Myra Falls Mine
Commodities: Zinc (Zn), Copper (Cu), Lead (Pb), Gold (Au), Silver (Ag)
Location: in the mountains ~80km southwest of Campbell River, BC at the south end of Buttle Lake in Strathcona-Westmin Provincial Park
Geology and Type of Deposit: Myra Falls is located in the Insular Belt of the Canadian Cordillera. The ore is classified as a volcanogenic massive sulphide deposit. The host rocks are part of the 370 myo (Upper Devonian) Myra Formation that formed in and around hydrothermal vents on the ocean floor. The ore minerals include sphalerite and chalcopyrite, which occur in a rock body that is more than 6km long and 450m thick.
The Operation: Myra Falls is the only mine in BC this is located within a provincial park. Mining began here in 1966 in an open pit, but after a short period of time went underground. The mine occupies 2km2 (3600 hectares) of land on the surface. However, under the ground there are more than 240km in drifts, shafts and stopes. Mining stopped from December 2001 to March 2002 because of low metal prices. It resumed until October 2015 when all production ceased again due to uneconomic market conditions, and the site was placed in Care & Maintenance status.
Myra Falls Mine has had a number of owner/operators. It was acquired by Nyrstar in 2011. In 2014, the last full year of production, the mill processed more than 463,000 tonnes of ore. Total concentrate production was 51,123 tonnes of zinc concentrate, 10,434 tonnes of copper concentrate and 4,578 tonnes of lead concentrate. Both the copper and lead concentrate contained significant gold and silver credits. At that time the ore reserve (measured and indicated) was 7.4 million metric tonnes, which at the current mining rate would extend the mine life for ten years. In spring 2017 the company began work to bring the mine back into production. A restart may begin as early as spring 2018.
Mining: Prior to suspended operations, mining at Myra Falls occurred in two distinct mines, the H-W and the Battle/Gap, which are connected underground by a 1.8 km drift. Ore is extracted by conventional underground bulk mining methods, including both room and pillar mining and long hole stoping. After the working face of the orebody is drilled with a jumbo, loaded with explosives and blasted, the muck is removed by diesel powered scoop trams and hauled to an underground crusher. After crushing, the ore is hoisted to the surface. There, it is moved on conveyor belts another 1km to the mill.
Processing: The crushed ore enters the mill where it enters one of several ball and rod mills that grind it with water into sand size particles. The slurry is piped to froth flotation cells which produce separate copper, zinc and lead concentrates. The copper, zinc and lead minerals are recovered using froth flotation cells. A gravity circuit using a BC-made Knelson concentrator was added to the mill to better recover the gold.
Tailings from the flotation cells are piped outside to a tailings impoundment behind a man-made dam. The coarser tailings (~50%) which settle out are removed, mixed with cement in the back-fill plant, and piped as a paste back down into the mine where it is used to back-fill mined out workings. The finer material is stored in the impoundment forever.
Markets: When Myra Falls Mine is in operation, the copper and zinc concentrates are trucked daily the 90km to Discovery Terminal, a deep-sea port in Campbell River, where they are offloaded and stored under cover. Once a month, conveyors at the terminal are used to load the concentrate on to ships for shipping overseas to smelters in Japan and Korea.
Community and Employment: Prior to suspended operations in 2015, around 375 people were employed at Myra Falls Mine. About 250 employees work underground as miners, mechanics, equipment operators, electricians and millwrights. The rest work in the mill, in geology and engineering, and in administration. Although there is room at the mine site for mine workers to live, most employees live in Campbell River or Comox and travel daily by a company chartered bus.
Environmental Considerations: Myra Falls is located within Strathcona - Westmin Provincial Park on Vancouver Island in an area designated with a Class B park status. The various ecosystems within the park support many large mammals, birds and amphibians as well as spawning beds for trout and char. Although the mine controls 3,600 hectares (36 square km) of land, it impacts only 198 hectares and obeys regulations that are in place to protect the park and minimize the operation's impact. The goal of Myra Falls reclamation plan is to return the land to a safe and stable condition as close to the site’s natural state as possible where water quality will meet standards of environmental protection in perpetuity.
Waste rock from the mining process is small compared to that of an open pit operation. Some is stored underground, placed in mined out stopes. Some has been brought to the surface, and dumped into the Lynx pit, one of the original mined out pits that permitted in 2008 to be a tailings impoundment.
Through 2014, tailings from the mill were dealt with in three ways. About 58% were piped directly to the tailings impoundment, about 29% (mostly the coarse fraction) were piped back into underground workings as fill, and the remainder placed in a sand stockpile at the surface. The older tailings impoundment was slated for closing and reclamation beginning in 2015.
The mine is located in a region of normally high rainfall, so one focus of the environmental team is to collect and treat all mine water. Due to the nature of the metallic deposit, contact waters become acidic and so carry more metals in solution that cannot be released to the environment. Structures have been built to capture surface and mine water, which are treated with lime to raise the pH and precipitate the metals as sludge. Those waters in turn are directed to polishing ponds – a series of six settling ponds – to clarify and purify the water that comes from the mine, mill, and tailings impoundment. Floating pumps move the metallic sludge out of the ponds into the tailings impoundment. Although most of the water used in the mill is pumped back and re-used over and over, water is still tested daily for toxicity to ensure whatever water is to be released will be neutral and safe for fish habitat.