INSIGHTS INTO SAPPHIRE FORMATION PROCESSES FROM YOGO GULCH, MONTANA AND BINGHAM CANYON, UTAH

Trace amounts of sapphire occur in several intrusive and extrusive igneous units at Bingham Canyon, Utah, the largest porphyry copper deposit in North America. Initial LA-ICP-MS analysis on these small (50-500 μm) sapphires indicates chemical similarities with sapphires from Yogo Gulch, Montana. Mg, Fe, and Ga contents are comparable for sapphires from both locations, but Bingham sapphires contain considerably higher Cr and V, and possible Ti-rich micro-inclusions. The high Mg, Fe, and Ti content of Bingham sapphires is significant, as this trace element signature is typically unique to Yogo sapphires. Bingham and Yogo sapphires are also similar in color and morphology.

At Yogo Gulch, sapphires occur in a lamprophyre dike; at Bingham, they occur in several Cu-Au-Mo mineralized porphyries as well as nearby unaltered block-and-ash flows believed to be the extrusive equivalents of the mineralized units. Primitive, alkaline magma was involved in the formation of sapphire-bearing units at both locations. In the Bingham system, the roughly parallel dike-like character of the mineralized units (which cross-cut slightly older unmineralized intrusions) suggests a small degree of extension may have allowed sapphire-bearing mafic magma to rise along Eocene normal faults to form these units, possibly also triggering volcanic eruptions through mixing with more felsic magmas. Similar tectonic processes have been observed at some modern volcanoes, such as Volcán de Colima in Mexico. Evidence exists for Eocene extension-related structural controls on the emplacement of the Yogo lamprophyre dike as well.

Our findings support a sapphire formation model in which ultramafic magma pools at the base of the crust and partially melts an Al-rich protolith. Refractory corundum forms as Al residue in these melts and is incorporated by the magma as it continues to rise and evolve. Sapphires formed in these settings may be considered xenocrysts, since they form at depth and are transported by a magma they may not ultimately be in equilibrium with. The occurrence of sapphire throughout a compositionally diverse range of igneous units at Bingham argues for a xenocrystic origin as well. Oxygen isotope ratios of Bingham and Yogo sapphires will be investigated to help determine their source and the processes involved in their formation.