The geology of kimberlites from the Fort a la Corne area, Saskatchewan, Canada

Abstract

Kimberlites have been recently discovered beneath 100m of glacial sediment at Fort a la Come, Saskatchewan, Canada. Crater and extra-crater facies have been intersected in borehole core, interstratified with coastal and marine sediments of Cretaceous age. Extra-crater kimberlite is very rare, and particularly well preserved at Fort a la Come. It is encountered in five borehole intersections drilled by Rhonda Mining Corporation, sponsors of the Operation Fish Scale project, which included kimberlite research at the University of Leeds. The regional setting and geological description of six kimberlite borehole intersections are presented. In addition, the broad geodynamic conditions and the stratigraphic context are described, and from these a model for kimberlite eruption is constructed. The kimberlites are then described at a range of scales from stratal thickness and disposition, to ultra-fine diagenetic mineral growth. A textural classification is then applied to the deposits. The volcanology of the Fort a la Cone kimberlites are then discussed: these are unusual in that the craters are preserved, and are broad and flat, rather than steep sided tapering cones. A new term, pateran crater, and process of evolution is proposed for these and other kimberlites of similar morphology. The survival of these volcanic edifices in the sedimentary environment is also considered. The geochemistry of the kimberlites is presented, both bulk rock, and over 450 analyses of individual mineral grains by electron microprobe. These minerals are mostly gamet, ilmenite and pyroxene, of megacryst, kimberlite, crust, mantle peridotite and eclogite origin. The mineral chemistries are compared to those found in other kimberlites around the world, and the nature of the cratonic lithosphere is described. The P-T and compositional characteristics of the lithosphere are further refined from diamond and garnet trace element chemistry in collaborative works with Taylor and Griffin, described herein. Economic aspects of the kimberlites are reviewed, and all the conclusions are presented in time order, from diamond growth in the Archean, crustal evolution in the Mid-Proterozoic, Early Cretaceous magma generation and eruption, to Late Cretaceous reworking and burial