The top half of my passenger side mirror once broke off too (casing & mirror). I cleaned the surface and used epoxy glue. It's held up for a few years now no problem. I should mention though that the car has been off road for seven years now so it does not see rain or snow. Hope that helps.
I use clear silicon. Thats what GM has been using for the last 20 years or so. I dont know why they didn't use it on our mirrors. Its the same stuff that they use to glue the wheel cap emblems onto the caps themselves.