Metal deposits are mined in a variety of different ways depending on their depth, shape, size and grade. Relatively large deposits that are quite close to surface and somewhat regular in shape are mined using open-pit mine methods (Figure 8.1.5 in Section 8.1). Creating a giant hole in the ground is generally cheaper than making an underground mine, but it is also less precise, so it is necessary to mine a lot of waste rock along with the ore, and that waste rock becomes an environmental liability. Relatively deep deposits, or those with elongated or irregular shapes are typically mined from underground with deep vertical shafts, declines (sloped tunnels) and levels (horizontal tunnels) (Figures 8.2.1 and 8.2.2). In this way it is possible to focus the mining on the orebody itself. In some cases, the near-surface part of an orebody is mined with an open pit, while the deeper parts are mined underground.
When ore is first processed (typically close to the mine) it is crushed to gravel-sized chunks and then ground to a fine powder and the ore minerals are physically separated from the rest of the rock to make a concentrate. At a molybdenum mine, for example, this concentrate may be almost pure molybdenite (MoS2). The rest of the rock is known as tailings. It comes out of the concentrator as a wet slurry and is typically stored near to the mine, in most cases within a tailings pond.
The tailings pond at the Volcanogenic Massive Sulphide copper-zinc-silver-gold Myra Falls Mine on Vancouver Island is shown on Figure 8.2.3. The tailings are contained by an embankment. Also visible in the foreground of Figure 8.2.3 is a pile of waste rock, which is non-ore rock that was mined in order to access the ore. Although this waste rock has low levels of ore minerals, at many mines it contains up to a few percent pyrite. The tailings and the waste rock at most mines are an environmental liability because they both contain pyrite plus small amounts of ore minerals. When pyrite is exposed to oxygen and water it generates sulphuric acid—also known as acid rock drainage (ARD). Acidity itself is a problem to the environment, but because the ore elements, for example copper or lead, are more soluble in acidic water than in neutral water the ARD is also typically quite rich in metals, many of which are toxic. The causes and consequences of ARD are summarized below in Box 8.1.
At the time of the failure of the retention dam work was underway to raise the level of the dam so that more tailings could be stored. Because of changes to the mining regulations the structure had not been inspected by government officials for at least two years. The released water and metal-rich tailings flowed first into Polley Lake, and then down Hazeltine Creek into Quesnel Lake. Surface water samples collected in the affected water bodies showed elevated levels of suspended solids, chromium, copper, iron and phosphorous during 2014, but the levels had returned to “normal” by 2016. The remediation project has cost the mining company $70 million, and the mine was out of operation for 3 years (2014 to 2017). Three engineers that were involved in the design, construction and maintenance of the dam are facing disciplinary hearings by Engineers and Geoscientists B.C.
Most mines have concentrators on site because it is relatively simple to separate ore minerals from non-ore minerals, and thus significantly reduce the costs and other implications of transportation. But separation of ore minerals is only the preliminary stage of metal refinement, for most metals the second stage involves separating the actual elements within the ore minerals. For example, the most common ore of copper is chalcopyrite (CuFeS2). The copper must be separated from the iron and sulphur to make copper metal and that involves complicated and very energy-intensive processes that are done at smelters or other types of refineries. Because of their cost and the economies of scale, there are far fewer refineries than there are mines.
Some of the potential environmental effects of metal mines are summarized on Table 8.2. All mining activities result in a loss of natural environment because land has to be cleared and is no longer available for or useful to the plants and animals that once lived there. Mining involves blasting, breaking and crushing rock, and so dust is created. Some of this material is metal-rich, and is fine enough to be wind-borne, and so can be dispersed widely. Some gets into water courses. Because sulphide minerals—especially pyrite—lead to acidification of water acid rock drainage is a common problem around mine sites (see Box 8.1). Materials, such as waste rock that is piled up around a mine, are subject to slope failure and such failures represent risks to people, infrastructure and to the environment. Tailings represent a special class of such materials because they are typically saturated with water, and are also rich in metals and so are potential sources of ARD. Tailings are a problem if their containment structure is breached (as at Mt. Polley), but can also be an ongoing issue of the water they contain is able to slowly leak from the storage facility. Finally, smelting processes involve application of enough heat to melt ore concentrates and that produces gases and ash that become airborne and hence are widely dispersed. In most cases the heat comes from fossil fuels.
The Sudbury region of Ontario is an example of the environmental risk of smelter stacks. Sudbury is home to the largest nickel smelting operation in the world. By the middle part of the 20th century a region of several hundred square km around mine district had either been completely de-vegetated or damaged by the contaminants released or the acidity they produced. In 1970 a 385 m tall superstack was built in Sudbury to allow smelter contamination to be dispersed further. This allowed for eventual revegetation of the region around the city but resulted in the spread of the contamination over a much wider area, even into other provinces and states.
|Potential Environmental Risks
|Loss of natural environment, wind-blown contaminated dust from open excavations, acid rock drainage (ARD)
|Waste rock storage
|Loss of natural environment, slope failure, wind-blown contaminated dust, ARD
|Loss of natural environment, potential slope failure, potential slurry spills, potential leakage of contaminated water, wind-blown contaminated dust, ARD
|Loss of natural environment, toxic particulates and gases from chimneys, leaching of metals from smelter waste material (slag), release of greenhouse gases
Any material that contains the mineral pyrite (FeS2), or some other iron sulphide mineral, has the potential to produce acidity in the surrounding surface environment. That’s because pyrite oxidizes quite readily at surface, and its oxidation leads to the release of hydrogen ions (H+) into the surface water and groundwater. A simplified equation for this process is as follows:
2FeS2 + 7O2 + 2H2O -> 2Fe2+ + 4SO42- + 4 H+
pyrite + oxygen + water -> dissolved iron + sulphate + hydrogen ions
Materials around a mine site that might contain pyrite include rock outcrops exposed by mining, road-building or construction, waste rock or any rock that was mined but doesn’t have enough metal to be considered ore, and tailings from a processing plant. The important point is that these materials may have been underground and so not susceptible to weathering, but, because of construction and mining activities they are now exposed to water and oxygen at surface.
Figure 8.2.5 shows part of the Mt. Washington porphyry copper mine on Vancouver Island. Waste rock piles are present in the background, and rock that has been exposed by mining is visible in the foreground. Both are producing acid rock drainage, and the pH of the water in this area is as low as 4.
Acid water alone has a negative effect on aquatic and terrestrial organisms, but in most cases the acid drainage from a mine also includes elevated levels of metals. This is because the rocks that are weathering tend to be metal-rich, and also because most metals are more soluble in acidic water than in neutral water.
Water draining from the Mt. Washington Mine flows into the Tsolum River an important salmon stream in the Comox Valley. In the 1930s and 1940s this stream had runs of up to 200,000 pink salmon, 15,000 coho, 11,000 chum and 3,500 steelhead. The mine operated for only 3 years from 1964 to 1966, but by 1987 only 14 coho could be found in the river and other fish stocks were also significantly depleted. In 1982 a fish hatchery released 2.5 million pink salmon fry into the Tsolum River; none returned.
Very low copper concentrations, in the order of 10 µg/L, can be toxic to Pacific salmon. In the 1980s and 1990s copper concentrations in the Tsolum River ranged from 20 to 400 µg/L, while copper concentrations in one of the tributaries leading from the Mt. Washington Mine were in the range 5,000 to 15,000 µg/L.
A remediation plan for the Tsolum River was established in 2003. The plan included steps to reduce the level of acid drainage at source, and also to naturally treat some of the water flowing into the river. The main stream draining the mine site was diverted into an existing wetland, and the flow out of this wetland was controlled so that most flow coincided with peak discharge periods of the Tsolum River (to achieve dilution). Reduction in ARD at source was achieved by diverting some surface drainage around the old mine workings and covering a large part of the workings with a bituminous liner.
By 2010 the copper level in the Tsolum River was consistently below 11 µg/L, and it has continued to drop since then. In 2018 over 220,00 Pink salmon, 32,000 Chum, 4,100 Coho and 600 Steelhead were counted in the river.
- Figure 8.2.1 Steven Earle, CC BY 4.0
- Figure 8.2.2 Photo by Steven Earle, CC BY 4.0
- Figure 8.2.3 Photo by Steven Earle, CC BY 4.0
- Figure 8.2.4 Steven Earle, CC BY 4.0, after Jesse Allen, 2014, based on public domain Landsat data from the U.S. Geological Survey, via Wikipedia, https://en.wikipedia.org/wiki/Mount_Polley_mine#/media/File:Mount_Polley_Mine_site.jpg; and https://en.wikipedia.org/wiki/Mount_Polley_mine#/media/File:Mount_Polley_Mine_dam_breach_2014.jpg
- Figure 8.2.5 Photo by Steven Earle, CC BY 4.0
- Epps, D. (2016). Quesnel Lake water quality for samples collected August 2014 to August 2016 compared to drinking water and aquatic life guidelines (Memorandum). BC Ministry of Environment. https://www2.gov.bc.ca/assets/gov/environment/air-land-water/spills-and-environmental-emergencies/docs/mt-polley/sample-monitor/2016-12-08_final_ql_memo.pdf ↵
- Wikipedia article on the Sudbury superstack built in 1970: Wikipedia contributors. (2021, September 11). Inco Superstack. Wikipedia, https://en.wikipedia.org/w/index.php?title=Inco_Superstack&oldid=1043763134 ↵
- White, W. & Healey, P. (2007). Mt. Washington Mine remediation project. Tsolum River Restoration Society & SRK Consulting (Canada) Inc. https://dx.doi.org/10.14288/1.0042513 ↵
- (White & Healey, 2007). ↵
- BC Ministry of Environment and Climate Change Strategy (2019). Copper Water Quality Guideline for the Protection of Freshwater Aquatic Life-Technical Report. Water Quality Guideline Series, WQG-03-1. Prov. B.C., Victoria, BC. https://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/water-quality-guidelines/approved-wqgs/copper/bc_copper_wqg_aquatic_life_users_guide.pdf ↵
- Regnier, R. (1999). Trend analysis of copper in the Tsolum River at Farnham, the Tsolum River 500 m downstream of Murex Creek, Murex Creek at Duncan Main and Pyrrohotite Creek (report for the BC Ministry of Environment and Climate Change Strategy). https://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/monitoring-water-quality/trend_analyses_of_copper_in_the_tsolum_river.pdf ↵
- (White and Healey, 2007) ↵
- Comox Valley Record. (2018, August 9). Tsolum River summer levels continue to drop with rising temperatures (online article). https://www.comoxvalleyrecord.com/news/tsolum-river-summer-levels-continue-to-drop-with-rising-temperatures/ ↵