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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