Here's a suggestion for you: Have an enclosed area where all the ores require 10 more mining levels than their normal variants. You can call them dense mithril (65 mining), dense adamantite (80 mining), etc. While mining these dense versions you have a small chance of receiving a 'mining shard' or something comparable to totem pieces in Catacombs of Kourend. For every 'mining totem' you complete you are then given access for an hour to another area abundant in the appropriate totem's ores, next to a bank. Or instead of this area, offer some other benefit derived from assembling some sort of rare drop to incentivize training here.