Definition of pathfinder

In geochemical exploration, a relatively mobile element or gas that occurs in close association with an element or commodity being sought, but can be more easily found because it forms a broader halo or can be detected more readily by analytical methods. A pathfinder serves to lead investigators to a deposit of a desired substance. Often called indicator element, but this latter term is restricted by some authors to elements that are important components of the ores being sought.

Ref: AGI