I attached screenshot. My metal mine has two sets of parentheses or brackets that are adding to the base modifier value. I'm a little confused.

Can the number in the brackets must be due to an iron vein that I control. I can see it on the map, and I can see that my mine is indeed within its area of influence. However, I'm confused about the "(1%)". What is that from? It's a minuscule amount, but still, I'm just curious where it's coming from.

I'm pretty sure that earlier in the game, this same mine did not have this extra (1%) modifier. It seems to have appeared at some point.

Ohh. This forum apparently doesn't accept .PNG images?? I clicked the "attach" button, and it made it look the attachment was uploaded but then it didn't actually appear in the post. (And I didn't notice it wasn't there.) Anyway, I tried saving it as .JPG, and it looks like it's fine now. My OP is updated with the screenshot.

I figured it out: it must be the specialization bonus, as you mentioned. At one point I "crossed over", where I started having more mines than sawmills, so the bonus appeared, and I had forgotten that I read about this in the manual.

BTW, the specialization doesn't differentiate between gold and metal mines. Looks like any mining is just one category, because I see the same (x%) bonus for both my gold and metal mines. But the bracket bonus (which comes from controlling a nearby vein) is separate.

PNG doesn't work, but png does. (If the file size is small enough.)