Quantitative XPS characterization of “invisible gold” in Carlin-type gold ores through controlled acid erosion

Abstract

The occurrences of gold as lattice-bound ionic Au⁺ and nanoparticulate Au⁰ inclusions, collectively termed “invisible gold”, in pyrite have long been recognized as a most important characteristic of Carlin-type gold deposits. Yet, till now, few studies have quantitatively resolved the two chemical states of Au, reflecting substantial challenges in the characterization of Carlin-type gold ores. In this work, quantitative XPS analysis of “invisible gold” was carried out for the first time, which revealed valuable results including the total Au concentration, percentages of Au⁺ and Au⁰, size of Au⁰ nanoparticles, and Au/As ratio at different depths of the pyrite surface. A simple acid etching approach, which effectively removed the signal-blocking outermost pyrite rim of low Au content and the signal-interfering Mg carbonates, was demonstrated to be the key to enhancing the Au signal in XPS. The ascending Au concentration (0.0017–0.24 at%) with increasing depth and the variation of Au⁰ fraction (31.2 to 59.8 at%) with relatively stable As concentration (∼2 at%) indicate heterogeneous distribution of Au chemical states in pyrite particles, while the small Au nanoparticle size (minimum 1–2 nm) confirms the challenge of microscopic observation of “invisible gold”. Unlike ion beam etching techniques potentially suffering from sample damages, the non-oxidative acid etching method used in our study is considered non-destructive and capable of obtaining chemical states with reasonable statistical significance. We believe that the new findings from this work will provide useful information to improve our understanding of the still puzzling ore-forming mechanisms and to boost the related metallurgical efficiencies. Furthermore, our methodology may also be applied to the characterization of poorer Au ores and even other geological samples.