Epithermal mineralization of the Bonanza-Sandy vein system, Masara Gold District, Mindanao, Philippines

Abstract

The Masara Gold District in southeastern Mindanao island is an area of prolific hydrothermal copper and gold mineralization. This study documents the mineralization characteristics of the NW-trending Bonanza-Sandy epithermal veins to constrain possible hydrothermal fluid sources and ore-forming mechanisms. Epithermal mineralization in the NW veins is divided into three main stages: Stage 1 - massive quartz-sulfide; Stage 2 - massive to amorphous quartz-carbonate (calcite); and Stage 3 - colloform-cockade quartz-carbonate (bladed rhodochrosite). Stage 1 is the main gold mineralization phase, with chalcopyrite, pyrite, sphalerite and galena occurring with native gold and tellurides. Stages 2 and 3 contain invisible gold in the sphalerite, galena, pyrite and chalcopyrite. The deposit exhibits mineralization characteristics typical of intermediate sulfidation epithermal deposits based on the dominant chalcopyrite-pyrite mineral assemblage; illite-muscovite-chlorite alteration mineralogy that point to neutral pH conditions; and sphalerite composition of 2.26 to 8.72 mol% FeS in Stage 1 and 0.55 to 1.13 mol% FeS in Stage 2. The K-Ar age date of illite separates from highly altered diorite porphyry of the Lamingag Intrusive Complex yielded an Early Pliocene age (5.12 ± 0.16 Ma). Hydrothermal fluid exsolved from the magma that formed the Lamingag Intrusive Complex probably formed the ore-forming Stage 1 veins. Stages 2 and 3 involved the deposition of quartz and carbonate veins possibly by boiling hydrothermal fluids. Precious and base metal deposition was controlled by the Masara Fault Zone. Exploration markers for gold mineralization in the Masara Gold District and vicinity include the presence of Lamingag Intrusive Complex and massive sulfide veins.

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Journal of Asian Earth Sciences: X 4 (2020) 100041
Available online 3 December 2020
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Epithermal mineralization of the Bonanza-Sandy vein system, Masara Gold
District, Mindanao, Philippines
Jillian Aira Gabo-Ratio
a
,
*
, Alfred Elmer Buena
a
, Barbie Ross B. Villaplaza
a
,
b
, Betchaida
D. Payot
a
, Carla B. Dimalanta
a
, Karlo L. Quea˜
no
c
, Eric S. Andal
d
, Graciano P. Yumul Jr.
e
a
Rushurgent Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, Philippines
b
Philsaga Mining Corporation, Upper Co-o, Bunawan, Agusan del Sur, Philippines
1
c
Department of Environmental Science, Ateneo de Manila University, Quezon City, Philippines
d
Apex Mining Corporation, Inc., Ortigas Center, Pasig City, Philippines
e
Cordillera Exploration Company, Inc., Bonifacio Global City, Taguig City, Philippines
ARTICLE INFO
Keywords:
Masara Gold District
Epithermal gold veins
Mindanao
Philippines
ABSTRACT
The Masara Gold District in southeastern Mindanao island is an area of prolic hydrothermal copper and gold
mineralization. This study documents the mineralization characteristics of the NW-trending Bonanza-Sandy
epithermal veins to constrain possible hydrothermal uid sources and ore-forming mechanisms. Epithermal
mineralization in the NW veins is divided into three main stages: Stage 1 - massive quartz-sulde; Stage 2 -
massive to amorphous quartz-carbonate (calcite); and Stage 3 - colloform-cockade quartz-carbonate (bladed
rhodochrosite). Stage 1 is the main gold mineralization phase, with chalcopyrite, pyrite, sphalerite and galena
occurring with native gold and tellurides. Stages 2 and 3 contain invisible gold in the sphalerite, galena, pyrite
and chalcopyrite. The deposit exhibits mineralization characteristics typical of intermediate suldation epi-
thermal deposits based on the dominant chalcopyrite-pyrite mineral assemblage; illite-muscovite-chlorite
alteration mineralogy that point to neutral pH conditions; and sphalerite composition of 2.26 to 8.72 mol%
FeS in Stage 1 and 0.55 to 1.13 mol% FeS in Stage 2. The K-Ar age date of illite separates from highly altered
diorite porphyry of the Lamingag Intrusive Complex yielded an Early Pliocene age (5.12 ±0.16 Ma). Hydro-
thermal uid exsolved from the magma that formed the Lamingag Intrusive Complex probably formed the ore-
forming Stage 1 veins. Stages 2 and 3 involved the deposition of quartz and carbonate veins possibly by boiling
hydrothermal uids. Precious and base metal deposition was controlled by the Masara Fault Zone. Exploration
markers for gold mineralization in the Masara Gold District and vicinity include the presence of Lamingag
Intrusive Complex and massive sulde veins.
1. Introduction
The Philippines hosts world-class epithermal lode gold deposits
found throughout the archipelago (Mitchell and Leach, 1991; Malihan
et al., 2015). The majority of gold endowment in the Philippines has
been produced from ve major gold districts, including Baguio-
Mankayan in northern Luzon, Paracale and Masbate in Bicol, and Sur-
igao and Masara in Mindanao (Fig. 1A) (Mitchell and Leach, 1991;
Yumul et al., 2003). These districts are within 515 km to the 1200-km
long sinistral strike-slip Philippine Fault Zone (Mitchell and Leach,
1991).
Among these districts, the Baguio-Mankayan Gold District (Fig. 1A)
in north Luzon is the most studied area (e.g. Cooke et al., 2011; Waters
et al., 2011; Yumul et al., 2020), with studies of world class deposits of
the Acupan-Sangilo (Cooke et al., 1996; Jabagat et al., 2020), Black
Mountain (Cao et al., 2018), Sto. Tomas II (Imai, 2001; Masangcay et al.,
2018) and the Lepanto-Far Southeast-Victoria systems (e.g. Arribas
et al., 1995; Hedenquist et al., 1998; 2017; Imai, 2000; Claveria, 2001;
Manalo et al., 2018). In contrast, the Masara Gold District in Mindanao
island is considered to be one of the least studied in terms of minerali-
zation characteristics. Recent publications describe the geology, struc-
tures, geophysics, geochemistry and alteration in the area (Malihan
* Corresponding author.
E-mail address: jgratio@nigs.upd.edu.ph (J.A. Gabo-Ratio).
1
Now with.
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Journal of Asian Earth Sciences: X 4 (2020) 100041
2
et al., 2015; Manalo et al., 2017; Villaplaza et al., 2017; Yumul et al.,
2017; Buena et al., 2019; Yumul et al., 2020). This paper examines the
ore-gangue mineralogy, textures, nature of the ore-forming uids, and
the mechanisms of ore formation for the northwest-trending Bonanza-
Sandy Au-bearing veins currently being mined by the Apex Mining Co.,
Inc. (AMCI) in the Masara Gold District. In addition to the vein samples
collected, illite mineral separates from the wall rock were dated by the
K-Ar method. The mineralization model proposed in this study may
serve as a guide in the exploration of other potential mineralization in
the vicinity.
2. Geologic framework
2.1. Regional geology
The Philippines is bound by subduction systems of opposing dips
(Fig. 1A). To the west are the Manila Trench, Negros-Sulu Trench and
Cotabato Trench, with subduction of the South China Sea, Sulu Sea and
Celebes Sea basins, respectively. To the east is the East Luzon Trough-
Philippine Trench system, which consumes the West Philippine Sea
basin. The kinematic interactions between these subduction zones led to
the formation of the sinistral Philippine Fault Zone (PFZ) in the Philip-
pine Mobile Belt (PMB) (Galgana et al., 2007; Aurelio et al., 2013). The
PFZ transects the entire archipelago and majority of the mineralized
areas (i.e., the gold districts of Baguio-Mankayan in Northern Luzon,
Paracale in Bicol Peninsula, Masbate island, and Surigao and Masara in
Mindanao) (Fig. 1A).
Eastern Mindanao (Pacic Cordillera) is separated from Central
Mindanao by the sinistral PFZ (Fig. 1B), bound by the Agusan-Davao
Basin to the west and by the southern portion of the Philippine Trench
to the east. Eastern Mindanao is underlain by a basement of Cretaceous
ophiolitic fragments and metamorphosed rocks intruded by Oligocene to
Plio-Pleistocene igneous rocks with early Miocene conglomerate, sand-
stone and limestone (Sajona et al., 1997; Suerte et al., 2009). Calc-
alkaline and adakitic magmatism played a major role in the hydro-
thermal mineralization prolic in the area (e.g. Suerte et al., 2009;
Sonntag et al., 2011; Yumul et al., 2017, 2020; Braxton et al., 2018). In
Eastern Mindanao, the major hydrothermal systems include the
Boyongan-Bayugo porphyry copper-gold deposit (e.g., Braxton et al.,
2012; 2018) and the sedimentary rock-hosted Siana gold deposit in the
Surigao District, the Co-O epithermal gold deposit in Agusan del Sur (e.
g., Sonntag et al., 2011), the Diwalwal epithermal gold deposit in the
Central District and the Masara epithermal gold and Kingking porphyry
copper deposits in the Masara District (e.g., Suerte et al., 2009; Yumul
et al., 2020) (Fig. 1B).
2.2. Masara Gold District
The Masara Gold District in the southern portion of Eastern Mind-
anao is known for prolic gold and copper mineralization. It is situated
in a dilational jog of the Philippine Fault Zone within caldera structures,
and hosts the Masara-Hijo-Amacan copper gold deposits (Malihan and
Flores, 2012; Yumul et al., 2017). The geologic units were investigated
in detail by Buena et al. (2019), building on the initial works of Lodri-
gueza and Estoque (1976) and Mercado et al. (1987). The district is
underlain by the Eocene Masara Formation, consisting of tuff deposits
intercalated with andesite ows. This basement unit is unconformably
overlain by the Miocene biohermal Agtuuganon Limestone. Various
Fig. 1. A.) Location map showing the major gold districts (red boxes) in the Philippines (Mitchell and Leach, 1991). The black box in the southeastern portion
represents the area shown in B. B.) Eastern Mindanao (Pacic Cordillera) being separated from the Agusan-Davao Basin by the Philippine Fault Zone. The yellow stars
represent the major hydrothermal systems in Eastern Mindanao: 1) Boyongan-Bayugo deposit in the Surigao District; 2) Co-O epithermal deposit; and 3) Masara
epithermal gold and Kingking porphyry copper deposits in the Masara District. The red box in B represents the location of the Masara Gold District as shown in Fig. 2.
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
3
stocks of intrusive units cut these older rocks. The Early Miocene equi-
granular Cateel Quartz Diorite is the dominant pluton in the area, sub-
sequently intruded by the Middle Miocene plagiophyric dikes of the
Alipao Andesite, followed by the Lamingag Intrusive Complex. The
Intrusive Complex is composed of multiple stages of intrusion of quartz
diorite stock, hypabyssal andesite and diorite porphyry. Our work, as
will be shown later, shows that the Lamingag Intrusive Complex extends
to a younger age. The youngest unit in the area is the Pliocene to Recent
Amacan Volcanic Complex, which is composed of dacite lava and py-
roclastic deposits.
These rock units record an evolution from tholeiitic magmatism
(Masara Formation) due to the subduction of the proto-Molucca Sea
Plate in the Eocene, to calc-alkaline Cateel Quartz Diorite in the Early
Miocene (Yumul et al., 2017; Buena et al., 2019). Eastern Mindanao is
postulated to have collided with a paleo-island arc in the Early to Middle
Miocene, resulting in the deposition of the Agtuuganon Limestone and
Fig. 2. Geologic map and cross-section of the Masara Gold District showing the distribution of the rock units (modied from Malihan et al., 2015; Buena et al., 2019).
The mineralized zones are associated with NW- and EW-trending epithermal veins, whereas porphyry mineralization occurs at Teresa and Kurayao (blue circles). The
yellow circles on the map are the surface projection of representative samples from the different mine levels, whereas those on the cross-section represent the sample
location. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
4
weak magmatism represented by the Alipao Andesite. The Late Miocene
multiple dikes and stocks of the Lamingag Intrusive Complex represent
reactivation of calc-alkaline magmatic activity and initial adakitic
magmatism attributed to the onset of the eastward subduction of the
Molucca Sea Plate. The Plio-Pleistocene unmineralized Amacan Volca-
nic Complex is characterized by adakitic signatures (Yumul et al., 2017).
Hydrothermal mineralization and associated alteration are hosted in
the units of the Masara Formation, Cateel Quartz Diorite, Alipao
Andesite and the Lamingag Intrusive Complex (Fig. 2) (Malicdem and
Pe˜
na, 1967; Mercado et al., 1987; Villaplaza et al., 2017; Buena et al.,
2019). The Plio-Pleistocene Amacan Volcanic Complex post-dates gold
mineralization in Masara (Yumul et al., 2017; Buena et al., 2019). At
least three mineralization styles have been recognized in the Masara
Gold District: epithermal gold-silver mineralization (NW- and EW-
trending mineralized veins related to the Masara Fault Zone);
porphyry-related copper–gold mineralization (e.g. Kurayao and Theresa
prospects); and minor skarn mineralization (Malihan and Flores, 2012;
Malihan et al., 2015; Yumul et al., 2020) (Fig. 2).
This study concentrated on the present mine operations of AMCI,
which are focused on the NW-trending gold-base metal epithermal veins
(140 kt at 8.4 g/t Au, given a 3 g/t cutoff grade) (Malicdem and Pe˜
na,
1967; Mercado et al., 1987; Malihan and Flores, 2012). The NW-
trending, NE-dipping mineralized faults serve as controls to the
bonanza-grade epithermal mineralization (e.g., Bonanza-Sandy veins
and their splits). The auxiliary E-W vein systems have associated WNW
and ENE segmented trends of varying dip directions (e.g., veins in
Wagas, Calixto, Dons and Saints), and are linked to the main NW-
trending Masara Fault Zone as relay structures.
The recent work of Villaplaza et al. (2017) described at least ve
major alteration zones in the Masara gold district: (1) regional propylitic
alteration zone; (2) potassic alteration zone with stockwork veins; (3)
early-stage chlorite-illite-muscovite ±epidote alteration; (4) overprint
of late-stage muscovite-illite ±chlorite alteration; and (5) minor
occurrence of quartz +kaolinite +magnetite +dickite ±illite ±calcite
localized along the Don Manuel Fault on the southwestern portion of the
study area. The rst three zones are linked to an earlier porphyry copper
system in the Kurayao and Teresa areas (Fig. 2), whereas the late stage
chlorite-illite-muscovite alteration in the eastern part are associated
with the NW- and EW-trending epithermal gold veins.
A reinterpretation of the airborne magnetic survey in the area
(Manalo et al., 2017) revealed magnetic low anomalies on the Reduced-
to-Equator (RTE) map. These coincide with the location of the porphyry
copper zones in Teresa and Kurayao. However, the early stage chlorite-
illite-muscovite zone delineated by Villaplaza et al. (2017) coincides
with the magnetic low RTE anomaly in the central portion of the Masara
District. Considering the non-magnetic signature of this alteration zone,
the observed anomaly must be caused by a deeper magnetic source. This
is suggested to be the signature of the deeper magnetite-rich potassic
alteration zone. It is further supported by the 500-meter upward
continued magnetic anomaly map implying that a magnetic source is
still present at ~250-meter depth (Manalo et al., 2017). Epithermal
veins that reect the Masara Fault Zone and the EW-trending faults in
the central part of Masara display linear anomalies in the tilt derivative
map.
3. Analytical techniques
3.1. Sampling methodology
Surface mapping and underground surveys were conducted in the
NW-trending Bonanza and Sandy veins to identify occurrences of the
epithermal veins, including ore and gangue mineralogy, sulde abun-
dance, textures and cross-cutting relationships. Approximately 30 vein
samples were taken from the Bonanza and Sandy vein systems from the
following mine levels corresponding to elevation (Fig. 2): L530, L575,
L605, L780, L795 and L810.
3.2. Ore and gangue mineralogy and mineral chemistry
Petrography was conducted to establish ore mineral assemblage,
textural relationships and paragenetic sequence. Double-polished, car-
bon-coated thin sections of vein samples were used for Electron Probe
Micro Analysis (EPMA) using the JEOL JXA-8230 Super Probe at the
University of the Philippines - National Institute of Geological Sciences
(UP-NIGS). Elemental composition and abundances were measured
using a Wave Dispersive Spectroscopy (WDS) analysis. Data obtained
were calculated using the ZAF correction matrix from the JEOL soft-
ware. Standard operating conditions for sulde analysis were set at 20
kV acceleration voltage, 20nA beam current with a beam diameter of 5
µm. Results of the analysis yielded L-values which were processed for
ZAF correction using the JEOL software. Data obtained from this
calculation produced elemental abundances expressed in weight %
(Reed, 1995). The detection limit per element varies per analysis and is
calculated as a function of peak intensity and background intensity
(Reed, 2000; Batanova et al., 2018). In this study, the detection limits
were calculated to range from ~100–200 ppm.
3.3. K-Ar dating
The most illite-altered host rock in the immediate vicinity of the
mineralized veins (Sample HW-1; Fig. 2) were dated by the potassium-
argon (K-Ar) method, using illite separates to estimate the age of alter-
ation. The diorite porphyry sample from the Lamingag Intrusive Com-
plex was analyzed by the Hiruzen Institute for Geology and
Geochronology, Co, Ltd., Okayama, Japan.
The rock sample was cut into one-centimeter thick rock slab then
crushed to obtain 10–30 mesh size fractions. Illite was separated from
the ner fractions following the mineral separation protocol reported by
Yagi (2006). The ne-sized particles were extracted using an ultrasonic
bath. The ner particles were extracted using a centrifuge and then
dissolved by HCl to eliminate any presence of carbonates. The treated
sample was viewed under the binocular microscope to have visual
conrmation of illite (Yagi, 2006). The illite separates were analyzed for
potassium content and argon isotopes. Qualitative analysis of potassium
was carried out by ame photometry using a 2000 ppm Cs buffer within
2% analytical error. Further details of this analysis are reported by
Nagao et al. (1984). For argon isotope analysis, the white mica samples
were heated to 180–200 ◦C by ribbon and mantle heater for about 72 h
to emit adsorbed gases under vacuum. The specimens that were wrap-
ped in Al foil were vacuumed out at 180–200 ◦C for 3 days, then argon
was extracted at 1500 ◦C in an ultra-high vacuum line. Calculations for
the age at two-sigma condence level were obtained following the
methods described by Nagao et al. (1984) and Nagao and Itaya (1988).
The decay constants for
40
K to
40
Ar and
40
Ca and
40
K content in potas-
sium from Steiger and Jager (1977) were used in the age calculation for
alteration/mineralization.
4. Results
4.1. NW-trending Bonanza-Sandy epithermal veins
The veins of the Masara Gold District are largely conned to the NW-
trending Masara Fault Zone. This structure is characterized by contin-
uous but segmented veins (e.g. Bonanza and Sandy veins) that split into
minor branches (Fig. 2). The vein system is characterized by a complex
history of multiple quartz-calcite-sulde veins and hydrothermal brec-
cias with late-stage carbonates (Fig. 3A-B). The Masara-Sandy veins
strike northwest to west-southwest and dip steeply to the northeast. The
vein system pinches and swells along its strike length, but thicknesses
typically range from 0.5 to 1 m, with the thickest veins measuring >3 m.
The vein phases are generally sulde-rich, containing pyrite, chalcopy-
rite, sphalerite and galena, with high gold grades generally correlated
with sulde abundance. The veins exhibit open-space ll textures
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
5
(crustiform, colloform, drusy and open space), brecciation (silicied
breccia in quartz matrix) and selective replacement of carbonates by
quartz (Fig. 3C-F). Alternating crustiform bands of sulde and quartz-
carbonate, as well as cockade and comb textures, indicate multiple
mineralization pulses.
4.2. Ore-gangue mineralogy and vein paragenesis
From the ore and gangue mineral assemblage, textures and cross-
cutting relationships of the NW-trending structures, three major stages
of vein formation are identied (Fig. 3A-B, Table 1). Stage 1 occurs as
Fig. 3. A.) Representative underground exposure (Sample L810) of the NW-trending veins showing the cross-cutting relationships of Stages 1 to 3. B.) Detailed
texture of the representative Masara epithermal veins exhibiting the three main paragenetic sequences (Sample L575S). C.) Colloform and bladed rhodochrosite with
quartz of Stage 3 (Sample L575). D.) Stage 2 small quartz crystals exhibiting drusy texture (Sample L530). E.) Stage 2 drusy quartz associated with calcite and coarse-
grained galena and sphalerite (Sample L795). F.) Stage 1 silicied breccia clasts in quartz-carbonate matrix (Sample L780) G.) Stage 1 massive sulde vein exhibiting
abundant chalcopyrite and pyrite with quartz and epidote (Sample L605). H.) Stage 2 vein (Sample L530) with massive to open-space quartz and calcite. I.) Stage 3
vein (Sample L575S) exhibiting colloform and cockade textures. J.) Stage 3 rhodochrosite veins cutting the Stage 1 massive sulde veins (Sample L575S).
Table 1
Ore and alteration mineral paragenesis of the different stages of the Masara NW-trending epithermal veins. Thickness of the black lines indicates relative abundance;
the solid red line refers to gold and telluride grains; red dashed line indicates invisible gold occurrence.
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
6
massive sulde vein with minor quartz and epidote (Fig. 3G), with
anhedral coarse-grained chalcopyrite surrounding subhedral ne- to
medium-grained pyrite (Fig. 4A). Within the chalcopyrite, medium-
grained subhedral sphalerite and galena exhibit interpenetrating grain
boundaries. Stage 1 is the main gold mineralization phase, where
precious metals occur as inclusions in pyrite and galena. Gold occurs
mostly as native gold that is 5 to 50
μ
m in size (Figs. 4B and 5) or as gold-
silver tellurides (Fig. 4C) identied as petzite and hessite (Figs. 6 and 7).
Stage 2 consists of massive to open-space ll quartz-carbonate vein
(Fig. 3H) that cuts the Stage 1 veins (Fig. 3B). The quartz associated with
Stage 2 exhibits massive texture with coarse-grained subhedral to
euhedral sphalerite and galena with interpenetrating boundaries
(Fig. 4D). Minor amounts of ne-grained anhedral chalcopyrite and
pyrite are sparse. Gold is not visible but was detected as a trace element
in sphalerite and pyrite, as quantied in the EPMA (Tables 3 and 4).
Stage 3 is characterized by a quartz-carbonate phase dominated by
rhodochrosite (Fig. 3B-C). Multiple bands of colloform-crustiform with
cockade-brecciated textures are commonly observed in this stage
(Fig. 3I-J), as well as rhodochrosite with platy textures (Fig. 3B-C).
Anhedral coarse- to medium-grained sphalerite is the dominant sulde
phase, with minor ne-grained euhedral galena and pyrite and anhedral
chalcopyrite (Fig. 4E). Stage 3 exhibits the same sulde assemblage as
Stage 2, but the sphalerite and galena grains are ner-grained with rare
chalcopyrite and pyrite.
4.3. Alteration mineralogy
The paragenetic sequence determined for the three vein stages were
incorporated with the observed alteration in the area following the study
by Villaplaza et al. (2017) (Table 1). The NW-trending veins, especially
Bonanza and Sandy, are dominantly hosted by altered intrusive bodies
of the Cateel Quartz Diorite and Lamingag Intrusive Complex, and vol-
canic rocks of the Masara Formation (Fig. 2). Veins hosted in the
intrusive units are typically thinner, whereas veins swell where hosted in
the Masara Formation. Wall rocks immediately adjacent to the epi-
thermal gold-base metal quartz veins of Stage 1 have halos of quartz +
chlorite +illite +muscovite +pyrite ±epidote ±smectite ±adularia ±
magnetite. Meanwhile, a later stage illite-muscovite alteration is mainly
observed in rocks predominantly hosting the quartz-carbonate Stage 2
and 3 veins. This late-stage illite-muscovite alteration assemblage
associated with the veins were observed to overprint an earlier alter-
ation zone of quartz +chlorite +illite +muscovite +pyrite ±biotite ±
smectite ±magnetite ±calcite, thought to be associated with an older
porphyry copper–gold system in the Kurayao and Teresa areas
Fig. 4. A.) Stage 1 quartz-sulde vein
(Sample L605) containing chalcopyrite (cpy)
and pyrite (py) as the dominant sulde
assemblage with minor disseminations of
galena (gal) and sphalerite (sphal). B.)
Native gold (Au) in pyrite found in Stage 1
vein (Sample L780) associated with the gold-
silver tellurides. C.) Stage 1 veins (Sample
L780) with hessite (hes) and petzite (petz)
tellurides occurring as pyrite inclusions. D.)
Stage 2 quartz-carbonate vein (Sample
L575B) mostly containing sphalerite and
galena with sparse distribution of pyrite. E.)
Stage 3 quartz-carbonate vein (Sample L795)
with sphalerite and minor occurrences of
galena and chalcopyrite. F.) Photomicro-
graph of plagioclase altered to illite in the
diorite porphyry unit of the LIC used for age
dating (Sample HW-1).
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
7
(Villaplaza et al., 2017).
The degree of host rock alteration varies from the immediate host
rock towards the periphery of the veins. Higher degrees of alteration by
chlorite-illite-muscovite occur proximal to the vein. Hornblende of the
diorite units of both the Lamingag Intrusive Complex and the Cateel
Quartz Diorite are partially to completely replaced by chlorite, and
plagioclase is typically replaced by minor patches of illite and muscovite
(Fig. 4F), and locally calcite. Where the veins are hosted in the diorite
porphyry unit of the Lamingag Intrusive Complex, sericitization is most
prevalent. A few meters from the vein, regional propylitic alteration
dominates.
4.4. Mineral chemistry
4.4.1. Gold and tellurides
Mineral chemistry analysis of the native gold grain yielded elemental
compositions of 85% Au and 15% Ag, a ratio of 5.7 (Figs. 5 and 6).
Quantitative analysis of 30 gold-silver telluride grains included by pyrite
and chalcopyrite in Stage 1 of sample L780 revealed hessite (Ag
2
Te) as
the dominant gold-silver telluride phase with minor occurrences of
petzite (Ag
3
AuTe
2
) (Table 2, Figs. 6 and 7). Analysis of 15 telluride
grains included in pyrite and chalcopyrite in the Stage 1 vein of sample
L605 also identied hessite and petzite.
4.4.2. Sphalerite
Sphalerite grains that coexist with pyrite in Stages 1 and 2 have core
compositions with higher Fe (1.50–5.99 wt%) in Stage 1 than Stage 2
sphalerite (0.38–0.74 wt%) (Table 3). Stage 1 sphalerite grains also
exhibit higher Cu content (1.53–6.16 wt%) compared to Stage 2 (nil
concentrations of Cu in most grains except for one at 0.27 wt%).
The mole % Fes of the sphalerite grains for each stage revealed Stage
1 values of 3.4 to 9.8 mol% FeS for sample L575, 2.3 to 2.5 mol% Fes for
sample L605 and 3.91 to 8.72 mol% FeS for sample L780 (Table 3). The
sphalerites from Sample L780 representing Stage 2 contain 0.55 to 1.13
mol% FeS; mol% FeS of Stage 3 sphalerite was not possible. Signicant
total values were not obtained for the sphalerites of this stage.
4.4.3. Pyrite
In several Stage 1 pyrites, gold is detected as invisible gold in-lattice
and ranges from 0.01 to 0.04 wt% (100–400 ppm) content (Table 4). The
gold also occurs as native gold, petzite and hessite telluride inclusions.
In Stage 2, gold was detected as invisible gold in the pyrite, galena and
sphalerite, ranging from 0.01 to 0.03 wt% (100–300 ppm). Only one
grain from Stage 3 had detectable gold content at 0.01 wt% (100 ppm).
The trace element content in pyrite from the three stages determined
that Co content ranges from 0.07 to 0.10 wt% for Stage 1, 0.09–0.11 wt
% for Stage 2, and 0.11–0.14 wt% for Stage 3 (Table 4).
Fig. 5. Multi-element compositional map of Sample L780 shows peak in concentration of gold and silver (pink color) of the native gold grain (Au) included by pyrite
(py). Microprobe analysis of the grain using Wave Dispersive Spectroscopy (WDS) yielded Au/Ag ratio of 5.7 at 85% Au and 15% Ag. CP-composite view, Fe-iron, As-
arsenic, Au-gold, Cu-copper, S-sulfur, Ag-silver, Te, tellurium, Zn-zinc, Pb-lead, sphal – sphalerite, qtz - quartz.
Fig. 6. Au-Te-Ag ternary system modied from Cabri (1965) showing the Stage
1 gold and telluride grains from sample L780 (green triangles) and the Stage 1
gold-silver tellurides of sample L605 (blue triangles). (For interpretation of the
references to colour in this gure legend, the reader is referred to the web
version of this article.)
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
8
4.5. K-Ar dating
White mica alteration of host rocks is cut by the NW-trending epi-
thermal veins and is used to approximate the timing of epithermal
mineralization (e.g., Arribas et al., 1995; Tassinari et al., 2008). Illite
separates from the diorite porphyry (Fig. 4F) of the Lamingag Intrusive
Complex (Sample HW-1, Fig. 2) yielded a K-Ar age of 5.12 ±0.16 Ma
(Early Pliocene) (Table 5).
5. Discussion
5.1. Ore-forming conditions
The alteration minerals associated with Stage 1 base metal veins are
quartz and epidote with halos of quartz, chlorite, illite and muscovite
(Table 1). Stages 2 and 3 quartz-calcite-rhodochrosite veins have halos
of illite-muscovite. The alteration minerals in the NW-trending Masara
epithermal deposit reveal typical neutral pH environment of formation.
In addition, the temperature of formation of epidote is at 240–300 ◦C
while illite and chlorite are estimated to form at ~220–300 ◦C in epi-
thermal environments (Henley and Ellis, 1983; Reyes, 1990; Simmons
et al., 2005). These temperatures may be roughly inferred to be the
conditions during vein deposition.
Stage 1 veins are dominated by massive sulde and quartz associated
with chalcopyrite and pyrite with galena and sphalerite. Gold mineral-
ization in the NW-trending vein system occurs as native Au and gold-
silver tellurides in Stage 1 massive sulde veins, which suggests an in-
uence of a magmatic source of Te (e.g. Gao et al., 2015). Stage 2
consists of massive to open-space ll quartz-carbonate with galena and
sphalerite along with minor pyrite and chalcopyrite. Stage 3 exhibits
colloform bands and bladed rhodochrosite with sphalerite, galena and
chalcopyrite as the primary minerals. The textures exhibited by Stage 3
indicate boiling as a possible mechanism for deposition (Dong et al.,
1995). For Stages 2 and 3, gold is commonly invisible, which refers to
the nonstructural ultramicroscopic and structurally-bound isomorphic
states of gold (Vikentyev, 2015). This is represented as submicron to
nanoscale inclusions of gold in suldes (e.g. Tauson et al., 1998; Deditius
et al., 2014), possibly due to gold substituting for arsenic in the iron sites
of pyrite crystal structures (Boyle, 1979; Cook and Chryssoulis, 1990). In
both stages, gold composition is up to 0.03% within the sphalerite and
pyrite minerals (Tables 3 and 4).
5.2. Suldation state and mineralization style
The suldation state can be identied based on temperature of for-
mation (estimated here from alteration mineralogy) and mol% FeS in
sphalerite grains (Barton, 1970; Czamanske, 1974; Einaudi et al., 2003)
(Fig. 8). In this study, Stage 1 veins are dominantly composed of chal-
copyrite and pyrite with galena and sphalerite. Chalcopyrite and pyrite
represent the upper and lower stability limit of intermediate suldation
state, respectively. Sphalerite grains from Stage 1 consist of 2.54 to 8.72
mol % FeS (Table 2), consistent with an intermediate suldation state at
epithermal temperatures (Einaudi et al., 2003).
Stage 2 quartz-carbonate veins (Table 1) consist mostly of sphalerite
and galena with minor chalcopyrite and pyrite. Sphalerite from this
stage has 0.55 to 1.13 mol % FeS, which corresponds to the upper
portion of intermediate suldation state (Einaudi et al., 2003) (Fig. 8).
Both Stages 1 and 2 veins of the NW-trending Masara epithermal veins
conform to the typical uid evolutionary path of base-metal veins
(Fig. 8).
Stage 3 veins are characterized by abundant rhodochrosite, common
in intermediate suldation deposits (Hedenquist et al., 2000; Wang
et al., 2019); there is no sphalerite in this stage. The alteration minerals
muscovite, illite and chlorite in the Masara Gold District NW-trending
veins (Villaplaza et al., 2017) are consistent with intermediate sulda-
tion characteristics that formed at near-neutral pH conditions (Heden-
quist et al., 2000).
The Masara Gold District formed during arc magmatism, with
neutral to compressive stress regime in the study area typically associ-
ated with porphyry Cu and associated high-suldation and
intermediate-suldation epithermal Au deposits (Einaudi et al., 2003;
Wang et al., 2019). The tectonic regime, combined with ore-gangue
mineralogy, suldation state and alteration mineralogy, indicates an
intermediate suldation style of epithermal mineralization for the NW-
trending veins of the Masara deposit.
Fig. 7. Representative multi-element compositional map of Sample L780 showing tellurides hosted in pyrite (py) with peak concentration in Ag, Au and Te (pink
colors). The tellurides were identied as hessite (hes) and petzite (petz) from WDS analyses (Table 2). CP-composite view, Fe-iron, As- arsenic, Au-gold, Cu-copper, S-
sulfur, Ag-silver, Te, tellurium, Zn-zinc, Pb-lead, sphal-sphalerite, qtz-quartz.
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
9
5.3. Mineralization model and implications for exploration
Integrating geologic and geochronological data with the nature of
the mineralizing uids and the ore-forming mechanisms, a
mineralization model is proposed for the Masara Gold District NW-
trending epithermal veins (Fig. 9). The Masara Gold District is under-
lain by the basement rock of the Eocene volcanic-pyroclastic basement
of the Masara Formation, which is cut by the NW-trending Masara Fault
Table 2
Summary of gold and tellurides mineral chemistry analysis from Sample L780 and L605 Stage 1 veins. ‘–’ means below detection limit. Minimum detection limits are
0.01–0.02 wt% for all elements.
L780
wt.% C0-1 C0-2 C0-3 C11-4 C1-3 C1-4 C1-5 C1-6 C1-7 C1-10 C1-11 C1-12 C2-1 C2-2 C2-4
Ag 61.68 59.23 57.95 55.48 59.14 62.72 60.50 60.77 63.62 59.17 57.65 59.80 58.39 58.80 44.95
Au 0.03 0.37 0.29 7.88 1.13 – – – 0.09 0.08 4.62 0.01 0.26 0.45 21.83
Bi 0.01 0.04 – 0.04 0.01 0.03 – 0.03 0.01 0.02 0.10 – 0.02 0.02 0.38
Co – – 0.01 – – – – 0.01 0.01 – – 0.02 – – 0.01
Cu – – 0.03 0.02 0.03 0.07 – 0.08 0.07 0.09 0.01 0.25 0.04 0.01 0.01
Fe 0.59 0.17 0.72 0.50 0.45 0.57 0.31 0.26 0.01 0.82 0.14 0.12 0.20 0.39 0.93
Ni – – – – – 0.08 – – – – – – – – –
S 0.17 0.07 0.34 0.14 0.26 0.12 0.12 0.14 0.11 0.17 0.10 0.15 0.18 0.21 0.15
Se – – – – – – – – 0.00 0.14 – 0.02 0.01 0.06 0.02
Te 40.45 41.70 42.25 37.65 40.47 38.54 39.49 40.77 38.85 40.67 38.68 41.26 43.01 42.03 34.12
Zn 0.02 – 0.12 0.05 0.06 0.06 – 0.03 0.02 – 0.13 0.07 0.00 0.05 0.04
Total 102.96 101.57 101.70 101.76 101.55 102.20 100.43 102.09 102.79 101.14 101.43 101.69 102.12 102.02 102.44
Mineral Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Petzite
L780
C2-5 C2-6 C2-7 C2-8 C2-9 C11-1 C11-2 C11-4 C11-10 C11-12 C11-13 C11-15 C11-16 C11-17 C11-18
Ag 61.92 49.89 50.48 45.22 62.21 58.89 61.09 62.57 61.17 62.05 58.81 61.02 61.05 60.63 60.23
Au 0.04 18.76 13.49 22.45 0.13 0.84 0.83 0.05 0.74 0.65 0.50 0.72 0.08 0.63 1.09
Bi 0.03 0.28 0.18 0.16 0.02 0.03 0.04 0.04 0.03 0.02 0.01 0.00 – – 0.07
Co – 0.02 0.02 – – 0.01 – 0.01 – – – – 0.01 0.01 –
Cu 0.06 – 0.03 0.01 0.33 – – 0.02 – – – 0.04 – – –
Fe 2.25 0.33 1.43 1.19 1.02 0.60 0.35 0.60 0.16 0.91 0.04 0.11 0.52 0.70 1.09
Ni 0.02 – 0.06 – – – – – 0.09 – – – 0.16 0.10 0.01
S 1.04 0.14 0.21 0.20 0.28 0.09 0.10 0.17 0.10 0.12 0.11 0.07 0.15 0.06 0.19
Se 0.01 0.02 0.04 – 0.04 0.07 – 0.01 – 0.04 – 0.00 – – 0.02
Te 37.38 32.01 36.55 33.51 38.21 38.38 39.19 39.31 39.30 38.93 37.31 40.14 40.00 39.74 39.66
Zn 0.03 – – – 0.03 – 0.07 0.05 0.03 – – – 0.02 0.03 –
Total 102.77 101.43 102.47 102.74 102.27 98.91 101.68 102.82 101.62 102.71 96.77 102.09 101.99 101.90 102.35
Mineral Hessite Petzite Petzite Petzite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite
L605
C2-1 C2-4 C2-5 C2-6 C2-7 C2-10 C2-11 C2-12 C2-13 C2-17 C2-18 C2-25 C2-29 C2-30 C2-33
Ag 59.10 40.95 41.45 41.06 61.60 58.59 56.52 59.34 59.82 59.86 57.74 59.11 61.65 58.51 53.85
Au 2.44 25.78 25.85 25.39 0.81 2.15 4.90 1.15 1.91 1.56 0.78 2.66 1.14 1.99 1.83
Co – 0.01 – 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.01
Cu – 0.05 – 0.00 0.04 0.13 0.29 0.03 0.02 1.13 0.00 0.02 0.14 0.12 0.02
Fe 0.55 0.35 0.12 0.15 0.25 1.23 0.34 1.93 1.14 0.72 0.36 0.78 0.38 1.32 6.56
S 0.38 0.14 0.11 0.13 0.21 0.51 0.57 0.93 0.13 0.12 0.16 0.56 0.38 0.49 6.94
Se – – 0.04 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.02
Te 37.49 31.98 32.95 32.95 37.03 37.22 35.66 36.60 38.05 37.18 38.54 37.40 37.32 37.43 33.17
Total 99.96 99.26 100.52 99.69 99.95 99.86 98.28 99.98 101.06 100.57 97.58 100.53 101.02 99.90 102.38
Mineral Hessite Petzite Petzite Petzite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite Hessite
Table 3
Summary of sphalerite mineral chemistry analysis of Stage 1 and Stage 2 veins from L575, L605 and L780. ‘–’ means below detection limit. Minimum detection limits
are 0.01–0.02 wt% for all elements.
No. L575-1 L575-2 L575-3 L605-1 L605-2 L780-1 L780-2 L780-3 L780-4 L780-5 L780-6 L780-7 L780-8 L780-9 L780-10
Stage Stage 1 Stage 1 Stage 1 Stage 2
(wt.%)
Ag – – – – – – – – – – – – – – –
Fe 2.23 3.16 5.99 1.5 1.68 3.56 2.38 2.9 4.53 5.33 0.44 0.39 0.74 0.37 0.38
S 33.57 33.27 33.96 33.36 33.46 33.72 33.66 33.65 34.38 34.31 33.56 33.56 33.56 33.41 33.53
Se 0.01 0.03 0.12 – 0.03 0.03 – – – – 0.02 – – 0.02 –
Cu 1.78 2.7 6.16 1.35 1.52 2.65 1.53 1.93 3.79 4.05 – – 0.27 – –
Zn 63.73 61.51 54.91 64.86 64.33 57.03 58.45 58.35 54.67 55.76 65.8 66.58 64.83 66.88 66.55
Au – – – – 0.04 – – 0.02 – – – – – – 0.03
Bi – – – – – – 0.02 – 0.04 0.02 – – – – 0.03
Co 0.01 – – – – 0.01 0.01 0.01 0.01 – – – – – –
Te – – – – – – 0.01 – – – – – – – –
Ni – – 0.01 – 0.01 0.01 – – 0.01 – – – – – –
Cd 0.21 0.22 0.21 0.23 0.23 0.18 0.17 0.16 0.17 0.18 0.46 0.39 0.23 0.47 0.44
Total 101.5 100. 9 101.4 101.3 101.3 97.2 96.2 97 97.6 99.7 100.3 100.9 99.6 101.2 101
FeS mol% 3.38 4.89 9.84 2.26 2.54 5.88 3.91 4.73 7.65 8.72 0.66 0.58 1.13 0.55 0.57
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
10
Zone (A in Fig. 9). After faulting, basement rocks were intruded by the
pre-mineralization equigranular diorite stocks of the Cateel Quartz
Diorite during the Early Miocene (B) along the Masara Fault Zone.
Intrusion of the Middle Miocene porphyritic Alipao Andesite (C) fol-
lowed, and then multiple stocks and dikes of the Late Miocene Lamingag
Intrusive Complex (D). The latter intrusion is considered to be associated
with mineralization (Buena et al., 2019), based on its close spatial and
temporal association with epithermal veins.
The age of the illite-altered diorite porphyry host is 5.12 Ma (Early
Pliocene), which coincided with the onset of formation of the Philippine
Fault Zone in Mindanao (Quebral et al., 1996). This reactivated earlier-
formed NW structures in southeastern Mindanao (Pubellier et al., 1994)
such as the Masara Fault Zone, which eventually served as conduit for
the mineralizing uids to alter the diorite porphyry host and form the
veins in the Masara Gold District.
The NW-trending Bonanza-Sandy veins exhibit intermediate sul-
dation state characteristics. Stage 1 is characterized by massive quartz-
sulde veins with native gold and tellurides (E), consistent with a
magmatic inuence (e.g. Gao et al., 2015). During Stages 2 and 3, quartz
and carbonate minerals deposited in veins (F). The colloform and
crustiform plus platy textures exhibited by this stage (G in Fig. 9) point
to the involvement of boiling.
In the Masara NW-trending veins, the sulde-rich veins contain the
highest grades of gold. Similar massive sulde veins and textures in
nearby structures may be indicators of mineralization. In the region,
other occurrences of the Lamingag Intrusive Complex may provide
exploration targets, as it was associated with mineralization, both
temporally and spatially, of the NW-trending Bonanza-Sandy veins.
Other Early Pliocene intrusive rocks in the region may also be
prospective.
6. Conclusions
Mineralization in the NW-trending Bonanza-Sandy epithermal veins
is hosted in the Masara Formation, Cateel Quartz Diorite and the Lam-
ingag Intrusive Complex. The NW-trending epithermal veins in the
Masara Gold District were formed in three major pulses which started
during the Early Pliocene, or possibly earlier during the late Late
Miocene: Stage 1 massive quartz-sulde vein dominantly composed of
pyrite, chalcopyrite, sphalerite and galena, with native gold, hessite and
petzite; Stage 2 massive quartz carbonate vein composed of sphalerite,
galena with minor pyrite and chalcopyrite; and Stage 3 colloform and
crustiform quartz-carbonate (platy rhodochrosite) vein with sphalerite
and minor galena and chalcopyrite.
The mineralization in the NW-trending Masara epithermal veins is
attributed to late Late Miocene to Early Pliocene uid circulation, which
is temporally associated with the formation of the Philippine Fault Zone.
The magmatic-hydrothermal uid is possibly sourced from the Late
Miocene Lamingag Intrusive Complex and eventually precipitated in the
reactivated NW-trending Masara Fault Zone.
Exploration guides include massive sulde veins and the presence of
the Lamingag Intrusive Complex in the surrounding areas. In addition,
the Early Pliocene rock units and structures in the vicinity of the Masara
Gold District might also be worth looking into.
Table 4
Summary of pyrite mineral chemistry analysis from sample L780 containing Stage 1 and Stage 2 veins and sample L575 Stage 3 veins. ‘–’ means below detection limit. Minimum detection limits are 0.01–0.02 wt% for all
elements.
No. L780-1 L780-2 L780-3 L780-4 L780-5 L780-6 L780-7 L780-8 L780-9 L780-10 L780-11 L780-12 L780-13 L780-14 L575-1 L575-2 L575-3 L575-4 L575-5 L575-6
Stage Stage 1 Stage 2 Stage 3
(wt%)
Ag – 0.01 – 0.002 – – 0.01 – – – – – 0.033 0.017 0.02 – – 0.01 – 0.02
Fe 47.5 47.3 47.25 47.21 47.05 46.68 46.37 47.21 47.32 47.25 47.31 46.88 46.53 46.62 47.25 47.2 46.72 46.97 46.27 47.86
S 54.2 54.31 54.22 53.5 53.57 54.09 54.03 53.69 53.55 53.49 53.64 53.45 54.27 54.16 53.83 53.71 54.32 53.96 52.7 53.8
Se – 0.01 – – 0.01 0.05 – – – – – – – – – – 0.02 – – 0.03
Cu 0.01 – 0.02 0.02 0.02 – – 0.34 0.08 – 0.35 0.01 – 0.06 – 0.01 0.01 – – –
Zn – – 0.02 0.03 – 0.03 – 0.01 0.03 0.02 0.05 – 0.04 0.03 0.01 0 0.01 – – 0.02
Au – 0.02 0.01 0.02 0.01 – 0.04 0.03 0.01 – 0.02 0.02 – 0.02 0.01 – – – – –
Bi – 0.02 – – 0.02 0.05 – – 0.01 0.03 – 0.05 0.04 0.02 – – – – – –
Co 0.09 0.1 0.07 0.07 0.08 0.07 0.1 0.07 0.09 0.11 0.09 0.11 0.1 0.11 0.11 0.11 0.14 0.12 0.11 0.12
Te 0.03 0.01 0.01 0.03 0.04 – – – – – – – 0.02 – – 0.01 – – 0.01 –
Ni – – – – – – – 0.003 – – – – – – 0.01 – – – – –
Cd 0.01 0.02 0.01 0.02 0.02 0 – – – – – – – 0.04 0.001 0.01 0.03 – 0.02 0.01
Total 102 101.8 101.6 100.9 100.8 101 100.6 101.4 101.1 100.9 101.5 100.5 101 101.1 101.2 101.1 101.3 101.1 99.11 101.9
Table 5
K-Ar age from sericite separates from a highly altered diorite porphyry sample of
the Lamingag Intrusive Complex.
Sample
No.
Mineral
(grain size)
K content
(wt%)
Rad.
40
Ar
(10–8 cc
STP/g)
K-Ar age
(Ma)
Non-
rad.
40
Ar
(%)
HW-1 Sericite
(0.2–2.0
μ
m)
2.751 ±
0.055
54.682 ±
1.299
5.115 ±
0.1586
48
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
11
Fig. 8. Temperature vs sulfur fugacity diagram
and the corresponding sulde mineral assem-
blage showing the elds for Stage 1 and Stage 2
veins plotting in the intermediate suldation
state. Temperatures were estimated from alter-
ation minerals. Diagram modied from Einaudi
et al. (2003), while the lines depicting mole% FeS
in sphalerite are from Barton and Toulmin
(1966). Also shown are paths depicting the evo-
lution pathway of hydrothermal uid associated
with porphyry copper and intermediate sulda-
tion (IS) base-metal environment (light orange;
from Einaudi et al., 2003). Cv-covellite; dg-
digenite; py-pyrite; bn-bornite; cp-chalcopyrite;
po-pyrrhotite; asp-arsenopyrite; lo-loellingite.
Fig. 9. Mineralization model of the NW-trending Masara Gold District showing the different intrusive units and the stages of mineralization represented by letters in
circles: (A) Formation of the Masara Fault Zone; (B) intrusion of Cateel Quartz Diorite; (C) intrusion of the Alipao Andesite; (D) multiple intrusions of the Lamingag
Intrusive Complex that resulted in hydrothermal uid exsolution and circulation; (E) Stage 1 massive sulde vein mineralization; (F) Stage 2 vein deposition; (G)
Stage 3 vein formation.
J.A. Gabo-Ratio et al.

Journal of Asian Earth Sciences: X 4 (2020) 100041
12
CRediT authorship contribution statement
Jillian Aira Gabo-Ratio: Conceptualization, Methodology, Investi-
gation, Formal analysis, Writing - original draft, Writing - review &
editing, Visualization. Alfred Elmer Buena: Methodology, Investiga-
tion, Writing - original draft, Formal analysis, Visualization. Barbie
Ross B. Villaplaza: Methodology, Investigation, Writing - review &
editing, Visualization. Betchaida D. Payot: Writing - review & editing,
Investigation, Methodology, Funding acquisition. Carla B. Dimalanta:
Writing - original draft, Writing - review & editing, Investigation,
Funding acquisition. Karlo L. Quea˜
no: Investigation, Writing - review &
editing, Visualization. Eric S. Andal: Investigation, Writing - review &
editing, Resources. Graciano P. Yumul: Writing - original draft, Writing
- review & editing, Investigation, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgments
This study forms part of the United States Agency for International
Development through the Science, Technology, Research, and Innova-
tion for Development (USAID STRIDE) research grant (Grant No.
0213997-G-2015-019-00). Logistical support provided by Apex Mining
Co., Inc. (AMCI) and the University of the Philippines - National Institute
of Geological Sciences (UP-NIGS) are gratefully acknowledged. The
EPMA at the UP-NIGS was procured through a Department of Science
and Technology grant. The support of Mr. Macky Barrientos in preparing
tables and gures is also gratefully acknowledged. The comments of an
anonymous reviewer and the signicant inputs of Dr. Jeffrey Hedenquist
are greatly appreciated.
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