Sentence:  The company is considering a site at Mandurang Valley and two others near its mine in Kangaroo Flat, south-west of Bendigo, for a <WSD>mullock</WSD> heap that could reach 21 metres over 25 years. 
Meaning of the word marked with <WSD> tags:  Waste rock from which the wanted gold, minerals, opal, etc., has been extracted; waste material generated while searching for minerals or while mining, such as when sinking a shaft.