Edgy,

for the nickel ore in the ROF, it is all sulfides mainly in the form of the mineral pentlandite containing the nickel, but also pyrithe(iron sulfides) and some pyrhothite(another form of iron sulfide).

Usually all iron ore is in the form of oxides(magnetite and hematite) with minimal sulfides. The same with nickel containing laterites with nickel oxide and no sulfides. These ores often are found in the tropics or sub-tropics.

World chromite ores are normally oxide ores with Cr2O3 and Fe2O3 with no sulfides. However, if these ores are mixed(normally not the case) you may also mine sulfides while trying to mine the chromite ore. That's why it would be much easier if the two forms of orebodies are fully separated in terms of location since the mining can be done completely separated.

The nickel ore in the ROF is all sulfides and yes tailings etc are all required.