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.

Silangan will employ an underground mining method called Sub-level Caving. Load-HaulDump (LHD) units will be utilized to extract the ore from the production drifts after which ore will be trucked from underground to the processing plant at the surface.

Mine production rate is determined by the rate of development and commissioning of production drifts and sublevels. In each production drifts, the critical path to sustained production rate is the precision in drilling and blasting production drill holes and equal drawing from the production drifts to promote desired caving. The mine will operate at an annual production rate of 4 Million metric tons.

Production will start after a 2.5-year development and construction period. A rapid ramp up to 4 Million tons wil happen on the first two years, after which the production rate will be maintained for most of the 21-year mine life until it wanes.

Mine Design
Industry standard panel dimensions are used at the Boyongan deposits i.e. extraction drive spacing of 15 m, level spacing of 25 m, ore drive width of 5 m. Standoff distances for work areas to caving zones are typically influenced by:
- yielding and stress relaxation due to caving;
- blast vibration.

PSM recommends that all capital development is located at least 20 m outside of a zone defined by a 45° subsidence zone.

For added conservatism, however the following parameters were applied based on industry standard practice and previous experience namely:
- 10 – 50 m for access drives;
- > 70 m for primary underground infrastructure (declines, ventilation rises etc.).

Ore drive dimension play a large part in the sub-level cave draw performance, with the width and height being a significant factor. The ore drive is designed with maximum width and minimum height with square shoulders maximizing wall strength and the active draw area. The effective extraction width of the drawpoints was estimated to be between 75% and 85%.

Sub level spacing was set at 25 meter hS and this is based on a review of existing SLC information from the Mining Plus (Ausenco’s mining sub-consultant) database. Sublevel spacing greater than 25 meter will lead to using specialized drill equipment.

Estimated caving draw height (ht) of 37.5 m for each draw point. The effective draw point extraction width (a) of 4 meter. An Extraction Ellipsoid Total Width (WT) of 13.2 meter.

Ore Extraction and Transport
Ore from the production drifts will be extracted by Load-Haul-Dump (LHD) units. The LHD selected for ore hauling has a capacity of 21 metric tons offered by industry leaders Epiroc’s ST18 model and Sandvik’s LH621 mode. These models fit the 5 meter by 5 meter (W x H) profile of the production drift.

Ore from the LHD is tipped to ore passes placed with metal grizzlies on top. Although it is not expected that large fragmentation of ore will report to the ore passes the grizzlies reduces the chance of hang-ups nonetheless. At the bottom of the ore pass, LHDs collects the ore and feed them to 60 metric tons capacity trucks, either Epiroc’s MT65 model or Sandvik’s TH663 model. At some instances, the LHD will transfer ore from the ore pass bottom into designated stockpiles prior to loading into trucks.

Trucks will haul the ore from underground to the Run-of-Mine (ROM) stockpile at the surface. Sufficient truck fleet will be maintained to achieve 12,000 metric tons per day of ore hauled to the ROM stockpile.

Some of the lower grade materials, after crushing at the surface will be hauled back underground to be backfilled as road base for the decline. A second materials handling decline will be commissioned at a later time to augment truck route as orebody is mined deeper. This materials decline will also serve the adjacent orebody Bayugo when development commences.

Crushing
The Run-of-mine ore (ROM) from Boyongan underground mine is delivered to the ROM pad by rear-dump trucks. During normal operation, material from the underground mile will be directly dumped into the ROM bin. The ROM pad is designed to stockpile approximately 24 hours of ore to allow for storage of ROM material during maintenance or outages.

ROM ore, with a top size of 550 mm, is fed into the 120 t capacity ROM bin by a front end loader (FEL). Coarse ore is reclaimed at a controlled rate by an apron feeder and discharged into the primary crusher sizer.

Crushed ore, at < 80% passing (P80) 80 mm, discharges onto the SAG mill feed conveyor fitted with a weightometer and belt monitoring systems.

In the event of primary crusher sizer maintenance, ROM ore is loaded onto a mobile grizzly with a screen aperture of 600 mm by a FEL. The grizzly oversize is returned to the ROM pad, while the undersize material is transferred to the 18 hour emergency stockpile using rear-dump trucks. Ore from the stockpile is fed onto the SAG mill feed conveyor by FEL via an emergency reclaim hopper and emergence emergency reclaim belt feeder.

Grinding
The grinding circuit consists of a SAG mill of 8.5 m diameter by 5.8 m EGL operating in closed circuit with a primary cyclone cluster. Crushed ore is fed to the SAG mill at a feed rate of 496 t/h (dry) at 4.0 Mt/y. Product from the grinding circuit (cyclone overflow) has a nominal density of 35% w/w solids and particle size P80 of 90 µm.

The SAG mill feed conveyor transfers crushed ore and scats recycle to the SAG mill feed chute where it is combined with mill feed dilution water to control the mill pulp density to approximately 70% w/w solids. The 6.2 MW SAG mill is fitted with a single pinion drive with variable speed drive.

The SAG mill product discharges via 40 mm discharge grates and trommel screen with 30 x 5 mm slotted aperture. Oversize from the SAG Mill trommel is transported by pebble recycle conveyors onto the SAG mill feed conveyor and returned to the SAG mill. Trommel undersize gravitates into the cyclone feed hopper and is pumped to the primary cyclone cluster using a variable speed cyclone feed pump.

The cyclone underflow reports to the SAG mill, while the overflow gravitates to the flotation circuit via a trash screen.

SAG mill grinding media is transferred, from the storage bunkers adjacent to the primary sizer, by FEL and loading into the SAG Mill Ball Feeder (2230-PK-004). The ball feeder has an indexer that feeds ball at a rate set by the plant operator. A SAG mill liner handler and liner removal tools are provided for relining the mill.

Initially copper and gold will be recovered thru a series of copper leaching and gold leaching process which produces copper cathode and gold-silver dore respectively. At year 3 of operations, copper flotation will be added to the process and will produce copper concentrate containing copper and gold as the sulfides parts of the orebody are started to be mined. The mill tailings will be stored in the TSF.

The crushing, grinding, copper flotation, copper leaching, gold leaching and tailings impoundment processes of the metallurgical plant will be designed to match the 4 Million metric tons per year output of the sub-level cave mine.

Silangan’s ore is a mix of copper oxide and sulfide minerals. This complex mineralogy dictated the employment of an intricate metallurgical flowsheet, which results to three saleable products. The Run-of-Mine (ROM) ore will average to 0.63 percent copper and 1.20 grams per metric ton gold. After processing, ROM will be converted to the ........