DIAMONDIFEROUS VOLCANICLASTICS OF THE BUFFALO HEAD HILLS KIMBERLITES, NORTHERN ALBERTA, CANADA

Exploration in the Buffalo Head Hills region has uncovered thirty-six diamondiferous kimberlites; however there is little previous work on the volcanology and characterization of these occurrences. In this study at least three distinctive volcaniclastic units are recognized, two of which are primary pyroclastic deposits, not typically preserved in most kimberlites. These pipes are distributed over 6000 km2 and are unusual in that they occur within the Proterozoic Buffalo Head Terrane rather than within an Archean craton. The kimberlites erupted through Proterozoic basement, Devonian sedimentary and Cretaceous sedimentary rock, and were covered by Quaternary till.

The three types of volcaniclastic kimberlite identified represent diverse depositional styles characteristic of distinctive processes. Type 1 comprises well-sorted, ash-sized, fine-grained, olivine crystal-rich layers interbedded with dominantly lapilli-sized, juvenile fragment-rich layers. This unit is characterized by both cross-stratified, finely bedded deposits similar to those generated by base surge, and finely bedded, well-sorted, normally graded deposits characteristic of pyroclastic fall. Type 2 is dominated by juvenile, accretionary and cored fragments, many of which show multiple magmatic rinds. This deposit type is interpreted as a series of eruptions through previously emplaced but unconsolidated pyroclastic kimberlite and is typical of proximal crater fill through pyroclastic fall or slumping. Type 3, consists of poorly-sorted, subtly bedded kimberlite. It is a crystal -rich deposit, depleted in fine grained matrix component, with little juvenile or accretionary material. Future work will discern the depositional process(es) for this deposit.

The deposits identified represent both intra-crater near vent (types 1&2) and either extra-crater or distal intra shallow crater deposits (type 1). Both types 1 and 2 have been observed within a single kimberlite pipe and further work will determine if these deposits are different facies of a single eruption style or whether there was a range of volcanic activity within this province.