Volcano-Tectonic Setting of Tertiary Ore Deposits, Southern Rocky Mountains

Abstract

Most Tertiary ore deposits in the southern Rocky Mountains are associated with eroded volcanoes or shallow underlying plutons. Distributions and compositions of the igneous rocks vary complexly with time, apparently reflecting evolving boundaries between the American, Farallon, and Pacific plates nearly 1, 000 km to the west. Loci of igneous rocks and associated ores also were influenced by structural zones of Precambrian ancestry, especially the mineral belt of Colorado and the Jemez zone in New Mexico.
In earliest Tertiary time, the southern Rocky Mountains were the site of Laramide compressional tectonics, limited igneous activity, and associated mineralization, all of which appear related to low-angle subduction extending the width of the cordillera. Igneous activity (calc-alkaline, dominantly intermediate compositions) and associated ore deposits (mainly base-and precious-metal veins) were largely confined to the Colorado mineral belt.
After an early Tertiary lull, volcanism, intrusive activity, and mineralization renewed on a large scale about 40 m.y. ago. A composite volcanic field extended across much of the southern Rocky Mountains; associated intrusive rocks range from isolated plutons to composite batholiths that underlie much of the presently high terrain. Compositions of the igneous rocks and associated ores differed little from those of Laramide activity. Some metals were probably derived from the magmas, but others-notably Pb, which is amenable to isotopic tracing-include major components from old upper crustal rocks. The largest erosional remnants of the volcanic terrane lie to the south, notably the San Juan, Thirtynine-Mile, and Latir volcanic fields; to the north, deeper erosion has exposed large mid-Tertiary intrusive complexes in the Sawatch Range and to the northeast along the mineral belt.
This igneous activity constitutes only a small sector of a regional sweep south and west across the cordillera, perhaps related to foundering of the descending Farallon plate as its size, thickness and rate of convergence decreased. About 25-30 m.y. ago, as intermediate-composition activity peaked, compositions became increasingly silicic, and northwest-trending extensional faulting marked inception of the Rio Grande rift. Especially rich ores are associated with such silicic subvolcanic intrusions; some-notably Mo deposits-are clearly derived from the magmas. This association of faulting and silicic magmatism, as exemplified by the Latir volcanic field and the Questa caldera, seemingly represents local expression of widespread inter-and back-arc extension as the subduction zone steepened and migrated westward: significant are coherent patterns of southward migration of silicic igneous activity and of northwest-trending normal faults across much of the western United States.
The volcano-tectonic association of the southern Rocky Mountains changed further after about 20 m.y. ago. Volcanism became dominantly basaltic or bimodal basaltic-rhyolitic, and dominant trends of normal faults shifted to nearly north-south, the trend of the modern Rio Grande rift, by about 5-10 m.y. ago. This association now characterizes much of the western United States, reflecting increasing interaction between Pacific and American plates along the San Andreas transform. Thus, igneous and structural patterns in the southern Rocky Mountains are local expressions of trends and sequences that affect the entire cordillera, a relation that supports platetectonic interpretations of the evolution of the southern Rocky Mountains despite great distance from the plate margin.