Automated Gold Grain Counting. Part 2: What a Gold Grain Size and Shape Can Tell!
Abstract
:1. Introduction
2. Database and Acquisition Procedures
2.1. Grain Size Measurement
2.2. Shape Analysis
3. Discussion
3.1. Similarity of Size Distribution
3.2. Fine-Grained Gold Elutriation
3.3. Morphology Changes with Transport
3.4. Meaning of the Different Pristine Morphofacies
4. Conclusions
- Grain size distribution approximately follows a log-normal distribution that is bounded by the instrumental limitations at 1 mm due to sieving and approximately 10 µm due to recovery limitations. This grain size distribution is relatively constant between surveys, areas and deposits. Discrimination of grains from dispersion train versus background signal requires detailed statistical analysis such as Student tests. However, it can efficiently be used without advanced statistics to detect sampling or processing issues.
- Gold grain size distribution obtained in lodgment tills is similar to size distribution in source rocks if such a material is process with the same laboratory procedure.
- No significant difference is noted between gold grain size distribution from various types of glacial sediments, except for high energy alluvial or beach sediments. It suggests that gold grain elutriation by natural processes occurs only at sizes smaller than what can be recovered by heavy mineral concentration procedures. Ablation and lodgment tills from an area constitute a uniform population in this regard and do not require separated statistical interpretations. No significant fine-grained gold loss is noted for samples collected by reverse circulation drilling.
- Gold grain shape is dictated by their relation with neighboring minerals in the source rock, which is rather well preserved by erosion and transport. Four morphofacies are here introduced: “Mutual” grains that are rounded droplet typically found in sulfides; “Crystalline” grains that have at the least partially developed the cubic crystalline habits and considered as originating dominantly from unconstrained growth in quartz or carbonates; “Intergranular” grains with complex shapes, which are dictated by neighboring minerals boundaries; and “remobilized” grains that are leaflets and flakes that filled cracks in other minerals. These four morphofacies are subdivisions of the pristine class.
- No significant difference is noted in grain morphofacies abundance between the various type of sediments, except for reworked till, alluvial sediments and beach sediments, where “reshaped” grains are more abundant.
- The relative morphofacies abundance can be used to forecast the association of gold with sulfides and orient geophysical survey and interpretation.
- The rate at which the shape of the grain is modified in the course of sediment transport is dependent on the size of the grain, small grains being less susceptible to deformation. It is also dependent on shape complexity, delicate grains such as remobilized leaflets being more readily modified than rounded mutual grains. This implies that estimation of the transport distance cannot be simply based on the “pristine” to “Modified plus reshaped” grains without taking into account the initial size and shape of the grains. Drawing exploration conclusions on such premises can be misleading.
- Regional gold grain backgrounds typically have a 50–30–20 ratio between “pristine,” “modified” and “reshaped” grains. The presence of a dispersion trains eroded from a local source will typically contribute dominantly to the “intergranular-pristine” grain population, without significant contribution to “reshaped” population. If counts are sufficient to circumvent the intrinsic variance induced by the Poisson distribution, the count of “intergranular” grains provides the best signal-to-noise ratio for interpreting the results.
Supplementary Materials
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Survey | Count | Mean | Std-Dev | Median | 1st Centile | 99th Centile | Log-Mean | Log-Std-Dev | Asymetry | Kurtosis |
---|---|---|---|---|---|---|---|---|---|---|
Opinaca #1, QC | 243 | 35.0 | 36.7 | 28.3 | 9.6 | 172 | 1.45 | 0.244 | 1.11 | 3.06 |
Eastmain, QC | 3156 | 37.5 | 22.6 | 32.3 | 11.0 | 125 | 1.52 | 0.216 | 0.32 | 0.48 |
Wawa, ON | 6501 | 36.1 | 32.3 | 28.5 | 8.9 | 183 | 1.47 | 0.249 | 0.71 | 1.71 |
Nottaway, QC | 737 | 42.8 | 25.3 | 36.5 | 11.8 | 141 | 1.57 | 0.229 | 0.17 | 0.19 |
Cape Smith, QC | 11,937 | 31.9 | 23.1 | 26.5 | 9.6 | 115 | 1.44 | 0.224 | 0.53 | 1.08 |
Opinaca #2, QC | 782 | 38.6 | 27.2 | 30.8 | 10.9 | 155 | 1.51 | 0.241 | 0.57 | 0.41 |
Appalachian, NB | 137 | 38.8 | 23.2 | 33.5 | 13.1 | 133 | 1.53 | 0.217 | 0.36 | 0.34 |
Abitibi #1, QC | 343 | 38.6 | 23.6 | 32.8 | 11.9 | 141 | 1.53 | 0.219 | 0.34 | 0.56 |
Temiskaming. ON | 355 | 43.9 | 27.0 | 37.4 | 12.5 | 146 | 1.58 | 0.234 | 0.16 | 0.46 |
Frotet, QC | 608 | 41.3 | 25.2 | 35.0 | 13.0 | 138 | 1.55 | 0.223 | 0.42 | 0.14 |
Abitibi, ON | 4025 | 39.3 | 44.2 | 29.6 | 9.5 | 189 | 1.50 | 0.263 | 0.79 | 1.70 |
Chibougamau, QC | 4742 | 42.2 | 37.3 | 34.7 | 12.6 | 160 | 1.56 | 0.225 | 0.65 | 1.47 |
Urban-Barry, QC | 809 | 34.6 | 22.3 | 28.9 | 9.6 | 130 | 1.48 | 0.227 | 0.35 | 0.51 |
Abitibi #2, QC | 504 | 36.4 | 22.5 | 30.6 | 11.2 | 111 | 1.50 | 0.218 | 0.53 | 0.46 |
LaGrande, QC | 253 | 33.2 | 24.0 | 28.2 | 8.9 | 119 | 1.45 | 0.233 | 0.29 | 1.11 |
Northern Abitibi, QC | 225 | 35.2 | 30.2 | 27.9 | 9.9 | 197 | 1.46 | 0.247 | 0.82 | 1.84 |
Total | 35,357 | |||||||||
Average | 37.8 | 27.9 | 31.3 | 10.9 | 147.2 | 1.5 | 0.2 | 0.5 | 1.0 | |
Std-dev | 3.51 | 6.53 | 3.34 | 1.49 | 26.71 | 0.04 | 0.01 | 0.26 | 0.80 | |
Coef.Variation | 9.3% | 23.4% | 10.7% | 13.7% | 18.1% | 2.9% | 5.9% | 50.9% | 82.6% |
Sediment Type | Samples | Gold Grains | Grains/Samples | ||
---|---|---|---|---|---|
Number | % | Number | % | ||
Total | 8107 | 100% | 89,613 | 100% | 11.1 gg/s |
Undiscriminated tills | 3568 | 44% | 27,634 | 28% | 8.0 gg/s |
Lodgment tills | 2151 | 26.5% | 35,086 | 35% | 16.3 gg/s |
Hybrid tills | 841 | 10% | 11,967 | 12% | 14.2 gg/s |
Ablation tills | 1059 | 13% | 13,275 | 13% | 12.5 gg/s |
Reworked tills | 79 | 1% | 920 | 1% | 11.6 gg/s |
Diamictons | 28 | 0.5% | 291 | 0.3% | 10.4 gg/s |
Fluvioglacial | 364 | 4.5% | 435 | 0.4% | 1.2 gg/s |
Alluvium and beach sand | 17 | 0.2% | 5 | 0.0% | 0.3 gg/s |
Metallurgical tests (Rocks) | 30 | 9970 |
Nb | Mean | δMean | Median | Std-Dev | Var.Co. | Asym. | Kurtosis | Axis Ratio | Complexity | |
---|---|---|---|---|---|---|---|---|---|---|
Total | 84,347 | 41.2 | 0% | 31.5 | 61.8 | 1.5 | 17.6 | 410 | 0.68 | 0.71 |
Mutual | 5198 | 20.0 | −51% | 17.9 | 10.0 | 0.5 | 2.9 | 17 | 0.72 | 0.75 |
Cryst. | 8591 | 27.1 | −34% | 23.1 | 15.9 | 0.6 | 3.3 | 20 | 0.71 | 0.75 |
Interg. | 21,406 | 33.0 | −20% | 28.4 | 20.2 | 0.6 | 4.9 | 57 | 0.66 | 0.68 |
Remob. | 11,201 | 36.7 | −11% | 32.1 | 23.0 | 0.6 | 5.7 | 87 | 0.60 | 0.67 |
Modif. | 27,775 | 48.3 | 17% | 37.9 | 43.7 | 0.9 | 11.0 | 290 | 0.69 | 0.71 |
Reshap. | 10,176 | 66.8 | 62% | 38.9 | 152.8 | 2.3 | 8.4 | 77 | 0.73 | 0.75 |
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Girard, R.; Tremblay, J.; Néron, A.; Longuépée, H.; Makvandi, S. Automated Gold Grain Counting. Part 2: What a Gold Grain Size and Shape Can Tell! Minerals 2021, 11, 379. https://doi.org/10.3390/min11040379
Girard R, Tremblay J, Néron A, Longuépée H, Makvandi S. Automated Gold Grain Counting. Part 2: What a Gold Grain Size and Shape Can Tell! Minerals. 2021; 11(4):379. https://doi.org/10.3390/min11040379
Chicago/Turabian StyleGirard, Réjean, Jonathan Tremblay, Alexandre Néron, Hugues Longuépée, and Sheida Makvandi. 2021. "Automated Gold Grain Counting. Part 2: What a Gold Grain Size and Shape Can Tell!" Minerals 11, no. 4: 379. https://doi.org/10.3390/min11040379