Gold Placers Formation
To understand alluvial deposits formation, we must go back to the gold’s genesis that appears in this type of deposits. Historically the most accepted theory by most experts in this field explains that the origin of the gold present in these types of deposits comes from a preexisting primary mineralized body, such us veins, reefs or porphyries; or secondary as paleo-placers.
In both cases the mineral release from the box rock in which it is found, is the result of both the physical and chemical weathering of the rock. When deposits come from mineralized bodies, gold particles are generated and completely released from the surrounding rock. Whereas in the paleo placers, particles are still included inside the rock being this last group more abundant.
The transport and the material concentration obtained is produced by the action of water in its solid and liquid states, by rivers, glaciers, lakes and seas. During this movement of the material, and due to the erosive action to which the rock is subjected, gold particles are finally released and molded. The material that is subjected to the action of the flow is integrated through a process of gravitational concentration. In that way, particles remain stratified inside the receiving sediment. The above, based on three parameters: density, size and shape.
Once the fluid loses speed, it also reduces its transport capacity. Then, the sediments are deposited in the following order: gravel, sand, silt and clays. Keeping relationship between the parameters described above and the gold particles size.
Tiempo
Currently, gold’s placers extraction technology is having an important development, which implies that deposits which were not profitable before, they could be now. Therefore, this research is developed in a period in which it could be very useful. So, to encourage the development of industries that promote the exploration and exploitation of this type of deposits.
Alluvial Gold Deposits in Chile
Mining Project Research UC - CORFO - ENAMI
Placers formation process considers the processes of weathering and erosion of the gold source which is in rock. This source, which is usually a vein or gold reef, is destroyed by forming fragments or debris of different sizes (i.e.; gravel or sand) which are transported and then deposited in placers.
Generally, in river placers, gold is usually deposited at the lagoons or rive mouths, in river meanders, at the bottom of a waterfall or in places where the different types of bottom rock generate traps where the metal is concentrated because it’s heavier than the rest of the sediments.
There are several forms of mineral concentration or accumulation, which depend on the sedimentation environments and the soil conditions to favor heavier metals deposition. Some traps that favor the concentration of gold prospects of fluvial origin are: Meanders, irregularities in the bottom or river bed, cataracts, lagoons. Gold deposition is mainly due to a loss in the fluids (water) transport capacity that drags the sediments.
In the figure, you can see the different levels or terraces of fluvial origin. These terraces are formed by the erosive action of the relief by the action of water runoff and by the relative variations of the river’s base level (generally denotes a relative decrease of the base level).
1. The current terrace is the area where the watercourse of the river flows and becomes its flood zone (they are at a similar level).
2. The hanging terraces are the old river levels (currently abandoned) which are at higher altitudes.
3. The paleoplacers are old hanging terraces that by effect of the pressure that exerts the overload were transformed in deposits of hard rock.
The alluvial placers have a stratigraphic column that usually has a vegetal layer in contact with the surface, followed by several layers of gravels of different thicknesses. Then they reach the paying mantle (false bedrock with the highest gold concentration). The mantle is located immediately above the bedrock.
What is alluvial mining?
