Ages of zircons from Jurassic sediments of Bluefish River slope, NWT Canada, and the possible age of kimberlite activity on the Lena West property

Abstract

This work is based on a study of 30 zircons extracted from the heavy mineral fraction of a sedimentary rock sample with anomalously high contents of well-preserved kimberlite indicator minerals (KIM). The sample was taken from the basal horizon of Jurassic‐Cretaceous sandstones on the slope of Bluefish River valley. The Bluefish River, located in the Northwest Territories (Canada) north of Great Bear Lake, is the right tributary of the Mackenzie River. The Bluefish River basin is a part of the large Lena West property that occupies an area of about 40 000 km 2 . Based on available tectonic data [1, 2], the area is situated within the Paleoproterozoic Hottah terrane and is separated from the Slave Craton to the east by Wopmay orogen. The Lena West property represents the sedimentary basin of a stable platform, which is filled with a thick sequence of mainly continental Cambrian‐Cretaceous sediments. The Bluefish area is dominated by undivided Jurassic‐Cretaceous rocks (clay shales, siltstones, and sandstones), which make up the Parsons Series and represent terrigenous sediments of the continental margin. Undivided Cambrian‐Devonian rocks are exposed only at the lower reaches and in the immediate vicinity of the river valley. Glacial deposits are subordinate. During prospecting of this area for diamonds, anomalously high contents of KIMs (primarily, Cr-pyrope and picroilmenite) were found in the alluvial samples of the middle reaches of the Bluefish River. Sedimentary rocks were sampled on the slope in detail (Fig. 1). The highest KIM contents (hundreds of grains) were found in samples from the basal horizon of Jurassic‐ Cretaceous sandstones. The Jurassic basal horizon rests on the limonitized gray siltstones of the Devonian Hare Indian Formation. The basal horizon (about 30 cm thick) includes three lithologically diverse interbeds: quartz gravelstones (upper interbed), limonitized coarse-to medium-grained sandstones (middle interbed), and yellow-green clays with a small admixture of pebbly material (lower interbed).