EVIDENCE OF METASOMATIC CORUNDUM (VAR. SAPPHIRE) FROM THE WHITEHORN STOCK METAMORPHIC AUREOLE IN CENTRAL COLORADO, USA

Corundum (var. sapphire) of a metasomatic origin occurs within a complexly sheared contact metamorphic aureole adjacent to the Late Cretaceous Whitehorn stock in Salida, Colorado. This study presents evidence of the metasomatic origin of corundum based on field relations, colored cathodoluminescence, core-to-rim trace element geochemistry by electron microprobe analysis, and isocon analyses. The corundum-hosting lithologies can be characterized as pelitic schists with a characteristic mineral assemblage of corundum + sillimanite + cordierite + plagioclase + hercynite ± K-feldspar ± biotite ± muscovite. The primary mineral-phase corundum presents as idioblastic platy hexagonal crystals (up to 6 mm) with a preferred planar orientation and can comprise more than 50 vol. %.

Field investigations indicate that corundum crystallization occurs within a ductile shear zone, creating a localized juxtaposed rock suite of corundum lenses, massive sillimanite mylonites, and a mineralized skarn. Colored cathodoluminescence depicts pronounced metasomatic zonation. Electron microprobe analysis on the corundum indicates core-to-rim depletion of iron, titanium, chromium, vanadium, and gallium; absolute analyses indicate iron decreases from 5,912 to 4,950 ppm; titanium, 406 to 96 ppm; chromium, 566 to 225 ppm; vanadium, 449 to 221 ppm; and gallium, 278 to 169 ppm. Electron microprobe analysis indicates core-to-rim silica enrichment of 40 to 152 ppm. Isocon analyses of corundum-hosting lithologies compared with a least-altered equivalent indicate absolute Al₂O₃ enrichment of up to 58 wt.%, SiO₂ depletion of 28 wt.%, and enrichment of the alkali and alkali earth metals.

Fluid channeling during ductile shearing allowed metasomatic processes to operate during contact metamorphism, transforming pelitic and carbonate protoliths to a juxtaposed rock suite of peraluminous metapelites and skarn. These metasomatic processes explain why corundum is constrained to the shear zone, where the mobilization of elements resulted in a greater compositional change of the pelitic protolith. The oscillatory zoning and changes in the core-to-rim trace element chemistry of corundum and the enrichment of both mobile and immobile elements of corundum-hosting lithologies are evidence of its metasomatic origin.