PROVENANCE OF THE MONTANA ALLUVIAL SAPPHIRES USING GEOLOGY, TRACE ELEMENT CHEMISTRY AND INCLUSION ANALYSIS

Alluvial sapphires in southwestern Montana have supported a mining industry for over 100 years. The Rock Creek deposit alone produced over 190 million carats. The source rock for the sapphires has never been found or identified. Geological mapping, trace element analysis and inclusion identification have been used to characterize the Montana alluvial sapphires. The formation environment for these sapphires is extrapolated by comparison with a corundum reference suite composed of samples from alkali basalt and metamorphic deposits. The placers occur in three areas in southwestern Montana: along the Missouri River east of Helena, Rock Creek southwest of Philipsburg, and Dry Cottonwood Gulch, east of Deer Lodge. Detailed mapping of the placers indicates that the present distribution is due to post-Quaternary reworking of Tertiary alluvium, which means the sapphires have been in the alluvial system for a minimum of 2 My. Corundum usually contains Fe, Cr, Ti, V, and Ga. There is no indication of a binary relationship between any of the elements. Ga displays different patterns for alkali basalt and metamorphic corundum, unrelated to colour. Alkali basalt corundum has Ga > 100 ppm, and a wide variation for samples from the same deposit. Metamorphic corundum has Ga < 100 ppm, and a consistency within each deposit. Since there is no crustal mechanism to fractionate Ga from Al, the dispersion in Ga values for alkali basalt corundum indicates a mantle origin in localized environments. The inclusion suite for alkali basalt corundum consists of ilmenite, goethite, hercynite, clinozoisite and secondary sulphate minerals. Metamorphic corundum hosts a wider variety of inclusions, including rutile, muscovite, plagioclase, clinozoisite, sapphirine, and zircon. The Dry Cottonwood sapphires contain plagioclase, muscovite, some clinozoisite, and rarely garnet. The Rock Creek sapphires contain significant zones of exsolved rutile, plus clinozoisite, labradorite, and muscovite. Using both the crn-an-cz stability field, and the composition of the garnet inclusion, the best estimate for T, P for the alluvial sapphires is 580 to 720oC and 9 to 13 kbar (~31-45 km depth).