§1. As long as they hold office validly, the local ordinary and the pastor can delegate to priests and deacons the faculty, even a general one, of assisting at marriages within the limits of their territory.
§2. To be valid, the delegation of the faculty to assist at marriages must be given to specific persons expressly. If it concerns special delegation, it must be given for a specific marriage; if it concerns general delegation, it must be given in writing.
Source: Code of Canon Law (Vatican.va)