The nanoparticulate nature of invisible gold in arsenopyrite from Pezinok (Slovakia)

Arsenopyrite is the most common sulfide host of invisible gold. Yet, despite many studies, the position of such gold in the structure of arsenopyrite has not been resolved conclusively. Here we report a multitechnique study of arsenopyrite samples from the Pezinok deposits (Slovakia) with moderate gold concentrations of 7-10 mu g/g. Secondary ion mass spectrometry showed that the invisible gold occurs as either (1) almost uniform, low-concentration of "dispersed" gold, or as (2) hot spots along fractures. X-ray absorption spectra at the Au L-III edge were collected from such hot spots. The spectra document metallic character of gold although no discrete gold particles were seen even after careful re-examining in back-scattered electron images. We conclude that such occurrences are most readily explained by the presence of gold nanoparticles. We suggest that the dispersed gold is the chemically-bound gold previously detected in these deposits by Au-197 Mossbauer spectroscopy. The concentration of the dispersed gold is too low for X-ray absorption spectroscopy.