Clear Creek County Colorado Gold Production
An unbroken succession of gold deposits extends from Idaho Springs in Clear Creek County to Central City and Blackhawk in Gilpin County. The deposits form a geologic entity, separated into the Idaho Springs and Central City districts only by the county line. The area has been the largest source of gold in both counties and includes the Chicago, Ute, and Cascade Creek camps.
Gold placers, which were found in early 1859 along Chicago Creek near Idaho Springs, attracted many prospectors who combed the nearby gulches and surrounding mountains and who soon uncovered additional placers in Nevada and Illinois gulches and Missouri Flats, as well as rich gold quartz veins, notable among which were the Gregory, Russell, Bates, Bobtail, and Mammoth lodes (Henderson, 1926, p. 27, 28). After the oxidized ores were depleted, the mines were shut down. In 1868 the Blackhawk smelter began treating the sulfide ores; the district was rejuvenated and experienced a long period of intense activity, exemplified by the 4y2-mile-long Argo tunnel, which was started in 1904. By 1918 a marked decline was evident, and this trend continued until 1932. The period 1932-42 was one of high production, and it was followed by a period of steady decline after World War II.
The total minimum gold production of that part of the district in Clear Creek County was about 1,805,000 ounces (R. H. Moench, written commun., 1963). The production of the Central City part of the district, in Gilpin County, is given in the Gilpin County section of this report (p. 100).
The area is underlain by interlayered metamorphic gneiss, migmatite, and roughly concordant sheets of granodiorite similar to Boulder Creek Granite and granite similar to Silver Plume Granite (Moench and others, 1962, p. 37-38), all of Precambrian age. These Precambrian rocks are intruded by a variety of Tertiary plutons and dikes of porphyritic igneous rocks of the granodiorite, quartz monzonite, bostonite, and hornblende granodiorite groups (Wells, 1960, p. 232).
Two episodes of Precambrian deformation are recognized: a plastic deformation that recrystallized the rock minerals and produced large open folds whose axes trend north-northeast, and a younger cataclastic deformation that was characterized by asymmetrical folds whose axes trend N. 55° E., weak recrystallization of minerals, and intense granulation. The major structures near Central City are the Central City anticline and smaller subparallel folds formed in the older period (Moench and others, 1962, p. 39-54). The younger folding is recognized only near Idaho Springs as part of a belt of shearing called the Idaho Springs-Ralston Buttes cataclastic zone (Tweto and Sims, 1960, p. B8). Large northwest-trending faults of Precambrian ancestry, displaying Laramide movement, and three sets of small faults of Laramide age also cut the Precambrian rocks.
Most of the ore deposits are mesothermal sulfide veins in fault fissures. Veins are grouped according to their mineral assemblages into (1) pyrite-quartz veins, (2) pyritic copper veins containing quartz, pyrite, chalcopyrite, tennantite, and minor galena and sphalerite, (3) pyritic lead-zinc veins contain¬ing quartz, pyrite, galena, sphalerite and subordinate chalcopyrite and tennantite, and (4) lead-zinc veins containing quartz, carbonates, galena, sphalerite, and small amounts of chalcopyrite, tennantite, and pyrite (Sims, 1956, p. 743-744; R. H. Moench, written commun., 1963). All four types contain gold, but the most important mines are in the pyritic lead-zinc veins. Gold occurs partly as discrete fine particles and is partly tied up in the sulfide minerals. A few gold telluride veins are found in the southeast part of the Central City district in Gilpin County.
The veins are arranged in a concentric pattern of zones with pyrite and pyritic copper veins occupying an elliptical central area which is 2 to 3 miles wide and extends from Blackhawk south to the Idaho Springs district. This is surrounded by successive zones containing pyritic lead-zinc veins and galena-sphalerite veins (Sims, 1956, p. 744-745; R. H. Moench, written commun., 1963).
About a mile southwest of Central City is the well-known "stockwork" named "The Patch"—a pipe or chimney of brecciated country rock cemented by ore minerals. This mineralized body extends from the surface to a depth of 1,600 feet where it is intersected by the Argo tunnel. At this depth the pipe has not decreased in size, but the grade of ore is diminished. The ore is of two types: one is characterized by pyrite, chalcopyrite, quartz, and a little tetrahedrite; and the other, by galena, sphalerite, chalcopyrite, and subordinate pyrite (Bastin and Hill, 1917, p. 96-97).