VOLCANIC ROCKS AND ORE DEPOSITS OF THE MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH

Igneous activity and associated mineralization in the Marysvale volcanic field occurred mainly during three intervals: ~34-22 Ma, 22-14 Ma, and 9-5 Ma. In the first interval, dacites were erupted above an evolving batholith and andesites were erupted peripheral to it. Periodic eruptions of crystal-rich ash-flow tuff resulted in the Three Creeks caldera at 27 Ma, Big John caldera at 24 Ma, and Monroe Peak caldera at 23 Ma. K, Rb, and U contents increased with time in dacitic rocks. About 23-22 Ma, igneous compositions changed from a calc-alkaline sequence to a bimodal sequence (alkali rhyolite and basaltic rocks) that erupted the late Quaternary. This change is interpreted to be a result of the change from subduction to initial extension. Voluminous rhyolites were erupted locally during the second and third intervals. The largest volume of alkali rhyolite was erupted during the second interval from two areas: (1) the 19 Ma Mount Belknap caldera and (2) an area about 20 km east of it that was active from 21 Ma to 14 Ma. From 9-5 Ma, basaltic compositions did not change, and rhyolites had major-element compositions like those erupted from 22 to 14 Ma but markedly different radiogenic isotope characteristics, reflecting derivation from different source regions. Mineralization was especially important near the change in eruption intervals. Gold-bearing quartz-pyrite-carbonate veins in the carapace of a quartz monzonite stock in the Kimberly district formed about 23 Ma. Large, replacement alunite deposits formed peripheral to similar stocks at about the same time. U-Mo-F veins in the Central Mining Area formed above a rhyolitic stock at 19 Ma. Large veins of coarse-grained alunite on Alunite Ridge formed by degassing of a rhyolitic stock at 14 Ma coeval with base-metal mantos of the Deer Trail mine and associated precious metal deposits. Small gold and alunite deposits formed at 9 Ma during a surge in basin-range extension.