AN EXPLORATION OF PRIMARY NATIVE COPPER IN OREGON SUNSTONES

Oregon Sunstones are copper-bearing feldspar most commonly found in highly porphyritic basaltic lavas of the Columbia River Basalt Group (CRBG). These native Cu inclusions occur within early-crystallizing plagioclase phenocrysts as discrete platelets that are typically ~1 µm thick and up to 100 µm in diameter. These inclusions are responsible for light reflection and the ‘schiller effect’, formed by post-crystallization exsolution and presumably during cooling. While preparing oriented sections of Sunstones samples for further SEM and TEM study from three different mines (Dust Devil, Ponderosa, and Little Eagle Butte). We observe that the vast majority of Cu platelets are indeed parallel to (010) as reported in previous studies, and thus closely parallel the plane for developing Huttenlocher intergrowths as commonly seen in other calcic plagioclase feldspars. Smaller portions of platelets (perhaps 10%) locally are aligned in planes closer to (001) and (100). Geochemically, Sunstones exhibit no zonation or distinct chemical variability. They have a uniform anorthite content (~An₆₇), remarkably homogeneous distributions of major and trace elements, and exhibit internally homogeneous ⁸⁷Sr/⁸⁶Sr ratios similar to plagioclase megacrysts in the Picture Gorge and Steens Basalt of the CRBG.

The existing geochronologic data are difficult to reconcile for assessment of the age of the Sunstones, their hosting basalt, and eruptive history. Preliminary data indicate an untenable age relationship, where the sunstones are younger than the hosting basalt. We are determining additional ⁴⁰Ar/³⁹Ar ages for both sunstones (plagioclase megacrysts) and their associated basaltic groundmass. We suggest that argon loss from megacrysts may be responsible for the consistently younger ages, while basaltic groundmass ages constrain the timing of eruption which is coeval with CRBG. Alternatively, though the basalts hosting the Sunstones share many geochemical similarities with the regional CRBG and thus can be expected to be derived from similar sources, they could have formed in later (‘post-Steens’) stages of regional volcanism.