Location: The DeCourcy Mountain Mine is at the head of Return Creek about 2 miles south of DeCoursy Mountain. The mine is shown on the USGS A-5, 1:63,360-scale topographic map. It is is the NW1/4 section 28 and SW1/4 section 21, T. 23 N., R. 50 W., of the Seward Meridian. The DeCoursey Mountain mine is locality 1 of Cobb (1972 [MF 363]); also described in Cobb (1976 [OFR 76-576]). Note that the mine is spelled 'DeCourcy' on the A-5 topographic map, but is spelled 'DeCoursey' in some published literature.
Geology: The DeCourcy Mountain Mine is one of the best studied lode deposits in the Iditarod quadrangle (Webber, 1944; Webber and others, 1947; Cady and others, 1955; Malone, 1962; Sainsbury and MacKevett, 1965). The rocks in the vicinity are shale and sandstone of the Upper Cretaceous, Kuskokwim Group that are cut by mafic sills and dikes. The mafic intrusions have been largely altered to a distinctive silica-carbonate rock that weathers bright yellow-orange. Major cinnabar and stibnite, minor arsenopyrite, and secondary cervantite occur in a gangue of fine-grained quartz, and carbonate and clay minerals in lenses, veins, and stockworks that cut the sedimentary and mafic intrusive rocks (Bundtzen and Miller, 1997; Miller, Bundtzen, and Gray, 2005). The sulfide minerals also occur in quartz cemented breccias. The veins and breccias generally strike N17E to N22E and dip steeply to vertically. Several of the veins have been identified by name including the 'A' Vein, the 'DeCourcy Orebody', the 'Upper' or 'Top' Vein, and the 'Retort' orebody. The A-vein, about 1,000 feet southeast of the DeCourcy vein, strikes north-northwest and, and dips steeply in sedimentary rocks adjacent to a mafic sill. According to Webber (1944), the ore bodies occur over a horizontal distance of about 1,800 feet through a vertical range of about 400 feet. The deposit was discovered in 1910, staked in 1919, and mined intermittently from 1921 to 1949. The ore as mined contained as much as 32.68 percent mercury and 0.86 percent antimony (Webber, 1944). Gray and others (1997) speculated that epithermal gold-silver deposits could be underneath the mercury-antimony mineralization. Resource Associates of Alaska, Inc. assayed portions of drill holes on the DeCourcy Mountain deposit for gold and silver but did not find elevated values of either (Jennings, 1975). Grab samples collected by the U.S. Geological Survey and Calista Corporation in the 1980s contained up to 650 parts per billion (ppb) silver, 200 parts per million (ppm) arsenic, 1.00 percent antimony, and 400 ppb gold in addition to abundant mercury (McGimsey and others, 1988; Bruce Hickock, oral commun., 1990). Malone (1962) estimated that through 1949, the DeCourcy Mountain orebodies produced 1,500 flasks of mercury from about 2,480 of ore, an average grade of about 2.29 percent mercury. The overall grade of processed ore was often higher; for example, the ore mined underground by R.F. Lyman from 1943 and 1944 averaged about 3.00 percent mercury (Webber, 1944).
Workings: The DeCourcy Mountain mercury deposit was discovered by Matt DeCourcy in 1910 and staked by him in 1919 (Cady and others, 1955). From 1921 to 1932, the Linfors adits were driven. The deposit was mined underground from 1942 to 1949 by R.F. Lyman of the the DeCourcy Mountain Mining Company. There has been no mining since but from 1953 to 1957, the deposit was explored with 2,600 feet of diamond drilling under a contract between the Defense Minerals Exploration Administration (DMEA) and DeCourcy Mountain Mining Company (Sainsbury and MacKevett, 1965). By 1961, the DeCourcy Mountain Mine was inactive though held by Alaska Mines and Minerals Company, which then operated the Red Devil Mine in the Sleetmute quadrangle. By the end of 1957, there was 1,370 feet of underground working in four adits, about 3,300 feet of trenching, and numerous prospect pits. Detailed descriptions of the ore bodies, including geologic maps, are provided in both Cady and others (1955) and Sainsbury and MacKevett (1965). There was additional sampling by the U.S. Geological Survey and the Calista Corporation in the 1980s (McGimsey and others, 1988; Bruce Hickock, oral communication, 1990).
Age: Unknown; an altered mafic dike near the base of DeCourcy Mountain which is thought to be a feeder for the Iditarod Volcanics has a 40K/40Ar age of 76.7 Ma (Miller and Bundtzen, 1994).
Alteration: Strong silica-carbonate alteration in mafic dikes.
Production: In 1921, the Thrift Mining Company installed a 4-tube retort and recovered an unknown amount of mercury (Cady and others, 1955). From 1924 to 1926, C.F. Linfors retorted ore from the 'Upper' or 'Top' vein but production was believed to be modest (Webber, 1944). In 1927, John and Harry Brink relocated the claims and mined the deposits on a small scale. From 1921 to 1932, about 157 flasks were produced (Joesting, 1942). In 1942, R.F. Lyman produced 80 flasks of mercury from surface float. From 1943 to 1944, about 720 flasks were produced and additional ore was stockpiled (Webber and others, 1947). By 1949, an additional 400 flasks were produced by the DeCourcy Mountain Mining Company. Based on examination of published and unpublished data, Malone (1962) estimated that through 1949--the last year of production, the DeCourcy Mountain orebodies produced 1,500 flasks of mercury from about 2,480 tons of ore, an average grade of about 2.29 percent mercury. The grade of the ore was often higher; for example, the ore mined underground by R.F. Lyman from 1943 and 1944 averaged about 3.00 percent mercury (Webber, 1944).
Reserves: According to Webber (1944), the reserves that remained at the end of 1944 included about 7,000 tons of indicated ore that averaged 1.61 percent mercury and and 7,600 tons of inferred ore that averaged 1.57 percent mercury. Subtracting the ore mined by the DeCourcy Mountain Mining Company through 1949, there may be about 4,600 flasks of mercury that remain in material with an average grade of about 1.6 percent.

Commodities (Major) - Hg; (Minor) - Ag, As, Au, Sb
Development Status: Yes
Deposit Model: Either a Hot Springs Hg-Sb or Silica-Carbonate Hg-Sb deposit (Cox and Singer,

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Mineral List

5 valid minerals.