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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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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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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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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