Although As-rich, hydrothermal overgrowths on pyrite have been recognized as the primary host phase for Au in Carlin-type deposits in Nevada, the chemical and structural state of the Au has remained unresolved. Spectroscopic and electron imaging techniques have suggested that Au is either structurally bound (e.g., Au1+) or occurs as particles of native Au (Au0), but the latter has never been observed directly. We have determined that Au is present in significant quantities as discrete nanoparticles of native Au (~5–10 nm) in As-rich overgrowths on pyrite from the Screamer deposit in the Carlin trend, Nevada, using analytical and high-resolution TEM and high-angle annular dark-field (HAADF) imaging in STEM mode. Electron microprobe and secondary ion mass spectrometry (SIMS) analyses of the As-rich rims containing the Au-particles reveal that these rims (1–20 μm) contain up to 0.8 wt% Au, among the highest Au-contents ever reported for arsenian pyrite. These observations suggest two possible mechanisms for nanoparticle formation: that Au exceeded its solubility limit in arsenian pyrite causing it to be deposited as nanoparticles of native metal; or that exsolution of native metal from metastable arsenian pyrite was caused by a later event in the history of the deposit.

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