MAGMATIC FRAGMENTATION OF SAPPHIRE XENOCRYSTS IN A BASALTIC TRACHYANDESITE SILL NEAR HELENA, MONTANA

Most alluvial sapphire deposits in Montana formed by weathering of volcanic rocks, but it is very unusual to find sapphires in this source rock. A sapphire-bearing Eocene basaltic trachyandesite sill about 2 m thick exposed along the Missouri River near Helena, Montana provides a unique opportunity for examination of xenocrystic sapphires in situ. More than 40 kg of this rock was slabbed exposing 28 colorless sapphires from 1 to 12 mm. On the basis of microscopic observation in conjunction with SEM, XRD, and EDS analyses, the following sequence of events during magmatic transport is inferred.

1. Incorporation of sapphire-bearing metamorphic xenoliths.
2. Assimilation of sapphire-bearing xenoliths liberating sapphires.
3. Partial resorption of sapphires.
4. Fragmentation of some sapphires into as many as four individual fragments.
5. Reaction of sapphires with the magma to form reaction rims 20 to 250 µm thick. These reaction rims were replaced by clay minerals including montmorillonite.
6. Accretion of biotite onto the surface of the initial reaction rim on the sapphires.
7. Liberation of sapphires by weathering of basaltic trachyandesite sill.

This study illustrates the importance of in situ examination of sapphires in the host rock in understanding the processes that occur during and after their magmatic transport.