The main Gahcho Kué kimberlite cluster comprises four kimberlite bodies: Hearne, 5034, Tuzo, and Tesla. Hearne, most of the 5034 Pipe, and the Tuzo and Tesla occur under Kennady Lake, which has an average depth of 8 m. The Tesla kimberlite body is not part of the current declared resources or reserves.

Gahcho Kué kimberlites are overlain by varying thickness of glacial boulder outwash and lake sediments (averaging 10 m thick), and have a combined water and sediment cover as much as 25 m thick.

Drill information suggests that Tuzo and 5034 are located on an inclined feeder dyke system, the GK dyke, which dips roughly 25 degrees NNE. A vertical feeder dyke, as it is common for most maar-diatreme volcanoes, was not identified. Hearne is located on another feeder dyke system which dips to the North. The feeder dyke systems were repeatedly active during emplacement, resulting in a complex facies architecture of the kimberlite bodies rising from the feeder dykes.

The kimberlite bodies are steep-sided comprising of several texturally distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite (HK) to diatreme facies tuffisitic kimberlite (TK). TK displays many diagnostic features including abundant unaltered country rock xenoliths, pelletal lapilli, serpentinized olivine and a matrix composed of microlitic phlogopite and serpentine without carbonate. HK contains common fresh olivine set in a groundmass composed of monticellite, phlogopite, perovskite, serpentine and carbonate. A number of texturally hybrid kimberlite rocks display a textural gradation from TK to HK, which is characterized by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity, proportion of fresh olivine and the degree of xenolith digestion (Hetman et al., 2004).

Hearne Kimberlite
Scott Smith (2005) modelled two bodies to comprise the Hearne kimberlite, Hearne South and Hearne North. Further analysis and additional drilling in 2018 has shown that these bodies connect as lobes of a single kimberlite body (Fulop & Pell, 2019). The Hearne kimberlite has smooth, steep-sided walls, and covers an area of about 1.5 ha. At surface, Hearne measures a maximum of 380 m x 90 m from north to south with its largest width occurring at the south end in the former south pipe. Average widths are approximately 40 m at surface. The south lobe is dominantly TK, and the north lobe consists of approximately equal amounts of HK and TK. The present pipe geological model for the Hearne South extends to 125 masl. Additional drilling has proven that the body does not extend beyond this depth; Its limited depth is most likely controlled by a steep N-dipping feeder dyke system. The north lobe narrows to less than 10 m width in the centre of the body at approximately 130 m depth below lake-surface. There is also evidence that the north lobe extends below 100 masl.

5034 Kimberlite
The 5034 kimberlite is a highly irregularly-shaped pipe and dyke complex that is comparable to kimberlite root zones elsewhere and has a surface area of approximately 2.1 ha.

The main part of the 5034 occurrence that reaches the surface occurs under Kennady Lake and can be divided into four lobes: West, Southwest Corridor, Centre and East. These lobes are joined at the surface, but separate at depth. The Centre and East lobes are modelled separately at shallow depth, but rejoin at greater depth producing what appears to be a window of granite within the kimberlite. The East and North lobes are joined at depth, geologically continuous, and are collectively referred to as the Northeast Lobe, now including the Northeast Extension (Kurszlaukis et al., 2019). The surface measurements of the four lobes of the 5034 Main Pipe are approximately as follows:
• West Lobe – 125 m x 45 m;
• Centre Lobe – 125 m x 80 m;
• North-East Lobe – 85 m x 65 m; and
• Southwest Corridor – 330 m x 40 m.

Tuzo Kimberlite
The overall surface area of the Tuzo body is roughly 1.2 ha, which is covered by as much as 25 m of water of and glacial overburden. The kimberlite body comprises various fragmental and coherent kimberlites, and it contains abundant inclusions of the surrounding granitic country rock. The 2007 drill program results improved the definition of the shape of the kimberlite body, which is unusual as it widens towards depth from 125 m in diameter near the surface to roughly 225 m diameter at 300 m depth.

Mineralization
Hearne Kimberlite
Five different phases of TK were recognised within the Hearne kimberlite.

Hearne North Lobe
A major TK unit in Hearne North is the HNTKN that occupies the upper northern part of the main pipe. This TK contains <15% of granite xenoliths, but does contain autolith-like bodies and magmaclasts. The TK grades with depth into transitional textures resembling HK. The transition zone was termed HNTKNt. Below the transition zone is HK, some of which appears to be of the same phase of kimberlite as the overlying TK and TKt. The internal contact separating the TKN and TKNt is sub-parallel to the contact with the underlying HK. Both internal contacts dip at approximately 50° to the north. The HK immediately underlying the HNTKNt is thought to be part of the same phase and was termed HNHKN. This interpretation is supported by the similarity in macro-diamond grade between the textural varieties of kimberlite. These three textural units (HNTKN, HNTKNt, and HNHKN) represent the transition from the diatreme to the root zone within a single phase of kimberlite.

Hearne South Lobe
Based on geological interpretations from limited core drilling, this body appears to be composed mainly of uniform diatreme-facies TK, containing as much as 50% granite xenoliths. The TK unit was named HSTKM. A separate transitional HK/TK was proposed and named HSTKW. The macro-diamond grades in both of the above units are similar.

5034 Kimberlite
Kryvoshlyk (2008) reported that the diamond distribution in the 5034 North Lobe appears to follow the layered character of the kimberlite overall. Maximum concentrations of diamonds appear often located close to the “Orange Marker” — a specific petrological layer generally found between the two units comprising the majority of the pipe infill: the upper HKt and the lower HK units. Diamond count maxima in the East Lobe appear to create a lens-like body at a depth of 85 to 131 m towards its flanks and 107 to 211 m in its centre.

Tuzo Kimberlite
The kimberlite units of the Tuzo kimberlite pipe are characterised by a large variation of diamond counts in both micro as well as macro grain size classes, likely due to varying levels of dilution within the kimberlite.

Volumetrically significant lithologies with elevated diamond counts are found in the coherent HK and HKt units at depth. The fragmental lithologies in contrast have the lower stone counts, which seem to correlate negatively with country rock dilution.

The Gahcho Kué Mine employs conventional open pit truck/shovel mining methods. Waste and ore are blasted and loaded using a fleet of diesel powered trucks, shovels, drills and ancillary equipment. Waste rock will be stored in two surface mine rock piles as well as in two of the excavated pits at later stages of the mine life. Kimberlite ore is hauled to a run-of-mine storage pad where the ore is stockpiled and loaded into the primary crusher via a front end loader. Kimberlite processing creates two additional waste streams of coarse and fine processed kimberlite. Coarse processed kimberlite (CPK) is loaded into haul trucks and stacked in a pile north of the plant, while the fine processed kimberlite (FPK) is deposited via slurry into a settlement pond known as Area 2. Non-acid generating (NAG) and potentially-acid generating (PAG) waste rock is differentiated using an on-site sampling system of blast hole cuttings. PAG rock is encapsulated within the surface mine rock piles and eventually below the restored final lake elevation of Kennady Lake during period of pit backfill.

The mine design and consequent mine plan considers conventional truck / shovel mining utilizing 29 m3 bucket diesel hydraulic front shovels, a 17 m3 front-end loader and 218 t class haul trucks to mine kimberlite and waste. This large fleet is augmented by 12 m3 bucket front-end loaders, scaling excavators and five 90 t haul trucks. Production drill and blast activities are supported by a fleet of rotary blast hole drills drilling 251 mm diameter holes. Pre-shear and auxiliary drilling is conducted by down the hole percussion drills drilling 171mm diameter holes.

Pit designs were developed using optimized Whittle shells as a basis, and these were used to develop the mine production plan and schedule. The mining sequence is optimized to smooth waste stripping requirements, while ensuring adequate kimberlite exposure to meet kimberlite feed requirements, as well as mine rock storage considerations within the Hearne and 5034 pits throughout the mine life.

Pre-stripping began on land in the northern half of the 5034 pit in 2014, with the majority of the granite waste used for road, dyke and infrastructure pad construction. Unsuitable overburden material was placed in the South mine rock pile. Mining continued in the north side of 5034 until Q1 2016, when mining in the south half of the ultimate 5034 pit began. Mining in the 5034 pit will be completed in Q2 2024. From this point, mine rock from the adjacent 5034 NEX Pushback will be placed in the mined-out 5034 pit.

Mining of Hearne pit started in December 2017 and will be completed in mid-2022. Priority has been placed on mining Hearne in these years to open up in-pit waste storage capacity. Once Hearne is complete, fine processed kimberlite will be diverted from the FPK storage facility to the empty pit.

In Q2 2020, equipment will begin stripping the 5034 NEX Pushback which also incorporates the first phase of Tuzo. Tuzo ore from this first phase that will be mined with the NEX Pushback will be released starting in 2021 and will continue through to mid-2023. In mid-2024, NEX ore will be released and it will be the primary kimberlite production source until mining in the NEX Pushback is complete in Q1 2026. At this stage in the mine life, the combined mined-out area of the NEX Pushback and the 5034 pit will be used for Tuzo mine rock storage. Waste stripping in the third and final phase of Tuzo pit will begin in Q3 2023 and ore release will start at the end of 2025. Kimberlite production in Tuzo will be sustained until mid-2030.

Commissioning of the treatment plant was completed in March 2017 with steady-state production expected through to mid-2030. The annual production is a planned 3.35 million tonnes in 2020, and approximately 3.23 million tonnes from 2021 to the end of the mine life.

All three kimberlite deposits exist under Kennady Lake, and required substantial dewatering efforts prior to mining. Dewatering of the Southern portion of Kennady Lake (Area 8, 7 and 6) was completed in 2015 along with construction of the primary dewatering infrastructure exposing the 5034 and Hearne deposits. Completion of the remaining dewatering dike network and substantial dewatering of Area 4 was conducted in 2018 and 2019, which exposed the Tuzo mining area.

The mining fleet must deliver 3.2 Mt of kimberlite annually to the process plant during production and strip an average of 32 Mt of waste per year during the same period. Peak waste stripping is approximately 44 Mt per year in 2024 when stripping is conducted at both 5034 and Tuzo.

The Gahcho Kué Mine operates 365 days per year, 24 hours per day. The mine operation is run using two12 hours shifts, with the majority of operations and operational support personnel working a 14 day on / 14 day off rotation. A portion of Yellowknife based mine management and mine administrative staff work a four day on / three day off schedule working 12 hour shifts and provide a consistent management presence at the mine site.

The current mine production plan for Gahcho Kué has been developed to maximize the value of the 5034, Hearne and Tuzo reserves through a strategic mining sequence. This sequence considers internal phasing to balance strip ratios during pre-strip activities, concurrent mining of all three pits, ore stockpiling and blending as well as in-pit backfill of the Hearne and 5034 pits during the later years of the mine life. The three pits are mined in the following order and for the following durations:
• 5034 – commenced in 2014 and extends to 2026;
• Hearne – commenced in 2017 and extends to 2022; and
• Tuzo – 2020 to 2030.

The process plant is oriented along an east-west axis. Plant feed is introduced near the middle of the plant length. In the middle of the plant is the secondary cone crusher, the scrubbers and primary screening. On the west side of the plant the high pressure grinding roll crusher (HPGR) is positioned, along with the water tanks and the thickener.

The mine extracts kimberlite ore from three different kimberlite pipes: 5034, Hearne, and Tuzo. The process plant was designed to treat 3.0 million tonnes per annum of ore, however several throughput improvements have been implemented since commercial production was achieved in Q1 2017.

The 2019 Strategic Business Plan includes an increase to the process plant throughput to 3.35 million tonnes in year 2020. From 2021 onwards, the plant rate is scheduled at 3.23 million tonnes per annum to align with the ore release rate that the mine plan is able to maintain based on the required strip ratios. In years 2024 and 2026 however, the annual plant throughput will be reduced to 3.18 and 3.13 million tonnes per annum respectively when waste stripping requirements are higher and the stockpile is depleted. The planned throughput is in line with demonstrated plant operation performance, as in 2019, the year-to-date plant throughput performance to the end of October averaged 3.53 millio ........