Waste rock dilution causes mill feed grade to be lower than mineral
resource grade. In nickel mines relative grade decrease (dilution) may be tens of per
cent because of irregular ore boundaries and weak wall rocks. Five per
cent dilution costs millions of dollars annually in medium size
underground mine. In Outokumpu Group selective bench and fill stoping,
cable bolting/shotcrete-skin support and fast ore boundary definition
(OMS-logg in-hole logging or sludge sampling) are the key tools in
dilution control. Commitment of the whole organisation to keep dilution
low is also needed. Dilution sources and effects are discussed with examples of the
dilution control methods applied in Outokumpu mines.