ALLUVIAL EXPLORATION & MINING
FEW OBSERVATIONS ABOUT THIS SUBJECT
After centuries, the alluvial mining activity is still very much alive, especially in South America.
It requires a lesser investment to start up, and the operational costs are low. To explore an alluvial (placer) deposit costs much less than to explore a primary deposit.
The gold or diamond alluvial (placer) deposits are controlled by a topography.
The tropical alluvial deposits (placers) are saturated with water. Floating dredges are used to mine alluvial deposits. This type of exploitation is the least expensive method of mining. The operational cost is 0.1 to 0.2 mg/m3. Only few paleochannels are more than 50 feet (15 m.) deep. A mobile (floating) recovery plant also can be loaded with a dragline.
Much paper and saliva was wasted on the subject of gold recovery. A sluice box is still the best tool to recover gold. It is tolerant to flow variations. It does not clog easily. It can take high volumes of flow. It is very inexpensive to make. It can trap very fine gold when is set and used with care. I was working an alluvial (placer) gold deposit where over 70% of the gold particles were less than 150 MESH size! A carpet equipped sluice box was recovering up to 65% of the gold during first pass.
I had many opportunities to see a miracle "systems" clogging, dumping gold, breaking down, and finally gathering rust. A wasted investment, which costs more than it does, pays back. A sluice box is still the best recovery tool in most cases.
Since majority of tropical alluvial (placer) deposits have a limonite-cemented layer, under which the most interesting values are, the dredge must use a cutter head or wheel. Many areas, where the gold or diamond alluvial (placer) deposits are found in South America, have undergone processes of rejuvenation. Most interesting gravel layers are found under a "false-bottom" (endured clay) layer.
In Venezuela, in Cuyuni river basin, up to three such layers are found. Often, deeper layers are still untouched by previous mining activity. The previous mining stopped at a first "false-bottom" clay layer. In Guyana, I had an opportunity to put a dredge pipe in a gravel layer located under such "false-bottom"... a 10-inch free-suction dredge produced, in first three rows of riffles, 10 ounces of gold every 3 hours!
The majority of well-explored alluvial gold deposits were in cold (Alaska, Siberia), dry (Australia), or temperate (North America, Asia) climates.
The evaluation of such a deposit is much more precise than the evaluation of a nuggety one. My experience, backed by figures, shows that an alluvial deposit with fine gold particles, in a large river system, can be evaluated with less than a 10% error, and often with less than a 5% error.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.