China
Africa
Explore chapters and articles related to this topic
Resources and Sustainable Materials
Published in Stanley E. Manahan, Environmental Chemistry, 2022
Extraction of minerals from placer deposits formed by deposition from water has obvious environmental implications. Placer deposits can be mined by dredging from a boom-equipped barge. Another means that can be used is hydraulic mining with large streams of water. One interesting approach for more coherent deposits is to cut the ore with high-pressure water jets and then suck up the resulting small particles with a pumping system.
Mineral Deposits
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Placer deposits are a type of ore deposit formed by physical processes and forces: weathering, erosion, water flow, and gravity. Figure 13.8 shows how such deposits form. The process starts with weathering and erosion that disaggregate rocks to form clastic sediment. In mountainous regions with a significant topographic gradient from high elevation to low elevation, runoff over land and then running water in streams transport clastic sediment from the mountains toward the oceans. When the streams are near their sources, high in the mountains, stream velocities are high, and streams can transport a variety of particles. Clasts of different sizes may be carried and, most important for ore deposits, clasts of different densities may be carried. As streams leave the mountains and slow, they lose some ability to carry coarse and heavy materials, so these materials may be left behind. This winnowing process continues farther downstream as the smallest and lightest clasts continue to be transported while other denser clasts remain behind and concentrate in stream bottoms, beaches, or gravels. Placer deposits, also just called placers, form when one or more of the minerals concentrated in this way becomes an ore resource. The word placer is Spanish for alluvial sand. Typically, placer deposits form where a stream’s velocity slows on point bars, in braided streams, or in alluvial fans. Other kinds of placer deposits are found in beach sands or gravels on ocean and lake shores, and some placers, although not commonly mined, form in offshore marine environments on continental shelves.
Mining Methods Vary Widely
Published in Karlheinz Spitz, John Trudinger, Mining and the Environment, 2019
Karlheinz Spitz, John Trudinger
Jewels or gemstones are sought for their beauty, durability, and rarity. Beauty may be due to color, brilliancy when properly faceted, or other special optical effects. Rarity of course is a psychological necessity. Much literature on gemstones is available (Ali 2003; Burke 2006) and the subject is only briefly covered in this text. Diamonds are undoubtedly the king of all jewels. The primary source of diamonds is kimberlite, a strong igneous rock that occurs as funnel-shaped pipes that extend deep into the Earth’s crust (Case 6.3). Diamond crystals are scattered sparsely through the rock. The high price of diamonds makes it economic to follow kimberlite pipes deep into the Earth. Once eroded from kimberlite, diamond is highly durable, remaining in the sediment in which it may, to some extent, be concentrated. It is commonly mined from placer deposits in stream and beach gravels. Mining of these placer deposits may be by conventional excavation equipment or dredging. Vacuum devices are operated offshore of South-West Africa to extract diamonds from sea bed placer deposits. Diamond grains are separated from other minerals by various physical processes, with the final sorting carried out manually.
Process Applications and Challenges in Mineral Beneficiation and Recovery of Niobium from Ore Deposits – A Review
Published in Mineral Processing and Extractive Metallurgy Review, 2022
Nnaemeka Stanislaus Nzeh, Samson Adeosun, Abimbola Patricia Popoola, Abraham Adeleke, Daniel Okanigbe
On the other hand, Pegmatitic deposits occur from primary cabonatitic deposits, through leaching of the carbonate minerals present. They may contain between 1% and 7%, about 2 to 10 times the head grade of primary carbonatitic deposits. Howbeit, the enriched ore deposits have been reported to generally be associated with Fe oxides, barite, phosphates, zircon, quartz, or silicate impurities. Typical examples of such pegmatitic Nb deposits are located mainly at the Catalao and Araxa mines, both at Brazil (Butalovic 2010; Gibson, Kelebek and Aghamirian 2015; Perrault and Manker 1981). However, the occurrence of the some minerals in alluvial/placer deposits is as a result of the weathering of primary hard rocks with subsequent transportation of the material by erosion (Angadi et al. 2015; Falcon 1982).
Gold placer characteristics in marine sediments of Bayah, West Java, Indonesia
Published in Australian Journal of Earth Sciences, 2023
H. Kurnio, Y. Dahlan, J. Kanily
The term ‘placer’ is derived from Spanish, ‘alluvial sand’. Placer deposits form from an accumulation of valuable minerals by gravity separation from a specific source rock during sedimentary processes (Gerhard & Pat, 2006). Gold derived from placer mining was the main source of gold in pre-industrial times. Placer gold, which has a specific gravity greater than 2.58 and resistance to weathering (Gerhard & Pat, 2006), occurs as native Au. The high specific density relates to the hydraulic behaviour that occurs with coarse sediments (Clifton et al., 1967).