Depth profiling of arsenian pyrite in Carlin-type ores through wet chemistry

Enrichment of As and Au at the overgrowth rims of arsenian pyrite is a distinctive feature of Carlin-type gold ores. Revealing distribution of such key elements in high resolution is of fundamental importance yet often proves challenging. In this study, repeated non-oxidative acid etching of ore samples from Shuiyindong gold deposit was applied to enable elemental depth profiling of gold-bearing arsenian pyrite grains. ICP-OES and AAS were used to determine the dissolved Fe, As, and Au concentrations in each of the etching solutions, and XPS was carried out to exam the etched mineral surfaces. In contrast to conventional ion beam etching that may cause substantial sample damage, our acid etching method does not seem to significantly alter the composition and chemical state of the samples. The etched depths directly converted from the measured elemental concentrations can reproducibly reach a very high resolution of ~ 1 nm, and can be conveniently controlled through varying the etching time. While the Fe and As depth profiles consistently reflect the surface oxidation property of arsenian pyrite, the Au profile displaying an obvious upward trend reveals the ore fluid evolution at the late stage of mineralization. Based on our experimental results, we demonstrate that our wet chemistry method is capable of effective depth profiling of gold ore and perhaps other geological samples, with advantages surpassing many instrumental techniques including negligible sample damage, nanoscale resolution as well as isotropic etching.