What are they? The erosion of weathered rocks and minerals results in the mechanical concentration of the more resistant higher specific gravity minerals. Or in lay mans terms the tougher heavier material stays behind as the less resistant material moves away. When the conditions are right, valuable minerals can concentrate to form placer deposits.
Placer deposits are classified according to the way they have formed. Mechanical concentration starts where the rocks and minerals are exposed to the elements at the surface. Little by little, the vein or rock in broken down, forming soils and related concentrations of valuable minerals forming what are called residual placers. Now gravity, rain, snow, and wind aid in the movement of the soil downslope, further concentrating of the valuable minerals forming eluvial placer deposits
As the soils work their way down hill into washes and stream channels, water plays a more important part in the continued concentration of the valuable minerals within them. These are called stream placers, glacial stream placers and bajada placers. The ordinary stream placer is by far the most important of them all.
Concentration of the same valuable minerals can also be accomplished by wind forming eolian placers and by wave action along coast lines forming beach placers.
FURTHER CLASSIFICATION OF PLACER DEPOSITS
The long continued weathering of mineralized veins and surface rocks and minerals breaks down the constituents. The more resistant minerals will collect nearer to the outcrop while the softer and less restant minerals are carried away by the forces of erosion. These deposits can mean big rewards to the crafty electronic prospector. Look for quartz outcrops and red iron stained areas and detect them for gold nuggets.
Now the minerals and soil of the residual placers on a hillside will slowly begin to "creep" down slope into rivulets and gulleys toward the nearest stream channel. The concentrations of valuable minerals along the hillside, downslope from the outcrop are called eluvial placers. These placers are also very important to the electronic prospector and can produce some large patches!
Stream placers consist of valuable minerals concentrated in sand and gravel sorted by the action of running water. Placers formed in recent time will occupy the active stream channel. The deposits will form where the water slows or around obstructions in the channel. If formed in ancient times, the placer deposits may occupy benches elevated well above the present day stream channel. Dry desert washes form placers in a very similar fashion and all exposed and shallow bed rock should be detected as well as benches and other likely spots.
Desert washes and dry stream channels can and are large producers for electronic prospectors. Work inside bends in the wash as well as around any obstructions in the channel. Also work the area 3 feet from the edge of the channel up the bank and always look for and check benches.
These placer deposits are formed in arid climates and are related to alluvial fans. Alluvial fans form at the base of a mountain range and are the result of flash flood erosion and transportation of rock and mineral material. During the rains material is washed down the narrow canyons and when the waters emerge there is a very sudden decrease in water velocity. All of the boulders, gravel, sand, and valuable minerals are then deposited building alluvial fans.
Placer deposits in alluvial fans are very erratic due to the wandering of the wash channels. With each separate flow the water takes a different coarse depositing it's load of rocks and minerals haphazardly throughout the fan. Now if there is still gold coming down the washes these areas are well worth checking for nuggets as are the exposed sides in the gulches in historically rich placer areas. As a rule though most of the gold is buried well out of reach of our detectors in large fans.
Eolian placers are formed by wind action. Bajada placers can show surface enrichment due to the lighter materials being removed by wind and sheet flooding in desert regions. These areas offer very good potential for the modern nugget-shooter, as after ages of wind whipping away surface materials gold nuggets are left well within the range of metal detectors. In some areas there can be a couple inches of soil movement in just a few years leaving new nuggets within our reach.
Glaciers do not concentrate minerals into placer deposits. The waters flowing from melting glacial ice may be sufficient to cause the formation of placer deposits downstream from the glacier. These deposits are of no interest to the electronic prospector as most if not all of the gold is ground to fines.
By William Southern Jr
Gold occurs in various locations along coast lines as the result of the action of shore currents and waves. Materials broken down from cliffs by waves or washed into the sea by streams are sorted and distributed by their size and specific gravity. These placers in most cases are of little interest to the electronic prospector although nuggets are sometimes found in ancient elevated beach sands.