The Cariboo-Bell or Mount Polley copper deposits are located approximately 57 kilometres northeast of Williams Lake. Copper was first discovered on Mount Polley in 1964.
In the period from 1966 to 1972, Cariboo-Bell Copper Mines Limited completed 18,341 metres of diamond drilling and 8533 metres of percussion drilling in 215 holes. In 1981, E & B Explorations Inc. optioned the property from Highland Crow and that year completed 1746 metres of diamond drilling, 1295 metres of rotary drilling and a soil geochemical survey. Work completed from 1982 to 1987 included 3585 metres of diamond drilling and 4026 metres of reverse circulation overburden drilling, as well as soil geochemistry, geological mapping, magnetics, ground geophysics and induced polarization. In 1988 Imperial Metals Corporation completed an induced polarization survey and trenching, plus an additional 99 diamond drill holes totalling 8878 metres. In 1989, a further 139 holes totalling 18,639 metres of diamond drilling were completed to detail reserves in the Central and West zones. A total of 535 percussion, rotary and diamond drill holes, comprising of 62,482 metres of drilling, were completed to the end of 1989.
The deposits occur within felsic Jurassic-Triassic Polley stock rocks which have intruded Nicola Group volcanic rocks. The Nicola Group in the area comprises a sequence of alkali basalt breccias and flows of Upper Triassic (Norian) age overlain by polylithic breccias characterized by the presence of felsic clasts of Lower Jurassic (Pliensbachian(?)) age. The stock which hosts the copper mineralization is a complex of several intrusive phases ranging in composition from diorite to syenite. Pyroxenite and gabbro have been intersected in drill holes while nepheline syenite dated at 201 Ma occurs to the west (the Bootjack stock) and presumably represents a more differentiated phase of the Cariboo-Bell intrusions.
Alteration is zonal with an outer propylitic zone, consisting of a calcite-epidote-chlorite-pyrite assemblage, surrounding a potassic zone characterized by secondary biotite and pink orthoclase with diopside. Between the inner potassic zone and the outer propylitic zone is an intermediate garnet-epidote zone. Zeolites are ubiquitous within altered rocks and, although some may be the result of metasomatism associated with hydrothermal fluids, most zeolitic alteration, especially in the outer alteration zone, may be the result of burial metamorphism of regional extent.
Copper-gold mineralization occurs within a variety of breccias and extends into the surrounding volcanic rocks. The two dominant breccia types are crackle breccias, typical of porphyry systems, and intrusion breccias. Six zones of significant mineralization have been defined within the breccias.
Hypogene minerals in ore zones include chalcopyrite (1 to 3 per cent), magnetite (4 to 8 per cent) and minor pyrite while supergene minerals include malachite, native copper, cuprite, chalcocite, neodigenite and covellite. Gold occurs as microscopic inclusions in chalcopyrite. The abundance of copper-gold mineralization is reported to be proportional to the intensity of brecciation.
The two main zones of interest are the Central and West zones. The tabular sill-like Central zone is 1,100 metres in length and up to 450 metres in width. This zone strikes north and dips east. The circular West zone has been drilled to 275 metres depth and is 450 metres in diameter. It plunges to the west and is open at depth below 275 metres.
The Mount Polley deposit was first discovered as a result of follow-up prospecting of an aeromagnetic anomaly highlighted on a government aeromagnetic mapsheet issued in 1963. Mastodon Highland Bell Mines Limited and Leitch Gold Mines first staked claims in 1964. In 1966, the two companies merged to form Cariboo-Bell Copper Mines Limited. The property was mapped, soil and geochemical surveys and, airborne and ground based geophysical surveys were conducted, followed by bulldozer trenching and drilling. In 1969, Teck Corporation assumed control of Cariboo-Bell.
During the period from 1966 to 1972, a total of 18,341 metres of core drilling and 8,553 meters of percussion drilling was completed in 215 holes. In 1970, magnetic, seismic and induced polarization (IP) surveys were conducted. Teck continued to work the property in 1972, 1973 and 1975. In 1978, Highland Crow Resources, an affiliate of Teck, acquired control. In 1979, Teck completed six percussion holes for 354 metres.
In 1981 E&B Explorations Inc. optioned the property from Highland Crow and completed 1,746 meters of core drilling, 1,295 meters of rotary drilling, and soil geochemical and ground control surveys. In 1982, E&B acquired a 100 per cent interest and continued to work the property with joint venture partners Geomex Partnerships and Imperial Metals Corporation. From 1982 to 1987, E&B completed soil geochemistry, magnetic, VLF-EM and IP surveys, geological mapping, 3,585 meters of core drilling and 4,026 meters of reverse circulation drilling
In 1987, Imperial Metals merged with Geomex Partnerships and purchased the remaining interest in the property from Homestake Canada and others. (E&B had merged with Mascot Gold Mines that subsequently merged with Corona Corporation and finally became Homestake Canada).
During the period between 1988 and 1990, Imperial Metals Corporation conducted a comprehensive exploration program consisting of 238 core holes totaling 27,566 meters, the collection of six bulk samples from surface trenches totaling 130 tonnes, geological mapping and IP surveys. In 1990 Wright Engineers completed a positive feasibility study that incorporated new ore reserve calculations, metallurgical testing, geotechnical evaluations, and environmental impact assessments.
Drilling outside the main pit area has identified four other areas of interest. Of these, the Northwest Extension zone was tested by one drillhole. The hole intersected 67 metres grading 0.33 per cent copper and 0.3428 gram gold (Property File - Imperial Metals Corp. Annual Report, 1991). The Road Zone (093A 202) occurs north of the pit area and several hundred metres south of the Lloyd-Nordik (093A 160) mineralization. It consists of magnetite and chalcopyrite-bearing breccia that may resemble Lloyd 2 mineralization.
Pit S-19 measured geological reserves are 48,983,400 tonnes grading 0.38 per cent copper and 0.54 grams per tonne gold. Inferred (geological) reserves at Mount Polley are 230,403,400 tonnes grading 0.25 per cent copper and 0.34 gram per tonne gold (George Cross News Letter #45, 1991).
In 1992, Imperial Metals Corp. received a mine development certificate from the B.C. Ministry of Energy, Mines and Petroleum Resources for a 13,700 tonne-per-day open pit mining operation and covers all elements of the mining plan including the open pit, processing plant, water supply, tailings pond and a power transmission line. The mine development recommended by Fluor Daniel Wright Engineers in its feasibility study calls for 13,700 tonnes-per-day based on an initial 10-year mining reserve of 48,983,400 tonnes grading 0.38 per cent copper and 0.54 gram per tonne gold to produce 13,608,000 kilograms of copper per year. Gold production will exceed 3,428,000 grams per year initially and gradually decline to 1,714,000 grams per year in year 10 (George Cross News Letter #199, October 15, 1992).
In 1994, Gibraltar Mines Ltd., under an option agreement with Imperial Metals drilled seven core holes for 1,216 metres. Upon evaluation of the project Gibraltar declined further participation. Following a merger with Bethlehem Resources Corporation in 1995, Imperial completed an in-house feasibility study. Financing was arranged with Sumitomo Corporation through a joint venture with SC Minerals Canada that culminated in the formation of Mount Polley Mining Corporation (MPMC) in April 1996.
In 1995, Imperial Metals Corporation with support from the Explore B.C. Program carried out an exploration diamond drilling program consisting of 230.1 metres in 2 holes on the Kay Lake Basin zone, 806.2 metres in 4 holes on the Road zone and 737.0 metres in 5 holes on the Pit areas as well as 935.4 metres of rotary drilling in 7 holes on other geochemical and geophysical targets in an effort to increase the resource base. This program confirmed the existence of mineralization which require further definition by induced polarization survey and drilling (Explore B.C. Program 95/96 - M35).
Imperial Metals Corporation and Sumitomo Corporation completed soil stripping on the mill site, road access route and tailings dam site, in anticipation of construction start-up in the spring of 1996. Production will commence in the fall of 1997 (Information Circular 1996-1, page 10). Mineable reserves are reported to be 82,300,000 tonnes grading 0.30 per cent copper and 0.417 gram per tonne gold at a stripping ratio of 1.16 to 1 (Information Circular 1997-1, page 14). This includes the Central pit with 43,022 tonnes grading 0.501 gram per tonne gold and 0.285 per cent copper; the North pit with 9428 tonnes grading 0.329 gram per tonne gold and 0.260 per cent copper; and the West pit with 29,875 tonnes grading 0.324 gram per tonne gold and 0.333 per cent copper. The total geological resource stands at 133 million tonnes grading 0.36 gram per tonne gold and 0.27 per cent copper (Northern Miner June 24, 1996). Annual production, at a daily throughput of 18,000 tonnes is estimated to give Mount Polley a mine life of between 12 and 15 years.
Approximately 750,000 tonnes of ore and waste have been mined from the starter pit, located between the Cariboo and Bell pits. The concentrator/service/office complex and crusher building are roofed and clad (October 18, 1996). Fine-tuning of the mill is underway, while processing close to 800 tonnes of ore per hour. Concentrate has been trucked daily from the site for several weeks to Vancouver. The 1190 bench is nearly complete and drilling on the 1180 bench has begun.
In 1996, seven core holes for 992 meters were drilled in areas peripheral to the proposed pits, such as the Road Zone, the Northwest Zone and the S Zone. Lithogeothemical samples were collected from road cuts and new bedrock exposures.
The mine officially opened on September 13th, 1997. The deposit will be mined in three pits: Cariboo (first), Bell and Springer (T. Schroeter, personal communication, 1997).
In 1997, fifteen core holes for 1,614 meters were drilled to define the margins of the Cariboo Pit and 17 percussion boles for 702 meters were drilled to provide better ore definition for mine planning. Surface and pit wall geological mapping east of and in the Cariboo Pit were conducted concurrently. Three water well holes for 351 meters were drilled to provide source water for milling and mining operations. Rock chip samples from new road cuts were collected and analyzed.
Imperial Metals Corporation (February 1998 merger of Imperial Metals and Princeton Mining) operates the Mount Polley mine. The mine is owned 52.5 per cent by Imperial and 47.5 per cent by SC Minerals Canada Limited, a wholly owned subsidiary of Sumitomo Corporation of Japan.
During 1998, 12.6 million tonnes of material were mined from the Cariboo Pit, of which over 6.0 million tonnes was ore. The bulk of the ore originated from the southern, high oxide, high gold, high value portions of the Cariboo Pit. In the latter half of 1998 it was decided to mine the north portion of the Cariboo Pit that provides better metal recoveries but is generally lower grade material; the intention is to preserve some of the higher grade material in the southern zones for better market conditions.
During 1998, nine core holes for 1,993 meters were drilled within and along the margins of the Cariboo Pit. These holes were designed to prove continuity of mineralization to depth, to determine the orientation of mineralization, to provide definition in under-drilled areas and to determine rock quality for pit design. Core from previously drilled holes within the Cariboo Pit area was relogged and reinterpreted.
Exploration in 1999 included drilling in the Bell Pit and at the south end of the Cariboo Pit. In the Bell Pit, immediately north of the Cariboo Pit, diamond drilling totalling 1946 metres in eight holes tested the Bell deposit to depth and along the north and east limits. Immediately south of the Cariboo Pit, five diamond-drill holes totalling 1011 metres were completed in the recently discovered C-2 zone and an additional five holes totalling 1110 metres were drilled under the south end of the Cariboo Pit to test the Deep Cariboo zone. Finally, 33 short percussion-drill holes totalling 1385 metres were drilled south and east of the Cariboo Pit. Reserves are reported as 76,470,300 tonnes grading 0.47 gram per tonne gold and 0.3 per cent copper in 1999 (Imperial Metals Corporation, 1999).
During 1999, 15.04 million tonnes of material was mined from the Cariboo Pit, of which over 6.65 million tonnes were ore. In addition 99,417 tonnes of material was mined from the upper bench of the Bell Pit, of which 89,353 tonnes were ore. At the end of 1999 a total of 896,793 tonnes of low grade material had been stockpiled for future processing (Imperial Metals Corporation Annual Report 1999, page 6).
In 2000, a total of 226 percussion holes for 10,652.5 meters and twenty-six core holes of 4,875.3 meters were completed. The areas that received work were the 207, Bell, C2, Cariboo, MP-071, Road/Rad, Southeast and Springer. This drilling was successful in defining previously discovered copper and gold mineralization in the C2, 2O7 and Southeast zones, and in discovering high-grade copper mineralization north of the proposed Springer Pit that has been named the North Springer Extension zone. At year end in 2000, Imperial Metals Corporation completed an agreement with Sumitomo Corporation that resulted in a restructuring of the mine’s long term debt and Imperial acquiring 100 per cent ownership of Mount Polley Mine.
In June 2001, Imperial Metals Corporation announced that mining and milling operations at Mount Polley mine would be suspended in September 2001 due to continued depressed copper and gold prices. The mine would be maintained on standby pending an improvement in metal prices. In 2001, the Mount Polley Mining Corporation drilled 6696 metres in 41 NQ2 holes and 9421.4 metres in 170 percussion holes.
Total probable ore reserves as of April 30, 2001 are 30,245,122 tonnes grading 0.36 per cent copper and 0.374 gram per tonne gold with a strip ratio of 1.96. This total includes 1,687,227 tonnes grading 0.269 per cent copper and 0.487 gram per tonne gold with a strip ratio of 0.48 at the Cariboo Pit; 5,099,907 tonnes grading 0.355 per cent copper and 0.37 gram per tonne gold with a strip ratio of 1.88 at the Bell Pit; and 23,457,988 tonnes grading 0.367 per cent copper and 0.367 gram per tonne gold with a strip ratio of 2.09 at the Springer Pit (Imperial Metals Corporation 2000 Annual Report). In 2000, reverse-circulation drilling on the Southeast zone was successful in identifying potentially economic mineralization.
As of September 30, 2001 the probable reserve of the Bell pit is 3,422,940 tonnes at 0.365 per cent copper and 0.364 grams per tonne gold. This reserve is calculated using a strip ratio of 1.620, a copper price of US$0.75 per pound and a gold price of US$325 per Troy ounce.
As of September 30, 2001 the probable reserve of the Springer pit is 15, 272,770 tonnes at 0.404 per cent copper and 0.390 grams per tonne gold. This reserve is calculated using a strip ratio of 2.140, a copper price of US$0.75 per pound and a gold price of US$325 per Troy ounce.
In 2003, Imperial Metals drilled four holes to test the potential below the current planned pit on the unmined Springer zone. The first hole, which was vertical, was mineralized over its entire 466.3 metre length. It averaged 0.61 per cent copper and 0.49 grams per tonne gold for the 267.5 metres it extended beneath the existing pit design depth (Exploration and Mining in BC 2003, page 26). The company recently raised $10 million (Canadian) to support an expanded exploration drill program on the Wishbone (093A 164) area, and to further explore the potential for economic mineralization beneath both the Springer zone and the Bell pit. The Cariboo pit has been mined out. If column leach testing of high oxide copper mineralization from the upper part of the Springer zone is successful it may lead to development of a heap leach facility at the mine.
Late in the 2004, Imperial announced that it will reopen the 20,000 tonne per day Mount Polley copper-gold mine in the first quarter of 2005. The Northeast zone (093A 164), which will be called the Wight Pit when developed, is an integral part of the mine restart. The present reserve base for the three zones (Bell, Springer and Northeast) is 40.7 million tonnes grading 0.432 per cent copper and 0.309 grams per tonne gold (Exploration and Mining in BC 2004, page 46,47).
Imperial Metals Corporation reopened its Mount Polley copper-gold mine in March, 2005, after a 3.5 year hiatus. Imperial Metals also undertook the largest exploration program in the Cariboo in 2005 on and adjacent to its Mount Polley mining lease. By mid-year close to 40 000 metres of diamond drilling had been completed. It further appraised the Northeast and Southeast zones and tested the Pond showing. The the Northeast zone, a highgrade ‘end member’ of the alkalic porphyry copper-gold system at Mount Polley, is now in production as the Wight pit.
As of January 1, 2006 the overall Mount Polley reserves and resources were revised (Press Release, January, 23, 2006). Total proven and probable reserves in the Wight, Bell, Springer and Southeast open pits are 40.9 million tonnes, grading 0.448% copper and 0.318 g/t gold, which contain 405 million pounds of copper and 419 thousand ounces of gold. Measured and indicated resources (which are additional to proven and probable reserves) increased to 79.2 million tonnes, from 68.5 million tonnes in 2005, grading 0.35% copper and 0.28 g/t gold (a copper equivalent grade of 0.58%) containing 615 million pounds of copper and 732 thousand ounces of gold. Inferred resources of 27 million tonnes, primarily contained in the Springer deposit, grade 0.30% copper and 0.29 g/t gold (a copper equivalent grade of 0.53%) containing 179 million pounds of copper and 254 thousand ounces of gold.
In 2010 Imperial Metals Corp. released updated reserve and resource estimates for Mount Polley incorporating the Southeast, C2, Pond, Boundary and Springer pits. The combined proven and probable reserves are 40.5 million tonnes grading 0.318 per cent copper, 0.282 gram per tonne gold and 0.606 gram per tonne silver. Combined measured and indicated resources are 167.7 million tonnes grading 0.317 per cent copper and 0.293 gram per tonne gold. Inferred resources are 3.7 million tonnes grading 0.302 per cent copper and 0.168 gram per tonne gold (Press Release, Stockwatch, May 17, 2010).
In 2012 Imperial Metals Corp. released an updated combined measured and indicated resource estimate of 361.14 million tonnes grading 0.284 per cent copper, 0.297 gram per tonne gold and 0.846 gram per tonne silver and an updated inferred resource estimate of 33.28 million tonnes grading 0.188 per cent copper, 0.242 gram per tonne gold and 0.592 gram per tonne silver (Press Release, Imperial Metal Corp., May 29, 2012).
Imperial Metals Corp. also completed a drill program on the Springer zone in 2012. Highlights of this program include drillhole SD12-152 which returned 67.5 metres grading 1.27 per cent copper and 0.90 grams per tonne gold (Press Release, Imperial Metals Corp., February 5, 2013).
Mount Polley suspended operations on August 4, 2014 due to a breach of the tailings storage facility. Operations were resumed on August 5, 2015.
On January 7, 2019 Imperial Metals Corporation announced suspension of the mine due to declining copper prices. Milling of stockpiles will continue until May 2019 at which time the mine will be placed on care and maintenance.