EPITHERMAL GOLD DEPOSITSA LECTURE ONAntamok Mine, 2005

OUTLINEINTRODUCTIONWhy Is Epithermal Deposit Important?Historical PerspectiveWhat 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 IMPLICATIONSQuartz - truscottite Vein, Lebong Donok, 2007

WHY IS EPITHERMAL DEPOSIT IMPORTANT?Saunders 2010Bonanza 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 2010Peak Au discoveries 1988Steady decrease from 1990- 2010Discoveries in late1980s >200M oz Au/yrDiscoveries in 2010 estimated

Gold from Lebong Tandai Drillcore, 2007Epithermal Gold DepositsA 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/tHishikari, Japan 264 t Au, Honko veins 70 g/t Au, 49 g/t AgWhite, 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, PNGPORGERA, PNGPorgera 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 mines 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% Witwatersrand12% Epithermal10% Porphyry (+ intrusion hosted)12% Sediment hosted (incl. 4% Carlin)9% Greenstone lode (mesothermal or orogenic)7% Other (Fe Fm, VHMS, etc.)

Arribas, 2000

HISTORICAL PERSPECTIVEMining 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 miningDe 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 ofmineralogy of ores and alterationtextures of ores and alterationand inferences abouttemperature of depositiondepth of formationQuartz-amythest vein, Lebong Tandai

Quartz-amythest vein cut bywhite quartz vein, Lebong TandaiWhat is an Epithermal Deposit?Epithermal deposits are recognizable by theirCharacteristic minerals and texturesCharacteristic hydrothermal alteration mineralogy and zoningThese characteristics aided by fluid inclusion data indicate that epithermal depositsFormed at low temperatures (100 to 320C, typically 160 to 270 Celsius)Developed at shallow crustal levels (

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 typeRansome (1907)Alunitic kaolinic gold veinsSericitic zinc-silver veinsGold-silver-adularia veinsFluoritic tellurium-adularia gold veinsEmmons (1918)Gold-alunite depositsArgentite-gold quartz veinsArgentite veinsBase metal veinsGold quartz veins in rhyoliteGold telluride veinsGold selenide veinsLindgren (1933)Secondary quartziteFedorov (1903); Nakovnik (1933) AcidAlkalineSillitoe (1977)EpithermalBuchanan (1981)Enargite-goldAshley (1982)Hot-spring typeGiles and Nelson (1982)High SulfurLow sulfurBonham (1986, 1988)Acid sulfateAdularia-sericiteHayba et al. (1985)Heald et al. (1987)High sulfidationLow sulfidationHedinquist (1987)Alunite-kaoliniteAdularia-sericiteBerger and Henley (1989)Type 1 adularia-sericiteType 2 adularia-sericiteAlbino and Margolis (1991)High sulfidationHigh sulfide + base metals, low sulfidationLow sulfide + base metals, low sulfidationSillitoe (1993)High sulfidationWestern andesite assemblage, low sulfidationBimodal basalt-andesite assemblage, low sulfidationJohn et al. (1999), John (2001)High sulfidation (HS)Intermediate sulfidation (IS)Low sulfidation (LS)Hedinquist et al. (2000)

Three Classes of Epithermal DepositsThree classes based on the fluids that formed the epithermal deposits:

High Sulfidation (HS) - Magmatic

2. Intermediate Sulfidation (IS) - Magmatic-Meteoric

3. Low Sulfidation (LS) - Meteoric

N. White, 2009

CharacteristicsFluids: Magmatic dominant in core mixed with meteoric on marginsMetal Associations:I-type: a) Cu-Au-Ag b) Zn-Pb-AgS-type: Sn-Ag (Zn-Pb)A-type: Au-AgAlteration:1a, b and 2: proximal very acid3 proximal not seen; distal neutralExamples:1a) Lepanto, Philippines1b) Cerro de Pasco, Pero Cerro Rico de Potosi, Bolivia Porgera, PNGN. White, 2009

CharacteristicsFluids: Dominantly meteoric, with high salinity magmatic fluids at depthMetal Associations:Ag-Zn-Pb-(Au)Ag-Zn-Pb-(Cu-Sn)Alteration:Mostly neutral pHExamples:Fresnillo, MexicoComstock, USAAcupan and Antamok, PhilippinesCikotok, IndonesiaAisasjur, IndonesiaModified from N. White, 2009

N. White, 2009CharacteristicsFluids: Meteoric ( magmatic)Metal Associations:Au-Ag (very minor Zn, Pb)Alteration:Hypogene - neutral pH;Gas condensates - acidExamples:McLaughlin, USAHishikari, JapanLebong Donok, IndonesiaGunung Pongkor, IndonesiaWaihi, New ZealandDiwalwal?, Philippines

Epithermal Deposits Characteristics

Tectonic Setting of Gold-rich Epigenetic Mineral DepositsGroves et al., 1998

Styles and Geometries of Epithermal DepositsDiagram 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 Australia3.5 billion year-old epithermal vein textureTarutung, North Sumatra, Indonesia Very young opaline vein deposited from hot spring coming out of a vertical fracture

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 g/kgAu concentration in hot spring waters:

Porgera Zone 7, PNGBonanza epithermal quartz gold-silver mineralization with wire gold, quartz and roscoelite.Sleeper Deposit, NevadaHigh grade bonanza colloform banded gold and chalcedonyCorbett, 2002Saunders, 2010

Silica deposition by coolingHedinquist et al., 1998Tarutung 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 AluniteIts formation requiresAcid conditionsHigh sulfateAvailable alkalisThese conditions can occur fromMagmatic gases (HS)Near-surface condensation of boiled off gases (HS, IS, LS)From supergene oxidation (any sulfide rich rock)Lithocap from the Baguio District

Pearly radial cluster of pyrophyllite, Hillsboro District, North Carolina, USARadiating fans of golden-brown pyrophyllite needles, Champion Mine, California, USA

High Sulfidation Deposits

Satsuma volcanichydrothermal system. 870C 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: Worlds Largest H2SO4 Crater LakeIjen Crater, East Java, Indonesia 1 km wide & 200 m deep lake filled with a sol-ution of H2SO4 & HCl with a pH of 0.5 & temperature of about 33C. 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, IndonesiaCondensation of magmatic vapor + HCl + SO2 generates acidic waters (pH ~1 or less):Causes leaching of rocks (vuggy quartz), and hypogene advanced argillic alteration (alunite, kaolinite)

Kawah Ijen, East JavaSulfur miners

Lepanto lithocap outcrop to southFrom Palidan slideMohong HillLithocapSurface projections: Lepanto Far Southeast Victoria

Epithermal Vein Deposits Low- and Intermediate-Sulfidation

Hishikari gold mines 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, IndonesiaSumatra 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 19383.24 Mt @ 12.8 g/t Au & 70.5 g/t Ag

Donok Vein 3 - colloform banded quartz-adularia & vughsFootwall of Main Donok Vein (qtz-cal-truscottite) inside Lubang Kacamata (Eyeglasses Dutch Tunnel)Donok LS Epithermal Veins

Au - BLEGAg - BLEGAisasjur, West Papua, Indonesia

ASD-28: 6m @ 2.54 g/t Au from 139.3m 145.3m depthASD-5 at 266.5m depth: 5.36 g/t Au & 1570 ppm AsASD-5 at 270.35m depth: 15.6 g/t Au & 4790 ppm AsAisasjur IS VeinsWest Papua, IndonesiaHydro-brecciated siltstone with stibnite cement

Geothermal WatersSteam-heated ZonesWater Table Movement

Champagne Pool, Waiotapo, New Zealand

Tarutung Hot Spring & Silica Sinter North SumatraR. Gonzales, 2006

Tarutung Silica Sinter North SumatraR. Gonzales, 2006

Tarutung Silica Sinter North SumatraMicro terracesStalactitesSpiky tubes and flakeR. Gonzales, 2006

Mud Pool, Rotorua, New Zealand

Epithermal Quartz Vein Textures

EPITHERMAL QUARTZ VEIN TEXTURESDong, Morrison & Jaireth, 1995

PRIMARY GROWTH TEXTURESPhotos: Dong, Morrison & Jaireth, 1995

EPITHERMAL QUARTZ VEIN TEXTURESDong, Morrison & Jaireth, 1995

RECRYSTALLIZATION TEXTURESMosaic aggregates of microcrystalline quartz crystals with highly irregular and interpenetrating grain boundaries.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 rimsFeathery 2: a feathery appearance seen as patches throughout quartz crystalsFlamboyant 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. Flamboyant 2: flamboyant extinctions seen through the crystals with rounded surface in bands.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 TEXTURESDong, Morrison & Jaireth, 1995

REPLACEMENT TEXTURESLattice bladed: a network of intersecting silica blades with polyhedral cavities.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, ueenslandGhost 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.Ghost bladed: aggregates of quartz crystals with superimposhed bladed texture identified by outlines of impurities and finer grain size. Parallell bladed: silica blades are parallel within each group but adjacent groups have different orientation.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, HISHIKARILattice-type Bladed QuartzParallel-type Bladed QuartzPhotos: Etoh, Izawa & Watanabe, 2002

REPLACEMENT TEXTURESa. 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 occursA. Lattice-typeB. Parallel-typeBladed quartz, Tambang Sawah, Bengkulu, IndonesiaEtoh, et al., 2002

REFERENCESDong, 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. 18411851.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.

*FIGURE 1. Photograph of a broken surface of super high-grade banded electrum(and silica) ore from the Sleeper deposit, Nevada, showing multiple gold-depositingevents, which sequentially laminated the developing vein wall (top of photograph istoward the vein center). Field of view is ~12 cm; See Saunders (1994) for detaileddescriptions of these textures (Saunders, 2010). Electrum-rich bands typically contain opaline silica or microcrystalline quartz, which is typically different from silica precipitated before or after gold deposition Saunders et al., 1994).***Lihir = 595 tonnes or 19.13 Moz AuPorgera = 493 tonnes or 15.85 Moz Au & 2,805 tonnes Ag*Truscottite, (Ca,Mn)14Si24O58(OH)8 2H2O - Silicates not Containing Aluminum; silicates of Mn and Na, K, Mg, Ca or Fe.*Aisasjur IS, West Papua - Hairline to 2 cm dark gray quartz-clay-carbonate veins with very fine-grained sulphides (pyrite, marcasite, arsenopyrite) and thin adularia selvage are typical of the gold-bearing intermediate sulphidation epithermal veins in Aisasjur prospect. A. Sample taken from ASD-5 at 266.50m depth assayed 5.36 g/t Au and 1570 ppm As and B. Another sample from ASD-5 at 270.35m depth assayed 15.60 g/t Au and 4790 ppm As.

*Laser ablation geochemistry imageFigure 1. Laser ablation grain analysis of a disseminated pyrite in MGDP from ASD27. Note the distribution Au and As at the rims of the pyritePyrite samples were sent to CODES for an exercise in the geochemistry course. The sample I sent was fortunate enough to be included for pyrite mapping. Instead of laser ablation spot analyses, the whole pyrite was blasted with laser and analysed, thus effectively mapping the distribution of elements, metals of interest, in the grain. Figure 1 shows the distribution of elements in a disseminated pyrite in MGDP interpreted to be associated with epithermal mineralisation. Au and As reside at pyrite rims in contrast to Co and Ni. One possible interpretation is that Au and As are part of a later event, which would imply that there is an overgrowth. Or Au and As were assimilated at a later stage during the formation of pyrite. Another possibility is that Au and As may have been present in the pyrite earlier in its growth stage but were subsequently pushed toward the rims as the pyrite grew. The varying interpretations are due to the current state of understanding of pyrite geochemistry but the author is leaning toward the idea that Au and As were incorporated in the pyrite lattice late in its growth as overgrowth rims were not observed.***Schematic models of the formation process of lattice-type (A) and parallel-type (B) bladed quartz. Precipitationof bladed calcite (A1, B1), precipitation of adularia and quartz (A2, B2), and dissolution of calcite (A3, B3). Subsequentquartz overgrowth sometimes occurs (A4). A single, platelike interstice in lattice-type bladed quartz is sometimes formed bydissolution of several parallel-bladed crystals of calcite, as shown in Figure 8C.