Bonanza Geology 26Jul 14
July 26, 2014
Geology and Mining History of Bonanza, and the Restoration of Kerber Creek
Trip leaders: Geologist Jim Cappa, Environmental Scientist Laura Archuleta, and Geologist Pete Lipman
In 1880 rich veins of silver and gold were discovered in the Kerber Creek area of Saguache County, reportedly by Tom Cooke of Salida. Prospectors and miners quickly came to the area to strike it rich in the Kerber Creek Mining District, also called the Bonanza Mining District. Businesses soon arrived to provide needed supplies and entertainment, and the town of Bonanza suddenly sprang into existence. Although the population during its first one or two years is uncertain, population estimates based on the 36 saloons and 7 dance halls suggest one to two thousand people lived in Bonanza in the 1880s. Several dozen named mines once worked the veins associated with the Bonanza volcanic caldera.
The rich ore was depleted fairly quickly, but the remaining relatively low grade ore was intermittently mined up until the 1970s. The low grade ore was first processed in a 100-ton/day mill built in Rawley Gulch in 1902. It operated from 1902 to 1905, but the stream flow in the gulch was inadequate for year around milling. From 1905 to 1910 efforts in the Rawley Mine, the largest mine in the district, focused on development of the mine. By 1910 the mine was 600 feet deep (to the 6th or 600 level). In 1911 and 1912 a 6,200-feet-long drainage and haulage tunnel was driven from Squirrel Creek to the Rawley Vein. This tunnel was 600 feet below the 6th level and was called the Rawley 12 or 12th level. With the draining of the Rawley Vein, the mine workings were further developed below the 6th level. By 1923, construction of a 300-ton/day concentrating mill located at the portal of the Rawley 12 was completed. An aerial tram over 7 miles long was built to haul the concentrated ore northward to the now-abandoned townsite of Shirley, where the ore was loaded into rail cars. A 40-feet-tall timber cribbing dam was constructed below the Rawley 12 portal to contain the mill tailings.
Mining and milling activities in the 1920s were conducted by the Colorado Corporation, which went bankrupt in about 1925. ASARCO Incorporated and other creditors to the Colorado Corporation formed a new company to operate the Rawley Mine and Mill. In 1925 they remodeled the mill and increased its capacity to 350 tons/day. The greatest production from the Rawley Mine occurred between 1925 and 1930. Three large tailings dams were built across Kerber Creek downstream of the town of Bonanza during this time. Water draining from the underground mines and seeping from the mill tailings, as well as tailings eroded from the dams and deposited in the creeks and on their floodplains, eventually caused major environmental impacts to Kerber Creek, Squirrel Creek, and Rawley Gulch. The primary metals of concern were zinc, copper, cadmium, iron, lead, manganese, and arsenic.
In the early 1990s federal and state regulatory agencies began evaluating the mining district for placement on the National Priorities List as a Superfund Site. In response, ASARCO initiated efforts with the regulatory agencies and other private parties historically involved in the Bonanza Mining District that eventually led to the formation of the Bonanza Mining District Group. ASARCO voluntarily conducted several reclamation and restoration efforts to address environmental impacts associated with the drainage from the Rawley 12 and the Rawley 4 levels; the mill tailings in Squirrel Gulch at the Rawley 12; and the tailings in the 3 tailings impoundments below the town of Bonanza. Subsequent voluntary reclamation and restoration activities were conducted by others at the Superior Mill, Minnie Lynch Mine, and at several locations along Kerber Creek. Restoration work continues.
The field trip on July 26 will be led by Jim Cappa, a consulting geologist who retired from the Colorado Geological Survey, Laura Archuleta, an environmental contaminants specialist with the U.S. Fish & Wildlife Service, and Pete Lipman, a U.S. Geological Survey emeritus who is an expert on the ancient San Juan volcanoes and recently completed studies of the Bonanza caldera. The trip will include stops at the Minnie Lynch Mine, at the Rawley 2, 3, 4 and 12 levels, at the Cocomongo and Bonanza Mines, and at locations along Kerber Creek downstream of Bonanza where old tailings impoundments have been reclaimed and restored.
The trip begins at 8:30 AM near Villa Grove. Further details on the meeting location will be available upon registration, and a handout will be provided during the field trip. A high-clearance vehicle is needed to drive on the rocky and sometimes steep roads that access the mines. There will be opportunities to collect samples of the ore. Please wear glasses, sunglasses, or better yet safety goggles to protect your eyes if you plan to break any rocks. And be sure to bring appropriate outdoor clothing, as well as your lunch and drinks. There will be several short hikes on steep and rocky ground. Parking is limited at some stops, so car pooling will be arranged at the meeting location.
► Everyone is welcome on this CFI field trip but current 2014 CFI members will receive a member discount on this and future field trips. Your membership dues also help support our great winter lecture series.
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