The geology and mineralogy of the Anuri kimberlite, Nunavut, Canada

Abstract

The 613±6 Ma Anuri kimberlite is a pipelike body comprising two lobes with a combined surface area of approximately 4–5 ha. The pipe is infilled with two contrasting rock types: volcaniclastic kimberlite (VK) and, less common, hypabyssal kimberlite (HK).The HK is an archetypal kimberlite composed of macrocrysts of olivine, spinel, mica, rare eclogitic garnet and clinopyroxene with microphenocrysts of olivine and groundmass spinel, phlogopite, apatite and perovskite in a serpentine–calcite–phlogopite matrix. The Ba enrichment of phlogopite, the compositional trends of both primary spinel and phlogopite, as well as the composition of the mantle-derived xenocrysts, are also characteristic of kimberlite. The present-day country rocks are granitoids; however, the incorporation of sedimentary xenoliths in the HK shows that the Archean granitoid basement terrain, at least locally, was capped by younger Proterozoic sediments at the time of emplacement. The sediments have since been removed by erosion. HK is confined to the deeper eastern parts of the Anuri pipe. It is suggested that the HK was emplaced prior to the dominant VK as a separate phase of kimberlite. The HK must have ascended to high stratigraphic levels to allow incorporation of Proterozoic sediments as xenoliths.Most of the Anuri kimberlite is infilled with VK which is composed of variable proportions of juvenile lapilli, discrete olivine macrocrysts, country rock xenoliths and mantle-derived xenocrysts. It is proposed that the explosive breakthrough of a second batch of kimberlite magma formed the western lobe resulting in the excavation of the main pipe. Much of the resulting fragmented country rock material was deposited in extra crater deposits. Pyroclastic eruption(s) of kimberlite must have occurred to form the common juvenile lapilli present in the VKs. The VK is variable in nature and can be subdivided into four types: volcaniclastic kimberlite breccia, magmaclast-rich volcaniclastic kimberlite breccia, finer grained volcaniclastic kimberlite breccia and lithic-rich volcaniclastic kimberlite breccia. The variations between these subtypes reflect different depositional processes. These processes are difficult to determine but could include primary pyroclastic deposition and/or resedimentation.There is some similarity between Anuri and the Lac de Gras kimberlites, with variable types of VK forming the dominant infill of small, steep-sided pipes excavated into crystalline Archean basement and sedimentary cover.