No ore veins past ten levels

If you go to an area that is more than ten levels below your character, you don't get any ore veins.

At a glance this might seem perfectly reasonable, as it prevents players from farming easy ore by just going to low level areas. But only ten levels is too small a difference, and primarily because the exp gain from kills is so high that throughout much of the campaign, you will typically be at least 5-10 levels above the content. Multiple times now I have had ore fail to spawn in zones which were barely behind my main content, and finally in act 8 I find myself 11 levels above the current place where I am supposed to be. I have not stopped and done excess level grinding. I have not even always full-cleared zones. Sometimes I just blaze through and try to get to the end as soon as possible.

I don't want this mechanic to force me into a narrow range of area choices (or occasionally none at all) just to keep my already pitifully weak ore stream going.

P.S.: A reduction in experience gain in the mid levels by up to 60-80% sounds to me like an entirely workable solution. The player character simply levels up way too fast in the 30-70 range while killing enemies at their own level.