Bimodal distribution of gold in pyrite and arsenopyrite: Examples from the Archean Boorara and Bardoc shear systems, Yilgarn craton, Western Australia

This study investigates the microstructures, geochemistry, and hydrothermal evolution of gold-bearing pyrite and arsenopyrite from six orogenic gold deposits in the Archean Eastern Goldfields Province, Western Australia. Scanning electron microscope (SEM), electron microprobe (EMP) and laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) analyses show that the gold-bearing minerals possess a number of similar textural features, including the occurrence of invisible gold within initial phases of growth, and later-stage visible gold associated with alteration rims. The alteration rims are characterized by a higher-than-average atomic mass (mainly owing to arsenic enrichment) and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite. These observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite. Textural observations and ILA-ICP-MS data suggest that some invisible gold was remobilized from early-formed pyrite and arsenopyrite to form visible gold during development of these alteration rims. Gold may also have been added by hydrothermal fluids during a later stage of mineralization. In situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures. Based on sulfide stability relations, our data indicate that development of arsenopyrite alteration rims associated with late-stage visible gold formation was related to an increase in temperature (maximum increase from 310° to 415°C) and up to of sic orders of magnitude increase in sulfur fugacity, whereas changes in oxygen fugacity were less important. LA-ICP-MS analyses show that the relative and absolute variations in selected trace element (An, Ag, Sb, Bi, Ba, Te, Pb, Co, and Mo) concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite. In general, trace elements within pyrite and arsenopyrite have a relatively uniform distribution, whereas later-stage alteration rims have more variable trace element distributions. Although the observed textures are typical of prograde metamorphic coronae, we suggest that they are the consequence of variations in fluid conditions and chemistry, and that mineralization occurred in response to syn- and/or postpeak metamorphic fluid infiltration. © 2008 Society of Economic Geologists, Inc.