Consider the stated properties of mithril:
-- Lighter and stronger than steel
-- Does not corrode
-- Looks a lot like silver
-- can be "beaten like copper and polished like glass"
Now consider the properties of titanium:
-- Strong as steel but weighs less
-- Highly resistant to corrosion
-- Looks a bit like silver
-- More ductile than many metals
Maybe the folk of Khazad-Dum were sitting on upon a massive deposit of titanium?