Bonanza Mine, Kennecott, Nizina District, Valdez-Cordova Borough, Alaska, USA
The locality is in the Wrangell-Saint Elias National Park and Preserve.
Location: The Bonanza mine is on the ridge between McCarthy Creek and lower Kennicott Glacier (MacKevett, 1970 [GQ 899]). It is at an elevation of about 5,950 feet, 3,000 feet north-northwest of elevation 6184, and 2,500 feet south of Bonaza Peak (elevation 6983). The mine is in the northwest corner of section 23, T. 4 S., R. 14 E. of the Copper River Meridian and its location is shown on the McCarthy C-5 quadrangle (1993 edition). This is locality 89 of MacKevett (1976), and Cobb and MacKevett (1980) included it under the name 'Kennecott Copper Corp.'. This site is often described with the Mother Lode (MC090), Jumbo (MC091), and Erie (MC083) mines; some of the above MRDS numbers may be for these other mines.
Geology: The Bonanza, Erie (MC083), Mother Lode (MC090), and Jumbo (MC091) mines, all on the ridge between McCarthy Creek and Kennicott and Root Glaciers, produced significant amounts of high-grade copper ore when they were operated by Kennecott Copper Corporation between 1911 and 1938. These mines developed several different orebodies but their underground workings were interconnected. Together they produced 4 million metric tons of ore with a grade of 13 percent copper. The estimated 536,000 tons of copper that was recovered was accompanied by about 100 tons of silver (MacKevett and others, 1997). No other metals were of economic importance in these orebodies. Bateman and McLaughlin (1920) and Lasky (1929) provide important descriptions of the geology, mineralogy, and structure of these deposits. Cobb and MacKevett (1980) refer to the many Federal government publications, dating from the time of the Bonanza discovery in 1900, that contain information about them. MacKevett and others (1997) provide an excellent synthesis and interpretation of the structure, stratigraphy, economic geology, and geochemistry of these deposits. This record largely summarizes information provided by MacKevett and others (1997).
The Bonanza mine produced 1,383,000 tons of ore containing 12.79 percent copper. The largest orebody in this mine, the Bonanza vein, produced 653,000 tons containing 13.44 percent copper. This ore was accessed from 12 levels between surface exposures at about 1,800 meters in elevation to a depth of about 1,450 meters in elevation. The Bonanza vein and other nearby orebodies are localized in the lower part of the Upper Triassic Chitistone Limestone. The base of the mineralization was commonly 27 to 37 meters stratigraphically above the contact of the Chitistone Limestone with the underlying Upper Triassic Nikolai Greenstone. The development of intertidal carbonate-facies rocks with stromatolites, bacterial mats, gypsum, and anhydrite in the lower Chitistone Limestone is one important control on the development and location of the orebodies. Steep, northeast-trending fissures up to 300 meters long are another important control on the location of the major orebodies. These fissures show minor displacement of bedding in the Chitistone Limestone and localize breccia and trangressive dolomite alteration. The breccia zones, thought by MacKevett and others (1997) to be early collapse breccia along solution-enlarged fissures, laterally envelop the orebodies and extend stratigraphically upward above them. The Bonanza vein orebody is about 580 meters long, 0.5 to 15 meters wide at its base, and 50 to 60 meters high (Bateman and McLaughlin, 1920). It strikes N 45-60 E and dips 75 SE to vertical. The width decreases upwards from a sharp base on a bedding plane in the Chitistone Limestone about 30 meters stratigraphically above the Nikolai Greenstone. The orebody merges into narrow and weakly mineralized structures both laterally and upward. Other orebodies at the Bonanza mine include the narrow, 400 meter long and 60 meter high Birch vein that contains copper sulfide-bearing veinlets, pods, and disseminations. The Birch vein produced 125,000 tons of ore with 9.37 percent copper. The Flat orebody and other small deposits in the Bonanza mine are mineralized zones localized along bedding in the Chitistone Limestone. Typically the large high-grade copper deposits of the area, like the Bonanza vein, contain many minerals in the Cu2S-CuS system. Chalcocite and djurleite are abundant, with minor amounts of covellite, bornite, chalcopyrite, digenite, anilite, luzonite, idaite, malachite, azurite, chalcanthite, and orpiment. Other minerals reported by Bateman and McLaughlin (1920) in minor or trace amounts include tennantite, antlerite, sphalerite, galena, pyrite, and copper arsenates. Enargite reported by Bateman and McLaughlin was not identified by MacKevett and others (1997). Although the Chitistone Limestone-hosted, copper-rich ores are mostly chalcocite and djurleite, remnant clots of earlier minerals provide a definition of the mineral paragenesis. Early pyrite, now found only in traces, was replaced by chalcopyrite, which in turn was replaced by bornite and minor covellite. The temperature of sulfide deposition fell during these stages from near 200 to 150 degrees centigrade. The main-stage ore minerals, chalcocite and djurleite, made up 95 percent of the ore and were deposited at temperatures of 90 +/- 10 degrees centigrade. Later, oxidized ore fluids overwhelmed reductants in the host rock and chalcocite was partly replaced by anilite and covellite, and finally by malachite and azurite. The common alteration at the Bonanza and other Chitistone Limestone-hosted, high-grade copper deposits in the area is trangressive dolomitization. Dolomite replacement is approximately coincident with the breccia zones that laterally surround the orebodies and extend vertically above them. The replacement dolomite is coarser and lighter gray than original dolostone and it lacks any evidence of bedding (Armstrong and MacKevett, 1982; MacKevett and others, 1997). The mineralogy and geochemistry of the high-grade copper deposits combined with fluid inclusion and stable isotope data indicate that the high-grade copper ores were deposited by reactions between oxidized copper-rich brines which moved through Nikolai Greenstone and sulfur-rich fluids derived from the thermal reduction of gypsum in the presence of organic matter in the lower part of the Chitistone Limestone. The migration of the oxidized copper-rich brines to the site of deposition is thought to have accompanied regional deformation and low-grade metamorphism in the Late Jurassic or Early Cretaceous (MacKevett and others, 1997). Related copper-bearing minerals were deposited in the underlying Nikolai Greenstone at about 112 Ma (Silberman and others, 1980).
Workings: The ore produced from the Bonanza mine was mined from 12 levels between surface exposures at an elevation of about 1,800 meters to a depth of about 1,450 meters in elevation.
Age: Cretaceous? The migration of the oxidized copper-rich brines to the site of deposition is thought to have accompanied regional deformation and low-grade metamorphism in the Late Jurassic or Early Cretaceous (MacKevett and others, 1997). Related copper-bearing minerals were deposited in the underlying Nikolai Greenstone at about 112 Ma (Silberman and others, 1980).
Alteration: The common alteration at the Bonanza and other Chitistone Limestone-hosted, high-grade copper deposits in the area is trangressive dolomitization. Dolomite replacement is approximately coincident with the breccia zones that laterally surround the orebodies and extend vertically above them. The replacement dolomite is coarser and lighter gray than the original dolostone and it lacks any evidence of bedding (Armstrong and MacKevett, 1982; MacKevett and others, 1997). Oxidation of deposits is not related to the present land surface and practically the entire deposit has been partially oxidized, even in the deepest levels of the mine.
Production: The Bonanza mine produced 1,383,000 tons of ore containing 12.79 percent copper. The largest orebody in this mine, the Bonanza vein, produced 653,000 tons that contained 13.44 percent copper. Some of the production was from natural surface accumulations.
Commodities (Major) - Ag, Cu
Development Status: Yes; large
Deposit Model: Kennecott-type copper deposit (after MacKevett and others, 1997)
21 entries listed. 21 valid minerals.
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Armstrong, A.K., and MacKevett, E.M., Jr., 1982, Stratigraphy and diagenetic history of the lower part of the Triassic Chitistone Limestone, Alaska: U.S. Geological Survey Professional Paper 1212-A, 26 p. Bateman, A.M., and McLaughlin, D.H., 1920, Geology of the ore deposits of Kennecott, Alaska: Economic Geology, v. 15, p. 1-80. Cobb, E.H., and MacKevett, E.M., Jr., 1980, Summaries of data on and lists of references to metallic and selected nonmetallic mineral deposits in the McCarthy quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-885, 156 p. Lasky, S.G., 1929, Transverse faults at Kennecott and their relation to the main fault systems: American Institute of Mining and Metallurgical Engineers Transactions, v. 85, p. 303-317. MacKevett, E.M., Jr., 1970, Geologic map of the McCarthy C-5 quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-899, 1 sheet, scale 1:63,360. MacKevett, E.M., Jr., 1976, Mineral deposits and occurrences in the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-773-B, 2 sheets, scale 1:250,000. MacKevett, E.M., Jr., Cox, D.P., Potter, R.W., III, and Silberman, M.L., 1997, Kennecott-type deposits in the Wrangell Mountains, Alaska--High-grade copper ores near a basalt-limestone contact, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 66-89. Silberman, M.L., MacKevett, E.M., Jr., Connor, C.L., and Mathews, A., 1980, Metallogenic and tectonic significance of oxygen isotope data and whole-rock potassium-argon ages of Nikolai Greenstone, McCarthy quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-2019, 31 p.
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