AcknowledgementsWe thank Alain Mauviel for making the thin sections, Glenn Poirier and David Diekrup for SEM-EDS and EPMA analysis, and G.T.V. Valera (University of the Phil-ippines) and Seungmin Lee for their help in sample collection. The study was supported by a NSERC Discovery Grant to KH.
Conclusions- Dizon whole rock and amphibole compositions are similar to those of 1991 AD Pinatubo eruption products.- Compositional zoning of amphibole suggests an introduction of ma�c melt into an evolved magma reservoir. Dusty zones in plagioclase phenocrysts indicate the destabilization of Na-rich portion during the injection of hot ma�c melt into a felsic magma chamber at Dizon. -Temperatures of magmas are around 820 °C for 2.5 Ma Dizon samples, which are similar to those of the magma for 1991 AD eruption products.- The water contents of magmas at Dizon and 1991 AD magmas were elevated, at ~6 wt%. - Parental magmas for 2.5 Ma Dizon rocks are highly oxidized, around FMQ +3, similar to the magma resevoir for the 1991 AD eruption products.
Consistent magma conditions at Mount Pinatubo, Philippines, over 2.5 million years
W. Midea and K. HattoriDepartment of Earth and Environmental Sciences, University of Ottawa, Ottawa, Canada
Email: [email protected], [email protected]
Introduction
Agglomerate
Hydrothermal breccia
Dacite porphyry
Pua diatreme
Welded breccia
Quartz diorite
Quartz diorite porphyry
Diorite
Basal volcanics
Qz-Prl-Dsp
Qz-Kln-Alu
Qz-WM-Chl
Bt
Fault
Sp-Gn-Ttr-Stb-Eng (Au-Ag)
1 km
Mount Pinatubo is in the Luzon arc in the Philippines. The study area, the Dizon mine porphyry Au-Cu deposit is located ~ 15 km south of the current summit on the southern foothill of the volcano. The mineralization is hosted in quartz diorite of ~2.5 Ma (Imai, 2005). Samples from Dizon were compared to younger eruption products of Mount Pinatubo to characterize the magma evolution in the volcano through time.
Below: Geologic map of Dizon mine displaying the di�erent lithologies, alteration zonation and structures. There is advanced argillic alteration with quartz/pyrophyllite/diaspore (Qz-Prl-Dsp) and quartz/kaolinite/alunite (Qz-Kln-Alu), white mica-chlorite alteration (WM-Chl), biotite alteration (Bt) and veins containing sphalerite (Sp), galena (Gn), tetrahedrite (Ttr), stibnite (Stb), enargite (En), gold and silver. Modi�ed after Imai (2005).
Pinatubo
Manila Trench
East
Luzo
n Tr
ench
Manila
Luzon Island
Dizon
LegendLand
Ocean/Lake
Fault
Trench
Above: Image of Mount Pinatubo’s summit and the Dizon mine ~15 km to the south.
Below: Map of Luzon Island (Philippines), Pinatubo and Dizon are northwest of Manila.
Mount Pinatubo
Dizon
Amphibole Zonation
40.5
41
41.5
42
42.5
43
43.5
44
44.5
0 2 4 6 8
SiO2
Series1
Series2
300 um
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
0 2 4 6 8
FeO
Series1
Series2
11
11.5
12
12.5
13
13.5
14
0 2 4 6 8
Al2O
3
Series1
Series2
1.35
1.4
1.45
1.5
1.55
1.6
0 2 4 6 8
TiO2
12.5
13
13.5
14
14.5
15
15.5
16
16.5
17
0 2 4 6 8
MgO
2.5
2.55
2.6
2.65
2.7
2.75
2.8
0 2 4 6 8
Na2
O +
K2O
Amphibole grains in diorite at Dizon (07-06) show compositional zoning. including variations in Mg, Fe, Si and Al. Three zones are recognized: core, mantle and rim. The core has higher Mg and Si, dark in back-scattered electron image, compared to the mantle (bright) which has higher Fe and Al contents. The rim, shown in grey, has a similar composition to the core. The zoning suggests the normal zoning of amphibole and the introduction of a new batch of magma to form the rim. The texture of amphibole in the Dizon diorite is consistent with destabilization textures on plagioclase from the andesite, such as dusty zones and overgrowths. In 1991 AD eruption products these plagioclase textures indicate injection of ma�c magma (Hattori & Sato, 1996).
SamplesSamples include altered and mineralised quartz diorite porphyry and andesite, and unaltered porphyritic andesite (07-02) and diorite (07-06) at the Dizon mine, and a 5100 BP dacite (06-15). Compositions of amphibole and Fe-Ti oxides in the unaltered samples are used to calculate the oxidation state, temperature and water content of magmas.
500 um
1 mm
Left: Hornblende and Fe-Ti oxides in Dizon andesite (Sample 07-02). Middle: Hornblende and Fe-Ti oxides in 5100 BP dacite (Sample 06-15). Right: Amphibole in Dizon diorite (Sample 07-06) in the upper right and plagioclase showing dusty zones and overgrowths in Dizon andesite in the bottom right.
Ilm
Mag
MagIlm
Amphibole CompositionThe composition of amphibole in Dizon samples of andesite, diorite and dacite is compared with that of 1991AD eruption products reported by Bernard et al. (1996). Decreasing Al and alkalis with increasing Si in amphibole is consistent with evolution in a calc-alkaline magma (Ridol� et. al., 2010).
0
2
4
6
8
10
12
14
16
40 42 44 46 48 50
Al2O
3w
t.%
SiO2 wt.%
Andesite
Dacite
Diorite
1991 AD
0
2
4
6
8
10
12
14
40 42 44 46 48 50
CaO
wt.%
SiO2 wt.%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
40 42 44 46 48 50
MnO
wt.%
SiO2 wt.%
6
7
8
9
10
11
12
13
14
15
40 42 44 46 48 50
FeO
wt.%
SiO2 wt.%
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
40 42 44 46 48 50
TiO 2
wt.%
SiO2 wt.%
0.0
0.5
1.0
1.5
2.0
2.5
3.0
40 42 44 46 48 50
Na 2
O w
t.%
SiO2 wt.%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
40 42 44 46 48 50K
2O w
t.%SiO2 wt.%
0.6
0.64
0.68
0.72
0.76
0.8
40 42 44 46 48 50
Mg#
SiO2 wt.%
Oxidation State, Water Content, Temperature
El Salvador
Yanacocha
Butte
Dizon
Bingham
Yerington
Santa Rita
FMQ-1 0 +1 +2 +3 +4
MORB
Oxidation conditions and temperatures of parental magmas for Dizon andesite and diorite, and 5100 BP dacite obtained using the Fe-Ti oxide geothermobarometry of Spencer & Lindsley (1985). The values for 1991 AD oxides are from Hattori (1993). The Ridol� et al. (2010) amphibole geothermobarometry was used for all amphibole grains.
Oxidation state of Dizon magmas is compared to other deposits. The magmas of porphyry Cu deposits are oxidised, ~FMQ +2 to +3 (Dilles et al., 2015 & Hattori, 2018).
Pressure calculated from amphibole composition using the geothermobarometry of Ridol� et al. (2010). The pressure corresponds to depths of ~16 km for diorite amphibole and ~4 km for andesite amphibole using a rock density of 2.9 g/cm3.
Water content of andesite, dacite and diorite is shown. The amphibole in diorite yields higher water contents, due to greater depths.
Porphyry Cu deposits
1991 AD
Dacite and Andesite Fe-Ti oxide
Dacite, Andesite and Basalt amphibole
5100 BP
Dacite Fe-Ti oxide
Dacite amphibole
Dizon
Andesite Fe-Ti oxide
Andesite amphibole
Diorite amphibole
0
100
200
300
400
500
600
700
750 800 850 900 950 1000 1050
P (M
Pa)
T (°C)
750
800
850
900
950
1000
1050
4 5 6 7 8
T (°
C)
H2O wt.%
-14
-13
-12
-11
-10
-9
-8
750 800 850 900 950 1000 1050
logf
O2
T (°C)
FMQ +3
FMQ +1
ReferencesAndersen, D.J., & Lindsley, D.H. (1985). New (and �nal!) models for the Ti-magnetite/ilmenite geothermometer and oxygen barometer. Ab-stract AGU 1985 Spring Meeting Eos Transactions. American Geophysical Union 66 (18), 416.Bernard, A., Knittel, U., Weber, B. Weis, D., Albrecht, A., Hattori, K., and Oles, D. (1996). Petrology and geology of the 1991 eruption prod-ucts of Mount Pinatubo (Luzon, Philippines). in Newhall, C.G., & Punongbayan, R.S. (eds.) Fire and Mud: Eruptions and Lahars of Mount Pina-tubo, Philippines. 767-798, Philippine Institute of Volcanology and Seismology, Quezon City, and University of Washington Press, Seattle.Dilles, J. H., Kent, A. J. R., Wooden, J. L., Tosdal, R. M., Koleszar, A., Lee, R. G., & Farmer, L. P. (2015). Zircon compositional evidence for sul-fur-degassing from ore-forming arc magmas. Economic Geology, 110(1), 241-251.Hattori, K. (1993). High-sulfur magma, a product of �uid discharge from underlying ma�c magma: Evidence from Mount Pinatubo, Philip-pines. Geology, 21, 1083-1086.Hattori, K. (2018). Porphyry copper potential in Japan based on magmatic oxidation state. Resource Geology, 68(2), 126-137.Hattori, K., & Sato, H. (1996). Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products. American Mineralogist, 81, 982-994.Imai, A. (2005). Evolution of the hydrothermal system at the Dizon porphyry Cu-Au deposit, Zambales, Philippines. Resource Geology, 55(2), 73-90.Ridol�, F., Renzulli, A., & Puerini, M. (2010). Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes. Contributions to Mineralogy and Petrology, 160, 45-66.
Top Related