Pyrite geochemistry in the north arm epithermal Ag-Au deposit, Queensland, Australia; A proton-microprobe study

The North Arm epithermal Ag-Au deposit, in Queensland, Australia, contains pyrite that displays spectacular fine-scale cryptic zoning defined mainly by arsenic content; some grains and zones contain up to 6 wt.% As. Proton-microprobe analysis shows evidence of widely variable concentrations of Cu, Sb, Se, Ag, Zn, Pb and Mo. Au contents are below detection limits (10-30 ppm), even in grains with inclusions of electrum. Cu, Sb and Se are broadly correlated with As, and with precious-metal grades in drill-core intervals. High contents of As require either the deposition of a metastable arsenian pyrite, which also accepts high levels of other elements, or a submicroscopic mixture of pyrite and other phases. Interelement correlations are consistent with a mixture of pyrite, arsenopyrite and a sulfosalt phase. The zoning of the pyrite may reflect rapid changes in fluid composition due to repeated boiling in the near-surface environment, but may also reflect chaotic behavior of very local subsystems.