Possibly Dwemer? Not as in unrefined ore per se, but smelted down/reused from scrap and such found in Dwemer ruins?
Judging by its appearance, I'd assume it's simply Bronze or Brass. Bronze if you mix copper and tin, Brass if you mix copper and zinc. Regardless, the armor has a very coppery sheen typical with those two materials. I think Brass is more likely, because Bronze oxidizes with time and turns a http://www.visitingdc.com/images/statue-of-liberty-picture.jpg. Brass, on the other hand, retains its quality. Nothing in the dwarven ruins looked corroded like Bronze does, so it is safer to rule out Bronze as a material used by them.
However, that is just the armor in Oblivion. The armor of Morrowind was different because it could rust and http://members.shaw.ca/rado907/armor/heavy/mor-dwemerarmor.jpg. Rust, officially known as Iron Oxide, requires Iron, as the name implies. Neither Bronze nor Brass contain iron, and thus cannot rust. The Dwemer armor in Morrowind must have been made of a different substance that contained Iron. Possibly Cast Iron which can occasionally have a very dark-brown color. But the pristine coppery sheen of Oblivion's armor leads me to believe it is made of Brass.