Park County Colorado Gold Production
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).