Lets assume building the initial Mine gives you only 1 Iron per turn. This represents simply having the initial backbone of the mining shafts.
Then later, you can build an expanded mine which increases the base output to 2 per turn.
Finally, (with engineering or some such) you can build a deep-shafted mine, which increases base output to 4 per turn.
"expanded mine" and "deep-shafted mine" aren't additional buildings, but upgrades to the original -> thus built over the original mine.
Now, somewhere in the Civilization Housing Techs ... you get the tech for slums. This also unlocks Mining Camps. A Mining Camp has roughly the same stats as a Slums (though perhaps less housing/more cost?), and can only be built adjacent to the mine. The mining Camp doubles the output of the Mine because it fills the mine with labor, where before the mine was working with largely a skeleton crew of the few, the proud, the brave volunteers.
This means Mine without Mining Camp -> 1 ore, 2 ore, 4 ore (w/ tech)
Mine WITH Mining Camp -> 2 ore, 4 ore, 8 ore (w/ tech).
NOW you have a much more realistic background of increasing Ore output where you can add a diminishing returns Foundry system.
Example: Foundry one *doubles* ore production (deep-shaft + Camp + Foundry = 16).
The next foundry additively increases output by 50% (deep-shaft + Camp + 2 Foundries = 20)
The third and fourth foundries would also increase output by 50% (additively) ... for a total of 28 ore.
// the 2nd, 3rd, 4th were all 50% for the case of simplicity as opposed to a descending staircase
Three more Foundries (6th, 7th, 8th) can now be built (if at size 5?) at an additive 25% increase ... for a max total of 34 ore.
Now then ... if there are TWO mines (both with mining camps) in a city, then this doubles the effectiveness of foundries (but doesn't increase total foundries allowed).
If two deep-shaft mines + mining camps ... then that is 16 total base Iron ore. First two foundries each additively add 100% Iron (32 -> 48) ... while the other 6 can additively add 50% (56->62->70->78->84->92)
If there are three mines within a city, then three foundries can additively add 100%.
Alternatively, you could choose the (1 mine) style for any number of mines, with only one foundry allowing for 100% increase. It have the benefit of providing a more linear Ore output per city, as opposed to giving the great advantage to two mine cities and three mine cities (I feel a two mine or three mine city would be able to mine more efficiently, as well as indicate a richer area of ore in general ... everyone knows how to mine in the city, so there is more applied effor).
//What do you think?