REFINING SAPPHIRE GENESIS MODELS FOR YOGO GULCH, MONTANA, AND BINGHAM CANYON, UTAH: EVIDENCE FROM TRACE ELEMENTS, MELT INCLUSIONS, AND OXYGEN ISOTOPE RATIOS

Processes involved in the genesis of many primary gem corundum deposits are relatively poorly understood. We present a geochemical comparison of sapphires from the porphyry Cu-Au-Mo deposit at Bingham Canyon, Utah and from Yogo Gulch, Montana in order to investigate their respective origins. Sapphires occur as trace components throughout a section of pyroclastic deposits erupted from the Bingham magma system around 38 Ma, and have also been found within the porphyry itself. Yogo sapphires occur in a single lamprophyre dike dated at 48.2 ± 1.3 Ma.

LA-ICP-MS shows that Bingham and Yogo sapphires have broadly similar trace element signatures. SIMS analysis gives ẟ¹⁸O values of 4.4-5.9‰ for Yogo sapphires, consistent with a lower crustal origin as proposed by earlier workers. In contrast, values for Bingham sapphires range from 6.4-15.7‰. Sapphires collected from the base of the volcanic section near Bingham have a much narrower range (8.2-10.1‰), while samples from stratigraphically higher units contain a mix of sapphires with high, intermediate, and low ẟ¹⁸O values, suggesting multiple sapphire populations formed over a period of time and were well-mixed into the Bingham magma system before eruption.

Both Bingham and Yogo sapphires contain glassy silicate melt inclusions. EPMA shows that these inclusions differ compositionally from the sapphires’ igneous host rocks, and many inclusions in Bingham sapphires are quartz normative. This is enigmatic, as corundum normally cannot coexist with quartz. Previous work on melt inclusions in Yogo sapphires led to a proposed model in which ultramafic magma supplied heat to partially melt an Al-rich protolith (possibly an anorthosite or troctolite), producing corundum as a solid phase of a peritectic reaction. Melt inclusions represent a liquid phase of this reaction trapped during corundum growth, and sapphires were then transported by the rising lamprophyre. We suggest a similar explanation for Bingham sapphires, where sapphire protoliths could have included peraluminous granites and high-ẟ18O sedimentary rocks partially melted by intruding mafic magmas. Although O-isotope and melt inclusion data suggest different protoliths for Bingham and Yogo sapphires, corundum may have been produced through partial melting of Al-rich country rock in both systems.