Geology topics

Research about San Juan volcanic field

Source-linked reports with geographic coverage including San Juan volcanic field.

3 recordsLinked to original sources

USGS2011

A geologic and anthropogenic journey from the Precambrian to the new energy economy through the San Juan volcanic field

The San Juan volcanic field comprises 25,000 km 2 of intermediate composition mid-Tertiary volcanic rocks and dacitic to rhyolitic calderas including the San Juan–Uncompahgre and La Garita caldera-forming super-volcanoes. The region is famous for the geological, ecological, hydrological, archeological, and climatological diversity. These characteristics supported ancestral Puebloan populations. The area is also important for its mineral wealth that once fueled local economic vitality. Today, mitigating and/or investigating the impacts of mining and establishing the region as a climate base station are the focuses of ongoing research. Studies include advanced water treatment, the acid neutralizing capacity (ANC) of propylitic bedrock for use in mine-lands cleanup, and the use of soil amendments including biochar from beetle-kill pines. Biochar aids soil productivity and revegetation by incorporation into soils to improve moisture retention, reduce erosion, and support the natural terrestrial carbon sequestration (NTS) potential of volcanic soils to help offset atmospheric CO 2 emissions. This field trip will examine the volcano-tectonic and cultural history of the San Juan volcanic field as well as its geologic structures, economic mineral deposits and impacts, recent mitigation measures, and associated climate research. Field trip stops will include a visit to (1) the Summitville Superfund site to explore quartz alunite-Au mineralization, and associated alteration and new water-quality mitigation strategies; (2) the historic Creede epithermal-polymetallic–vein district with remarkably preserved resurgent calderas, keystone-graben, and moat sediments; (3) the historic mining town of Silverton located in the nested San Juan–Silverton caldera complex that exhibits base-metal Au-Ag mineralization; and (4) the site of ANC and NTS studies. En route back to Denver, we will traverse Grand Mesa, a high NTS area with Neogene basalt-derived soils and will enjoy a soak in the geothermal waters of the Aspen anomaly at Glenwood Springs.

Colorado↗

USGS1979

Genesis of ore deposits in the San Juan Volcanic Field, Southwestern Colorado-lead isotope evidence

In the San Juan volcanic area of southwestern Colorado, the isotopic composition of lead in ores and ore prospects of Cenozoic age ranges widely: 17.72 to 21.13 for 206 Pb/ 204 Pb; 15.50 to 15.81 for 207 Pb/ 204 Pb; and 37.21 to 38 for 208 Pb/ 204 Pb. Examination of the lead isotope data indicates that once deposition of lead minerals begins, further exchange of lead between fluid and wall rock is insignificant. This conclusion is supported by the relatively constant isotopic composition of the lead in these ores, which is not affected by the grade of ore mineralization or type of wall rock. The values of 206 Pb/ 204 Pb in some vein-type deposits exceed the maximum value known for all Mesozoic and Cenozoic igneous rocks of the Rocky Mountain region. These isotopic relations show that if ore-forming solutions are composed of meteoric water, as indicated in studies of light stable isotopes, they must have penetrated deep enough to acquire lead from Pre-cambrian rocks or sediments derived from them. The fact that some of these vein ores are now in Cenozoic igneous rocks indicates the ore fluid had an upward vertical component to its movement. The lead isotope data therefore support a circulating cell hypothesis for these kinds of ores, as suggested by many recent studies of light stable isotopes as related to mineralization. Some other deposits (Summitville, Jasper, Red Mountain district) have values of 206 Pb/ 204 Pb similar to those of the large volumes of altered rock that enclose them ( 206 Pb/ 204 Pb ∼ 18.5), suggesting that in places the lead may have been locally derived by leaching of the adjacent rocks or from magmatogenic fluids. When the lead isotope data are treated in detail, the rocks and galenas of the Platoro caldera complex, of the central San Juan caklera complex, and of the Baughman Creek center appear to contain significant components derived from 1,400- to l,500-m.y.-old and 1,700- to 1,800 m.y.-old source materials. These also are the ages of the two main groups of rocks that comprise the Precambrian basement under the San Juan volcanic area. Although the data from the western San Juan caldera complex scatter considerably, the only obvious source for the lead seems to be the 1,700- to l,800-m.y.-old rocks or detritus of such age in Phanerozoic sediments. Where the involvement of 1,400- to l,500-m.y.-old sources is greatest, the Th/U ratio in the source material is small (calculated to be about 0.7), whereas the Th/U ratio of the 1,700- to l,800-m.y.-old source material appears to be 2.3 to 3.3. The largest Th/U values are from the western San Juan caldera complex. © 1979 Society of Economic Geologists, Inc.

Colorado↗

USGS1974

Relation of mineralization to calderas in the San Juan volcanic field, southwestern Colorado

A review of the calderas in the San Juan volcanic field, southwestern Colorado, indicates that lead, zinc, copper, gold, and silver mineralization took place in and near calderas which had complex histories of postsubsidence intrusive and extrusive igneous activity. Most known ore occurs in veins that follow fractures formed during the different caldera cycles, but some intrusive bodies were extensively mineralized and possibly may host disseminated, porphyry-type deposits. The mineral deposits seem to be related most commonly to the youngest and most highly differentiated late intrusives of a caldera cycle.

Colorado↗