By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Rio Grande County is in south-central Colorado in the southeastern San Juan Mountains, on the west side of the San Luis Valley. The Rio Grande River crosses the northern part of the county.
In the early 1880's Rio Grande County ranked third in the State in the production of gold, the county's chief metal. Gold output of the county through 1947 was about 257,600 ounces (about $7 million worth) ; however, from 1949 through 1958 it was only 135 ounces. Small amounts of silver, copper, and lead valued at about $330,000 have been recovered from the gold ores.
The Summitville district is in the southwest corner of Rio Grande County, high in the San Juan Mountains.
The earliest discoveries of gold in the San Juan Mountains were in 1870 in the Summitville district and in Arrastre Gulch in San Juan County. The news of these finds started a rush to the region, and within a few years most of the major mining camps were established. The early discoveries in the Summitville district were of placer gold, but available records indicate that placer production in the area was minor, probably less than $10,000 worth. The first lodes were located in 1871, the richest deposits were located by 1873, and large-scale mining began in 1875.
The ore near the surface was oxidized, rich, and easy to beneficiate. By the end of 1887 most of the known oxidized ore had been mined out, and the underlying sulfide ores were much lower grade and more difficult to mill and concentrate. During 1873-87 the Summitville district produced, mostly from the Little Annie mine, about $2,064,000 in gold and silver, 95 percent of which was gold (Steven and Ratte, 1960b, p. 6).
Production declined sharply in 1888 and fluctuated considerably in the years through 1925. A deposit discovered in 1926 in the Little Annie group of mines produced, by 1930, about $500,000 in gold from 864 tons of sorted ore. This ore shoot was mined out by the end of 1930, and during 1931-33 the total metal production from the district was only slightly more than $5,000.
In 1934 most of the properties in the district were brought under.one control, and the most productive period in the history of the district followed. From 1934 through 1947 the total production of the district exceeded $4 million, most of which was in gold (Steven and Ratte, 1960b, p. 6-7). Although considerable exploration work was done after 1947, there was little or no production from 1948 through 1959.
The total gold production of the district from 1873 through 1959 was about 257,600 ounces.
According to Steven and Ratte (1960b, p. 9-10), bedrock in the Summitville district consists of volcanic rocks and related shallow intrusive rocks, all of middle or late Tertiary age. The oldest rocks, known as the Conejos Formation, are a thick succession of dark, fine-grained porphyritic rhyodacite flows cut by a large quartz monzonite stock. The north margin of the stock and the adjacent flow rocks were intensely altered by solfataric action. Erosion dissected the area and produced relief of at least 2,000 feet.
Volcanic eruptions were renewed and quartz latite lavas known as the Fisher Quartz Latite were extruded on the irregular erosion surface. These rocks were intruded by dikes of similar composition, altered by hydrothermal solutions, and in part covered by later eruptions of quartz latite and rhyolite flows. Mineralization was related to the second period of alteration, and all known ore deposits are in the Fisher Quartz Latite (Steven and Ratte, 1960b, p. 38-40).
The ore bodies are resistant pipes and veinlike masses of vuggy quartz and quartz-alunite rock that commonly contain pyrite and enargite and some galena and sphalerite. The resistant veins are surrounded by irregular envelopes of soft argillized ground in which illite, montmorillonite, and locally occurring kaolinite are the most abundant minerals.
Beyond the argillized envelope the rocks are pervasively bleached, owing to the extensive alteration of the matrix and ferromagnesian minerals to montmorillonite, chlorite, and quartz; only the quartz and feldspar phenocrysts are relatively unaltered (Steven and Ratte, 1960b, p. 41-48).
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