By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Dolores County, in southwestern Colorado, contains deposits of silver, lead, and zinc in the mountainous eastern part, which is a dissected laccolithic dome. The western part of the county is within the Colorado Plateaus. A relatively unimportant gold mining area, Dolores County has had a total gold production through 1959 of about 104,500 ounces; almost all production has been a byproduct of silver, lead, and zinc deposits of the Rico district.
The Rico (Pioneer) district lies near the southwest end of the Colorado mineral belt (Fischer and others, 1946) near the headwaters of the Dolores River.
Prospectors first came into the region in 1861 (Ransome, 1901a, p. 240-242), but the first claims were not located until 1869. In 1872 the first smelting furnace was erected and produced three bars of bullion. The early results were not encouraging, however, and the area was abandoned. Prospectors came into the region again in 1877 and, in the spring of 1879, discovered rich oxidized silver ore on Nigger Baby Hill and in the Chestnut vein on Newman Hill. Prospectors from neighboring camps rushed to the region, and the town of Rico was founded. Other major discoveries were made soon afterward, smelters and mills were built, and in 1880 the Rio Grande Southern Railroad reached Rico. The rich Enterprise blanket deposit was found in 1887, which insured some permanence to the district (Ransome, 1901a, p. 240-242). Mining was almost continuous through 1959 but it fluctuated considerably. Metal production was relatively high during the periods 1889-94, 1924-29, and during World War II. Before 1904 silver was of greatest value in the ore (Ransome, in Cross and Ransome, 1905, p. 14), but after 1904 lead and zinc were the important products. Most of the gold, which was a byproduct, was produced before 1910, especially between 1889 and 1894 (Henderson, 1926, p. 117). After 1910, gold production declined at an irregular rate, and for most years through 1959 it was below 500 ounces.The total gold production of the Rico district is unknown, but of the 104,000 ounces credited to Dolores County through 1959, it seems reasonable to credit the Rico district with about 100,000 ounces.
The Rico district is in the central part of a laccolithic dome comprising the Rico Mountains in the southeast corner of Dolores County. In the central part of the uplift, Precambrian quartzite and schist are exposed. These are flanked on all sides by sedimentary rocks of Cambrian, Devonian, Mississippian, Pennsylvanian, Permian (?), Triassic, and Jurassic ages. All the rocks are intruded by sheets and sills of hornblende monzonite porphyry and by a stock of quartz monzonite of Tertiary (?) age. The central part of the dome is complexly faulted (Cross and Ransome, 1905, p. 2-11).
Much of the ore mined in the district has come from mineralized solution breccia in gypsum beds of the Hermosa Formation of Pennsylvanian age. These are known as blanket deposits. Other ore deposits are replacement bodies in Devonian and Pennsylvanian limestone beds and fissure veins. The age of the mineralization is thought to be late Tertiary (Cross and Ransome, 1905, p. 14-19).
The ore is of two general types: (1) pyritic ore, most of which is of too low grade to be mined, and (2) silver-bearing galena ore, in which galena, sphalerite, chalcopyrite, pyrite, argentite, proustite, polybasite, and silver-bearing tetrahedrite are the characteristic minerals. Much of this silver ore contains small amounts of gold that is associated with sphalerite and chalcopyrite and locally is associated with tellurium and bismuth (Cross and Ransome, 1905, p. 14-15).
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