Clear Creek County Colorado Gold Production
The Empire district is in north-central Clear Creek County about 9 miles west of Idaho Springs and 4 to 5 miles south-southwest of the Alice district. It includes an area of about 8 square miles, but the main productive veins are about IV2 miles north of Empire.
Oxidized and disintegrated gold-bearing quartz veins were discovered in the district in 1860 by prospectors from the Central City district (Spurr and others, 1908, p. 172-173). The disintegrated material could be easily sluiced and treated the same way as placer gravel and these operations proved very profitable. The oxidized material extended to a depth of about 40 feet and there gave way to sulfide ore which carried some free gold. Amalgamation of the sulfide ore presented difficulties, and after 1875 activity declined, although sporadic activity continued to 1924 (Henderson, 1926, p. 31). The district gained prominence again in 1934 when a group of veins north-northwest of Empire was developed in the Minnesota mine. Production increased from 272 ounces in 1932 to 16,693 ounces in 1940. The mines closed during World War II, and there was only minor sporadic production from 1945 through 1957. Total minimum gold production of the district was 165,000 ounces, roughly half of which came from the Minnesota mine from 1934 through 1943.
The following summary of the geology and ore deposits of the district was taken from Spurr, Garrey, and Ball (1908, p. 383-386) and from Lovering and Goddard (1950, p. 156-161).
The country rock of the Empire district consists chiefly of Precambrian rocks intruded by stocks and dikes of early Tertiary age. The Precambrian rocks include schist of the Idaho Springs Formation, hornblende gneiss, granite gneiss, and both the Boulder Creek and Silver Plume Granites. The Boulder Creek Granite occupies the greater part of the area. The Tertiary stocks are quartz monzonite, and the dikes range in composition from bostonite to alaskite.
The ore deposits of the district are in pyritic quartz veins that are chiefly in the Boulder Creek Granite near the contact of the granite with a quartz monzonite stock. The chief metal produced is gold but in some veins copper also is of value. Few veins have been traced for more than 1,000 feet along the surface or to depths greater than 500 feet. The chief minerals are pyrite, chalcopyrite, and quartz, although small amounts of sphalerite and galena are found in some veins. The gold seems to be associated with the chalcopyrite. The ore ranges from about 0.2 to 0.4 ounce of gold per ton, and silver ranges from a few ounces to 20 ounces per ton.