The term ‘coherent’ is used in a general non-genetic sense to describe kimberlite characterised by a crystalline groundmass and lacking readily discernable evidence of fragmentation. Coherent kimberlite has been encountered in three main geological settings worldwide: sheet intrusions (dykes and sills), root zone intrusions (irregular pipes) and high-level pipe fills (steep-sided pipes). The EKATI property in the Northwest Territories of Canada provides a unique opportunity to study coherent kimberlite from a range of emplacement settings. The majority of EKATI kimberlites are small pipe-like bodies filled dominantly by volcaniclastic kimberlite (VK), including mainly resedimented and lesser primary deposits, as described by Nowicki et al. (2004, 2008). Coherent kimberlite (CK) occurs as minor sheet intrusions, classified as hypabyssal kimberlite (HK), and as volumetrically significant high-level pipe fills, termed pipe-fill CK (pfCK). A recent study of the geochemical trends in a variety of volcaniclastic and coherent EKATI kimberlites (Nowicki et al., 2008) provides insight on the formation of these diverse bodies. Variations in bulk composition of selected kimberlites were found to correlate with the emplacement settings and textural character of analysed samples. In this contribution, we confirm, expand and build on this work.
Scope and objectives of study While Nowicki et al. (2008) analysed CK samples from a variety of emplacement settings, the results were grouped and interpreted in terms of the two endmember types (HK and pfCK). This study investigates a larger set of CK samples from fourteen localities representing six contrasting geological settings. The settings are interpreted based on drill core intersections, in some cases supported by geophysics and mining excavations. The petrographic and geochemical characteristics of these samples are examined in detail in relation to their geological setting and are compared to five previously analysed samples of primary volcaniclastic kimberlite (PVK) from the Koala and Panda kimberlites.
The primary objectives of this work are to: (i) establish the extent, if any, to which the sheet intrusions within host rocks and within VK pipe fills differ from each other; (ii) determine the range and type of textural and component characteristics that can be considered diagnostic of specific emplacement settings; (iii) assess whether variations in geochemical composition of the samples can be shown to correspond with the spectrum of petrographic characteristics observed; and (iv) elucidate possible emplacement processes involved in the formation of pipe-fill coherent kimberlite at EKATI and the relationship of these to volcaniclastic processes. The study focuses in particular on petrographic and geochemical evidence indicative of varying degrees of fragmentation of the kimberlite magma, as well as the extent to which this has interacted with volcaniclastic material.
