Alluvial (placer) gold diamond exploration and mining

ALLUVIAL EXPLORATION & MINING
PHOTOGEOLOGY | SEISMIC SURVEY | BANKA DRILLING | MANAGEMENT | TRAINING

Alluvial Placers
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FEW FACTS ABOUT ALLUVIAL EXPLORATION AND MINING Mining gold and diamonds on Mazaruni River, South America.

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.

A cased drill sampling is guided by a refraction seismic study, to limit sampling to the most significant places, and to limit initial exploration expenses.

The gold or diamond alluvial (placer) deposits are controlled by a topography.
A photogeomorphology study should precede any fieldwork. In reality, the fieldwork should be limited to testing areas identified and delimited on aerial photographs. In tropical areas, covered by jungle, vegetation is a good guide to locate paleo-channels. It takes experienced eyes to see a paleochannels on the ground. A good placer geologist can do much of his work by walking a prospecting grid. He must know well his environment.

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 paleo-channels 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.
In tropical climate (Central Africa, Central and South America), where processes of laterization are predominant, the gold behaves differently. While in former climates the gold moves as physical particles, in lateritic environments the gold often moves as a solution and precipitates in areas with specific geochemical conditions. (I can imagine some eyebrows rising.) Plants play an essential role in a gold mobilization. Many tropical plants produce, during metabolism, cyanide and humic acids. Precipitated gold particles are very small, and are distributed like a cloud in the alluvium. The gold in such alluvial deposit tend to be distributed through a complete vertical section.

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.

The survey of alluvial deposits of gold or diamonds, using the modern equipment, offers:

Rafal Swiecki, geological engineer:
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