Iolite from the Thor-Odin Dome, British Columbia, Canada: Geology, Chemical Composition, Inclusions, and Cause of Chatoyancy

Abstract

Two gem-quality cordierite (iolite) localities, the Blue Arrow and Blue Bear occurrences, were recently discovered (in 2016 and 2021, respectively) in the mountainous Thor-Odin dome, southwest of Revelstoke in British Columbia, Canada (figure 1). They have so far produced small quantities of faceted stones (generally <2 ct, though the author has seen faceted windowed stones up to 4.76 ct from Blue Bear) and attractive cat's-eye cabochons (up to 12.02 ct, Blue Bear only). The present study seeks to gain new insights into: (1) the geological origin of the occurrences in a regional context and how they compare to the geology of other iolite deposits, (2) the nature of iolite mineralization at the Blue Arrow and Blue Bear localities, (3) the chemical composition and inclusions of Thor-Odin dome iolite, and (4) the nature of the inclusions responsible for chatoyancy. GEOLOGY OF IOLITE DEPOSITSDeposits of iolite (gem-quality cordierite) occur in magnesium-and aluminum-rich metamorphic rocks formed at amphibolite to granulite facies conditions. Many iolite occurrences are found around the world, but relatively few have been comprehensively characterized from a geological (i.e., petrological, geochemical, and geochronological) perspective.Significant deposits occur in the Bandarguha and Orabahala areas of Kalahandi District, India, where gem cordierite (with X Mg = [Mg]/[Mg + Fe] = 0.82-0.88) is hosted in granulite facies cordierite-biotite schists and gneisses, which can contain the following accessory minerals: quartz, potassium feldspar, plagioclase, sillimanite, corundum, and orthopyroxene (Das and Mohanty, 2017).