Pyroclastic kimberlite deposits from the Victor Northwest pipe (Ontario, Canada): The transition from phreatomagmatic to magmatic explosivity

摘要

Magmas of all compositions, including kimberlites, may undergo both magmatic and phreatomagmatic fragmentation during emplacement. In this contribution we assess the extent of phreatomagmatism at different stages of the pipe formation for the Victor Northwest kimberlite (northern Ontario, Canada). Detailed drill core and petrographic observations of all volcanic facies within the pipe suggest emplacement in two volcanic cycles, each consisting of three repeated eruptive phases. Pyroclastic kimberlite formed at the start of both cycles is characterized by the presence of fine-grained, poorly sorted deposits containing broken olivine crystals, angular country rock fragments, accretionary lapilli, and variably vesicular irregular-shaped juvenile pyroclasts. All observations indicate these deposits formed by phreatomagmatism. Subsequent clastogenic coherent kimberlite deposits were formed as a result of Hawaiian fire-fountaining during the second phase of both cycles. These phases were followed by mass wasting into the partly filled crater. The evolution from phreatomagmatic to magmatic fragmentation is commonplace in volcanic systems, and our study provides the first indication that the evolution from a phreatomagmatic to magmatic eruption style is also present in kimberlite volcanoes, suggesting kimberlite volcanism is not significantly different from more common basaltic to rhyolitic systems. In addition, this research indicates that high fragmentation intensities recorded by the presence of broken olivines, accretionary lapilli, abundant (angular) country rock clasts, and retention of ash are important textural criteria for recognition of phreatomagmatic kimberlite deposits.