By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Pitkin County, in mountainous west-central Colorado and west of the Continental Divide, has produced silver, lead, and zinc valued at more than $100 million, but only about 28,200 ounces of gold has been produced. The Aspen district was the major source of this mineral wealth; however, the bulk of the gold came from the Independence Pass district.
INDEPENDENCE PASS DISTRICT
The Independence Pass district is in southeastern Pitkin County, about 20 miles southeast of Aspen.
In 1879, miners from Leadville crossed the Sawatch Range and found ore in this district on West Aspen Mountain (Henderson, 1926, p. 45). In the same year a belt of gold-bearing veins was found which, during 1881-82, yielded $190,000 (9,192 ounces) in gold.
The veins apparently terminated abruptly, either against a fault or contact, and mining was halted. The mines were reopened in 1891, again in 1897-99, and possibly for a short period in 1900 (Henderson, 1926, p. 45). Although the output of the district is unknown for these years, Pitkin County is credited with a gold production valued at $300,000 (Henderson, 1926, p. 201) ; much of this probably came from the Independence Pass district. There were only small sporadic operations in the district from 1932 through 1959. Total gold production through 1959 was probably about 25,000 ounces.
The formations of the Independence Pass district consist of Precambrian gneiss and granite covered by Tertiary breccia or rubble and rhyolite flows, all intruded by a granite porphyry stock (Vanderwilt and Koschmann, 1932).
Not much is known about the veins. They occur in the Precambrian rocks or in the Tertiary intrusive rocks. Howell (1919, p. 75-102) described some of the veins in the Twin Lakes district east of the county line which are probably similar and are genetically related to those in the Independence Pass district. They consist of gold-bearing quartz associated with pyrite and galena and locally associated with some chalcopyrite and sphalerite. Silver is present in small amounts.
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