Fireplaces, Heating Stoves, Flues and Chimneys - How to attach mantle??

We just had a new direct vent gas fireplace installed and I'm going to put cultured stone on the walls around it but am reviewing different ways to attach the mantle above it. Does anyone have suggestions on the best ways to do this? The chase is closed off now that the fireplace is in. (I know- I probably should have done it before the fireplace was installed)

What kind of mantle is it? Wood, stone, granite?

sorry- I left out that little all-important detail.
It's wood from a 100-year old barn that I'll have planed down at the mill (to about 8"wx4'l) then stain & finish it myself.

As you said, easier, earlier. I make sure the chase has adequate framing at the location I want to install the mantel, usually a 2x8 or so laying on edge and securely fastened to the other members. Then you can install 1/2" pipe flanges to that board and screw 1/2" black iron pipe in a length that will fit at least 2/3rds the distance into your mantel. then drill 9/16 holes in your mantel back at the location of the pipes and epoxy the mantel to the pipe by pouring the holes about 1/2 full of epoxy before inserting onto the pipes.
Make sure your installer provided for a fire stop in the chase if you have more than one level. It is code in most localities.

thanks chandler- installer did install fire stop; there's a 2"x10" header between the studs above the fireplace unit itself so there's definitely something solid there to mount the mantle onto. I'm thinking that if I use black pipe then a black iron flange would be best to screw that pipe into, but when I mount that flange on the wall above the fireplace won't the flange prevent the mantle from being flush against the wall? Another concern: I was going to have the wood planed down to about 3" thick to try & keep the weight down a little; won't that flange stick out under/ or above the mantel?

You could router or saw a slot in the back of the mantel about 1/8 - 1/4" wide. (not going to the ends) Then lag bolt a 36" or so piece of angle iron to the header. Load up the slot with polyurethane glue (spray slot on wood with water to speed cure time) or 5 min. epoxy and press slot over angle iron. Hold or clamp until set. You will have to recess the lag bolt heads with a drill bit. The angle iron will only hold the mantel out a little bit and should be covered up buy the stone.