COMPARISON OF SURFACE MORPHOLOGY OF MONTANA ALLUVIAL SAPPHIRES BY SEM

Alluvial sapphire deposits in southwestern Montana account for more than 90% of the quantity of historic Montana production; production continues from two of these three deposits. Corundum in the Rock Creek and Dry Cottonwood Creek deposits has an unusual origin, as it occurs in post-Quaternary reworked alluvium and is believed to have weathered from nearby Eocene rhyolite flows. The Missouri River deposits are currently being investigated.

Comparison of surface morphology by SEM provides insight into the magmatic history of these sapphires and enables the differentiation of sapphires from these three districts. Morphology is interpreted to be largely caused by resorption and fracturing during magmatic transport from a presumed crustal source. Approximately 40% of the sapphires examined from the Rock Creek District exhibit prominent irregular hillocks standing above smooth surfaces of the basal pinacoid with relief from 100 µm to several hundred micrometers. Slopes on the sides of hillocks are characterized by steps, some attaining 0.5 µm in height. Adhering rhyolite indicates that at least some of these surfaces were produced by fracturing before liberation of the sapphire from rhyolitic lava. Small (< 2 mm) sapphires from this district are typically subrounded and covered with a finely-etched rough surface with relief of 6 µm. Sapphires from the South Fork of Dry Cottonwood Creek are characterized by the occurrence of deep, smooth grooves developed along basal and rhombohedral partings. Some of these sapphires are decorated with sawtooth ridges of 100 µm relief. Sapphires from the strath terraces along the Missouri River near Helena are generally deeply sculpted on the basal pinacoid and on relict prism faces with some depressions > 1 mm deep. Small black to dark green spinel crystals preserved in these depressions are attributed to reaction between the sapphire and transporting magma.