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8/14/2019 1 Active Seg
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Active hydrothermal systems and geochemical processes SEG 2010
J.W. Hedenquist 1
White Island, NZ:White Island, NZ: 300 t Au, 1 Mt Cu flux over ~10,000 yr discharge300 t Au, 1 Mt Cu flux over ~10,000 yr discharge Exploration implicationsExploration implications
Porphyry to epithermal transitions:processes and products
Jeffrey W. Hedenquist Jeffrey W. Hedenquist Ottawa, CanadaOttawa, Canada
SEG: 2010SEG: 2010
Introduction: Active analoguesIntroduction: Active analogues TerminologyTerminology Magmatic-hydrothermalMagmatic-hydrothermal systemssystems Processes and productsProcesses and products
PorphyryPorphyry systems (tops)systems (tops) Transitions to epithermal environTransitions to epithermal environ
Advanced Advanced
argillic lithocapsargillic lithocaps
andand
high-sulfidationhigh-sulfidation ore depositsore deposits Features and variationsFeatures and variations
Epithermal veins: Dave JohnEpithermal veins: Dave John
epithermalepithermal
porphyryporphyry
MagMag [mat][mat] icic fluidfluid
W.F.W.F. GiggenbachGiggenbach , 1992, 1992
Upper crustal sub-volcanic magma
chamber formed atbasement-
supracrustal contactdue to buoyancy rheology contrastsNo verticalexaggeration
Richards (2007), from Richards (2005)
marginalIS veins
AA host toHS deposits
SulfidationSulfidation state definitionstate definition
EinaudiEinaudi et al., 2003et al., 2003
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Active hydrothermal systems and geochemical processes SEG 2010
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SulfidationSulfidation statestate evolutionevolution
W a l l r o
c k
W a l l r o
c k b u f
f e r b u f
f e r
F e F e 3 + 3 + / F e
/ F e 2 + 2 +
RiftRiftmagmasmagmas
Ar c Ar cmagmasmagmas
EinaudiEinaudi et al., 2003et al., 2003
Goldfarb, 2010
Several Mexican Ag-Au
Analogues of epithermal deposit s: Analogues of epithermal deposit s:
Settings of active hydrothermal systemsSettings of active hydrothermal systems
HS & IS deposit sHS & IS deposit s (some related to porphyry systems):(some related to porphyry systems): volcanic arc setting:volcanic arc setting:
andesite-daciteandesite-dacite eruptive rocks and domeseruptive rocks and domes
active volcanic analogues:active volcanic analogues: high-T fumaroles, acidic t o neutral pH, v ariable salinitieshigh-T fumaroles, acidic to neutral pH, variable salinities
LS deposits:LS deposits:
extensional setting (rifts):extensional setting (rifts): rhyolite-basalt bimodal volcanic associationrhyolite-basalt bimodal volcanic association
geothermal system analogues:geothermal system analogues: neutral pH, low salinities (variable gas, locally high )neutral pH, low salinities (variable gas, locally high )
Mt.Mt. SakurajimaSakurajima , Kyushu:, Kyushu: Daytime, ash + vapor Daytime, ash + vapor
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Active hydrothermal systems and geochemical processes SEG 2010
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Mt. Sakurajima, Kyushu: Eruption of ash + vapor White Island, NZ:White Island, NZ: Quiescent eruption, 1988Quiescent eruption, 1988
High-temperaturehypogene vapors,
850 oC with HCl, SO 2
White Island, New Zealand: fumarolesWhite Island, New Zealand: fumaroles
acidic stream,pH ~0.6
pH 0.2
White Island, New Zealand: drowned fumaroles, 2007
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Active hydrothermal systems and geochemical processes SEG 2010
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SatsumaSatsuma IwojimaIwojima , S. Kyushu:, S. Kyushu:passive degassing (no eruption)passive degassing (no eruption)
877877 ooCCvaporsvapors withwithHClHCl , SO, SO
22
Summit crater Summit crater Sampling of 770 C vapor with acidic gases
dissolved rock
Residual (Residual ( vuggyvuggy )) qtzqtz
Hedenquist et al., 1994
Satsuma Iwojima, Japan
pH 1.1pH 1.1
pH1.7 -- 1.1
SatsumaSatsuma IwojimaIwojima , S. Kyushu:, S. Kyushu:
0.4 Mt0.4 Mt rhyoliterhyolite leached per year =leached per year =200 m thick, 1x1 km in 1000 yrs200 m thick, 1x1 km in 1000 yrs
No longer a magic fluidNo longer a magic fluid
Where do the acidWhere do the acidcondensates comecondensates comefrom?from?
HH22O,O, HClHCl ,,SOSO 22
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Critical fluidCritical fluidbulk salinitybulk salinity
CriticalCriticalfluidfluid
FluidFluid exsolvedexsolved in meltin melt
vapor vapor hypersalinehypersalineliquidliquid
COUPLEDCOUPLED brine + vapor brine + vapor
H2O, NaCl,SO 2 , HCl, CO 2 , H 2S, ...
H2SO 4, HCl
Lithocap formed byhypogene condensate,pH~1
Formation of advanced argillic alteration
HCl, SO 2, CO 2 , H 2S:hypogene alteration
4 SO 2 + 4 H 2O =>3 H 2SO 4 + H 2S
80% 20%
Steam-heatedSteam-heated zone,zone,~100~100 o CC (CO(CO 2 , H, H 2S)S)
High-temperaturehypogene vapors,
850 oC with HCl, SO 2
White Island, New Zealand: fumarolesWhite Island, New Zealand: fumaroles
~110~110 o CC
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H2S + 2 O 2 = H 2 SO 4
H2O, NaCl,SO 2, HCl, CO 2, H 2S, ...
H2SO 4, HCl
CO 2 , H 2 S
4 SO 2 + 4 H 2O =>3 H 2SO 4 + H 2S
pH >2
pH
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ElEl ChichonChichon , Mexico:, Mexico: post 1982 eruption,post 1982 eruption, hypogenehypogene vaporsvapors Kawah IjenKawah Ijen , Java:, Java: volcanic crater with acidic lakevolcanic crater with acidic lake
pH 0.0pH 0.0
Photograph:Photograph:PierrePierre DelmelleDelmelle
Condensation of acidCondensation of acidmagmaticmagmatic volatilesvolatiles
Colloidal silica in suspension,Colloidal silica in suspension,and deposited in laminated sedimentsand deposited in laminated sediments(also(also enargiteenargite ,, covellitecovellite ,, stannitestannite in native S)in native S)
Volcan Pos, Costa Rica LakeLake ToyaToya , Hokkaido:, Hokkaido: StratovolcanoStratovolcano , caldera, domes, caldera, domes
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Mt. Pinatubo, Philippines:Mt. Pinatubo, Philippines: Explos ive, cataclysmic erupt ion, 1991: 9 hrs, 4 kmExplos ive, cataclysmic erupt ion, 1991: 9 hrs, 4 km 33 d.r .e., + 100d.r.e., + 100 Mt vapor Mt vapor
Satsuma WhiteIwojima Island
Critical fluidCritical fluidbulk salinitybulk salinity
CriticalCriticalfluidfluid
FluidFluid exsolvedexsolved in meltin melt
vapor vapor hypersalinehypersalineliquidliquid
80% COUPLED 20%
Pinatubo?
Sillitoe and Hedenquist, 2003
Schematic relations: HS and IS epithermal deposits,Schematic relations: HS and IS epithermal deposits,and porphyry systemsand porphyry systems
evolved fluid (en)phyllicphyllic fluidfluid-shallow barren--shallow barren-vapor (AA)vapor (AA)brine (brine ( potassicpotassic ))critical fluidcritical fluidmeltmelt