Origin of gem corundum in calcite marble: The Revelstoke occurrence in the Canadian Cordillera of British Columbia

Abstract

The calcite marble-hosted gem corundum (ruby, sapphire) occurrence near Revelstoke, British Columbia, Canada, occurs in the Monashee Complex of the Omineca Belt of the Canadian Cordillera . Corundum occurs in thin, folded and stretched layers with green muscovite + Ba-bearing K-feldspar + anorthite (An 0.85–1 ) ± phlogopite ± Na-poor scapolite . Other silicate layers within the marble are composed of: (1) diopside + tremolite ± quartz and (2) garnet (Alm 0.7–0.5 Grs 0.2–0.4 ) + Na-rich scapolite + diopside + tremolite + Na,K-amphiboles. Non-silicate layers in the marble are either magnetite- or graphite-bearing. Predominantly pink (locally red or purple) opaque to transparent corundum crystals have elevated Cr 2 O 3 (≤ 0.21 wt.%) and variable amounts of TiO 2 ; rare blue rims on the corundum crystals contain higher amounts of TiO 2 (≤ 0.53 wt.%) and Fe 2 O 3 (≤ 0.07 wt.%). The associated micas have elevated Cr, V, Ti, and Ba contents. Petrography of the silicate layers show that corundum formed from muscovite at the peak of metamorphism (~ 650–700 °C at 8.5–9 kbar). Because the marble is almost pure calcite (dolomite is very rare), the corundum was preserved because it did not react with dolomite to spinel + calcite during decompression. The scapolite-bearing assemblages formed during or after decompression of the rock at ~ 650 °C and 4–6 kbar. Gem-quality corundum crystals formed especially on borders of the mica-feldspar layers in an assemblage with calcite. Whole rock geochemistry data show that the corundum-bearing silicate (mica-feldspar) layers formed by mechanical mixing of carbonate with the host gneiss protolith ; the bulk composition of the silicate layers was modified by Si and Fe depletion during prograde metamorphism . High element mobility is supported by the homogenization of δ 18 O and δ 13 C values in carbonates and silicates for the marble and silicate layers. The silicate layers and the gneiss contain elevated contents of Cr and V due to the volcanoclastic component of their protolith. • First detailed characterization of prograde gem corundum formation in calcite marble • Corundum present in mica-feldspar layers in calcite marble • Common protolith of Cr,V-bearing marble assemblages and host rock • Isotope homogenization between the host rock and the marble • Corundum formed by prograde muscovite breakdown reactions