Tertiary Intrusion-Related Copper, Molybdenum, and Tungsten Mining Districts of the Eastern Great Basin

摘要

The eastern Great Basin hosts about 70 Tertiary intrusive-related mining districts, of which about 50 contain recognized porphyry Cu, porphyry Mo, W skarn, and/or related mineral deposits. The recognized porphyry/skarn systems range in age from 40 Ma to 11 Ma. The style of mineralization in these districts is strongly influenced by the age and composition of the mineralizing intrusive event. During the Tertiary, magmatism in the eastern Great Basin evolved from (1) mid-Eocene to early Oligocene calc-alkaline, intermediate to felsic, subduction-related magmas, to (2) late Oligocene to early Miocene, and (3) mid-Miocene to Quaternary, bimodal, extension-related magmas. The Eocene and early Oligocene (about 40 to 29 Ma) magmatic suites range in composition from andesite to dacite to low-silica rhyolite, and this magmatism produced several calderas in west-central and southwestern Utah. The late Oligocene to early Miocene (about 26 to 17 Ma) bimodal suite is predominantly andesite and rhyolite accompanied by extension and a few calderas in southwestern Utah. The final mid-Miocene to Quaternary (about 16 Ma to present) suite is strongly bimodal, basalt and rhyolite associated with significant extension. The earliest Tertiary mineralization in the eastern Great Basin occurs in the mid-Eocene in western Utah and easternmost Nevada including scattered districts primarily near the state line. These districts are associated with granodiorite and quartz monzonite stocks and include numerous small W skarns in the Gold Hill (40 Ma) and Rosebud (40 Ma) districts. The recognized and suspected Tertiary calc-alkaline porphyry Cu ±Mo and related systems in Utah fall into three clusters: an Eocene group ranging from 40 to 36 Ma, an Eocene-Oligocene cluster from 35 to 33 Ma, and a middle Oligocene group at 31 to 29 Ma. The ten Eocene porphyry Cu and related systems cluster in the Oquirrh Mountains, including the giant Bingham Canyon porphyry Cu-Au-Mo, and western Utah. The early Oligocene group of five porphyry Cu systems occurs in the Tintic, Cottonwood, and Park City mining districts of central Utah. The four middle Oligocene porphyry Cu systems cluster in central Beaver County of southwestern Utah. These calc-alkaline-centered districts include the most productive mining districts in Utah. No mineralization is recognized in the eastern Great Basin between about 29 and 26 Ma. Following this hiatus, a pulse of early Miocene bimodal magmatism and related mineralization occurred in the eastern Great Basin from about 26 to 17 Ma. This period of andesitic and rhyolitic magmatism produced seven known and suspected porphyry Mo and related mineral deposits. The giant Pine Grove Climax-type porphyry Mo deposit and Spor Mountain, the world's largest beryllium deposit, both belong to this group. A second episode of strongly bimodal, basalt and rhyolite magmatism and related mineralization flared up in the mid-Miocene. These systems are similar to the early Miocene systems, but have been less deeply eroded. The mid-Miocene systems include six more suspected porphyry Mo and related mineral deposits ranging in age from about 16 to 11 Ma.