite Albino and Margolis (1991) High sulfidation High sulfide + base metals, low sulfidation Low sulfide + base metals, low sulfidation Sillitoe (1993) High sulfidation Western andesite assemblage, low sulfidation Bimodal basalt-andesite assemblage, low sulfidation John et al. (1999), John (2001) High sulfidation (HS) Intermediate sulfidation (IS) Low sulfidation (LS) Hedinquist et al. (2000) Three Classes of Epithermal Deposits Three classes based on the fluids that formed the epithermal deposits: 1. High Sulfidation (HS) - Magmatic 2. Intermediate Sulfidation (IS) - Magmatic-Meteoric 3. Low Sulfidation (LS) - Meteoric N. White, 2009 Au-Ag-Zn-Pb Au-Ag-Cu Au-Ag N. White 2009 Characteristics Fluids: Magmatic dominant in core mixed with meteoric on margins Metal Associations: 1 I-type: a) Cu-Au-Ag b) Zn-Pb-Ag 2 S-type: Sn-Ag (Zn-Pb) 3 A-type: Au-Ag Alteration: 1a, b and 2: proximal very acid 3 proximal not seen; distal neutral Examples: 1a) Lepanto, Philippines 1b) Cerro de Pasco, Pero 2 Cerro Rico de Potosi, Bolivia 3 Porgera, PNG N. White, 2009 Characteristics Fluids: Dominantly meteoric, with high salinity magmatic fluids at depth Metal Associations: Ag-Zn-Pb-(Au) Ag-Zn-Pb-(Cu-Sn) Alteration: Mostly neutral pH Examples: Fresnillo, Mexico Comstock, USA Acupan and Antamok, Philippines Cikotok, Indonesia Aisasjur, Indonesia Modified from N. White, 2009 N. White, 2009 Characteristics Fluids: Meteoric (± magmatic) Metal Associations: Au-Ag (very minor Zn, Pb) Alteration: Hypogene - neutral pH; Gas condensates - acid Examples: McLaughlin, USA Hishikari, Japan Lebong Donok, Indonesia Gunung Pongkor, Indonesia Waihi, New Zealand Diwalwal?, Philippines Epithermal Deposits Characteristics Quartz-adularia-clay veinlets, Acupan Tectonic Setting of Gold-rich Epigenetic Mineral Deposits Groves et al., 1998 Styles and Geometries of Epithermal Deposits Diagram shows the influence of structural, hydrothermal and lithologic controls on permeability or fluid conduits. Sillitoe, 1993 Hydrothermal Alteration – LS & HS Epithermal Systems North Pole Mining District, Pilbara, Western Australia 3.5 billion year-old epithermal vein texture Tarutung, North Sumatra, Indonesia Very young opaline vein deposited from hot spring coming out of a vertical fracture Ages of Epithermal Deposits • Epithermal deposits are most common in young environments (due to preservation, not process) • They can be found whenever favourable geology is preserved • Youngest deposits are forming NOW; oldest known are Early Archean Epithermal Deposits Key Processes Boiling is the critical process to deposit high amount of gold in LS epithermal deposits. •Au concentration in deep water prior to boiling & gas loss: 10 µg/kg •Au concentration in hot spring waters: <0.1 µg/kg •Au precipitates during ascent & boiling Colloform-crustiform banded quartz- adularia choking a geothermal pipe. Porgera Zone 7, PNG Bonanza epithermal quartz gold-silver mineralization with wire gold, quartz and roscoelite. Sleeper Deposit, Nevada High grade bonanza colloform banded gold and chalcedony Corbett, 2002 Saunders, 2010 Silica deposition by cooling Hedinquist et al., 1998 Tarutung opal vein, North Sumatra Low sulfidation vein texture, Mc Laughlin, California, USA (Photo by Y. Matsuhisa) Silica deposition is affected by pH • Neutral pH - Quartz, chalcedony and amorphous silica deposit - Spectacular textures! • Acid pH - Silica deposition suppressed - No siliceous veins Significance of Alunite Its formation requires 1. Acid conditions 2. High sulfate 3. Available alkalis These conditions can occur from 1. Magmatic gases (HS) 2. Near-surface condensation of boiled off gases (HS, IS, LS) 3. From supergene oxidation (any sulfide rich rock) Lithocap from the Baguio District Pearly radial cluster of pyrophyllite, Hillsboro District, North Carolina, USA Radiating fans of golden- brown pyrophyllite needles, Champion Mine, California, USA High Sulfidation Deposits Lepanto Quartz-Alunite Zone Satsuma volcanic–hydrothermal system. 870°C fumaroles vent from summit of rhyolitic dome; acidic hotspring (pH 1.5) rich in Fe and Al leached from host rock discharge from volcano flanks to the sea. Kawah Ijen: World’s Largest H 2 SO 4 Crater Lake Ijen Crater, East Java, Indonesia – 1 km wide & 200 m deep lake filled with a sol- ution of H 2 SO 4 & HCl with a pH of 0.5 & temperature of about 33°C. Photo: Ulet Ifansasti/Getty Images, 2009 Aluminum can dissolving in the acid water of Ijen Crater. Somebody having fun rafting in the highly acidic water of Ijen Crater. KAWAH IJEN, JAVA Kawah Ijen, East Java, Indonesia Condensation of magmatic vapor + HCl + SO 2 generates acidic waters (pH ~1 or less): Causes leaching of rocks (vuggy quartz), and hypogene advanced argillic alteration (alunite, kaolinite) Kawah Ijen, East Java Sulfur miners Lepanto lithocap outcrop to south From Palidan slide Mohong Hill Lithocap Surface projections: Lepanto Far Southeast Victoria Lepanto Spanish Adits Outcrop of Lepanto enargite-luzonite ore Epithermal Vein Deposits Low- and Intermediate- Sulfidation Aisasjur, Indonesia Hishikari gold mine’s Keisen No. 3 vein (left). The gold grade at Hishikari is 10X the average of worldwide deposits and contains 264 tonnes Au (8.5 Moz Au). The Hishikari mine (above) is located in northern Kagoshima Prefecture. The hot water seeping into mineshafts is supplied to the nearby hot spring spa. Photo credits: National Institute of Advanced Industrial Science and Technology (AIST) and Sumitomo Metal Mining Co., Ltd. HISHIKARI MINE, JAPAN Lebong Donok Gold Mine, Bengkulu, Indonesia Sumatra Copper and Gold, Ltd. Lebong Donok Stope Grades Sumatra Copper and Gold, Ltd. More than 1.3 Moz Au and >70.5 Moz Ag produced to 1938 3.24 Mt @ 12.8 g/t Au & 70.5 g/t Ag Donok Vein 3 - colloform banded quartz-adularia & vughs Footwall of Main Donok Vein (qtz-cal- truscottite) inside Lubang Kacamata (“Eyeglasses” Dutch Tunnel) Donok LS Epithermal Veins Au - BLEG Ag - BLEG Aisasjur, West Papua, Indonesia ASD-28: 6m @ 2.54 g/t Au from 139.3m – 145.3m depth ASD-5 at 266.5m depth: 5.36 g/t Au & 1570 ppm As ASD-5 at 270.35m depth: 15.6 g/t Au & 4790 ppm As Aisasjur IS Veins West Papua, Indonesia Hydro-brecciated siltstone with stibnite cement Macalalad, 2009 Laser ablation of pyrite from Aisasjur drillhole ASD27. Micron size Au and As occur at the rims of the pyrite. Aisasjur, West Papua, Indonesia Geothermal Waters Steam-heated Zones Water Table Movement Neutral pH chloride water •The deep geothermal fluid – neutral, low salinity •Alters rock to illite, illite- smectite or smectite (depending on T) •At surface produces boiling pools with clear water and silica sinters •This is the potential ore fluid for low sulfidation deposits Tarutung hot spring North Sumatra, Indonesia Geothermal Plant, Wairakei, New Zealand Geothermal Plant, Tiwi, Albay, Photo: NPC Champagne Pool, Waiotapo, New Zealand Photo: N. Macalalad, March 2010 Tarutung Hot Spring & Silica Sinter – North Sumatra R. Gonzales, 2006 Tarutung Silica Sinter – North Sumatra R. Gonzales, 2006 Tarutung Silica Sinter – North Sumatra Micro terraces Stalactites Spiky tubes and flake R. Gonzales, 2006 Travertine deposits, Mammoth Hot Springs, Yellowstone National Park, USA Acid bicarbonate water • Boiling deep fluids expel steam, CO 2 and H 2 S •Gases condense into groundwater to produce weakly acid water (H 2 CO 3 and H 2 S) •Alters rock to produce illite-smectite, calcite and pyrite – can be very wide- spread •Water is clear, spring deposits travertine •NOT related to ore Acid sulfate water •Boiled off CO 2 and H 2 S condense above water table •Atmospheric O 2 oxidizes H 2 S to H 2 SO 4 – strongly acidic •Rock alters to kaolinite and alunite, partly dissolves •Form steaming ground, collapsing ground and mud pools at surface •Colored water •Responsible for “silica cap” •NOT related to ore Yellow-green colored lake, NZ (top) and the colored lakes of Kelimutu, Indonesia (bottom) Collapsing ground, Rotorua, NZ Mud Pool, Rotorua, New Zealand Epithermal Quartz Vein Textures Lebong Donok LS Vein Textures EPITHERMAL QUARTZ VEIN TEXTURES Primary Growth Textures Dong, Morrison & Jaireth, 1995 Crustiform PRIMARY GROWTH TEXTURES Cockade Moss Moss, plane polarized light Comb Zonal Photos: Dong, Morrison & Jaireth, 1995 EPITHERMAL QUARTZ VEIN TEXTURES Dong, Morrison & Jaireth, 1995 RECRYSTALLIZATION TEXTURES a. Mosaic aggregates of microcrystalline quartz crystals with highly irregular and interpenetrating grain boundaries. b. Feathery 1:a feathery appearance in the rims of the crystals with euhedral cores seen only as slight optical differences in maximum extinction positions. In another position (e .g. bottom center) the quartz crystal displays a very similar interference color between the euhedral core and rims c. Feathery 2: a feathery appearance seen as patches throughout quartz crystals d. Flamboyant 1: radial or flamboyant extinction of individual quartz crystals with more or less rounded crystal outline.In this sample, the flamboyant texture is well developed in the rims of crystalline quartz crystal with more or less euhedral cores. e. Flamboyant 2: flamboyant extinctions seen through the crystals with rounded surface in bands. f. Ghost spheres: solid and/or fluid inclusion defined spheres with thin microcrystalline quartz crystals. All samples from Pajingo, Afarti and Crakow (Queensland, Australia), crossed polars. Scale bars = 0.2 mm EPITHERMAL QUARTZ VEIN TEXTURES Dong, Morrison & Jaireth, 1995 REPLACEMENT TEXTURES a. Lattice bladed: a network of intersecting silica blades with polyhedral cavities. b. Lattice bladed: in thin section, each blade consists of a series of parallel seams separated by quartz crystals or crystallites which have grown symmetrically about the seams and perpendicular to them.,B imurraQ, ueensland c. Ghost bladed: blades are identified on the polished surface of the hand specimens by the concentration of impurities. This texture commonly occurs in crustiform bands and lacks the cavities between blades. d. Ghost bladed: aggregates of quartz crystals with superimposhed bladed texture identified by outlines of impurities and finer grain size. e. Parallell bladed: silica blades are parallel within each group but adjacent groups have different orientation. f. Parallel bladed: each group is composed of a set of parallel-oriented quartz crystals which have more or less rectangular shape. All samples from Bimurra and Woolgar (Queensland, Australia). Scale bars = 0.2 mm REPLACEMENT TEXTURES – BLADED QUARTZ, HISHIKARI Lattice-type Bladed Quartz Parallel-type Bladed Quartz Photos: Etoh, Izawa & Watanabe, 2002 REPLACEMENT TEXTURES a. Pseudoacicular aggregates of silica minerals commonly associated with adularia or its weathered products (kaolinite or illite) display radial acicular appearance caused by differences in color and/or relief in hand specimens. b. Pseudoacicular: acicular appearance is indicated under the microscope by linear arrangement of fine-grained quartz crystals and linear distribution of clay minerals. Crossed polars. c. Saccharoidal: loosely packed fine-grained quartz aggregate, having sugary appearance in hand specimens. d. Saccharoidal: under the microscope slender subhedral crystals are randomly distributed in a matrix of smaller anhedral grains. Crossed polars. All samples from Queensland, Australia. Scale bars = 0.2 mm, metric bars = I cm. Schematic models of the formation process of lattice-type (A) and parallel-type (B) bladed quartz. A1, B1 - Precipitation of bladed calcite A2, B2 - Precipitation of adularia and quartz A3, B3 - Dissolution of calcite A4 - Subsequent quartz overgrowth sometimes occurs A. Lattice-type B. Parallel-type Bladed quartz, Tambang Sawah, Bengkulu, Indonesia Etoh, et al., 2002 REFERENCES Dong, G., Morrison,G and Jareth, S., 1995. Quartz textures in epithermal veins, Queensland – classification, origin, and implications; Economic Geology, Vol. 90, 1995, pp.1841 – 1856. Etoh, J., Isawa, E. and Watanabe, K., 2002. Bladed quartz and its relationship to gold mineralization in the Hishikari low sulfidation epithermal gold deposit, Japan; Economic Geology, Vol. 97, 2002, pp. 1841–1851. Hedinquist, J., Arribas, A., Einaudi, M. and Sillitoe, R., 2002. Abstract: Exploration for and assessment of epithermal precious-metal deposits: critical characteristics, and their variations, Denver Region Exploration Geologists’ Society. Saunders, J., 1994. Silica and gold textures in bonanza ores of the Sleeper Deposit, Humbolt County, Nevada: evidence for colloids and implications for epithermal ore forming processes; Scientific Communications, Economic Geology, Vol. 89, 1994, pp. 628-638. White, N., 2009. Epithermal gold deposits: presentation at Gold Deposit Workshop 2009, 11-12 October, 2009, Semarang, Indonesia. White, N. and Hedenquist, J., 1995. Epithermal gold deposits: styles, characteristics and exploration; SEG Newsletter, 1995, No. 23, pp. 1, 9-13.