Chemical and physical heterogeneity within native gold: implications for the design of gold particle studies

Abstract

Studies of populations of gold particles are becoming increasingly common; however, interpretation of compositional data may not be straightforward. Natural gold is rarely homogenous. Alloy heterogeneity is present as microfabrics formed either during primary mineralization or by modification of pre-existing alloys by chemical and physical drivers during subsequent residence in either hypogene or surficial environments. In electron-probe-microanalysis (EPMA)-based studies, the combination of Cu, Hg, and Pd values and mineral inclusion suites may be diagnostic for source style of mineralization, but Ag alone is rarely sufficient. Gold characterization studies by laser-ablation-ICP mass spectrometry linked to both quadrupole and Time-of-Flight (ToF-MS) systems show that only Ag, Cu, and Hg form homogenous alloys with Au sufficiently often to act as generic discriminants. Where present, other elements are commonly distributed highly heterogeneously at the micron or submicron scale, either as mineral inclusions or in highly localized, but low concentrations. Drawing upon our own data derived from individual inspection and analyses of approximately 40,000 gold particles from 526 placer and in situ localities worldwide, we show that adequate characterization of gold from a specific locality normally requires study of a minimum of 150 particles via a two-stage approach comprising spatial characterization of compositional heterogeneity, plus crystallographic orientation mapping, that informs subsequent targeted acquisition of quantitative compositional data by EPMA and/or laser-ablation ICP-MS methods. Such data provide the platform to review current understanding of the genesis of gold particle characteristics, elevating future compositional studies from empirical descriptions to process-focused interpretations.

Native Gold in the Chudnoe Au-Pd-REE Deposit (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Genesis

“…The mechanism of formation of native gold of varying chemical composition is very complicated and depends on many factors [22][23][24][25][26][27][28]. The content of elements in native gold is determined by their concentrations in hydrothermal solutions, which are governed by temperature, redox conditions, pH, presence of ligand elements Cl, S, and elements such as Se, Te, As, Sb.…”

“…The precipitation of these metals from hydrothermal solutions is the main process of concentrating of metals and they can form their own mineral phases or occur as an isomorphic impurity in other minerals. The fineness and set of impurity elements in native gold, and associated minerals are the indicators of different genetic origin [28,[38][39][40][41][42][43].…”

Challenging the traceability of natural gold by combining geochemical methods: French Guiana example

“…hydrothermal or metamorphic event) and extractive processes (Roberts et al, 2016). It is important to note that composition of gold is generally heterogeneous at sub-µm level (Chapman et al 2021), which makes the use of multivariate data more challenging. The previous traceability studies only focus on the identification of processed and/or refined non-natural gold (i.e.…”

“…114 Cd and 118 Sn are underestimated by 25% (Appendix A.2). Chapman et al (2021) have highlighted that trace elements can be heterogeneously distributed within a gold particle at 5-10 µm scale. But our LA-ICP-MS analytical conditions, that use a larger ablation spot (85 µm), yielded reproducible analyses within a same gold particle, meaning that our gold grains can be considered as homogeneous at the scale of our ablation spot.…”

“…However, to develop a robust interpretation based on mineral inclusion data, a large number of gold grains must be collected (i.e. about 150 particles for Chapman et al (2021)) and characterized because most are devoid of such inclusions (e.g. only 5% of gold grains host inclusions in the Klondike Gold District; Chapman et al, 2010a).…”