Uranium and Gold are elements that occur naturally on earth. Gold had an atomic mass of 79. On the other hand, Uranium has an atomic mass of 238 and portrays radioactive properties. In the majority of rocks, soils, and waters, the elements occur in small qualities. It is challenging find the element in huge quantities, where it can be mined. The three major types of Uranium include paleo placers and sandstone-type deposits. However, all Uranium deposits have similar aspects; they undergo remobilization from one region and precipitation in the host rock, where the environment favors its concentration. Uranium is among the most common elements found in the Earths crust (Wyant & Klepper, n.d). It is 40 percent more prevalent as compared to gold. In general, the term placer deposit is used to refer to concentrations of heavy metals especially those of Uranium, Gold, and Platinum. In reference to age, the deposits are categorized as paleo placers if their formation occurred in the ancient coarse siliciclastic rocks (Eckstrand, Sinclair & Thorpe, 1995). Gold and Uranium are the only elements whose mining occurs from paleo-placer deposits. In some cases, both elements can be mined from the same deposit; however, at times, only one is present. One of the major regions where paleo-placer gold has been mined includes Witwatersrand, South Arica. This paper seeks to explore the formation, geological history, and deposit composition of placer uranium and gold deposits and mining in the modern times.
Before the 1940s, the value of Uranium was unknown and detailed information lacked about its existence and characteristics. The process of Placer formation is dependent on varied geologic aspects. The geochemical activities that take place are mainly driven by orogenic processes. As a result, it is significant to assess geochemical characteristics associated with uranium and the environment that favors its deposition. Up to date, detailed information about the precise concentration of Uranium still lacks. Hence, it means that data on the occurrence and distribution of these elements are just estimates. On the earth, reports have indicated that Uranium is among the rare elements; however, its concentration is high in the Earths crust. Also, research has indicated that over 100 minerals may comprise of Uranium as the major element where is occurs as tetravalent and hexavalent. The geochemical process is believed to be the activity that leads to the concentration of Uranium. It also explains the regions where the concentrations of Uranium are found.
The geochemical process comprises of three stages that form a single integrated cycle. The first process comprises of weathering and sedimentary activities; the second stage comprises of metamorphic activities (Wyant & Klepper, n.d). At the metamorphic stage, there is the formation of magma, which later undergoes emplacement and consolidation. Rocks that preexisted undergo metamorphosis, which leads to the deposition of Uranium of igneous and metamorphic origin. During weathering and the sedimentary stage, there is the weathering and erosion of uranium deposits, which are moved and sediment in a different region where they may accumulate. Igneous activities lead to concentration of Uranium in late phases differentiates of magma and mainly occurs in carbonites, pegmatites, plutons, and in fissures of epigenetic deposit.
In the generation of magma by the igneous processes, igneous rocks are exposed, which is an aspect of the orogenic cycle. The formation of magma is a complex one, and it marks the first step of concentration of the minerals. Uranium levels in rocks that have undergone crystallization from magma increase continuously. The most common minerals found in magma include proxene, olivine, and calcic plagioclase. Eventually, rocks that comprise of the three minerals have only a negligible amount of Uranium. Afterward, the common minerals include monazite, zircon, and xenotime. These minerals consist of larger sections of Uranium. Hence, the increase of Uranium in rocks is a progressive activity. Studies have also revealed that Uranium increased is directly linked to the levels of potassium in rocks and it is most common in effusive rocks. In the process of weathering and erosion, Uranium is released and may be transported to a different region. Its retention of exportation is dependent on a range of factors such as climate, lithology, and landscape.
In regions of high humidity, where water tables are high, weathering is extreme and deposits tend to be exported to the direction of the drainage. In contrast, dry regions tend to have lower water tables. In these regions, Uranium deposits from the process of erosion and weathering are not transported to different regions. However, in some cases, it may be exported to different regions through subterranean waterways. The concentration of molten Uranium may be concentrated in the tissues of organisms, as secondary minerals, or accumulated clays and phosphates through adsorption (Wyant & Klepper, n.d). It is possible for aquatic animals to extract Uranium from the sea and incorporate it in their skin. It is because a significant proportion of molten and solid Uranium is transported to the seal through water channels. Nevertheless, studies have revealed that molten Uranium does not remain in solution for a long duration. Uranium occurs in many minerals such as uraninite, graphene, autunite, and coffinite.
The metamorphic process is vital in the formation of Uranium. After the burying of sedimentary rocks following geosynclines formation, the rocks change into rocks of molten fluids that balance with its environment. Different factors cause a metamorphosis of gneiss, schist, and a number of rocks. Consequently, it may lead to the collection of Uranium particles. When rocks melt, there is a connection between the last and the first liquids to form and have high concentrations of silica, alumina, and potash. In such a case, a significant proportion of the magma formed tends to contain high levels of Uranium. This claim is supported by the finding that many of the Uranium vein depositions in the globe tend to appear in paleo shields, and rocks are believed to have undergone instances of ultra metamorphism. Also, it is evident that the exportation of Uranium may occur in conditions that are less harsh. A significant number of marine black shales contain a notable amount of syngenetic Uranium. The concentration of Uranium may occur in a number of activities. The existing environment is dependent on the orogenic aspects of the area.
Placers originate from the sedimentary phase of the geochemical cycle. Placer means that the mineral deposits originate from the concentration of particles in motion. During weathering, solid particles of the element are transported through water channels. Their formation occurs when vigorous water and airwaves sort out between heavy and light particles and lead to accumulation of heavy particles in large concentrations. As stated earlier, the term is placer applies to the accumulation of heavy minerals that consist of valuable elements such as gold and Uranium. In most cases, placers consist of weak uraniferous minerals and while others consist of strong uraniferous minerals (Eckstrand, Sinclair & Thorpe, 1995). Nevertheless, the level of uranium in many placers is extremely minute and is not regarded important unless it occurs as a byproduct of other valuable minerals such as gold. There are different types of placer deposits that include alluvial, eluvial, beach placers, elonian placers, and paleo placers. Eluvial describes placer deposits, which originate from actions of water in the natural channels. Eluvial describes placer deposits that form of the slopes of sloppy hilly regions. Beach placers form as a result of activities of shoreline currents. Elonian placers are formed by actions of air currents. Lately, paleo placers describe prehistoric placers, which are deeply concealed and have metamorphosed into solid rocks. An example of paleo placers includes Gold and Uranium from Witwatersrand, South Africa. For minerals to concentrate in placers, weight is a determinant factor. Only heavy deposits can be accumulated in placer deposits. Also, it should be able to resist the forces of pounding waters.
For the formation of placer gold, the major requirement is a source rock that bears the gold. It is mainly referred to as the mother lode. In the source, the gold particles are extremely small in size and invisible to the naked eyes. The process of weathering leads to the disintegration of the rock source, and the gold is left behind because it is heavy. If no additional processes occur after the weathering, placers of lower grade may arise (Placer Gold, 2007). However, exportation and gravitational impact lead to concentration of larger amount of good and high-grade placers. As moving water flows over free gold particles, they are transported to different regions. As waves reduce at the shores of water bodies, gravity force causes these particles sediment. It leads to concentration of the gold particles. Nevertheless, for Uranium, there are no beaches that comprise of large amounts of minerals. Brazil and India beach placers consist only low-grade sources of Uranium. Placers are transient, and only a few endure. Those that endure become lithified. It is reported that uraniferous conglomerates are placers that are concealed deep in the ground and are modified by hydrothermal actions. Uranium deposits occur only in rocks that are billions of years old. Gold paleo placers are believed to originate from the archaic age.
Gold usually occurs in the metallic state, and it is mainly linked to sulfide minerals such as pyrite. However, it is impossible for gold to form a mineral solely. For instance, in Australia, gold becomes of economic value when it occurs in association with tellurium and telluride minerals. The highest proportion of gold mined in this region is invisible on rocks. It occurs as fine grains, and the concentration is five grams per ton of rocks (Placer Gold, 2007). The highest percentage of mines in Australia occurs from open cut regions. Earth moving tools are utilized in removing host rocks that cover the ore. Pyritic paleo placers and hematitic paleo placers occur in thickened sequences of clastic sedimentary rocks that comprise of large quantities of thick quartz arenite components that are sub-mature and super-mature (Eckstrand, Sinclair & Thorpe, 1995).These deposits are believed to originate from the fluvial environment.
The Witwatersrand Basin in South Africa accounts for over 40 percent of the total gold mined in the world. Also, it accounts for a significant source of Uranium. It represents the largest gold placer deposits in the globe. The Witwatersrand Basin is located on the Kaapvaal Craton and it transverses in the Free State for a distance of over 400 kilometers. Gold is mainly found in the Northern and Western edges of the basin (Eckstrand, Sinclair & Thorpe, 1995). Young sedimentary and volcanic rocks cover the region. About 40,000 t of gold has been extracted from an estimate of 4 billion tons of auriferous pyritic quartz pebble conglomerate and quartzites. The year 1970 recorded the highest level of gold extracted from the region. However, in the recent years, the level of gold production per year has reduced significantly. Since the 1950s, Uranium has been extracted from gold ores in the South Africa.
Geologists have attempted to explain the origins of Gold and Uranium in Witwatersrand. It is reported that gold and Uranium in this region originated from weathering of archaic rocks and later accumulated as grains in conglomerates. According to an article Sedimentary Origin (2013), both Ura...
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