Mount Polley Mine
Commodities: Copper (Cu), Gold (Au), and Silver (Ag)
Location: 56 km northeast of Williams Lake, BC and 8 km southwest of Likely, BC.
Geology and Type of Deposit: Mount Polley is located in the Intermontane Belt of the Canadian Cordillera. The ore is a copper-gold porphyry type deposit that occurs in a Jurassic-Triassic age granodiorite that intrudes older volcanic rocks. Copper-gold mineralization occurs in breccias along the contacts and extends into the volcanic rocks. The main mineral is chalcopyrite, with lesser bornite, malachite, chrysocolla, and azurite. Gold occurs mainly in the sulphide minerals.
The Operation: The Mount Polley deposit was discovered in 1964. Construction of the open pit mine began in 1996 and was complete in 1997 at a total cost of C$115 million. Low metals prices forced a shut down from September 2001 to March 2005. Mining resumed until August 2014 when a failure in the tailings pond dam halted all operations and work focussed on repairing the environmental damage and fixing the dam. Full operations resumed in June 2016. This mine also has limited underground workings that extend from the bottom of the Wight Pit; the ore from these workings is also processed in the mill.
Mount Polley is owned and operated by Mount Polley Mining Corporation. The mine currently processes 20,000 tonnes per day. In 2016 it produced 25,300,000 pounds of copper, 46,400 ounces of gold and 90,100 ounces of silver. Based on current production, proven and probable reserves allow mining to continue until 2026. Exploration drilling to determine the underground extension of this deposit, including from underground workings at the bottom of the Wight Pit, has been ongoing since surface drilling in 2004 showed good mineralization. This may prove to be economic and extend operations.
Mining Method: Surface mining at Mount Polley uses conventional open pit methods and is currently focussed in one pit, the Cariboo Pit. Blasted rock is loaded into haul trucks by using an electric shovel, a front end loader, or an excavator. Trucks haul the ore to a crusher located next to the mill. Waste rock is hauled to a designated waste rock dump that stores either non-acid generating rock or potentially acid-generating rock which must be managed differently at the end of the mine life.
Mineral Processing: The ore runs through a primary gyratory crusher then a cone crusher before being conveyed into the mill. In the mill it passes through a series of rod, ball and pebble mills that pulverize the ore to fine sand. It exits the final mill as a slurry that is piped to flotation cells. In the cells (or tanks) the copper concentrate floats to the surface. It is collected and cleaned in a sequence of float cells before being piped to a thickener cell and on to pressure filters that reduce water in the concentrate to 8%. The concentrate is stored under cover until being trucked to market
Markets: Most of Mount Polley concentrates are trucked to Vancouver Wharves in North Vancouver (Port of Vancouver). There it is loaded onto bulk carriers and transported to smelters in Asia. A portion of the concentrate is trucked to Ashcroft for loading on rail cars bound for smelters in Timmins, Ontario or Rouyn, Quebec.
Community and Employment: Mount Polley employs 370 full time staff. Approximately 40% of employees live in Williams Lake. The remaining 60% live in the surrounding communities of Big Lake, Likely, 150 Mile House and Horsefly.
Environmental Considerations: Mount Polley is located on the Fraser Plateau within the Southern Interior Forest Region Central Cariboo District. Prior to mining the main use of the land was forestry. The reclamation plan is to restore wildlife habitat and commercial forests, areas that will also support hunting, guide-outfitting, trapping and outdoor recreation.
The main environmental considerations at this open pit operation have been protecting natural waters and air quality. Annual precipitation in the region is high and there is commonly excess water to manage at the mine. To protect the surrounding watersheds surface waters are collected in strategically built ditches and directed to the tailings storage facility (TSF). Ground waters that naturally seep into the pits are piped to and used in the milling process. Excess pit water and water used in the mill are also piped to the TSF. Air quality is protected in the dry seasons by water trucks that spray the roads and by water sprayers that are strategically placed near stock piles and on the TSF.
Prior to the TSF dam failure, reclamation was focussed on the no-longer-used waste rock dumps. Test plots with different amendments and types of vegetation have been created and monitored for several years to determine the most successful approach. In most cases the plan is for a progression of activities: 1) overburden covers are amended with a light application of biosolids, 2) the cover is reseeded with native grasses to establish early cover and 3) seedlings of deciduous shrubs and trees suited to the site are planted, 4) conifer seedlings are planted.
The TSF failure in 2014 that released 17 million cubic meters of water and 8 million cubic meters of tailings down Hazeltine Creek into Quesnel Lake did enormous damage to the creek and adjacent areas. After assessment of the impacts and repair of the dam reclamation of the creek began. The channel was redefined and armoured with boulders. The slopes to the creek were mulched, reseeded with native vegetation, including grass, shrubs and trees. Fish habitat was reconstructed in upper part of Hazeltine Creek in 2016. The project involved moving more than 75,000 toads and other amphibians form the area and constructing a fence to keep them from returning. The creek bed was re-contoured to create meanders and important habitat features, including gravel beds, pools, riffles, and woody covers, were added. Thousands of sitka alder seedlings were planted on along the banks to establish the more complex natural setting.
Restoration of habitat, wildlife monitoring and, water quality testing in the creek and Quesnel Lake is ongoing.
BCMEM's Mount Polley MINFILE profile.