![a) Total alkali vs. silica discrimination diagram showing the compositions of the gabbro diabase, tonalite-granodiorite, and hornblende quartz diorite. b) AFM diagram [12] showing a tholeiitic character for the gabbro-diabase and calc-alkaline signatures for the tonalite-granodiorite and hornblende quartz diorite. c-d) Adakite discrimination diagrams [13] revealing typical arc rocks signature for the gabbro-diabase and tonalite-granodiorite but an adakitic character for the hornblende quartz diorite. e) Diagram [14] classifying the hornblende quartz diorite as a fertile or productive intrusive while the two older units are considered as barren.](https://www.researchgate.net/publication/363106524/figure/fig1/AS:11431281081871833@1661875850027/a-Total-alkali-vs-silica-discrimination-diagram-showing-the-compositions-of-the-gabbro_Q320.jpg)
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Geochemistry and geochronology of intrusive units
in the Suyoc epithermal deposit: Constraints on
magma fertility in the Mankayan Mineral District,
Philippines
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RCGEOE-2021
IOP Conf. Series: Earth and Environmental Science 1071 (2022) 012021
IOP Publishing
doi:10.1088/1755-1315/1071/1/012021
1
Geochemistry and geochronology of intrusive units in the
Suyoc epithermal deposit: Constraints on magma fertility in
the Mankayan Mineral District, Philippines
J A S Gabo-Ratio1*, K D Jabagat2, O B Soberano3, K Yonezu4, and Y H Lee2
1National Institute of Geological Sciences, College of Science, University of the
Philippines, Diliman, Quezon City, Philippines
2Department of Earth and Environmental Sciences, National Chung Cheng University,
Chia-Yi, Taiwan, R.O.C.
3Graduate School of International Resource Sciences, Akita University, Japan
4Department of Earth Resources Engineering, Kyushu University, Fukuoka, Japan
*Corresponding author’s e-mail: jgratio@nigs.upd.edu.ph
Abstract. The Mankayan Mineral District in northern Luzon, Philippines is host to world-class
Cu-Au hydrothermal deposits such as the Far Southeast porphyry copper deposit and the
Lepa nto, Victo ria, and Teresa epith ermal deposits. It also ho sts the Suyoc epithermal p rospect
in the southern portion of the district. This study focuses on the petrography, whole rock
geoc he mist ry, m ine ral c he mist ry and U-Pb da ting o f t he th ree mas sive ba tholit ic in tru siv e units
(gabbro-diabase basement, tonalite-granodiorite, and hornblende quartz diorite) in Suyoc. U-Pb
dating results reveal that the tonalite is Late Eocene in age (37.20 ± 4.70 Ma), while the
hornblende quartz diorite was dated 3.18 ± 0.77 Ma (Middle Pliocene). The gabbro-diabase
exhibits tholeiitic signature while the tonalite-granodiorite and hornblende quartz diorite are
calc-alkaline. Trace element plots for the three host rocks indicate formation in a subduction
setting. Furthermore, discrimination diagrams point to an adakitic character for the younger
hornb lende quartz d io rite a nd typ ical a rc rock signa tures for th e o lder ga bb ro a nd tonalite-
granodiorite. The results indicate that hydrothermal mineralization is attributed to the hornblende
qua rtz diorite, wh ich c oinc ides with t he Plio cen e mineraliza tion recogn ized in the Ma nkayan
Min era l Distric t.
1. Introduction
Recent research emphasized the importance of understanding the geochemistry of igneous rocks in
determining fertility of magmatic systems that form porphyry copper deposits [1, 2]. Barren and fertile
intrusive rocks would have formed under varied geochemical compositions and magmatic conditions,
which would provide constraints on ore deposit formation [3]. An ideal natural laboratory to study these
magmatic systems are in highly mineralized areas like the Mankayan Mineral District.
The Mankayan Mineral District (MMD) in northern Luzon, Philippines is considered as one of the
most productive mineral districts in the southwest Pacific [4]. It hosts a world-class fossil hydrothermal
system consisting of the Far Southeast porphyry copper deposit, Lepanto high sulfidation epithermal
deposit and the Victoria intermediate sulfidation epithermal deposit. In addition, the MMD also consists
of other hydrothermal prospects that have yet to be investigated in detail, and this includes the Suyoc
epithermal vein prospect located ~4 kms south of the Far Southeast-Lepanto-Victoria system (Figure
1a). Intrusive rocks to the west of the Suyoc epithermal veins were investigated in this study to
understand magma fertility in relation to the various stages of igneous activity in the area.
RCGEOE-2021
IOP Conf. Series: Earth and Environmental Science 1071 (2022) 012021
IOP Publishing
doi:10.1088/1755-1315/1071/1/012021
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Figure 1. a) Distribution of porphyry copper and epithermal gold deposits in the Mankayan Mineral
District. Suyoc is in the southern portion of the study area. b) Geologic map of the district adopted from
[5].
2. Geologic Outline
The Mankayan Mineral District (MMD) is part of the Northern Luzon segment of the Taiwan-Luzon
volcanic arc. It was formed from subduction of the South China Sea Basin along the Manila Trench [6].
In the Pliocene, subduction of the Scarborough Seamount Chain, an extinct mid-oceanic ridge, resulted
in tectonic uplift and magmatic activity [6]. This is believed to result in the formation of Pliocene-
Pleistocene hydrothermal mineralization in the MMD [7]. The geology and mineralization of the MMD
shares similarities with the well-known Baguio Mineral District, which is 55 km south of Mankayan [7].
The spatial distribution of mineralization in the the MMD is believed to be controlled by the Abra
River Fault, which is a splay of the Philippine Fault Zone [8]. The NS- to NNW-trending Abra River
Fault is a left-lateral fault with thrust component. One of its NW-trending splays is the Lepanto Fault,
which hosts the Lepanto epithermal deposit.
The district is underlain by Late Cretaceous to Early Eocene Lepanto Metavolcanics basement
(Figure 1b) composed of basaltic to andesitic volcanic rocks that have undergone low grade greenschist
metamorphism [10]. Middle Eocene to Early Oligocene volcanic and shallow sedimentary rock
sequences of the Balili Formation overly this basement rock. Major magmatism, which formed
intermediate composition batholithic intrusions of the Bagon Intrusive Complex, occurred during Late
Oligocene [11]. Associated with the magmatism is the deposition of terrestrial to shallow marine
volcano-sedimentary sequences of the Suyoc Conglomerate comprised of conglomerate, sandstone,
shale, limestone and interbeds of volcanic flows and tuffs [5]. Magmatism in the Pliocene resulted in
the intrusion of Quartz Diorite and the deposition of the Imbanguila Dacite. Continued magmatism
during the Pleistocene formed the Bato Dacite. The youngest unit in the area is the Late Pleistocene
Lapangan Tuff [5].
RCGEOE-2021
IOP Conf. Series: Earth and Environmental Science 1071 (2022) 012021
IOP Publishing
doi:10.1088/1755-1315/1071/1/012021
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3. Methodology
Least altered igneous rocks were collected from surface exposures in the study area. Sample preparation
of double-polished thin sections was done at the University of the Philippines – National Institute of
Geological Sciences (UP-NIGS) using a Pelcon Automatic Thin Section machine and a Topper
Diamond Polishing unit. Petrographic analysis was conducted using the Olympus BX53-P polarizing
microscope.
Samples for whole rock major composition determination were powdered and made into pressed
pellets. These were analyzed using a Rigaku RIX3100 X-Ray Fluorescence (XRF) spectrometer at the
Department of Earth Resource Engineering, Kyushu University, Japan. Prior to analysis, loss on ignition
(LOI) values wer e measured. For tr ace element com positions, powdered samples were subjected to four -
acid digestion technique then analyzed using an Inductively Coupled Plasma Optical Emission
Spectrometer (Agilent ICP-OES 5100) and Inductively Coupled Plasma Mass Spectrometer (Agilent
7700x ICPMS) at the Department of Earth and Environmental Sciences, National Chung Cheng
University, Taiwan.
Zircons separated from the igneous rocks were subjected to U-Pb isotopic dating using a laser
ablation inductively coupled plasma mass spectrometer (LA-ICPMS) at the National Chung Cheng
University in Taiwan. The laser ablation was set at 70 seconds with less than 1000 counts per second.
Calibration was made prior to analysis using a GJ-1 standard. Isotopic ratios were calculated using the
GLITTER 4.4.2 (GEMOC) software. Probability curves, density plots, and weighted mean ages were
prepared using Isoplot v. 3.0.
4. Results and Discussion
Three major igneous units were identified in Suyoc: the gabbro-diabase, tonalite-granodiorite, and
hornblende quartz diorite. The gabbro-diabase is part of the Cretaceous Lepanto Metavolcanics, the
basement unit in the area. It exhibits low-grade deformation and alteration. The gabbro-diabase is
composed of allotriomorphic-granular plagioclase and actinolite with minor magnetite and pyrite grains.
The tonalite-granodiorite is considered part of the Bagon Intrusive Complex. It consists of varying
amounts of plagioclase, quartz, potassium feldspar, biotite, and hornblende with minor magnetite.
Zircon grains from this unit yielded an age of 37.20 ± 4.70 Ma (Late Eocene). Meanwhile, the
hornblende quartz diorite is dated as Middle Pliocene (3.18 ± 0.77 Ma). This is the first time that a
Middle Pliocene intrusive is recognized in Mankayan. This age can be correlated to the Black Mountain
Diorite in the Baguio Mineral District. The hornblende quartz diorite dominantly consists of plagioclase,
hornblende, and quartz ± magnetite.
The gabbro-diabase plot in the basalt to basaltic andesite fields in the total alkali vs silica diagram
(Figure 2A) consisting of 48-55 wt% SiO2. The granodiorite-tonalite are classified as dacites and
rhyolites with 72-76 wt% SiO2 while the hornblende quartz diorite plot in the andesite field with 57-63
wt% SiO2. The AFM diagram [12] indicates that the gabbro-diabase exhibit tholeiitic signatures (Figure
2b). In contrast, both the granodiorite-tonalite and hornblende quartz diorite show a calc-alkaline
fractionation trend. Furthermore, the two younger intrusive units reveal negative correlation of FeO and
TiO2 in Harker diagrams. Trace element compositions normalized to primitive mantle values indicate
that all the intrusive rocks indicate strong subduction signatures with enriched large ion lithophile
elements and strong negative Nb and Zr anomalies. When plotted in the chondrite-normalized REE
diagram, the tonalite-granodiorite and hornblende quartz diorite rocks have elevated LREE with the
hornblende quartz diorite exhibiting a negative Eu anomaly. In contrast, the gabbro-diabase exhibits
positive Ti anomaly and depleted LREE values.
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IOP Conf. Series: Earth and Environmental Science 1071 (2022) 012021
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doi:10.1088/1755-1315/1071/1/012021
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Figure 2. a) Total alkali vs. silica discrimination diagram showing the compositions of the gabbro
diabase, tonalite-granodiorite, and hornblende quartz diorite. b) AFM diagram [12] showing a tholeiitic
character for the gabbro-diabase and calc-alkaline signatures for the tonalite-granodiorite and
hornblende quartz diorite. c-d) Adakite discrimination diagrams [13] revealing typical arc rocks
signature for the gabbro-diabase and tonalite-granodiorite but an adakitic character for the hornblende
quartz diorite. e) Diagram [14] classifying the hornblende quartz diorite as a fertile or productive
intrusive while the two older units are considered as barren.
When plotted in discrimination diagrams for adakites [13], the older gabbro-diabase and tonalite-
granodiorite units plot in the field for typical arc rocks while the hornblende quartz diorite are classified
as adakites (Figure 2c-d). In addition, the elevated Sr/Y values of the hornblende quartz diorite when
plotted against SiO2 [14] classify it as a fertile intrusive in contrast to the two older intrusive rocks that
plot in the barren intrusive field.
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IOP Conf. Series: Earth and Environmental Science 1071 (2022) 012021
IOP Publishing
doi:10.1088/1755-1315/1071/1/012021
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The gabbro-diabase unit of the Lepanto Metavolcanics represents the basement rocks in the
Mankayan Mineral District. I ts tholeiitic character points to a mid-oceanic ridge basalt (MORB) or back-
arc setting for Mankayan in the Cretaceous. Meanwhile, the calc-alkaline tonalite-granodiorite of the
Bagon Intrusive Complex represents a subduction-related magmatic event in the Late Eocene. The
recognition of a Late Eocene intrusive in Mankayan supports tectonic models indicating Eocene
subduction along the proto-East Luzon Trench beneath Luzon, which later experienced arc reversal that
resulted in subduction at the Manila Trench [15]. The calc-alkaline hornblende quartz diorite represents
an adakitic pulse of magmatism in the Pliocene. This magmatic event is associated with subduction of
the Scarborough ridge along the Manila Trench. It represents the fertile magmatism associated with the
ubiquitous hydrothermal mineralization in the Mankayan Mineral District.
5. Summary and Conclusions
The results of the study are summarized in Table 1. The tholeiitic Cretaceous Lepanto Metavolcanics
gabbro-diabase unit was possibly generated in a MORB or back-arc setting. On the other hand, the calc-
alkaline signatures of the Late Eocene Bagon Intrusive Complex tonalite-granodiorite could represent
subduction along the proto-East Luzon Trough. Meanwhile, the Pliocene hornblende quartz diorite
correlate with the Black Mountain Diorite could represent subduction in the Manila Trench that is
affected by the Scarborough Seamount Chain. Furthermore, the results indicate that hydrothermal
mineralization is only attributed to the hornblende quartz diorite, which coincides with the Pliocene
mineralization recognized in the Mankayan Mineral District.
Table 1. Summary of the characteristics of the intrusive units in Suyoc.
Gabbro-
diabase
Tonalite-
granodiorite
Hornblende
quartz diorite
Age
Cretaceous
Late Eocene
Pliocene
Geochemical
affinity
Tholeiitic Calc-alkaline
Calc
-alkaline; a
dakitic
Tectonic Setting
MORB/
back-arc?
Island arc Island arc
Potential for
mineralization
غ
غ
ض
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Acknowledgements
The authors would like to acknowledge the Itogon Suyoc Resources Inc. for the logistical support during
the fieldwork component of the study. Members of the Rushurgent Working Group from UP NIGS are
also thanked for their assistance and guidance.
