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 FrontRange. Summit County ranks first in Colorado in placer gold production and tenth in the State in total gold production. 1,010,670 ounces of gold has been produced through 1959.
Mineralization in the Breckenridge area consists of both placer deposits and primary lode deposits. The placer deposits near Breckenridge were some of the richest in Colorado. Virtually every creek bed and dry gulch in the area contained gold. A few of them (like French Gulch, Illinois Gulch, and Georgia Gulch) were legendary producers.
Lode deposits consisting of gold and silver-bearing polymetallic sulfide veins are also found in the Breckenridge District. By far, the most important of these occur on Farncomb Hill. On the western side of the hill, veins containing native gold, pyrite, chalcopyrite, and galena are hosted within a Laramide quartz monzonite intrusion. On the southern side of the hill, the same ore minerals occur in a shattered and brecciated portion of the intrusion and in the surrounding Cretaceous shales. Other lode deposits occur on Gibson Hill, Humbug Hill, and Mineral Hill.
Native gold occurs in the Breckenridge district both in veins and placers. The principal locality for vein gold is Farncomb Hill, whence have come the innumerable specimens of "Breckenridge gold" found in museums and private collections the world over. It is probable that from no locality equally productive has so large a proportion of the gold mined been saved from the melting pot on account of the beauty and interest of its form.
Leaf gold is in exceedingly irregular masses, many of which consist of thin sepia meeting at angles that strongly suggest deposition controlled in part by the cleavage planes of a rhombohedral carbonate of the calcite group. As a rule the sides of these sepia and other less regular surfaces of the mass are covered with small crystal faces, generally the triangular face of the octahedron, up to about 3 millimeters in diameter. In some places these faces are irregularly crowded, distorted, and associated with planes not so clearly belonging to the octahedron. In other places the little triangular facets are in parallel orientation and suggest a regular imbrication. of golden scales. Skeletal crystals are common, especially octahedrons, in which each face has a triangular depression or panel.
The wire gold, of which no good illustration has been obtained, is fully as beautiful as the leaf gold, but being more fragile it is not often secured in as large specimens. It consists generally of a porous mass of curved and intricately felted or tangled crystalline xvires, which may individually be 3 inches or more in length. There are all gradations between coherent spongy masses of felted wires and single filaments, but as a rule, where the gold occurs in commercial quantity, it is aggregated into masses of considerable size. The wires vary greatly in thickness and shape. Some are short, stout, and rodlike; others are of hairlike slenderness. Some are longitudinally striated and resemble curved capillary shavings such as might be made with a graving tool; others are transversely ridged and are evidently chains of crystals regularly grown together, or may be regarded as one tremendously elongated crystal exhibiting the phenomenon known as oscillatory growth and illustrated familiarly by the striated prisms of slender quartz crystals; still others are rough or frosted in appearance and as seen under the microscope are covered with tiny crystal facets much as are the surfaces of the leaf gold. In cross section the wires may be square, ribbon-like, or irregular, and they may vary in thickness from point to point. Some are simple; others branch and send off little curled tendrils. In fact no wire is straight, and most of them are bent and twisted in various directions. Specimens of the wire gold, while all possessing a character distinctive of Farncomb Hill, are never monotonous. Each has its individuality of texture, crystallization, aggregation, shape, and shade of color.
Notwithstandhng the extreme tenuity of many of the golden filaments, these are generally so interwoven as to give the mass a coherency greater than would at first appear to belong to so delicately beautiful a mineral aggregate.
The thickness of the masses of leaf or wire gold is generally less than an inch, and, as a rule, nearly or quite equals the thickness of the vein in which they are found. The gold, however, does not occur in the clean, bright condition familiar in cabinet specimens, but is normally embedded in a reddish earthy matrix consisting largely of limonitc with oxides and carbonates of copper and various earthy impurities. The removal of this matrix by baths of acid, particularly of hydrofluoric acid, is a process requiring exceptional patience and skill--qualities which, when "gold strikes" were more frequent than now, brought their possessor into much demand. So far as could be learned all the important masses of gold taken from the veins have been associated with this oxidized matrix. The association of gold and galena, however, is not unusual. Some specimens show crystals of galena implanted on or inclosing threads of gold.
Some of the deepest gold found in Farncomb Hill, from stopes above the Fair tunnel, is associated with sphalerite in calclte. Wire gold in sphalerite is reported also from the I. X. L. mine, on the Swan near Browns Gulch, and some of the nuggets dredged from the gravels of French Creek show a similar association. Wire gold embedded in calcite has been obtained from the Key West vein at a depth of about 250 feet.
The rich gold veins of Farncomb Hill are in a class by themselves and are not known outside the area that embraces the northeast slopes of Farncomb and Humbug Hills. The important veins are limited to the western part of Farncomb Hill, in an area about 2,500 feet long and less than 1,500 feet wide. The general trend of the veins is nearly north- east, and the principal veins from west to east are the Ontario, Key West, Boss No.2, Boss, McQuery, Reveille, Carpenter, Gold Flake, Graton, Silver, Bondholder, and Fountain. They fall into two groups, which are separated by an interval of 700 to 800 feet between the Reveille and Carpenter veins.
The vein material is generally wholly or partly oxidized. The unoxidized veins contained varying amounts of pyrite, chalcopyrite, sphalerite, galena, and native gold in a calcite gangue. The chief sulfide in some of the veins is chalcopyrite, and in others it is sphalerite. In the oxidized parts of the veins the calcite and sphalerite have largely disappeared, though some of the pyrite, chalcopyrite, and galena remain.
The vein material on the whole is a rather spongy earthy limonite in which the flakes and wires of native gold occur. The pockets of native gold are closely related to the small bedding faults that dislocate the veins near the porphyry sills. Nearly all the pockets in a vein occur where it crosses a porphyry sill.
The gold is remarkably segregated in these veins. Throughout most of their extent the veins contain too little metal to be of value, but here and there are the famous "pockets" where a section of the vein 2 or 3 feet in diameter and as much as an inch thick may consist of a nearly continuous hackly mass of crystalline gold ramifying through a matrix of limonite. In 1909, in a section about 3 feet long and 2 feet broad, $1,000 worth of gold was taken from the Reveille vein. Another small pocket contained $3,000 to $4,000 worth of gold.
USGS Proffesional Paper 75
Summit County Mindat