New Mexico Mineral Symposium — Abstracts

The greater Alma mining district occupies an area in northwestern Park County, Colorado. The crest of the Mosquito Range, with four peaks over 14,000 ft, forms the western boundary of the district, and the valley of the South Platte River lies on the east.

Gold prospecting by Spanish explorers dates to before the 1850s (Misantoni et al., 1998). The first wave of mining in the area was for placer gold, which started in 1859 at Buckskin Gulch and lasted through the early 1860s. The second wave of mining was for silver and started in the 1870s.

Most of the district's more than 100 adits, shafts, prospects, and named mines are above tree line and were developed in the 1860s and 1870s. These high-elevation mines were called "aerial mines" by newspapers at the time. Mining at these elevations required new mining methods and technologies to be developed. Miners also had to adapt to the extreme winter conditions at high elevations.

The small town of Alma, established in 1873, is the only important community in the district, though Park City and a few other towns had significant populations during the boom years that extended from 1860 to the early 1940s. Significant production of gold, silver, lead, and zinc extended into the 1950s and later in some mines.

General geology
The Mosquito Range is an east-dipping tilted fault block consisting mostly of Paleozoic sedimentary rocks and Cretaceous and Tertiary felsic sills and stocks (Scarbrough 2001). Thrust faults, reverse faults, and normal faults offset the sedimentary rocks and act as loci for gold-silver-lead-zinc mineralization dating to about 35 Ma. Major faults include the steeply northeast-dipping London fault, a reverse fault in the southwest corner of the district, and the northeast-dipping Frontal fault, a reverse fault in the northeast corner of the district. The London fault is flanked by an anticline to the northeast and a syncline to the southwest, both of which preceded and were modified by the faulting. Many of the highest gold and silver values in the district occur within a few tenths of a mile to a few miles of the major faults. Placer deposits occur in both Quaternary and Pleistocene deposits along streams draining the east flank of the Mosquito Range.

Mining subdistricts
Scarbrough (2001) divided the greater Alma mining district into six smaller units: the Alma placers, Consolidated Montgomery, Buckskin, Horseshoe, Mosquito, and Pennsylvania subdistricts. Boundaries of these subdivisions are somewhat arbitrary, and the deposit types overlap from one subdivision to the next. Of the six, we will focus on those that were most productive: the Mosquito, Alma placers, Buckskin, and Consolidated Montgomery subdistricts (arranged in order of decreasing gold production). Production figures came from Scarbrough (2001).

Production from the Mosquito subdistrict, 5-7 mi west of Alma, included at least 570,296 ounces of gold, 446,698 ounces of silver, 331 tons of copper, 7,237 tons of lead, and 300 tons of zinc through 1956. (Production figures for the periods between 1860 and 1932 and after 1956 are not available for the district as a whole.) Paleozoic sedimentary rocks, including the Mississippian Leadville Limestone and Pennsylvanian Minturn Formation, host most of the individual deposits. Included are veins, fault/fissure deposits, and mantos associated with Tertiary (?) porphyry sills (Scarbrough 2001). Minor production came from placers and some vein-type uranium deposits in Proterozoic granite and metamorphic rocks.

Major mines of the Mosquito subdistrict include those of the London mine group (London, North London/Vienna, South London, London Extension, and Butte mines), the Hock Hocking, Orphan Boy, and the Fanny Barret. Mining commenced in 1873, and intermittent production continued through 1989. Johansing and Misantoni reported proven, probable, and inferred resources of 502,000 short tons of ore ranging between 0.10 and 0.41 ounces per ton of gold (Scarbrough 2001).

The Alma placers occupy an area on the east side of the South Platte River immediately northeast, east, and southeast of the town of Alma. Total production is estimated at 20,271 ounces of gold and 4,140 ounces of silver between 1932 and 1953. Gold occurs mainly in till and outwash related to the Wisconsin Stage of glaciation. Concentrations of gold occur in localized channels and at the boundary between Pleistocene gravels and Paleozoic shale and sandstone bedrock. Currently, production at the Alma Placer mine averages about 600 ounces per year of 82% gold and 18% silver.

The Snowstorm dredge was built in San Francisco between 1939 and 1941 and assembled at the mining site in Park County. The four-level dredge was in use by 1941 and then shut down during World War II, and then resumed operations after 1947 for 16 months. The dredge operated briefly in 1976. The Snowstorm dredge is the largest and last dragline dredge in Colorado. It processed 600 tons of gravel an hour in the goldfields between Alma and Fairplay.

The Buckskin (or Buckskin Joe) subdistrict includes the mines of the Phillips group, the Excelsior, the Ten-Forty, and the Criterion. The Buckskin subdistrict is located approximately 2-2.5 mi west of Alma. Settled in 1860, the area produced at least 19,782 ounces of gold, 131,219 ounces of silver, 182 tons of copper, 730 tons of lead, and 4,500 tons of zinc between 1932 and 1957. Deposits include mantos in the Leadville Limestone; veins and mantos in the Devonian Parting Quartzite, the Cambrian Sawatch Quartzite, and the Peerless Shale; and minor placers.

The Consolidated Montgomery subdistrict occupies an area at or near the crest of the Mosquito Range, approximately 5-7 mi west and northwest of Alma. Important mines include the Russia, Moose, Dolly Varden, Atlantic-Pacific, the Ling, and the Magnolia. Production came mainly from precious- and base-metal-sulfide veins in the Sawatch Quartzite; silver-lead mantos and veins in the Leadville and (Ordovician) Manitou limestones; and minor oxidized deposits and placers. Metals recovered between 1932 and 1957 included nearly 6,000 ounces of gold, 79,131 ounces of silver, 19 tons of copper, 100 tons of lead, and 41 tons of zinc. Today the Montgomery Dam, part of Colorado Springs water supply, fills the valley with water where the town of Montgomery once stood.

Minerals
A search of Eckel 1997; MINDAT.org; articles in The Mineralogical Record (including Murphy and Hurlbut 1998 and Misantoni et al. 1998); and other sources suggests that more than 87 minerals have been reported for the deposits of the greater Alma mining district. Of these, at least 60 are reported to occur in the Sweet Home mine (Moore et al.
1998). In part, the variety of minerals in the Sweet Home probably reflects the careful collection of samples by Collector's Edge Minerals, who exploited the spectacular rhodochrosite finds of the 1990s. Relatively few documented samples are preserved from other Alma district mines, and most of those are not aesthetically attractive.
The following minerals are reported to occur in the district, but more exhaustive research would no doubt add more to the list. 

The district's mining activity, combined with other mining camps in the region, contributed to the growth of Colorado and accelerated the settlement of the West. Today old mining headframes, abandoned tramways, deserted mills, and rusting mining equipment remain as symbols of Colorado's mining heritage.

References:

1. Eckel, E. B., et al., 1997, Minerals of Colorado: Golden, Colorado, Fulcrum Publishing.
2. Misantoni, D., Silberman, M. L., and Lees, B. K., 1998, Geology of the Sweet Home mine and Alma district: The Mineralogical Record, v. 29, no. 4, pp. 101-114.
3. Moore, T., et al., 1998, The Sweet Home mine, Park County, Colorado: The Mineralogical Record, v. 29, no. 4,192 pp.
4. Murphy, J. A., and Hurlbut, J. F., 1998, Minerals of the Sweet Home mine: The Mineralogical Record, v. 29, no. 4, pp. 115-122.
5. Scarbrough, Jr., L. A., 2001, Geology and mineral resources of Park County, Colorado: Colorado Geological Survey, Resource Series 40.