BEDROCK SOURCE OF SAPPHIRES IN ALLUVIAL DEPOSITS IN THE ROCK CREEK, SAPPHIRE DISTRICT, WESTERN MONTANA

The bedrock source of Montana’s alluvial sapphire deposits has been a long-standing geologic puzzle. A recent detailed field study of the southwestern part of the Rock Creek sapphire district in the Sapphire Mountains in western Montana contributes significantly to our understanding of alluvial sapphire deposits in Montana and their bedrock source. Historic mining yielded an estimated 65 tonnes of sapphires from alluvial deposits within the 11 km² area that was studied.

Bedrock geology within the study area consists of rhyolitic lavas, tuffs, and volcaniclastic rocks that overlie metasedimentary formations of the Mesoproterozic Belt Supergroup. Biotite from one of the lava flows gives an ⁴⁰Ar/ ³⁹Ar plateau age of 50.2 ±0.4 Ma. Small remnants of tuff and more widely spread volcaniclastic rocks are interpreted to be younger than the lava flows on the basis of field relationships, but geologically are coeval with the lavas. Seventeen analyzed specimens of the volcanic rocks plot in the rhyolite field on a total-alkali-silica diagram. Evidence that rhyolitic volcanic rocks are the bedrock source for these sapphires includes: a tremendous concentration of sapphires confined to alluvial deposits within a small area proximal to rhyolite outcrops, productive sapphire-bearing gulches entirely surrounded by rhyolite, adhering rhyolite found on some sapphires, and general lack of fractures and abrasion textures as shown by SEM that indicates limited fluvial transport.

Sapphires appear to have been transported by rhyolitic magma from either a crustal or mantle source to the surface, where they were subsequently liberated during extended weathering of these volcanic host rocks.