Abstract

Extremely fine-grained, hypocrystalline, microporphyritic dacite (whole-rock SiO2 = 65–70 oxide wt.%), called “glassy basalt” by archaeologists, was commonly used to manufacture lithic artefacts found in the British Columbia (B.C.) Interior. Geochemical fingerprinting of dacite minerals can help identify the geologic source of these artefacts. Multiple (∼300) mineral analyses show that mafic orthopyroxene (En65–80), plagioclase (An30–70), augite (Wo30–45, En40–45, Fs10–15), and olivine (∼Fo85) (in that order) represent the most abundant and commonly occurring microphenocrysts. Relative abundances vary among sources. Clustering of averaged mineral data reveals at least five distinct dacite sources for lithic artefacts in the B.C. Interior. Discriminant analysis separates individual mineral analyses according to these five areas with 90% efficiency and provides functions for “sourcing” new artefacts in the future. Two sites represent “quarry” locations and their scope (e.g., geographic area 4 km2, archaeological stratigraphic depth locally 2 m at Cache Creek) implies prolonged use and trade. However, fingerprinting suggests that in the B.C. Interior, tools were made from local dacite. Mineral fingerprinting uses small (∼0.1 g) samples, which is important when analyzing valuable artefacts. Hypocrystalline, “knappable,” microporphyritic dacite is probably common around the Pacific due to Cenozoic subduction. Thus, mineral-based sourcing could have wider application outside of western Canada.

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