Getting a better understanding of the Mount Polley accident

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One of the most frequently asked questions after the Cariboo mine's tailings dam spill was, 'What is in the tailings?! And furthermore, are they 'toxic', as has been claimed repeatedly in media coverage? Now that new details have been provided from the mine owner, we are better able to answer these questions


As Peter Severinson wrote in an article on August 7, “tailings” are the water and rock particles left over after the mined rock has been crushed and processed to recover, in this case, valuable copper, gold and silver. So, the tailings are mostly made up of water and minerals of essentially the same composition as the surrounding rocks that the mine is built on, but crushed into very small, sand- to silt-sized, particles.

So what are these rocks? In the Mount Polley area the rocks are mostly what geologists call basalts, which are volcanic rocks that were erupted by an ancient volcano about 200 million years ago, and granitic rocks, that intruded into the basalts a few million years later.

Basalts are primarily made up of minerals called pyroxene, amphibole and feldspars, while granites are primarily composed of quartz and feldspars. These are common rock-forming minerals found in many of the rocks all around you. These minerals, ground up into fine particles, make up the majority of the Mount Polley tailings.

Imperial Metals has recently released their 2013 chemical analyses of the solids in their tailings impoundment and the water quality analyses of the water in the tailings from 2009 to 2014, which show what elements make up the minerals in the tailings. If you total up the amounts in their analysis of the solids, you will see that it sums to less than 12 percent. That is because the majority of the elements in the tailings will be silicon and oxygen, which are the two primary chemical components of rock-forming minerals, and they are not reported here.

The analyses measure certain elements as a percent of the mineral content of the tailings. These include iron, calcium, aluminum, magnesium, potassium, titanium, sodium and phosphorous – all elements found in common rock-forming minerals. All of these elements are within or below the range for average basalt and granite. Which is to say that there is no greater concentration of these elements in the tailings than there are in the surrounding rocks in that area.

All the other elements in the tailings register in the less than 0.1-percent range. Most of these elements will be bound up in the crystal structure of the minerals that you find in the tailings, both the common rock-forming minerals and what geologists call accessory minerals, such as titanite, zircon and apatite. If you were to take a chunk of average granite or basalt, you would probably find very similar values for most of these elements, with the exception of the copper, arsenic, selenium, antimony and silver, which are higher in these rocks because of the enrichment process that created the mineralization at Mount Polley and made it worth mining.

The analyses of the water in the tailings pond shows it comes very close to drinking water standards. This suggests that even the elements that are above the average for normal granite or basalt are bound up in the minerals in the tailings and do not appear to be leaching out into the surrounding water.

So, in terms of immediate concerns about the chemistry of the tailings that have escaped into the environment, it looks like the solids are essentially equivalent to the composition of the rocks in the area, but ground down to sand and silt-sized particles. This is pretty much what receding glaciers in the area did 12,000 to 15,000 years ago when they created naturally occurring sand and silt.

The water in the tailings pond was very close to drinking water standards before it leaked, and as soon as it entered the surrounding watershed it was diluted many times over by the water that it mixed with, which would further reduce the concentrations of the elements listed in the water quality analyses.

The big question most people still have is, what will be the impact of the sand and silt material that was deposited in Polley Lake, Hazeltine Creek and the Quesnel River? One of the most important factors will be how quickly the solid particles react with the air and water into which they have been deposited.

As several people have noted, the Mount Polley tailings are not acid-generating. They do not contain an abundance of minerals that react with oxygen in the atmosphere or water to create acid, which could break down minerals in the tailings and release metals to the environment. That means the solid materials in the tailings are relatively stable, and will probably poses no more future metal contamination hazards than the glacial deposits of silt, sand and gravel in the area.

The water in the tailings is not acidic. Acidity is measured on the pH scale, where seven is neutral, lower values are acidic and higher values are alkaline. The lowest value for the water in the Mount Polley tailings is 7.30, making it just about neutral, and even slightly alkaline.

I am not an expert in tailings remediation nor in assessing the biological impacts that a mudflow event of this magnitude will have on a creek bed or salmon spawning grounds. However, I do have a good understanding of geochemistry and geology. And while there is no doubt that the aesthetic and physical impacts on Hazeltine Creek and Lake Polley have been profound, the present indications are that, given the chemical composition of the tailings from Mount Polley, the future chemical hazard to the environment is probably very low.


Dr. 'Lyn Anglin, PhD, PGeo, is the chair of the Resource Works Advisory Council and the former president and CEO of Geoscience BC.

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