What Is Placer Mining?

Placer ^[1] refers to a deposit that exists in the gravel deposits. This is due to the weathering and erosion of rocks or ore deposits in the surface environment. Some of the minerals with high chemical stability are not retained, and after a certain distance of handling, they are stacked with gravel in low-lying areas to form recoverable sand. mine. Sand ore is widely distributed, and it is widely distributed in coastal and continental shelf areas in many countries. China's seashore sand deposits are abundant, and are distributed along the coasts of Liaodong Peninsula, Shandong Peninsula, Guangdong, and Taiwan. Sand deposits are stabilized by rock or L-shaped bed during surface weathering, erosion, and transportation. Into a mineral deposit. The minerals related to sand ore are: gold, tin, diamond, rutile, etc. They have important positions in national defense, metallurgy, and cutting-edge science. Therefore, many countries pay more attention to the exploration and development of sand ore.

Under the action of solar radiation, the atmosphere, water, and organisms, rocks and minerals undergo a series of physical and chemical changes, such as disintegration, fragmentation, and decomposition. It can be divided into three types according to the factors and properties of weathering ^[2]

A. Climatic conditions ^[3]

From the perspective of dynamics, the process of sand ore formation can be divided into the following three stages ^[2]

Coastal sand deposits are sand deposits that are concentrated in the loose sediments of modern and ancient coastal zones, and are mostly surface deposits. In terms of definition, it is different from surface sedimentary minerals concentrated in loose sediments in the shallow seabed and paleocean sandstones buried in ancient sediments under modern sediments ^[1]

Exploration methods of sand ore There are two methods for sand ore exploration abroad: shallow well method and drilling method ^[5]

Here are some of the drilling techniques used in sand ore exploration ^[6] .

Placer drilling

The basic working principle of a Banga drill is to manually drill a 4-inch or 6-inch casing into a formation, or use a human to slowly rotate the casing into a formation.
Then use a spoon-shaped drill, auger, sand extraction cylinder and other sampling tools to carefully take out the sand sample into the casing. When sampling, generally keep a small amount of sand sample in the lower part of the casing, and then continue to drill the casing to another depth, and then take a part of the sand sample, and then repeat this cycle until it penetrates the ore layer and sees the floor. Sample size is 200 ~ 300 mm. The depth of casing return is about 0.5 meters, and the maximum length is 1,5 meters. The length of re-sampling controls the instantaneous stringency of the mattress. According to Soviet regulations, the length of the re-sampling of ore-bearing layers does not exceed 0.2 meters, and the non-ore-bearing layers are 0.5 to 1.0 meters. If large gravel is encountered in the lower part of the casing, use an impact drill to break and then drill and sample. Banga drilling is reliable for sand ore exploration. It has the characteristics of light equipment, simple structure and low cost. It is very popular for exploration and construction in areas with inconvenient transportation. The disadvantages are high labor intensity and low efficiency, and the construction effect is not ideal in some complex sand ore formations. Therefore, some countries have improved it and changed the manual impact rotation to steel rope hammer impact and mechanical rotation, such as the Dutch pioneer sand drill and the Soviet ysP sand drill. Its caliber is generally 100 ^ 219 mm, the drilling depth is about 30 meters, and the efficiency can reach 100 meters / month.

Placer vibratory rotary drill

The use of vibratory drills or vibratory rotary drills in core drilling for sand ore exploration is still a recent issue. The Ural Geological Bureau of the Soviet Union constructed shallow holes with vibratory drills from 1973 to 1976, with an average of more than 40,000 meters per year and an efficiency of 420 meters per table per month. With vibratory rotary drill, the average efficiency can reach 930 meters per month, and the maximum hole depth is 45 meters.
Vibration rotary drills are mostly used for coastal sand ore exploration, and the effect is more prominent with one drill-up sampling. It is believed that this method is very promising.

Coreless Rotary Drill for Rinsing

In 1955, the Soviet Bird Geological Bureau used the non-washing fluid core rotary drilling method to explore sand ore for the first time. This method is limited to use in sand ore formations with less gravel, more clay, better cementation of the formation, and no need for wall protection. '

Large caliber impact grab drill

Practice has proven that in the complex placer gold deposits, it is difficult to obtain reliable geological data for small-diameter boreholes. Sampling with sand extraction can not avoid the disturbance, sorting and mixing of sand samples, and it is difficult to construct shallow wells. Therefore, in the late 1960s, a large-caliber impact grab drill attempted to replace shallow wells in sand ore exploration. Typical drilling rigs include the YBCP-5 sand drill from the Soviet Union and the Jost grapple rig from the United States.