Variations of olivine abundance and grain size in the Snap Lake kimberlite intrusion, Northwest Territories, Canada: A possible proxy for diamonds

Abstract

The Snap Lake hypabyssal kimberlite is a 15 degrees inclined intrusion that intrudes Archaean rocks of the Slave craton. Mapping has identified two main lithofacies within the intrusion, olivine-rich kimberlite (ORK) and olivine-poor kimberlite (OPK). Extensive bulk sampling has demonstrated that diamonds vary in both abundance and size across the intrusion. Since fresh forsteritic olivine and diamond possess similar densities they can be expected to be hydraulically equivalent. This study investigates the local scale variation of olivine grain size and content using image analysis, and whether olivine can be used as a proxy for diamond concentration and size. Direct measurement of diamonds is made difficult by extremely low concentrations of around 0.4 ppm and by the inability of the sampling methodology to sample selectively. Olivines from the two kimberlite varieties possess distinctly different crystal size distributions and this, together with contrasting groundmass textures, mineralogy and phlogopite compositions, indicates that the OPK and ORK represent two distinct batches of kimberlite magma. The general lack of sorting of olivine grains and the presence of decussate phlogopite textures in the ORK indicate that this magma crystallized rapidly, but was probably preceded by the OPK which had a more complex crystallization history as revealed by complex zoning patterns of its phlogopite grains. A methodology for comparing olivine and diamond size distributions has been developed, and a strong correlation between diamond and olivine size distributions is demonstrated. The correlation is used to demonstrate that diamond size distributions can be predicted from the proportions of OPK and ORK that comprise each bulk sample. It can be concluded that the two textural varieties of kimberlite possess distinctly different olivine (and diamond?) populations and that olivines are a good proxy for understanding diamond distributions at a local scale in the Snap Lake intrusion. (C) 2009 Elsevier B.V. All rights reserved