What Treesock said. When WoW says you can only locate one type of item at a time, they mean that you can't have "locate minerals" and "locate herbs" or "track animals", or any of those other nice tracking devices that put little yellow dots on your minimap, not that your "locate minerals" will only locate one type of mineral. It should locate any mineral deposit if your close enough, but you have to be in higher-level areas to find tin/silver and even higher level areas to find iron/gold.