M.H. Delevaux J. P. Albers B. R. Doe 1985 <p><span>The tectonic setting comprising the West Shasta mining district has often been compared with that of primitive island arcs. Concentrations of uranium, thorium, and lead and lead isotope compositions were determined for Devonian ores and rocks of the West Shasta district, eastern 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 possible. A lead isotope point for the Devonian oceanic mantle is now well established from this study on the West Shasta district. This isotopic composition is in agreement with that suggested by Slawson in 1983--i.e., from a sample of massive sulfide ore from the Golinsky deposit--with a&nbsp;</span>²⁰⁶<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb of 17.830, a&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb of 15.450, and a&nbsp;</span>²⁰⁸<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb of 37.313. If the Devonian mantle is analogous to the modern mantle, the determined value is probably at the more radiogenic end of the Devonian oceanic mantle array.Some samples of volcanic rocks and ores that have elevated values of&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb and&nbsp;</span>²⁰⁸<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb indicate that subducted pelagic sediments or interaction of the magmas with other sediments downsection have been involved to varying degrees in the generation of magmas. The high ratios were not attained by addition of lead to the volcanic rocks or ores during subsequent magmatic events, such as intrusion of the Permian Pit River stock to the east or of the Cretaceous Shasta Bally batholith that crops out to the south and west and may underlie part of the district. Some values of&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb are sufficiently high as to suggest that West Shasta may have developed nearer a continent than was previously supposed. Lead isotope data for Quaternary volcanic rocks are somewhat more radiogenic than for Cretaceous ores and plutonic rocks. Permo-Triassic ores are again a bit less radiogenic. The isotopic differences between the Permo-Triassic and Quaternary data could be accounted for by a value for&nbsp;</span>²³⁸<span>&nbsp;U/&nbsp;</span>²⁰⁴<span>&nbsp;Pb of 10.42 in common source material. Using this value of&nbsp;</span>²³⁸<span>&nbsp;U/&nbsp;</span>²⁰⁴<span>&nbsp;Pb to calculate even further back to 400 m.y., the resulting ratios are found to fall very close to the "best value" for the lead isotope mixing line between the Devonian mantle and pelagic sediments. We interpret this intersection to be the "mean value" for the assimilated Devonian sediment lead--18.250 for&nbsp;</span>²⁰⁶<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb and 15.582 for&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb. As deduced for the lead isotope composition for the Devonian mantle at this location, the pelagic sediment, lead isotope composition also is toward the more radiogenic end of the values expected for pelagic sediments in the Devonian. Lack of sediments in the observed volcanic section of the West Shasta district suggests a submerged volcanic-arc setting (as interpreted by others for the Troodos and Samail ophiolites) rather than an island-arc situation (such as the Green Tuff area of Japan) or an incipient spreading ridge setting (that might have grown to be a subaerial Iceland). The elevated values of&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb for some samples suggest a submerged volcanic are whereas the exceedingly low values of thorium--which is not easily mobilized during alteration events--for both basaltic andesites and plagiorhyolites is suggestive of depleted source material such as is found for most oceanic spreading centers. Elevated values of&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb are exceedingly rare in midocean ridge basalts and low contents of thorium are known in island arcs, so a submerged volcanic arc is the preferred interpretation. Some similarity exists between the&nbsp;</span>²⁰⁶<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb values of Devonian ores having elevated values of&nbsp;</span>²⁰⁷<span>&nbsp;Pb/&nbsp;</span>²⁰⁴<span>&nbsp;Pb and those of the Permo-Triassic ores of the East Shasta district, as mentioned by Slawson in 1983. This similarity is now confirmed to be a coincidence. We find it due first of all to some incorporation of sediment lead in the magmas, followed later by some migration of radiogenic lead--probably locally derived--into the ores during subsequent events. Because there was some postdepositional lead migration into the ores, some migration of copper also into the ores cannot be excluded. Lead, especially radiogenic lead, however, is more mobile than copper. As the migrating lead is deduced to have been locally derived from the observed section rather than exotically derived, any copper added was probably locally derived also.</span></p> application/pdf 10.2113/gsecongeo.80.8.2136 en Society of Economic Geologists The plumbotectonics of the West Shasta mining district, eastern Klamath Mountains, California article