One thing that they clearly haven't taken into account, is the fact that gold mining causes environmental damage on many other levels as well.
You shouldn't just look at carbon emissions, but also at other forms of pollution caused by gold mining.

In most of the western world, there are pretty strict regulations on how you can mine for gold, but that isn't the case in most of the third world.
There they still use mercury to recover gold, which heavily pollutes the soil.