Went a place where you can mine 3 iron rocks at one spot, mined, mined, dropped while the ores respawned, took so many concentration, but I actually had better exp per hour than when I tried powermining granite. There's no possible way that you got more xp mining iron in ardougne or alkharid or wherever you mined iron where you didn't have to move than mining granite. Iron is max of like 33k an hour, maybe a little more. With granite you can get close to 50k an hour. Please, I got average of 46k - 49.2k exp per hour powermining iron at my spot, and around 45k exp at granite.