Automated Gold Grain Counting. Part 1: Why Counts Matter!

Abstract

The quantitative and qualitative assessment of gold grains from samples of glacial till is a well-established method for exploring gold deposits hidden under glaciated cover. This method, which is widely used in the industry and has resulted in numerous successes in locating gold deposits in glaciated terrain, is still based on artisanal gravity separation techniques and visual identification. However, being artisanal, it is limited by inconsistent recoveries and difficulties associated with visually identifying the predominantly small gold grains. These limitations hinder its capacity to decipher subtle or complex signals. To improve detection limits through the recovery of small gold grains, a new approach has recently been introduced into the industry, which is commercially referred to as the “ARTGold” procedure. This procedure involves the use of an optimized miniature sluice box coupled with an automated scanning electron microscopy routine. The capabilities of this improved method were highlighted in this study by comparing till surveys conducted around the Borden gold deposit (Ontario, Canada) using the conventional and improved methods at both local and regional scales. Relative to that with the conventional approach, the improved method increased the recovery of gold grains from samples (regional and down-ice mineralization) by almost one order of magnitude. (regional and down-ice mineralization), dominantly in regard of the small size fractions. Increasing the counts in low-abundance regional samples allows for a better discrimination between background signals and significant dispersions. The described method offers an alternative for improving the characterization of gold dispersal in glaciated terrain and related gold deposit footprints.

Counting a pot of gold: A till golden standard (AuGC-1)

“…The preconcentrate was wet-sieved at 1 mm, yielding about 80% of <1 mm material (Appendix A). The material was then processed for gold grain recovery using a fluidized bed, a proprietary technology used for till processing (Girard et al, 2021). This technique enables the efficient recovery of gold grains as fine as 5 µm.…”

“…We then counted gold grain abundance in the aliquots by direct sorting using the automated SEM-based ARTGold™ method (Girard et al, 2021). The aliquots were sieved at 50 µm using a disposable single-use mesh.…”

“…1b). The <10 m grains, however, are absent from till data owing to either the difficulty of isolating these very fine grains when using a binocular microscope (Averill, 1988) or a recovery collapse for grains <20 mm (Girard et al, 2021). The second challenge for technicians is related to the possible occurrence of gold alloys involving other metals.…”

“…Gold grain size distribution in A) orogenic gold deposits (Golden Mile and Nalunaq), a Carlin-type deposit (Getchell), a gold-bearing volcanogenic massive sulfide (VMS) deposit (Trout Lake), a porphyry deposit (Pebble), a gold-bearing platinum-group element (PGE) deposit (Skaergaard), and undifferentiated Canadian gold deposits. B) Cumulative gold grain size distributions for tills from different areas of Canada: Western Cordillera (Plouffe et al, 2013;Plouffe, 2015;Plouffe and Ferbey, 2016;Ferbey et al, 2016), Northwest Territories (Kerr, 2002), Quebec-Ontario and Appalachian tills (Girard et al, 2021). The Canadian ore size distribution of Haycock (1937) is included for comparison.…”

“…Gold grain size is important when producing a reference material, as gold recoveries depend heavily on size (Girard et al, 2021). Fig.…”