Laser ablation ICP-MS trace element systematics of hydrothermal pyrite in gold deposits of the Kalgoorlie district, Western Australia

The Kalgoorlie district in the Archean Yilgarn Craton of Western Australia contains two world-class gold deposits: the giant Golden Mile shear-zone system and the Mt Charlotte quartz-vein stockworks. Mineralization occurs in three styles: (a) Fimiston style is characterized by ankerite-pyrite ± hematite-magnetite-gold replacement, (b) Oroya style overprints Fimiston ore in the shear zones and is characterized by silica-ankerite-V-muscovite-pyrite ± pyrrhotite-gold-telluride replacement and (c) Mt. Charlotte style is characterized by veins with ankerite-sericite ± albite-pyrite-pyrrhotite-gold selvages. Hydrothermal pyrite is ubiquitous in all styles and occurs in several stages. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analyses (n = 652) were collected on 54 representative samples of pyrite from three deposits. Smooth sections in the ablation spectra were selected for quantitative analysis excluding peaks caused by micron-sized inclusions. Linear mixed effects (LME) modeling of the analytical results indicates no systematic differences between the Fimiston, Oroya and Mt Charlotte styles. The variance introduced to the dataset by geological variability reflected in random differences between samples and deposits is large. This may be a major reason for difficulties in distinguishing the differences due to mineralization style. However, there are clear differences between pyrites co-existing with different mineral assemblages. These indicate a strong control on pyrite chemistry by the composition of the hydrothermal fluids. Finally, Au-Te-As systematics show that a substantial proportion of the analyzed pyrites in all deposits fall into the field of gold saturation consistent with the known metallurgical character of the ores. Mineralogical studies, ultra-fine grinding and recovery by cyanide leach show that > 82% of all gold is present in native grains or in Au-Ag-tellurides. The refractory nature of the Fimiston pyrite concentrates is due to clusters of micron- to nano-sized inclusions rather than due to abundant lattice-bound gold.