The property straddles the Piché Group volcanic rocks that separate Pontiac Group metasedimentary rocks to the south from Cadillac Group metasedimentary rocks to the north. In the property area, all lithologies strike east-west and dip steeply south at approximately 85°.

The The Cadillac–Larder Lake Fault Zone (CLLFZ) is a major regional crustal break that consists mainly of chlorite-talccarbonate ultramafic schist, and ranges in thickness from 100 to 300 ft (30 to 100 m) in the mine area, and narrows significantly to about 40 ft (12 m) wide to the east of Zone 36 East. Across the property, the fault is subparallel and close to the Piché Group-Cadillac Group contact, but is generally enveloped by Cadillac Group sedimentary rocks (argillites, greywackes and, to a lesser extent, chert).

Greenstone-hosted quartz-carbonate vein deposits occur as quartz and quartzcarbonate veins, with valuable amounts of gold and silver, in faults and shear zones located within deformed terranes of ancient to recent greenstone belts commonly metamorphosed at greenschist facies (Dubé and Gosselin, 2007). Greenstone-hosted quartz-carbonate vein deposits are a subtype of lode gold deposits (Poulsen et al., 2000). They are also known as mesothermal, orogenic. They consist of simple to complex networks of gold-bearing, laminated quartz-carbonate fault-fill veins in moderately to steeply dipping, compressional brittle-ductile shear zones and faults, with locally associated extensional veins and hydrothermal breccias. They can coexist regionally with iron formation-hosted vein and disseminated deposits, as well as with turbidite-hosted quartz-carbonate vein deposits. They are typically distributed along reverse-oblique crustal-scale major fault zones, commonly marking the convergent margins between major lithological boundaries such as volcanoplutonic and sedimentary domains. These major structures are characterized by different increments of strain, and consequently several generations of steeply dipping foliations and folds resulting in a fairly complex geological collisional setting.

Typically, the alteration haloes are zoned and characterized, at greenschist facies, by iron-carbonatization and sericitization with sulphidation of the immediate vein selvages (mainly pyrite, less commonly arsenopyrite).

The main gangue minerals are quartz and carbonate with variable amounts of white micas, chlorite, scheelite and tourmaline. The sulphide minerals typically constitute less than 10% of the ore. The main ore minerals are native gold with pyrite, pyrrhotite and chalcopyrite without significant vertical zoning. (Dubé and Gosselin, 2007).

Due to the narrow vein nature of the orebody, two (2) underground mining methods were considered in the study, modified Avoca and long-hole mining with captive sublevels.

The mineralized material will be transported to surface using a combination of 3.5-cubic-yard to 6-cubic-yard scoop trams and 30-tonne trucks. Waste material will be used to backfill mined out stopes as much as possible or will be brought to surface and stored on a dedicated waste pad.

The current PEA is based on an underground mine with access by decline to a vertical depth of 550 metres in the 36E area and 250 metres in the Kewagama area.

The expected average daily production rate during the production period is estimated in this PEA between 450 and 500 t/day. The overall project mine life is expected to be approximately 6 years, including a two-year pre-production period.

The modified Avoca mining method was mostly used in the present study.

The modified Avoca mining method consists of drilling a series of vertical holes downward into mineralization from one level to another. The mineralization is then blasted in vertical slices, and the broken material ends up in the bottom sill and is extracted using LHDs. For every sill and sublevel horizontal slice, a primary slot opened by drop raise method is excavated at each extremity of the level, and blasting of a first stope 18 to 22 metres in length along strike is achieved using a longitudinal retreating process. All the broken mineralized material is extracted before another slice is blasted to ensure maximum recovery of the mineralized material should any unplanned caving occur. Once the stope is completely mined out, waste rock is dumped in the empty stope as uncemented rock fill. To be able to blast the second stope of the same level, a void must first be created by pulling out some of the backfill of the first fully-backfilled stope. The second stope is then blasted, mucked and backfilled. The process is repeated until all sublevels are mined out. This mining method can also be referred to as longitudinal long-hole retreat mining method. Some parts of the mucking and backfilling steps are performed with remotely operated LHDs for safety reasons.

In some areas, the captive long-hole method was an economically better choice than the modified Avoca mining method. Long-hole stopes will be mined from 3-metre-high sublevels at ±15-metre vertical intervals. It is assumed that stopes will be backfilled. Pillars will be left in place between panels and mining horizons. It was assumed that pillars will have a minimum width of metres or 1.5 times the width of the stopes.

The method consists in drilling and blasting 63.5-mm-diameter holes in a pattern parallel to the walls. Holes are drilled upward or downward depending on the context.

The development sequence consists in accessing the mineralized zone and excavating a level cut in the mineralized zone. The mining sequence will require the excavation of a raise opening, which is either developed as a conventional raise or as a drop raise when a top access is available. Once development is completed, the mineralized zone is surveyed with precision for the preparation of the drilling and blasting pattern.

The recent metallurgical testwork has demonstrated the amenability of O’Brien mineralized material to the gravity, leaching and flotation processes.

The O’Brien Project is planned for a five-year period at a production rate of 500 tpd. Five gold concentrators located within a 75-km radius were then identified as being able to potentially process the O’Brien material: the Kiena Mill, the Sigma-Lamaque Complex, the Camflo Mill, the Westwood Mill and the Aurbel Mill.

The Westwood and Aurbel mills have shown interest. This PEA is based on the use of the Westwood mill.

The ore is first dumped on a heavy-duty grizzly at the mine site. The oversize rock (> 18 inches) will be crushed by a contractor. The ore will be loaded and trucked to the Westwood mill.

The plant will process ore at a rate of 2,400 tonnes per day for an entire month. During this period, the circuit will be dedicated to custom milling. Ore extracted at the Westwood mine will be stockpiled for later processing.

The O’Brien Project will accumulate material during approximately 145 days before conducting a milling campaign. At this rate, there will be 2 to 3 milling campaigns per year.

Process description
The crushing circuit is composed of a jaw crusher. The product is transported by conveyor belt to an ore bin for storage.

The grinding circuit is composed of a SAG mill and a ball mill.

The SAG mill and the ball mill run in closed circuit with their cyclones. The underflow product from the ball mill feeds the gravity recovery circuit.

The gravity circuit is composed of a Knelson concentrator to recover free gold. The Knelson concentrate is treated on a shaking table. The gold concentrate is then further treated in the refinery.

The cyclone overflow is sent to a trash screen. The underflow goes to the leach circuit after it has been thickened by a thickener.

In the leach circuit, cyanide is used to dissolve the gold. Each leach tank is equipped with an agitator mechanism and oxygen lines.

The discharge of the leach circuit flows to the carbon-in-pulp (CIP) circuit. The slurry goes from one tank to the other by gravity. Interstage screens prevent carbon from being carried away with the slurry. Carbon is pumped counter-current to the slurry.

The combined volume of the leach and CIP circuits provides a residence time of approximately 72 hours.

Loaded carbon is pumped from the CIP tank onto a screen, which returns the underflow slurry to the tank. Carbon is then sent to an acid wash column to eliminate carbonates. From there, the carbon goes into an elution column.

The cooled pregnant solution is sent to the electrolysis cells located in the refinery. The drying oven and furnace are also used to treat the shaking table concentrate. The doré ingots are stored in a safety vault.