The $11.86m deferred tax asset is the result of the following,...

The $11.86m deferred tax asset is the result of the following, imho.

First they looked five years forward, and estimated the amount of taxable profits they expect @ nominal tax rate of 30%. That would be the amount of tax payable during that five years period.

Then they estimated how much tax payable can be offset by the fractional losses. The fraction is approx. 11% (this is information from the company). $11.86m equals this 11%. So if we reverse engineer this, they expect to pay a nominal amount (before applying the fractional losses) of 107m$ (11.86/0.11). GIven a 30% nominal tax rate, they have estimated 107m$/0.30 is 356m$ of taxable profits over these five years [PS: to me that seems very prudent, but I expected them to be on the prudent side, and they didn't know as per year end Bisie going offline and tin prices to bubble, and also don't know the tax amortization profile of PP&E that might play a role here as well).

So, in other words, if we assume this 356m$ of taxable profits to be equally spread over the five years, this would be the yearly payable
Y1-5:
Taxable profit before tax losses utlilization: 71m$ (being 356m$/5)
less; Fractional use of tax losses : 7m$ (being 11% of 71m$)
Taxable profit : 64m$
Tax rate : 30%
Tax to be paid in the year : 19m$ (aka cash tax)

Tax that would have to be paid if no fractional losses: 30%x71m$ is 21m$, the 2m$ difference is due to the loss utilization. All number rounded ofcourse.

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Yes, they can (and will have to) change the estimates, as this depends on production profile, and margins (hence costs and tin price and payabilities). Also if they keep a rolling five year period, they will add a year of estimate every year, as long as the remaining mine life is five years or more. All other estimates being equal a higher assumed tin price of this 5 year period would trigger an extra addition to the deferred tax asset. And vice versa.

The thing to take away is that the fractional tax losses lower the corporate income tax payable per year by approx. 11% (11% being the fraction indicated by the company). But you will still have to pay tax every year just a bit less, basicly 89% of what you would have had paid without these losses. So the way I like to think about it that they will pay not 30% cash tax rate but effectively 89% of 30% is approx. 27% rate.

So it is not like they can just offset the whole deferred tax asset in year 1. They can offset roughly a fifth every year (assuming taxable profit estimated over the five year to be equally spread over these five years). But they will be tax paying every year, just a little less than nominal.