Mechanical milling of auriferous arsenopyrite and pyrite

The behaviour of pyrite and arsenopyrite in the process of mechanical milling and its potential for exsolution of gold were examined. Both reactants and milled products were characterised by powder X-ray diffractometry and by scanning electron microscopy of polished sections. In the presence of pyrite, arsenic-rich arsenopyrite can readily be transformed to sulphur-rich arsenopyrite, a process which does not readily occur by heating methods. The transformation mechanism can be explained by relating it to similar structural relationships between pyrite, marcasite and arsenopyrite. Detailed structural refinement of powder XRD data by the Rietveld method confirmed changes in unit cell parameters of arsenopyrite as a result of changes in stoichiometry. While the milling process may possibly liberate gold from the sulphide structures, the fracturing and welding processes occurring during milling can also potentially lock the gold up in the large aggregate arsenopyrite.