Kensington Mine, Juneau District, Juneau Borough, Alaska, USA
|Latitude & Longitude (WGS84):||58° 51' 51'' North , 135° 4' 55'' West|
|Latitude & Longitude (decimal):||58.86417,-135.08194|
The Kensington project, that is currently (2001) controlled by Coeur Alaska, covers an area in the Berners Bay district that includes numerous mines and prospects described separately (see JU026-028, JU030-034, JU036-041, and JU044-053). Coeur is currently (2001) limiting expenditures to focus on securing the permits required for development and optimization of the entire project to enhance the economic return of the project in response to declining gold prices. Work continued throughout 2000 to develop a revised, selective mining plan that would reduce the tons of material to be mined but significantly increase the gold grade. This revised plan also lowers the estimated capital costs (Coeur d'Alene Mines Corporation, corporate website, www.coeur.com; July 31, 2001).
Location: The Kensington Mine is at an elevation of approximately 2,000 feet, about 2 miles northeast of Pt. Sherman on Lynn Canal and 1 mile southwest of Lions Head Mountain in the Kakuhan Range. It is marked on the Juneau D-4 topographic map. It is in the center of section 4, T. 35 S., R. 62 E. of the Copper River Meridian. The location is accurate.
Geology: The deposit at the Kensington mine consists of a quartz vein in Jualin Diorite. The vein strikes N-S and dips 55-70 east. It has been traced for 1500 feet along strike and 3,200 feet vertically, and averages 43 feet thick. The vein is bounded on the north by a metavolcanic rocks ; to the south, the limit of ore is defined by declining metal values (Echo Bay Mines, 1993). The Kensington vein was discovered in 1886. It was explored and mined from 1897 to 1900. Total production was 2,600 ounces of gold from 10,342 tons of ore (Redman and others, 1989). Placid Oil Co. acquired rights to the property in 1980 and completed nearly 22,000 feet of core drilling on the Kensington and other nearby veins by the end of 1985 (Kucinski and others, 1985). The property was acquired in 1987 by a joint venture consisting of Echo Bay Mines and Coeur Alaska, who implemented an extensive exploration and development program that totaled over $80 million by 1992 (Swainbank and others, 1993). In 1995, Coeur paid $32.5 million, plus a scaled royalty to gain 100% interest in the Kensington Mine. The Kensington vein system contains reserves of 11.5 million tons of ore with 0.143 ounce of gold per ton (Bundtzen and others, 1991). The Kensington project, as defined by Coeur, contains over 1.96 million ounces of gold in the proven and probable categories. Mine life is expected to be 12 years, mining at a rate of 4,000 tons per day by long-hole open-stoping. Annual production is expected to be 259,000 ounces of gold. Ore will be beneficiated by flotation, augmented by carbon-in-leach processing (Bundtzen and others, 1996). The Kensington Mine is in the Kensington project area, that in 2001 was controlled by Coeur Alaska. It is in the Berners Bay district at the north end of the Juneau Gold Belt. The district is characterized by a series of structurally-controlled, mesothermal, gold-bearing quartz veins. Most of the veins are in Early Cretaceous (105 Ma) Jualin Diorite, which intrudes Upper Triassic metabasalt. The Jualin Diorite is generally massive, jointed, blocky, quartz monzonite to quartz monzodiorite. Gold occurs in low-sulfide, quartz-carbonate veins that contain pyrite and tellurides; the veins are marked by distinctive ankeritic alteration zones. There are both extensional and shear veins that generally strike north to northwest and dip east. Discrete vein systems are defined by one or more through-going quartz veins, many of which are in shear zones. Levielle (1991) and Knopf (1911) describe other gangue minerals near vein margins including albite, chlorite, muscovite, and lesser tourmaline, rutile, and apatite. Hydrothermal alteration adjacent to the veins is characterized by reddish-brown ferroan dolomite (Miller and others, 1995). Other alteration includes sericitization of plagioclase, chloritization, sulfidization of mafic minerals, and albitization of feldspars (Leveille, 1991). Pyrite is the most abundant sulfide mineral, with lesser amounts of chalcopyrite, galena, sphalerite, arsenopyrite, and tetrahedrite. Gold occurs in the native state, in pyrite, and in various telluride minerals such as calaverite, hessite, and petzite (Leveille, 1991; Redman and others, 1989). The vein paragenesis consists of early quartz, carbonates, albite and pyrite, followed by deposition of base and precious metals. Gold, galena and the tellurides were the last to be deposited (Leveille, 1991). The age of hydrothermal muscovite from veins at Kensington Mine (JU029) varies from 53.4 Ma to 56.5 Ma (Miller and others, 1994). This coincides with the 55 Ma age of the other mesothermal gold vein deposits in the Juneau Gold Belt (Goldfarb and others, 1997).
Workings: Since 1987, the Kensington mine has been extensively developed including: drilling to define resources and reserves, a 5,300-foot lower adit; drifts, crosscuts, and footwall ramps, metallurgical testing; and bulk sampling. A tailings impoundment and semi-permanent camp are on-site. The project has a complete 'Environmental Impact Statement', a 'Large Mine Permit' from the City and Borough of Juneau, and a preliminary 'Coastal Zone Consistency Determination' from the State of Alaska (Swainbank and others, 1995).
Age: Hydrothermal muscovite from veins at the Kensington Mine gives ages of from 53.4 to 56.6 Ma (Miller and others, 1994). This coincides with the 55 Ma age of the other mesothermal gold-quartz-vein deposits in the Juneau Gold Belt (goldfarb and others, 1997).
Alteration: Hydrothermal alteration adjacent to the veins is characterized by reddish-brown ferroan dolomite alteration (Miller and others, 1995). Other alteration includes sericitization of plagioclase, chloritization and sulfidization of mafic minerals, and albitization of feldspars (Leveille, 1991).
Production: The Kensington vein was explored and mined from 1897 to 1900. Total production was 2,600 ounces of gold from 10,342 tons of ore (Redman and others, 1989). Should production resume under Coeur Alaska, the estimated mine life is expected to be 12 years, mining at a rate of 4,000 tons per day by long-hole open-stoping. Annual production is expected to be 259,000 ounces of gold. Ore will be beneficiated by flotation, augmented by carbon-in-leach processing (Bundtzen and others, 1996).
Reserves: The Kensington vein has reserves of 11.5 million tons of ore that contain 0.143 ounce gold per ton (Bundtzen and others, 1991). The Kensington project, as defined by Coeur, includes the Kensington, Horrible (JU027) and other properties that collectively contain over 1.96 million ounces of proven and probable gold (Bundtzen and others, 1996).
Commodities (Major) - Au; (Minor) - Ag, Cu, Pb, Zn
Development Status: Yes
Deposit Model: Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)
17 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Albian - Cretaceous|
100.5 - 145 Ma
|Granitic rocks of central and southeast Alaska|
Age: Early Cretaceous (100.5 - 145 Ma)
Description: Hornblende diorite, quartz diorite, and minor gabbro; medium to very coarse grained; color index 15 to 50; weathers light to dark green; highly altered to epidote and chlorite rich rock. Crops out southeast of Berners Bay, south of Herbert River, and on Douglas and Lincoln Islands, at Treadwell, and Woewodski and Annette Islands. Differs from other Cretaceous granitic rocks in the high degree of alteration and (or) metamorphism. Metamorphosed and unmetamorphosed diabase, gabbro, hornblende diorite, metadiorite, and amphibolite exposed only west of Point Bishop on Taku Inlet. Gold mineralization at Treadwell and Jualin shows evidence for 2 stages of mineralization, interpreted by Newberry and others to represent late metamorphic remobilization of primary mineralization
Reference: Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M. (compilers). Geologic map of Alaska. doi: 10.3133/sim3340. U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196.