Stratigraphy of the intra-crater volcaniclastic deposits of the Victor Northwest kimberlite, northern Ontario, Canada
Section snippets
Introduction and regional geology
Kimberlites generally occur as pipe-shaped deposits that are formed by explosive volcanic eruptions of crystal-rich, low viscosity, low volume and intra-cratonic magmas. Worldwide, these kimberlite pipes are the most important source for diamond, however their emplacement mechanisms are as yet not fully understood. The key in unravelling the emplacement history of these volcanic pipes is establishing the detailed volcanic facies architecture of the deposit. This type of research can provide
Terminology
In this study, we use the following terms. Volcaniclastic kimberlite (VK) refers to all rocks or unconsolidated deposits that consist of volcanic fragments, irrespective of the mode of fragmentation or final deposition (Cas et al., 2008). Hence, these deposits encompass both primary pyroclastic (pyroclastic kimberlite, PK) and resedimented volcaniclastic kimberlite (RVK) deposits. After careful study, some of the volcaniclastic kimberlite units are interpreted to be pyroclastic kimberlite (PK).
Methods
The external shape and internal geology of the Victor Northwest (VNW) kimberlite pipe have been reconstructed using a total of 22 drill cores (Fig. 1, Fig. 2), with a total cumulative length of ~ 4.2 km. All drill cores intersecting VNW were drilled in between 1998 and 2003 by De Beers Canada Inc. Initial drill core logging for advanced exploration, evaluation and geotechnical purposes was carried out by De Beers and associated personnel, and has been presented in Webb et al. (2004). For this
Pipe morphology
The contact between the Victor Northwest kimberlite and the host country rock has been intersected in only five drill cores (Fig. 1B). Of these five drill cores, only one drill core intersects the kimberlite to country rock contact above ~ 100 mbs. The four other drill cores intersect the contact between ~ 200 and 300 mbs. As a result, the confidence in the external pipe shape is low at shallow levels and moderate at deeper levels. Geophysical surveys have been used to aid the reconstruction of the
Lower stratigraphy
The lower stratigraphy comprises units BK-L1, DCK-L1, DCK-L2, BK-L2, CRB-L1, CRB-L2 and RVK-L (Fig. 2, Fig. 3, Table 1). BK-L1 is found at the pipe wall (Fig. 2, Fig. 3), and we interpret it to be the first unit to form after the pipe excavation. This unit is bedded, contains both VK and DCK, is variably country rock fragment-rich, broken olivine-rich, contains clastic (ash-armoured) pyroclasts (Fig. 9A, B) and the matrix is inferred to be ash-rich. Based on the presence of broken olivines at
Discussion and conclusions
There is a striking similarity between the order of kimberlite units that comprise the lower and the upper stratigraphy of the pipe (Fig. 3, Table 1). Both stratigraphic sequences start with an early volcaniclastic kimberlite containing abundant broken olivines, followed by dark and competent kimberlite, and capped by country rock breccia deposits. We interpret that the Victor NW kimberlite pipe was formed by two similar eruptive cycles that are described below. Cycle 1 comprises all six units
Acknowledgements
The results presented in this paper were collected as part of a Ph.D. project by BvS on the volcanology and petrology of the Victor North kimberlite pipes. We would like to thank De Beers Canada Inc. for its financial support, and its permission to publish this paper. This research would not have been possible without the extensive logistical assistance of De Beers' employees in Sudbury and Toronto; in this regard, we would especially like to thank Stephan Kurszlaukis, Brad Wood, Cliff
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2017, Gondwana ResearchCitation Excerpt :As a result, much of our existing knowledge and understanding about the petrology, geochemistry and genesis of kimberlites is strongly influenced by studies on hypabyssal kimberlites. Pyroclastic kimberlites, within crater as well as extra-crater, are relatively the least preserved of all kimberlite types but are known from (i) Mwadui, Tanzania (Stiefenhofer and Farrow, 2004), (ii) Orapa A/K/1 and Jwaneng, Botswana (Field et al., 1997; Brown et al., 2008), (iii) Tokapal, Central India (Mainkar et al., 2004), (iv) Igwisi Hills, Tanzania (Willcox et al., 2008), (v) Mbuji-Mayi, Democratic Republic of Congo (Demaiffe et al., 1991), and (vi) Ontario, NW Territories, North Alberta and Fort à la Corne, Canada (Eccles et al., 2004; Zonneveld et al., 2004; Nowicki et al., 2008; Grunsky and Kjarsgaard, 2008; van Straaten et al., 2009). However, a great majority of the studies yet carried out on them addresses various aspects of their geology, volcanology and emplacement mechanism, discriminating various volcanic facies within the larger pipes, diamond prospectivity and geochronology.
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Metal binding to dissolved organic matter and adsorption to ferrihydrite in shallow peat groundwaters: Application to diamond exploration in the James Bay Lowlands, Canada
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