Columnist Walt Humphries describes the mineralization that appears in rocks around Yellowknife, including arsenic. He says many people focus on the arsenic dangers from mining, not realizing those same dangers are present when the city allows for rock to be blast without testing its arsenic content first. Above is an aerial photo of the Chateau Nova Hotel, a project that required a fair amount of rock blasting. (NNSL file photo)


This week I am going to talk about some things that deserve a lot more care, consideration and respect than they tend to get – rocks and minerals.

They form the foundation or crust of the earth. Our lives depend on them and much of the trappings of civilization come from them. Everything from salt, to our vehicles, to our electronics.

Yellowknife, Ndilo and Dettah are all sitting on a belt of volcanic rock that runs north-south. To the west are granites and to the east is a mixture of volcanic rocks, granite and pegmatites. They all contain minerals because minerals are the building blocks of rocks. Volcanic rocks tend to carry minerals like pyrite, arsenopyrite, sphalerite, galena and other sulphides. These minerals contain iron and sulphur. As the rocks weather, the minerals weather and their elements are released into the environment.

Imagine you have an outcrop area 100 metres by 100 metres. That’s 10,000 square metres of surface and as it weathers, let’s imagine it releases 10 ppb (parts per billion) arsenic into the environment every year. Now blast it down to three metres’ depth and use it as fill. The surface area of the rock you have know exposed has increased 50-, 100- or 10,000-fold, depending on the size you crush it down to. As it weathers it now releases a whole lot more arsenic and other elements into the environment.

Also, minerals aren’t distributed evenly in rock. You can get real concentrations of the sulphide minerals. As they weather, they are also releasing iron, which oxidizes and gives some sections that rusty look.

You are also releasing sulphur, which forms sulphuric acid and that breaks down the rocks even faster. If you get a heavily mineralized or rusty zone, you will notice nothing grows on it because the area is so acidic. So now, not only do you have an area high in arsenic and other minerals, but you also have acid rock drainage. Blast that rock into fill or gravel and you have made the problem a whole lot worse.


People like to blame our problems all on the mines, but the city has a role to play in all this because they blast rock without testing its mineral content and use that rock for fill. Drainage from that fill is increasing the arsenic content in our local lakes and it is changing the pH, or acidity, of the lakes as well. A double whammy to the environment.

At the mine sites, some of the so-called contaminated areas are nothing more than crushed mineralized rock that was used as fill. Several of the places in Yellowknife, Dettah and Ndilo, that are called contaminated seem to be the same thing. Personally, I don’t like to use the word contaminated to describe this because nothing was added to the fill, it is just naturally high in arsenic, iron, sulphur and other elements.

So where did this fill come from? The mines stopped selling broken or crushed rock as fill years ago. However, when I look at the rock cut up Tin Can hill, the quarry out by the dump or some other areas in town, I see rusted, heavily mineralized rock being used as fill. In some cases the very people complaining about arsenic levels around Yellowknife are the ones directly and indirectly creating them and/or affected by them. For years I have tried to warn the city about blasting outcrops needlessly, particularly without testing them first. Before using that crushed rock as fill, it should be tested.

Apparently, the federal and territorial governments have rules that fill must be tested before it is used, particularly in or near lakes, but no such rules seem to exist for municipalities. Why is that? The same rules and regulations should apply for the city.

Frankly, I think a geologist or prospector should test all outcrops before they are blasted and it should be mandatory that the fill be tested. Of all the places in Canada where this is important, Yellowknife would be near the top of the list.

Rocks and minerals should be given the respect they deserve.