Summary:
The Fox is an underground project planned as part of the Ekati Diamond Mine.
The Fox pipe is hosted by granodiorite of the Koala Batholith. The Fox open pit has been mined out, but kimberlite mineralization extends at depth under the pit floor.
The Fox kimberlite pipe is roughly rectangular with dimensions of approximately 530 m (north-south) by 435 m (east-west). The pipe walls dip inwards with typical angles of about 75°, except the north wall which has a shallower dip.
The Fox pipe consists of two major domains: the Crater and Diatreme domains, respectively. The approximate Crater/Diatreme geological boundary occurs at an elevation of 300 to 315 masl; the planned run-of-mine material is entirely contained within the Diatreme domain. Large rafted blocks of granite are entrained internally throughout the pipe.
The Crater domain is dominated by mud-rich resedimented volcaniclastic kimberlite (RVK). This kimberlite is massively to crudely bedded, and contains variable amounts of fine- to medium-grained altered olivine macrocrysts and scattered small mudstone clasts set in a very fine-grained muddy to silty matrix. Large rafts of broken xenolithic material occur at the base of the crater domain.
The Fox Diatreme domain can be categorised into four main material types:
• Tuffisitic kimberlite breccia (TKB);
• Tuffisitic kimberlite (TK);
• Volcaniclastic kimberlite (VK);
• Granite or granite-rich zones.
TKB is distinctly different from kimberlites in any of the other development pipes at Ekati. It is characterised by a massive texture, high in-situ granite content (mostly small, altered clasts), relatively high olivine content, and a matrix dominated by variably clay-altered serpentine (as opposed to the clastic silt/mud that dominates the VK and RVK varieties). Granite is highly fragmented and characteristically includes 30%, small (less than 1 cm) microxenoliths or xenocrysts of feldspar and biotite derived from disaggregation of the granite. All olivine is pervasively altered to serpentine. Several varieties of TKB have been identified:
• TK is texturally similar to TKB but is characterised by a relatively fine-grain size with respect to olivine, xenocrysts and xenoliths. Granite occurs mostly as fine (less than 1 cm) altered fragments with relatively minor, scattered larger xenoliths. Xenoliths exceeding 5 cm are rare. Clay content within TK appears to be intermediate between those in VK (high) and TKB (moderate);
• VK is a volumetrically small component mostly associated with the contact zones and granite breccia zones in the lower part of the open pit. VK is characterised by a dark silty to muddy, typically friable matrix with varying amounts of fine- to medium-grained altered olivine. Based on core logging and comparison with similar rock types of other Ekati kimberlite pipes, the clay content of VK is expected to be high. The xenolith content of VK is variable but generally low or very low. This is particularly true of xenoliths exceeding 5 cm, which are rare (on average 1 every 5 m in drill core) and xenoliths greater than 10 cm, which are extremely uncommon. Microxenoliths comprise approximately 15% of VK on average. In contrast with TK and TKB, small xenoliths (less than 5 cm) within VK are often not significantly altered. All larger xenoliths (greater than 5 cm) are fresh.
Internal granite occurs as evenly distributed small, commonly altered fragments within kimberlite (1 mm to 10 cm) and as large fresh xenolith blocks (20 cm to approximately 20 m in size). The latter are not evenly distributed and are concentrated primarily in a granite-dominated breccia zones occupying the lower portion of the open pit. They are also prevalent at the margins of the diatreme, close to the wall-rock contacts.
The Fox pipe contains distinctive crater and diatreme facies, which will be waste and ore, respectively. The crater facies is 100 to 150 m thick, defined by an assemblage of resedimented volcanoclastic kimberlites very similar to that observed in other Ekati kimberlites. The black to brown material is made up primarily of variable amounts of loosely packed, angular olivine grains set in a very fine-grained, mud-dominated matrix with lesser amounts of serpentine, phlogopite, and minor calcite. The small, altered olivine grains average 1 to 2 mm in size and comprise 25% to 35% of the kimberlite. Small mudstone clasts, granodiorite xenoliths and fresh to carbonised wood fragments are scattered throughout, but are most abundant at the top of the crater. Also, a small number (<1%) of shale lenses containing as much as 6% sulphides occur in the crater facies (Fox kimberlite typically contains < 0.5% sulphides). An interval dominated by large granodiorite boulders (up to approximately 30 m) occurs in the lower part of the crater phase on the north side of the pipe. The contact between the crater facies material and the underlying diatreme phase is sub-horizontal and sharp with no evidence of intermixing.
The diatreme facies of the Fox pipe is a distinctive magmaclastic kimberlite, unique at Ekati in that it is the only phase identified to date that comprises consistently high (40% to 50%) proportions of xenolithic wall-rock materials, mostly as small fragments. The upper portion of the diatreme (upper 80 m) is a greyish-brown to brown tuffisitic kimberlite with a ground mass mineralogy similar to that of the crater facies kimberlite, but with 30% to 35% coarse olive grains (up to 5 mm). The rocks are described as highly fragmented and intensely clay-altered with a homogeneous distribution of olivine, very high concentrations (>40%) of altered, finely comminuted (>4 mm) commonly angular granodiorite xenoliths (minimum 5% of the rock), an absence of matrix carbonate and pervasive olivine serpentinization. The intensely clay-altered Fox kimberlite contains high percentages of clay dominated by sodium and potassium enriched smectite/montmorillonite clays.
Approximately 80 m below the base of the crater phase, the lower diatreme zone is a tuffisitic kimberlite breccia similar to the upper diatreme material, but with >15% xenolithic material and greenish-grey to light grey in colour due to a greater proportion of serpentine (up to 30%). Large granodiorite boulders (up to 30 m) occur sporadically throughout the diatreme facies to the limit of drilling (approximately 550 m depth). An interval dominated by the large boulders also occurs between the two diatreme zones.