Oxidation state of gold and arsenic in gold-bearing arsenian pyrite

XANES measurements on gold-bearing arsenian pyrite from the Twin Creeks Car lin-type gold deposits show that gold is present as both Au-0 and Au1+ and arsenic is present at As1-. Au-0 is attributed to sub-micrometer size inclu sions of free gold, whereas Au1+ is attributed to gold in the lattice of th e arsenian pyrite. STEM observations suggest that As1- is probably concentr ated in angstrom-scale, randomly distributed layers with a marcasite or ars enopyrite structure. Ionic gold (Au1+) could be concentrated in these layer s as well, and is present in both twofold- and fourfold-coordinated forms, with fourfold-coordinated Au1+ more abundant. Twofold-coordinated Au1+ is s imilar to gold in Au2S in which it is linearly coordinated to two sulfur at oms. The nature of fourfold-coordinated Au1+ is not well understood, althou gh it might be present as an Au-As-S compound where gold is bonded in fourf old coordination to sulfur and arsenic atoms, or in vacancy positions on a cation site in the arsenian pyrite. Au1+ was probably incorporated into ars enian pyrite by adsorption onto pyrite surfaces during crystal growth. The most likely compound in the case of twofold-coordinated Au1+ was probably a tri-atomic surface complex such as S-pyrite-Au1+-Sbi-sulfideH or Au1+-S-Au 1+. The correlation between gold and arsenic might be related to the role o f arsenic in enhancing the adsorption of gold complexes of this type on pyr ite surfaces, possibly through semiconductor effects.