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Alluvial gold flakes

Updated: Sep 9

SOME NOTES ON BEHAVIOR AND RECOVERY OF ALLUVIAL GOLD FLAKES

“…Most of the gold contained in alluvial sediments is made up of small and thin flakes, improperly called pailettes in French language. Their shape is very varied, from roughly quadrangular to circular, oval, starry, rarely elongated in the form of thin specks. The edges can be more or less regular or very jagged, rounded or blunt, often with more or less developed and evident folding: sometimes the folding concerns most or all of the surface (sandwich shapes) and, in some cases, can be repeated many times. The dimensions vary from microscopic to more than a centimeter, with a prevalence of minute powder (flour gold, color) and elements with a diameter or maximum elongation of less than 0.5 mm; however, elements with dimensions ranging from 0.5 to 1 mm are fairly abundant, those with dimensions from 1 to 2 millimeters are frequent, the larger ones rare.

To the naked eye the surface appears smooth, but under the microscope it is always very bumpy, hammered, sometimes with evident streaks.

Given the extreme ductility of the metal and the inevitable abrasions due to transport, the flakes are always very thin, with extremely variable thickness, from a few microns to less than a millimetre. This determines a great variability of absolute weight and, given the high specific weight (16-19 depending on the purity), flakes of the same dimensions can take on considerably different weights for minimal variations in thickness, while generally remaining very low: for the purposes of absolute weight of the individual particles, the extreme thinness ends, in fact, by canceling the effects of the high specific density. The flakes with greater thickness begin to take on granulometric consistency (nuggets) and, therefore, they assume absolute weights of a certain importance.

During the washing, the characteristics of the flakes determine great recovery difficulties. The smallest and thinnest flakes of gold tend to float due to the surface tension, a phenomenon which increases with the presence of fats and clay and which makes the recovery of this "floating gold" problematic. But it is not just a matter of flotation, on the contrary this problem can be partly solved by increasing the volume of water, in the turbidity, and creating turbulence: more problematic is instead the sail effect caused by the water current on larger flakes, the which, depending on their thickness and the speed of the water, are dragged to considerable distances. So, it is not always true that, in a stream of water, the larger flakes precipitate before the smaller ones, as stated by many authors, on the contrary, experience teaches that dust and minute flakes precipitate first and, during the washings, their recovery is proportionally greater than the larger flakes. In practice, it takes a lot of skill, and attention, to recover what I, paraphrasing the Anglo-Saxon term relating to flotation, I usually call "fucking gold".

Furthermore, the professorial theorems on the precipitation of the various minerals commonly present in alluvial deposits, based on specific weight and diameter, are completely unenforceable in the case of gold: with the same diameter, in fact, a flake of gold can weigh less than a grain of medium and high specific weight mineral, such as quartz (2.6), garnets (3.4-4.3), ilmenite (4.7), magnetite (5.2); and nor is it possible to include a factor relating to the shape and thickness of the flakes, as these are extremely variable...”

Taken from Giuseppe Pipino: “ Aurifodine e sfruttamento dei terrazzi auriferi” (2013)

downloadable at the link:

(99+) AURIFODINE E SFRUTTAMENTO DEI TERRAZZI AURIFERI | giuseppe pipino - Academia.edu

The gold samples illustrated in the picture comes from the Orba stream (Liguria-Piemonte, Italy). Tthe physical characteristics are quite similar to many other samples taken all over the world: in the second picture Dr. Pipino (right) shows a gold sample recovered during the hydrauliking of an alluvial terrace near Dawson City, Yukon (1990).

ASSOCIAZIONE VAL D’ORBA


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