Summit County Colorado Gold Production
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The Tenmile district includes the mining camps of Kokomo, Robinson, and Frisco, which lie in the Tenmile valley on the west side of the Tenmile Range. The district has chiefly produced silver, lead, and zinc. Small amounts of gold have been recovered as a byproduct from the base-metal ores, and small amounts have been derived from placer deposits in McNulty Gulch near Kokomo.
Gold placer deposits were discovered in McNulty Gulch in 1861 (Hollister, 1867, p. 326), and some time later oxidized gold ore in lodes was discovered in the Tenmile Range on the east side of the valley. Neither the placers nor lodes proved significant, and no important mining development took place until 1879-80 after the discovery of rich silver deposits in 1877 in the Leadville district 12 miles to the south (Emmons, 1898, p. 1). Rich silver ore was discovered on the west side of the Tenmile valley during the summer of 1878, and as a result of this discovery many claims were located and the district developed rapidly (Henderson, 1926, p. 11, 36). By the spring of 1881 most of the properties that were to become the large producers had been located. The total mine output in 1881 was estimated to be worth about $2 million, and the district became the leading producer in Summit County (Henderson, 1926, p. 237). The blanket of oxidized and secondarily enriched ore in the district was rapidly mined out between 1880 and 1890 and as a consequence the bonanza period of the camp came to a close.
In 1896, the Wilfley table was put into operation. This device separated the components of the lead-zinc sulfide ores and enabled these ores to be exploited profitably. The district entered a long period of prosperity that lasted until 1923 (Koschmann and Wells, 1946, p. 55-56). The demand for base metals during World War II rejuvenated the mines, and from 1942 through 1949 the district was one of the most productive in the State. Falling metal prices caused most of the mines to close in 1950, and the district was virtually inactive from 1950 through 1959. Total gold production through 1959 was about 52,000 ounces, most of which was a byproduct of base-metal ores. An unknown but small amount of gold was mined from placers.
The Tenmile district is in a belt of folded and faulted Paleozoic sedimentary rocks bounded on the east and west by high mountain ranges of Pre-cambrian gneiss, schist, and granite (Koschmann and Wells, 1946, p. 57-96). The Paleozoic rocks are of sedimentary origin and lie unconformably on a Precambrian basement. Because of tectonic activity during Paleozoic time, the Paleozoic section is incomplete from place to place. The most persistent sedimentary rocks are the Minturn Formation of Pennsylvanian age and the Maroon Formation of Pennsylvanian and Permian (?) age. Tertiary intrusive rocks, which are predominantly sills and a few small irregular bodies of several textural varieties of quartz monzonite, cut the Paleozoic formations. The rocks were folded into a north-trending syncline that plunges north. The east limb of the syncline is cut by the north-trending Mosquito fault, one of the major faults in the region.
The ore deposits of the Tenmile district are of two types: massive sulfide replacement bodies in limestone beds of the Minturn Formation and sulfide veins in Tertiary igneous rocks and Precambrian rocks. The most productive ore has been in the replacement deposits which consist of aggregates of pyrite, pyrrhotite, marcasite, sphalerite, galena, and chalcopyrite. Quartz and siderite are the chief gangue minerals. The veins are in fissures and faults and contain galena, sphalerite, pyrite, and local molybdenite in a gangue of calcite, barite, and quartz (Koschmann and Wells, 1946, p. 100-105).