Kimberlites and lamproites are volcanic and subvolcanic varieties of ultramafic rocks and are the main hosts for terrestrial diamonds. The vast majority of global primary diamond mines are hosted in kimberlite, and this rock type is the target at the Chidliak project. Kimberlites are mantle-derived, volatile-rich ultramafic magmas that transport diamonds from depths of 150 to 200 km to the earth’s surface, together with fragments of mantle rocks from which the diamonds are directly derived (primarily peridotite and eclogite). Kimberlites occur at surface as volcanic pipes, irregular shaped intrusions, or sheet-like intrusions (dykes or sills). Due to the wide range of settings for kimberlite emplacement, as well as varying properties of the kimberlite magma itself (most notably volatile content), kimberlite volcanoes can take a wide range of forms and be infilled by a variety of deposit types, even within a single kimberlite field, like Chidliak.

The Chidliak kimberlites are stratified bodies and different pipes contain different types of infill ranging from VK (Volcaniclastic Kimberlite) only to mixed VK, ACK (Apparent Coherent Kimberlite) and CK (Coherent Kimberlite) deposits (referred to as combined infill pipes). None of the Chidliak pipes contain massive VK-type infills like observed in many southern African kimberlites and in Canadian pipes at Gahcho Kué or Renard (Field and Scott Smith, 1999; Field et al., 2008, Fitzgerald et al., 2009; Hetman et al., 2004). The Chidliak kimberlites also differ from many other Canadian kimberlites, such as those found at Fort à la Corne and Lac de Gras. The Fort à la Corne kimberlites are large, shallow, champagne-glass-shaped pipes infilled entirely with pyroclastic kimberlite. The Lac de Gras pipes are small, steep-sided pipes characterized by an abundance of resedimented volcaniclastic kimberlite (RVK) and associated PK (Pyroclastic Kimberlite) (Field and Scott Smith, 1999; Scott Smith, 2008).

The Chidliak kimberlites do however have similarities to those at Victor in the Attawapiskat region (van Straaten et al., 2009) with respect to their general emplacement and types of pipe infill. The timing of kimberlite magmatism at Chidliak roughly corresponds with that of some of the younger intrusions in the Attawapiskat province (Heaman et al. 2012), which were also intruded into a Paleozoic carbonate-dominated sequence. Unlike at Chidliak, some of the Paleozoic strata are preserved in the Attawapiskat region and the Chidliak bodies may be deeper analogues of Victortype PKs (Pell et al., 2013).

The diamond content of the Chidliak pipes is controlled by the efficiency of sampling diamondiferous mantle material at depths of 150 to 200 km, and rapid transport to surface. At Chidliak, any kimberlite with significant total mantle-derived garnet content is assessed as potentially having significant diamond content, especially if eclogitic or websteritic garnets are present (Pell et al., 2013).

It is planned to open pit (OP) mine the Chidliak CH-6 and CH-7 deposits. Mining of the deposits will produce a total of 9.5 Mt of processing plant feed and 75.1 Mt of waste (7.9:1 overall strip ratio) over a 13-year mine life.

The current life of mine (LOM) plan focuses on achieving consistent plant feed production rates, and early mining of higher value material, as well as balancing grade and strip ratios.

The pit shells for Chidliak were further analyzed and optimizations were conducted in order to better define the possible stage shapes within the ultimate pit limits. It was decided to divide the pit sequence into three stages at CH-6 and two stages at CH-7 for the mine plan development to maximize the grade in the early years, reduce the pre stripping requirements, and to maintain the process facility at full production capacity.

The current LOM plan focuses on achieving consistent processing feed production rates, mining of higher value material early in the schedule, balancing grade and strip ratios, while trying to maximize NPV. Mining would commence at CH-6 and then moves onto CH-7 later in the mine life.

Year -1 represents the planned commencement of pre-stripping at CH-6. The average mining rate over the LOM will be 16,500 t/d, reaching a maximum of 28,200 t/d from Year 1 to Year 6. During full production, the mine, on average, is estimated to produce 1.3 Mct/a with a LOM average mining head grade of 1.8 ct/t.

Waste material from CH-6 will be used to construct the TMF containment facility and also be placed in a waste rock storage facility (WRSF) adjacent to the pit. Waste material from CH-7 will be placed in a separate WRSF near the CH-7 deposit.

Stockpiling of run-of-mine ore is intentionally limited, with small amounts of plant feed stockpiled adjacent to the process plant in the pre-strip period (111 kt).

The process plant site will be located near the CH-6 deposit and plant feed from both CH-6 and CH-7 will be hauled to the plant with the mine truck fleet.

The Chidliak process plant will have a capacity of 2,000 t/d (0.7 Mt/a). The preliminary flowsheet is based on existing Canadian diamond mine operations and will be optimized at a later engineering stage when process and equipment specific test work results become available. Kimberlite processing and diamond recovery at Chidliak will involve:
- Run of Mine (ROM) stockpiles - to allow kimberlite blending and oversize waste rock removal;
- Primary and secondary crushing - jaw and cone crusher plants;
- Crushed feed stockpile - to provide buffer capacity between primary crushing and the main plant;
- Scrubbing and de-gritting – fines removal, friable rock and clay deagglomeration;
- Tertiary crushing – high-pressure grinding rolls (HPGR), diamond liberation, fines generation;
- DMS - heavy mineral, diamond concentration;
- Recovery:
o Wet high intensity magnetic separation;
o Wet X-ray sorting;
- Drying;
- Dry single particle X-ray sorting;
- Grease table;
- Diamond cleaning; and
- Diamond preparation - for valuation prior to shipment off-site.

A batch plant planned to be located near the process plant building will be used for main plant audits, mine grade control and exploration sample treatment.

Run of Mine (ROM) crushing to prepare sized process plant feed will consist of a conventional primary jaw and secondary cone crushing circuit with closed circuit screen. The CH-6 and CH-7 kimberlites are classified as medium-hard plant feed containing little clay, even in the upper weathered kimberlite zones.

To reduce on-site installation and construction costs, portable jaw and cone crushing with screening plants are proposed. These will be enclosed within a building structure to provide equipment and personnel protection from the Arctic climate. Heating control will maintain the crushing section operating temperature at approximately 0°C during the sub-zero temperature winter months.

The crushing plant will be fed by a front-end loader from the ROM stockpiles. This will allow plant feed to be pre-sorted for the removal of large oversize material and waste rock and also for plant feed blending capabilities ahead of the crushing plant.

Crushed plant feed from the primary and secondary crushing section product conveyor discharges into a 50 t process plant feed surge bin equipped with belt weigh feeder to control feed to the process plant feed conveyor. Excess plant feed from the bin discharges via a bypass chute onto the crushed plant feed stockpile stacking conveyor. Sufficient covered storage will be provided for approximately 4,000 t or 2-days plant feed capacity. Crushed stockpile material will be fed to the process plant by front-end loader (FEL) via a static grizzly, feed hopper and belt weigh feeder onto the process plant feed conveyor. Ferrous tramp metal removal will be allowed for at the discharge of the plant feed conveyor.

Initial mini-bulk and bulk sample processing data (refer to Section 11) indicates that the amount of clay, even in weathered kimberlite, to be expected in the plant feed is very low. When additional metallurgical and equipment vendor data is available the flowsheet location of the drum scrubber(s) will be determined at a later engineering stage.

The plant feed material will be sized and washed on horizontal deck and banana type vibrating screens. The -1 mm fines report to the fines pumpbox and the -75 + 30 mm size fraction will be conveyed to a HPGR feed bin equipped with a belt feeder fitted with metal detection to feed the HPGR.

HPGR product, loosely compacted flake, will be conveyed to a rotary drum scrubber for deagglomeration. Scrubber discharge will be sized on horizontal deck and banana type vibrating screens, +30 mm material reporting to the HPGR feed bin, -30 +1 mm to the DMS feed bin and -1 mm to the fines effluent pumpbox.

The HPGR has been the main comminution technology used in Canadian diamond plant designs since the mid-1990s and has considerably improved process efficiencies and diamond recoveries, when compared to previous flowsheet designs, due to the increased diamond liberation and reduced diamond breakage.

A single size fraction DMS plant has been proposed to minimize the overall plant footprint, reduce DMS plant capacity redundancy and section over sizing requirements.

The proposed 150 t/h DMS plant will use three 420 mm diameter pump fed cyclones to reduce the material conveying and elevation required for gravity fed cyclones.

Material will be fed at a controlled rate from the DMS feed bin to the dense medium cyclone feed mixing box where the kimberlite material is mixed with ferrosilicon (FeSi) medium at the correct density. The cyclone feed pump transports the kimberlite material and medium slurry from the mixing box to the dense medium cyclones, where particles will be separated based on their density.

The higher density particles move down the inner wall of the cyclone and discharge through the apex. The lower density particles move towards the central axis of the cyclone and exit from the vortex finder. Both cyclone products report to medium drain and rinse screens to recover the ferrosilicon, the drain section to the correct medium circuit and the rinse section to the dilute medium circuit where the FeSi is recovered by a wet drum magnetic separator and returned to the correct medium circuit.

Correct medium densification, non-nuclear density measurement with Programmable Logic Controller Proportional-Integral-Derivative (PLC PID) control will be used to provide an operating medium density to within 0.01 SG units of the medium set-point density.

DMS concentrate will discharge into a Recovery Plant feed surge bin, DMS concentrate areas will be secured to limit access and will be CCTV monitored.

Dense medium cyclone float material will be sized on a double deck drain and rinse screen, -30 + 8 mm oversize returning to the HPGR feed bin.

The recovery plant flowsheet and supply requirements need further development when additional metallurgical and equipment vendor data is available. Recovery plant supply options will include:
- A dry X-ray plant fabricated off-site;
- A wet X-ray plant fabricated off-site; and
- A simplified wet X-ray and grease table plant constructed on-site. This option was used in the order of magnitude cost estimate and based on Northern diamond mine recovery plant costs.