Haile geological history includes several major events, as listed below from oldest to youngest:

Late Pre-Cambrian to Early Cambrian (580 to 530 Ma)
• Carolina Terrane formed as part of a subduction zone-oceanic island arc complex
• Persimmon Fork Formation deposited with metavolcanic and metasedimentary rocks - foliated laminated siltstone unit is the primary host rock for Haile mineralization
• Gold mineralization at Haile assumed at ~549 Ma (Mobley, et al., 2014) by close association with molybdenite dated using Re-Os
• Transition from volcanism to basinal sedimentation ~548 Ma (Persimmon Fork-Richtex boundary)
• Richtex Formation deposited (mudstones and siltstones) conformably on the Persimmon Fork

Carboniferous (320 to 290 Ma)
• ENE structural fabric developed during NW-SE compression and emplaced granite plutons and
lamprophyre dikes within 5 km of Haile

Triassic - Early Jurassic (250 to 200 Ma)
• Diabase dikes intrude the Carolina Terrane as magnetic NW-SE-trending anomalies Cretaceous to Present (100 Ma)
• Coastal Plain clayey sands deposited over all units, thickens southeastward

Mineralization and Alteration
Haile gold mineralization occurs as en echelon clusters of moderately to steeply dipping ore lenses within a 4 km x 1 km area. Nine named gold deposits are recognized at Haile. From west to east, these deposits include Champion, Small, Mill Zone, Haile, Ledbetter, Red Hill, Palomino, Snake and Horseshoe that often show ‘pearls on a string’ alignmen. Ledbetter is by far the largest orebody (approximately 1 Moz) and includes the shallow Chase and deep Mustang deposits. Orebody geometry, depth, size, grade, mineralogy, and alteration are variable. The orientation of gold mineralization generally parallels the regional NW dipping foliation but is concentrated along the metavolcanic metasediment contact. Orebody geometry is partly controlled by orientation of volcanicsediment contacts and location of barren dacite sills. Ore lenses are typically 50 to 300 m long, 20 to 100 m wide, and 5 to 30 m thick. Ore zones are separated by barren siltstone, dacite sills and diabase dikes. The Mv/Ms contact and gold mineralization gradually deepen from west to east across the Haile district. The Mv/Ms contact at Champion has been partly removed by erosion in the west portion of the district and is over 500 m deep at the Horseshoe deposit, 4 km east of Champion. Depth and position of the contact are complicated by faulting and folding. Drilling in southeast areas around Palomino has encountered gold mineralization up to 1 km deep.

Gold mineralization at Haile is mostly hosted by laminated siltstone in the Upper Persimmon Fork Formation and is capped by less permeable coherent dacite. Mineralization is typically within 100 m of the dacite-siltstone contacts. Gold mineralization is disseminated in silicified and fine grained, pyritic siltstone with local K-feldspar and molybdenite. Small, mineralized zones at Ledbetter, Red Hill and Snake are hosted in dacite along fault zones within 15 m of the Mv/Ms contact. Gold grades in mineralized dacite are typically weaker than in the underlying siltstone and sericite alteration is stronger in the dacite. Hydrothermal brecciation is common in portions of the Ledbetter, Horseshoe, Small and Champion deposits where milled, silicified siltstone clasts occur in a fine-grained quartzpyrite matrix intruded by fingers of quartz feldspar porphyry with quartz stockwork veinlets.

Mineral zonation grades outward from quartz-pyrite ± K-feldspar + gold (QS) ? quartz-sericite pyrite ± gold (QSP) ? sericite + pyrite ± pyrrhotite ? chlorite-calcite ± epidote (propylitic). QS and QSP mineralized zones are tens of meters thick. Sericite envelopes range in thickness from tens to hundreds of meters and are controlled by protolith, permeability, and weathering. Within the mineralized zones, quartz is dominant (60% to 80%), pyrite is moderate (1% to 10%), and sericite is variable at 5% to 40%. Semi-massive pyrite zones are locally observed over thicknesses of 0.5 to 5 m, especially in the Mill Zone, Red Hill and Haile pits.

Two silicification events are observed in the mineralized zones. Early massive silicification is finely disseminated to diffuse with less than 1% pyrite. This pyrite-poor silicification correlates roughly with the 0.1 g/t Au shell. Phase two silicification is manifested as matrix fill in tectonic and hydrothermal breccias with 1% to 5% pyrite and is associated with grades more than 0.5 g/t Au. High-grade gold zones more than 3 g/t are characterized by intense silicification and more than 1% fine-grained disseminated pyrite. Pyrite grain size is typically less than 20 microns in ore zones. Pre-ore silicapyrite zones are frequently stretched, boudined and sheared. A second phase of barren, coarse, cubic, undeformed pyrite overprints the fine-grained pyrite that formed during regional greenschist metamorphism. Pyrite cubes in chloritic metamorphosed rocks are 0.5 to 1 mm in size but can be as large as 1 to 2 cm. Pyrrhotite occurs in 5 to 25 m thick halos around and on the edges of ore zones. Its ductile nature produces length: width ratios more than 5:1 in foliated rocks. Pyrrhotite formation is interpreted to be coeval with early, fine-grained pyrite precipitation.

Supergene sericite-kaolinite alteration forms large bleached, cream to white halos around the ore zones with little to no pyrite that was removed during intense acidic leaching. Strong supergene alteration caps and flanks most of the district. Strong sericite alteration is rarely observed deeper than 40 to 50 m. Numerous shallow sericite-kaolinite bodies were mined historically for paint filler.

Propylitic alteration is characterized by increased chlorite (25% to 50%) and a mottled texture with blebs of 1 to 5 mm calcite/ankerite aggregates (2% to 10%) and stockwork. Late calcite ± quartz veining is often focused along fault zones and along shear zones within strongly deformed rocks. Sigmoidal pods of strained quartz are often observed. Oxidation at Haile extends to depths of 20 to 60 m and is deepest along faults and in volcanic rocks. Hematite and goethite are strongest near surface in the saprolite and decrease at depth as weak joint stains.

Gold spatially correlates with silver, arsenic, molybdenum, and tellurium. Base metals are rare at Haile. Thin section petrography and scanning electron microscopy show that the gold occurs a native gold, gold-pyrite and gold-pyrite-pyrrhotite clusters in fine-grained silicified zones. Smeared molybdenite occurs primarily on foliation surfaces and as fine-grained aggregates in silicified zones. Molybdenite at Haile has been dated by Re-Os isotopes at 553.8 ± 9 Ma (Stein et al., 1997), which is coeval with the zircon crystallization age of 553 ± 2 Ma reported by Ayuso et al. (2005). This age correlation indicates that molybdenite mineralization was concurrent with Persimmon Fork volcanism. Seven Re- Os molybdenite ages from Haile (Mobley et al., 2014) yielded ages ranging from 529 to 564 Ma. Four of these samples produced an average age date of 548.7 ± 2 Ma (Mobley et al., 2014).

Open Pit mining
Haile is currently being mined using conventional open pit methods and OceanaGold plans to continue with current mining methods. The material encountered at Haile is a combination of soft (Costal Plains Sands [CPS] and saprolite) and hard (metavolcanics and metasediments) rock units.

CPS is loosely consolidated sand which can be mined without the need for drilling and blasting. Mineralization is not present in CPS thus drilling for the purposes of ore control and waste classification is not necessary. Saprolite is mined without blasting where possible. Saprolite is sampled for waste classification to meet the requirements in Haile’s Overburden Management Plan (OMP).

Drilling and blasting are required in all hard rock. Drilling and blasting are performed on 10 m benches. Multiple bit sizes (115 mm, 171 mm, 200 mm) are used depending on material type and application. Blast hole depth is 10 m plus subdrill; subdrill ranges from 1.4 m to 1.9 m.

The number of samples taken per blasthole is material-type dependent. Blastholes in waste are typically sampled twice on 5m intervals (top half and bottom half). Blastholes in ore are typically sampled three times at 3.3 m sample intervals.

Flitch height is variable. Waste is typically mined on a 10 m flitch and ore is typically mined on a 3.3 m flitch. Ore is usually mined with hydraulic backhoe excavators, while the majority of waste is mined with hydraulic shovels. Front-end loaders may be used in either application in back-up capacity. The haul truck fleet is a mix of 175 t and 140 t payload units.

Pit Design
The major design parameters used are as follows:
• Ramp grade = 10%
• Full ramp width = 32 m (3x operating width for 730E)
• Single ramp width = 20 m for up to 60 m vertical or six benches
• Minimum mining width = 40 m but targets between 150 to 300 m
• Flat switchbacks
• Bench heights, berm widths and bench face angles in accordance with current site-specific design criteria

Underground mining
The Project is currently being mined as an open pit mine. Economic mineralization extends below and outside of the pit extents. Mineralization is concentrated in two main zones based on vertical position which form a “horseshoe” geometry over a vertical distance of 350 m. Both zones strike NE adjacent to the siltstone-dacite contact, however, the upper zone dips about 400NW and the lower zone is nearvertical. The upper zone NW-dipping high-grade lenses of mineralization are focused along beddingparallel foliation with intense silicification. The Horseshoe fault (NE strike, 400NW dip) juxtaposes the hanging wall of upper Horseshoe against barren dacite with a sill-like geometry. This geometry extends southwestward into the nearby Snake pit. The steeply dipping Lower Zone is adjacent to the subvertical contact with barren dacite. This mineralization will be mined as an underground mine and is referred to as Horseshoe.

Based on the orientation, depth, and geotechnical characteristics of the mineralization, a transverse sublevel open stoping method (longhole) with has been selected. The stopes will be 20 m wide and stope length will vary based on mineralization grade and geotechnical considerations. A spacing of 25 m between levels is used. Cemented rock fill (CRF) will be used to backfill the stopes. There will be an opportunity for some non-cemented waste rock to be used in select stopes based on the mining sequence. The CRF will have sufficient strength to allow for mining adjacent to backfilled stopes. Paste backfill could be used instead of CRF and there is ongoing work investigating this possible change.

Mine Design
Stope Design
For the design, vertical slices were created through the orebody along 2 m strike length intervals. The model was then interrogated, filtered on cut-off, and then the slices were combined to create minable stopes.

Both top and bottom stope access are designed, as mucking will occur from the lower access and drilling/backfilling will occur from the upper access. Slashing of drifts in necessary areas has also been included in the design. Stope accesses are expected to be in waste until they intercept the stoping block, but grade control will be used to determine the exact ore/waste boundary during mining. A typical level in the upper zone is made up of approximately eight stopes across, while the lower zone has approximately two to four stopes across. The length of stopes is limited by geotechnical stability and often several stope cuts are taken. A primary/secondary stoping sequence will be used where, on any given level, primary stopes must be separated by a secondary stope. Extraction of the secondary stope can only occur after the two immediately adjacent primary stopes have been mined, backfilled and have had time to cure. Backfilling will be an integral part of the mining cycle as there is a limited quantity of stopes available on each level.

Stopes are developed using a slot. Separate slot triangulations were not constructed for each stope, but the slot tonnage of each stope is separated out and a slot activity is used for scheduling.

A relatively compact Run of Mine (ROM) area is provided for storage and re-handling of ore allowing blending to minimize variation of head grade (sulfur and gold) and rock type, into the crusher. Ore is rehandled into the crusher dump pocket by Front End Loader (FEL).

Ore is reclaimed by an apron feeder onto a vibrating grizzly that delivers scalped oversize to the primary jaw crusher to reduce the ore size from RoM to minus 100 mm. Crushed ore is conveyed for surge and storage of the recombined grizzly undersize fines and primary crushed ore in a coarse ore surge bin or diverted on to an open conical emergency stockpile for later reclaim by Front End Loader (FEL) into a reclaim bin.

Ore is reclaimed from either the surge or reclaim bins, separately or simultaneously, using apron feeders onto a SAG mill feed conveyor belt delivering into the SAG mill feed chute.

Ore is milled in the SAG–Ball Mill-Pebble Crusher (SABC) circuit. The SAG mill operates in closed circuit with a vibrating discharge screen and a pebble return circuit incorporating a surge bin and Sandvik CH-440 cone crusher. The ball mill operates in closed circuit with hydrocyclones to produce the desired grinding product size of 75 microns.

Selected flotation reagents are added in the grinding circuit. A portion of the grinding circuit ball mill circulating load is treated in a flash flotation cell with the concentrate going to the regrind circuit. The grinding circuit product passes to a bank of bulk rougher flotation cells to recover the balance of the sulfide mineralization. Thereafter, the combined flash and rougher flotation concentrates are reground in a two-stage circuit utilizing an ETM-1500 tower mill in closed circuit with cyclones to a P80 under 40 microns and then followed by an M10000 Isamill in closed circuit with cyclones to a target P80 of 13 microns.

Progressive debottlenecking and upgrades to the processing plant proceeded following successful commissioning of the process plant. The flowsheet and unit operations did not change as part of the upgrades with the target of 3,800,000 t/a capacity increase achieved with a reduced scope than that expected during the optimization study completed in 2016.

The process plant consists of the following major components:
• Crushing and conveying;
• Storage and stockpiling of ore and reclaim;
• Grinding;
• Flotation;
• Fine grinding of concentrate;
• Carbon in leach (CIL) recovery of precious metal values from reground flotation concentrate and flotation tailings;
• Acid washing and elution of precious metal values from CIL loaded carbon;
• Electrowinning and refining of precious metal value;
• Thermal regeneration of eluted carbon and recycle to CIL;
• CIL tailing thickening, cyanide recovery, detoxification and pumping of slurry to s ........