I can see the possibility of this theory and will have to think about it some more.
The transcript for the Price of Gold states that they converted the mines to hold Rumpelstiltskin and that a lot of dwarf blood had gone into making his cell. I don’t understand why the mine would hold him unless it was enchanted somehow or the unmined diamonds they used to make fairy dust somehow have a magical property that could keep him from using his magic. Although the EQ visited him in the form of a mouse and morphed into Regina right there. So, how the mines rendered Rumple powerless I am not sure. I always thought it more likely that it was where he wanted to be when the curse was enacted… that somehow it would not affect him the same or something.
Somehow I seem to remember the mines having been abandoned, but I cannot find such a comment in the transcripts. Only this one about the mines being converted to hold Rumple, which perhaps alludes to them being abandoned because the dwarves were helping Snow. But I thought I heard in one of the epiosdes that they were no longer producing fairy dust. Anyone else?