sadly, it doesn't actually turn other metals into gold
massive amounts of energy required
You weren't expecting a nuclear mushroom were you?
(Not Hiroshima - I mean a fungus that enacts nuclear reactions by moving protons and neutrons from one atom to the next.)
It essentially catalyzes this reaction somehow:
Au(s) + O2- + 2 H2O(l) = Au(OH)4- (log K25 = -1.91)
Ah, well, disappointing indeed.
maybe in old mines that the previous tech didn't allow for a final refining process?
Or feeding it tons and tons of sea water? That's the first source of gold on earth, as I suspect many of us know. Unfortunately, it's so diluted that we'd need a really huge pile of hungry fungus, without any cardiac problems, so not afraid of a salt-rich diet.

EDIT
a viable en-devour
I hadn't noticed this. The WO seldom disappoints 