By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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Eagle County, in mountainous west-central Colorado, produced about 359,900 ounces of gold through 1959.Although some prospecting was done in Eagle County in the 1860's it was not until 1879, after the great rush in 1877-78 to the Leadville district about 20 miles to the south (Henderson, 1926, p. 41), that rich oxidized silver-lead carbonate ore was discovered on Battle Mountain in the Gilman district. Many claims were located that year, and in 1880 silver valued at $50,000 was produced (Henderson, 1926, p. 47).
In the early 1880's prospectors swarmed over the county and opened small silver and gold mines here and there, but most of these early camps were short lived. Only the Gilman district became a major producer. More than 99 percent of the total metal output credited to Eagle County has come from this district.
GILMAN DISTRICT
The Gilman (Battle Mountain, Red Cliff) district, in southeastern Eagle County on the northeast flank of the Sawatch Range, is between Gilman and Red Cliff, about 20 miles north of Leadville. Gold has been extracted from pyritic gold ores and as a byproduct of base-metal ores.
The initial discovery in 1879 of silver-lead ore in limestone was followed in 1884 by finds of gold ore in the underlying quartzite. The relative importance of the metals mined varied periodically in the history of the district. Before 1905 silver and gold were the major commodities, whereas lead and copper were of minor importance.
From 1905 to 1930 zinc was the principal product and from 1931 to 1941 silver-copper ore containing considerable gold was mined. Zinc again regained importance from 1942 through 1959. The total gold production of the district through 1959 was roughly 348,000 ounces.
Precambrian granite, schist, and gneissic diorite are exposed in the bottom of Eagle Canyon and are overlain by thin lower Paleozoic formations that include the Sawatch Quartzite of Cambrian age, the Harding Sandstone of Ordovician age, the Chaffee Formation of Devonian age, and the Leadville Limestone of Mississippian age. Overlying these units is a thick section of Pennsylvanian and Permian (?) sedimentary rocks.
A sill of Cretaceous or Tertiary quartz latite appears a few feet above the Leadville Limestone throughout the district. The sedimentary rocks dip about 12° NE. and are cut by bedding-plane faults and a few weak high-angle faults (Ogden Tweto and T. S. Lovering, in Vanderwilt and others, 1947, p. 379-381).
Ore bodies occur in (1) veins in Precambrian rocks and in the Sawatch Quartzite and (2) in replacement deposits in quartzite and limestone of Devonian and Mississippian age. Most production has come from replacement bodies in the Leadville Limestone and the Dyer Dolomite Member of the Chaffee Formation. The veins in the Precambrian rocks contain pyritic gold and complex sulfide ores in which sphalerite, chalcopyrite, galena, and pyrite are the chief minerals.
The pyrite-gold veins are almost entirely of pyrite with a little gold. Most of the veins terminate or become thin at the base of the Sawatch Quartzite, but those that extend into the quartzite contain scattered pockets of gold-silver tel-lurides, chief among which are petzite and hessite.
A large part of the early output from the Gilman district came from oxidized manto or bedding-vein deposits in a breccia zone in the Sawatch Quartzite, about 180 feet above the base. Two distinct stages of mineralization are recognized in these deposits. The first deposited largely pyrite, a very little chalcopyrite, and inconsequential amounts of gold and silver. During the second stage, manganosiderite, pyrite, chalcopyrite, galena, sphalerite, and pyrite were deposited. Gold and silver are chiefly associated with the chalcopyrite of the second stage.
The replacement deposits in the limestones consist of chimneys of pyritic silver-copper ore and mantos of base-metal sulfide ores. The chimneys are downward-tapering pipes that extend from the ends of manto ore bodies near the top of the Leadville Limestone downward into the Parting Quartzite Member of the Chaffee Formation. The chimneys are roughly circular or elliptical and are as much as 300 feet in diameter at the top and taper downward.
There is no physical break between the two types of ore bodies, but there is a pronounced mineralogic difference. The chimney ore, the chief source of the gold, consists of a core of pyrite containing minor quantities of other minerals which make them valuable for silver, copper, and gold. The chief copper mineral is chalcopyrite. Silver and gold are associated with chalcopyrite, galena, and with a group of late copper and silver minerals including tetrahedrite, freibergite, polybasite, stromeyerite, bournonite, and schapbachite. Late accessory minerals include manganosiderite, dolomite, barite, apatite, and quartz. The galena contains small inclusions of hessite and a little petzite, and the petzite contains minute blebs and veinlets of free gold.
The manto ore bodies are valued mainly for zinc. The minerals of these deposits are sphalerite, pyrite, manganosiderite, minor galena, and accessory chalcopyrite, barite, dolomite, and quartz. Oxidized parts of these deposits contain considerable lead, silver, and gold. The mantos are 50 to 300 feet wide, 5 to 150 feet thick, and as much as 4,000 feet long. All are in the Leadville Limestone (Ogden Tweto and T. S. Lovering, in Vanderwilt and others, 1947, p. 381-385).
