Dura-ace FD7800 front derailleur , plus Speen Umlenker , plus carbon cycles eXotic Carbon Front Derailleur Clamp with a few aluminium bolts thrown in for good measure weighs 82g.
![Dura ace FD tuned for MTB use](/gfx/pubs/1312934265_r0.jpg)
This is much, much lighter than any readily-available MTB front derailleur. For example, the latest XTR front derailleur weighs around 150g.