DISTRIBUTION OF GOLD IN PYRITE AND IN PRODUCTS OF ITS TRANSFORMATION RESULTING FROM ROASTING OF REFRACTORY GOLD ORE

We have characterized gold-bearing pyrite and products its transformation to iron oxide during roasting of refractory gold ore at the Goldstrike roaster in Nevada in order to understand the mineralogical transformations taking place during high-temperature oxidation of pyrite and their implications for gold recovery. The distribution of gold within pyrite is heterogeneous and somewhat mimics the distribution of arsenic within the host. Gold concentrations range from the minimum detection limit of 20 ppb to 5,212 ppm. In general, fine-grained and inclusion-rich pyrite crystals tend to be richer in Au than the coarse-grained crystals. Iron oxides in calcine products from the roaster include maghemite and hematite, which may occur in alternating concentric bands within the host particle. Hematite is highly porous and contains minor or negligible amounts of As ( i.e., <2.9 wt%), whereas maghemite appears massive and impervious, and can contain significant amounts of As, up to 18.6 wt%. Gold concentrations range from 30 ppb to 260 ppm in iron oxides. Gold appears to be confined to impervious bands of maghemite within iron oxide particles, and its distribution appears to mimic that of arsenic. Mineral transformations during roasting involve significant changes in volume; evidently, the formation of maghemite as an intermediary product of roasting is detrimental to subsequent cyanide leaching of gold. For efficient extraction of gold, calcines should be: (1) highly porous, (2) highly fractured and permeable, (3) enriched in magnetite as an intermediate product and devoid of impervious bands of maghemite within and around the particles of iron oxide, and (4) dominantly hematite.