Summary:
The main Gahcho Kué kimberlite cluster comprises five kimberlite bodies: Hearne, 5034, Tuzo, Wilson and Tesla. Hearne, most of the 5034 Pipe, and the Tuzo, Wilson 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 and is excluded from further discussion.
Gahcho Kué kimberlites are overlain by varying thickness of glacial boulder outwash and lake sediments (averaging 10 metres (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. 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.
In the near-surface, 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 have 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 Hearne South extends to 125 masl. Additional drilling has proven that Hearne South does not extend beyond this depth; Its limited depth is most likely controlled by the steep N-dipping feeder Dunn Dyke system.
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 (‘NEX’; Kurszlaukis et al., 2019), but excluding NEX deep, which has insufficient drilling to be modelled for inclusion. 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
• Southwest Corridor: 330 m x 40 m.
Tuzo Kimberlite
The overall surface area of the Tuzo body is roughly 1.2 ha, which was covered by as much as 25 m of water and glacial overburden. The kimberlite body comprises various fragmental and coherent kimberlites and contains large inclusions of the surrounding granitic country of several tens of metres in size. 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.
Wilson Kimberlite
In 2019 the Wilson kimberlite was discovered during drill testing of geophysical and geological anomalies near the Tuzo kimberlite. In contrast to the Curie kimberlite, drilling at Wilson shows no connection to Tuzo, and as such Wilson is a distinct, new kimberlite discovery. Located roughly 200 m east of Tuzo, the Wilson kimberlite lies within the open pit mine plan for the Tuzo pit. It is a roughly north-south trending, north-northeast plunging series of interconnecting sheets.
Mineralization
Hearne Kimberlite
Five different phases of TK were recognised within the Hearne kimberlite.
Hearne North
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. This unit was geologically modelled using the upper limit of HK and the lower limit of TK textures logged in both core and reverse circulation holes. 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
Based on geological interpretations from limited core drilling, this part of Hearne 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
Tuzo country rock breccias with kimberlite (CRB + K) have the lowest average diamond counts, whereas the Upper TKB, Upper/Mixed TKB and Mixed TKB all have high diamond contents, with the Mixed TKB being the highest.
Wilson Kimberlite
On the basis of 273 - 8 kg micro-diamond samples, diamond counts are roughly double in the HK modelled unit relative to the TK unit.