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: wmide063@uottawa.ca, khattori@uottawa.ca

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

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