Gold deposits commonly are found as primary lode deposits in the bedrock or as superficial placer deposits resulting from erosional reworking of bedrock sources. Lode gold deposits are formed by hydrothermal fluids and commonly take the form of veins and sometimes are associated with arsenic. Placer deposits are formed by selective concentration of heavy minerals with specific gravity greater than 2.7. These minerals are found in eroded bedrock deposits within river systems and shorelines. To survive erosional environment, these minerals must resist chemical attack. It’s common to find in placers gold, rutile, zircon, monazite, and magnetite. Placer deposits are quite variable in size, ranging from 10s to 1000s of meters in width, and with pay streaks of 1 to 10s of meters thickness, occurring above bedrock.
Placer deposits have been responsible for near 70% of gold production in the world, as well as major sources of other minerals. Much of this gold, with the exception of the giant Witwatersrand deposits in Africa has been retrieved from preglacial gravels by washing operations. Many classic gold rushes such as Klondike were focused ion rich placers in active river systems. Unfortunately, these types of deposits are small by current mining standards and environmental concerns now restrict extraction from many rivers.
Placer deposits may be categorized according to their age, size, means of formation, mineralogy, physical and chemical characteristics, and geomorphological forms. The most common classification is based on deposit geomorphology that comprises residual, eluvial, stream/alluvial, eolian, glacial stream and beach placers. There are three variables very important in placer formation, first tectonic regime, second composition of the substrate, and third the climate. The last one has a very strong control in the process formation. The gold content in placer ores is usually low compared with the associated primary hard rock deposit from which they were formed. However, due to the easy operation and low costs, placers are often commercially significant and may be the forerunner to further underground mining.
Gold particles once released by erosion from their bedrock, undergo physical attrition and deformation to a degree proportional to the distance of transport. Metallic gold is very malleable and can form several contrasting physical characteristics dependent on the original style of mineralization, the mode and distance of transportation, and any subsequent weathering. Grain characteristics are described by flatness, roundness, shape, and surface texture. Primary gold can be found as platy, square and irregular shapes. During the secondary process formation, gold particles round edges and smooth protrusions, even can be possible to observe a new flattening process.
It is common to find rimming of gold grains. Rims can be 5 to 18 µm thick. This phenomenon is the result of the different solubility of gold and silver in the low temperature weathering environment that leads to dissolution of silver leaving a gold metallic residue. These rims are characterized for containing gold particles of high purity.
Residual placers are accumulated over the main rock source. This formation is due to decomposition and elimination of lighter materials than the rock. Sometime, these formations are oriented to eroded veins.
Eluvial placers are found in sloped ways and can contain minerals liberated from the closer rocks. Heavy minerals are concentrated on the surface and lighter minerals are dissolved or transported. These actions produce a partial concentration due to a volume reduction. Obviously, deposits with economical interest, the source rock must have an important content of gold.
Alluvial placers have been a very important source of gold and the first miners worked these deposits. The gold extraction is usually easy and they have been the reason of passionate gold extraction in California and El Yukon two centuries ago.
Beach placers are formed by waves. The formation of zones with heavy minerals is due to movements in two directions, towards the beach and to the ocean. In this way, waves can classify light and heavy minerals in the coast. A very important factor is the sea level because the exposed area is variable. This phenomenon occurs in Australia and Africa where there are important mining operations.
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| Placers | Concentration in potholes |
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| Effect of water flow | Beach placer formation |
When two streams are joining, the water flow promotes points with gold content. In this case one stream has higher water flow than the other. The degree of gold liberation and the surface chemical properties of placer gold are important in the effectiveness of gravity concentration and amalgamation. As most gangue minerals are lighter than gold, unliberated gold grains are recovered less efficiently by gravity concentration. Detailed surface chemical data on gold grains in placer deposits are not abundant, however, it is known that sulphur and hydrocarbon adsorption can occur and the presence of impurities in the gold significantly affects amalgamation.