Summary:
The vast majority of global primary diamond mines are hosted in kimberlite, and this rock type is the target at the Chidliak project. 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).
CH-6 Geology
The updated CH-6 geological model has an ellipsoidal shape in plan view with steep-sided walls that dip slightly to the south-west at approximately 77°. The surface dimensions are 120 m by 100 m with an approximate area of 1.0 ha. The top of the pipe is covered by 3 m of glacial overburden that thickens to 25 m at the southeast of CH-6. The pipe expands with depth in north-south dimension down to approximately 280 mbs, below which it begins to contract, such that at 300 mbs it measures 70 m by 110 m and occupies an area of 0.5 ha. In areas where the external pipe shape is not well-constrained by drilling, the morphology was interpreted using projections of angles between drill hole contacts from higher and lower elevations, combined with knowledge of kimberlite pipe emplacement models and typical shapes for kimberlites of mixed VK and ACK, and the types of shapes of pipes observed at Chidliak.
The CH-6 kimberlite pipe is a steep-sided, slightly south-west plunging, kidney shaped to elliptical body with a surface area of approximately 1.0 ha. It was emplaced into basement paragneisses and now-eroded Paleozoic carbonate rocks. The body does not outcrop and is overlain by approximately 3 m of overburden, deepening to 25 m in the southeast. It was discovered in 2009 by core drilling the southwestern edge of a magnetically reversed geophysical anomaly.
The pipe infill comprises two volumetrically significant kimberlite units: KIM-L and KIM-C. Generally, the KIM-L and KIM-C units can be distinguished megascopically by the respective presence or paucity of carbonate xenoliths. Sharp contacts between KIM-L and KIM-C have not been observed.
The KIM-L unit is the volumetrically dominant pipe infill, comprising 89% by volume of the pipe and occurring between the base of the overburden at depth of from 3 mbs to 25 mbs and the base of drilling at 540 mbs. The upper portion of KIM-L (to approximately 40 mbs) is weathered and is referred to as wKIM-L in drill core logs. KIM-L is a dark grey to greenish black, competent, texturally variable rock with conspicuous and diagnostic Paleozoic carbonate xenoliths. These xenoliths are generally 2 to 3 cm in size, rarely up to 15 cm, with very rare blocks up to 13 m. Dilution by carbonate and country rock xenoliths is low (< 5% by volume) but locally may exceed 10% by volume. Melt-bearing pyroclasts are present in variable amounts, as are broken melt-free olivine and broken, primarily fresh garnet macrocrysts. With depth, the texture of KIM-L changes from PK to ACK, where it either lacks or contains diffuse melt-bearing pyroclasts. KIM-L is interpreted to have been emplaced by explosive volcanic processes that varied from high to low energy that resulted in pipe infill ranging from pyroclastic to apparently coherent clastogenic deposits, respectively.
The KIM-C unit occurs along the north and northeast margin of the pipe below 80 mbs (600 masl) to a drilled depth of 315 mbs (365 masl) and occupies 11% by volume of the pipe. It is a dark grey to black to greenish black, massive, homogeneous CK, distinguished macroscopically by having few or no Paleozoic carbonate xenoliths and a low gneissic basement xenolith content. No intrusive contacts are observed between KIM-L and KIM-C, and KIM-C is interpreted to be extrusive. The whole rock geochemistry and groundmass chrome spinel signature of KIM-C is identical to that of KIM-L. However, microdiamond sampling of KIM C completed in 2014 was sufficient to establish a clear difference in diamond content compared to KIM-L.
A variety of other minor units occur within CH-6, such as thin intercalated CK intervals of uncertain origin, blocks of carbonate, and rare gneissic blocks.
CH-7 Geology
The CH-7 kimberlite is a steep-sided, southwest-plunging body comprised of at least two coalescing lobes with a combined surface area of approximately 1.0 ha. It was emplaced into basement paragneisses and now-eroded Paleozoic carbonate rocks. The north-eastern part of the body outcrops and elsewhere CH-7 is overlain by an average of 3 m of overburden. CH-7 was discovered in 2009 as outcrop/subcrop during prospecting.
Five main kimberlite units, KIM-1 to KIM-5, with distinct physical, and in some cases geochemical, characteristics, have been identified at CH-7. The KIM-2 unit predominates, and occupies 61% by volume of the CH-7 pipe model. The units were named in the order that they were identified, and the numbering does not have any implications as to the genesis or the order of emplacement of the units. In addition to the five main kimberlite units, a variety of other minor units are present, such as a gneiss xenolith-bearing CK and internal blocks of both fresh and brecciated gneiss and carbonate rocks. The upper approximately 50 to 60 m of the kimberlite pipe is weathered, friable and clay-altered.