By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Park County is in the mountainous central part of Colorado and includes a central broad basin, called South Park, rimmed on the east and north by the Front Range, on the west by the Mosquito Range, and on the south by an unnamed range of low hills. The county is drained by the South Platte River and its tributaries, most of which head in the Mosquito Range.
Gold is the principal mineral mined in the county, which ranks eighth in the State in gold production. Gold, silver, and other metals totaling $49,172,800 in gross value had been produced through 1959. Of this total about $36,725,000 (1,364,430 ounces) represents gold - $27,305,000 in lode gold and $9,417,000 in placer gold. Most of the production has come from the northwest part of the county along the east slope of the Mosquito Range.
Placer gold was discovered in Park County in 1859; this was one of the earliest mineral discoveries in Colorado. Prospectors, some arriving by way of the Arkansas River and others arriving by way of the South Platte River, found gold in the streams of South Park and later rich lodes at the headwaters of the South Platte and its tributaries. Nearly all the lode gold has come from the Alma district, and the placer gold has come from the Fairplay, Tarryall, and the Alma districts.
The Alma district lies east of the Leadville district along the east slope of the Mosquito Range and includes the Mosquito-Buckskin, Montgomery, Horseshoe, and Alma placers.
The earliest discoveries in the Alma district appear to have been of lode deposits along the headwaters of Buckskin Gulch and the South Platte River (Henderson, 1926, p. 36-38). The Phillips lode in Buckskin Gulch was discovered in 1860, and other lodes were discovered in rapid succession soon after. The gold obtained from the lodes from 1860 to 1867 was worth about $710,000 (Henderson, 1926, p. 37, 196).
After the easily disintegrated and oxidized ores near the surface were exhausted, the mines were closed because the sulfide ores at depth could not be successfully treated by amalgamation.
In the summer of 1871, silver ore was discovered on Mount Lincoln and Mount Bross, and mining activity in the district increased. Silver was the chief metal mined through 1885, and high silver production was maintained until 1892 when the price of silver began to drop. In later years production of silver fluctuated considerably.
In 1873, outcrops of the London vein were discovered (Singewald and Butler, 1941, p. 36), but the London mine was not located and opened until 1875. It became the largest producer in the district and, though production fluctuated, it operated almost continuously until 1942.
The output of lode gold of the Alma district through 1959 was about 1,320,000 ounces valued at about $27,275,000.
Gold placers in the Alma district are found along the South Platte River east of Alma. These were first mined in the early 1870's, and during the first 3 years they produced $19,000 in gold (Singewald, 1950, p. 145). The greatest productivity was from 1904 to 1942 when about 27,600 ounces was recovered. There was some production during 1947-52, but the amount has been reported with that of other districts. The placers were virtually inactive from 1952 through 1959. Probably less than 200 ounces of gold has come from some of the other streams in the district. The total minimum placer production is about 28,000 ounces.
Exposed bedrock in the district is of Precambrian, Paleozoic, and Tertiary (?) ages. The Precambrian rocks are contorted gneiss, schist, and granite. They are overlain by Paleozoic sedimentary rocks consisting of the Sawatch Quartzite of Cambrian age, the Manitou Limestone of Ordovician age, the Chaffee Formation of Devonian age, the Leadville Limestone of Mississippian age, the Minturn Formation of Pennsylvanian age, and the Maroon Formation of Pennsylvanian and Permian age.
Intrusive sills and dikes of quartz monzonite porphyry of Tertiary (?) age cut the older rocks. The stratified rocks dip eastward, but the dip is locally modified by folds and faults. The largest fault is the London, a reverse fault along which the beds have been displaced 1,600 feet. Most of the major ore deposits are near the London fault or the Cooper Gulch fault, another major reverse fault (Singewald and Butler, 1941, p. 7-28).
The principal types of ores are (1) gold-bearing sulfide veins in or adjoining porphyry sills near the base of the Lower Pennsylvanian strata, (2) silver-lead deposits in limestones, and (3) gold deposits in the Sawatch Quartzite of Cambrian age. The gold deposits, both in the porphyry sills and in the Sawatch Quartzite, are flanked by small silver-lead deposits.
The gold-bearing sulfide veins are composed of milky quartz with subordinate pyrite, sphalerite, galena, and chalcopyrite, and native gold which is seen only in exceptionally rich ore. The gold deposits in the Sawatch Quartzite contain the sulfides previously mentioned in a gangue of quartz and iron and manganese-bearing dolomite. The silver-lead deposits in limestones are unimportant as a source of gold (Singewald and Butler, 1941, p. 38^0).
The gold placers are glacial outwash deposits. The mineralized area on North Star Mountain, near the head of South Platte River along the Continental Divide, probably contributed to the bulk of the gold in the Alma placer as well as in minor placers farther north (Singewald, 1950, p. 149).
Gold production from the Fairplay (Beaver Creek) district was entirely from placer deposits and included production from the Snowstorm and Fairplay placers along the South Platte River and small placers along Sacramento and Beaver Creeks.
Gold placers were discovered in the district about 1859, and through 1872 their output was valued at about $1 million (48,380 ounces) (Henderson, 1926, p. 36-38). From 1872 through 1938 the placers were worked sporadically without any spectacular results. From 1939 to 1951, the district was rejuvenated and about 125,000 ounces of gold was produced. These operations were terminated in 1952, and the district was virtually idle from 1952 through 1959. The minimum total gold yield of the district through 1959 was about 202,000 ounces.
By far the most productive placers are outwash gravels which extend downstream from the moraines formed by the South Platte glacier; smaller deposits have been found upstream (Singewald, 1950, p. 146-161).
Almost all the gold production from the Tarryall district has come from placer deposits along the upper reaches of Tarryall Creek and its tributaries, northwest of the town of Como. Placer gold was discovered in August 1859, probably a little earlier than the discovery in the Fairplay district. These placers are credited with an output from 1859 to 1872 of about $1 million (48,380 ounces) (Henderson, 1926, p. 36, 187).
Unrecorded and probably small-scale activity continued into the early 1900's. A brief resurgence occurred in 1941-42 and again in 1947, but the district was dormant from that time through 1959. The total minimum gold production from the Tarryall placers through 1959 was about 67,000 ounces. Lode mines in the district yielded less than 250 ounces of gold.
The placer deposits are of two kinds: glacial moraines and outwash gravel deposits downstream from the moraines (Singewald, 1950, p. 147-148, 162-168). The bulk of the gold has been mined from outwash deposits where the gold is concentrated just above bedrock; however, all the gravel contains some gold.
The gold in the placers was probably derived from lodes in the mineralized area at the heads of Montgomery and Deadwood Gulches, tributaries of Tarryall Creek (Singewald, 1950, p. 148).