- © The Mineralogical Society
Mesothermal gold ores in the Archaean Yilgarn Craton of Western Australia are dominated by a pyrite ± arsenopyrite ± pyrrhotite sulphide assemblage. Many of these ores are refractory to varying degrees and require treatment by roasting, bacterial oxidation or finer milling. The most common sulphide ore types can be sub-divided broadly into pyritic (pyrite±pyrrhotite) and arsenical types (pyrite+arsenopyrite±pyrrhotite). Arsenical ores vary from highly refractory to free-milling. Arsenopyrite in highly refractory ores is finer grained, As-deficient (27–32.5 at.% As), contains high average concentrations of submicroscopic gold (60–270 ppm), but does not contain inclusions of particulate gold. Arsenopyrite in free-milling ores is coarser grained, less As-deficient to slightly As-rich (30–35 at.% As), contains low or negligible concentrations of submicroscopic gold, but contains inclusions and fracture fillings of particulate gold. In some refractory arsenical ores, pyrite also contains moderately high levels of submicroscopic gold (20–40 ppm), the concentration of which is directly related to As content of the pyrite.
Pyritic ores are free-milling to mildly refractory, or rarely moderately refractory. Pyrite in pyritic ores contains negligible to low levels of submicroscopic gold (<5 ppm). Other reasons for refractory behaviour in pyritic ores include very fine-grained native gold inclusions in pyrite, or the presence of gold-bearing tellurides.
It is concluded that submicroscopic gold is incorporated into the crystal lattices of arsenopyite and arsenical pyrite at sub-greenschist to lower greenschist-facies temperatures, and is progressively expelled as inclusions and fracture fillings of native gold in sulphides, and ultimately into the gangue, as recrystallization proceeds through upper greenschist- into amphibolite-facies temperatures, during deformation and burial. Submicroscopic gold is expelled more rapidly from pyrite than arsenopyrite. Pyrrhotite progressively replaces primary pyrite at higher temperatures, but rarely contains gold.
Finally, a metallurgical classification scheme for refractory ores is presented which incorporates the above mineralogical conclusions.