The uranium abundances in middle Tertiary volcanic rocks of the Mogollon-Datil volcanic field, southwestern New Mexico, have been determined as part of a major petrogenetic study. Over 350 samples of middle Tertiary to Quaternary volcanic rocks have been analyzed for their uranium content by delayed neutron activation analysis.

Of the volcanic associations previously proposed for southwestern New Mexico, calc-alkalic andesite, ±43 to ±35 m.y., has a mean of 2.3 ppm U (range 0.9 to 5.4 ppm); calc-alkalic quartz latite to rhyolite, ±35 to ±29 m.y., has a mean of 3.9 ppm U (range 1.7 to 6.2 ppm); basaltic andesite and associated rocks, ±32 to ±18 m.y., has a mean of 2.3 ppm U (range 0.8 to 6.9 ppm); and high-silica rhyolite, ±32 to ±18 m.y., has a mean of 5.2 ppm U (range 1.6 to 9.4 ppm). Anomalous values in the range of 14 to 35 ppm U were found for a riebeckite-bearing lava from the central San Mateo Mountains, a sample of intrusive andesite from the Alum Mountain area, and a lithophysal rhyolite lava and associated ash-flow tuff from the Sierra Cuchillo. Post-13-m.y. b omodal basalt-rhyolite is sparse within the Mogollon-Datil volcanic field. A few determinations from this study, and published and unpublished data for other localities in New Mexico, indicate U abundance from 0.3 to 1.5 ppm U in post-13-m.y. basalt and about 7 to 8 ppm U in rhyolite.

End_of_Article - Last_Page 755------------