I am doing exactly the same as you are and am having the same problem finding a
stud. I agree 16" oc is normal however my stud finder did find one stud, with
some reluctance, and it was 17-1/2" from the corner stud. No idea why. Drilling
holes through the mortar will work and you don't have to worry about patching,
since I would assume you would drill in the location where the mantle will be
anyway - which will of course cover the holes.
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