EVIDENCE FOR A METAMORPHIC ORIGIN FOR SAPPHIRES IN AN ALLUVIAL DEPOSIT NEAR BUTTE, MONTANA

The bedrock source and ultimate origin of the more than 50 tonnes of sapphires mined from numerous alluvial deposits in southwestern Montana remain topics of continuing speculation. However, recent observations suggest that the sapphires contained within these alluvial deposits weathered out of local volcanic rocks. The inference is that the sapphires were refractory xenocrysts that had been liberated by the assimilation of aluminous metamorphic basement rocks containing corundum porphyroblasts. A recent study of the Silver Bow sapphire occurrence located 8 km west of Butte, Montana supports this hypothesis. At this locality, sapphires, garnets, spinel, and schist pebbles were recovered from a debris flow consisting of altered tuffaceous volcanic rock. The debris flow can be traced up gradient to a point near a weathered felsic lapilli tuff of the Eocene Lowland Creek Volcanic field. The typical mineralogy of 21 schist pebbles examined in thin section is sillimanite, spinel, biotite, and either plagioclase or K-spar. One of these pebbles also contains corundum that suggests that schist of similar mineralogy was the probable source rock for the sapphires. Because metamorphic rocks are not exposed in the present drainage basin and the closest exposed metamorphic rocks are in the Flint Creek Range 45 km to the northwest, we infer that these schist pebbles originated as xenoliths within the sapphire-bearing tuff. During the ascent of this magma through the Precambrian basement, the magma intruded and assimilated corundum-bearing schist liberating the more refractory porphyroblasts of corundum to form xenocrysts. Subsequent weathering of the corundum-bearing tuff liberated the corundum xenocrysts and xenoliths that were later incorporated within the debris flow.