is it the original alternator? The pulley has a small diameter, it makes the alternator rotate much faster than the engine.
I've restore mine last year and changed the main bearing. Its quite a large one for such a relatively small device, may be yours is damaged/corroded and need be replaced.
it's easy and inexpensive to replace it and can't hurt, even if the cause of the failure would be something else.
The shaft is pretty heavy, and if the bearing locks somehow, the weight/inertia could at the speed it turns snap your belt just like that..