Abstract

Igneous activity in the Marysvale volcanic field occurred from about 32 Ma to present and was accompanied episodically by formation of a wide variety of mineral deposits. Most of the volcanic rocks were erupted during the mid-Cenozoic, between 32 and 22 Ma, as calc-alkaline lavas and ash-flow tuffs from shallow magma chambers beneath stratovolcano and caldera sources. These rocks were emplaced along tectonically controlled, east-west igneous belts where volcanoes coalesced and were intruded by epizonal stocks. Hydrothermal gold deposits formed in the carapaces of some of the stocks at about 23 Ma, and both coeval replacement alunite and brick-clay deposits formed peripheral to others. About or shortly after 23 Ma, basin-range extension began and the composition of the associated volcanic rocks changed to a bimodal assemblage of alkali rhyolite and basaltic rocks that continued to be erupted episodically into the Holocene. Uranium-molybdenum-fluorine-bearing deposits formed at about 18 Ma in association with small rhyolite stocks and volcanic domes. Lead, zinc, copper and precious metals in mantos at Deer Trail Mountain and hydrothermal gold deposits in Bullion Canyon and Cottonwood Creek were deposited on the periphery of a related, but unexposed, 14 Ma stock. The hydrothermal system vented to the surface above the stock where spectacular coarsely crystalline vein alunite was deposited from magmatic-vapor-dominated fluids in open fractures. Episodic extensional faulting at 9 to 5 Ma was accompanied by scattered rhyolite lava flows with minor associated gold deposits. Quaternary basalts near Cove Fort are associated with a small fluorite deposit and several active fumerolic deposits of native sulfur; a nearby and related geothermal steam system is used to generate electricity. In the northern Mineral Mountains, another steam system associated with nearby rhyolitic lavas generates electricity.