deposit costs much less than to explore a primary deposit.

A cased drill sampling is guided by a shallow refraction seismic study, to limit sampling to the most significant places, and to limit overall exploration expenses.
Gold or diamond placer deposits are controlled by a topography. A careful photogeomorphology study should precede any field work. In reality, the field work should be limited to testing areas discovered and delimited on aerial photographs. In tropical areas, covered by jungle, vegetation is a good guide to paleo-channels. It take experienced eyes to see paleochannels on the ground. A good placer geologists can do most of his work by walking a prospecting grid. He must know his environment well. An "in-and-out" geologist lacks a "feel" for the area.

Tropical placers are saturated with water. Floating dredges are used to mine alluvial deposits. This type of exploitation is the least expensive method of mining. Operational cost is 0.1 to 0.2 mg/m3. Few paleo-channels are more than 50 feet deep. A mobile recovery plant can also be loaded with a drag line.

Much paper and saliva was wasted on the subject of recovery. A sluice box is still the best. It is tolerant to flow variations. It does not clog easily. It can take high volumes of flow. It is very inexpensive to set up. It can trap very fine gold when used with care. I was working an alluvial gold placer where over 70% of the particles were bellow 150 MESH size!
A carpet equipped sluice box was recovering up to 65% of the gold during one pass.
I had opportunities to see miracle "systems" clogging, dumping gold, breaking down, and finally gathering rust. A wasted investment which takes more than it pays back. Sluice box is still the best in most cases.

Since majority of tropical placers have limonite cemented layer, under which the most interesting values are, the dredge must use a cutter wheel. Many areas, where gold or diamond placers are found in South America, have undergone processes of rejuvenation. It is my experience, that the most interesting gravel layers are found under "false-bottom" of endured clay.

In Venezuela, in Cuyuni river basin, up to three layers of "false-bottoms" are found. Often, all deeper layers are still maiden. Previous mining stopped at a first "bottom". In Guyana, I had an opportunity to put a pipe in a second layer... a 10 inch suction dredge produced, in first three rows of riffles, 10 oz. per 3 hours!

The minority of explored gold placer 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 laterisation are predominant, gold behaves differently. While in former climates gold moves as physical particles, in lateritic environments gold predominantly moves as a solution and precipitates in areas with a specific geochemical condition. ( I can imagine some eyebrows rising.) Plants play an essential role in a gold mobilization. Many tropical plants produce cyanide. Precipitated gold particles are very small, and are distributed like a cloud in the alluvium. The gold in such tropical placer is often distributed through a whole vertical section.

The evaluation of such deposit is much more precise than an evaluation of a nuggety one.
My experience, backed by figures, shows that a tropical fine gold particles placer, 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

Bryson Burke Diamond Corporation
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