By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gilpin County, on the east slope of the Front Range in north-central Colorado about 30 miles west of Denver, ranks second among the counties of Colorado in gold production. Through 1959 it produced, in round numbers, 4,255,000 ounces of gold valued at $89,785,000. Of this production, 4,207,000 ounces was lode gold and 47,900 ounces was placer gold.Some of the first significant gold discoveries in Colorado were in Gilpin County. In May 1859, John H. Gregory found a rich and easily worked, oxidized, gold-bearing lode at the Gregory diggings near Blackhawk. Early in June 1859, W. Green Russell discovered gold placers and decomposed outcrops of lodes in what is now called Russell Gulch. By July 1, 1859, about 100 sluices were at work in the vicinity of Gregory's discovery, and toward the end of September about 890 men were at work in Russell Gulch. Some lodes averaged $100 a day for months at a time and yields as large as $400 for a day's work were not uncommon.
In June 1859 the miners at the Gregory diggings met and adopted resolutions denning the boundaries of the district (now known as the Central City district) and the conditions under which claims could be taken and held. In July a provisional local government was formed at the Gregory diggings.
All the veins of the region were oxidized and gold bearing at the surface, and the decomposed ores could be easily and cheaply mined and treated in sluices and crude stamp mills. The underlying sulfide ores, which commonly contained silver and base metals as well as gold, were not amenable to such simple treatment, and many mines were closed when the oxidized ore gave way to sulfide ores at depths of 40 to 100 feet. Stamp mills saved only about one-fourth of the gold and wasted all the other metals in the sulfide ores (Henderson, 1926, p. 30).
In 1868, the Hill matting smelter at Blackhawk opened and the first matte was shipped that same year. The successful smelting of ores stimulated the mining industry and made possible a long period of development and production from lodes. In 1872, additional impetus was given to mining in Gilpin County when the Colorado Central (later the Colorado and Southern) Railway was completed from Denver to Blackhawk. Records show that Gilpin County had a steady output of gold from 1859 through 1908, a peak production valued at $3,237,346 being recorded in 1871 (Henderson, 1926, p. 122). From 1909, output gradually declined and in 1920 it dropped below $100,000 for the first time since gold was discovered in the county. Production rose in the late 1920's and during the depression in the early 1930's, and after the price of gold was raised in 1934 there was a marked increase in output. Since 1944, mining in the county has been on a reduced and fluctuating scale.There are many small mining camps in the county but they are in general grouped into two districts referred to as the Northern Gilpin district and the Central City district.
CENTRAL CITY DISTRICT
The Central City district is along the southern border of Gilpin County in the vicinity of the towns of Central City and Blackhawk. This district is the northern segment of the rich chain of ore deposits between Central City and Idaho Springs.
Both the mining history and geological setting of Idaho Springs and Central City are so entwined that they are considered an entity and are discussed together in the Clear Creek County section of this report (p. 96).
The production of the Central City district rightfully belongs under the Gilpin County heading. The district is credited with 95 to 99 percent of the $84,114,389 worth of gold mined in the county through 1923 (P. K. Sims, oral commun, 1959). Total gold production through 1959 was about 4,200,000 ounces; all but about 30,000 ounces was from lode mines.
NORTHERN GILPIN DISTRICT
The Northern Gilpin district lies north of North Clear Creek in the central part of the county and extends north to the Boulder County line. The most important mines are just south of Apex and in the vicinity of Gilpin.
Gold was first discovered in the district in June 1859 in Gamble Gulch in the Perigo area, and lodes in both the Dirt and Perigo mines were discovered in 1860 (Bastin and Hill, 1917, p. 68, 197-198). Some of the ore was very rich near the surface, and within a short time 10 or 12 stamp mills were operating (Henderson, 1926, p. 31). However, the rich and easily worked ore was soon exhausted and in 1867 only four or five companies were still operating in the district. After 1868 the camp was almost deserted until 1879 when the Perigo mine again became active and continued activity at least until 1888. The district was revived briefly during the 1930's, but from 1943 through 1959 very little activity was reported.
There was some placer mining near Rollinsville in 1897 (Henderson, 1926, p. 31). Many of the gulches in the district have placer ground, but production was small until 1937-39, when dredging operations along South Boulder Creek recovered 7,724 ounces of gold.
Though the early production is unknown, it was probably small, and the total amount of gold mined in the district through 1959 was probably 35,000 ounces.
The bedrock of the district is a complex of Pre-cambrian rocks, comprising schist of the Idaho Springs Formation and quartz monzonite gneiss, cut by Boulder Creek Granite and by quartz diorite. The Precambrian rocks are intruded by irregular stocks and dikes of quartz monzonite porphyry and dikes of bostonite porphyry of Tertiary age.
The ore deposits are pyritic gold veins in fractures, most of which trend northeast; a few strike west or northwest. The primary ores are in general low grade and contain less than half an ounce of gold and 1 ounce or less of silver to the ton, but some veins have higher grade ore in the supergene-enriched upper parts. Many of the deposits are discontinuous lodes along shear zones that carry pyrite disseminated through several feet of sheared rock; however, fissure fillings are more abundant. Besides pyrite the ore contains variable amounts of chalcopyrite and locally, some galena and sphalerite. Quartz is the common gangue mineral in the veins (Lovering and Goddard, 1950, p. 193-194).