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Kimberlite volcanic facies and eruption in the Buffalo Head Hills, Alberta (Canada) Boyer, Liane Patricia

Abstract

An analysis of volcanic facies in kimberlites from the Buffalo Head Hills (BHH) of Northern Alberta, Canada, is undertaken to discover the mechanisms of kimberlite eruption and deposition in this region. Thirty-eight kimberlites have been discovered in the BHH to date; six were chosen for this study. Kimberlite bodies K6, K11, K252, K281, K296 and K300 were selected to represent the range of deposits found in BHH with the object of constraining the eruptive style of these kimberlites. The lithofacies observed in bodies K6 and K11 have characteristics consistent with deposition through pyroclastic fall and synvolcanic slump. Rare accretionary pyroclasts, abundant juvenile pyroclasts, bomb sags, and weak bedding, attest to a pyroclastic origin and the geometry of the deposits suggest proximal deposition within a crater. Body K281 lithofacies are characterized by abundant accretionary pyroclasts and alternating intervals of pyroclastic surge and pyroclastic fall deposits. Similar to bodies K6 and K11, the stratigraphy of body K281 suggests deposition within a crater, however the better developed sorting and structures suggest a less proximal, crater edge location. The deposits and geometry of body K296 include massive pyroclastic fall occurring with a crater, and finely bedded, relatively well-sorted pyroclastic surge deposits occurring in an outer tephra ring. The deposits of body K300 are also surge dominated with a geometry that implies a tephra ring location, however no intra-crater deposits have been discovered in this body. Finally, outside of the tephra ring, distal pyroclastic fall deposits and reworked pyroclastic kimberlite occur, and these are recorded in the deposits of body K252. The characteristics of each of the bodies indicate similar eruption and depositional mechanisms. Information from each of these bodies can be combined to create a composite model of an idealized kimberlite in this region. According to this model the kimberlite is composed of a pyroclastic surge-dominated tephra ring surrounding a crater that is partially infilled with pyroclastic material, and subsequently with material reworked from the tephra ring. These deposits and clast types, particularly the presence of pyroclastic surge deposits and accretionary pyroclasts, are good indications that phreatomagmatism was an important mechanism of kimberlite eruption in this region. However, the high volatile contents traditionally associated with kimberlite, in concert with an alteration assemblage indicating abundant CO₂, suggest that exsolution driven explosive volcanism is likely the dominant mechanism in kimberlite emplacement. Frequently these two end-member models have been considered mutually exclusive, however deposits in BHH require both mechanisms. The model proposed includes the concept of magmatic fragmentation through exsolution, nucleation and expansion of volatiles, as well as the incorporation of external water or wet sediments resulting in phreatomagmatic interactions. In this model phreatomagmatism is the result of turbulent mixing of a dispersed flow of magma and gases with surrounding wet sediments. This model accounts for the range of features observed in Buffalo Head Hills kimberlites, while allowing for the presence of reasonable volatile contents within the kimberlite magma.

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