By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Saguache County lies in south-central Colorado. Its east boundary is the crest of the Sangre de Cristo Mountains, and its western part includes the northeastern San Juan Mountains. The north end of the San Luis Valley occupies the central part of the county. Through 1959, Saguache County produced about 20,590 ounces of gold and significant quantities of silver, lead, zinc, and copper. Most of the gold has come from the Bonanza district.
The Bonanza (Kerber Creek) district is about 12 miles north of Saguache in the northeastern part of the county.
The first mineral-bearing lode was located in the district in May 1880 on Kerber Creek, where large silver-lead fissure veins were discovered (H. C. Burchard, in Henderson, 1926, p. 208). After the discovery, prospectors swarmed into the area, and the town of Bonanza was founded. Most of the mining operations were small, however, and by 1882 the town had lost most of its population (Patton, 1915, p. 10).
In 1911-12 the Rawley drainage tunnel was driven to tap the silver-lead-copper-zinc ore shoot of the Rawley vein (W. S. Burbank, in Vanderwilt and others, 1947, p. 445), and in 1912 annual production increased from $9,600 to $80,000. Most of the ore in the Rawley shoot above the tunnel level was mined and milled between 1923 and 1930.
District production dropped abruptly in 1931 but was revived in the late 1930's and early 1940's. The mines were relatively inactive from 1952 through 1959. Total gold production through 1959 was about 17,000 ounces; virtually all production was a byproduct of base-metal and silver ores.
Though Tertiary volcanic rocks cover much of the Bonanza district, older rocks are discernible in places, and they consist mainly of Precambrian schist, gneiss, and granite overlain unconformably by a thick succession of Paleozoic formations ranging in age from Ordovician to Permian. The Tertiary rocks overlie the older rocks with pronounced unconformity. They are about 4,000 feet thick, are chiefly lava flows, and include the Rawley Andesite, Bonanza Latite, Squirrel Gulch Latite,
Hayden Peak Latite, Brewer Creek Latite. These rocks are cut by dikes and small intrusive bodies of granite porphyry, diorite, monzonite, rhyolite, and latite. Numerous small closely spaced faults cut the Tertiary rocks, and most of the ore deposits are in fissure fillings along these faults in the Rawley Andesite, the basal unit of the Tertiary section (Burbank, 1932, p. 5-60).
The ore deposits of the Bonanza district are chiefly complex base-metal ores containing pyrite, sphalerite, galena, chalcopyrite, bornite, enargite, tennantite, and stromeyerite in a gangue of quartz, calcite, rhodochrosite, and barite.
Two principal classes of ore deposits have been distinguished: (1) quartz veins of rather high sulfide content that contain lead, zinc, copper, silver, and a little gold, and (2) quartz-rhodochrosite-fluorite veins, valuable mainly for their silver, with only minor quantities of sulfides. A few veins in the northern part of the district contain small shoots of gold and silver tellurides (Burbank, 1932, p. 60).
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