Epithermal Gold Deposits (천열수형 금광상)

Summary  Ore deposits in magmatic arc  Epithermal ore deposits Porphyry copper-gold deposit : deeper setting Epithermal gold deposit : relatively shallow depth (< 1km), low temperature (< 300 °C)  Epithermal ore deposits Divided into low and high sulfidation based on their mineralogies and textures Low sulfidation epithermal gold deposit - derived from dilute near neutral pH fluids - subdivision arc low sulfidation : mineralogies derived dominantly from magmatic source rocks rift low sulfidation : mineralogies derived mainly from circulating geothermal fluids High sulfidation epithermal gold deposit - derived from hot acidic magmatic fluids - zoned alteration : changes in wall rock alteration and ore mineralogy with depth of formation - permeability controls

Arc low sulfidation Rift low sulfidation

Epithermal Gold Deposit A. Low sulfidation epithermal gold deposits (저유황형)  Characteristics Reduced, near neutral pH, dilute fluids developed entrainment of magmatic components within circulating groundwater Sulfur series reduced to H2S 열수는 천부로 상승함에 따라 magmatic components의 영향이 감소하고 groundwater의 유입이 증가하면서 dilution 지속 광체의 형성은 상승하는 OBF(광화유체)와 groundwater와의 혼합, water/rock interaction에 따른 유체의 냉각에 의해 가속 Individual ore deposit은 반복된 광화작용의 산물로, telescoping 결과 천부일수록 후기 광화작용에 해당  Subdivision : arc/rift low sulfidation Arc low sulfidation - mineralogies derived dominantly from magmatic source rocks - ore mineralogy : pyrite(FeS2), sphalerite(ZnS), galena(PbS), chalcopyrite(FeCuS2), arsenopyrite(AsFeS, 황비철석) - gangue mineralogy : quartz, carbonate, clay - wall rock mineralogy : clay, chlorite - mineral associations : quartz-sulfide Au±Cu, polymetallic Au-Ag veins, carbonate-base metal Au, epithermal quartz Au-Ag zoning in time and space with shallower styles overprinting the deeper high Cu at depth, Au and Ag dominant in elevated crustal settings - Sulfur series reduced to H2S Rift low sulfidation - adularia-sericite epithermal Au-Ag ores (veins) - gangues : chacedony, adularia, quartz pseudomorphing platy carbonate - dilatant structures, typically rift within magmatic arc or back arc environments

Epithermal Gold Deposit A. Low sulfidation epithermal gold deposits (저유황형) a. Arc low sulfidation Quartz-sulfide Au±Cu deposits Dominantly iron sulfides and quartz (veins and vein/breccias) Mineralogy Wall rock alteration - retrograde sericite-illite-pyrite(shallowing order)/chlorite-carbonate assemblages Gold grade - generally 1~3g/t - higher grades by fluid mixing or repeated mineralization - telescoped mineralization by bicarbonate water typical of carbonate-base gold deposits, resulting in higher gold grades Structural controls - mineralization exploiting pre-existing regional structures - thicker intersection in local flexures (forming pipe or chimney) – higher grades/ore shoot Deep level Shallow level Iron sulfides pyrite, pyrrhotite, arsenopyrite, chalcopyrite marcasite, arsenean pyrite, galena, sphalerite Gangue mineralogy quartz, Ksp(alkaline silica poor rocks) chalcedony, opal Anomalies Bi As, Hg, Sb

Epithermal Gold Deposit A. Low sulfidation epithermal gold deposits (저유황형) a. Arc low sulfidation Polymetallic Au-Ag veins Distinguished from the carbonate-base metal Au system by the paucity of carbonate gangue minerals Mineralogy : quartz, pyrite, galena, sphalerite, lesser chalcopyrite, some carbonates, various lesser phases including many silver minerals Partly transitional between quartz-sulfide Au±Cu and carbonate-base metal Au Higher grade in more dilational ore setting by repeated mineralization Correlations between ore and gangue minerals - galena-sphalerite dominant : silver ore - chalcopyrite rich : gold rich and transitional to quartz-sulfide Au±Cu Complicated metal zonation by repeated activity of dilational structures and telescoped minealization

Epithermal Gold Deposit A. Low sulfidation epithermal gold deposits (저유황형) a. Arc low sulfidation Carbonate-base metal Au Most prolific gold producers in the SW Pacific rim (undeveloped prospects) High crustal levels Gangue minerals : carbonate > quartz > pyrite > sphalerite > galena > chalcopyrite - fracture/vein/brccias - zonation depending on the crustal levels Partly overprinted by epithermal quartz Au-Ag ores or telescoping deeper level quartz-sulfide Au±Cu mineralization Promotion of mineralization by mixing of rising mineralized fluids with bicarbonate waters Examples of zonation * partly silver rich at high crustal level, forming carbonate-base metal Ag/indication of low T(high level) by pale red sphalerite Grade - higher than quartz-sulfide Au±Cu and polymetallic Au-Ag vein systems - extremely irregular, especially in case of the presence of epithermal quartz Au-Ag mineralization Fineness(순도) : lower than quartz-sulfide Au±Cu ores/extremely variable High crustal level Low crustal level Intermediate (shallow) (deep) Fluids cool, acidic hot, neutral Carbonate mineralogy Fe-carboate Mg, Mn-carbonate Ca-carbonate Sphalerite Fe < Zn sphalerite Fe > Zn sphalerite Wall rock alteration illite-smectite illite chlorite with sericite

Epithermal Gold Deposit A. Low sulfidation epithermal gold deposits (저유황형) a. Arc low sulfidation Epithermal quartz Au-Ag Characterized by the presence of bonanza gold grade (hundreds of g/t) and coarse free gold Vein mineralogy : quartz (crystalline to opal/chalcedony), chlorite, illite-smectite very minor gangue material causing difficulty in identifying fracture/vein Mineralization exploiting fracture/vein/breccias, locally overprinting earlier veins Common in dilatant structural settings Increased Ag contents over the deeper intrusion-related low sulfidation mineralization styles Wall rock alteration : chlorite and illite-smectite dominant/low T Ksp in alkaline rocks Only well preserved in younger poorly eroded magmatic arc (why? formation in elevated crustal settings) Several styles of epithermal quartz Au-Ag deposits - direct overprinting carbonate-base metal silver - peripheral other intrusion related styles such as porphyry Cu-Ag or carbonate-base metal, etc. - transition to adularia-sericite epithermal Au-Ag - evolution of some high sulfidation deposits to lower sulfidation styles (epithermal quartz Au-Ag) Exploration implication - coarse free gold leading to many discoveries of this style by gold panning (alluvial gold resources) - need of testing carbonate-base metal gold for associated epithermal quartz Au-Ag mineralization

Epithermal Gold Deposit A. Low sulfidation epithermal gold deposits (저유황형) b. Rift low sulfidation Adularia-sericite epithermal Au-Ag Occurrence - banded fissure veins and local vein/breccias composed of colloform banded quartz, adularia, quartz pseudomorphing carbonate, and ginguro (dark sulfidic material) bands - calcite : in some veins, late stage cutting quartz or filling remaining open space - identified by gold panning and characteristic banded quartz Depostional settings - closer associations with extensional structures and rift setting than those with intrusion source rocks - fossil back arc rifts / local rift structure within magmatic arc - fissure veins (~1km depth) are the main gold producer (colloform banded quartz vein ores) Depositional mechanism - mixing of rising pregnant fluids with oxygenated or collapsing acid sulfate (low pH) groundwaters - silver rich with Au:Ag ratios greater than 1:10 - anomalies : Cu (chalcopyrite), Hg (cinnabar, HgS), Sb (stibnite(휘안석), Sb2S3) Wall rock alteration - sericite (illite) – peripheral smectite with pyrite and chlorite – marginal chlorite/carbonate (propylitic) alteration - surficial acid sulfate alteration (silica (chalcedony, opal), kaolin, local alunite) by low T acid waters developed by the condensation of volatiles in the vadose zone Structure and host rock - high grade ore shoot : dilational jogs or flexures, intersection of fault splays, fluid quenching sites changing rock competency

Epithermal Gold Deposit B. High sulfidation epithermal gold deposits (고유황형) Acid sulfate High sulfidation in the early geological literature Alteration formed by collapsing surficial cooler acidic waters within low sulfidation system Outlines Major gold producers in the Andes of South America, partly significant Cu-Au producers in the SW Pacific Divided into ore-bearing system and zones of non-economic acid alteration Exploration implications of some barren acid alteration styles Lithocaps or barren shoulders - barren high sulfidation alteration cropping out in the vicinity of low and high sulfidation Au epithermal and porphyry Cu-Au - non-mineralized advanced argillic (silica-alunite±pyrophyllite etc.) alteration derived from magmatic volatiles - high T minerals (pyrophyllite-diaspore(AlO(OH)), locally corundum (Al2O3) and andalusite (Al2SiO5) - silica of massive form / lack of vuggy silica, typical of mineralized high sulfidation deposits - coarse grained and higher T layered silicates (alunite, pyrophyllite, dickite, etc.) Steam heated alteration - upper portion of high sulfidation epithermal systems, laterally extensive sheets within the vadose zone - mostly friable rocks (opal, powdery alunite, kaolin, sulfur), rare Au-Ag mineralization - acidic warm waters by oxidation of magmatic sulfur bearing volatiles Magmatic solfataras - direct venting of high T magmatic volatiles to the surface within active magmatic arc - silica-alunite-kaolin alteration, locally sulfur production

Epithermal Gold Deposit B. High sulfidation epithermal gold deposits (고유황형) Mineralized high sulfidation systems Dominant in younger poorly eroded magmatic arc Fluid characteristics Enriched in magmatic volatiles(SO2, HCl, CO2, HF, etc) with limited dilution by groundwaters or interaction with host rocks Evolution of fluids decrease in P of rising fluids resulting in reaction of magmatic volatiles(coming out of solution) with water and oxygen increase in concentration of H2SO4 and production of hot acidic fluids(SO2) by dissociation of H2SO4 (shallow, <300°C) zoned high sulfidation alteration by the progressive cooling and mineralization of the hot acidic fluid by interaction with host rocks Two phase fluid flow model Stage I : volatile-rich event - interaction with host rocks of more rapidly migrating volatile-rich component of the high sulfidation fluid - residula silica, Sil-alunite±pyrophyllite/kaolin, clay outwards Stage II : liquid-rich event - sulfide and Au-Ag-Cu mineralization Permeability Dilatant structures or phreatomagmatic breccia pipes (pathways of hot acidic fluids) Structural controls in deeper portions, lithological controlss in elevated crustal settings Ideal ore setting : intersection of dilatant structures and diatreme margins

Epithermal Gold Deposit B. High sulfidation epithermal gold deposits (고유황형) Mineralized high sulfidation systems Fluid characteristics Two phase fluid flow model

Epithermal Gold Deposit B. High sulfidation epithermal gold deposits (고유황형) Mineralized high sulfidation systems Alteration Zoned alteration formed as a result of the progressive cooling and neutralization of the hot acidic fluids by reaction with host rocks and groundwaters Core of high sulfidation ore systems - leaching of many components form the host rocks by hot acidic fluids leaving mainly only silica and some rutiles - altered rock termed residual silica or vuggy silica (important secondary permeability) Zonation - progressive outwards zonation attributed to crustal level of formation : alunite, pyrophyllite, kaolin, illitic and chloritic clays - pyrophyllite-diaspore-dickite in higher T (deeper) condition, pervasive silicification or alunite-kaolin in lower T condition - thin alteration zone in rapidly changing fluid condition at higher level (distal to the fluid upflow) wide zonation in slower changing fluid condition at deeper level (more proximal to the fluid upflow) Mineralization Induced after alteration into the central portion of the zonation by feeder structure or breccia pipes Sulfur assemblages - pyrite and enargite (Cu3AsS4), lesser covellite (CuS, deeper level), peripheral tennantite-tetrahedrite ((Cu,Fe,Zn,Ag)12Sb4S13) Textures - filling of open space (vuggy silica), fissure veins, matrix to breccias (breccias associated with dilatant structures or later sulfide matrix in hydrothermal injection breccias) Vertical zonation - higher Cu contents at deeper level - greater abundances of Au or Au-Ag with local Hg, Te, Sb in the upper portions of poorly eroded systems

Epithermal Gold Deposit B. High sulfidation epithermal gold deposits (고유황형) Mineralized high sulfidation systems Mineralization Au:Ag ratios - order of 1:100, locally anomalous Pb and Zn in Andean systems - Ag-poor in SW Pacific Gold grades : 1~3.5g/t * improved mechanism causing higher gold grades - change to an ore fluid of a lower sulfidation style (epithermal quartz Au-Ag style) - mixing of rising pregnant fluids with oxidizing acid sulfate waters Exploration implications Difficult metallurgy in sulfide ore and environmental difficulties associated with abundant pyrite and mercury Needing early field recognition of rock texture such as vuggy silica (high sulfidation), and careful mapping of breccias

Conclusion Low sulfidation High sulfidation Ore fluid Origin Salinity pH S/base-metal contents Meteoric water Low salinity Near neutral Low Magmatic water Mostly low salinity(some high) Acid High Mineralization Veins Base metals Chemical indicator Open-space/cavity filling, stockworks Au, Ag, minor Pb, Zn, Cu Zn, Pb high, Cu low, Ag/Au ratio high Open-space/cavity filling rare, scattered Au, Cu, minor Ag As, Cu high, Ag/Au ratio low Textures Crustification/colloform banding Vuggy silica Chalcedony vein common Adularia Alunite, pyrophyllite minor Enargite-luzonite absent Chalcedony mostly absent No Adularia Alunite, pyrophyllite abundant Enargite-luzonite present Mineralogy