The evolution of geological models for the DO-27 kimberlite, NWT, Canada: Implications for evaluation

This contribution reviews the evolution of geological models for the DO-27 kimberlite, located in the Lac de Gras kimberlite province, Northwest Territories, Canada. The DO-27 kimberlite was discovered as a geophysical anomaly in 1992, and early models of the kimberlite (Models 1-2) were based largely on geophysical surveys, with early core drilling confirming the presence of kimberlite. Core drilling completed in 1993 was used to create the first geological model, Model 3. Model 3 interpreted the dominant pipe infill to be 'diatreme-facies' kimberlite. Influencing the interpretation of Model 3 was the understanding of kimberlites at the time, which assumed most kimberlites were comparable to southern African-style kimberlites; these kimberlites were typically large, steep-sided bodies infilled by 'diatreme-facies' kimberlite. However, this was inconsistent with the much smaller kimberlites being discovered in the early 1990s in the Lac de Gras area. Bulk sample evaluation in 1994 was conducted based on the interpretation in Model 3, targeting the 'diatreme-facies' kimberlite. The resulting diamond grades were below expectations and the project was not evaluated further. However, several partners in the project questioned the validity of Model 3 and acquired a re-assessment of the same 1993 drill cores. The resulting geological model, Model 4, interpreted 'pyroclastic kimberlite' to be the dominant pipe infill; this pipe was located south of where the 1994 evaluation was focused. The 'diatreme-facies' kimberlite of Model 3 was reinterpreted to be low-volume intrusive sills, not infilling a pipe-like body. Based on this re-interpretation, DO-27 was re-evaluated from 2005-2007 by Peregrine Diamonds Ltd. through bulk sampling and core drilling. These new data were used to create Model 5, which builds on the interpretations of Model 4. Diamond grades obtained from the 20052007 bulk sampling were substantially higher than those from the 1994 evaluation, which has significant implications for the economic viability of the DO-27 kimberlite. The progression in understanding and geological modeling of DO-27 demonstrates the importance of reliable and accurate geological models in the economic assessment of kimberlite pipes. (C) 2009 Elsevier B.V. All rights reserved.