OF-01-05 Geologic Map of the Georgetown Quadrangle, Clear Creek County, Colorado


The Georgetown Quadrangle is located in Clear Creek County. Includes cross section, map unit correlation, shaded-relief map with geology overlay, booklet with extended descriptions of map units, geologic hazards, structural geology, ore deposits and alteration products, economic geology, and selected references. 22 pages. 1 color plate (1:24,000). Digital PDF download. OF-01-05D

From the Author’s Notes:

The Georgetown quadrangle is located in Clear Creek County in the Colorado Front Range. It is characterized by rugged terrain ranging in elevation from about 8,400 ft in Georgetown to 13,040 ft at the southern edge of the quadrangle. The Interstate Highway 70 transportation corridor (I-70) passes through Georgetown and Silver Plume in the northern part of the quadrangle. The Georgetown-Silver Plume mining district, known primarily for its silver production, generally extends throughout the western and northern parts of the quadrangle.

Geologic mapping of the Georgetown 7.5 minute quadrangle was undertaken by the Colorado Geological Survey (CGS) as part of the STATEMAP component of the National Cooperative Geologic Mapping Program, authorized by the National Geologic Mapping Act, which is administered by the U.S. Geological Survey. Partial funding for this project came from Colorado Severance taxes, which are derived from the production of oil, gas, coal, and metals. Geologic maps produced by the CGS through the STATEMAP program are intended as multi-purpose maps useful for land-use planning, geotechnical engineering, geologic-hazards assessment, mineral-resource development, and ground-water exploration.

The Georgetown 15-minute quadrangle, which includes the Georgetown 7.5-minute quadrangle, was previously mapped by Spurr and others (1908). The Montezuma 15-minute quadrangle to the west was mapped by Lovering (1935). Other previously mapped 7.5-minute quadrangles adjacent to the Georgetown quadrangle include Berthoud Pass (Theobald, 1965), Empire (Braddock, 1969), Central City (Sims, 1964), and Idaho Springs (Widmann and others, 2000).

Proterozoic metamorphic and igneous rocks crop out beneath most of the quadrangle. The oldest are Early Proterozoic metasedimentary and metavolcanic rocks that were originally deposited up to 1,800 Ma (Tweto, 1987) and later metamorphosed during a period of intense deformation about 1,726 Ma (Selverstone and others, 1997). This metamorphic complex was subsequently intruded by granitic rocks of the Berthoud plutonic suite (Tweto, 1987) between about 1,448 and 1,420 Ma (Aleinikoff and others, 1993; Peterman and others, 1968). Multiple pulses of igneous intrusion from 65 to about 20 Ma resulted in the emplacement of numerous porphyry dikes and other small intrusive bodies throughout the Colorado Mineral Belt. Porphyry bodies in the Georgetown quadrangle are predominantly associated with a magmatic pulse occurring between 43 to 35 Ma. The ore minerals of the Georgetown-Silver Plume mining district were precipitated during the later stages of this 43 to 35 Ma intrusive event. The youngest deposits in the quadrangle include glacial till of Pinedale and possibly Bull- Lake age, as well as other Quaternary surficial deposits.