COPPER AND LITHIUM DIFFUSION IN EASTER OREGON SUNSTONES

Sunstones are large plagioclase feldspar crystals (An₆₅ to An₆₇) that contain microscopic inclusions of native copper (Cu). Sunstone-bearing lavas occur in two mining localities in eastern Oregon (Ponderosa and Plush), located approximately 150 km apart within the spatial extent of the Columbia River Basalt Province. Sunstones exhibit a variety of colors, including pink, yellow, red, and green; and areas of dense color correspond to higher concentrations of copper within the crystal. While the close spatial proximity and uniqueness of these lavas suggest a similar magmatic source, age data indicates that the basaltic lavas containing sunstones have a notable age discrepancy. Basaltic lavas at the Ponderosa Mine yield a groundmass age of 16.2 Ma, basaltic lavas from the Plush area mines yield groundmass ages of 8.9-9.2 Ma. It is unlikely that the same anomaly would occur only 150 km apart and be unrelated in source, yet the time difference between the groundmass implies otherwise. Heavy weathering and oxidation present in the groundmass of the basalt flows hosting the sunstones suggests that hydrothermal activity played a role.

We analyzed 20 sunstones from the two source localities using laser ablation inductively coupled plasma-mass spectrometry (LAICP-MS). We report trace-element concentrations and Cu/Li ratios along transects within sunstones and compare the results between mine locations. Our preliminary results indicate that Cu and Zn are anticorrelated, while Cu and Li are positively correlated. An anticorrelation between Zn and Cu is surprising, as Zn inclusions have been identified in a sunstone from the Plush location. Higher concentrations of Cu and Li correspond to the densest regions of color, commonly observed within the core of the crystal. Since Li is a fast-diffusing element in plagioclase, higher concentrations of Li and Cu in the center of the crystal suggest that the color zonation in the crystal may result from Li and Cu diffusion out of the crystal (rather than in).