Evaluation of the contributions of gold derived from hypogene, supergene and surficial processes in the formation of placer gold deposits

Abstract

Placer gold particles have traditionally been considered as either detrital products of weathering or authigenic minerals growing within placers. Recent advances in understanding of gold chemistry/bio-geochemistry demonstrate that gold growth in specific environments is plausible, but opinions differ on the importance of ‘new’ gold in the overall placer inventory. Here we draw upon visual inspection over 40,000 polished gold particle sections from locations worldwide to evaluate the implications of gold alloy composition and particle heterogeneity in determining the contributions of detrital and authigenic gold to fluvial placers. We conclude i. the detrital model of placer gold formation is widespread and demonstrable, ii. supergene gold may be a locally important constituent of fluvial placers, iii. gold-rich rims on placer gold particles comprise two distinct components: a surface micron-scale addition of pure Au and a tens- of- micron- scale inner rim formed by Ag depletion, iv. the importance to placer inventories of gold particle formation and modification by biogenic processes is considerably overstated.

Archaeological applications of natural gold analyses

“…Rim thicknesses are usually fixed, such that the proportion of rim gold is greater in small or flaky particles that are more mobile in fluvial settings. This relationship explains both the observation that distal placer gold is relatively Au rich with respect to that in proximal placers in the same drainage, and the assertion that gold from placers exhibits a higher Au/Ag ratio with respect to that of the source (Boyle 1979;Chapman et al 2021a). Studies that have a focus on mineral exploration and/or geological characterization of the bedrock source of detrital gold (Fig.…”

“…The chemical and mineralogical characteristics of gold particles liberated by erosional processes are inherited by the resulting placer deposit (Chapman et al 2021a). This principle has underpinned numerous studies of the relationship between placer and lode gold (e.g.…”

“…However, Au/Ag may be modified to a small extent through the formation of gold-rich rims (e.g. dos Santos Alves et al 2020;Chapman et al 2021a;Combes et al 2021), as discussed in the following section. The composition of artefacts is itself a weighted average of the component gold particles, and to gain meaningful comparison with natural gold, notwithstanding the potential complicating factors mentioned above, it is necessary to replicate the outcomes of fabrication processes by either melting or dissolution of an appropriate quantity of particles.…”

“…McCready et al 2003). The subject is discussed in detail in Chapman et al (2021a). The reader is referred to that text for a more detailed evaluation that leads to the conclusion that the outer portions of gold particles comprise alloy with two different origins.…”

“…Shuster and Reith 2018). Whilst the overall importance of this process is contentious (Chapman et al 2021a), it is the case that Ag is absent from biogenically-precipitated gold.…”