To begin with I don't know the detail of how this tax is to be applied.  If based on what has been reported so far and taking everything at face value.  That is how I intrepet its effects. 

What I did was a very "back of the envelope" calculation done on assuming a $850 contract rate and now gold price is at $950, so we pay the windfall tax on the $100 overage.  But without any details you can look at this as the worst case scenario. 

I also assume that this tax is levied regardless of profitability, in that case, based on my illustration, the value of ARU's 13.7 million oz just got reduced by $42 @ oz. for a total of $575.4 million (if the income tax rate is 40%).  In fact if the income tax rate is lower, say 30% instead of the 40% used, we are looking at $49 per oz. for a total of $671.3 million.  If you have no income to used up the deductibility of this tax, then the whole $70 per oz. goes directly into the bottomline.  This tax is nasty!

Your train of thought and mine on how this tax work is along the same line.  You are right, unless the details spells out otherwise, it is not pretty the way it is.