Mineralization parameters and exploration targeting for gold – copper deposits in the Baguio (Luzon) and Pacific Cordillera (Mindanao) Mineral Districts, Philippines: A review

Journal Pre-proofs Mineralization parameters and exploration targeting for gold – copper deposits in the Baguio (Luzon) and Pacific Cordillera (Mindanao) Mineral Districts, Philippines: A review Graciano P. Yumul Jr, Carla B. Dimalanta, Jillian Aira S. Gabo-Ratio, Leo T. Armada, Karlo L. Queaño, Karl D. Jabagat PII: DOI: Reference:

S1367-9120(20)30007-9 https://doi.org/10.1016/j.jseaes.2020.104232 JAES 104232

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Journal of Asian Earth Sciences

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18 February 2019 7 November 2019 4 January 2020

Please cite this article as: Yumul, G.P. Jr, Dimalanta, C.B., Gabo-Ratio, J.A.S., Armada, L.T., Queaño, K.L., Jabagat, K.D., Mineralization parameters and exploration targeting for gold – copper deposits in the Baguio (Luzon) and Pacific Cordillera (Mindanao) Mineral Districts, Philippines: A review, Journal of Asian Earth Sciences (2020), doi: https://doi.org/10.1016/j.jseaes.2020.104232

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Mineralization parameters and exploration targeting for gold – copper deposits in the Baguio (Luzon) and Pacific Cordillera (Mindanao) Mineral Districts, Philippines: A review Graciano P. Yumul, Jr.1, Carla B. Dimalanta2*, Jillian Aira S. Gabo-Ratio2, Leo T. Armada2, Karlo L. Queaño3 and Karl D. Jabagat2

1Cordillera 2Rushurgent

Exploration Co., Inc., BGC, Taguig City, Philippines

Working Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, Philippines

3Department

of Environmental Science, School of Science and Engineering,

Ateneo de Manila University, Loyola Heights, Quezon City, Philippines

ABSTRACT The Baguio Mineral District in Luzon, Philippines is known to host several world-class epithermal gold – porphyry copper deposits. The interplay of tectonic setting, magma composition, structural control and hydrothermal system contributed to the generation of these deposits. Ridge subduction (Scarborough seamount) resulting to flat subduction and a transpressional regime could also be related to the formation of epithermal gold - porphyry copper deposits in Baguio. Subduction processes leading to the formation of calc-alkaline rocks associated with high water pressure, oxygen fugacity and late sulfur saturation are almost always associated with the gold-copper deposits in the district . Compared to the Baguio Mineral District, less exploration work, mine development and production were done in the Pacific Cordillera Mineral District, Mindanao in southern Philippines. It is worth noting, however, that both mineral districts show similarities and overlapping features in terms of geological,

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geophysical and geochemical characteristics. This leads one to conclude that the Pacific Cordillera Mineral District has ore deposits waiting to be discovered. Keywords: Gold-copper, subduction, mineralization, tectonics, Philippines *Corresponding author: [email protected]

1. Introduction Specific combinations of tectonic, structural, erosion level, magmatic and hydrothermal features are conducive to the formation of epithermal gold and porphyry copper deposits (e.g. Richards 2011a; 2013; Cao et al. 2018; Vigneresse 2018). The presence of specific melt compositions resulting in the generation of calc-alkaline magmas, some of which may be adakitic or potassic in character, is identified to be closely associated with precious and base metal deposits (Dreher et al., 2005; Loucks, 2014; Farner and Lee, 2017). The adakitic rocks need not be a product of slab melting but could also be products of slab window magmatism, high pressure fractionation, lower crust melting, sediment melting or fault-related melt generation (e.g. Jego et al., 2005; Macpherson et al., 2006; Dimalanta and Yumul, 2008; Ribeiro et al., 2016). For that matter, adakites can just be plain fractional crystallization products that involve the presence of elevated water content (e.g. Richards, 2011b; Castillo, 2012). The common denominator among the different models relating adakitic rocks with gold-copper mineralization is the calc-alkaline nature of the melt marked by high pH2O and high fO2 (e.g. Sajona and Maury, 1998; Imai, 2002; Sun et al., 2016; Hattori, 2018). In terms of geological setting, areas in releasing bends of faults, a generally compressive regime, presence of stratovolcanoes and dome complexes, and areas above subducted ridges have been found to be commonly associated with epithermal gold – porphyry copper mineralization (e.g. Waters et al.,

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2011; Hedenquist et al., 2018; Sillitoe, 2018). Apart from geologic setting, magmatic, structural and geochemical controls, it is also believed that a thick crust contributes to the formation of copper and gold mineral deposits (Kay and Mpodozis, 2001; Shafiei et al., 2009). Ultimately, fluid and mass fluxes in the subduction zone influence the magmatism associated with mineralization in the overlying arc (Richards and Holm, 2013; Richards, 2018). The nature of the subducting oceanic slab (e.g. presence of aseismic ridge, seamounts, fracture zone, sediment cover) as well as the slab angle have effects on the release and evolution of fluids that control mineral deposit emplacement (Cooke et al., 2005; Manea et al., 2014; Holm et al., 2019). The Philippines, because of its geological setting, is richly endowed with precious and base metal deposits (Figures 1A and 1B) (e.g. Sillitoe and Gappe, 1984; Mitchell and Leach, 1991; Hammarstrom et al., 2014). The most studied among the different mineral districts in the Philippines is the Baguio Mineral District (BMD) located in northern Luzon. In this region, extensive works on the regional tectonic setting, district and mine host rock geology and mineralization characteristics of epithermal gold – porphyry copper deposits have been reported (e g. Imai, 2001; Polve et al., 2007; Hollings et al., 2011) (Figure 1B). More recently, the Pacific Cordillera Mineral District (PCMD) in eastern Mindanao, southern Philippines has also been investigated for the exploration and production of epithermal gold (+Ag and Cu) and porphyry copper (+Au or Mo) (Figure 1B) (e.g. Suerte et al., 2007; Braxton et al., 2012; Taguibao and Takahashi, 2018). The possibility of new gold-copper discoveries in eastern Mindanao is high given its favorable geological setting. To further assess the mineralization potential of eastern Mindanao and to identify additional exploration targets that could lead to new ore discoveries, the PCMD is compared to a similarly-situated mineral district, in this case, the BMD. In the

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process, such comparison could further enhance our understanding of the mineral potential and prospectivity of these two mineral districts in the Philippines.

2. Geologic background 2.1. The Philippines: A composite terrane The Philippine island arc system is a composite terrane resulting from the accretion and amalgamation of continent, island arc and ocean basin fragments. This island arc system is made up of two major blocks – the seismically-active Philippine Mobile Belt and the aseismic Palawan Microcontinental Block of mainland Asia origin (e.g. Yumul et al., 2009; Dimalanta et al., 2018). The arc is surrounded by oceanic marginal basins that are being subducted along bounding trenches. These include the Oligocene-Miocene South China Sea (SCS) subducting along the Manila Trench; and the Oligocene-Miocene Sulu Sea and the Eocene Celebes Sea descending underneath the arc along the Miocene Negros-Sulu Trench and Cotabato Trench, respectively (Figure 1A). The Eocene West Philippine Basin east of the archipelago subducts westward along the Pliocene East Luzon Trough – Philippine Trench. The Palau Basin, presumably of Eocene age, located on the southeastern offshore area of Mindanao, also subducts along the Pliocene Philippine Trench (e.g. Taylor and Goodliffe, 2004; Sasaki et al., 2014). Subduction of these surrounding oceanic plates is the dominant geologic process responsible for the observed magmatism in the Philippine arc. Several oceanic bathymetric highs composed of oceanic plateaus and ridges, relict spreading centers, micro-continental fragments and fracture zones have collided with the Philippine arc. Other geologic features, however, are in the process of colliding with or subducting underneath the Philippines. The oblique subduction of the oceanic plate along the

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eastern part of the archipelago resulted in the formation of the Philippine Fault and its splays. The Philippine Fault, which transects the whole length of the archipelago, generally has a sinistral movement (e.g. Galgana et al., 2007; Perez et al., 2015) (Figure 1A).

2.2. Baguio Mineral District: From marginal basin to an island arc setting The Baguio Mineral District (BMD) is underlain by Cretaceous ophiolitic to primitive island arc complex covered by a thick sedimentary sequence (Peña, 1992; Queaño et al., 2008) (Figure 2A). The sedimentary rock suites include the Early Oligocene to Early Miocene marginal basin-related volcaniclastic deposits with characteristic alternating red and green beds and associated non-clastic rocks; the Early to Middle Miocene reefal limestone; and the Middle to Late Miocene submarine fan deposits that are capped by Plio-Pleistocene shallow-water reefal limestones and pyroclastic flow deposits (e.g. Peña and Reyes, 1970; Payot et al., 2007; Dimalanta et al., 2013). Extrusive rocks as well as intrusive bodies of gabbro, tonalite, diorite and quartz diorite are also present (Bellon and Yumul, 2000) (Figure 2A). Some of these intrusive bodies are related to the formation of hornfels, amphibolites and chlorite schists (e.g. Bellon and Yumul, 2001; Cooke et al., 2011). Structures related to the Philippine Fault trend NW-SE south of the BMD, to almost N-S in areas surrounding the BMD (Barrier et al., 1991; Ringenbach et al., 1993). Conjugate to these structures are NE-SW and E-W dilational jogs mostly observed at the local scale (e.g., mine areas). In northern Luzon, the Philippine Fault assumes a horse-tail structure (Pinet and Stephan, 1990) (Figures 1A and 2A). The geochemistry of the igneous and sedimentary rocks in the BMD manifests the evolution of the district from a restricted, oceanic marginal basin to a mature island arc setting following an arc polarity reversal origin for Luzon (Yumul et al., 2008; Suzuki et al., 2017).

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Such tectonic origin considers an earlier westward subduction along the proto-East Luzon Trough possibly until the Late Oligocene, and subsequent eastward subduction of the SCS plate along the Manila Trench. This latest phase of subduction attributes for the continuous uplift of the Luzon Central Cordillera following intrusive activities starting from Early Miocene as recorded in the succession of the rock units: submarine fan-related turbidite deposits through shallow-water limestones to pyroclastic rocks that blanket the district (De Leon et al., 1991). Hollings et al. (2011) reported that dioritic and gabbroic plutons were emplaced synchronous with calc-alkaline volcanic activity related to the subduction of the SCS crust along the Manila Trench during the Early Miocene. This was followed by a resurgence of mafic to intermediate calc-alkaline intrusive activities during the Pliocene and Pleistocene. These magmatic events are causative to the district’s mineralization based on the reported isotopic ages of mineralized intrusive bodies (e.g. Imai, 2002; Waters et al., 2011). Adakitic rocks in the BMD were recognized by Imai (2002) based on their elevated whole rock Sr/Y ratios, such as in the Lobo-Boneng and Santo Nino porphyry Cu prospects. In the same study, however, the Santo Tomas II porphyry Cu deposit did not present adakitic characteristics while other adakitic rocks in the western Luzon arc have no recognized porphyry copper mineralization, implying that there is no one to one correlation between adakitic magmatism and mineralization.

2.3. Pacific Cordillera Mineral District: Basin closure to uplifted island arc setting The Pacific Cordillera Mineral District (PCMD) in eastern Mindanao is bounded on the east by the Pliocene Philippine Trench and on the west by the Agusan-Davao Trough. The evolution of the PCMD is closely related to the closure of the proto-Molucca Sea Plate in

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Mindanao (e.g. Pubellier et al., 1991; Rangin et al., 1996; Bader & Pubellier, 2000). Several models account for the tectonic setting of the PCMD. These range from a west-facing- to an east facing-arc attributed to the subduction of either the proto-Molucca Sea Plate or the West Philippine Basin and Palau Basin (e.g. Lallemand et al., 1998; Bader et al., 1999; Hall, 2018). The PCMD has, for its basement, Cretaceous ophiolitic bodies intruded by Oligocene to PlioPleistocene igneous rocks. Deep-water early Miocene clastic rocks and Early Miocene to Pliocene reefal to massive limestones are also noted, reflecting the shallowing water conditions through time (Figure 2B) (e.g. Sajona et al., 1997; Sonntag et al., 2011). The oceanic lithospheric fragments exposed in the PCMD exhibit supra-subduction zone to mid-ocean ridge geochemical characteristics (Hawkins et al., 1985; Tamayo et al., 2004). Tholeiitic basalts to calc-alkaline andesites, diorites and dacites (some with adakitic geochemical characteristics) host the mineralization (Sajona et al., 1993; Imai et al., 2009; Yumul et al., 2017). The Philippine Fault Zone cuts the western side of the PCMD (Figure 2B). Although the trend of the major faults in eastern Mindanao is NNW-SSE, NE-SW and E-W structures are also recognized (e.g. Quebral et al., 1996; Kolb & Hagemann, 2009). At its southern terminus, the Philippine Fault is expressed as horsetail structures. Strike-slip faults with thrust component are also noted from north to south (in Lianga, Cateel and Mati areas) of the Pacific Cordillera (Figure 2B). These structures divide the district into three blocks: Surigao (North Pacific Cordillera District), CoO (Central Pacific Cordillera District) and Masara (South Pacific Cordillera District) (inset in Figure 2B). Like the BMD, precious and base metal prospects and deposits as well as active mine areas straddle the Philippine Fault Zone in Mindanao. In addition to porphyry copper and epithermal gold deposits, jasperoid-hosted ‘Carlin-like’ mineralization were recognized for Surigao (Lascogon prospect) and Masara (Hijo prospect), but were

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concluded to be ‘distal sediment-hosted epithermal gold deposits’, which are also related to the widespread hydrothermal mineralization in the area (Maglambayan et al., 2005; Kirwin and Royle, 2019). The age of gold–copper mineralization in the PCMD ranges from Oligocene through Miocene to Plio-Pleistocene (e.g. Villaplaza et al., 2017; Braxton et al., 2018; Buena et al., 2019). Movement along the splays of the Philippine Fault Zone resulted in extensional regimes both on a regional and local scale. Lake Mainit in the northern Pacific Cordillera is one of the extensional features related to such movement (Figure 2B). It is worth noting that the prePliocene mineral deposits are not associated with the Pliocene to present-day Philippine Fault Zone (e.g. Mitchell and Leach, 1991; Quebral et al., 1996; Yumul et al., 2003). Instead, they are concentrated in reactivated structures that predate the formation of the Philippine Fault (Pubellier et al., 1991; Quebral et al., 1996; Yumul et al., 2003).

3. Methods Published whole rock geochemical data were compiled for both the BMD and the PCMD. These data are compared with new whole rock geochemical data of additional samples collected from the study areas. Sample preparation was conducted at the University of the PhilippinesNational Institute of Geological Sciences (UP-NIGS). Crushing and powdering were done using an agate mortar and pestle. Whole rock major, trace and rare earth element chemical analyses were carried out using the X-ray Fluorescence (XRF) and Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) ELAN 9000 at the Bureau Veritas Commodities Canada Limited. Accuracies are within 1% for major elements and 5% for both trace and rare earth elements. Additional XRF analysis was also done at Kyushu University using a Rigaku RIX 3100 and

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following the methodology of Soejima (1999). The accuracy is 0.001-0.03% for the major oxides. A comparison of the results from the two laboratories shows a 0.01-1.04% standard deviation with a mean percent relative difference (MPRD) of ±0.22%. Table 1 shows the whole rock major, trace and rare earth element compositions of igneous rocks from the BMD and PCMD. Mineral chemistry analyses were done at the UP-NIGS using a JEOL JXA-8230 electron probe micro-analyzer (EPMA). Wavelength dispersive x-ray spectroscopy (WDS) analysis was utilized to identify the major oxide compositions of mineral grains, mostly amphiboles and plagioclase. An acceleration voltage of 15 kV, beam current of 20 nA and beam diameter of 15µm were used. Natural minerals and synthetic standards were used during the analysis. The ZAF correction was applied to account for the atomic number, absorption and fluorescence correction. Major oxide compositions of representative minerals are listed in Table 2. Crustal thickness estimates for both mineral districts were derived from gravity anomalies using the relation proposed by Milsom et al. (1996) (Figure 3). A 16 mGal gravity anomaly corresponds to a 1-kilometer change in the depth to Moho based on a 30-kilometer thick crust. The gravity anomalies used in this study were extracted from the global marine gravity data of Sandwell and Smith (2009). Focal mechanism solutions, on the other hand, were plotted using the methodology of Heuret and Lallemand (2005) (Figures 4A and 4B). The Centroid Moment Tensor (CMT) profiles were generated such that earthquakes up to 100 kilometers away to the north and south of the transect line are included in the plots. The profiles were generated using the Generic Mapping Tool software (Wessel et al., 2013). The earthquake focal mechanism solutions were sourced from the Global Centroid Moment Tensor Catalogue

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(Ekström et al., 2012). The topography and bathymetry information were extracted from the database of Sandwell and Smith (2001).

3. Results Results of the whole rock geochemical analyses, when plotted in the AFM diagram adopted from Irvine and Baragar (1971), show that samples of the BMD and PCMD are mostly calc-alkaline rocks (Figure 5A). On the other hand, the SiO2 versus FeOtotal/MgO plot (Miyashiro, 1974) shows a well-spread distribution of the sample sets from tholeiitic to calcalkaline, although majority of the samples plot in the calc-alkaline field (Figure 5B). Whole rock trace and rare earth element spidergrams of samples collected from BMD and PCMD normalized to primitive mantle show negative Nb, Ta and Ti anomalies with some exhibiting negative Zr anomaly, typical of subduction-related magmatism (Figures 6A and 6B). Positive Pb and Sr anomalies can also be observed on the andesites, dacites, and diorites from the two mineral districts. The Sr/Y vs. Y diagram (adopted from Defant and Drummond, 1990) shows that a substantial number of samples from the PCMD plot in the adakite field while BMD samples straddle the boundary between the adakite and typical arc rock fields (Figure 7A). In the (La/Yb)N vs. Yb(N) diagram (Martin, 1986), most of the PCMD samples lie on the overlapping region between the adakite field and typical arc field (Figure 7B) whereas BMD samples plot in the typical arc rock field. In the diagram of (K2O/Na2O) vs. Yb adopted from Kamvong et al. (2014), most of the samples from BMD and PCMD samples cluster in the hybrid or contaminant melt-derived adakites and slab-melt derived adakites fields, although several samples still plot in the crust-derived adakites and normal calc-alkaline rocks field (Figure 8).

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The plagioclase from samples collected from the two mineral districts range in composition from anorthite to oligoclase (Figure 9A). The amphiboles from the BMD are classified as magnesiohornblende to pargasite whereas PCMD amphiboles are mostly edenite with a few pargasite (Figure 9B). All these intrusive rock units that contain amphiboles with low Fe/(Fe+Mg) have consistent high fO2 (Figure 9C) (Anderson and Smith, 1995). In addition to magmatic and geochemical controls on the formation of major precious and base metal deposits, some authors have cited the role of thick crust in the formation of gold and copper deposits (e.g. Kay and Mpodozis, 2001; Barley et al., 2002; Kay et al., 2013). As deep seismic refraction data are not available in the Philippines, this work examined gravity data to obtain estimates of crustal thickness. Crustal thickness beneath Luzon varies between 12 and 35 kilometers (Figure 3). Bouguer anomalies in the BMD translate to crustal thicknesses of ~29 to 31 km. In Mindanao island, the thickness of the crust ranges from ~12 to 35 kilometers. The Agusan-Davao Basin, bounding the western side of the PCMD, is characterized by “thick” crust due to the thick accumulation of sediments. The northern and central parts of the PCMD are underlain by thin crust (from 12 to 25 kilometers). However, the southern end of the Masara block has thicker crust in excess of 26 kilometers and reaching a maximum thickness of 35 kilometers (Figure 3).

Plots of earthquake focal mechanism solutions beneath Luzon Island, particularly beneath the BMD, show a relatively flat slab subducting down to depths of ~50-60km (Figure 4A). Based on the distribution of CMT plots, the Wadati-Benioff zone clearly delineates the subducting SCS slab beneath western Luzon. In turn, this observation clearly shows that the BMD is associated with the present subduction along the Manila Trench. The distinct flat subduction beneath the

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BMD is attributed to the subduction of a more buoyant, relict spreading ridge (segment of the Scarborough seamount chain) (Figure 4A). On the other hand, portions of the West Philippine Basin and Palau Basin oceanic crust are currently subducting beneath eastern Mindanao along the Philippine Trench. The CMT plots of earthquakes in this portion of the Philippine Trench indicate the subduction of the oceanic slab down to depths exceeding 190km. Beneath eastern Mindanao, the Wadati-Benioff zone is ~80-90km deep. The CMT section cuts across the area where the Mindanao Fracture Zone is being subducted westward beneath Mindanao (Figure 4B).

4. Discussion 4.1. Subducted slabs, faults, and crustal thickness: Geological controls

Porphyry-epithermal mineralization is closely linked with complex tectonic and magmatic processes in the subduction zone. Mineralization events are often associated with evolving subduction zone systems, particularly subduction of oceanic bathymetric highs and fracture zones as well as changes in the slab angles (Holm et al., 2019). Subducted aseismic ridges or any other subducted oceanic bathymetric highs (e.g. submerged continent, guyot) can result in volcanic arc gaps, emplaced ophiolites, uplifted accretionary complexes, fault structures, slab window magmatism and flat slab subduction among others (e.g. Yumul et al., 2013; Manalo et al., 2015). Subduction of these bathymetric highs can also result in a compressive stress regime within the overriding plate (Kimura et al., 2019). Compressive regime, combined with the appropriate structural features, magma characteristics and hydrothermal systems can result in the deposition of porphyry copper deposits (± Au and Mo) (e.g. Hollings et al., 2013; Richards, 2018; Sillitoe, 2018).

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In the BMD, a spatial and temporal link between mineralization and flat subduction of the aseismic Scarborough seamount of the SCS is reported particularly for the Plio-Pleistocene deposits (Cooke et al., 2011; Hollings et al., 2011) (Figure 4A). These include the porphyry CuAu deposits emplaced in the southern and western part of the district, such as the Sto. Tomas II Cu-Au (~1.5 Ma), the Black Mountain Cu-Au porphyry and skarn deposits (~3 Ma) and the Ampucao-Hartwell-Balatoc cluster (~0.51 to -1.09 Ma) (Imai, 2001; Imai, 2002; Garwin et al., 2005; Waters et al, 2011). In addition to porphyry Cu-Au deposits, the Scarborough seamount chain subduction has also been associated with the low sulfidation deposits in the district (Sun et al., 2010; Hollings et al., 2011). Earthquake hypocenter data show that the depth of subduction beneath the BMD is ~50-60 km (Figure 4A). The relatively flat subduction of the Scarborough seamount chain underneath the Luzon Central Cordillera is attributed to the relatively buoyant and young (~15 to 18 Ma) character of the ridge associated with the last spreading event in the SCS (Briais et al., 1993; Barckhausen and Roeser, 2004; Fan et al., 2015; Zhao et al., 2018). Focal mechanism solutions (Figure 4A) of the Wadati-Benioff zone earthquakes as well as tomographic images indicate this flat slab subduction beneath the Baguio area. The slow subduction of the buoyant Scarborough seamount chain affects the evolution and migration of melts from the mantle wedge to the upper crust (Fan et al., 2015; Bishop et al., 2017). Coupled with these subduction events are accretion and/or subduction of Mainland Asia continent-derived sediments deposited on the SCS. These events facilitated crustal thickening and melting of this relatively young oceanic lithosphere consistent with the calc-alkaline and adakitic geochemical signatures of the BMD igneous rocks (Figure 8). Hollings et al. (2011) reported higher isotopic ratios of

143Nd/144Nd

for Pliocene rocks in the BMD, compared to those of Miocene age,

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consistent with melting of the Scarborough seamount. Flat subduction and consequent generation of fertile melts (including adakites) produced from the interaction of primitive mafic melts with earlier generated calc-alkaline rocks were critical in the formation of porphyry copper and epithermal gold mineralization in the BMD (Waters et al., 2011). This is similar to flat slab subduction associated with adakites known to host porphyry copper – epithermal gold deposits as demonstrated elsewhere (e.g. Bissig et al., 2003; Carrasquero et al., 2018). In the BMD, adakitic rocks (those with high whole rock Sr/Y ratios) were recognized in the Miocene LoboBoneng and the Pliocene Santo Nino porphyry Cu prospects (Imai, 2002). However, this adakitic character was also identified for intermediate to silicic rocks elsewhere in the Luzon Arc (e.g. Agno Batholith, Mount Pinatubo range) but was not present in the Santo Tomas II porphyry Cu deposit in the BMD, which implies that adakite presence is not necessarily related to mineralization in the deposit scale (Richards 2011b; Yumul et al., 2017).

Zones of weaknesses which may include boundaries between igneous complex – sedimentary rock formation are also favorable sites for metal deposition (e.g. Fernandez and Damasco, 1979; Dimalanta, 1996; Mitchell and Leach, 1991; Imai, 2001). In the BMD, the distribution of prospects and mines follows this boundary (Dimalanta, 1996). Compressional deformation of the district accompanying flat subduction of the Scarborough seamount chain. underneath the BMD was also vital for the formation and exhumation of porphyry copper systems. In the same way, continuous subduction and its accompanying relaxation phase resulted in extensional regimes within the near-surface environment of the overriding plate. These events made conditions conducive to the formation of epithermal gold vein-type deposits. The superposition of porphyry copper and epithermal systems commonly observed in the BMD is

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also attributed to post-mineralization strike-slip faulting related to the Philippine Fault (Fernandez and Damasco, 1979; Imai, 2001; Yumul et al., 2008). At a district- or local-scale, intersecting faults and dilational structures formed in between fault splays are associated with porphyry copper deposits and bonanza-type epithermal gold vein type mineralization in the BMD.

In the PCMD, the Eocene West Philippine Basin (WPB) and Palau Basin oceanic plates subduct obliquely along the Philippine Trench (Table 3). These oceanic plates are separated by the Mindanao Fracture Zone (MFZ), a zone of curvilinear and multi-stranded fractures that merge westwards, subducting together with the bounding plates underneath Mindanao. The formation of the MFZ follows the counterclockwise rotation of the WPB at around 49 Ma and 33 Ma (Taylor and Goodliffe, 2004). The current subduction of the MFZ beneath eastern Mindanao directly affects the entrenchment of fluids in the mantle and the geochemical signatures of arc magmatism (Macpherson, 2008; Manea et al., 2014). Unlike in the Manila Trench fronting Luzon island, earthquake hypocenter data do not indicate any slab shallowing associated with the subduction of the MFZ. Marked differences in the age and the nature of subducted materials provide a plausible explanation for such variation in the geometry of the present subducting slabs. In contrast to the subduction activity along the Manila Trench, subduction erosion is expected along the segment of the Philippine Trench fronting Mindanao owing to the low rates of sediment supply (relative to SCS subduction underneath Luzon) and the high rate of convergence (~9 cm/yr) as also demonstrated elsewhere (e.g. Stern, 2011). The role played by the MFZ in the observed mineralization in the PCMD has not yet been fully explored. However, it has been demonstrated that subducted fracture zones enhance the production of slab-derived

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fluids and concentrate melts derived from the mantle wedge (e.g., Richards and Holm, 2013; Gartman and Hein, 2019). These zones represent regions where the flux of hydrothermal fluids is significantly greater relative to the adjacent unfractured lithosphere (Richards and Holm, 2013). These provide favorable conditions and additional mechanisms for the formation of porphyry copper and epithermal deposits, especially in the presence of structures that could host mineralized fluids in the overriding plate. In the Pacific Cordillera, the splays of the Philippine Fault Zone and its antecedent structure provided conduits for mineralization, similar to those in the BMD. Active propagation of the fault zone is observed as it is coupled with the southward propagation of the Philippine Trench (e.g. Quebral et al., 1996; Lallemand et al., 1998; Yumul et al., 2003). In terms of magmatism and crustal thickness, the nature of the subducting slab, particularly the age and sediment cover, controls the erosion or accretion in the convergent margin (Scholl and von Huene, 2007). Tectonic accretion or subduction erosion can significantly alter the structure of the upper plate (Ranero and von Huene, 2000; Clift and Vannucchi, 2004). Global compilations of crustal accretion and erosion of major convergent margins indicate that the Manila Trench subduction is dominated by accretion while the Philippine Trench is dominated by subduction erosion (Clift and Vannucchi, 2004). Subduction erosion leads to the removal of some volume of material from the crust resulting to a thinner crust (Condie, 1997). Major copper-gold mineralization in Central Andes, Central Kalimantan, New Guinea and southeastern Iran have been influenced by crustal thickening (e.g. Kay and Mpodozis, 2001; Barley et al., 2002; Kay et al., 2013). Thickened crust plays a crucial role through the release of fluids, by changing source mineralogy or through melt-modifying processes necessary for mineralization (Kay and Mpodozis, 2001; Shafiei et al., 2009). In their investigation of the

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copper and gold deposits in the Central Andean region, Kay and Mpodozis (2001) noted that crustal thickening favors the release of fluids for mineralization. This is due to the breakdown of hydrous, amphibole-bearing mineral assemblages into drier, garnet-bearing assemblages. In northern Luzon, including the BMD, crustal thickness is estimated to be around 30 km based on Bouguer anomalies (Figure 3). The observed crustal thickening in the BMD is explained by the series of arc magmatic additions as a consequence of multiple subduction-related magmatic episodes.

The crustal thickness map derived from the gravity dataset indicates slightly varied thicknesses beneath the PCMD (Figure 3). The crustal thickness is around 20km in the north and is thicker at the southern part (Masara block). Based on the distribution of Wadati-Benioff zone earthquakes, the depth of the subducting slab reaches almost 200km (Figures 3 & 4B). The variations in crustal thickness along the PCMD can be explained by the dominance of subduction erosion at the northern portion (Surigao-CoO area). In contrast, the southern block (Masara) preserves a record of more extensive magmatic activities (Mercado et al., 1987; Macpherson et al., 2006; Suerte et al., 2007). The off-scraping of materials along the subduction interface can be attributed to the scarcity or absence of sediments in the subducting WPB oceanic crust. The rough subduction interface is conducive to erosion at the base of the overriding plate (Clift and Vannucchi, 2004).

4.2. Magmatism and associated mineralization: Geochemical constraints

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Mineralization in the BMD is associated with both tholeiitic and calc-alkaline magmatism with a preference for the latter (Figures 5A and 5B). This is consistent with the observation of the dominant role played by calc-alkaline magmas in mineralization as observed in other areas (e.g. Dreher et al., 2005; Loucks, 2014). High water content, which characterizes calc-alkaline magmas, suppresses the crystallization of plagioclase and enhances magnetite and amphibole crystallization (e.g. Zimmer et al., 2010; Farner and Lee, 2017). The crystallization of magnetite results in high oxygen fugacity conditions, which induces breakdown of sulfide phases to release metals into arc magmas. High oxygen fugacity (fO2), in combination with high water pressure (pH2O) and high activity of oxidized species of sulfur, is conducive to the formation of epithermal gold – porphyry copper deposits such as in the BMD (Imai, 2002). The high oxygen fugacity responsible for the precious – base metal mineralization in the BMD. is evidenced by the low amphibole Fe content and high whole rock V/Sc (Figures 9D and 11). The BMD samples contain V/Sc values that are greater than the fayalite-magnetite-quartz buffer (FMQ), with logfO2 values up to and even exceeding FMQ +2, implying fO2-rich conditions. Dehydration melting, due to the subduction and partial melting of the upper mantle wedge, with contributions from the hydrothermally-altered oceanic crusts and their pelagic to continent-derived sediment carapace, accounts for the strong subduction imprints recognized among the BMD rocks (Figure 6).

The PCMD has geochemical controls on mineralization that are almost the same as those of the BMD, with subduction playing a major role (Table 4). The presence of hydrous minerals, early crystallization of magnetite and amphibole and dominant diorite to dacite host rocks to orebodies are all consistent with a high pH2O and high fO2 associated with calc-alkaline rocks that host the mineralization in both districts. Adakites, attributed to different processes of

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formation, have been identified associated with some of the gold-copper mineralization in the district (Suerte et al., 2009). Although an Oligocene age of mineralization is recognized, most of the mineralization in the PCMD is within the Miocene to Plio-Pleistocene age range (Suerte et al., 2009; Braxton et al., 2012; Taguibao and Takahashi, 2018; Buena et al., 2019) (Table 2). Aside from slab melting, fractionation coupled with mixing, assimilation, storage and homogenization (MASH) is suggested by the (K2O/Na2O) vs. Yb (Figure 8) (Kamvong et al., 2014). Rocks from the BMD are either normal calc-alkaline rocks or adakites with low K2O/Na2O, implying that they are products of either slab melting or secondary processes like crustal contamination. Meanwhile, most of the adakites from PCMD have slightly higher K2O/Na2O values, pointing to crustal contamination and melting of crustal materials as possible source of adakitic magmatism.

Pelagic and continent-derived sediments subduct along the Manila Trench. The (Th/Yb) versus (Ta/Yb) (Pearce, 1982) shows that most of the igneous rocks from the BMD have active continental margin signature (Figure 10). As there is no evidence that continental crust underlies the BMD, the active continental margin signature could have resulted from the subduction of mainland Asia-derived sediments (Yumul et al., 2008). The igneous rock bodies in the PCMD also exhibit active continental margin to oceanic island arc signatures as reflected by their enriched Th/Yb content (Figure 10). The high Th content is attributed to Th-enriched subduction zone fluids (Wilson, 1989). The active continental margin signature is derived from melting subducted sediments that were eroded from the Australian margin. Pelagic sediments are found in both the West Philippine Basin and Palau Basin, with the latter receiving sediment inputs from the Australian continental margin. This is also consistent with the K2O/Na2O content of the

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PCMD adakitic rocks exhibiting crustal contamination and derivation from crustal sources rather than being direct products of slab melting (Figure 8).

4.3. Finding the next gold-copper deposits: Exploration targeting

It is said that almost all of the outcropping giant and world-class ore deposits have already been discovered. Some of these deposits have actually been developed and mined. In recent years, factors such as lower exploration budget, increasing resource nationalism, environmental concerns and unstable political regimes affect the conduct of mineral resource exploration and mine development, thus making the search and discovery for gold-copper deposits difficult. As a consequence, there has been a noticeable shift in doing exploration from greenfield to brownfield areas (specifically those in the vicinity of operating mines), including those in the so-called elephant countries where epithermal gold – porphyry copper deposits are known to exist in abundance. With this realization, it is important to identify the different factors that could make an area prospective for epithermal gold – porphyry copper deposits.

Relative to the PCMD, the BMD is more mature in terms of exploration, prospect identification and development coupled with ongoing mining operations. This resulted in the recognition and delineation of factors responsible for the formation and deposition of epithermal gold – porphyry copper deposits in the BMD which include the following:

1. Subduction processes that generate fertile calc-alkaline magmas, including adakitic magmas, associated with elevated water pressure (e.g. high Sr/Y), oxygen fugacity (early

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magnetite crystallization), and and high activity of oxidized species of sulfur (e.g. Bellon and Yumul, 2000; Imai, 2002; Chiaradia et al., 2012);

2. Subduction of oceanic bathymetric highs that translates to compressive stress into the overriding plate which is conducive to the formation of porphyry copper deposits. Subsequent relaxation, at the later part of the orogeny, develops tensional gashes that are filled up by epithermal gold vein-type deposits (e.g. Yumul et al., 2003; Hollings et al., 2011);

3. Faults and fractures which convey magmatic, meteoric and metal-carrying hydrothermal fluids that subsequently deposit metals. Fault intersections are conducive for porphyry copper deposit formation. Dilational jogs induce boiling, which destroy metal solubility and ultimately initiating metal deposition (e.g. Kolb and Hagemann, 2009; Manalo et al., 2017);

4. Igneous rock complex – sedimentary rock formations, being zones of weakness, are known to localize ore deposits (e.g. Mitchell and Leach, 1991; Dimalanta, 1996);

5. Thickened crust with multiple pulses of magmatism is more susceptible to host ore deposits (e.g. Chiaradia, 2015; Meffre et al., 2016). The thick crust associated with the BMD results in more fractionated magmas as they propagate upwards, in the process increasing the chances of accumulating metals through intense rock-melt interaction or increased pH2O conducive to mineralization;

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6. Miocene to Plio-Pleistocene igneous rocks host most of the gold-copper deposits in the BMD. Chances of finding new deposits within this time horizon is high as the realignment of the geological parameters conducive to ore deposition is enhanced (e.g. Cooke et al., 2011; Richards, 2013; Bray, 2017).

With these factors identified in the BMD, it is no surprise that this district is host to several giant and world class gold-copper deposits. These parameters can be applied in identifying other mineral prospects in the PCMD. Areas surrounding existing mines, localities in between existing well-defined prospects or operating mines and areas characterized by thicker crust under a transpressional regime are the better targets.

5. Conclusions

A comparison of the geological, geophysical and geochemical features of the BMD and PCMD shows a great extent of overlap between the two. Considering that the factors responsible for the formation of giant and world-class gold-copper deposits in the BMD are also found in the PCMD, additional ore discoveries can be expected in the latter. Areas surrounding abandoned or operating mines, regions in between mines or known gold-copper prospects, igneous complex – sedimentary rock formation boundaries and associated zones of weaknesses and areas with thicker crust present bigger chances and possibilities of ore discoveries in the PCMD. The need to find bigger and relatively easy to mine deposits that translate to less cost per pound Cu or

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ounce Au is imperative considering the other challenges (e.g. social, legal, economic, environment) facing the industry.

Acknowledgements The assistance extended by the different mines and mineral deposit claimants in Baguio and eastern Mindanao during the course of this study are acknowledged with thanks. Special acknowledgment with thanks to Dr. Walter W. Brown of Apex Mining Co. Inc. who had supported the search for new knowledge and ideas. Logistic and laboratory support from the University of the Philippines - Diliman and Kyushu University are very much appreciated. Fieldwork through the years, with colleagues and students, had helped us in understanding the intricacies of the geology of the two mineral districts. Discussion with members of the UP-NIGS Rushurgent Working Group helped clarify some of the issues we have encountered. We appreciate the comments and sugestions of two anonymous reviewers and Dr. Ryohei Takahashi for their thorough and insightful reviews. Prof. Khin Zaw is also acknowledged for handling the manuscript and for editorial inputs. This work is offered in memory of the late Rolando E. Peña, a geologist and a friend.

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Waters, P.J., Cooke, D.R., Gonzales, R.I., Phillips, D., 2011. Porphyry and epithermal deposits and 40Ar/39Ar geochronology of the Baguio District, Philippines. Economic Geology 106, 1335–1363. Wessel, P., Smith, W.H.F., 1991. Free software helps map and display data, EOS Transactions American Geophysical Union 72, 441-446. Wessel, P., Smith, W.H.F., Scharroo, R., Luis, J., Wobbe, F., 2013. Generic Mapping Tools: Improved version released. EOS Transactions AGU 94, 409-410. Wilson, M., 1989. Igneous petrogenesis: A global tectonic approach. Unwin Hyman, London, 466 p. Yumul, G.P.Jr., Dimalanta, C.B., Maglambayan, V.B., Tamayo, R.A.Jr., 2003. Mineralization controls in island arc settings: Insights from Philippine metallic deposits. Gondwana Research 6, 767-776. Yumul, G.P.Jr., Dimalanta, C.B., Tam, T.A.III, Ramos, E.G.L., 2008. Baguio mineral district: an oceanic arc witness to the geological evolution of northern Luzon, Philippines. Island Arc 17, 432–442. Yumul, G.P.Jr., Dimalanta, C.B., Marquez, E.J., Queaño, K.L., 2009. Onland signatures of the Palawan microcontinental block and the Philippine mobile belt collision and crustal growth process: A review. Journal of Asian Earth Sciences 34, 610-623. Yumul, G.P.Jr., Dimalanta, C.B., Tamayo, R.A.Jr., Faustino-Eslava, D.V., 2013. Geological features of a collision zone marker: The Antique Ophiolite Complex (Western Panay, Philippines). Journal of Asian Earth Sciences 65, 53-63. Yumul, G.P.Jr., Brown, W.W., Dimalanta, C.B., Ausa, C.A., Faustino-Eslava, D.V., Payot, B.D., Ramos, N.T., Lizada, A.N., Buena, A.E., Villaplaza, B.R., Manalo, P.C., Queaño, K.L.,

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List of Figures Figure 1. A. Oceanic marginal basins surrounding the Philippines are presently subducting along the different trenches bounding this island arc system. Oceanic bathymetric highs have collided with and have subducted beneath the Philippines. The whole archipelago is traversed by the left-lateral Philippine Fault Zone (red lines). Box 1: Baguio Mineral District; Box 2: Pacific Cordillera Mineral District; MT: Manila Trench; NT: Negros Trench; ST: Sulu Trench; CT: Cotabato Trench; PT: Philippine Trench; ELT: East Luzon Trough. B. Porphyry copper and epithermal gold deposits are found throughout the country. The Baguio Mineral District in Luzon and the Pacific Cordillera Mineral District in eastern Mindanao are prolific areas in terms of gold-copper mineralization. Bathymetric map was generated using the Submap 4.2 tool of Heuret and Lallemand (2005) and the GMT software of Wessel and Smith (1991). Mineral distribution modified from Sillitoe and Gappe (1984) and Mitchell and Leach (1991). Figure 2. A. The Baguio Mineral District consists of a Cretaceous basement overlain by a thick volcaniclastic sequence intruded by young magmatic bodies. These intrusions are responsible for the hydrothermal mineralization in the area. B. The Pacific Cordillera Mineral District is underlain by a Cretaceous ophiolitic basement cut by Neogene intrusives that are overlain by volcano-sedimentary sequences. It is divided into 3 blocks - Surigao, CoO and Masara, which are bounded by the Lianga, Cateel and Mati Faults (inset). Map of the Baguio Mineral District is modified from Bellon and Yumul (2000) and that of Pacific Cordillera is modified from Pubellier et al. (1991). Figure 3. Crustal thickness map of the Philippines generated from Bouguer anomalies extracted from the global gravity data of Sandwell and Smith (2009). Labeled boxes represent the

41

Baguio Mineral District (1) and the Pacific Cordillera Mineral District (2). The crust beneath the Baguio Mineral District is ~30 km thick. The Pacific Cordillera Mineral District has a thinner crust (<25 km) except for the southern end where crustal thickness varies from 26-35 km. Figure 4. The index map shows the location of the Baguio Mineral District (1) and the Pacific Cordillera Mineral District (2) and the traces of the Philippine Fault Zone (black lines). A. Focal mechanism solutions beneath Luzon show the South China Sea (SCS) slab subducting along the Manila Trench and indicate typical reverse faulting mechanism. The subduction of a segment of the Scarborough seamount chain led to a distinct flat subduction beneath the Baguio Mineral District. B. The West Philippine Sea slab subducts along the Philippine Trench down to depths of ~190 km. The CMT section is taken at the latitude where the Mindanao Fracture Zone is subducting beneath Mindanao. The focal mechanism solutions from the Global Centroid Moment Tensor Catalogue (Ekström et al. 2012) were plotted using the methodology of Heuret and Lallemand (2005) and the Generic Mapping Tool software of Wessel and Smith (1991). Figure 5. The dominantly calc-alkaline nature of the intrusive and volcanic rocks in both the Baguio Mineral District (red symbols) and the Pacific Cordillera Mineral District (green symbols) is exhibited in the: A. AFM diagram of Irvine and Baragar (1971) and B. SiO2 vs. FeO/MgO diagram of Miyashiro (1974). Symbols in bold represent data from this study and the rest of the symbols represent data from previous works. Sources: adakites from Eastern Mindanao (Macpherson et al., 2006); volcanic rocks from CoO (Sonntag et al., 2011; Taguibao and Takahashi, 2018); diorites from CoO (Sonntag et al., 2011); diorites from Kingking (Suerte et al., 2009); diorite porphyry samples from Boyongan

42

and Bayugo (Braxton, 2007); volcanic and intrusive rocks from Surigao (Sajona, 1995); Masara dacite (Yumul et al., 2017); Masara diorite and porphyritic andesite (Villaplaza et al., 2017); Central Cordillera Diorite Complex and Itogon quartz diorite (Polve et al., 2007); Balabac andesite and Black Mountain quartz diorite (Hollings et al., 2011). Figure 6. The multi-element diagrams normalized to primitive mantle reveal distinct negative Ti, Nb, Ta and Zr anomalies as well as positive Pb and Sr anomalies for both the BMD (A) and PCMD (B) samples. Primitive mantle values are from Sun and McDonough (1989). The above boxes refer to data from this study while the boxes below are data from previous works. Symbols and references are the same as in Figure 5. Figure 7. A. The Sr/Y vs. Y diagram (Defant and Drummond, 1990) and B. (La/Yb)N vs. Yb(N) diagram (Martin, 1986) show a large number of samples, particularly from Mindanao, plot in the adakite field. Symbols and references as in Figure 5. Figure 8. Igneous rocks from both districts plot mostly in the slab-melt derived and hybrid adakite fields in the (K2O/Na2O) vs. Yb diagram of Kamvong et al. (2014). Symbols and references are the same as in Figure 5. Figure 9. A. Analyzed plagioclase grains from the Baguio Mineral District and Pacific Cordillera Mineral District have a wide compositional range from anorthite to oligoclase (field from Elkins and Grove, 1990). B. The amphibole grains from Baguio Mineral District are magnesiohornblende and pargasite whereas those from the Pacific Cordillera Mineral District are edenite to pargasite (diagram from Leake et al., 1997). C. In the FeO/(Fe+Mg) diagram (Anderson and Smith, 1995), samples from both districts exhibit high oxygen fugacity. Symbols and references as in Figure 5.

43

Figure 10. Whole rock Th/Yb vs. Ta/Yb diagram of Pearce (1982) shows the active continental margin to oceanic island arc signatures (fields from Wilson, 1989) of the samples from the Baguio and Pacific Cordillera Mineral Districts. Symbols and references as in Figure 5. Fig 11. Whole rock V/Sc vs. MgO diagram (Lee et al., 2005) manifests relatively high oxygen fugacity for the sample sets. FMQ = fayalite–magnetite–quartz. Symbols and references are the same as in Figure 5.

List of Tables 1. Whole rock major and trace element compositions of igneous rocks from the Pacific Cordillera Mineral District and Baguio Mineral District (data from this study). 2. Plagioclase and amphibole compositions of igneous rocks from the Baguio and Pacific Cordillera Mineral Districts from this study. 3. Geological comparison of the Baguio and Pacific Cordillera Mineral Districts 4. Comparison of the geochemical features of the Baguio and Pacific Cordillera Mineral Districts

44

Table 1. Whole rock major and trace element compositions of igneous rocks from the Pacific Cordillera Mineral District and Baguio Mineral District (data from this study). Pacific Cordillera Mineral District Alipao andesite

L500 DNC

Amacan Volcanic Complex

STFLUMTERBPNR JMI UUP 604-01 509-03 510-03

Major elements (wt.%) SiO2 49.5 47.7 Al2O3 18.9 18.3 Fe2O3 8.22 14 FeOt 7.40 12.62 MgO 4.31 3.59 CaO 7.53 7.82 Na2O 3.84 3.86 K2O 1.64 0.23 TiO2 1.03 1.13 P2O5 0.22 0.24 MnO 0.19 0.25 Cr2O3 0 0.01 LOI 4.3 2.4 Sum 99.77 99.74

71.8 15.5 0.66 0.59 0.13 0.71 7.29 1.94 0.09 0 0 0 1.6 99.82

67.03 17.2 1.89 1.70 0.96 1.78 7.43 2.41 0.20 0.11 0.07 0 0.6 99.77

Cateel Quartz Diorite

L530 MST2

60.6 17.8 4.67 4.20 2.53 4.76 5.09 1.64 0.49 0.21 0.1 0.01 1.7 99.81

L575 SDN ODW

60.7 17.1 3.56 3.20 1.72 4.32 3.57 4.22 0.37 0.15 0.3 0.01 3.6 99.83

Baguio Mineral District Lamingag Intrusive Complex

MAP- APEXGIN 129908 515-06

57 15.3 8.42 7.58 2.48 3.75 1.87 1.53 0.44 0.14 0.04 0 8.8 99.86

58.5 13.4 7.78 7.00 3.74 0.35 0.16 7.38 0.56 0.19 0.7 0.03 5.5 98.30

L455 BNZ

55.6 16.5 6.46 5.81 5.88 2.81 2.59 3.27 0.75 0.23 0.55 0.05 4.9 99.76

L500 BHWS

55.9 7.63 16.4 14.78 0.4 0.65 0.03 2.29 0.23 0.06 3.43 0 12.1 99.17

Itogon Central Quartz Cordillera Diorite Diorite Complex SG DC-3 SG DC-9 SG DC-4

54.4 15.7 10.2 4.01 7.86 2.79 1.40 0.77 0.1 0.25 0.01 2.2 99.77

53.7 17.1 8.42 3.87 9.21 2.81 0.84 0.53 0.07 0.22 0.01 2.9 99.81

55 17.9 6.94 2.77 6.95 3.69 2.71 0.78 0.23 0.14 0.01 2.60 99.79

Balabac Andesite

SG DC-10

51.3 17.3 7 2.97 5.97 2.78 3.99 0.77 0.23 0.19 0 7.00 99.64

Baguio Silicic Rocks

BG-10 BG-25 BG-27 BG-29 BG-4

BG-30 BG-44

54.4 19

55.2 17.0

55.7 18.7

52.2 17.9

56.6 17.4

55.5 18.1

58.7 14.6

10.9 5.38 3.35 5.08 0.33 1.02 0.12 0.24

8.13 5.74 8.19 4.15 0.13 1 0.17 0.21

8.66 3.39 5.47 6.52 0.12 1.05 0.13 0.17

7.88 8.14 8.88 3.54 0.30 0.79 0.08 0.19

6.85 4.49 7.89 4.04 1.61 0.68 0.19 0.17

8.46 4.84 8.12 3.11 0.67 0.72 0.13 0.16

9.42 2.49 13.1 0.22

3.69 100

3.21 100

3.17 100

1.87 100

1.52 100

1.07 0.09 0.14

2.54 2.63 99.98 99.98

Trace elements (ppm) Rb 36.8 2.7 Ba 207 47 Th 0.8 0.7 U 0.2 0.2 Nb 2.6 1.2 Ta <0.1 0.1 La 8.5 5.4 Ce 20.2 13.2 Pb 2 2.1 Pr 2.7 2.1 Sr 485 360 Nd 13.6 11.3 Sm 3.36 3.11 Zr 85.6 60.8 Hf 2.2 2 Eu 0.91 0.97 Ti 6174 6774 Gd 3.73 3.64 Tb 0.62 0.6 Dy 3.83 3.62 Ho 0.87 0.71 Y 21.4 20.1 Er 2.49 2.18 Tm 0.37 0.31 Yb 2.49 1.94 Lu 0.37 0.31

30.5 534 0.9 1.4 4.9 0.3 1.2 1.4 2.2 0.14 946. 0.4 0.09 66.1 2.2 0.1 539 0.15 0.02 0.2 0.04 1.4 0.15 0.03 0.26 0.05

45.1 601 1.8 2 5.3 0.3 5.8 10.2 2.8 1.22 1139 4.6 1.04 93.8 2.5 0.32 1199 1 0.14 0.92 0.2 6.2 0.58 0.09 0.75 0.11

25.7 321 1.5 0.8 3.4 0.1 10.3 17.6 2.8 2.26 853 9.8 2.08 79.8 2.5 0.7 2937 2.04 0.26 1.3 0.21 6 0.58 0.07 0.54 0.08

77.9 819 2.1 1.2 4.7 0.2 12.1 22 23.9 2.46 793 9.5 2.02 95.7 2.6 0.68 2218 1.78 0.27 1.58 0.33 10 1.06 0.16 1.01 0.17

29 97 1.6 0.6 2.4 0.1 8.7 16.9 1.9 2.24 136 9.8 2.37 90 2.3 0.81 2637 2.57 0.41 2.65 0.6 16 1.77 0.28 2.09 0.33

133 2278 1.5 1 4.3 0.3 9.5 17.8 1489 2.32 161 10 2.21 87.8 2.3 0.85 3357 2.16 0.35 2.01 0.4 11.7 1.21 0.18 1.11 0.19

70.7 599 1.9 0.9 5.1 0.3 13.3 25.4 8.7 2.89 584 11.6 2.63 99.9 2.4 0.92 4496 2.5 0.37 2.31 0.48 12.6 1.4 0.18 1.27 0.19

62.7 156 0.7 0.4 1.2 <0.1 5.1 8.1 1258 1.08 9.2 5.2 1.49 39.8 1 0.51 1378 2.32 0.38 2.63 0.66 20 2.09 0.29 2.03 0.32

30.1 241 2.7 1 2.3 0.2 8.8 18.8 3.7 2.48 385 11.3 3.17 71.2 2 0.97 4616 3.61 0.61 3.88 0.87 23.4 2.52 0.38 2.58 0.38

16.2 199 2.8 0.9 1.9 0.2 9.4 17.6 5.5 2.31 375 10.3 2.7 41.6 1.4 0.71 3177 3.17 0.52 3.49 0.71 19.9 2.11 0.32 2.02 0.34

50.6 450 3.3 1 4.6 0.3 12.7 27.2 1.5 3.59 571 15.9 3.55 118. 3.1 1.1 4676 3.75 0.58 3.66 0.75 20.7 2.21 0.32 2.04 0.32

105 874 3.1 1.1 4.4 0.2 11.8 25 5.1 3.22 369 14.1 3.18 113 2.8 0.89 4616 3.29 0.53 3.23 0.72 18.7 2.1 0.3 1.94 0.29

2

1 2 30 7 1 193.00 194.00 340.00 251.00 748.00 106.00

0.3

1.5

0.5

261

278

247

157

599

743

106

88 2.71

87 1.62

139

36 0.8

98 2.84

90 1.5

99 3.42

6150

4.1

4350

Table 2

Table 2. Plagioclase and amphibole compositions of igneous rocks from the Pacific Cordillera Mineral District and Baguio Mineral District from this study). Pacific Cordillera Mineral District

Sample code

Lamingag Intrusive Complex (amphibole) MAI MAI MAI MAI UUP UUP UUP UUP 0507- 0507- 0507- 050711E1 11E2 11E3 11E4

wt% SiO2 46.13 TiO2 1.32 Al2O3 7.84 FeO* 17.13 MnO 0.70 MgO 12.87 CaO 11.30 Na2O 1.67 K2O 0.92 Total 99.87 Atoms (O = 23) Fe* 2.11 Mg 2.82 Aliv 1.29

Alipao Andesite (amphibole)

Cateel Quartz Diorite (amphibole)

A10C1-1 amp

A10C1-3 amp

A10C2-1 amp

A10C3-3 amp

A10C4-3 amp

A10C5-1 amp

A10C6-1 amp

A10C7-1 amp

A10C8-2 amp

A10C9-1 amp

A10C101 amp

A4C6-1 amp

A4C6-2 amp

A4C6-3 amp

A4C6-4 amp

A4C6-5 amp

A4C6-8 amp

A4C6-9 amp

A4C610 amp

A4C611 amp

A4C612 amp

42.96 1.21 10.04 19.00 0.64 11.32 11.31 1.99 1.15 99.61

42.07 1.35 9.93 19.02 0.55 10.78 11.37 2.00 1.15 98.22

44.77 1.90 9.03 12.02 0.30 15.51 11.34 2.31 0.67 97.84

43.64 1.62 8.52 17.45 0.59 11.61 11.12 1.72 1.06 97.32

44.18 1.54 8.55 18.98 0.45 11.22 11.17 1.75 1.01 98.84

42.90 1.35 8.88 19.76 0.53 10.59 11.05 1.67 1.17 97.89

43.40 1.11 8.93 18.56 0.51 11.04 11.15 1.65 0.96 97.31

44.65 1.20 8.59 19.48 0.65 11.11 11.15 1.72 1.06 99.62

44.58 1.14 7.77 19.00 0.57 11.27 11.23 1.64 1.00 98.17

42.84 1.46 9.69 18.51 0.49 10.76 11.05 1.82 1.09 97.69

44.20 1.01 7.91 17.50 0.61 11.23 11.18 1.59 1.05 96.29

43.46 1.12 8.50 18.31 0.54 10.99 11.04 1.60 0.99 96.55

44.83 1.16 7.71 17.47 0.57 11.40 10.99 1.59 0.91 96.63

43.89 1.06 8.17 18.52 0.58 11.27 11.06 1.56 0.92 97.02

44.13 1.38 7.91 16.76 0.50 11.26 10.91 1.61 1.00 95.47

41.53 1.44 10.05 17.94 0.51 9.90 10.63 1.76 1.17 94.93

43.92 1.27 7.93 17.79 0.61 11.32 10.71 1.63 0.99 96.16

44.76 1.09 7.86 17.62 0.57 11.38 10.71 1.52 0.91 96.42

43.51 1.14 8.34 17.64 0.54 10.76 10.86 1.54 1.02 95.35

43.82 1.19 7.86 17.68 0.56 11.16 11.02 1.47 0.92 95.68

43.88 1.12 7.82 17.28 0.64 11.16 10.83 1.41 0.95 95.08

42.88 1.14 8.62 17.90 0.56 10.65 10.87 1.53 1.00 95.14

44.47 1.07 7.68 17.37 0.60 11.17 10.86 1.49 0.85 95.56

43.54 1.13 8.06 17.57 0.60 11.08 10.88 1.56 0.96 95.38

2.38 2.53 1.68

2.42 2.45 1.70

1.48 3.39 1.49

2.22 2.63 1.45

2.38 2.51 1.44

2.53 2.41 1.53

2.37 2.51 1.50

2.43 2.47 1.43

2.40 2.54 1.33

2.35 2.44 1.59

2.24 2.57 1.31

2.35 2.51 1.43

2.22 2.58 1.26

2.36 2.56 1.39

2.16 2.58 1.28

2.35 2.31 1.59

2.29 2.59 1.34

2.25 2.59 1.28

2.28 2.48 1.35

2.28 2.57 1.33

2.24 2.58 1.30

2.33 2.47 1.43

2.23 2.56 1.25

2.27 2.56 1.36

Table 2. Continued

Itogon Quartz Diorite (amphibole) Sample code

SG7578DC-03C1-hbl02

wt% SiO2 46.62 TiO2 0.57 Al2O3 6.21 FeO* 18.43 MnO 0.32 MgO 11.29 CaO 11.63 Na2O 0.73 K2O 0.59 Total 96.39 Atoms (O = 23) Fe* 2.34 Mg 2.56 Aliv 1.01

Baguio Mineral District Central Cordillera Diorite Complex (amphibole)

Balabac Andesite (amphibole)

SG7578DC-03C2-hbl

SG7578DC-03C2-hbl03

SG7578DC-03C2-hbl04

SG7578DC-09C1-03

SG7578DC-09C1-02

SG7578DC-09C1-04

SG7578DC-09C2

SG7578DC-09C2-02

SG7578DC-09C3-03

SG7578DC-04C1-04

SG7578DC-04C1-05

SG7578DC-04C2-04

SG7578DC-04C2-07

SG7578DC-04C3-03

SG7578DC-04C3-06

46.32 0.74 6.74 19.92 0.32 11.34 10.87 1.05 0.75 98.09

45.36 0.73 6.99 18.46 0.38 10.98 10.91 1.07 0.79 95.98

45.67 0.72 7.08 18.46 0.34 11.07 10.88 1.03 0.86 96.44

45.74 0.69 6.62 22.36 0.68 11.01 10.59 1.02 0.77 99.47

45.58 0.65 6.54 21.32 0.62 10.36 10.91 1.00 0.74 97.73

46.11 0.73 6.92 24.13 0.63 10.66 10.75 1.05 0.71 101.68

45.77 0.64 6.64 24.08 0.59 10.23 11.02 0.96 0.67 100.60

45.30 0.70 6.57 21.57 0.56 10.68 10.67 1.08 0.68 97.80

45.15 0.73 6.96 21.74 0.59 10.48 10.08 1.05 0.65 97.43

41.93 2.40 10.88 16.07 0.29 13.82 11.21 2.35 0.48 99.42

42.34 2.87 10.29 13.67 0.34 13.48 11.22 2.36 0.51 97.07

40.51 2.42 12.11 13.63 0.23 13.13 11.55 2.37 0.48 96.44

39.84 2.30 12.88 13.65 0.21 13.50 11.80 2.43 0.48 97.10

39.94 2.44 12.44 14.58 0.20 13.21 11.43 2.52 0.46 97.20

44.06 2.45 8.93 17.04 0.37 14.27 10.97 2.01 0.75 100.85

2.51 2.55 1.12

2.37 2.52 1.13

2.36 2.52 1.11

2.82 2.47 1.19

2.72 2.36 1.14

2.99 2.36 1.24

3.02 2.29 1.21

2.75 2.43 1.17

2.78 2.39 1.19

1.99 3.04 1.19

1.71 3.01 1.88

1.72 2.96 1.73

1.72 3.03 1.99

1.92 3.09 2.14

2.26 3.37 2.10

Table 2. Continued Pacific Cordillera Mineral District Lamingag Intrusive Complex (plagioclase)

Alipao Andesite (plagioclase)

LMN LMN LMN QMN LMN QMN LMN QMN MAI UUP MAI UUP Sample Code QMN 514 QMN 514 03 514 03 514 03 0507-11E 0507-11E 03 514 03 wt% SiO2 57.00 56.93 61.25 61.97 61.15 60.02 59.80 TiO2 0.13 0.00 0.00 0.00 0.02 0.03 0.00 Al2O3 26.73 26.95 23.74 22.76 23.24 23.74 23.93 FeO 0.27 0.20 0.27 0.23 0.24 0.25 0.14 CaO 8.49 8.69 5.15 4.30 4.56 5.65 6.01 Na2O 6.47 6.39 8.46 8.76 8.63 7.90 7.63 K2O 0.40 0.38 0.73 0.81 0.83 0.66 0.65 Total 99.49 99.53 99.60 98.83 98.67 98.25 98.16 Atoms (O = 5) Al 1.42 1.43 1.25 1.21 1.23 1.27 1.28 Ca 0.41 0.42 0.25 0.21 0.22 0.27 0.29 Na 0.57 0.56 0.73 0.76 0.75 0.69 0.67 K 0.02 0.02 0.04 0.05 0.05 0.04 0.04 An% 41.08 41.98 24.16 20.35 21.53 27.25 29.19

MAI UUP 0507-11E

Cateel Quartz Diorite (plagioclase)

MAI UUP MAI UUP APEX APEX APEX APEX APEX 0507-11E 0507-11E 129923 129914 129914 129914 129914

73.65 0.00 13.81 0.88 1.17 6.29 0.57 96.37

60.30 0.00 23.36 0.12 5.48 8.06 0.67 97.99

61.79 0.00 20.79 0.02 2.87 9.60 0.15 95.21

55.74 0.00 24.29 0.23 9.42 6.04 0.27 95.99

55.44 0.04 24.53 0.36 11.18 5.63 0.34 97.52

63.54 0.00 21.72 0.19 5.69 8.19 0.39 99.72

59.34 0.00 22.60 0.21 7.66 7.33 0.48 97.62

57.15 0.04 23.70 0.43 7.89 6.60 1.04 96.85

0.72 0.06 0.54 0.03 8.85

1.25 0.27 0.71 0.04 26.27

1.13 0.14 0.86 0.01 14.04

1.34 0.47 0.55 0.02 45.57

1.34 0.56 0.51 0.02 51.35

1.14 0.27 0.71 0.02 27.13

1.22 0.38 0.65 0.03 35.63

1.30 0.39 0.59 0.06 37.44

Table 2. Continued Baguio Mineral District Itogon Quartz Diorite (plagioclase) Sample Code wt% SiO2 TiO2 Al2O3 FeO CaO Na2O K2O Total Atoms (O = 5) Al Ca Na K An%

T17-1

T17-2

T17-3

T17-4

Central Cordillera Diorite Complex (plagioclase) H24-1

H24-2

H24-3

H24-4

H24-5

Balabac Andesite (plagioclase) A08-1

A08-2

A08-3

A08-4

55.01 0.00 25.98 0.23 9.52 5.54 0.23 96.52

57.89 0.-27 23.70 0.20 6.77 7.11 0.33 95.98

60.42 0.02 22.32 0.09 4.90 8.31 0.24 96.29

54.58 0.01 26.26 0.28 9.60 5.47 0.23 96.44

53.53 0.02 28.26 0.16 10.12 4.99 0.17 97.25

54.24 0.02 27.73 0.21 9.89 4.93 0.17 97.19

56.91 0.00 25.59 0.21 8.15 6.08 0.23 97.17

55.98 0.00 26.49 0.20 8.00 6.12 0.22 97.00

56.65 0.01 25.66 0.20 8.00 6.12 0.22 96.85

42.86 0.00 33.64 0.59 17.01 1.23 0.07 95.40

42.34 0.00 34.03 0.50 17.81 0.87 0.03 95.57

41.00 0.00 34.78 0.48 18.30 0.60 0.01 95.17

43.41 0.03 34.93 0.62 16.47 1.60 0.03 97.08

1.43 0.48 0.50 0.01 48.03

1.30 0.34 0.64 0.02 33.78

1.21 0.24 0.74 0.01 24.22

1.45 0.48 0.50 0.01 48.55

1.55 0.50 0.45 0.01 52.26

1.52 0.49 0.45 0.01 52.01

1.40 0.40 0.55 0.01 41.95

1.45 0.42 0.52 0.01 44.13

1.40 0.40 0.55 0.01 41.39

1.91 0.88 0.12 0.00 88.05

1.94 0.92 0.08 0.00 91.70

1.99 0.95 0.06 0.00 94.33

1.88 0.85 0.15 0.00 84.88

Table 3

Table 3. Geological comparison of the Baguio and Pacific Cordillera Mineral Districts Geological Features

Baguio

Pacific Cordillera

Subduction depth

≥150km

≥100km

Subducted ridge

Scarborough Seamount

Mindanao Fracture Zone

Crustal thickness

≥30km

≥20km

Oceanic plate age

Oligo-Miocene South China Sea

Eocene Philippine Sea Plate and Palau Basin

Subducted sediments

Pelagic, continental

Pelagic, continental

Philippine Fault Zone

Horsetail, dilational jogs

Horsetail, dilational jogs

Table 4

Table 4. Comparison of the geochemical features of the Baguio and Pacific Cordillera Mineral Districts.

Geochemical Features

Baguio

Pacific Cordillera Porphyry copper, vein-type gold

Type of deposits

Porphyry copper, vein-type gold

Magma type

deposits Tholeiitic, calc-alkaline

Adakite origin

Slab melting, fractionation, ridge

Fractionation, strike-slip

subduction

faulting, high P fractionation

High (elevated V/Sc; low

High (elevated V/Sc; low

amphibole Fe)

amphibole Fe)

Water pressure

High (amphibole, biotite)

High (amphibole, biotite)

Gold sulfidation state

Intermediate to high

Low, intermediate to high

Oxygen fugacity

deposits Tholeiitic, calc-alkaline

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

45

Highlights  Cu-Au deposition is related to calc-alkaline and adakitic magmas with high fO2. 

Deposits in Baguio and Pacific Cordillera are Miocene to Plio-Pleistocene.



Crustal thickening and bathymetric high subduction contribute to mineralization.

46