I would think that an alien race advanced enough to travel here from great distances would be smart enough to look for resources closer to home, as there is a huge cost in time, effort, and energy requirements, such that the value of any minerals taken from Earth would be almost worthless, or used up, by the time they got them back home. As some have pointed out, there are easier ways to get useful minerals without mining them from here. Unless there is some specific and quite unique mineral or life form here on the planet they are desperate for, then I do not see a benefit to traveling great intergalactic distances to acquire what can be found closer to home. It just doesn't make sense to me.