Athas likely has many deposits of metallic ore deep underground. It’s the surface mining sources that are spent. Mining without machinery this deep is probababy impossible for humans and probably even for dwarves. If a Sorcerer Monarch could locate a source through divination (they won’t even try because they don’t know about modern geology, however), they could use undead creatures, golems, or other created for purpose constructs to mine the ore.
I’ve had some adventuring done in the Tyr iron mine and a party that tried to start up their own mine. One of the party members was a former templar of kalak who oversaw the mines. He knew a modified Detect Minerals and Metals spell from Races of Faeurn that increased range in 60ft increments per caster level.
Most of the serious mining was done by dwarves and kobalds in the Green Age. Every other race was far behind them and none of the Sorcerer Monarchs were miners so a lot of information of geology was lost. In the Tyr region (and most of the rest of the world) the easy to reach metal was mostly mined out, deeper mines were collapsed and the knowledge to access the deep metals was lost. A few still operate such as in Tyr but without advanced mining knowledge they are very limited.
I use some of the old Roman mining techniques as a baseline for current techniques:
Surface Mining - rivers and exposed metals, all used up
Opencast - following rivers and exposed areas to the source and “strip mine” with short tunnels, all used up
Deep Vein - Deeper and longer tunnels (usually from an Opencast area but occasionally found using the spell up to a half mile underground. These use narrow vertical shafts and wide horizontal galleries with long tunnels following veins.
Like the Romans, force, not skill is used for mining in current times. Iron picks cut grooves around the veins, hammers smash the rocks into chunks along with wedges and chisels then the chunks are carted to the surface. Supports of uncut rock and mellikot bone are used along with some wood beams to stabilize the tunnels.
This boils down to the question “how much metal do you want in Athas?”
I personally like to go with “primitive materials are the standard” “bronze is for specific regions and is still expensive” and “iron/steel feels like getting a low end magic item or full plate in other settings”
So I’d say deep mining could be introduced as long as it is a rare undertaking and the iron extracted is treated as a precious resource.
Also I’d like to point out that a lot of this depends on the Sorcerer King in question. Some will have far less interest in such a venture such as Lalali-Pui (trees > deep mining), Nibenay (books > deep mining), Abalalech-Re (apathy > deep mining), or Kalak (rainbow ziggurat > deep mining). However someone like Andropinis or Hamanu I do think might go for this idea. Additionally if you want a different city state to try this, a high enough ranking templar might be able to get the okay for such a project and get the royal defilers to help him out in place of the Sorcerer Monarch personally dealing with it.
In my head canon mining is a plus but not essential for any of the Sorcerer Monarchs, all of whom can magically produce a certain amount of metals for industrial and military purposes if needed. Having a mine increases wealth but because of the SM’s own capabilities they don’t look into deep earth mining.
You could argue that the Nibenese efforts and mine in Black Spine are the result of a templar trying to put some esoteric knowledge she found in a book on ancient forgotten mining techniques to good use in an attempt to curry favor and raise her city-state’s prestige in the region.