Indicator mineralogy of a kimberlite cobble from an esker on the southeastern flank of the Buffalo Head Hills, northern Alberta

Abstract

This report describes the indicator mineral abundance and composition of kimberlite cobble 03-MPB-009 from an esker on the southeast flank of the Buffalo Head Hills, northern Alberta. The Buffalo Head Hills kimberlite field consists of 38 kimberlites intruded into poorly indurated Cretaceous host rocks. The source esker is the 2 km southern segment of a discontinuous chain of ice-contact sediments extending 7 km to the north. Numerous kimberlite fragments have been recovered from pits in this esker segment since 1997. Olivine and chromite are the most abundant indicator minerals in the kimberlite cobble, Cr-pyrope and Crdiopside occur only in minor amounts and Mg-ilmenite is absent. The cobble also contains pink chromian corundum and hercynite (Mg, Fe-Al-spinel), which are known minor minerals in Buffalo Head Hills' kimberlites and can also be used as kimberlite indicator minerals in the region. The indicator mineral suite found in the cobble confirms the presence of several lithologies of mantle xenoliths in the Buffalo Head Hills kimberlites: fertile garnet-lherzolite and garnet-harzburgite, sheared garnet-lherzolite, garnet-wehrlite, garnet-pyroxenite, garnet-websterite, Group II eclogite, spinel lherzolite and chromian corundum-bearing high Al assemblages that also include Mg-Al-spinel. Although several studies document mantle xenolith and indicator mineral compositions from some of the Buffalo Head Hills kimberlites, insufficient data are available to characterize the indicator mineral abundance and composition of individual kimberlites in the Buffalo Head Hills field. Thus comparison between the kimberlite cobble studied here and potential source kimberlites is vague and the cobble's source cannot be determined. Southward paleocurrent flow directions for the host glaciofluvial sand deposits combined with late-stage ice flow towards the southeast and south suggest that the likely up-flow direction to search for the cobble's source is north. The nearest kimberlites up-flow and up-ice are K5, K6, and K14, and one of these may be the bedrock source of the cobble.