Elsevier

Lithos

Volume 112, Supplement 1, November 2009, Pages 488-500
Lithos

Stratigraphy of the intra-crater volcaniclastic deposits of the Victor Northwest kimberlite, northern Ontario, Canada

https://doi.org/10.1016/j.lithos.2009.06.010Get rights and content

Abstract

The Victor Northwest (VNW) kimberlite is one of several steep-sided pipes in the Victor kimberlite complex. In this paper detailed logging of ~ 4.2 km of drill core and petrographic studies of hundreds of samples and thin sections are used to reconstruct the intra-crater stratigraphy of the VNW kimberlite and to constrain the emplacement history of the pipe. The results show that the VNW pipe comprises numerous contrasting small-volume volcanic facies, including dark and competent kimberlite, sedimentary country rock breccias, pyroclastic kimberlite and resedimented volcaniclastic kimberlite.

We interpret that the VNW pipe was formed by two separate eruption cycles. During the first eruption cycle, the main VNW crater is excavated and partly filled. The second eruption cycle excavates a smaller nested crater within the existing lithified pipe fill. Both eruption cycles can be subdivided into three comparable stages.

The first stage in both cycles comprises a highly explosive eruption involving crater excavation and deposition of pyroclastic kimberlite. The eruption products contain abundant broken olivines, small angular country rock fragments and kimberlite ash. All these features indicate high degrees of fragmentation resulting from high eruption intensities. The high proportion of country rock fragments in these deposits suggests continuous pipe wall erosion. The second stage in both cycles is represented by dark and competent deposits, which have low country rock fragment abundances and mostly intact olivines. Based on contact relationships and textures within these units it is suggested that these rock types are formed by lower energy eruptions, during which no major pipe wall erosion took place. The eruption came to an end during the third and last stage. In both cycles, the uppermost deposits record resedimentation of kimberlite by water. Major pipe wall collapse results in the formation of voluminous sedimentary country rock breccia deposits that cap all previous deposits.

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

References (18)

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