Аннотация (русский):
The book contains materials on the search for modern and buried alluvial and primary deposits of diamonds. Much attention is paid to prospecting testing of potentially diamondiferous deposits and provides information on all types of diamondiferous rocks currently known. It is addressed primarily to young geologists who have embarked on a search for diamond deposits. It will find the answer to many questions by many geologists, prospectors and prospectors, leading the search for gold and diamonds. While this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Ключевые слова:
diamonds, deposits, Basaltoids, Impactites, Tuffisites, Metamorphites, Lamproites, Kimberlites, placers, Angarida

One of the most important questions facing a researcher studying the diamond content of a particular area is the question - Where, in what place is it safer and more reliable to sample rocks? To answer this question correctly means not to miss the diamond-bearing placer and to achieve reliable research results with optimal costs. Before touching on this one of the most important issues of placer geology, we will try to find out what place diamond occupies among placer-forming minerals, and very briefly describe its features. In his wonderful book "Fundamentals of the doctrine of placers" N. А. Shiloh (2002) established a quantitative assessment of the behavior of minerals released in the hypergene zone, which is necessary to understand the essence of placer formation. The coefficient of hypergene resistance, or, as the author calls it, “constant of hypergene resistance”, was used as an indicator characterizing the process of placer formation. This coefficient is equal to the logarithm of the product of the hardness (H) of the mineral on the Mohs scale by its specific gravity (P). According to the data presented (Table 3) on the coefficients of hypergene resistance of minerals, it follows that diamond is one of the most mobile minerals. In addition, if we take into account the absolute hardness of the minerals, and not the relative, then it sharply stands out from all for this indicator. Without going into the numerous legends about diamonds, it is necessary to remind the reader of the extraordinary strength of a diamond, recorded in the following curious information. Diamond is a crystalline modification of carbon of the cubic system. It has the highest hardness of all known natural minerals and artificial alloys. The density of diamond ranges from 3.01 to 3.51 g/cm3. A characteristic feature of most diamonds is their luminescence when irradiated with ultraviolet, X-ray, cathode and gamma rays, as well as thermoluminescence (when heated), triboluminescence (when squeezed) and electroluminescence (under the influence of the potential difference of an electric charge). With different excitation, diamonds has different luminescence both in intensity and spectral composition. This property is used to extract diamonds from ores and concentrates. In nature, diamond occurs mainly in the form of separate well-formed flat-faced or curved crystals (single crystals) of octahedral, rhombic-dodecahedral and cubic forms, less often in the form of crystalline aggregates. Among the crystalline aggregates, three types are usually distinguished: boart, ballas and carbonado. Mineralogists include irregular aggregates of small crystals and poorly cut diamond grains on the board; in technology, a boart is called an opaque and semi-transparent poorly formed crystals. Table 3 Physical properties and coefficients of hypergene resistance of minerals according to N. A. Shiloh (2002). Mineral Sp. gr. (Р) Hardness (Н) Р•Н К=1g (РН) Platinum 21.5 4 86 1.93 Gold 16.9 2.7 45.63 1.65 Cassiterite 7 6.5 45.5 1.65 Corundum 4 9 36 1.56 Wolframite 7.1 5 35.5 1.55 Zircon 4.7 7.5 35.25 1.54 Diamond 3.5 10 35 1.54 Spinel 3.9 8 31.2 1.49 Magnetite 5.2 5.8 30.16 1.47 Ilmenorutile 4.8 6 28.8 1.46 Topaz 3.6 8 28.8 1.46 Monazite 5.1 5.3 27.03 1.43 Scheelite 6 4.5 27 1.43 Pyrope 3.8 7 26.6 1.42 Ilmenite 4.7 5.5 25.85 1.41 Rutile 4.2 6 25.2 1.4 Brookite 4.1 5.5 22.55 1.35 Anataz 3.9 5.5 21.45 1.33 Quartz 2.6 7 18.2 1.26 Amber 1.07 2.3 2.46 0.39 Ballas is most often called spherical radial-radiant spherulites. Carbonado includes dense, fine and cryptocrystalline diamond aggregates. The grain size is usually from microscopic to 1–2 cm; the mass of most crystals does not exceed 1–2 ct. (1 ct. is equal to 200 mg), rare stones reach hundreds of carats and more (the world's largest diamond "Cullinan" from the Premier pipe had a size of about 10 cm and a mass of 3106 carats - more than 620 g). Diamond has high thermal conductivity and usually low electrical conductivity (dielectric), belongs to the hydrophobic minerals, adheres to some fats. It is chemically stable, insoluble in acids and salt solutions. It is widely used in industry as an abrasive material, diamond dies, for reinforcing cutting tools, in measuring instruments (hardness testers). Diamonds play a leading role in the production of jewelry. For technical purposes, rejected and small non-standard diamonds are used (microcrystals less than 1.2 mm in size, crystal aggregates, as well as fragments with a large number of defects and inclusions). Depending on the dimension, diamonds are further classified (graded) by shape, quality and color. The coarsest is the classification of diamonds of conventional sieve class - 3, which are classified as suitable only for technical purposes. They are classified according to a simplified scheme: crystals, debris, attachments and aggregates. The most interesting information about diamonds (Shafranovskiy, 1964; Ward, 1979; Orlov, 1984) consists of the following data: 1) a diamond cutter, when working on a lathe without sharpening, is able to remove steel shavings 700 km long; 2) with continuous wear for 40 days (950 hours), a 50-carat gem diamond loses no more than 1 mg of its weight. 3) The diamond can only be cut with a diamond saw, which is a thin disc with diamond grit reinforced with hard alloy. The diamond saw cuts through a diamond with a thickness of 4.3 cm in 6 weeks; 4) diamond is as much harder than quartz as quartz is harder than calcite (about 9 times). Thus, given the high coefficient of hypergene stability of diamond, its hydrophobicity (non-wettability with water), elasticity, colossal strength of crystals and a relatively low specific gravity (close to the specific gravity of pyrope and almost 1.5 times higher than that of quartz), we can say that he has the highest degree of migratory ability. The series of migratory activity of minerals proposed by N. A. Shiloh (2002), looks like this (in descending order): diamond → zircon → ilmenite → monazite → magnetite → scheelite → cassiterite → wolframite → gold → platinum. Other researchers (Sokolov, 1982; Beskrovanov and Shamshina, 2000) prove the opposite, claiming that diamond is one of the most inert minerals in relation to the transfer of minerals. In this case, a one-stage movement of diamond-bearing material (source - placer) is carried out at a distance of the first hundreds of meters, and multistage redeposition (source → terrace → placer) increases the distance of their demolition to the first thousand meters. S. A. Grakhanov et al. (Placers of diamonds ..., 2007) give the following multi-stage scheme of redeposition or the history of the formation of placers in the north of Yakutia: root source → ancient reservoir → neogene reservoir → reservoir of buried valleys and above-floodplain terraces → modern channel placers. In all likelihood, both are right to some extent. The transfer of diamonds entirely depends on the transporting force of the water flow, and the stronger it is, the farther from the source the crystals migrate. At the same time, due to their high migration ability, they can freely move not only over the area, but also vertically in the transported mass of the water flow, which determines their relatively fast paddling under the rubble and prevents one-stage movement over distances beyond the first hundreds of meters from the source. This is also evidenced by the long established fact that the process of movement of clastogenic material is accompanied by its differentiation in size and specific gravity, during which heavy minerals are fractionated together with the corresponding class of clastic rocks corresponding to them (Fig. 28, table. 4). Table 4 Distribution of diamonds by granulometric classes in placers, % Granulometric class, mm Placer alluvial deluvial -16 + 8.0 1.0 - - 8 + 4.0 17.0 9.1 - 4 + 2.0 47.2 53.6 -2 + 1.0 27.7 30.3 -1 + 0.5 7.1 7.0 B. N. Sokolov (1982), studying one of the watershed type placers, confined to the deposits of the Early Jurassic lacustrine-boggy basin, found that the distance of mass removal of diamonds from the feeding kimberlite pipe along the log and valley of the main river is limited by a distance of 2600-2800 m. The maximal diamond size was noted at 400 m, and the peak of the maximum concentration was shifted downstream of the gutter by 200 m. Analyzing the range of diamonds transfer, B.N. Sokolov assumes their sevenfold redeposition, if we take into account the average migration capacity of diamonds - 400 m per one completed act. At the same time, for one completed act, it takes the time of formation of one of the river terraces. Interesting studies on the safety of pyrope, depending on the conditions of its transportation, were carried out by B.I. Prokopchuk (1979), who came to the following conclusions: 1) the presence of pyrope grains with a kelyphite shell (pyrope in a kimberlite “jacket” or, more simply, leather coat of a binder kimberlite substance are preserved on the surface of pyrope grains) is the most accurate indication that a kimberlite body is located nearby. This is because in the process of transportation the kelyphite shell is worn out and is rarely preserved when transferred to a distance of 10 km from the location of the pipe. At the same time, the presence of green pigmentation spots on highly mechanically worn diamond crystals is characteristic of very ancient (Proterozoic) diamond-bearing deposits; 2) pyrope grains with a sculpted surface also indicate that kimberlites are located nearby, as it smoothes out already over 8-10 km and disappears at a distance of 40 km from the kimberlite body; 3) at a distance of 35-50 km from the kimberlite pipe, fractured pyrope grains are completely destroyed; 4) the least stable in alluvium are orange pyropes, or, as Chinese geologists call them, tangerine yellow pyropes (Kaminsky, 1988). As V. S. Trofimov (1960) noted, according to the location, transfer conditions and concentration of useful components in them, alluvial placers can be divided into two groups. The first group of placers is formed in the upper and middle reaches of rivers at the beginning of the accumulation area due to the concentration of useful components, which are moved by rolling, dragging, saltation and, to a lesser extent, carried in a suspended state. The second group of placers is formed in the lower reaches of rivers (placers of wide alluvial plains, land deltas, etc.) due to the concentration of useful components that reach the river mouth in a suspended state. Their concentration occurs in areas with a slow flow. Valley placers of the first group are usually composed of coarser-grained material (boulders, pebbles, gravelstones and coarse-grained sands) compared with placers of the second group, which are found only among sands containing clay material. The main part of valley placers is formed at the beginning of the accumulation area, where psephite deposits usually predominate. Large debris creates natural barriers for the diamond crystals being carried along the bottom and contributes to their concentration. Deposits located near the raft are especially rich, which is sometimes called “bedrock” or “placer bed”. FIg. 28. Distribution of diamonds (filled in gray) depending on the granulometric classes of diamond-bearing deposits (according to V. L. Batalov, 1967). The fissure is a bed on which the sedimentary rocks that host the placer lie. Under the raft is meant not only the bedrock itself, but also their heavily destroyed upper layers, turning into eluvium (Fig. 29). The surface of the raft may coincide with the bottom of the placer, as well as be located below or above it, in the latter case, the upper part of the rocks of the raft is part of the productive formation. It should be noted that the rim rock (rock bed) and the rock floor (bottom) do not always coincide, especially if we are dealing with a weathered rock bed. Rock bed is the most important element of any placer, therefore, we will dwell on its features in more detail. According to the type of surfaces, rafts are divided into five types: 1) smooth - soft (clay) or grit (gravel); 2) more or less even - dense (rocky); 3) uneven - dense with grooves and projections (“rock brushes”); 4) uneven - rough with deep pockets (karst) and 5) uneven - loose (coarse-block or boulder-pebble). The shape of the surface and structure of the raft determine its ability to trap the moving diamonds and their satellite dikes, and also affect the way the placer is mined. For example, a smooth, soft raft, represented by kaolin clay, acts as a beautiful red precipitant for diamonds. It is like a sticky dressing plant, where diamonds easily adhere to the greasy surface of the drum due to their remarkable hydrophobicity. It should be noted that the clear contact of loose sediments with the rocks of the raft is called a seam. The spike is characterized by an increased concentration of heavy minerals, including diamonds, and therefore is the primary target for sampling. Fig. 29. The most favorable places for sampling the sediments exposed by the ditch (bedrock-adjacent): 1 - soil and vegetation layer; 2 - eluvial-deluvial deposits; 3 - sand and pebble formations; 4 - limestone; 5 - kimberlites; 6 - weathering crust on a kimberlite dike; 7 - the most favorable places for sampling. In addition to the raft and the seam, a productive stratum takes part in the structure of the alluvial placer, which lies immediately behind the raft and completes the productive deposits. Above are top soil - deposits devoid of diamonds or containing them in insignificant quantities. It is generally known that the primary sources of diamonds are closely related to ancient platforms, and therefore we will focus on platform deposits. So, in the platform areas, where the relief is relatively weakly dissected, the flow of rivers is slow and smooth, the river profile is developed among the rocks of the sedimentary cover, soft in strength. There are usually no rock (trap, etc.) materials and erosion processes are weak. That is why such conditions are not very favorable for placer formation. Rare diamond placers found in such areas are formed mainly by repeated washing of previously formed diamond-bearing deposits (Placers of diamonds ..., 2005). In platform areas with a noticeable dissection of the relief, the rivers have canyon-like valleys. The presence of trappean bodies, as well as strongly cemented rocks, creates favorable conditions for the rapid flow of rivers, the formation of rapids, and sometimes small waterfalls. In the valleys of these rivers, channel placers appear below various obstacles (trap bodies of various morphology, crossing river channels, etc.). Sometimes alluvial placers are formed when the channel meets with rocks weakly resistant to erosion. An example of such placers is diamond placers in craters in the river. Tibazhi in Brazil, formed as a result of erosion of weakly stable rocks crossed by the river (Trofimov, 1960). On the eastern side of the Siberian platform, under similar conditions, a placer “watershed gravels” was formed. This diamond-bearing placer is a depression of erosionala and karst origin up to 40 m deep and up to 15 km2 in area (Fainshtein, 1968). It is directly adjacent to the Mir tube, which is, in all likelihood, the only source of its power. The climate has a significant impact on the development of alluvial deposits, and, consequently, placers. In areas of arid climate, the river network is poorly developed. This can be seen most clearly if you look at this problem globally. Thus, the alluvial coefficient, reflecting the ratio of the area of the river network of a certain climatic zone to the area of the entire climatic zone, expressed as a percentage, in arid zones (tropical zone) is 0.09%, and in the climatic zone of temperate latitudes - 0.24% (Akulov, 1988). The alluviality coefficient reaches its maximum value (0.32%) in the equatorial zone. In this regard, alluvial placers are most widespread in a humid climate favorable for the intensive development of river drainage. Similar climatic conditions existed on the Siberian platform (coal mining time). Quite often, in the study of modern alluvial deposits, there are multiply enriched alluvial placers, similar to the Chukshinskaya (Chuksha river) or Ebelyakhskaya (Ebelyakh river) placers. Placers of this type are characterized by a relatively large average weight of diamonds due to the loss of small diamonds along the migration routes, and sometimes their paragenetic satellite dikes (Chuksha River). Diamonds from multiply enriched placers are usually of high quality and, in combination with the relative size, have a great jewelery value and a very high cost. The price of pink diamonds on the world market reaches 46 thousand US dollars per carat (Temporary methodical ..., 1988). On average, the cost of mined diamonds, taking into account the technical ones, is significantly lower and varies depending on the quality in the transition from one kimberlite province of the world to another (Table 5). Diamonds over 0.5 carats in size are considered gem-quality diamonds, which are of high quality (transparency, color, absence of impurities, etc.). Table 5 Average annual production and cost of diamonds from primary deposits of the most important kimberlite provinces of the world (without the USSR, data by M. A.Milashev, 1989) Provinces Average annual production (million carats) Average annual production cost (US $ million) Average cost per carat (US dollars) Transvaal 2.64 155.3 58.8 Kalahari 19.30 1042.0 54.0 Congolese 19.48 380.0 19.5 Tanzanian 0.40 39.0 97.5 Liberian 0.30 40.0 136.0 Guiana 0.25 100.0 100.0 Indian 0.02 2.4 120.0 According to published data, the lion's share of diamond transactions in the world is carried out by the Swiss firm "De Beers Sentineri" from Lucerne (Kossinsky, 1990). In the spring of 1990, the Soviet Union, represented by "Glavalmazzoloto", entered into an agreement with this company. The Swiss gave our state a loan of $ 1 billion, which was then repaid by the supply of diamonds. According to Swiss experts, even then the USSR ranked fourth in the world in the production of rough diamonds (in 1989, 93 million carats of diamonds were mined on Earth, including: Australia - 34, Zair - 23, Botswana - 15, USSR - 12 and South Africa - 9). Summarizing the above, it should be emphasized that when searching for alluvial diamond placers, the most promising are river sediments located in areas with a noticeable dissection of the relief, moreover, in the upper and middle reaches of rivers. In addition, the rivers flowing from the side parts of the platform (Siberian) into its limits are of great interest. Their waters have a high hydrodynamic capacity, allowing differentiation of terrigenous material and form placers (if there is a primary diamond source or an ancient placer in the river basin). Particular attention should be paid to multiply enriched diamond-bearing placers. Even with poor grades, but with high quality and size of crystals (jewelry), they can successfully compete even with rich placers containing small technical diamonds, and sometimes it is even more profitable to develop them than diamondiferous kimberlite bodies. In the diamond mining industry in Russia and abroad, technological schemes are used that make it possible to extract diamonds of a certain minimum size, depending on their quality, cost, content and extraction costs, which should not exceed the recoverable value. Since the mid-1990s. changes in the economy and successes in the production of synthetic diamonds have led to a sharp decrease in the cost of diamond powder (23 times). World mining enterprises have stopped commercial extraction of diamonds finer than 1.2 mm, and the Australian company "Argyle Diamond" - finer than 1.5 mm. 8.1. Channel placers In an elementary approximation, a river channel is a giant leaching sluice, in which the entire mass of terrigenous sediments is empty sand and pebble deposits, consisting mainly of light fraction minerals. Spits, islands, overwashes, reaches and rifts serve as paths and steps in this sluice on which diamonds and their satellite minerals are retained, and as mats - underlying bedrocks (raft) along which the entire alluvial rock assemblage “flows”. For this reason, we are not interested in the channel formations themselves, but in the deposits of some parts of the spits, the frontal parts of the islands, reaches, rifts, spoons and plumes (eroded terrace outliers) (Fig. 30, 31). When sampling modern or buried valley sediments, the upper and middle parts of the rivers should be studied in detail (Salikhov et al., 2020). At the same time, great attention must be paid to oblique deposits, among which placers of long-range transport and redeposition are often found. The spit is usually represented by sandy-pebble riverbanks or alluvial islands. They are easily recycled by water flow and can be displaced downstream during floods. Fig. 30. Scheme of the location of various types of placers in the river valley (block diagram according to A. P. Bobrievich et al. (1957), with minor changes): terrace placers: 1 - fifth terrace, 2 - fourth terrace, 3 - third terrace, 4 - second terraces, 5 - first terrace; valley placers: 6 - inundable, 7 - overwashes, waterfronts, 8 - trail eroded terrace outliers, 9 - spits, shoals, 10 - actual channel, 11 - spoon. Four types of river bars are conventionally distinguished (Prokopchuk, 1979). The first type includes coastal spits located on straightways of channels (Fig. 32). Usually their front section is much wider than the tail part. They are mainly composed of pebble material. The largest terrigenous formations and the largest number of diamond crystals are confined to the front section of the spit (Fig. 33). The spits of the second type are located on the convex banks of the bends and are coastal crescent bodies. In size, they are much larger than spits of the first type. The increased diamond content in such spits tends to its convex middle part. The front and tail sections of these streamers are usually poor in diamonds. Spits of the third type are insular. They are composed of sand and pebble deposits. The largest amount of diamonds in the island spits is associated with coarse-clastic deposits located, as in the previous case, in their convex parts. Fig. 31. Layout of various types of alluvial diamond placers in the river valley section: 1 - bedrock, 2 - source of supply with diamond crystals (kimberlite pipe), 3 - diamond-bearing placer, 4 - gravel-pebble deposits, 5 - sands, 6 - modern river bed; I-V - types of alluvial placers: I - uplifted valley net, II - terraced, III - bench, IV - channel, V - buried incision. The spits of the fourth type are channel shallows filled with water even during minor floods, and they are usually stretched across the channel. Although rare, spits of this type are characterized by a high diamond content. In all types of the above-described spits, useful components occur in the form of lenses, streams and layers, alternating with interlayers of “empty” deposits (peat). Spit placers are not of industrial interest, but they serve as a reliable indicator that there are other types of diamond placers in the river valley, and indicate the presence of their supplier (supply province) in the upper reaches of the river. Fig. 32. The main types of spits of diamondiferous rivers and patterns of distribution of diamonds and HDC in them (according to B.I. Prokopchuk, 1979 with changes): 1-3 - sections of spits, consisting of the following deposits: 1 - boulder-pebble, 2 - pebble, 3 - sandy; 4-7 - pyrope content in concentrates: 4 - from 4 to 10 grains, 5 - from 5 to 50 grains, 6 - from 50 to 100 grains, 7 - more than 100 grains; 8 - directions of movement of water flows. Fig. 33. The nature of the concentration of heavy minerals in the moving ridge of alluvium (according to N. V. Razumikhin and Z.N. Timashkova, 1960): I-IV - successive stages of transfer and concentration: I - heavy fraction, 2 - light fraction, 3 - river flow direction, 4 - sediment movement paths. For most of the quaternary placers of the Siberian platform, an increased concentration of diamonds is usually confined to the alluvial banks of river bends. The washed-out shores are characterized by poor diamond content. In all modern placers of the Ebelyakh river bassin, the distribution of diamonds is streaky in nature (Placers of diamonds ..., 2007). On the straightened sections of the channel, the front parts of the jets are located within the core subfacies of the alluvium in the transition zone of the reach-roll. The central and tail parts of the jets are confined to the latter. It should not be forgotten that the first Vilyui diamond was also found on a spit (Sokolinaya spit, August 1949). The importance of this find was that it was the first real evidence of the presence of diamonds on the Siberian platform. Spit deposits are usually sampled from ditches (pits, no more than 0.3 m deep). Firstly, its upper part is sampled in relation to the river flow, which is called the front section of the spit. This is the most favorable part of the spit for the concentration of diamonds and their companions. If diamonds or their paragenetic satellite dikes are found in the front part of the spit, sampling is carried out in other parts of the spit. Then it continues upstream of the river and its tributaries until the disappearance of the required minerals or their exit to their root source. Fig. 34. The sequence of the search for the primary source of diamonds by taking placer samples using the search “fork” method: 1 - kimberlite pipe, 2 - schlich sample and its serial number (the sample contains minerals-companions of diamonds), 3 - sample, not containing heavy diamond concentrates. Throughout this work, a rigorous count of pyropes and other HDCs is being carried out. N. N. Sarsadskikh (1958) called the search for diamonds by counting pyrope grains in concentrates pyrope survey. When shooting pyrope, it is recommended to study in detail the class of size –2 + 1 mm, which is obtained by wet screening on sieves with holes of 2 and 1 mm. In order not to lose material smaller than 1 mm, sieving is carried out over the tarpaulin. After washing off the gray concentrate, all pyropes and other diamond satellite dikes are selected from it, and their number is calculated. The data obtained are marked in a schlich journal and on a map or plan. The closer to the primary source of diamonds, the greater the amount of HDC and the more noticeably their size increases. So, when one of the kimberlite pipes of the Yakutian diamondiferous province was discovered, the researchers initially counted only a few dozen pyrope grains per 10 liters of the initial sample, then their number reached many tens, then the first hundreds and thousands (Feinstein, Lebed, 1988) ... According to operational data obtained during the work of "Nizhne-Lenskoye" OJSC on quaternary placers in the north of Yakutia, up to 65% of mined diamonds are concentrated in the grain size class –2 + 1 mm, and when calculating the primary diamond reserves, diamonds less than 0.5 mm in size are not at all are taken into account (Placers of diamonds ..., 2007). G.Kh. Feinstein jokingly called the method of searching for primary sources of diamonds by pyropes "the artillery fork method" (Fig. 34). The first "fork" contains three samples, while the second sample contains no heavy concentrates. The second “fork” combines the fourth, fifth (empty) and sixth tests, and the third - the seventh, eighth (empty) and ninth tests. The tenth placer sample completes the exit to the primary source of diamonds. In the future, when describing a particular stratum, information is provided on the content of paragenetic satellite dikes of diamonds, indicating their amount per tray sample (for example, 5 signs of pyropes per 10 liters of sample, etc.). This must be done to obtain statistical material that is entered into a computer and processed using various programs, including the method of cluster analysis. It is important to note that when sampling both spits and other valley deposits, it is necessary to strive to take not the surface layers of sediments, but the deepest (about 0.6 m). This is because the main useful components due to their relatively large specific gravity and high migration capacity are always deposited in the lower parts of alluvial strata. Channel sediments are sampled along the thalweg - a line connecting the lowest points of the bottom. For this purpose, factory catamarans are usually used or they are made from several rubber boats. As a last resort, you can use a log raft. In the manufacture of a catamaran, two or three six-seater boats are taken, a carrier frame is laid on them and the floor is laid. The frame is attached to boats with nylon ropes. The catamaran obtained in this way is characterized by good stability and surpasses a log raft in all respects, including mobility. On the recommendation of V. M. Kreiter (1940), the raft should be knitted from 12-15 logs with a size of 8 x 0.2 m. In the middle of the raft, an elongated slot (0.4 x 0.8 m) is left above which a hand gear is installed, for raising and lowering a large iron scoop mounted on a long (5-6 m) handle (fig. 35). It is desirable to make a ladle with a capacity of 0.02 m3. The raft is usually installed with its long side across the river. Samples are taken at separate points or in a continuous furrow along the thalweg, and sometimes in a continuous furrow across the entire channel. The choice of this or that method of sampling channel sediments entirely depends on the physical and economic capabilities of the researchers, as well as the specific conditions in which the sampling is carried out. So, when sampling small rivers (up to 30 m wide), it is desirable to carry out sampling in a continuous furrow along the thalweg until the required sample volume is obtained (Kreiter, 1940). On rivers of large size, sampling is carried out with a continuous furrow across the channel. During reconnaissance prospecting for diamonds, samples are taken at separate points with a frequency (3-5 km), which allows, in given specific circumstances, to identify and trace the "chain" of distribution of paragenetic satellite dikes of diamonds. It should be noted that testing in such cases is often based on the intuition of the researcher and his “volitional” decisions. When testing from a raft (catamaran), a calculation of three people is required. V. M. Crater (1940) in this sequence describes the methods of conducting this type of sampling. Two workers, turning the knob, unwind the rope, and the third, holding on to the handle of the scoop and pressing on its crossbars, plunges the scoop as deep as possible into the channel sediments. Then, rotating the shaft of the wrench in the opposite direction, the bucket is torn off the bottom, lifted together with the rock to the surface and the rock is poured into a measuring vessel or directly into a shaker or cradle. In the latter case, the volume of the washed sample is determined by the number of unloaded scoops, the capacity of the scoop and the degree of its filling. In winter, channel sediments are sampled from pits, which are passed through with the help of freezing. It should be noted that structural traps or “pockets” for diamonds are often associated with the alluvial deposits of the valleys. They are usually confined to the places of intersection of valleys with a block structure. In such cases, the block structure is always refreshed by neotectonic movements, which contribute to the long-term existence of a flat with stepped bends. Under such conditions, placers of tectonic scarp zones or micro-grabens are formed (Fig. 36). The structural trap causes local variations in the productivity of growths, an increase in the content and reserves of the useful component, as well as the redistribution of the latter by size classes, in particular, outside the placer recharge areas, the capture of active classes of the useful component capable of moving in the longitudinal direction. Structural traps are often characterized by exceptional diamond richness (Figure 37). An example is the traps found in the valleys of California (USA), widely known as “pits of millions” (Fig. 38). Eversion boilers formed in river beds under waterfalls are also very interesting. B.I. Prokopchuk (1979) writes that many rivers in Brazil are characterized by diamondiferous eversion boilers located within the channels below the waterfalls. According to I. Burdet, from one such boiler (Fig. 38) with a depth of 6 m and a volume of 50 m3, 2500 carats of diamonds (50 carats / m3) were mined. It should be noted that eversion boilers are formed when water containing rock fragments rapidly rotating in a depression in bedrock is grinded. Rich concentrations of placer-forming minerals, including diamonds, are confined to the reaches and rifts, but to those places along or near which the line of the core runs (a conventional line passing along the surface of the water flow and connecting the points with maximum velocities of water flow in the river) (Fig. 39). It should be recalled that river reaches are deep sections of the river channel washed out by the river in the concave part of the meanders during the flood period, separated by shallow areas called rifts. Rifts are positive forms of the river bottom relief, usually crossing the riverbed diagonally. The upper part of such a diagonal is called the upper side, and the lower part is the lower side. Watercourses are sampled outside the influence of the river valley into which they flow. It is advisable to carry out sampling for each watercourse below and above the nearest side tributary. While sampling the channel sediments, one should not forget about those places where the channel alluvium comes out above the river level, and especially about those where the raft and near-channel formations are exposed (Fig. 40). The labor expended on the search for the exposed near-bedrock areas is fully justified by the possibility of dispensing with the production of pit works, the classic size of which is determined by a section of 1.25 m2 (1.0x1.25 m) (Fig. 41). The shape of the pit is rectangular, with the long side located across the valley or strike of the supposed placer. The place for the rock thrown into the dump should be at least one meter from the edge of the pit. The rock is removed from the pit to a depth of 2.5 m for ejection, and later with the help of a mechanical or manual crank. Fig. 35. A raft for sampling channel sediments (according to V. M. Kreiter, 1940): 1 - log raft, 2 - iron scoop, 3 - scoop handle, 4 - hand crank, 5 - cradle. Digging a pit on a pick consists in loosening the soil with a pick and then unloading it from the mine. When driving promising areas, the rocks are laid out by drifts near the pit (see Fig. 41 b). Pick axe is one-sided, 3-5 pieces for each ground finisher (see Fig. 17). Collecting shovels, with a shortened handle, total length no more than 0.8 m. After documentation and testing, the well is liquidated by backfilling with rock removed during its sinking. Lithological sections are drawn up along prospecting lines and plans for the location of mine workings. For these purposes, a field book or a geologist's diary is used. This is the primary document in which all notes are made with a simple pencil or ballpoint pen. The date, the name of the stream or river, the number of the line and pit, the number of samples taken, their location and number are indicated. The surname of the feller must be indicated. All results of video and photography and all finds must be indicated. HDCs are registered by the result of washing (“empty”, “marks”, “diamonds”). A pit that has not exposed the underlying or bedrock is considered “not finished off”. It needs to be deepened. The pits are deepened at intervals of 0.2 m or multiples (0.2; 0.4; 0.6 m, etc.). V. M. Kreiter, (1940, p. 176) wrote about the importance of testing such places. He emphasized “… sampling of near-river banks gives more valuable material than sampling of spits”. Fig. 36. The structural trap built along the exploration lines (with their numbers) is represented by a micrograben (according to L. Z. Bykhovsky et al., 1981 with changes): 1 - slope deposits, 2 - late Pleistocene-Holocene alluvium, 3 - deluvial-alluvial deposits, 4 - poorly sorted micrograben alluvium, 5 - gold-bearing placer, 6 - bedrocks, 7 - micrograben, outlined by neotectonic faults. Fig. 37. Fragment of a structural trap (pit million): 1 - soil-vegetation layer, 2 - conglobreccias, 3 - diamonds, 4 - millionth pit (pocket with diamonds), 5 - Devonian limestones. Fig. 38. Eversion boiler in the alluvial valley. 2500 carats of diamonds were recovered from this boiler, which has a volume of 50 m3 (Bardet, 1973): 1 - sand with the inclusion of pebble material is weakly diamondiferous, 2 - gravel with an average content of 160 carats / m3, 3 - pebble with a content of 6 carats/m3. Assessing the overall morphology of the studied bodies of alluvial placers in the valleys of different rivers, one can see their great diversity and uniqueness. It was found that in the section they form single-tier and multi-tiered formations. Their most typical forms in plan are as follows: single-striate, ribbon-like, lenticular, nested, isometric, multi-striate and dissected (Fig. 42). Fig. 39. Scheme of distribution of placer-forming minerals in channel alluvium (according to N. V. Razumikhin and Z. N. Timashkova, 1960): 1-2 - placer-forming minerals: 1 - large classes, 2 - small classes, 3 - core line, 4 - accumulative forms. Fig. 40. Favorable places for sampling on the edge of the river (longitudinal and transverse (A-B) cleaning): I - the most favorable places (bedrock-adjacent), 2 - favorable places, 3 - sand-gravel-pebble mixture with the inclusion of individual boulders, 4 - stratified sands, 5 - clays, 6 - traps, 7 - topsoil. Summarizing all of the above, it should be emphasized that sampling of valley deposits is essential not only for prospecting for diamond placers, since it allows one to judge the content and nature of valley deposits, but also for the search for their primary deposits. 8.2. Terrace placers There are many mysteries in the formation of terraced placers. One of them is the formation of the terraces themselves. In the geological literature, several points of view are stated, which, in general, come down to that some researchers consider tectonic movements to be the primary cause of the appearance of terraces, others associate terraces with climatic fluctuations. Still others associate it with changes in the level of the World Ocean. Most likely, the formation of terraces is influenced by tectonics. Thus, the terraced placer formation is the result of neotectonic fluctuations in the region. Neotectonics caused the rise of the river erosion baseline, which contributed to the deepening of the river network and the formation of terraces. Usually, terrace placers are formed from valley placers, which were preserved during the incision of the valley and the formation of an erosion and accumulative terrace. They are subdivided into three types of placers: 1) placers on ground terraces; 2) placers of accumulative baseless terraces and 3) terraced placers (formed due to the transformation of terrace placers as a result of denudation and transformation of a terrace into a altiplanation terrace). The sections of the terraces are often exposed by the river net, and therefore they are easy to sample without significant excavation work (Fig. 43). After clearing the surface layer (rockslide), samples are taken from the bedrock-adjacent and raft areas at intervals of 0.25-0.5 m, depending on its thickness and the thickness of the terrace itself. The layers from the middle part of the section can also be enriched, therefore coarse-grained rocks of the middle parts of the terraces are also sampled, but with less detail. It should be remembered that clay layers and interlayers often trap diamonds and their satellite dikes, being at the same time a “screen”, in connection with which there are enriched areas above them. Analyzing the ratio of reserves of terraced and valley placers (Table 6) N. А. Shilo (2002) found that the bulk of the reserves of terraced placers (more than 75%) is associated with the first and second above-floodplain river terraces, and the first contains almost half - 40.9% (or 18.2% of all reserves). Surely, this example is not a rule confirming the general regularity of the distribution of useful components among terrace and valley deposits, but it can give a general idea of this. Fig. 41. Favorable places for sampling potentially diamondiferous deposits: a - opened with a pit to the raft, b - the form of laying out rocks when driving a pit; 1 - topsoil, 2 - sands, 3 - sand-gravel-pebble mixture, 4 - clays, 5 - argillites of the terrace basement, 6 - the most favorable places for sampling (bedrock-adjacent), 7 - favorable places for sampling. In our opinion, the confinement of the main reserves (more than 90%) to the first three above-floodplain terraces is explained by the fact that rocks from the seventh to the fourth terraces participated in their creation. Consequently, as a result of repeated washing and redeposition of sedimentary material of ancient valleys, each stage of the formation of terraces was accompanied by intensive enrichment of newly created placers due to the influx of new portions of gold from the feeding province and destruction of the previous valley placers. The presence of satellites of diamonds in the deposits of the terraces is far from always able to orient in the search for primary deposits, but orientates to the finding of valley placers, as it indicates the place and sources of the useful product. The practical value of terraced placers is much less than that of modern alluvial placers. The clastic-river method is of great importance for the search for primary sources of diamonds. Entire sections of various researchers are devoted to the description of this method (Crater, 1940; Prokopchuk, 1979; and others), so we will very briefly touch on this issue. Fig. 42. Forms of placers in plan (according to Yu. N. Trushkov, 1972): a - single-striate, b - ribbon-like, c - phacoidal, d - lenticular, e - nested, e - isometric, g - irregular shape, h - multi-striate, and i - dissected along terrace levels; 1 - placers; 2 - contours of the valleys; 3 - the edges of the terraces. As you know, the clastic-river method consists in searching for pebbles and fragments of kimberlite and other, diamondiferous rocks among alluvial deposits, and then in tracing them up the river, until they reach the source of their demolition (feeding province). Kimberlites are unstable breeds. In rare cases, kimberlite fragments are transported by water streams further 5-10 km. Fragments of lamproites are transported much further (15-20 km). Fig. 43. Sampling of potentially diamondiferous deposits, exposed by clearing (step ditch): 1 - topsoil; 2 - sands; 3 - stratified sands; 4 - clays; 5 - aleurites; 6 - sand and pebble deposits; 7 - talus; 8 - limestones of the basement; 9 - the most favorable places for sampling (bedrock-adjacent); 10 - favorable places for testing. The clastic-river method is used at all stages of prospecting for diamond placers. For these purposes, the tested pebble material is classified into 5 groups according to the degree of its roundness (0, 1, 2, 3, 4). Roundness scores are given according to the method proposed by Crumbain (1941). There is some disagreement in the geological literature on this issue. Some researchers, citing this classification, refer to Khabakov, others to Rukhin. L.B. Rukhin (1962) himself points to Crumbain. Having determined the shape of 50-100 pebbles, then the average value is displayed, multiplying the number of pebbles by the corresponding point. The sum of the products multiplied by 25 and divided by the total number of pebbles will be the average roundness of the given specimen, expressed as a percentage. Table 6 The ratio of reserves of terraced and valley placers in one of the provinces according to N.A. Shilo (2002), % Total in terraced placers In the placers of various terraces террас 1 2 3 4 5 6 7 44.5* 100.0 18.2 40.9 15.3 34.5 6.8 15.3 3.0 6.7 0.8 1.7 0.3 0.7 0.1 0.2 *Total reserves in terraced placers from the total amount of reserves (valley and terraced). where P is the average roundness, %; — the number of pebbles with round-ness, respectively, in 1, 2, 3 and 4 points; K is the total number of pebbles. After that, the pebbles in each rock group are broken to obtain a fresh fracture, which is used to determine the petrographic type of rocks. Fragments reminiscent of kimberlites and lamproites are being investigated especially closely. Kimberlites are an igneous ultrabasic rock with a breccia-like texture. Usually it is a dark gray, greenish and bluish gray, dark green, dark blue, black or brown rock with a porphyry structure of matter. Lamproites are massive porphyric rocks in which two generations of olivine phenocrysts are immersed in a fine-grained groundmass consisting of diopside, phlogopite, and devitrified glass (Temporary methodical ..., 1988). The bulk is usually glassy, but sometimes well crystallized (diopside, phlogopite, chlorite, apatite, perovskite, potassium richterite, vyidit, praiderite and ilmenite). It is important to note that, according to A. A. Frolov and his colleagues (Frolov et al., 2003; Belov et al., 2008), diamondiferous diatreme-dike alkaline-ultrabasic rocks are widely developed in the northeastern Siberian platform within the Udzhinsky uplift. They revealed intrusive rocks of the ijolite-carbonatite formation, represented by alkaline-ultrabasic lamprophyres, the heavy fraction of which contains chromite, magnetite, magnesioferrite, chrome spinels, picroilmenite, pyrope and diamond. In this type of rocks, diamonds are found as accessory minerals and they could not significantly affect the placer diamond content in this region (Placer diamonds ..., 2007). 8.3. Deluvial-proluvial placers Placers of this type are formed due to erosion and destruction of both direct bedrock diamond-bearing rocks and intermediate diamond collectors. More than half of all known world reserves are associated with such placers. Fig. 44. A typical section of a deluvial diamond placer in Yakutia (according to B. I. Prokopchuk, 1979 with changes): 1 - topsoil; 2 - clay loam with dolomite gruss; 3 - deluvial clays; 4 - deluvial placers of diamonds; 5 - weathering crust; 6 - dolomites; 7 - increased content of diamonds; 8 - mine workings: a - pit and its number, b - well and its number. B. I. Prokopchuk (1979), studying deluvial-proluvial placers in the Anabar Shield region, noted that they were formed as a result of erosion and destruction of ancient intermediate reservoirs. He found that the productive layers are confined to karst sinkholes formed in the Cambrian dolomites. The general nature of the placer is shown in Fig. 44. The length of deluvial-proluvial placers usually does not exceed 1.5-2.0 km, the width is 250-300 m. The thickness of the placers depends entirely on the slope relief and is on average 3-5 m, although in some cases it can reach several tens of meters. Their structure is usually simple, although there are exceptions (Fig. 45). The most favorable places for sampling deluvial-proluvial deposits are bedrock-adjacent layers, which are best exposed by pits. Fig. 45. The structure of one of the deluvial-proluvial placers. In the upper part, it has a simple single-layer structure, and in the lower part, it has a two-layer structure (according to N. N. Armand et al., 1985): a - cross sections, b - longitudinal section; 1 - loose sediments, 2 - bedrock, 3 - weathered bedrock, 4 - pay bed. According to Yu.I. Goldfarb and his colleagues (https://zolotodb.ru/article/11134), it is necessary to determine what types of alluvial placers are possible in a particular valley before starting the search for gold-bearing placers in places with favorable metallogenic characteristics. In gold-bearing areas with equilibrium valleys on wide terraced valleys, only erosional placers are usually developed, and in the middle - trail, erosional and primary perluvial (perluvial deposits are part of alluvial formations, and are formed in situ near the concave bank of the river channel). Strigillate placers are found only on steep sections of erosional gorges. They are exposed, found by schlich sampling, are explored in the process of mining and can be regenerated, therefore, their multiple (annual) mining is useful. Erosional placers have the shape of rather rectilinear (with rare sharp bends) monolithic ribbons 0.3 to 15 km long and 5–10 to 40–70 m wide; in cross-section - the shape of an inverted triangle, less often - a trapezoid. Talus placers in the plan consist of several jets, each of which has the shape of a flat lens in cross-section, the maximum sustained power and width. In valleys of medium orders, the jets often touch each other, forming continuous strips up to 2 km wide and tens of kilometers long. In large valleys, the jets are usually scattered and very winding. Gold of medium and small grades is flattened, most sorted by hydraulic size and evenly distributed across and along the streams. Heavy mineral sand sampling can only indicate the areas where placers of this type are located, since their primary sources are usually unknown, they can be destroyed or be located tens of kilometers from them. It is useful to start your search by determining the thickness of loose sediments by GPR profiles. Distance between profiles - 500-1000 m, between observation points - 20-40 m. 8.4. Buried placers of Angarida According to the prevailing views, Angarida is an ancient continent that existed in North Asia at the interfluve of the Yenisei and Lena rivers (Fig. 46, 47). For the first time Angarida was identified by E. Suess (Obruchev and Zotina, 1937), who named it after the river. Angara, the current bed of which is located near the central part of the mainland. The continent existed for about 160 million years, including the Devonian, Carboniferous, and Permian periods (Akulov, 1990, 1991, 2003a, 2010a). During this time, a number of cataclysms occurred on its territory, including the introduction of diamondiferous kimberlite magma, numerous centers of which were found on its eastern margin (Akulov, 2003b). Due to the warm seas surrounding Angarida on all sides, the climate in its territory in the early Carboniferous was humid and relatively hot. The weathering crust was formed on the flat areas, and the processes of erosion and denudation developed intensively on the uplifts and in the places of intrusion of kimberlite bodies, which served as the basis for the formation of paleorrhoids. They were formed mainly coastal-marine, alluvial, basin (lacustrine) and paleo placer deposits. Long-term processes of weathering of Middle Paleozoic sediments, including exposed kimberlite bodies, contributed to the formation of diamond-bearing placers. Fig. 46. Paleogeographic scheme of Angarida (Early Carboniferous era) and diamond-bearing provinces: 1-3 - land: 1 - lowlands, plains and hills; 2 - Stanovoye highlands; 3 - mountains: 1 - Yenisei, 2 - Sayan, 3 - Baikal; 4 - sea; 5 - modern outline of the continent; 6 - diamond-bearing provinces (according to G. Kh. Fainshtein, 1968): A - South Siberian, B - North Siberian, C - East Siberian (Aldan). The average thickness of the residual weathering crust on the Middle Paleozoic rocks reaches 20 m (Almazonosnye placers ..., 1967). It was intensively eroded, as evidenced by traces of deep erosion on kimberlite bodies. Thus, diamondiferous paleo placers associated with redeposited products of the weathering crust of this age could have formed, but only in the presence of a thick (several hundred meters) kimberlite pipe. The well-known "Vodorazdel'nye galechniki" placer can serve as an example of this type of deposits. 8.5. Riphean sources of diamonds in the south of Angarida A comparative analysis of the location of the main primary diamond-bearing provinces and fields located on the Siberian craton and on the Kimberley craton (Australia) showed their surprising similarity (Fig. 48, Fig. 49). In western Australia, four diamondiferous provinces have been identified, which they call ore regions: Ellendale, Big Springs, Calvinyard and Nornverdach (Michel, 1988). Australian lamproites contain a large amount of phlogopite, and their age ranges from 1100-1200 Ma. Despite the fact that the content of diamonds in them is not high and ranges from 1 to 5 carats per 100 tons and rarely more, they are of gem quality, and their average size does not exceed 0.1-0.2 carats. Similar mica diamondiferous rocks were discovered by B. M. Vladimirov (Vladimirov, Znamerovsky, 1960) in the Uriko-Tumanshetskaya zone of the Prsiayanye. It is quite possible that the modern alluvial Shelekhovskaya placer adjacent to this zone, containing a large amount of gem-quality diamonds weighing up to 7.5 carats, was formed due to their erosion. The Riphean lamproites are about 800 m long, the thickness does not exceed 0.5 m, the diamond content is less than 0.05 ct / t, and the weight of individual diamond crystals revealed in them reaches 12.7 mg. These veinstone did not receive their own name, therefore, they were assigned to one of the varieties of strongly mica (the content of flogopite is up to 87%) kimberlites. From the surface to a depth of about 6 m, mica kimberlites are strongly weathered and are represented by “yellow earth” with numerous rounded olivine crystals up to one centimeter in size. There are also numerous flakes of phlogopite, pyrope and orange garnet grains. To wash a small-volume sample, they had to carry water to the mountains at a distance of 1.5 km. Nevertheless, they managed to wash up a concentrate, from which diamond crystals were isolated in a grain size class of –2 +1 mm. Fig. 47. The layout of the main fields with "primary" diamond content in Angarida: 1 - mountain-folding structures from the Yenisei ridge (western part of the scheme) to the Baikal mountainous region (eastern part); 2 - zones of epiplatform folding: A - Angarskaya, B - Nepskaya, V - Muiskaya; 3 - low and hilly plains; 4 - Taimyr coastal zone or development area of the coastal-marine (Tychan) diamond-bearing placer; 5 - structural-sedimentary zones or Middle Paleozoic sedimentary basins: 1 - Kyutingdinsky, 2 - Ygyattinsky, 3 - Kempendyaisky, 4 - Angara-Tungusky, 5 - Poimo-Biryusinsky, 6 - Rybinsky, 7 - Minusinsky; 6 - kimberlite fields: 1 - Malo-Botuobinskoe, 2 - Alakitskoe, 3 - Daldynskoe, 4 - Verkhnemunskoe, 5 - Chomurdakhskoe, 6 - West Ukukitskoe, 7 - East Ukukitskoe, 8 - Ogoner-Yuryakhskoe, 9 - Merchimdenskoe , 10 - Molodinskoe, 11 - Toluopskoe, 12 - Kuoyskoe, 13 - Srednekuonapskoe, 14 - Ebelyakhskoe, 15 - Tomtorskoe; 7 - Ilimo-Kangskaya diamondiferous area. Fig. 48. The layout of the main fields with "primary" diamond content in the Siberian craton: 1 - Siberian craton; 2-6 - mountainous formations: 2 - Yenisei ridge, 3 - Taimyr, 4 - Verkhoyanya, 5 - Baikal, 6 - Eastern Sayan; 7 - zones of the main faults; 8-9 - diamond provinces and their diamond fields: 8 - North Siberian (1 - Malo-Botuobinskoe, 2 - Alakitskoe, 3 - Daldynskoe, 4 - Verkhnemunskoe, 5 - Chomurdakhskoe, 6 - West-Ukukitskoe, 7 - East -Ukukitskoe, 8 - Ogoner-Yuryakhskoe, 9 - Merchimdenskoe, 10 - Molodinskoe, 11 - Toluopskoe, 12 - Kuoyskoe, 13 - Srednekuonapskoe, 14 - Ebelyakhskoe, 15 - Tomtorskoe); 9 - South Siberian (Ingashinskoe "lamproite field"). It should be noted that vein diamondiferous bodies were exposed only by shallow pits, and no drilling work was carried out on them and they were not exposed by deep pits (25-30 m). Therefore, there is still no information about their behavior at depth. 8.6. Coastal-sea placers of Angarida The Angarida coastline, which existed in the Early Carboniferous, is clearly traced in places along the outliers of coastal-marine conglomerates (see Fig. 47). The first finds of blocks of conglomerates were made back in 1966 by geologists of the Amakinskaya expedition of YSTU (I. M. Koryakin, I. P. Plakin, Yu. P. Belik and others) and SRIAG (L. I. Rubenchik and others) in Ebelyakh river basin (northern part of Angarida). In 1967, the same conglomerates were found by B. I. Prokopchuk, V. A. Skosyrev, I. A. Bukhmillir and others in the basin of the rivers Kumakh-Yurekh, Billyakh and Mayat. Later, I. A. Galkin and B. I. Prokopchuk found similar conglomerates on the left bank of the river. Anabara and in the valley of the Popigay river. Fig. 49. Scheme of the location of the main fields with “primary” diamond bearing on the Kimberley craton (Australia) and its periphery (after J.-C. Michel, 1988): 1 - Kimberley craton; 2-3 - folded region: 2 - Halls Grick, 3 - King Leopold; 4 - breaking violations; 5 - kimberlite fields: 1 - King George, 2 - Lighting Grick; 6 - lamproite fields: 3 - Argil, 4 - Ellendale, 5 - Big Springs, 6 - Calvinyardach, 7 - Norverdach. Such close attention to these conglomerates is due to the fact that a diamond crystal and its numerous satellite dikes (picroilmenite, pyrope, etc.) were found in their composition. Conglomerates are a very dense light gray rock containing gastropods and algal colonies, which allowed them to be attributed to the Early Carboniferous (Prokopchuk et al., 1983). Thin Lower Carboniferous coastal-marine deposits of the ancient Taimyr Sea were also found to the east of the Yenisei Ridge (basin of the Tychana River, right tributary of the Podkamennaya Tunguska). According to A. V. Kryukova and L. N. Peterson (Kryukov and Peterson, 1978), on the Shushuk uplift they are composed of siltstones, fine-grained sandstones, clayey dolomites and marly limestones, combined into the Shushuk Formation 25-40 m thick.On the eastern side of the uplift, the deposits of the formation are completely eroded. Siltstones and fine-grained sandstones prevailing in the formation consist of fragments of quartz, hornfels, microquartzites, feldspars and mudstones. The cement of the rocks is usually basal of clay-lime composition with limonite and kaolinite. According to the peculiarities of the lithological composition, the sediments of the formation are classified as shallow-water, coastal-marine (Kryukov and Peterson, 1978). It is important to note that in the basin of the river flowing here many diamonds have been found in Tychany river. Thus, the northeastern section of Angarida continues to remain promising for prospecting for kimberlites, adjacent to the coastal boundary of the Taimyr Early Carboniferous Sea, along which the Tychanskaya area of alluvial diamonds discovered by Krasnoyarsk geologists stretches. Fig. 50. Zones favorable for the accumulation of placer-forming minerals in the coastal zone; generalized profile by J. Mero (Armand et al., 1985): 1 - beach sands and pebbles, 2 - deposits of heavy minerals, 3 - tidelines: A - upper, B - lower; S.b - sand bar, U.c. - underwater clough; 1-3 - zones: 1 - sea, 2 - frontal, 3 - rear. Usually, in coastal-marine placers, industrial diamond concentrations are recorded in the basal layers of coarse-grained material, which are sometimes overlain by less diamondiferous pebble-gravel-sand formations. The width of coastal-marine placers entirely depends on the size of the water basin and varies from the first meters to the first hundred meters (Fig. 50). According to the study of the regularities in the distribution of heavy diamond concentrates and diamonds themselves, it is possible to outline the places of the introduction of diamond-bearing material. The large size of the satellite mineral grains, the preservation of relics of their primary surface, the presence of such unstable minerals as chrome diopside and olivine indicate that the sources of the coastal basin placer were located at a small distance from the coastline. The nature of the enrichment of diamondiferous layers in one of the coastal-marine placers in Africa is shown in Fig. 51. Most of the diamonds mined from this placer belong to gem grades, and their content ranges from 0.4 to 3.6 carats/m3 with an average weight of 0.5 carats. It should be emphasized that the African coastal-marine placers are confined to the mouths of large rivers and extend in the areas of the near shelf. Fig. 51. The nature of the enrichment of diamondiferous layers of coastal-marine placers (according to B. I.Prokopchuk, 1979): 1 - silt, 2 - sand, 3 - boulder-pebble material, 4 - bedrock, 5 - diamond-bearing deposits. When reading this section, the reader may have a question, is it not possible to search for buried kimberlite pipes through buried placers? Answering this question, it should be said that the search can and should be carried out, but this requires a significant amount of mining and drilling operations. The difficulty also lies in the thickness of the exposed sediments for tracing the paleoplacer. Therefore, such work is justified to a depth of no more than 10 m. Carrying out independent searches for overlapped kimberlite bodies using numerous drilling of wells with a depth of more than 100 m is unprofitable. It justifies itself only when there is a search for buried kimberlite or lamproite bodies near the already known pipes, according to the principle - “look for ore near ore”. Published data (Methodical Recommendations ..., 1985) indicate that 75% of buried kimberlite bodies were found by direct penetration of wells into a kimberlite body, 20% of pipes were found by HDC halos and only 5% by geophysical anomalies and HDC halos on them. It should be emphasized that only one of all the open kimberlite pipes (the basin of the upper reaches of the Alakit River) was uncovered at a depth of more than 130 m. Along with this, the issue of the Jurassic coastal-marine placers should be raised. As B. I. Prokopchuk (1966), in the late Triassic and early Jurassic, intense tectonic movements on the Siberian platform led to an uplift of the territory, which caused an increase in erosion and denudation processes in the northeast of the Siberian platform. At this time, the Ural-Mongol-Okhotsk fold belt (Western Sayan, Altaids, Uralids, etc.) and the West Siberian plate joined the Angarida. A huge epiplatform continent was formed, which is called Laurasia. The formation of Laurasia began in the Triassic period and was accompanied by epiplatform tectonic-magmatic activity on the Siberian platform. This is evidenced by the vast lava fields of traps, thick strata of tuffs, and numerous intrastratal dolerite bodies - sills. Many kimberlite pipes were eroded to a considerable depth (200-300 m), and diamond-bearing material was carried to the coastal zones of the Lower Jurassic epicontinental sea.

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