In the southernmost main island of the Philippines (Figure 1), the Co-O mine sits in the Central Pacific Cordillera of the Eastern Mindanao Province (Figure 2). This region is saddled between the North Pacific Cordillera to the north and the South Pacific Cordillera to the south. Structural features separating these regions of the north-south trending cordillera are the Lianga Fault and Cateel Bay Fault. These regional structures including the Mati Fault to the south constitute the southern horsetail structure of the left-lateral strike-slip Philippine Fault that runs along the entire stretch of the Philippine archipelago [1] (Figure 1 and Figure 2).

The Co-O gold underground mine is currently being operated by the Philsaga Mining Corp., which is a Philippine subsidiary of the Medusa Mining Ltd. As described in earlier consultancy works and company surveys, the Co-O deposit is characterized as an intermediate sulfidation epithermal gold (+Ag ± Cu ± Pb ± Zn) quartz vein type (e.g., [2] [3] ). The Co-O mine was first developed in the late 1980s and has since then been currently developed to about 350 meters below the adit level (Level 1) that is at 150 meters above mean sea level. East-west length of underground workings reaches to about 1000 meters per level. The reported reserve of the Co-O mine for mid-2017 is 1.64 M tonnes at 6.54 g/t for 345,000 ounces of gold [4] .

With a purpose to reconstruct the tectonic history of the Co-O epithermal gold deposit and vicinity, this paper presents the results of recently conducted whole-rock geochemical analyses on the basis of X-ray fluorescence (XRF) spectroscopy and inductively coupled plasma-mass spectrometry (ICP-MS) and K/Ar dating of selected representative samples of host rocks of the Co-O epithermal gold vein deposit.

As in earlier works (e.g., [1] [5] [7] ), the Philippine archipelago has been generally described as an amalgamation of magmatic rocks, ophiolitic suites,

sedimentary basins and metamorphic units that are grouped into two distinct tectono-stratigraphic terranes, namely: Philippine Mobile Belt (PMB) and Palawan-Mindoro Continental Block (Figure 1). The ophiolitic and metamorphic rocks constitute the pre-Tertiary basement of the Philippine archipelago [1] .

The Philippine Mobile Belt is an active deformation zone between the Philippine Sea Plate to the east and the eastern margin of the Eurasian Plate to the west. It comprises majority of the whole length of the Philippine archipelago, from the whole Luzon island to most parts of the Visayas and Mindanao islands.

It is bounded on both sides by subduction zones of opposing polarities and traversed by the ~1200-km left-lateral strike slip Philippine Fault System (Figure 1). Tectonically separated from the PMB is the Palawan-Mindoro Continental Block at the western side of the archipelago. This block is believed to have been rifted from mainland Asia and drifted to its present position as it collided with the PMB (e.g., [1] [5] ).

Previous works on the eastern Mindanao Island are mainly focused on the Surigao and/or Masara District including other areas of the island and southern offshore regions with implications on the island’s tectonic history (e.g., [11] [12] ). According to these studies, the pre-Miocene geological units of Eastern Mindanao were formed in southern latitudes and have moved towards the northwest until it collided with the pre-Miocene units of the Western Mindanao that were derived from the eastern Eurasian margin generally of continental affinity. Studies on the reconstruction of paleo-latitudes using onshore and offshore paleomagnetic declinations (e.g., [13] [14] ) reveal that the Eastern Mindanao during Oligocene (30 Ma) was located at 10˚S latitude, where a northeast-dipping subduction system was forming most of the Eastern Mindanao arc units.

Main lithologies associated with the Co-O gold deposit are volcanic and volcaniclastic rocks intruded by diorite, dacite and andesite porphyries, all in turn cut and overlain by a diatreme-maar complex typified by polymictic volcanic breccias (Figure 3).

These units are composed of typical mineral assemblages of volcanic and intrusive rocks of basalt-andesitic to andesitic composition. The porphyritic andesitic to basaltic-andesitic volcanic rocks consist of phenocrysts mainly of plagioclase partly altered to K-feldspar, calcite and/or clay (smectite, interstratified illite-smectite, chlorite ± kaolinite; confirmed through X-ray diffractometry), with subordinate hornblende and/or clinopyroxene altered into chlorite, and quartz, set in a microcrystalline groundmass. Intrusive andesite porphyries include similar assemblage and alteration minerals, with some containing xenoliths of porphyritic andesite. These rocks exhibit similar alteration assemblage to

those of the volcanic rocks but of higher degree of alteration. Pyrite, ilmenite, and chalcopyrite are found in the volcanic rocks, while hematite, magnetite, pyrite, chalcopyrite and ilmenite are in the intrusive rocks. In both units, quartz ± calcite veins/veinlets are present. Polymictic diatreme volcanic breccias, as defined by the Philsaga Mining Corporation (unpublished internal report), generally contain clasts of the abovementioned units with detrital grains of quartz and feldspars set in a fine-grained (flour) or patches of quartzofeldspathic, calcitic, and chloritic minerals as matrix.

Geochronological analysis using K/Ar dating of the andesite porphyry and polymictic diatreme breccia was outsourced to Activation Laboratories (Actlabs) in Ancaster, Ontario, Canada. Prior to K/Ar dating, bulk samples of these rocks were sent to Actlabs for quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and mineral separation. Based on the modal mineralogical study, the mineral selected for age dating is feldspar, which may have partly replaced primary plagioclase (e.g., [20] ).

Selected representative sample of the andesite porphyry plots on the sub-alkaline basalt composition in the Zr/TiO₂ vs. Nb/Y discrimination diagram and on the island arc tholeiitic basaltic andesite-andesite in the Th vs. Co diagram.

For the polymictic diatreme breccia, geochemical analysis of the selected representative sample of this unit plots on the basaltic andesite composition in the Zr/TiO₂ vs. Nb/Y discrimination diagram and on the calc-alkaline basaltic andesite-andesite in the Th vs. Co diagram.

Results of the K/Ar dating of the feldspar concentrates yielded ages of 28.6 ± 0.9 Ma (Late Oligocene) for the andesite porphyry and 31.7 ± 1.9 Ma (Early Oligocene) for the polymictic diatreme breccia (Table 2).

In principle, for ideal settings (closed system), age dating of alteration products indicates the age of hydrothermal activity or alteration event, which is typically associated with the mineralization event forming the deposit system (e.g., [21] ). Age dating of the hydrothermal minerals therefore gives the age of the hydrothermal activity associated with the mineralization.

As in previous works (e.g., [17] ), the volcanic host rocks of the Co-O deposit is dated to be of Oligocene (~32 Ma) age, possibly attributing to the Eocene to

Standards Bern 4M Muscovite and 1/65 “Asia” rhyolite matrix were measured for ³⁸Ar spike calibration.

Oligocene (45 - 25 Ma) island arc tholeiitic magmatism. This implies that the hydrothermal activity related to the Co-O epithermal vein system occurred immediately after or during the Oligocene magmatism.

Main points of this study are summarized in the following:

1) Whole-rock geochemical analyses of the polymictic diatreme breccia, intrusive and volcanic rocks indicate general island arc tholeiitic and calc-alkaline magma series signatures, associated with an island arc subduction setting.

2) General composition of the polymictic diatreme breccia, intrusive and volcanic rocks ranges from sub-alkaline andesite, basaltic-andesite to basalt and alkaline basalt.

3) Typical alteration of the polymictic diatreme breccia, intrusive and volcanic rocks associated with the Co-O epithermal gold deposit follows the chlorite-carbonate and chlorite-pyrite (-sericite) alteration trend lines defined in the “alteration box plot” by [17] , comparable with the dominant chlorite alteration observed through petrographic analysis of these rocks.

4) K/Ar dating of hydrothermal minerals from the andesite porphyry and polymictic diatreme breccia samples yielded ages of 28.6 ± 0.9 Ma (Late Oligocene) and 31.7 ± 1.9 Ma (Early Oligocene), respectively. These ages correspond to the age of the hydrothermal activity in the area, which is in turn related to the timing of the mineralization.

Integrated with earlier works on age dating and geochemistry of similar rock units along eastern Mindanao, whole-rock geochemical analysis of the volcanic host rocks and an Oligocene age date of the hydrothermal activity from this study corroborate an island arc setting of formation of the rock units corresponding to paleo-latitude reconstruction studies [13] [14] , which situate the eastern Mindanao magmatic province to be forming with the Indian and Australian plate subduction along the Sunda-Java Trench system at about 10°S latitude during the Oligocene times, followed by or contemporaneous with the formation of the Co-O epithermal gold vein system prior to the accretion of the different terranes constituting the Philippine archipelago around Miocene.

This study further confirms the claim that the Co-O deposit is (one, if not) the oldest epithermal gold deposit in the Philippines [2] [3] . Since most of the deposits in the Philippines are dated to be of Miocene and Pliocene to Pleistocene ages, with some associated with the Philippine Fault that was formed in response to the present geodynamic setting of the archipelago since Miocene times (e.g., [22] [23] [24] ), the study of older deposits such as the case of the Co-O epithermal gold deposit could provide supplementary lines of evidence in understanding the occurrence and preservation of such deposits in an actively deforming region. In line with this, additional studies, particularly on the characterization of the Co-O epithermal vein system with tectono-kinematic analysis of structural controls and other related structural features, still need to be conducted on the deposit. Additionally, further study on the occurrence of the hydrothermal products must be conducted to ensure the results of the age dating, since this bears a significant impact on the interpretation for the research.

The authors would like to express their gratitude to the assistance of the Medusa Mining Ltd. and Philsaga Mining Corp. for permitting the research study to be conducted within their mining tenements, and providing technical and logistical support throughout the research. Technical support from the members of the Economic Geology Laboratory in Akita University, with funding and assistance from the New Frontier Leading Program, its staff, and faculty members, and the Japanese government (Monbukagakusho) scholarship program are greatly appreciated. The authors would likewise wish to express their appreciation to Dr. Akira Imai for imparting his knowledge on the subject matter.

Taguibao, K.J.L. and Takahashi, R. (2018) Whole-Rock Geochemistry of Host Rocks and K/Ar Age of Hydrothermal Mineral of the Co-O Epithermal Gold Deposit, Mindanao, Philippines. Open Journal of Geology, 8, 383-398. https://doi.org/10.4236/ojg.2018.84022