I haven't run into (or tried looking for) for lead or coal, but I have never found such a large vein of iron as I have now. I've made 1000 rebar frames, probably another 600 pieces of forged iron, 100 or so iron bars, I currently have just under 25,000 pieces of iron smelted in the furnace, and I have 7 stack (1200) pieces of raw iron sitting in the chest, plus 2x 1200's in the furnace that's shutoff. I probably have around 40% iron left in the vein I found. This is A LOT of iron, more than I've had access to. Just doing the math, tha's a lot more iron that could be smelted down.
I dug down from the base level of a mound probably 22 blocks high, probably...30 blocks deep to find the start of the vein. It extends another 10-15 blocks down, and...very wide, maybe 25-30?