What's worth mining in space? Everything that we can mine in space. That seems glib but I'll elaborate.
By moving mining off planet to the moon and asteroids we eliminate the carbon emissions and environmental damage that occurs from these processes.
So how do we do it? We don't need to start sending fleets and fleets of crafts into space which is energetically expensive and time consuming, what we need to do is develop self-replicating mining equipment that we can send to turn the things we want to mine into more things that can mine for us. We won't achieve full self-replication immediately but we can definitely send machines that can do partial self-replication at first and work on improving the ratio of material sent to material returned.
What do we send back? titanium, platinum, aluminum, nickel, iron, whatever we find there. If platinum was as cheap and plentiful as aluminum was now we would have all sorts of catalytic uses for it that would significantly clean up our Earth based industrial practices.
How do we send it back? If we're sending it back from the moon that's easy -- a space elevator or rail gun. If we're sending it back from asteroids inefficient but cheap iron ion thrusters. Use iron from the asteroids as fuel.
Bonus points: Send it back covered in ablative material like mined olivine that can react with CO2 in the atmosphere to sequester it, that way the whole process isn't just moving environmentally damaging industrial practices to other solar bodies but we're undoing some of the damage from past human activities.