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The eruption of kimberlite : insights from the Victor North kimberlite pipes, Northern Ontario, Canada van Straaten, Bram Ivo

Abstract

This dissertation describes the volcanic facies, petrology and economic aspects of the diamondiferous Victor North kimberlite complex (Northern Ontario, Canada) using detailed drill core logging, petrographic observations, electron microprobe analysis, and physical volcanological calculations. This research project is aimed at improving our understanding of kimberlite emplacement models, as fragmentation and eruption mechanisms for these deposits are controversial. The results of this study show that the youngest kimberlite pipe (Victor Northwest) is filled by two similar eruption cycles. Each cycle starts with explosive crater-excavation forming predominantly pyroclastic deposits, followed by crater-filling with dark and competent rocks, and ends with volcanic quiescence resulting in formation of pipe wall collapse breccias and minor resedimented volcaniclastic kimberlite. Textural observations and eruption duration calculations suggest that the second crater-excavating eruption is phreatomagmatic in nature. This is based on the presence of fine-grained, well-mixed country rock fragment-rich, broken olivine-rich pyroclastic deposits containing small variably vesicular irregularly shaped juvenile pyroclasts as well as clastic pyroclasts. The crater-excavation stage is followed by formation of spatter-fed dark and competent clastogenic rocks. Evidence for a clastogenic origin includes the deposit morphology, presence of remnant pyroclasts, angular broken olivines, as well as the gradational nature of contacts with the enveloping pyroclastic units. Estimated eruption durations for each cycle range from days to months. The cross-cutting kimberlite pipe (Victor Main) comprises two macroscopically similar country rock fragment-poor pyroclastic kimberlites that have contrasting macro diamond sample grades. This study explains the variation in diamond grade within Victor Main by differential sampling of mantle material (incl. diamond) by two different magma batches that formed the high- and low-grade domains. Victor Main lacks textures indicative of phreatomagmatism, and the relatively long calculated phreatomagmatic eruption duration suggests that magmatic eruptions are most likely responsible for the formation of these deposits. This study concludes that, despite the generally more extreme range of physical properties of kimberlite melt, kimberlites erupt in a similar fashion as common basaltic-rhyolitic volcanoes and display a similarly diverse range of fragmentation processes and deposition styles. The geological and emplacement models presented here have broad economic implications for kimberlite exploration and mining.

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