An XPS study on the valence states of arsenic in arsenian pyrite: Implications for Au deposition mechanism of the Yang-shan Carlin-type gold deposit, western Qinling belt

The enrichment of gold in arsenian pyrite is usually associated closely with the enrichment of arsenic in the mineral, generally known as As1−-pyrite [Fe(As, S)2]. Direct analyses of the valence state of Au in pyrite are, however, difficult due to generally low (∼ppm level) Au concentrations. By means of X-ray photoelectron spectroscopy (XPS), this study obtained reliable valence states of As in pyrite from the Yang-shan gold deposit, a giant “Carlin-type” Au deposit in the western Qinling orogen, central China. The arsenian pyrite specimens were sputtered with Ar+ beam in the vacuum chamber of an XPS to obtain pristine surfaces and to avoid As oxidation during sample preparation. Analyses before and after sputtering show that the As3+ peak are only present on surface that was once exposed to the air. In contrast, the peak of As−1 was essentially unchanged during continuous sputtering. The results indicated that As− is the predominant state on the pristine surface of arsenian pyrite; the peak of As3+ previously reported for Au-bearing arsenian pyrite was probably due to oxidation when exposed to air during sample preparation. It is unlikely that the coupled substitution of (Au+ + As3+) for 2Fe2+ takes place in the pyrite lattice. The so-called As3+-pyrite proposed by previous studies may occur in some special (oxidizing) geologic settings, but it is not observed in the Yang-shan gold deposit, and is unlikely to be important in typical orogenic or Carlin-type gold deposits, in which arsenian pyrite is a dominant Au carrier. Combining previous studies on Carlin-type Au deposits with our XPS experimental results, we suggest that the most likely state of Au in the Yang-shan Au deposit is lattice-bounded Au with or without nanoparticles (Au0).

► Arsenic in As-pyrite can be easily oxidized during sample preparation.
► Pristine unoxidized surface of As-pyrite obtained by sputtering with Ar+ beam bombardment under vacuum, show no oxidized As.
► The experiment results provide new understanding for Au deposition mechanism in As-pyrite.