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Received: 30 August 2018 / Revised: 8 October 2018 / Accepted: 15 October 2018 / Published: 19 October 2018
Precious stones form in metamorphic, igneous and sedimentary rocks. In sedimentary units, these minerals were placed by organic and inorganic chemical processes and also found in clastic deposits as a result of heating, erosion, transport, and deposition leading to what is called the formation of placer deposits. Of the approximately 150 gemstones, roughly 40 can be recovered from placer deposits for profit after passing through the “natural processing plant” that includes the aforementioned stages in an aquatic and aeolian regime. It is mainly the group of heavy minerals that plays the largest part among the placer-type gemstones (almandine, apatite, (chrome) diopside, (chrome) tourmaline, chrysoberyl, demantoid, diamond, enstatite, hessonite, hiddenite, kornerupine, kunzite, kyanite, peridot, pyrope, rhodolite, spessartine, (chrome) titanite, spinel, ruby, sapphire, padparaja, tanzanite, zoisite, topaz, tsavorite and zircon). Silica and beryl, both light minerals by definition (minerals with a density less than 2.8-2.9 g/cm
, minerals with a density greater than this are called heavy minerals, also sometimes abbreviated to “heavies”. This technical term has no connotation regarding the presence or absence of heavy metals), can also appear in some placers and won for a profit (agat, amethyst, citrine, emerald, quartz, rose quartz, smoky quartz, morganite, and aquamarine, beryl). This also applies to the fossilized tree resin, which has a density similar to the light minerals. Going down from the source area to the basin, in fact, means separating the wheat from the chaff, display case of the jewelry quality, because only the flawless and strongest competitors among the gems survive it all. On the other hand, gem minerals can also be used as marker minerals for their own primary or secondary gem deposits along with a range of other non-gemmy material genetically linked to these gems in igneous and metamorphic gems. All placer types known to be relevant for the collection of non-gemmy material are also found as trap-sites of gemstones (residual, eluvial, colluvial, alluvial, deltaic, aeolian and marine shelf deposits). Running water and wind can separate minerals according to their physicochemical characteristics, while glaciers can only transport minerals and rocks, but do not sort and separate placer-type minerals. However till (unconsolidated mineral matter transported by the ice without redeposition from fluvio-glacial processes) exploration is a technique that is successfully used to delineate ore bodies of for example diamonds. The general parameters that are important when collecting precious stones in placers are their intrinsic value governed by the size and hardness and the extrinsic factors that control the evolution of the landscape through time, such as weather, erosion, and vertical movements and fertility of the hinterland regarding the minerals aimed at. Morphoclimatic processes are particularly effective in the humid tropical and mid-humid mid-latitude zones (chemical heating) and in the periglacial / glacial and the high-altitude / mountain zones, where mechanical weathering and the paleogradients are high. Some tectono-geographical elements such as unconformities, hiatuses and sequence boundaries (often with incised valley fills and karstic landforms) are also known as planar architectural elements in sequence stratigraphy and applied to marine and correlative continental environments where they play an important role in progression. modeling of gemstone accumulation. The current study on gems and gemstone placers is a reference example of the fine-tuning of the “Chessboard classification scheme of mineral deposits” (Dill 2010) and a sedimentary supplement to the digital maps that form the core of the overview “Gems and geosciences in space” and time” (Dill and Weber 2013).
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Precious stones; placer heavy and light minerals; landforms; climate; geodynamic setting gemstones; placer heavy and light minerals; landforms; climate; geodynamic setting
Inorganic raw materials and mineral deposits are divided in the most convenient way using a tripartite classification scheme into (1) ore minerals and metallic resources, (2) industrial minerals and rocks, and (3) precious stones and ornamental stones . Category 1 is self-explanatory and serves as a source for metals such as Pb, Zn, and Fe, category 2 includes mineral raw materials that are essential for humans based on their physical and chemical properties such as insulating capacity or fire-resistant properties, for example perlite, kaolin, bentonite, zeolites, diatomite or vermiculite, while category 3 stands out for the aesthetic value of its items, such as a diamond in a necklace or a statue sculpted from a block of marble. Precious stones and precious stones as part of category 3 are known for their increased hardness expressed by the Mohs hardness numbers. The three top scorers in the Mohs hardness scale diamond (10), corundum (9), and topaz (8) are famous gemstones (Table 1). All but three gemstones (varieties of beryl, quartz modifications, amber) are heavy minerals by definition (minerals with a density greater than 2.8-2.9 g/cm
, which is the density range of the most common rock-forming minerals, quartz, plagioclase, alkaline feldspar and calcite) (Table 1).
Placer deposits are a subtype of clastic sedimentary rocks composed of rock fragments or mineral particles that were transported from their source by water, wind, ice, or simply by gravity to the depocenter. While ordinary siliciclastic rocks the light minerals feldspar s.s.s. (=series of solid solutions), quartz, phyllosilicates and carbonate minerals as main components and contain heavy minerals only as accessory minerals, and in placer deposits the latter group of minerals predominates in places over light minerals, to such an extent that it can be mined for the three commodity groups mentioned at the beginning, for example, magnetite placers (ore), phosphate placers (industrial minerals), and diamond placers (precious stones) (see section 3.8). Gemstones meet all the physical requirements to survive transport from the source to the depositional environment; they have the hardness to resist chemical heating in the area of origin, to endure the wear and tear and to continue the enlargement processes during the transport, so that they appear at the place of deposition as a raw material to be mined for profit. The separation of the gems from the diluting thrash or gangue minerals (quartz, feldspar) due to their different specific gravities and hardness takes full effect, leading to a grain size and quality that justify the minerals being called a gem of economic relevance for cutters and jewelers – jeweler quality (table 1, figure 1). Lower grades are recovered, in places, for the showcase attractive to mineral dealers and collectors, and the lowest grade can still be used because of its hardness as an industrial mineral, used as abrasives in industry, for example, corundum.
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Placer deposits are surface mineral deposits that result from mechanical processes operative in different depositional environments from the source to the basin (Figure 1). These accumulations consist, mainly of heavy minerals, have been intensively investigated by several authors, who have paid most attention to the depositional environments [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 , 14, 15 , 16, 17]. Conclusion from the aforementioned studies, a first-order subdivision can be achieved in modern placers and paleoplacers. Modern placers are unconsolidated to loosely bound accumulations embedded in mostly Neogene to Quaternary near surface deposits of various clastic lithofacies types and lacking post-sedimentary diagenetic alteration. The conglomerates at Blind River-Elliot Lake, Canada, and Witwatersrand, South Africa occur in stratigraphic series that developed when the Earth’s atmosphere was different from today’s oxidizing conditions and are called paleoplacers. Their quartz pebble conglomerates formed from