AHD3 has separate entries for mantel, the fireplace adjunct and mantle, the covering of whatever sort. Mantel is derived from manteltree which is derived from mantle + tre, beam. As if that weren't enough, we note from Dr Bill's post above, that an older spelling of mantle was, in OE, mentel and, in OF, mantel, both from Latin mantellum.

I have a slip of paper in my Junk Drawer Memory® with an almost legible scribble on it that seems to say something about a bird called a nickle, but I can't find anything a-googling.