Mother Lode Mine, Kennecott, Nizina District, Valdez-Cordova Borough, Alaska, USA
The locality is in the Wrangell-Saint Elias National Park and Preserve.
Location: The Mother Lode mine is at the head of a small cirque valley on the west valley wall of McCarthy Creek (MacKevett, 1970 [GQ 899]). It is at an elevation of about 5,300 feet, 2,600 feet east of Bonanza Peak (elevation 6983) and about 300 feet east of the center of section 14, T. 4 S., R. 14 E. of the Copper River Meridian. This is locality 91 of MacKevett (1976); Cobb and MacKevett (1980) included this mine under the name 'Kennecott Copper Corp.'. The Mother Lode mine is shown on the McCarthy C-5 quadrangle (1993 edition).
Geology: The Mother Lode, Bonanza (MC093), Jumbo (MC091), and Erie (MC083) 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 recovered was accompanied by the recovery of 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 Mother Lode mine produced 1,216,000 tons of ore containing 12.39 percent copper. The largest orebody is the Mother Lode vein, which produced 507,000 tons of ore containing 12.05 percent copper. This ore was accessed from both the Mother Lode and Bonanza mines, which developed 12 levels between surface exposures at about 1,800 meters elevation on the Bonanza vein (MC093) to depths at elevations of about 1,450 meters. The Mother Lode vein and other nearby orebodies are localized in the lower part of the Upper Triassic Chitistone Limestone. The base of the mineralization is usually about 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 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 Mother Lode vein orebody is about 1000 meters long, up to 8 meters wide at its base, and 90 meters high. It strikes 34-40 E and dips steeply to the southeast. The width decreases upwards from a sharp base on a bedding-plane fault near a stromatolite layer 30 meters stratigraphically above the Nikolai Greenstone. The orebody has many similarities to the nearby Bonanza vein (MC093), including becoming larger and richer in its lowermost, southwestern parts. The orebody is bordered by copper-bearing disseminations and veinlets except at its base. Some higher grade parts are associated with reddish sandy limestone, breccia, and gouge, suggesting ore deposition within a chimney or similar cavern. The Marvelous fissure was also developed at the Mother Lode mine. It is a small orebody but it occurs stratigraphically higher than other deposits in the area. It is localized in the upper member of the Chitistone Limestone and is intermittently mineralized over a strike length of 335 meters and a height of about 100 meters. It strikes about N-35 E and dips between 70 NW and 70 SE. Typically, the large high-grade copper deposits of the area, like the Mother Lode 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 allow the determination 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. Temperatures 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 Mother Lode 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). 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 was accessed from both the Mother Lode and Bonanza mines, which developed 12 levels between surface exposures at about 1,800 meters elevation on the Bonanza vein (MC093) to depths at elevations of about 1,450 meters.
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 Mother Lode 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 mine.
Production: The Mother Lode mine produced 1,216,000 tons of ore containing 12.39 percent copper. The largest orebody is the Mother Lode vein, which produced 507,000 tons of ore containing 12.05 percent copper.
Commodities (Major) - Ag, Cu
Development Status: Yes; large
Deposit Model: Kennecott-type copper deposit (after MacKevett and others, 1997)
ReferencesArmstrong, 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.
21 entries listed. 21 valid minerals.
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Locality Updated: Pataias, Alcobaça, Leiria District, PortugalFrom Rui Nunes, 24th Oct 2014 15:23:24