Boulder County Colorado Gold Production
The Grand Island-Caribou district is in southwest Boulder County, about 17 miles west of Boulder and 4 miles northwest of Nederland.
Silver is the chief metal produced in the district, but moderate amounts of lead and some lode gold have also been produced. Prospectors discovered veins near Caribou in about 1860 (Henderson, 1926, p. 38) ; however, they did not recognize the silver ore until 1869, when one of them, after seeing some silver ore from Nevada, returned to the district and made the first location. Other claims were staked the same year, and by the end of 1871 most of the rich lodes in the district had been found and production increased rapidly. Ore was produced from the district until 1893, when a drop in the price of silver forced most mines to close; however, some of the richer gold mines resumed operation in 1898. Since 1900 activity in the area has been limited to sporadic attempts to reactivate certain mines or to mill dump material from some of the larger mines (Moore and others, 1957, p. 521-522). The district was almost dormant from 1952 through 1959.
Nearly all the output of the district has come from lead-silver veins containing a little gold, though some gold-silver ore has been mined in outlying areas. The ore in the upper levels of many of the mines was very rich in silver, probably because of secondary enrichment (Moore and others, 1957, p. 521).
There is no record of the early gold production from the Grand Island-Caribou district. It has been estimated (Moore and others, 1957, p. 522) that the total value of lead and silver produced before 1924 was about $6 million, but no figures are given from which to estimate the gold production, though it was probably small. The gold production from 1932 through 1959 was 10,006 ounces.
The eastern and western parts of the district are underlain by schist and gneiss of the Idaho Springs Formation and by small bodies of Boulder Creek Granite. These rocks, which are of Precambrian age, were intruded by a composite stock of calcic monzonite and quartz monzonite which occupies the central part of the district. A striking feature is the occurrence of numerous masses of pyroxenite, titaniferous magnetite, and hornblendite in the stock (Smith, 1938, p. 171, 174).
Lead-silver veins in the monzonite stock in the vicinity of Caribou Hill have been the most productive in the district. The gold veins are in the Precambrian rocks and are older than the lead-silver veins. Quartz, pyrite, chalcopyrite, covellite (?), and minor galena and sphalerite are the predominant minerals of the gold veins. The lead-silver veins chiefly contain quartz, pyrite, sphalerite, galena, chalcopyrite, argentite, pyrargyrite, carbonates, and secondary azurite, malachite, native silver, and limonite (Moore and others, 1957, p. 526-528).