SUNSTONES: A GEMSTONE FORMED BY A MAGMATIC OR HYDROTHERMAL PROCESS?

Sunstones are feldspar crystals that contain macroscopic inclusions of native copper within basaltic lavas in eastern Oregon. Sunstones are found and mined in two primary places located approximately 150 km apart in Oregon, the Ponderosa Mine and the Plush claims, with the larger of the two at Plush. The sunstone-bearing lavas at the Ponderosa Mine sunstones are 16.2 Ma, while Plush’s sunstones are much younger with a groundmass age of 9 Ma.

Since silicate minerals are not compatible with metals similar to copper, timing and the right conditions for copper enrichment remain unconstrained. The close proximity of the sunstones suggests they share a similar magmatic source, but the hosting basaltic lava is heavily weathered and suggests post-emplacement hydrothermal alteration. Preliminary oxygen isotope data suggest a magmatic origin, with values between 5.8-6.2 ‰. This study aims to quantify oxygen isotope values sourced from sunstones at both localities and determine if there is evidence for hydrothermal processes.

We analyzed 10 sunstone samples to test if different parts of the sunstone reflect variable oxygen isotope values. These analyses compare source locality (i.e. Ponderosa versus Plush), color variation (i.e. red, pink, clear), and color density (i.e. core versus rim of the crystal) within sunstones. Hydrothermal alteration would result in diverse and possibly lower 𝛿18O values, and provide insight on post-emplacement processes associated with copper diffusion and/or exsolution in feldspar. These oxygen data do not reflect direct evidence of alteration via hydrothermal processes but may help constrain timescales for alteration processes.