Résumé

Dans le domaine fluviatile, la morphologie des grains d’or change depuis la source à son milieu de dépôt, amenant à une augmentation de l’aplatissement et de l’arrondi. Cependant, cette évolution est basée sur des observations morphologiques semi-quantitatives et sur des grains d’or de taille supérieure à 200 µm. Cette étude vise à quantifier en 3D la morphologie des grains d'or de tailles comprises entre 42 et 909 µm en utilisant le microtomographie aux rayons-X pour les grains de taille supérieure à 85 µm et la photogrammétrie MEB pour les grains de taille inférieure à 85 µm. Les descripteurs de forme en 3D sont quantifiés sur des grains d'or de la Rivière du Moulin (Québec, Canada) afin d'étudier le changement morphologique. Au cours d’une augmentation de la distance de transport (i) les indices d’aplatissement, de sphéricité et de compacité ne montrent pas d’évolution, (ii) l'ellipticité diminue et (iii) l’arrondi augmente significativement.

Mots clés: Microscope à rayons-X 3D – MEB 3D – Morphométrie – Descripteurs de forme – Sédiments fluviatiles – Exploration minérale

3.2. Abstract

Gold is a malleable material and the morphology of gold particles changes during fluvial transport. The study of the change in shape of gold particles is commonly used to estimate the distance of transport from the source. Increase of flatness and roundness indices with increasing distance of transport is commonly observed in rivers several kilometers long. However, these estimates are based on subjective observations or semi-quantitative 2D studies, and focus on gold grains greater than 200 µm in size. This study aims to quantify in 3D the morphology of gold grains ranging from 42 to 909 µm in size. The microtomography and SEM photogrammetry were used to reconstruct the shape of gold grains in 3D. The microtomography is preferred for grains greater than 85 µm, whereas SEM photogrammetry is used for grains smaller than 85 µm. The flatness, roundness, convexity, sphericity, and ellipticity shape descriptors of gold grains from the Rivière du Moulin (Québec, Canada) are computed in order to quantify the morphological particle change along 9 km of fluvial transport. Gold grains have a moderate to high flatness, compactness, and sphericity indices that do not change significantly with distance of transport. However, the ellipticity of gold particles slightly decreases during transport, whereas the roundness increases significantly along the Rivière du Moulin.

Keywords: X-ray 3D microscope – 3D SEM – Morphometric analysis – Shape descriptors – Fluvial sediments - Mineral exploration

3.3. Introduction

Native gold is a malleable material of which the shape changes during transport (Hérail et al., 1989; Knight et al., 1999). In mineral exploration, the change in gold grains shape as a function of the distance of transport can help to locate the source of the mineralization (Giusti, 1986; Hérail et al., 1990; Grant et al., 1991; Knight et al., 1999; Youngson and Craw, 1999; Townley et al., 2003). Visual charts and semiquantitative approaches are used to estimate a gold grain shape (Hérail et al., 1990; Dilabio, 1991; Minter, 1999). The general shape, outline, surface texture, and mineral inclusions have been shown to characterize gold particle morphology (Townley et al., 2003). The surface texture and gold particle size are transformed after several kilometers (> 5 km) of transport, depending on the abrasion, hammering and folding processes (Knight et al., 1999). The change in shape is generally studied on gold grains larger than 200 µm, and the quantification of a smaller size fraction is not undertaken (Hérail et al., 1989; Knight et al., 1999; Youngson and Craw, 1999; Craw et al., 2013; Barrios et al., 2015; Craw et al., 2017). However, in alluvial sediments, an important proportion of gold particles is smaller than 200 µm (e.g. North Saskatchewan and Athabasca rivers, Giusti, 1986) and it has been suggested that the particle size decreases as the distance of transport increases (Knight et al., 1999; Craw et al., 2013). Thus, the investigation of various particle sizes including the smallest fractions (i.e. < 200 µm) would be beneficial for the determination of the morphological change of gold grains during fluvial transport.

In the fluvial domain, the morphology of gold grains is measured using the length, width, and thickness of the particle in order to compute flatness shape descriptors (Hérail et al., 1990; Barrios et al., 2015), whereas the roundness is determined with a visual chart (Powers, 1953). The Wentworth flatness index (Wentworth, 1922) and the Corey shape factor (Corey, 1949), are two descriptors commonly used to represent flatness. An increase in flatness and roundness is a typical change in gold grain shape during alluvial transport (Giusti, 1986; Hérail et al., 1990; Knight et al., 1999; Youngson and Craw, 1999). Another shape descriptor, the Hofmann shape entropy (Hofmann, 1994), is used as a settling velocity descriptor or an estimation of sphericity (Le Roux, 1997) but is not commonly used to describe shape change during alluvial transport. An automated computation of shape descriptors was introduced to

surface and triaxial lengths are used to compute quantitative descriptors of the morphology of sedimentary particles (Wadell, 1932; Wadell, 1933; Riley, 1941; Aschenbrenner, 1956; Hayakawa and Oguchi, 2005; Fonseca et al., 2012; Alshibli et al., 2014; Masson et al., 2020).

In this paper, we combine µCT and SEM photogrammetry to reconstruct in 3D the morphology of fluvial gold grains, with a long axis between 42 and 909 µm. We compute 3D shape descriptors that use shape parameters such as triaxial length, surface area and volume of the particles. A study case of gold grains from Rivière du Moulin (Québec, Canada) leads to estimate the change of gold particle morphology along 9 km of fluvial transport.

3.4. Geological setting

The Rivière du Moulin was sampled over 9 km, from the Lac Volet to Rivière Chaudière at Beauceville because of limited anthropogenic disturbances such as bridges, and a limited number of tributaries, which prevents contamination by exotic gold grains (Fig. 3.1). In this section, the river has an average width of 5.0 m and an average slope of 0.7 degrees.

The river erodes mainly glacial and alluvial sediments. Under the sedimentary cover, the basement is composed of shales of the Ordovian Magog Group (Hébert, 2001). The Magog Group consists of four formations, but only two of them are present in the study area, the St-Victor and Beauceville formations (Hébert, 2001). The St-Victor Formation is composed of turbiditic sequences and felsic volcanoclastic rocks. The Beauceville Formation is mainly composed of graphitic schists as well as felsic volcano- sedimentary rocks and cherts, and hosts most of the gold mineralization north and near the river (Fig. 3.1). Mineralized occurrences around the Rivière du Moulin are documented as: 1) quartz vein stockwork associated with arsenopyrite and pyrite hosted in volcano-sedimentary rocks and disseminated gold bearing pyrite in black shales (Hébert, 2001); 2) gold placers, such as Ruisseau des Meules (Fig. 3.1) and Rivière Gilbert to the northeast (Shilts and Smith, 1986). Till cover (Fig. 3.1) is composed of three units from the oldest to the most recent (Shilts and Caron, 2018): 1) the Johnville till that was deposited by a southeast flowing glacier and that contains saprolite clasts targeted for gold exploration; 2) the Chaudière till that was deposited from a glacier flowing toward the southwest; 3) the Lennoxville till, associated with the last major glacial episode, that was deposited by a glacier flowing in a southeasterly direction. The Rivière du Moulin flows from south to north, in the opposite direction of glacial movements.