Big Sky Country Sapphire: Visiting Montana's Alluvial Deposits
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FIELD REPORT BIG SKY COUNTRY SAPPHIRE: VISITING MONTANA’S ALLUVIAL DEPOSITS Tao Hsu, Andrew Lucas, Robert E. Kane, Shane F. McClure, and Nathan D. Renfro apphire has been mined in the western U.S. state cussion continue. The glamour of the sapphires, the of Montana for more than a century and a half. mysteries of their origin, the area’s mining history, Historically,S gem-quality sapphires have been mined and the natural beauty of Big Sky country are an in- commercially in four main areas in southwestern triguing combination (figure 3). Montana, shown in figure 1: the upper Missouri River gravel bars (1865), Dry Cottonwood Creek MISSOURI RIVER (1889), Rock Creek (1892), and Yogo Gulch (1895). Gold miners first discovered Montana sapphire crys- Today, the first two areas remain quite active, while tals in 1865, in the gravel bars along the upper Mis- operations in Yogo Gulch and Dry Cottonwood souri River, about 15 miles northeast of Helena. Creek have been suspended for many years. To better Kunz (1894) reported that the sapphires were found understand the characteristics of Montana sapphire by an “earnest and reliable prospector” named Ed R. and record current mining and commercial activities, Collins in 1865. Collins had some of the Eldorado Bar GIA sent a team to visit the placer deposits at the sapphires cut in New York by Tiffany & Co. and M. upper Missouri River and Rock Creek areas in Au- Fox & Co., as well as overseas, as part of an effort to gust 2015. locate a potential market. The first scientific refer- Since the latter half of the 19th century, Mon- ence to these stones was in 1873, when Dr. J. tana’s history has been intertwined with gold, silver, Lawrence Smith described them in the American and copper mining. Corundum was discovered dur- Journal of Science. Smith had some of the sapphires ing the course of gold mining activities in southwest- faceted and stated “my opinion is that this locality ern Montana. Before the 1940s, the state’s alluvial is a far more reliable source for this gem variety of sapphire deposits were exploited mainly to supply corundum than any other in the United States that I the watch industry, but production fell dramatically have yet examined.” Today, seven different small- with the use of synthetic sapphire in watch bearings scale commercial operations are active in the Mis- (Emmett and Douthit, 1993). Among Montana’s sec- souri River area, some of which mainly cater to ondary deposits, Rock Creek (figure 2) is the only area mined specifically for sapphire from its discovery in 1892 until World War II (Clabaugh, 1952). Figure 1. Sapphire deposits are clustered in south- While Yogo Gulch is a primary deposit, the placer western Montana. Active mining operations are at deposits at Rock Creek, Dry Cottonwood Creek, and alluvial deposits along the upper Missouri River and Rock Creek. Mining at Dry Cottonwood Creek and the upper Missouri River near Helena have been a Yogo Gulch is suspended. matter of speculation among researchers trying to work out the origin of these sapphire crystals. Re- search indicates that they were carried to the surface M O N TA N A by volcanic activity, but their ultimate origin is still an open question, even after about 125 years of Missouri searching (e.g., Pratt, 1906; Clabaugh, 1952; Garland, Dry River Lewistown 2002; Berg and Dahy, 2002; Berg, 2014; Zwaan et al., Cottonwood Yogo Creek • Gulch 2015). Due to the lack of significant sapphire-bearing Philipsburg Helena Billings host rock outcrops in these areas, the search and dis- Rock Creek Bozeman Sapphire deposits See end of article for About the Authors and Acknowledgments. N • Capital GEMS & GEMOLOGY, Vol. 53, No. 2, pp. 215–227, http://dx.doi.org/10.5741/GEMS.53.2.215 City or town ã © 2017 Gemological Institute of America FIELD REPORT GEMS & GEMOLOGY SUMMER 2017 215 Figure 2. Among all of Montana’s secondary sapphire deposits, Rock Creek has been the most productive. Poten- tate Mining is actively operating on its property in this area. Photo courtesy of Potentate Mining. tourists. Some of the finer rough is sold to jewelers, On October 8, 1942, President Roosevelt issued gem dealers, and hobbyists who facet these gems. In War Production Order L-208, which closed all gold addition, there are several other areas along the Mis- mines in the United States. The order was designed souri River where sapphires are mined by hobbyists, to shift gold miners to commodities that were essen- including McCune Bar and Gruell’s Bar. tial to the war effort, such as copper, and to allow the shipment of mining equipment to U.S. allies, includ- ing the Soviet Union (Hammett, 1966). Figure 3. A selection of heat-treated Rock Creek sap- The Perry-Schroeder Mining Co. was given special phires produced by Potentate Mining shows various permission to continue dredging gold on Eldorado Bar shades of blue and green. The majority of alluvial throughout the war, because its significant sapphire by- sapphires from Montana have bluish and greenish product had military applications (Ball, 1943). The var- pastel colors prior to heat treatment. Photo by Jeff ious non-gem uses included bearings for bombsights Scovil, courtesy of Potentate Mining. and torpedoes, as well as abrasives in grinding wheels. From 1940 to 1944, the Perry-Schroeder gold dredge on Eldorado Bar recovered approximately seven million carats of sapphires of all qualities, most of which were sold for industrial purposes (Clabaugh, 1952). During this trip, we visited the Eldorado Bar (figure 4) and Spokane Bar deposits, which are active sapphire mining sites. Cass Thompson, owner of the Spokane Bar deposit and one of the six independently owned mines at Eldorado Bar, took the team on a boat tour of Hauser Lake, a reservoir on the Missouri River near Helena. The Missouri River flows from southeast to northwest in this portion of Montana. In addition to 216 FIELD REPORT GEMS & GEMOLOGY SUMMER 2017 Figure 4. Eldorado Bar is one of the sapphire- bearing gravel bars that have been mined for over 150 years. The op- eration is visible from the Missouri River. The small gravel mound in the foreground is a tail- ings pile left by previ- ous miners. Due to the higher river level caused by the 1910 re- construction of Hauser Dam, the lower part of the tailings pile is sub- merged. Photo by An- drew Lucas. other occurrences of known sapphire-bearing gravel, itan Bar, Spokane Bar, French Bar, Gruell’s Bar, and there are at least nine historical sapphire bars (geolog- Emerald Bar. Many of these deposits were exploited in ically known as strath terraces). These gravel bars, the late 1800s and early 1900s, and remnants of the shown in figure 5, have long been known as American old mining activities can still be found. All of the sap- Bar, Eldorado Bar, Dana’s Bar, McCune Bar, Metropol- phire deposits are distributed along this approximately Upper N Holter Lake American Bar MONTANA ² Figure 5. Sapphires have Smith River been discovered in grav- els in many areas—some very recently—along this northwest-flowing section Hauser Dam of the upper Missouri River. This map shows nine of the historic min- Hauser Lake Eldorado Bar ing areas, seven of which are distributed between the Canyon Ferry Dam and the Hauser Dam. All Dana’s Bar York nine occurrences of sap- phire-bearing gravel on Lake Helena Sapphire-bearing gravel this section of the river McCune Bar Roads have also been mined for Metropolitan Bar Dams Hauser Lake gold. Sapphires are very 0 1.5 3Miles rarely found downstream Gruell’s Bar from American Bar. Mod- 0 1.5 3Kilometers ified by R. Kane from Spokane Bar Berg (2015), with permis- Helena Valley sion, courtesy of the Mon- French Bar Canyon Regulating Ferry tana Bureau of Mines and Reservoir Dam Geology. Canyon Emerald Bar Ferry Lake Helena Valley FIELD REPORT GEMS & GEMOLOGY SUMMER 2017 217 14-mile section of the original river. The deposits ex- tend downriver to the northwest from Canyon Ferry Dam to Hauser Dam—essentially all of Hauser Lake—in gravel bars on both sides of the river channel, Topsoil as well as the riverbed itself. Sapphires are very rarely found northwest of American Bar. When each dam was built, the river level rose dramatically, so the Overburden sediments lower layers of many previously exposed gravel bars have been submerged ever since. Rocks in this area include argillites of the Precam- brian Belt series, Paleozoic sedimentary rocks, Terti- Volcanic ash ary lake sediments and volcanic materials, and Cretaceous or Tertiary intrusives (Clabaugh, 1952). Gravel layers sit directly above the Precambrian Belt series metasedimentary rocks (figure 6). The thick- ness of the gravel layers varies and at some localities Sapphire-bearing gravel is more than 40 feet. According to Thompson, some gravel bars have a very thick sapphire-bearing layer but are expensive to operate due to extremely thick overburden, as is the case at Dana’s Bar and some por- tions of Eldorado Bar. At Eldorado Bar, a mine run was prepared for GIA’s corundum collection. The team witnessed the entire Bedrock day’s operation as approximately 100 cubic yards of gem-bearing gravel were run through the mechanized processing plant. This process started with gravel ex- traction in the morning, during which a complete rock profile was exposed (figure 7). Topsoil, overbur- den sediments, a volcanic ash layer, a gravel layer, and Figure 7. A well-layered lithological profile was ex- metasedimentary bedrock were observed from top to posed at the Eldorado Bar mining site.