Indicator mineral and till geochemical dispersal patterns associated with the Ranch Lake kimberlite, Lac de Gras region, NWT, Canada

Abstract

The Ranch Lake kimberlite, in the Contwoyto Lake area of the northwest Territories, consists of volcaniclastic crater facies kimberlite which contains thousands of kimberlite indicator mineral grains in a 10 kg sample. Cr-diopside and pyrope are the most abundant indicator minerals in the kimberlite, accompanied by much less abundant chromite and rare Mg-ilmenite. Indicator minerals in till samples collected around the Ranch Lake kimberlite define a spectacular ribbon-shaped glacial dispersal train trending west for 70 km. At its head, the train is 500 m wide and gradually widens 2 km at 20 km down-ice. The lateral edges of the train are sharply defined by the presence or absence of indicator minerals in till. Background (up-ice) concentrations of all indicator minerals are zero. Patterns for Cr-diopside and pyrope abundance are similar and concentrations increase down-ice, reaching their highest levels between 15 and 19 km down-ice. Of the two size fractions of heavy minerals examined, most indicator minerals occur in the finer, 0.25 to 0.5 mm, size fraction. The <0.063 mm fraction of the till matrix displays weak geochemical signatures of the kimberlite down-ice, most notably for Ba, Cr, Ce and Th. The results of this study of indicator mineral and geochemical methods around a known kimberlite will aid in the design of mineral exploration programs for kimberlite-hosted diamond deposits in glaciated terrain around the world.