By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Summit County, in north-central Colorado, is bounded on the west by the crest of the Gore Range, on the northeast by the crest of the Williams River Mountains, and on the southeast by the Front Range.
Summit County produced, through 1959, gold, silver, lead, and zinc, all having a gross value of about $74,177,600. Of this total, a minimum of $21,673,180 (1,010,670 ounces) represents gold - $15,644,143 (739,511 ounces) in placer gold and $6,024,769 (271,159 ounces) in lode gold. Summit County ranks first in Colorado in placer gold production and tenth in the State in total gold production. Almost the entire output has come from the southern part of the county, in which the Breckenridge and Tenmile are the major districts.
The Breckenridge (Blue River) district includes the upper valley of the Blue River which lies between the Front Range on the east and the Tenmile Range on the west. The district has produced large amounts of placer and lode gold, silver, lead, and zinc. It is credited with an output of about 1 million ounces of gold, of which about 735,000 ounces came from placers and the remainder from gold lodes and as a byproduct of silver-lead-zinc ores.
Rich gold placers were discovered along Georgia Gulch on the north side of Farncomb Hill in 1859, and soon afterward placers were discovered along many streams and gulches in the district (Ransome, 1911, p. 16-20). From 1859 to 1862, the Georgia Gulch placers reportedly yielded $3 million in gold; most of the $5.5 million worth of placer gold produced in Summit County from 1860 to 1869 is credited to the Breckenridge district (Henderson, 1926, p. 33, 245).
Production of placer gold decreased in the 1880's and 1890's but in 1908 it was rejuvenated by dredging operations. By 1909, four dredges were operating in the district (Henderson, 1926, p. 35). After the late 1930's activity declined, and after 1947 there was only small sporadic placer production.
In 1880, about 20 years after the discovery of the placer deposits, gold was found in lodes on Farncomb Hill and elsewhere in rapid succession. Lodes were the chief source of gold in the Breckenridge district from about 1885 until 1908, when the large-scale placer dredging operations began.
Some gold in the Breckenridge district has also been derived as a byproduct from silver-lead-zinc ore, chiefly from the Wellington mine, which became an important producer in 1910 (Lovering, 1934, p. 3). This mine operated continuously to 1929 and sporadically thereafter until 1958. Lode mines in the district were virtually inactive in 1959.
Precambrian schist, gneiss, and granite, which underlie the sedimentary rocks of the district, are exposed in only a few places. The Maroon Formation of Pennsylvanian and Permian age rests directly on the Precambrian rocks. It is overlain by the Morrison Formation of Jurassic age and the Dakota Sandstone, Benton Shale, Niobrara Formation, and Pierre Shale, all of Cretaceous age.
Folding and faulting at the end of Cretaceous time was accompanied by intrusion of sills and dikes of monzonite and a small stock of quartz monzonite porphyry. The sedimentary rocks were folded into an asymmetric syncline that generally dips to the west (Lovering, 1934, p. 3-22).
Ransome (1911, p. 124-157) recognized three major types of ore deposits in the district: zinc-lead-silver-gold veins, gold-silver-lead stockworks and veins, and the gold veins of Farncomb Hill. The zinc-lead-silver-gold veins are in a stock of quartz monzonite porphyry and in the surrounding country rocks. Chief minerals are sphalerite, argentiferous galena, and pyrite in a gangue of carbonate and very little quartz. Gold is in the pyrite. The Wellington mine is the chief producer in this group.
The gold-silver-lead stockworks are masses of quartz monzonite porphyry interlaced with a network of veinlets of pyrite, sphalerite, and galena in a gangue of sericitized porphyry. The Farncomb Hill gold veins are in shale country rock near a mass of quartz monzonite porphyry on Farncomb Hill. Though some veins enter the porphyry, ore bodies do not. The Farncomb Hill veins are remarkably persistent considering their narrowness. They are rarely more than one-half inch wide, and some have been traced for distances of 300 feet. These veins are noted for their rich pockets of native gold which have supplied specimens of wire and leaf gold to museums and collectors throughout the world. The vein mineralogy is simple; pyrite, chalcopyrite, sphalerite, and galena are in a calcite gangue. Native gold occurs in local thickenings or pockets and also is embedded in calcite, sphalerite, or galena.
Placers, according to Ransome (1911, p. 175), are of three typesâ€”bench or high-level placers, deep or low-level placers, and shallow gravels in gulches. The gulch gravels, notably those on Farncomb Hill, were the first placers worked. The bench gravels were mined by hydraulic methods and were largely worked out by 1900. The deep placers, which are along the Swan and Blue Rivers, have been worked by dredges and were the source of most of the placer gold of the district after 1900.
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).