OF-04-03 Geologic Map of the Alma Quadrangle, Park and Summit Counties, Colorado


The purpose of this publication is to describe the complex geologic setting and mineral resource potential of this 7.5-minute quadrangle located in central Colorado. Staff geologist Beth L. Widmann, consulting geologist Paul J. Bartos, consulting geologist Richard F. Madole, students Kathryn E. Barba and Marilyn E. Moll completed the field work on this project during the summer of 2003. (1:24,000). Digital ZIP/PDF download. OF-04-03D

This mapping project was funded jointly by the U.S. Geological Survey through the STATEMAP component of the National Cooperative Geologic Mapping Program which is authorized by the National Geologic Mapping Act of 1997, Award number 03HQAG0095, and the Colorado Geological Survey using the Colorado Department of Natural Resources Severance Tax Operational Funds. The CGS matching funds come from the Severance Tax paid on the production of natural gas, oil, coal, and metals.

From the Author’s Notes:

The majority of the Alma quadrangle is located in western Park County; the southern edge of the quadrangle is less than 3 miles northwest of Fairplay. In the northern part of the quadrangle, the Park/Summit County line coincides with the continental divide at Hoosier Pass and along Hoosier Ridge. State Highway 9 bisects the quadrangle longitudinally. The highway passes through the town of Alma in the south-central part of the quadrangle, ascends Hoosier Pass, then drops down to Breckenridge 9 miles to the north. The town of Leadville is roughly 8.5 miles west of the quadrangle.

The principal geographic features in the Alma region are the Mosquito Range to the west and South Park to the southeast. The Mosquito Range extends along the western half of the quadrangle and has several peaks over 14,000 ft. Within the project area these include Mounts Lincoln, Cameron, and Bross. South Park is a broad flat basin, the bulk of which is generally south of the quadrangle, although a few fingers of the park do extend northwards along the eastern margin of the quadrangle. The Middle Fork of the South Platte River flows into the northwest corner of the quadrangle, is captured at Montgomery Reservoir, and is released again into the Platte River valley. State Highway 9 parallels the Platte River valley in the center of the quadrangle. Buckskin, Mosquito, and Pennsylvania Creeks originate in the Mosquito Range and feed into the Middle Fork of the South Platte River. Beaver Ridge and Beaver Creek parallel the Platte River Valley to the east. Mount Silverheels, at 13,822 ft, is the highest point in the eastern part of the quadrangle.

Several mining districts are found within or adjacent to the Alma quadrangle. The region was therefore the focus of considerable study, particularly west of the South Platte River valley, during the height of the mining period in the early part of the 20th century. One of the earliest geologic studies in the Alma region focused on the Alma mining district, which included the area west of the South Platte River Valley to the Park County line. A subsequent paper on the Leadville mining district outlined in detail the geology and ore deposits of the area encompassing the Alma mining district as well as parts of Lake and Summit Counties from south of Leadville to north of Copper Mountain. Singewald and Butler (1941) focused specifically on the ore deposits in the vicinity of the London fault, just southwest of the quadrangle, although their mapping covers much of the same area as the Patton and others report. Geologic studies by Bookstrom (1989) of the Climax ore body northwest of the Alma quadrangle are of note since he correlates many of the igneous rocks in the Climax area to those in the western part of the Alma quadrangle. More recently, several 15- and 7.5-minute-scale quadrangles have been mapped in the area.