The Silangan Project tenements consist of two main ore deposits: Boyongan and Bayugo.
Boyongan ore deposit is wholly owned by Silangan Mindanao Mining Co., Inc. (SMMCI).
Bayugo ore deposit: Bayugo-Silangan deposit is wholly owned by SMMCI, Bayugo-Kalayaan deposit is a joint venture with Manila Mining Corporation, its subsidiary Kalayaan Copper Gold Resources, Inc. (KCGRI), and Philex Mining which currently holds a 5% interest with an option to increase its stake up to 60%.
Philex Mining Corp. (PMC) owns 100% stake in SMMCI.
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Summary:
In the Silangan region, intrusions of andesite porphyry and dioritic plutons are widespread. Dating by ANU-PRISE (ANU-Prise is an externally funded group within the Research School of Earth Sciences at the Australian National University) for Anglo returned mid to late-Pliocene ages for most altered or mineralised intrusions (Anglo 2008a). The majority of intrusions from the Pacific Cordillera exhibit andesitic to trachyandesitic compositions, but also include a relatively minor component of basaltic andesites, dacites and monzonites of calc-alkaline and high-K calc-alkaline affinity.
Porphyry intrusions commonly occur as plugs, dykes and sills of hornblende andesite porphyry, generally within a 20 km wide belt, which includes the majority of the known epithermal gold and copper-gold porphyry deposits and prospects. The igneous activity is closely related to splays off the Philippine Fault, probably because the fault exploited hotter rocks along the axial zone of the magmatic arc (Anglo 2008a).
Intrusive clusters occur along major structural lineaments and at the intersection of lineaments. The Maniayao Highlands immediately north of Lake Mainit forms the largest igneous complex in the Surigao District and is situated in an area of maximum extension within a pull-apart basin. Other clusters of intrusive rocks in the area tend to comprise of five to ten individual intrusions occurring as individual conical hills. There are several such clusters in the Surigao District, each covering an area of 20 – 40 km2. Each cluster of intrusives is thought to represent a single magmatic plumbing system that converges at depth on a common magma chamber. The mineralogy of most intrusive rocks in the Surigao District suggests that they crystallised from andesitic to dacitic melts.
Boyongan Orebody
The blind porphyry deposits carry high-grade copper and gold values and are hosted by a complex of composite diorite porphyry stocks and diatreme breccias. The composite diorite porphyry stocks and diatreme breccias were emplaced into volcanic and sedimentary rocks of the Bacuag Formation (Upper Oligocene – Lower Miocene age basalts, volcanic breccias and limestones) and the Motherlode Turbidite Formation (Upper Miocene age, silty mudstones). Intense quartz stockwork veining developed within the stocks and the adjacent breccia wallrocks.
The Boyongan and Bayugo deposits are centred on two related early-mineralisation cylindrical composite diorite porphyry stocks and are typical of island-arc style deposits. Emplacement appears to have taken place in conjunction with the earliest phase of mineralising fluids. Since their late Pliocene emplacement (2.3–2.1 Ma; sensitive high resolution ion microprobe uraniumlead-zircon dating) at depths of 1.2 – 2 km, these deposits were exhumed, deeply weathered and buried (Braxton et al, 2009).
At both deposits, hypogene copper and gold are concentrated in two distinct eastern and western zones flanking discrete, high-aspect ratio ('pencil-shaped') stocks. Intense quartz stockwork veining has developed within the stocks and the adjacent breccia wallrocks.
The porphyry hosts underwent pervasive potassium-silicate alteration and have been affected by a weakly developed illite-chlorite-smectite alteration event and by localised zones of illitepyrite and vuggy quartz related to phyllic and advanced argillic overprints, respectively.
In the Boyongan Deposit, the deep supergene profile contains malachite, azurite, chysocolla and cuprite as the principal copper phases, and importantly, copper in silicates such as kaolinite, dickite, biotite and phlogopite. Chalcopyrite and bornite are the dominant hypogene sulphides and pyrite is minimal. The Boyongan Deposit is hosted by potassic altered diorite porphyry. Copper and gold mineralisation occurs as quartz veins disseminated within porphyritic intrusions. The sulphide zone is dominated by chalcopyrite and weak bornite, whilst the main gold species are pure gold, electrum, gold and silver tellurides locked in pyrite, bornite and on rare occasions within chalcopyrite.
An early middle Pleistocene supergene event followed a period of rapid uplift and exhumation in north-east Mindanao at a rate of 2.5 km/Ma. Rapid subsidence and burial followed at a rate of =3.4 km/Ma (Braxton 2007). During this period, debris flows, volcanic material and fluviolacustrine sediments accumulating in the actively extending Mainit Graben covered the weathered deposits, which preserved the supergene profiles beneath 50 – 500 m of cover.
Initially, the Boyongan Deposit formed by hypogene mineralisation processes, displaying mineralization characteristics and vein paragenesis consistent with the porphyry vein classification of Braxton (2007), Corbett and Leach (1997) and Sillitoe (2000).
The main geological features and controls on the mineralisation are the early coarse-grained diorite porphyries, which form the focus for the copper and gold mineralisation. Drill core logging enabled the definition of three principal intrusive phases emplaced in the early diorite complex. The ECD porphyries are the intrusive phases associated with intense quartz-vein stockworks and elevated copper-gold grades. Drillhole data indicates that these bodies are roughly cylindrical at depth and broaden towards the upper 250 m to 300 m. At Boyongan, the diatreme breccia complex and the ECD series are the main host to mineralisation. A series of moderately-altered inter-mineralisation and late-mineralisation diorite porphyry stocks and dykes cut the progenitor porphyry stocks. The mineralisation associated with the porphyry stocks terminate at the Quaternary paleosurface.
The deposits show typical porphyry style mineralisation with characteristic magmatic hydrothermal alteration patterns. A pervasive potassic alteration zone is clearly defined in the progenitor intrusive rocks and associated with the mineralising phase. Illite-smectite-pyrite alteration, usually associated with argillic alteration, overprints the potassic zone in various degrees. Skarn alteration is observed along the contacts between carbonate-rich units and intrusive rocks. Propylitic alteration is well developed along the periphery of the diorite stocks and dissipates outwards.
Primary mineralisation occurs as both sulphide dissemination and sulphide in quartz veins. Chalcopyrite dominates as the primary copper mineral in the sulphide zone with rare to weak bornite. Hypogene gold occurs as pure gold, electrum and gold-silver tellurides locked in pyrite, bornite and rarely in chalcopyrite. Pyrite is widespread though it is more common in the Bayugo Deposit than in the Boyongan Deposit.
The tendency is for the gold to be concentrated closer to the intrusions while the copper is found slightly "outboard" of the gold as well as being coincident with it. Later oxidation and weathering processes resulted in the remobilisation of the copper distribution in the ore body generating a greater dispersion of copper metal in upper levels of the ore deposit and variable secondary copper speciation. The remobilised exotic copper zone was deposited to the south-east of the Bayugo Deposit (north-west of Boyongan) where the groundwater was hypothesised to flow during that particular time (Braxton, et al. 2009).
Minor mineralisation is noted in the cover sequence material directly above the paleosurface, which is understood to have been transported in a sub-horizontal direction after the mineralisation was uncovered by erosion.