Geology of the Copper King uranium mine, Larimer County, Colorado

The Copper King mine in Larimer County, Colo., in the northern part of the Front Range of Colorado, was opened in World War I in an unsuccessful attempt to mine copper and zinc ore. In 1949, following the discovery of pitchblende on the dump, the mine was reopened, and it was worked until 1953 for uranium. A total of 652 tons of ore that contained an average of 0.28 percent U₃O₈ was shipped.

The bedrock consists predominantly of biotite granite, part of the Precamblian Log Cabin batholith, and minor metasedimentary rocks -- biotite-quartz-plagioclase gneiss, amphibole skarn, biotite schist, quartzite, amphibolite, and biotite sköls. The metasedimentary rocks occur as inclusions that trend northeast, essentially parallel to the prevailing foliation in the granite. In places the metasedimentary rocks are crosscut sharply by the granite and form angular, steep-walled blocks in the granite. Faults, confined to a narrow easterly-trending zone through the mine, cut all the Precambrian rocks.

Mineral deposits of two types are present at the mines 1) sulfide-magnetite ore and 2) a uranium deposit in a filled fissure that cuts the skarn. The sulfide-magnetite deposits are small and consist of pyrite, sphalerite, chalcopyrite, pyrrhotite, and at places magnetite. Alpha-helium age determinations on ore magnetite by Hurley indicate that the mineral assemblage is late Precambrian in age. The deposits are pyrometasomatic in origin and possibly related to the granite.

The uranium deposit consists of pitchblende and associated vein-forming minerals that occur in the Copper King fault and locally in pyrite boxwork adjacent to the fault. Three phases of black pitchblende have been identified -- uraninite, "coffinite", and UO₃-rich pitchblend. Colored secondary minerals are absent. The pitchblende occurs in a steeply plunging, tabular shoot between 45 and 135 feet below the surface that has a horizontal length of about 50 feet. Within the shoot the pitchblende occurs in pods or layers generally only a few feet in height and length and as much as a foot thick that are separated by nearly barren vein. The grade of the ore within the pods ranges from 0.2 percent uranium to as much as 20 percent but averages about 1 to 2 percent. Age determinations by the Pb²⁰⁶/U²³⁸ and Pb²⁰⁷/U²³⁵ methods on two samples of hard pitchblende from the vein, not from the pyrite boxwork, gave ages by the two methods after suitable common lead corrections, ranging from 55 to 76 million years, corresponding to an early Tertiary age.

Diamond core drilling and reconnaissance for radioactivity have not disclosed other uranium deposits the Prairie Divide region; nevertheless, it seems likely that other deposits are present.