Placing turntable on top of fireplace mantle?

Yes, that will work good! The mass of the marble acts as a solid foundation!

Looks good, except for the chimney. When you see the word "chimney" think great big tuning fork going through the roof picking up every vibration from wind, rain, etc. Not saying its bad just think about it, nowhere near as good as it might seem. Still, a good solid base probably better than most racks. Replace the turntable cones with springs, preferably Townshend Pods, and you should have the best of both worlds- stable base, isolation from structure-borne vibration.

Is that an active fire place, do you use it? That corner and a TT it is really gonna collect the bass? Am I missing something?  I think it looks beautiful, I'm just not sure for a TT.  How is the ol knock test. There is a lot of insulation for the flue and, shielded too, I bet.  Pros and Cons, ay

Great place for a horn loaded sub.. Big boom boom, in the room room..
If your not using it, if you're using the fire place, my TT really wouldn't be there, no way.. My TT are industrial, OK, they are gigantic rim drives..
Still to close to the fire to close to the corner... 10 foot ceilings would help.

I don't do TT in corners UNLES it's in the closet or the next room, the exception being a stepped corner. Kinda asking for trouble..
BUT I usually err on the side of EXTREME caution. 

Very clean, gosh...nice looking..man oh man..

Regards