Due to a PM, I'm taking an obvious route towards simplifying mining. Each mine will be given a richness rating, which will be multiplied by the number of miners and possibly a random number of some sort.
Here's the list of potential mines, including one or two per fief:
Redhills Mine: Iron, richness 3
Grayhills: Coal, richness 4; Diamonds, richness 0.5*(1d4)
Greenhills: Copper, richness 6
Blackhills: Silver, richness 1
Yellowhills: Gold, richness 0.07
Haigwaa Mine Alpha: Copper, richness 5
Haigwaa Mine Beta: Coal, richness 2
Torian Limonite Deposit: Iron, richness 0.6
Torian Tetrahedrite Deposit: Copper, richness 1; silver, richness 0.2
Northmine: Tin, richness 0.2
, richness
The White Hills: Marble, richness 50
Sparklestones: Assorted gems, richness 1
Galena Gorge: Lead, richness 1.75; Silver, richness 0.25
Old Eyes Gems and Jewelry: Aluminum, richness 0.9; sapphires, richness 0.3*1d8
Sound okay?
And everyone who wants to mine, please assign miners to mines. You don't need to assign miners to every mine, but you won't get metal from them if you don't.