Kensington (Coeur-Alaska)

Mine, Active

Commodities and mineralogy

Main commodities Ag; Au; Te
Other commodities Cu; Fe; Hg; Mo; Pb; Zn
Ore minerals altaite; bornite; calaverite; coloradoite; galena; gold; hessite; magnetite; molybdenite; petzite; pyrite; pyrrhotite; sphalerite; volynskite
Gangue minerals calcite; ferroan dolomite; quartz

Geographic location

Quadrangle map, 1:250,000-scale JU
Quadrangle map, 1:63,360-scale D-4
Latitude 58.8628
Longitude -135.0801
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents the consolidation by Coeur-Alaska of more than 12 mines and prospects, 53 patented claims, and 377 unpatented claims along a northwest-trending belt about 2 miles long between the head of Johnson Creek and Sherman Creeks. By 2008, Coeur-Alaska had done considerable new underground exploration along this belt, discovered new veins, and connected many of the old workings. The work is far advanced and Coeur-Alaska plans to mine the deposit as a single entity. This site is referred to as the Kensington deposit/property, which is part of Coeur-Alaska's 'Kensington Project' (which also includes the Jualin deposit/property (JU262) to the southeast which is described separately. The coordinates are near the center of the Coeur-Kensington deposit, which is near the site of the old Kensington Mine (JU029). The Coeur-Kensington deposit also includes such old mines and prospects as the Ophir, (JU026), Horrible (JU027), and Comet (JU036) Mines as well as several other mines and prospects (JU022, JU028, JU030, JU031, JU032, JU034, JU036, JU037, and JU039); however, these are still described individually in ARDF for their pre-Coeur-Alaska geologic and historical data. This site is located 0.2 mile southeast from the center of section 4, T. 35 S., R. 62 E., of the Copper River Meridian. The location is accurate, though degree of accuracy is not reported.

Geologic setting

Geologic description

The modern exploration history of the Kensington project began in 1980 when the Kensington Mine was optioned to Homestake Mining Company and from 1980 to 1985 to Placid Oil Company. Placid drilled 13,626 feet near the old Kensington Mine (JU029) and another 14,076 feet on other targets in the area. Coeur d'Alene Mines Corporation became involved in the property in 1987 in a joint venture with Echo Bay Mines; exploration continued that resulted in a feasibility study in 1993. In 1995, Coeur gained total control of the property under a subsidiary, Coeur-Alaska. By 1997, with the decline of gold prices, the project was considered economically unattractive, but work continued in 1998 that added 300,000 ounces of gold resources. Feasibility and exploration continued and in 2005 Coeur drilled 34,035 feet underground to better identify the resources. As of 2006 (Birak, 2006), the Kensington deposit consisted of more than old 12 mines and prospects, 53 patented claims, and 377 unpatented claims that occur in a northwest-trending belt about 2 miles long between the head of Johnson Creek and Sherman Creeks.
Simultaneous, exploration also started in 1978 to the southeast on a contiguous block of mines, prospects, and claims around the old Jualin Mine. From 1983 to 1993, the area was explored by a succession of companies including Bear Creek Mining Company, International Curator Resources, Granges, and Placer-Dome USA. In 1993, Coeur entered into a joint venture on the property with Curator and in 1994, Coeur acquired a 100 percent interest in the property. Coeur subsequently carried out major drilling and underground work on the property, which is now the Jualin deposit/property (JU262) of the Kensington Project.
In 2006, Coeur prepared an extensive NI 43-101 report (Birak, 2006) on the Kensington Project that is largely the basis of this description. As of 2006, the Kensington deposit had been defined by 342,000 feet of drilling; it and the continuous Jualin deposit (JU262) had been explored by 27,000 feet of underground workings. The resources of the Kensington Project, that is both the Kensington deposit described here and the contiguous Jualin deposit (JU262) are: 1) 617,000 tons of indicated resources with a grade of 0.436 ounce of gold per ton (at a cut off grade of 0.12 ounce of gold per ton); 2) 2,499,000 tons of inferred resources with a grade of 0.234 ounce of gold per ton (at a cut-off grade of 0.12 ounce of gold per ton); and 3) 4,206,000 tons of probable reserves with a grade of 0.250 ounce of gold per ton (at a cut-off grade of 0.16 ounce of gold per ton), all estimated at a gold price of $375 per ounce.
The mine plant for the project was largely completed by 2008 but the start of production was delayed to resolve permitting and legal issues. The new Kensington Mine began production in June of 2010 with a projected production rate of about 125,000 ounces of gold per year and a projected life of 12.5 years (Coeur Alaska, 2012). The mine produced 43,143 ounces of gold in 2010 from 174,028 tons of ore that was milled and 88,420 ounces of gold in 2011 (Coeur d'Alene Mines Corp., 2012).
The Kensington deposit is largely in the Cretaceous Jualin Diorite, a northwest-trending stock about 5 miles long and 3 miles wide that is bordered on the east by Triassic metavolcanic rocks of the Wrangellia Terrane and to the west by pelitic sedimentary rocks of the Cretaceous Treadwell Formation, part of the Gravina Belt. The structure of the area is dominated by two regional-scale, northwest-trending faults: the Gastineau shear zone which is southwest of the Kensington mineralization and the Kensington shear zone that trends through the mineralization. The Kensington shear zone has several strands and movement along them controlled the development of the veins in the Kensington deposit. Four periods of deformation are recognized in the Kensington shear zone that span the period from 110 Ma (D1) to 53 Ma (D4). The gold veins are coeval with the D3 deformation, which developed a system of right-lateral, reverse, wrench faults.
The Kensington deposit consists of seven mineralized bodies; they form an en echelon series that dips east and extends to at depth of at least 3,500 feet. The individual ore bodies are related to two general structures: 1) large swarms of discontinuous quartz-carbonate-pyrite veins in biaxial sets between right-lateral reverse, conjugate faults; and 2) small massive sulfide-bearing veins in dilation zones along individual shear zones. Large dilation zones typically have a tabular to sigmoidal shape and grade into barren diorite. The vein swarms typically are continuous across the entire width of the mineralized zones but locally contain poorly mineralized rock.
The vein mineralization typically consists of gold tellurides and gold-silver tellurides with minor base metal sulfides in quartz-carbonate gangue (Casey, 2000). Most of the gold is in calaverite (AuTe2) that also contains tiny gold inclusions. Gold also occurs interstitially and along microfractures in pyrite. The veins also contain trace amounts of petzite (Ag3AuTe2), hessite (Ag2S), coloradoite, (HgTe), volynskite (AgBeTe2), and altaite (PbTe), as well as minor chalcopyrite and rare bornite, molybdenite, sphalerite, tetrahedrite, galena, magnetite, and pyrrhotite.
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). Calcite is the dominant carbonate; quartz occurs in stages 2 to 4 of the mineralization; and minor ferroan dolomite occurs with the calcite in stage 4. The age of hydrothermal muscovite from veins at Kensington Mine (JU029) varies from 53.4 Ma to 56.5 Ma (Miller and others, 1994).
Geologic map unit (-135.081918014094, 58.8624704557345)
Mineral deposit model Te-Au pyrite-carbonate-quartz-carbonate-pyrite veins (Cox and Singer, 1986; model 22b).
Mineral deposit model number 22b
Age of mineralization About 50 Ma. Younger than the 1110 Ma, D1 deformation and younger than the Cretaceous Jualin Diorite. Hydrothermal muscovite from the Kensington Mine has been dated at 53.4 Ma to 56.5 Ma (Miller and others, 1994).
Alteration of deposit 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).

Production and reserves

Workings or exploration
The modern exploration history of the Kensington project began in 1980 when the Kensington Mine was optioned to Homestake Mining Company and from 1980 to 1985 to Placid Oil Company. Placid drilled 13,626 feet near the old Kensington Mine (JU029) and another 14,076 feet on other targets in the area. Coeur d'Alene Mines Corporation became involved in the property in 1987 in a joint venture with Echo Bay Mines; exploration continued that resulted in a feasibility study in 1993. In 1995, Coeur gained total control of the property under a subsidiary, Coeur-Alaska. By 1997, with the decline of gold prices, the project was considered economically unattractive, but work continued in 1998 that added 300,000 ounces of gold resources. Feasibility and exploration continued and in 2005, Coeur drilled 34,035 feet underground to better identify the resources. As of 2006 (Birak, 2006), the Kensington deposit consisted of more than old 12 mines and prospects, 53 patented claims, and 377 unpatented claims that occur in a northwest-trending belt about 2 miles long between the head of Johnson Creek and Sherman Creeks.
Simultaneous, exploration also started in 1978 to the southeast on a contiguous block of mines, prospects, and claims around the old Jualin Mine. From 1983 to 1993, the area was explored by a succession of companies including Bear Creek Mining Company, International Curator Resources, Granges, and Placer-Dome USA. In 1993, Coeur entered into a joint venture on the property with Curator and in 1994, Coeur acquired a 100 percent interest in the property. Coeur subsequently carried out major drilling and underground work on the property, which is now the Jualin deposit/property (JU262) of the Kensington Project.
In 2006, Coeur prepared an extensive NI 43-101 report (Birak, 2006) on the Kensington Project that is largely the basis of this description. As of 2006, the Kensington deposit had been defined by 342,000 feet of drilling; it and the continuous Jualin deposit (JU262) had been explored by 27,000 feet of underground workings.
The mine went into production in July 2010.
In 2012, Coeur-Alaska completed 143,796 feet of core drilling primarily devoted to in-fill drilling of Block K and the Raven veins, and identified 151,000 tons with initial proven and probable reserves of 50,400 ounces of gold. The average of 0.33 ounce per ton gold is approximately 51 percent higher than the overall average reserve grade at Kensington. Kensington produced 82,1225 ounces of gold in 2012 (Athey and others, 2012).
In 2016, Coeur Alaska, Inc.’s accelerated surface- and underground-exploration program was focused on potential resource conversion and expansion within the Kensington Main ore body, nearby Raven vein, and newly discovered high-grade Jualin deposit, as well as targeting for growth of several other new veins in the district discovered through surface-sampling programs. Exploration drilling at Kensington Main included four zones (lower Block M, and zones 12, 41, and 44) focusing on the potential expansion of the Kensington Main resource down-dip and to the south of the current resource model; one drill hole returned 3.7 meters of 41.5 grams of gold per tonne. Development of the Jualin decline (64 percent complete at year-end 2016) is allowing better exploration drilling access, and Coeur Alaska, Inc. reports the Jualin Vein #4 (1 of 5 known veins) contains a resource of 179,000 ounces of gold at 0.619 ounce of gold per ton. In addition, drilling in 2016 at the Raven vein targeted the down-plunge extension of a high-grade ore shoot. As of October 2016, a total of 31,586 feet (9,627 m) had been drilled in resource-infill with potential to convert to reserves, out of a total budgeted 60,000 feet (18,288 m). In addition, a total of 16,712 feet (5,094 m) was drilled underground in resource expansion by October 2016, with a total budgeted 25,000 (7,620 m) feet.
Exploration of the high-grade Jualin deposit accelerated following development of a new exploration station; 3 drill holes targeting the potential upgrade and expansion of the Jualin resource were completed. Surface drilling twinning historical Jualin drill holes began in August 2016, and this drill program had six planned holes for a total of 11,000 feet (~3,350 m). A surface-based winter drilling program followed the twinning program, and focused on potential expansion of the Jualin Vein #4; Vein #5 sits 300 feet (~90 m) below Vein #4 and has similar grade-thicknesses in 5 out of 6 holes drilled to this depth. Coeur Alaska, Inc. indicates accelerated surface and underground exploration will continue in 2017 at Kensington Main, Jualin, Raven, and other new vein discoveries in the district (Athey and Werdon, 2017).
Indication of production Yes
Reserve estimates
As of December 31, 2011, the Kensington Mine has proven reserves of 1,164,000 tons with a grade of 0.280 ounce of gold per ton; probable reserves of 4,842,000 tons with an average grade of 0.209 ounces of gold per ton; measured and indicated resources of 3,039,000 tons with an average grade of 0.193 ounce of gold per ton; and an additional inferred resources of 731,000 tons with an average grade of 0.232 ounce of gold per ton (Coeur d'Alene Mines Corp., 2012).
As of January 1, 2013, the Kensington Mine has proven reserves of 647,000 tons (short) with a grade of 0.28 ounces of gold per ton and probable reserves of 4,020,000 tons with an average grade of 0.21 ounces of gold per ton; 3) measured resources of 382,000 tons with an average grade of 0.24 ounces of gold per ton; 4) indicated resources of 2,224,000 tons with an average grade of 0.20 ounces of gold per ton; and 5) inferred resources of 704,000 tons with an average grade of 0.24 ounces of gold per ton. These reserved include the Kensington deposit and the Raven deposit (formerly Horrible, JU027) (Barry, 2013).
As of year-end 2016, Kensington mine contains proven and probable reserves of 2,616,000 short tons grading 0.190 ounce of gold per ton, for a contained metal content of 497,000 ounces of gold; measured and indicated resources of 3,125,000 tons grading 0.279 ounce of gold per ton, for a contained metal content of 871,000 ounces of gold, which is a 350,000-ounce upgrade from 2015; and, total inferred resources for Kensington are 1,579,000 tons grading 0.276 ounce of gold per ton, for a contained metal content of 436,000 ounces of gold (Coeur Mining, 2016).
Production notes
About 65,000 ounces of gold was produced in the Berners Bay district prior to WWII. That production is documented individually by site in the ARDF records for the old mines in the vicinity of the Kensington deposit. (See the location field for their ARDF numbers). The new Kensington Mine began production in June of 2010 with a projected production rate of about 125,000 ounces of gold per year and a projected life of 12.5 years (Coeur Alaska, 2010). The mine produced 43,143 ounces of gold in 2010 from 174,028 tons of ore that was milled and 88,420 ounces of gold in 2011 (Coeur d'Alene Mines Corp., 2012).
In 2012, Kensington produced 82,1225 ounces of gold (Athey and others, 2012).
In 2016, Kensington mine produced 124,331 ounces of gold, and 620,209 tons of material were processed through the mill at an average grade of 0.21 ounce of gold per ton, with a 94.7 percent recovery rate. Development of the new Jualin decline was 64 percent complete as of year-end 2016. Capital expenditures are expected to increase in 2017, primarily as a result of ongoing development of the Jualin deposit, where production is expected to begin late in 2017 (Athey and Werdon, 2017).

Additional comments

The Kensington mine area consists of two contiguous Property Groups controlled by Coeur Alaska: the Kensington Group and Jualin Group. These two Groups constitute the Kensington Consolidated Property Package (Property Package) (Barry, 2013).
The Kensington Group includes the Raven (formerly Horrible, JU027), Kensington (JU261), Eureka (JU031), Johnson, and Elmira mineral deposits (Barry, 2013).
The Jualin Group includes the Jualin and the Empire mineral deposits. Both Groups are held through a combination of federal patented and unpatented lode mining and mill site claims and state of Alaska claims, all of which are either owned or held by lease agreements. The combined land holdings total over 14,000 acres, approximately 1,150 of which are covered by Federal patented lode claims (Barry, 2013).

References

References

Kucinski, R., Porterfield, J., and Croff, C., 1985, Kensington Project summary report - 1985: Unpublished report for Placid Oil Co., 27 p.
Reporters D.J. Grybeck (Contractor, USGS); V.C. Zinno (Alaska Earth Sciences, Inc.); M.B. Werdon (DGGS)
Last report date 8/26/2017