Flour gold represents the smallest and most challenging form of placer gold encountered in streambeds and alluvial deposits. This material consists of extremely fine, often microscopic particles, resembling fine dust or glitter rather than flakes or nuggets. Although it is the most common form of gold found in many environments, its minute size makes it notoriously difficult to separate and capture using traditional mining techniques. Specialized knowledge and equipment are necessary for effective recovery.
The Physical Characteristics of Flour Gold
Flour gold is defined by its particle size, typically measured as minus 100 mesh, often extending down to less than 75 microns. Its defining characteristic is the extremely high surface area to volume ratio, which dictates how it behaves in water.
This high surface area makes the gold highly susceptible to water’s surface tension. Instead of settling quickly like heavier gold, the particles often become entrapped in the water’s surface layer. This phenomenon causes the flour gold to float or remain suspended in the water column, easily washing away with the current.
The gold’s behavior is further complicated when mixed with fine silts and clays, which can also become suspended. This mechanical suspension, combined with electrical charges on the tiny gold particles, prevents the material from settling efficiently. Successful recovery relies on overcoming these physical forces to allow the gold to drop out of suspension.
Geological Context and Deposition
Flour gold originates from the extensive erosion and weathering of primary gold-bearing lode deposits, such as quartz veins and hydrothermal zones in hard rock. Millions of years of natural processes, including freeze-thaw cycles and water action, pulverize the original gold, creating the fine particles that are then transported downstream by flowing water.
Because the particles are so light, they require very low-energy water environments to settle and concentrate. Flour gold is typically deposited in areas where the current slows significantly, allowing the fine material to drop out of suspension. These depositional environments include:
- Quiet eddies
- Inside bends of rivers
- Areas behind large obstructions like boulders
- Wide floodplains
The gold is almost always found mixed with other dense, fine materials, most notably the heavy minerals known as “black sand.” Black sand is primarily composed of magnetite and hematite, which concentrate in the same low-energy zones as the flour gold due to their relatively high density. Separating the gold from this heavy, dark concentrate is a major challenge in the recovery process.
Specialized Equipment for Recovery
Specialized recovery systems are necessary to process flour gold. These systems rely on carefully controlled water flow and enhanced capture surfaces to overcome the gold’s tendency to float, as standard panning and sluicing methods are largely ineffective.
A common approach involves using fine gold sluices equipped with high-traction matting, such as deep V-grooves or silicone mats with micro-riffles. These surfaces create a turbulent yet controlled water flow that traps the fine particles while allowing lighter materials to wash away. Efficiency is maximized by classifying the material down to a very fine size before processing.
Mechanical concentrators are highly effective for final clean-up of black sand concentrates. Equipment includes spiral concentrators, which use a rotating wheel to separate materials by weight, and specialized shaker tables, which use vibration and a thin film of water. These devices use centrifugal force or precise water flow to separate the ultra-fine gold from the heavier black sand, achieving high recovery rates for sub-100 mesh gold.
In some operations, chemical aids are used to improve separation. Surfactants, which reduce the surface tension of water, can be added to the slurry to help fine gold particles sink. Flocculants, such as high molecular weight polyacrylamides, can also be used to aggregate the microscopic gold particles into larger clumps, making them easier to trap. While mercury was historically used to recover fine gold, this practice is strongly discouraged due to its extreme toxicity and the availability of modern, safer alternatives.