AN INVESTIGATION OF NATIVE COPPER IN PLAGIOCLASE, LAKE AND HARNEY COUNTIES, OREGON
Type of DegreeMaster's Thesis
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Mafic igneous rocks are commonly associated with various metals and there are many examples of basalts that host small amounts of native copper. Such copper is more common among the matrix phases of basalts and could represent a final stage of a melt or a secondary alteration. Worldwide, there are only a few occurrences of copper within early-crystallizing phenocrysts of plagioclase, and this fact makes the ‘sunstones’ of Oregon as interesting as they are spectacularly beautiful. The native copper in these ‘sunstones’ occurs as thin platelets (copper schiller) with crystallographically-controlled orientations. These copper platelets appear to have formed via the exsolution of metallic copper and are typically found in the cores of the highest-grade gemstones. The age of the sunstone host basalts has previously been uncertain. Also, there has been little petrographic or geochemical characterization of the basalts hosting the sunstones. The objectives of this thesis were to determine the age and provide an improved petrographic, petrologic and geochemical characterization of the host basalt. These labradorite megacrysts (~An67) have strikingly homogeneous major and trace element distributions and internally homogeneous 87Sr/86Sr ratios, similar to those observed in plagioclase phenocrysts of the Columbia River Basalt Group's Steens Basalt (~16.7 Ma). The homogeneous nature of all these data suggests that following copper exsolution, the crystals have not experienced significant chemical change (diffusive mass transport, alteration, weathering). This research determined the age of the basalt hosting sunstones. The means of four matrix plateaus is 9.16±0.12 Ma (95% c.l., MSWD=1.13). These late Miocene plateau ages are comparable to lavas of the High Lava Plains Trend but are distinctly younger than the Steens Basalt. In addition, this research illustrated a simple sunstone development hypothesis.