1. Epithermal Gold Deposits
(천열수형 금광상)

2. 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

3. Arc low sulfidation
Rift low sulfidation

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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

13. 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

14. 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

15. Conclusion Low sulfidation High sulfidation Ore fluid Origin SalinitypH
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