I'd put 'basic metalworking' in mining, and then require 'advanced metalworking' to use, well, advanced metalworking.
The basic stuff would basically be cold-forging metal, giving access to cold forged copper and iron, although if the iron is better than the copper (which it should be) then it should probably take more labour to make, since cold forging iron is a bitch.
Your advanced metalworking would then be forging, and possibly metallurgy (although that could be a seperate research in and of itself), forging would improve the quality of the metal equipment and decrease the labour cost, while metallurgy (either seperate or a part of adv metalworking) would allow one to mix copper and tin to get bronze, or use flux stone with iron to get steel.
Ultimately; it depends on if 'metal weapons' are generalized, or if steel is better than bronze which is better than iron which is better than copper. If the former, then you can keep it pretty generic, but if the latter then you should really introduce an extra 'step' to the iron\steel mix to make it harder to get to, since locating both copper and tin to get bronze is a real bitch.
If you want to go for a 'realistic' approach, iron should be unusable until advanced metallurgy and forging techniques are developed; which would require usage of Bronze to test those techniques, the advanced forging techniques that come about through the use of purified coal (coke) would then give Iron, and the advanced metallurgical techniques that come about through experimentation with flux and coke would then give Steel. Bronze would require more labour than Steel, which would require more labour than Iron, which would require more labour than Copper.
Your call; personally i'd go for either the realistic approach or the abstracted approach, mostly because i feel that it would be nice if Copper and Bronze actually got used, unlike in DF where they never do because it's the same amount of work to use Iron, which is better.