The tectonic setting comprising the West Shasta mining district has often been compared with that of primitive island arcs. Concentrations of U, Th, and Pb and Pb isotope compositions were determined for Devonian ores and rocks of the West Shasta district, E Klamath Mountains, California, to help evaluate the tectonic classification. The lead isotope pattern is found to be complex. From comparison of the data with those on younger ores and rocks in the region and with those isotopic patterns found in modern tectonic terranes, however, a number of conclusions are described in detail. -after Authors