Normally it is automatic, it's for this reason that I'll check. But, to resolve problem, you have two ways:
Disable option "Harvest Minerals" and add names of mines at harvest in advanced general settings tab "Looting and Farming options" option "Harvest objects (one name or id by line)(case sensitive)".
Or you can add name of mines with hightlevel of you current zone like 'Tin ore' in advanced general settings tab "Looting and Farming options" option "Don't harvest objects (one name or id by line)(case sensitive)".