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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
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Singer et al., 2008; pers. obs.
epithermal Au, Ag (Pb, Zn, Cu)
Porphyry Cu-Au systems: * Intrusion centered, may have surficial expression
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Mt. Sakurajima, Kyushu: Eruption of ash + vapor
High-temperature hypogene vapors, !850 oC with HCl, SO2
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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
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Magmatic fluid: coupled vapor and brine from deep fluid Cataclysmic
eruption (bulk metals)
White Island (high metals)
Satsuma Iwojima (low metals)
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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
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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
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Volcan Poás, Costa Rica
Volcan Poás, Costa Rica Volcan Poás, Costa Rica
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Akaiwa high-sulfidation epithermal prospect, Japan
Akaiwa high-sulfidation epithermal prospect, Japan
A. Arribas, unpub.
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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
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Block and ash fall, overlain by pyroclastic flow
Montserrat 1997* Mt St Helens 2004 dome inflates from within isolated spines emplaced
along shear zones
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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
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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
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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...
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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
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Sillitoe and Hedenquist, 2003