Chemical Speciation of Gold in Arsenopyrite

Abstract

The speciation of Au in gold-bearing arsenopyrite (FeAsS) from four gold deposits (Olympiada, Sentachan, São Bento and Sheba) was determined by micro-X-ray absorption near-edge structure (XANES) on grains well characterized microscopically and by electron-microprobe and secondary-ion mass spectrometry analyses and images. "Invisible" gold in arsenopyrite occurs in two apparently mutually exclusive chemical forms: chemically bound and elemental. Arsenopyrite from the Sentachan, São Bento and Sheba deposits contains chemically bound gold. With comparable constituent electronegativities and a white-line feature in the XANES indicating unoccupied Au 5d-states, but absorption-edge positions comparable to Au 1+ species, the bonding is interpreted as being covalent rather than ionic. The invisible gold in arsenopyrite from 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 obtained by 197 Au Mössbauer spectroscopy on arsenopyrite concentrates. In some arsenopyrite crystals, the gold concentration is closely related to growth zoning. This feature represents conditions during crystallization and does not correlate with the chemical form of the gold. Similarly, selenium, where present, correlates with gold in some deposits and not in others, irrespective of the gold speciation. The finding of two types of invisible gold in arsenopyrite from different deposits has beneficial implications for extractive metallurgy.

A proposed new type of arsenian pyrite: Composition, nanostructure and geological significance

“…Furthermore, discussion of the oxidation state of gold and other metals in arsenian pyrite is simplified with respect to the formal valence states and settings of these elements. For example, Cabri et al (2003), on the basis of micro-XANES analyses, proposed a covalent type of bonding for Au in the FeAsS structure rather than defining its strict ionic valence. Both valences, nominally Au + and Au 3+ , have been suggested for gold in arsenian pyrite.…”

Oxidation of arsenopyrite and deposition of gold on the oxidized surfaces: A scanning probe microscopy, tunneling spectroscopy and XPS study

“…Much of the metal is concentrated as ''invisible'' gold forming submicrometer metallic particles and Au(I) bonded with sulfur (Fleet and Mumin, 1997;Genkin et al, 1998;den Besten et al, 1999;Simon et al, 1999;Tauson, 1999;Cabri et al, 2000;Palenik et al, 2004;Reich et al, 2005). Precipitation proceeds via the electrochemical mechanism involving anodic oxidation of the sulfide mineral and cathodic reduction of gold (Sakharova et al, 1975;Bou et al, 1998;Maddox et al, 1998).…”

Invisible gold in Colombian auriferous soils

“…Then it is very important to obtain a good characterization of the mineral gold ore to determinate the recovering technique. The term Binvisible^arises from the fact that one cannot distinguish, by conventional microscopy, between gold chemically bound in the mineral and submicroscopic inclusions [2]. The question of the chemical form of Binvisibleĝ old in sulfide and sulfarsenide minerals has also attracted much interest in the years following the report of chemically bound Au in sulfides [3,4].…”