Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example

Abstract

This study documents the bulk Au fineness (Au parts per thousand) of the bullion from a placer gold mine in southern New Zealand. The compositions of doré bars produced approximately every 10 days over nearly three years is compared to the range of compositions of gold particles which have been extracted. Silver is the principal impurity in the gold, and the doré bars contained 2-3 wt % Ag over the period examined. At the scale of a typical individual 0.5 mm gold particle, there are three different types of gold: an Ag-bearing core (2-9 wt % Ag), a 10-50 µm wide Ag-poor rim (typically <1 wt % Ag), and micron scale overgrowth gold (0% Ag). The overgrowths are volumetrically negligible, and the average Ag content of a gold particle is controlled principally by the proportions of core and rim gold. The rims have been formed by recrystallisation of deformed core gold, with associated leaching of the Ag from the recrystallised gold. The volumetric proportion of cores has decreased with increasing flattening of gold particles, and highly flattened and folded flakes have little or no remnant cores. The bulk Au fineness of doré bars from the mine has decreased from 980 to~970 as the mine progressed upstream in a Pleistocene paleochannel because the upstream gold has been less flattened than the downstream gold.

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

“…3I), but have seen no evidence that gold accumulation is progressive beyond these features. Consequently, we concur with Stewart et al (2017) and Craw et al (2017), who made a clear distinction between surface authigenic gold and a pre-existing rim formed by Ag depletion.…”

The Paleoproterozoic metaconglomerate-hosted Castelo de Sonhos gold deposit, Tapajós Gold Province, Amazonian Craton: a modified paleoplacer origin

“…The homogeneous chemical composition of the detrital gold present in the matrix of the metaconglomerate, including the narrow range of silver contents, indicates a single source, or sourcing from a single deposit class (e.g., Craw et al 2017), although multiple sources could be expected for a paleoplacer deposit, considering that the provenance study (Klein et al 2017) showed multiple sources for detrital zircon of the hosting metaconglomerate. The high Au/Ag ratios and absence of significant content of any other metal suggest that the gold particles might have come from orogenic gold source(s) (Minter 1991).…”

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

“…Chemical alteration to gold particle cores can occur during annealing following deformation as described above (Stewart et al 2017;McLachlan et al 2017;Kerr et al 2017b), and in our view is the most common origin of gold particles which appear to have multiple cores, such as illustrated by Knight et al (1999a). Some researchers view the addition of a gold rim to pre-existing particles as the first stage in a process of biologically mediated 'transformation'.…”

“…6a-c). Similarly, a mixture of gold particle morphologies can also occur in modern placers where gold Craw et al 2017). The progressive nature of gold particle deformation has been studied in the context of transport distance both qualitatively (Townley et al 2003) and quantitatively (e.g.…”

“…With further transport and physical deformation, gold particles become progressively flattened and their interior structure becomes progressively finer grained and eventually becomes fully annealed (Fig. 7j; Stewart et al 2017;Craw et al 2017).…”