Crazytrade,

the mode of mining is determined by the ore body and its shape, in this case for sure most of the mining will be underground. The tailings is another issue, they are formed after the ore is crushed to less than 0.1 mm or so in size. This means they do not have much mechanical stability and it is limited how much you can use as backfill underground. Some will for sure, but most will have to be filled in and stored as tailings dams. As I mentioned before this can be dealt with, but many factors such a appropriate location, drainage, max rain fall etc all play important roles. Tailings are just one part of the overall nickel sulfide business.