Chemical speciation of gold in arsenopyrite

The speciation of Au in gold-bearing arsenopyrite (FeAsS) from four gold de posits (Olympiada, Sentachan, Sao Bento and Sheba) was determined by micro- X-ray absorption near-edge structure (XANES) on grains well characterized m icroscopically and by electron-microprobe and secondary-ion mass spectromet ry analyses and images. "Invisible" gold in arsenopyrite occurs in two appa rently mutually exclusive chemical forms: chemically bound and elemental. A rsenopyrite from the Sentachan, Sao Bento and Sheba deposits contains chemi cally bound gold. With comparable constituent electronegativities and a whi te-line feature in the XANES indicating unoccupied An 5d-states, but absorp tion-edge positions comparable to Au1+ species, the bonding is interpreted as being covalent rather than ionic. The invisible gold in arsenopyrite fro m the Olympiada deposit, on the other hand, occurs as very small particles of Au-0, probably less than a few nanometers in diameter. Micro-XANES data for the Olympiada and Sentachan arsenopyrite support earlier results obtain ed by Au-197 Mossbauer spectroscopy on arsenopyrite concentrates. In some a rsenopyrite crystals, the gold concentration is closely related to growth z oning. This feature represents conditions during crystallization and does n ot correlate with the chemical form of the gold. Similarly, selenium, where present, correlates with gold in some deposits and not in others, irrespec tive of the gold speciation. The finding of two types of invisible gold in arsenopyrite from different deposits has beneficial implications for extrac tive metallurgy.