Alice Mining District (Yankee Mining District; Lincoln Mining District), Clear Creek County, Colorado, USAi

Regional Level Types
Alice Mining District (Yankee Mining District; Lincoln Mining District)	Mining District
Clear Creek County	County
Colorado	State
USA	Country

The Alice-Yankee Hill district is in the north-central part of Clear Creek County and the southwest part of Gilpin County, about 7 miles N. 75° W. of Central City. It ranges in altitude from 10,000 to 11,500 feet, but the productive mines are easily accessible by automobile roads. The most noteworthy ore deposit of the district is the great pyritic stockwork of the Alice mine, which was first worked in the early eighteen eighties by hydraulic methods but was abandoned for many years after the rich ore near the surface had been removed.

With the increased price of gold in 1933 the lower part of the supergene sulfide zone, which had previously been noncommercial, became of sufficient value to warrant exploitation, and a considerable output was made in the next few years.

The ores of the Alice-Yankee Hill district are chiefly of the pyritic type, but a few veins, such as the Stonewall, Ninety-Four, Cumberland, and Lombard veins, contain galena and sphalerite. The pyritic ores contain little silver and are mined for their gold content. They are relatively rich near the surface in the oxidized zone, but the unaltered sulfides are generally too low-grade to be worked. As a result, the individual pyritic deposits have not been long-lived.

ORE DEPOSITS

In three parts of the district - the ridge south of Fall River 2 miles southwest of Alice, the vicinity of Silver Lake, and Yankee Hill - there are many northeastward-trending veins, but none of them have been very productive. Most of them are of the pyritic gold type and consist chiefly of quartz and pyrite. The oxidized ore near the surface is said to have contained as much as 7 ounces of gold to the ton, although the average tenor of the better class of surface ore was approximately 1 to 2112 ounces. The unoxidized ore, consisting of quartz and pyrite, has been in general too low grade to be worked.

The depth of supergene enrichment probably varies slightly with the nature of the wall rock and to a somewhat greater extent with the position of the deposits relative to ancient erosion surfaces. At the Alice mine the oxidized zone was very shallow, and slightly oxidized sulfide ore was found at the surface at several places. The writers observed supergene sooty chalcocite on the main adit level about 100 feet below the surface but found none on the sub-level 40 feet deeper.

The grade of the ore was directly related to the amount of supergene copper sulfide present. Specimens rich in the secondary sulfides would assay more than an ounce of gold to the ton, but the primary pyrite in the sub-level directly beneath the large bodies of enriched ore assayed only 0.03 ounce per ton or less. The better grade of ore sent to the mill from stopes on the main adit level averaged about 0.20 ounce of gold to the ton, less than an ounce of silver, and about 0.5 percent of copper, suggesting that the gold content of the supergene ore is about seven times that of the primary sulfides.