In your example the company boosts the Canadian economy by $100 - not $50 - because all of the money stays in Canada with either the company or the employee and is presumably spent in Canada.
It doesn't matter which job the worker has, he still earns $50/hour, so the benefit to Canada from his own personal spending is the same whether he works for the mine or for someone else.
But it's not the mine that creates the second $50 value, it's the producers of the goods that the worker spends it on. It's inadmissible to chalk it up to the mine unless you're going to say that the producers of the food and clothes he buys don't produce any value in their goods themselves. Either way, however he still spends it, and it makes no difference.
Canadian owned with Canadian employee: benefit to Canada $50 / tonne
Canadian owned with foreign employee and Canadian doing something else: benefit to Canada $90 / tonne
(In both cases the Canadian employee still earns $50 / hour, so he gets the opportunity to contribute some or all of that $50 on further Canadian purchases.)
I didn't give you the figures if the mine is foreign-owned. Here's how that works. Let's say the mine is sold to China. It's full of Canadian workers, at the moment. The negotiation between China and Canada goes like this:
Chinese representative: we want to buy your mine.
Canadian: OK. The price is $CDN1bn. But you have to continue to use Canadian workers. You can keep the $50 per tonne the mine earns, but we want to keep the $50 / tonne that you're paying our guys, so they will continue to buy Canadian products.
Chinese: no - we want to earn more than $50 / tonne, we want to earn $90 per tonne, using cheaper foreign labour.
Canadian: OK, well, in that case, to offset the loss to our economy from you bringing in foreign labour, we want more. The price will be $1.8bn.
So option 1:
China pays $1bn for the mine, Canadian economy gets no direct contribution from the coal but continues to gain from the $50 / hr that the miners earn, in terms of buying goods from other industries
Or, option 2:
China pays $1.8bn for the mine, imports its own labour, and nothing more is spent in Canada.
There's an appropriate price in both cases. The government has to make sure that the right price is paid. But once it is, you can't quibble about foreign workers and damage to the economy any more.
And that also doesn't consider the cost to the Canadian economy of an unemployed Canadian worker.
The worker isn't unemployed. His labour is worth $50/hr, remember? that means that there's a pool of employers waiting to snap him up at $49.99 / hour. If not, then he was overpaid in the first place.