Boulder County Colorado Gold Production
The Gold Hill-Sugarloaf district, the largest gold producer in Boulder County, contains several small mining camps within an area of about 12 square miles in the central part of the county, 3 to 8 miles northwest of Boulder. The largest of these is Gold Hill; others are Sugarloaf, Rowena, Salina, and Sunshine. Gold was the chief metal produced in the district, although in most deposits silver is associated with the gold.
Placer gold was discovered in this district in January 1859, very early in the history of mining in the State, and $100,000 (4,838 ounces) worth of gold was worked from these placers during the first summer (Goddard, 1940, p. 106). Gold-bearing veins were discovered nearby during the summer of 1859, and in consequence several thousand people flocked to the district. The oxidized surface ore yielded free gold and recovery was made by sluice, arrastre, and stamp mill. When these ores were mined out after a few years, activity in the district declined sharply. Mining activity increased markedly in 1872 when the gold-silver telluride, petzite, was discovered at the Red Cloud mine at Gold Hill. In 1873 telluride ore was discovered in the Cold Spring mine. The ore was extremely rich, yielding an average of $1,500 per ton, and in 2 years these two mines produced about $600,000 in gold from about 400 tons of ore (Henderson, 1926, p. 39). Many more veins were found from 1875 to 1880 and activity was sustained at a high level until 1904, after which mining declined (Goddard, 1940, tables p. 108-109). The district was rejuvenated in the middle and late 1930's but slumped sharply during World War II; after the war it failed to regain its former importance and was almost inactive during 1950-59.
The lode-gold production from the Gold Hill-Sugarloaf district from the time of discovery through 1903 could not be ascertained. According to Henderson (1926, table, p. 106), Boulder County from 1859 through 1903 produced about $13,435,000 worth of gold. It seems reasonable to assume that the Gold Hill-Sugarloaf district produced at least one-fourth of this amount—about $3,360,000 or 162,500 ounces. The minimum total output of the district through 1959 was about 412,000 ounces, mostly from lodes. The placer production probably did not exceed 3,000 ounces (Lovering and Goddard, 1950, p. 240).
The following brief description of the geology and ore deposits of the district is mostly from Goddard (1940, p. 110-139).
Schists and gneiss of the Idaho Springs Formation are intruded by a batholith of Boulder Creek Granite and dikes of Silver Plume Granite, all Pre-cambrian in age. Sedimentary rocks of Pennsyl-vanian age unconformably overlie the Precambrian rocks about 2 miles east of the district. The Precambrian rocks have been cut by a series of porphyry dikes of Laramide age that range in composition from diabase to alaskite. The mineral deposits are chiefly in the northern part of the Boulder Creek batholith; most of the veins are in the granite and a few in the western part of the district extend into the schist.
The distribution of ore deposits was strongly influenced by conspicuous silicified, hematite-stained breccia zones, called breccia reefs. The most prominent of these are nearly vertical and trend N. 25°-50° W.; others can be grouped into sets that trend N. 70°-80° W., N. 60°-75° W., and N. 5°-30° E. The gold deposits are in telluride and pyritic veins that occupy fissures, most of which strike northeast. Ore is localized where these veins cross the breccia reefs. Most of the productive veins are more than half a mile long and from 1 to 5 feet wide, but some are from 10 to 30 feet wide. The order of deposition of the veins is not certain. Silver-lead veins appear to be the oldest in the district; these are followed by the gold telluride veins, and then by the pyritic gold veins. A few of the silver-lead veins, however, seem to be related to the pyritic gold veins.
Gold tellurides, the most abundant of which are petzite and sylvanite, are the most important ore minerals in the Gold Hill-Sugarloaf district, but free gold is also abundant. Other tellurides occurring in small amounts are hessite, altaite, and coloradoite. Fine-grained pyrite and very small amounts of galena and sphalerite are associated with the ore minerals. Horn quartz and sugary quartz are the chief gangue minerals. Roscoelite is closely associated with the tellurides and free gold. Anker-ite and other carbonates also are associated with the telluride ores but are younger than the telluride minerals.
In the pyritic gold veins, pyrite and chalcopyrite are the most abundant ore minerals, but free gold is abundant in some veins. The chief gangue mineral is sugary to glassy quartz; ankerite is found in some veins.