Cinnabar Creek lode Mine, Aniak District, Dillingham Borough, Alaska, USA

Location: The Cinnabar Creek lode mine is located at the head of Cinnabar Gulch (TA002), a north headwater tributary of Cinnabar Creek. The mine is at an elevation of about 1,350 feet in the NW1/4 of section 12, T 8 N, R 55 W, of the Seward Meridian. It is 0.75 mile north-northeast of the north end of the Cinnabar Creek airstrip. The site is accurately located. It is included locality 1 of Cobb (1972 [MF 384]; 1976 [OF 76-606]).
Geology: The Cinnabar Creek lode mercury deposit was discovered by following up the headwater source of the Cinnabar Creek placer deposit (TA002). Although initial headwater exploration, including 390 feet of U. S. Bureau of Mines dozer trenching, was unsuccessful in discovering lode deposits, the potential of an upslope area underlain by mineralized mafic intrusive rock was noted (Rutledge, 1950, p. 9; Cady and others, 1955, p. 115). By 1955, lode cinnabar mineralization had been delineated along or near this mafic intrusive. Between 1955 and 1959, an open pit 160 feet long and 20 to 40 feet wide was excavated for selective mining of high-grade ore averaging 3 to 4 percent mercury (Sainsbury and MacKevett, 1965, p. 38). Mapping of bedrock geology in the open pit shows that a branching mafic dike about 5-feet wide intrudes altered and sheared Triassic graywacke and siltstone (Sainsbury and MacKevett, 1965, Plate 4). Bedding in the pit strikes northwest and dips steeply west. Northwest-trending faults subparallel to bedding and to the contact of the mafic intrusive body shear the bedded rocks to the west and possibly hanging wall side of the mafic intrusive. The sheared zone is a few feet to 8 feet wide and over 200 feet long as exposed in the open pit. Striations on fault planes plunge 22 degrees northwest suggesting a significant component of lateral displacement. The mafic dike has been traced 1,000 feet to the northwest in surface pits and trenches. It is locally extensively altered and associated with quartz veining and stibnite mineralization along its northwest extensions, but alteration and mineralization appear strongest in the vicinity of the open pit. Mineralization includes irregular cinnabar and stibnite veinlets in graywacke, siltstone, and locally in the mafic dike. Cinnabar and native mercury accompany quartz along fault zones and in brecciated siltstone and graywacke. Some cinnabar is colloidally dispersed through cryptocrystalline quartz and disseminated through altered rocks. Cinnabar, stibnite, and some pyrite fill fractures in quartz and locally replace quartz and graywacke. Clay minerals including dickite, replace breccia fragments and in places are replaced by cinnabar. The graywacke, siltstone, and mafic dike are altered. The graywacke and siltstone are iron-stained, sheared, and argillized. The mafic dike is locally extensively replaced by combinations of clay (including dickite), quartz, dolomite or ankeritic dolomite, and some pyrite, and is cut by limonite veinlets. Selected high-grade ore averaging 3 to 4 percent mercury was recovered from the open pit and transported along a dozer trail to a small retort on Cinnabar Creek during the 1955 to 1960 mining operations. This period of operations produced several hundred flasks of mercury (Sainsbury and MacKevett, 1965, p. 35). A sample of ore from the Cinnabar Creek lode containing 6.53 percent Hg also contained 0.14 ppm Au and greater than 10,000 ppm Sb (Hawley and others, 1969).
Workings: The Cinnabar Creek mine was explored by surface pits and trenches in the 1950s and by at least some diamond drilling in 1961 (Sainsbury and MacKevett, 1965, p. 35). A dozer trail connects the mine with a retort, airstrip and other facilities less than a mile to the southwest along Cinnabar Creek. Additional surface examination, mapping, sampling, and trenching was completed in 1986-87. Clark and others (1971 [OF 458]) reported results of a reconnaissance stream sediment survey in the Taylor Mountains D-8 quadrangle. This survey included data on mercury, and replicate analyses confirmed anomalous Hg concentrations in several areas; no samples were collected in the Cinnabar Creek drainage.
Age: Cretaceous or Tertiary. Inferred to be similar in age to other mercury deposits of southwest Alaska that postdate deposition of mid-Cretaceous clastic sedimentary rocks and intrusion of Upper Cretaceous or Tertiary igneous rocks.
Alteration: Silicification, argillization, oxidation and iron-staining.
Production: Production from 1955-60 amounted to several hundred flasks of mercury (Sainsbury and MacKevett, 1965, p. 35).

Commodities (Major) - Hg, Sb; (Minor) - Au
Development Status: Yes; small
Deposit Model: Cinnabar and native mercury in veins, breccia, and replacements (Hot-spring Hg

References:
Cady, W.M., Wallace, R.E., Hoare, J.M., and Webber, E.J., 1955, The central Kuskokwim region, Alaska: U.S. Geological Survey Professional Paper 268, 132 p. Clark, A.L., Condon, W.H., and Hoare, J.M., and Sorg, D.H., 1971, Analyses of stream-sediment samples from the Taylor Mountains D-8 quadrangle, Alaska: U.S. Geological Survey Open-File Report 458, 60 p. Cobb, E.H., 1972, Metallic mineral resources map of the Taylor Mountains quadrangle: U.S. Geological Survey Miscellaneous Field Studies Map MF-384, 1 sheet, scale 1:250,000. Cobb, E.H., 1976, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Dillingham, Sleetmute, and Taylor Mountains quadrangles, Alaska: U.S. Geological Survey Open-File Report 76-606, 92 p. Hawley, C.C., Martinez, E.E., and Marinenko, John, 1969, Geochemical data on the South ore zone, White Mountain mine, and on the gold content of other mercury ores, southwestern Alaska, in Some shorter mineral resources investigations in Alaska: U.S. Geological Survey Circular 615, p. 16-20. Rutledge, F.A., 1950, Investigation of mercury deposits, Cinnabar Creek area, Georgetown and Akiak districts, Kuskokwim region, southwestern Alaska: U.S. Bureau of Mines Report of Investigations 4719, 9 p. Sainsbury, C.L. and MacKevett, E.M., Jr., 1965, Quicksilver deposits of southwestern Alaska: U.S. Geological Survey Bulletin 1187, 89 p.