Geology and evaluation strategy of the Star and Orion South kimberlites, Fort à la Corne, Canada

Abstract

Diamond-bearing kimberlites in the Fort à la Corne field, east-central Saskatchewan, consist primarily of volcaniclastic deposits which are interstratified with Lower Cretaceous continental, marginal marine and marine sediments. Seventy kimberlite occurrences have been drill-confirmed with the majority of those occurring in the Fort à la Corne main trend. The Star (352 ha) and Orion South (358 ha) kimberlite complexes are located at the south-eastern extremity of the main trend and contain some of the world's best examples of preserved primary volcaniclastic kimberlite deposits. The advanced exploration programs on both the Star and Orion South complexes offer a rare opportunity to examine a wide variety of in situ volcaniclastic kimberlite deposits. Both kimberlite complexes are composed of multiple eruptive units that were deposited episodically over 4 to 6 million years, separated by periods of volcanic quiescence marked by reworking of kimberlite and siliciclastic sedimentation. Detailed logging of core from 100 m spacing grid drilling has revealed well-defined tuff ring, tuff cone, crater and extra crater deposits. Evaluation of these large tonnage, lower grade, coarse diamond size frequency kimberlite complexes requires a methodical and multi-branched exploration strategy utilizing: 1. Grid-pattern core drilling to gain an understanding of the three-dimensional architecture of the kimberlite complexes; 2. Underground bulk sampling to provide appreciable carats for accurate estimates of diamond grade and price; and, 3. Grid-pattern mini-bulk sampling via large diameter drilling to obtain an understanding of the diamond grade across the breadth of each kimberlite.