Summary:
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).