1 Active Volcanic Santiago

Jeffrey HedenquistConsultorOttawa, Canada

Sociedad Geológica de Chile: July 2011

after R. Goldfarb, from Groves et al. (2005)

Cu-Au

Upper crustal sub-volcanic magma

chamber formed at basement-

supracrustal contact due to buoyancy ± rheology contrasts�No vertical

exaggeration�© Richards (2007), from Richards (2005)�

Porphyry systems

Sillitoe, 2010 Economic Geology

Singer et al., 2008; pers. obs.

epithermal Au, Ag (Pb, Zn, Cu)

Porphyry Cu-Au systems: * Intrusion centered, may have surficial expression

Mt. Sakurajima, Kyushu: Eruption of ash + vapor

High-temperature hypogene vapors, !850 oC with HCl, SO2

acidic stream, pH ~0.6

pH 0.2

White Island, New Zealand: drowned fumaroles, 2007

Satsuma Iwojima, Japan

H2O, HCl, SO2

Sampling of 770 C vapor with acidic gases

dissolved rock

Hedenquist et al., 1994

Satsuma Iwojima, Japan

pH 1.7 -- 1.1

Magmatic fluid: coupled vapor and brine from deep fluid Cataclysmic

eruption (bulk metals)

White Island (high metals)

Satsuma Iwojima (low metals)

H2O, NaCl, SO2, HCl, CO2, H2S, ...

H2SO4, HCl

Lithocap formed by hypogene condensate, pH~1

Formation of advanced argillic alteration

HCl, SO2, CO2, H2S: hypogene alteration

4 SO2 + 4 H2O => 3 H2SO4 + H2S

80% 20%

High-temperature hypogene vapors, !850 oC with HCl, SO2

H2S + 2 O2 = H2SO4

H2O, NaCl, SO2, HCl, CO2, H2S, ...

H2SO4, HCl

CO2, H2S

4 SO2 + 4 H2O => 3 H2SO4 + H2S

pH >2

pH <1

Steam-heated blanket

Lithocap formed by hypogene condensate

Formation of advanced argillic alteration

HCl, SO2, CO2, H2S hypogene alteration

CO2, H2S steam-heated

alteration

Volcan Poás, Costa Rica

Volcan Poás, Costa Rica Volcan Poás, Costa Rica

Akaiwa high-sulfidation epithermal prospect, Japan

Akaiwa high-sulfidation epithermal prospect, Japan

A. Arribas, unpub.

Showa Shinzan, Hokkaido: dacite dome, 1944-1945 100 m

pyroclastic flows generated by dome collapse

Unzen lava dome in Japan, surrounded by pyroclastic deposits The lava dome at Colima,

Mexico

Block and ash fall, overlain by pyroclastic flow

  Montserrat 1997* Mt St Helens 2004 dome inflates from within isolated spines emplaced

along shear zones

collapse scar

shear lobe Crater rim

Shear lobe extrusion, determines location/direction and timing of collapses

The character of the internal contacts are not clear and probably does not resemble this !

BUT : what must be the case, is that large domes are at least in part constructed over an unconsolidated base.

Lithocap

!  Horizontal to sub-horizontal body

!  Residual silicic core (± ore), halo of advance argillic (AA) alteration

"  Structure-controlled feeder zone

"  Lithology-controlled lithocap horizon

Mineralization

Structure- controlled

Lithocap

Summitville, Colorado

Steven and Ratté, 1960

Steven & Ratté, 1960

pH ~ >6 4 - 6 2 - 4 <2

10 - 100s m

Summitville, Colorado

Mohong Hill quartz-alunite

lithocap

quartz

Lithology-controlled lithocap alteration

Gonzalez, 1956, 1959; Garcia, 1991

in residual qtz host

Quartz-alunite ± Al-silicate halo

fluid:rock = 10:1

fluid:rock = 10:1

Alteration mineral stability

shallow qtz-alun

zone

deep alumino- silicate zone

diaspore pyrite gypsum alunite andalusite kaolinite pyrophyllite pyrrhotite hematite muscovite dickite

residual qtz core w/ py, S

qtz-alun halo

deep alumino- silicate zone

Alteration mineral stability and zoning

fluid:rock = 100:1

py

diaspore pyrite gypsum alunite andalusite kaolinite pyrophyllite pyrrhotite

Magmatic condensates more reactive at lower temperature, hence abundance of shallow, advanced argillic alter’tn

150 C 200 C 250 C pH~1.5 ~0.7 ~1

300 C pH~2

W:R <10 W:R >10

H2O, NaCl

SO2, HCl, CO2, H2S...

Einaudi, Hedenquist and Inan, 2003

Einaudi, Hedenquist and Inan, 2003

py + bn

cp

cvdg

Sulfur condensation

po

lo+po

poiro

n

tn

hm + py

mt

mt+qz+po

fayalite

T ( C)o100 300 500 800

1000/T (K)3.0 2.0 1.0

-2

-6

-10

-14

-18

2 arc magmas

py + bn

cp

cvdg

Sulfur condensation

po

lo+po

poiro

n

tn

hm + py

mt

mt+qz+po

fayalite

T ( C)o100 300 500 800

1000/T (K)3.0 2.0 1.0

-2

-6

-10

-14

-18

2 arc magmas

py + bn

cp

cvdg

Sulfur condensation

po

lo+po

poiro

n

tn

hm + py

mt

mt+qz+po

fayalite

T ( C)o100 300 500 800

1000/T (K)3.0 2.0 1.0

-2

-6

-10

-14

-18

2 arc magmas

FUMAROLES

MAGMATIC HYDROTHERMAL

py + bn

cp

cvdg

Sulfur condensation

po

lo+po

poiro

n

tn

hm + py

mt

mt+qz+po

fayalite

T ( C)o100 300 500 800

1000/T (K)3.0 2.0 1.0

-2

-6

-10

-14

-18

2 arc magmas

FUMAROLES

py + bn

cp

cvdg

Sulfur condensation

po

lo+po

poiro

n

tn

hm + py

mt

mt+qz+po

fayalite

T ( C)o100 300 500 800

1000/T (K)3.0 2.0 1.0

-2

-6

-10

-14

-18

2 arc magmas

FUMAROLES

HS

epith

erm

al b

ase-

met

al v

eins

Porphyry-Cu

Sillitoe and Hedenquist, 2003