M.A. Theses: Dorothy I. Godfrey-Smith, 1986

X-Ray Flouresence Characterization of the Obsidian Flows From the Mount Edziza Volcanic Complex of British Columbia, Canada

The purpose of this work was to determine how many chemically distinct obsidian flows exist in the Mount Edziza Volcanic Complex; to characterize these obsidian flows chemically; and to apply this information towards the understanding of prehistoric obsidian exploitation within the Complex. The Complex, located in northwestern British Columbia, constitutes the largest obsidian source area in Canada; by virtue of its size, accessibility, and location, it is also the most significant prehistorically exploited obsidian quarry in the Northwest. This is attested to by the wide distribution of artifacts made from Mount Edziza obsidian, extending hundreds of kilometres away from the Complex. This thesis reports the results of x-ray fluorescence analyses of both native obsidian rocks and of artifacts collected within the study area.

A set of 174 obsidian rocks from outcrop and gravel deposits was analyzed semi-quantitatively by energy-dispersive x-ray fluorescence, and the relative concentrations of a number of elements were determined. On the basis of these relative elemental concentrations, ten chemically distinct types of obsidian were identified. Each chemical type of obsidian was then correlated to a unique obsidian flow within the Complex.

The absolute concentrations of 28 major and minor elements in each of the ten obsidian flows were determined using wavelength-dispersive x-ray fluorescence. On the basis of these quantitative data, eight of the Edziza obsidians were found to have a peralkaline chemical composition; five of these fall within the class of rocks known as pantellerites, while three were classified as comendites. The other two flows were interpreted as sub-alkaline.

The semi-quantitative energy-dispersive x-ray fluorescence method was also applied to determine the sources of 169 obsidian artifacts from three archaeological sites situated within the Mount Edziza volcanic Complex. The results indicate preferred exploitation of the high-elevation obsidian sources from one central location. The results also suggest that the people who exploited the Complex ignored all the poorer-quality low-elevation obsidian scatters, and that they may not have exploited the high plateau region north of Raspberry Pass. This raises the possibility that the obsidian sources north of Raspberry Pass were not accessible at that time, perhaps due to harsher environmental conditions than at present.