Upper Union Mining District, Clear Creek County, Colorado, USAi

Regional Level Types
Upper Union Mining District	Mining District
Clear Creek County	County
Colorado	State
USA	Country

The Empire mining district occupies an area of about 8 square miles in the vicinity of Empire, in the southwestern part of the Central City quadrangle. It ranges in altitude from 8,500 to 11,500 feet.

The first activity in the Empire district was the discovery of the oxidized part of the Silver Mountain ore zone in 1862. This gossan was washed in sluices and treated in the same way as placer gravels, yielding a good profit, and for several years the district was very prosperous. Water was brought in a ditch from Mill Creek, 3 1/2 miles away, and most of the surface material was sluiced. These operations decreased in importance after 1875 but continued in a small way for many years.

The total gold recovered by this method amounts to a little more than 75,500 ounces. Lode mining has been intermittently productive since the seventies. After several years of desultory output the district recently assumed an important place with the development of the Crown Prince-Atlantic group by Minnesota Mines, Inc., from 1934 to 1943, when the mine was shut down, owing to World War II.

ORE DEPOSITS:

The ore deposits of the Empire district are almost entirely of the pyritic gold type. The chief metal produced is gold, although in places there is sufficient copper to be of value. The chief minerals of the veins are pyrite, chalcopyrite, and quartz. Rarely there are small amounts of sphalerite and galena. The veins have a general northeasterly trend and a steep dip to the southeast; they are Chiefly limited to a narrow belt surrounding the quartz monzonite porphyry stock 1 1/2 miles due north of Empire and to the area just northeast of the town.

Most of the veins are fissure fillings and occur both as simple fracture::; and as complex lodes. A mile and a quarter north of Empire is the strong zone of fracturing and alteration, about 3500 feet long and several hundred feet wide, called the Silver Mountain ore zone.

This zone is impregnated with pyrite throughout and contains irregular bodies of ore along certain fractures. It lies a few hundred feet southeast of a small stock of monzonite, which is elongated in a northeast direction. A similar complex lode, in the Comet-Little Johnny group, parallels it to the northwest. Where these lodes end northwesterly cross lodes occur in the area between them. It seems probable that this encircling zone was formed by the stresses related to the intrusion of the porphyry stock and probably reflects the collapse of the roof when some of the underlying magma, was withdrawn following a late stage in the consolidation of the stock. The most important mines in the district are found in the northeast parts of this zone. Just north of Empire several short veins follow a northeasterly zone of porphyry dikes.