The main trend in the Norwegian mineral industry, in recent years, is that industrial minerals in particular, but also aggregates and dimension stone, have become increasingly important. Norway has around 35 active mines and quarries for industrial minerals, with a total production of around 14 Mt annually. Around half of this production is exported. The use of domestically produced industrial minerals in Norway is, in general, restricted to low-value / high-volume commodities such as limestone/calcium carbonate for production of cement and agricultural and environmental applications, and quartz/quartzite for Fe-silicon alloys and silicon-manganese production. In addition, dolomite is used domestically in agriculture, metallurgy and fertilisers, and ilmenite-concentrate is used for production of TiO2-pigment and titanium slag. Concerning exports, most of the considerable volumes of olivine and nepheline syenite produced in Norway, as well as feldspar, are exported. Calcium carbonate, in the form of slurry, is the most important export mineral in economic terms. Export of dolomite, mainly as micronised products, is also of great importance, as is that of micronised talc and ilmenite. The total export value of industrial minerals in 1997 exceeded 2.26 Bill. NOK, representing an increase of 20.5% over 1996 (Karlsen, in press).
The possibility for development of new industrial mineral deposits in Norway is large, when one takes into account favourable geological settings for numerous types of deposits, combined with proximity to the European market, the long coastline with excellent all-year-round shipping possibilities and the low density of population (Sturt et al., 1995).
Due to this positive future outlook for industrial minerals exploitation in Norway, the Geological Survey of Norway (NGU) has intensified their work with prospecting, mapping and characterisation of such deposits. An annual expenditure of around 15 mill. NOK is used on the programme 'Mapping and development of industrial mineral deposits'. The programme includes subsidiary projects on calcium carbonate, titanium minerals, talc, dolomite, quartzite, high purity quartz, apatite, garnet and others. In many of the sub-projects, research and development is a significant part of the work and supplements traditional mapping. The projects are carried out either as internally-funded NGU-projects or as cooperative projects with industrial partners. Some of the goals for the programme include the establishment of new mineral based industries, to secure the reserves for existing companies and to increase the number of applications for raw materials. NGU has played an active role in the development of a variety of deposits, some of them now being exploited; others will probably be mined in the future.
Karlsen, TA, Industrial Minerals, (in press).
Sturt BA, Barkey H, Neeb PN & Heldal T, NGU-report, 95.116, (1995).
In the 1990´s the production and use of industrial minerals have increased significantly in Finland. The positive development is mainly based on the increasing demand of paper pigment minerals (Pekkala, 1995). The total consumption of these is over 2.6 m. tpa, and the major part comes from imports of kaolin, which alone is 1.3 m. tonnes (Pekkala, 1998). The Geological Survey of Finland (GTK) has carried out extensive search for domestic kaolin and ilmenite deposits (Sarapää, 1996; Kärkkäinen et al., 1997). Noteworth geological resources have been located but so far technical - ecomical studies have not given satisfactory results.
The imports of carbonates (calcite and chalk) are over 0.5 m. tpa while the domestic production of micronised calcite and precipitated calcium carbonate is 0.7 m. tpa. In recent years the GTK has looked for new calcite rock deposits in southern Finland and the results are encouraging (Sarapää et al., 1999). The investigation reports on two new deposits have been completed and will be offered to interested parties in early 1999.
Talc production is over 0.4 m. tpa, and about half is exported. The demand of talc is at present high and the search for new talc deposits in mid eastern Finland is intensive. Besides Finland´s sole operator in talc business, Mondo Minerals, also Talc de Luzernac is very actively looking for own deposits. As a whole it may be noted that the development of industrial minerals sector in Finland has been favourable in recent years.
Pekkala Y, Industrial Minerals, Dec., 33-39, (1995).
Sarapää O, Ahtola T, Kärkkäinen N, Seppänen H & Reinikainen JP, New Industrial Mineral Deposits in Southern Finland, Geological Survey of Finland Current Research 1997-1998, 27, (1999).
Kärkkäinen N, Sarapää O, Huuskonen M, Koistinen E & Sarapää O, Geological Survey of Finland Current Research 1995-1996, Special Paper, 23, 15-24, (1997).
Sarapää O, Geolocal Survey of Finland, Special Paper, 152, (1996).
Pekkala Y, Industrial Minerals, 374, 57-61, (1998).
In recent decades the industrial use of carbonates in Norway has expanded markedly. Carbonate rocks are used as a raw material in the cement, paint and paper industries, and in agriculture, as well as being widely utilised for building and decoration purposes. Rapidly growing demand for new resources combined with the higher quality of the raw materials and much lower production costs provide a challenge for prospectors.
Prospecting for carbonates differs from ore prospecting in that commercially valuable carbonates are common rocks. Norwegian carbonate deposits form parts of carbonate-schist sequences in the Caledonian nappes. They were initially deposited in basinal environments and were then affected by diagenesis, and several stages of metamorphism and deformation. All these processes contributed to the formation of commercial deposits. Consequently, disciplines such as basinal analysis, (tectono)stratigraphy, sedimentary and metamorphic petrology, and geochemistry should be considered as key components in exploration strategy.
Existing commercial carbonate deposits are distributed unevenly throughout the country. Whereas Finnmark and Troms counties lack commercial deposits, the Nordland-Trøndelag region exposes several, commercial, white dolomite marble deposits; and in southern Norway there are numerous calcite marble deposits. As no explanation has hitherto been given for such a distribution, this has resulted in a random search for new deposits.
Here we share our experience obtained in the three-year Project 'Carbonate formations of Norway: from basic research to industry'. The results discussed below are applicable to the Nordland-Trøndelag region.
Mapping, age constraints and stratigraphic subdivisions. Pb-Pb age determinations on calcite and indirect age constraints based on the 13C and 87Sr/86Sr evolutionary curves in seawater have been applied to date and subdivide carbonate sequences. Results show that some extensive carbonate successions previously considered as Ordo-Silurian are, in fact, of Neoproterozoic age. A scientifically based explanation for the areal/temporal distribution of the existing deposits has been suggested, e.g., all commercial, white dolomite marbles are now constrained between 620 and 550 Ma.
Sedimentology, isotope geochemistry and lithostratigraphic correlations. A multicoloured lithological (grey-variegated-white) and geochemical (13C=+5‰ for grey, -9‰ for variegated and white) marker has been established to correlate and subdivide strongly deformed and metamorphosed carbonate sequences. The variegated member of this tripartite marker unit has been dated indirectly to ~590 Ma and constitutes a valuable and inexpensive correlation tool. Numerous carbonate sequences are now recognised to be inverted, and carbonate formations of different ages have been distinguished.
Results applied to national exploration and prospecting. The observed close correlation between lithostratigraphy, depositional age and deposits of different kinds has led to objectively targeted exploration. White dolomite and calcite marble deposits can now be predicted as they have a specific relationship to the tripartite marker.
Research has been supported by the Geological Survey of Norway and Nordland County, Projects 270500-270510.
The Precambrian rocks in Finland are divided into Archean (3.1-2.65 Ga) and Svecofennian (1.93-1.82 Ga) Domains. The Uusimaa Belt forms the southwestern part of the Svecofennian Domain and it is metamorphosed predominately in higher amphibolite facies conditions. It consists mainly of felsic volcaniclastic rocks (leptites) with some mafic volcanics, migmatites, mica gneisses and pegmatite granites. Carbonate rocks are widely distributed and they form from few hundreds meters to several kilometers long units. They are intercalated with the leptites, while epiclastic sediments are rare. Leptites are usually companied by layer-like basic veins. The post-orogenic potassium granites cross-cut the other rocks of the Uusimaa Belt.
The carbonate rocks are metamorphic marbles, i.e. they are recrystallized, granoblastic, coarse-grained and greyish white in colour. They are mainly calcitic in composition, while alternating dolomite rock layers are typical in Kisko and Nummi-Pusula regions. The calcite rocks contain thin silicate layers composed of quartz, feldspars and metamorphic silicates: tremolite, diopside and phlogopite. Wollastonite may also be found associated to quartz layers. The dolomite rocks are interpreted to be pre-metamorphic, i.e. syndepositional in origin. The porous dolostones occurring within some calcite rock deposits represent post-metamorphic dolomitization by magnesium-rich fluids. The composition of carbonate rocks has changed due to decarbonation during their post-depositional history in diagenesis and metamorphism. However the prevailing composition of the carbonate marbles is interpreted to be mostly primary in origin. The calcite rocks in the Kemiö-Särkisalo region are interpreted to be precipitated in shallow sea, carbonate shelf facies and the calcite-dolomite rocks of Kisko and Nummi-Pusula regions in carbonate ramp-slope facies of tidal environment.
Geological Survey of Finland (GTK) has located several promising carbonate rock deposits within the Uusimaa Belt. The demand of paper pigments in Finland has increased rapidly during the 1990's. The studies comprised detailed geological mapping, geophysical profile measurements and drilling. The studied carbonate rock deposits are 500 to 2000 m long and 30 to 80m wide. They form steeply dipping layers, which are orientated owing to isoclinical F2-deformation. They may also be remobilized and refolded during F3-deformation. The thickness of the layers is obviously multiplied during deformation and they can be grouped as stratabound and metamorphically purified, epigenetic calcite ores. Reserves of the studied carbonate rock deposits are 4 to 10 Mt, calculated on the open-pit level. Benefication tests has been successful for GCC product and concentrates contain over 99% calcite. The whiteness of the calcite concentrates is 91-94 ISO-%, when measured from under 45 microns grain size.
Reinikainen JP, Geological Survey of Finland Current Research 1995-1996, Special Paper, 23, 37-41, (1997).
Sarapää O, Ahtola T, Kärkkäinen N, Seppänen H & Reinikainen JP, New Industrial Mineral Deposits in Southern Finland, Geological Survey of Finland Current Research 1997-1998, 27, (1999).
Reinikainen JP, Petrogenesis of Paleoproterozoic Sedimentary Carbonate Rocks in Finland, Svecofennian Domain, Geological Survey of Finland, Special Paper, 29, 112, (1999).
Ilmenite (TiFeO3) and rutile (TiO2) are the primary raw-materials for world production of Ti. Ilmenite is the most important by volume, but rutile is 4-5 times more valuable in TiO2- eqivalents. Rutile-bearing eclogites in West Norway have, since the 1970's, been considered to represent a titanium resource for the future. This contribution gives a general geological overview of the occurrence of rutile in eclogites and associated rocks in the Sunnfjord region, and shows how the character of the rutile ore is influenced by changes in the metamorphic conditions.
Eclogites in West Norway are mafic metamorphic rocks formed during Caledonian high-pressure metamorphism at approx. 400 Ma. It is generally agreed that the eclogite metamorphism is related to continental collision, although the details remain uncertain. Mafic eclogite paragenesis in the Sunnfjord region comprises garnet, omphacite, amphibole, phengite, clinozoisite, quartz, dolomite, rutile and pyrite. Retrogression of the eclogites is often dependant both on deformation and fluid infiltration and occurred predominantly along shear zones and margins of bodies.
Changes in oxide mineralogy can be related to the P-T evolution for the Sunnfjord eclogites, as exemplified by the P-T path for the Naustdal eclogite (Krogh 1980). In this evolution, four stages have major effects on the oxide mineralogy: (1) Ilmenite is the stable titanium mineral at the protolith stage, represented by Proterozoic Fe-Ti oxide-bearing metagabbroic rocks in the southern part of the region, which are only partly eclogitised. (2) During eclogitisation, the titanium remaining when iron from ilmenite enters garnet, forms rutile. The eclogitisation period was dynamic in terms of deformation, fluid activity and associated metasomatic processes, leading to a variety of metasomatic rutile-bearing quartz - omphacite - amphibole - garnet - mica rocks. (3) During uplift, fractures and shear-zones opened for influx of fluids that triggered amphibolite-facies retrograde reactions in which rutile altered to ilmenite. (4) During the final metamorphic stage, alteration of rutile and ilmenite to titanite occurred under lower epidote-amphibolite to greenschist facies conditions. The proportion of titanium expressed as rutile varies widely between eclogite bodies as well as within individual bodies, mainly due to large differences in the degree of retrogression. Eclogites with insignificant retrogression have 90-95% of total TiO2 as rutile. In most eclogite bodies, however, the retrogression is significant, and a large part of the TiO2 occurs in ilmenite.
Krogh, E.J., Lithos, 13, 355-380, (1980).
A large number of ultramafic lenses, each up to 1 km wide, occur in the Rödingsfjället Nappe Complex, Uppermost Caledonide Allochthon, along the Helgeland coast, northern Norway. In Altermark, the lenses show distinct zoning patterns; some are exploited for talc-carbonate. The lenses (Karlsen, 1995) are associated with internal thrust faults within the Rødingsfjället Nappe Complex and are hosted by amphibolite, graphite-mica schists and calcitic marble. Other associated rocks are staurolite-kyanite-garnet-mica schist and dolomitic marble. Three deformation events (D1, D2 and D3) are recognised. The ultramafites are isofacial with prograde P-T development in the country rocks during D1, with a metamorphic peak around 640°C, 7-10 kbar. Mineralogical zoning pattern in the ultramafic lenses are, from core outwards: (1) Serpentinitic core, (2) Talc-carbonate zone, (3) Monomineralic rocks. The serpentinite consists predominantly of interpenetrating textured antigorite and 5-20% magnetite. Ferro-chromite, in particular, but locally also relics of olivine and clinopyroxene, are present in subordinate amounts. Some ultramafic cores carry lenses of primary clinopyroxenite, dunite, chromitite, and rodingite. The latter, probably representing metasomatised mafites, has been described only once previously from Norwegian ultramafites (Bøe, 1985). They consist of epidote + amphibole + chlorite ± hydrogrossular ± serpentine. The talc-carbonate zone consists of about 40-70% talc, 30 - 45% carbonate, and trace magnetite, chlorite and chromite. In the innermost parts of the zone, trace amounts of antigorite occur, either as porphyroclasts that predate talc-carbonate formation, or as porphyroblasts post-dating talc-carbonate. The carbonate is dominated by texturally and chemically zoned breunnerite; dolomite may be present locally in subordinate amounts. The process of talc-carbonate formation leads, in its ultimate stage, to magnetite breakdown, with resulting consequences for both prospecting and ore quality. The monomineralic rocks in the rim consists (outwards) of talc-schist, ± tremolitite, chloritite and biotitite. The tremolite in the tremolitite occurs predominantly as green idioblastic grains. The ultramafites show distinct chemical zoning patterns. The most distinct changes during serpentinization and formation of talc-carbonate are addition of H2O and CO2. The typical «ultramafic» components MgO, Cr and Ni generally decrease from ultramafic core towards country rocks. Cr, Zr, Ti, Ba, Y, V, Fe2O3(total) and Al2O3 show abrupt changes at the boundary between tremolitite and chloritite/biotitite, interpreted to be the original contact betweem ultramafic and country rock. A volume increase of ca. 14% took place during formation of talc-carbonate from serpentinite. Structural evolution of the area, combined with volume increase, led to development of talc deposits in areas of reduced stress, analogous to quartz pressure shadows around garnets.
Bøe, R, Norsk Geologisk Tidsskrift, 65, 301-320, (1985).
Karlsen, TA, Geological and geophysical studies of ultramafite associated talc deposits, Altermark, northern Norway. Unpubl. Dr. Ing. thesis, Norwegian Institute of Technology, Trondheim, (1995).
The Minas-Bahia Graphite Province (Northeast of Minas Gerais State and Southeast of Bahia State, Eastern Brazil) has 52,000,000 ton of graphite ore reserve (Morais, 1997). Approximately, 1000 ton/month of flake-type graphite concentrate and 1500 ton/month of fine ore concentrates are produced. The graphite mines are located in the vicinities of Pedra Azul, Salto da Divisa, and Maiquinique towns. The graphite deposits are found in two Proterozoic units of the Araçuaí Belt (Faria, 1997, Reis and Pedrosa-Soares, 1998, Pedrosa-Soares et al., 1998). The Schistose Unit consists of sillimanite-graphite schist, graphite-quartz schist, graphitic quartzite, garnet-muscovite-biotite schist and quartzite. The Kinzigite Suite consists of sillimanite-graphite gneiss, graphite schist, graphite-sillimanite-cordierite-garnet-biotite gneiss (kinzigite), cordierite-garnet-biotite gneiss, garnet-biotite gneiss, leptite, calc-silicate granulite, and quartzite. Two mineralization types of flake graphite were characterized in the Minas-Bahia Graphite Province (Faria, 1997). The Schist-type graphite mineralization occurs in the Schistose Unit. It is tectonically controlled by way of high-angle dip, ductile shear zones. Intense folding is observed in some deposits. The metamorphic association, together with indicators of partial melting, are evidence of metamorphism in the amphibolite facies, initial anatectic zone (600° C to 700° C). The analysis of graphite flakes shows that, on the basal cleavage plane, graphite is stretched, preferentially taking on losangular forms. In sections perpendicular to the basal cleavage, a ragged aspect is observed. The medium grain-size is less than 1 mm. The Gneiss-type graphite mineralization occurs in the Kinzigite Suite. The metamorphic association indicates metamorphism at the amphibolite-granulite facies transition, intense anatectic zone (700° C to 800° C). Ductile shear zones control the morphology and continuity of the graphitic bodies. The Gneiss-type graphite mineralization is subdivided into three subtypes (Faria, 1997). The subtype G1 includes graphite-poor gneiss to graphite-rich schist. The principal characteristic is endowed by the flakes that exclusively, or together with biotite, define the gneissic foliation. On the basal cleavage plane, the flakes are stretched and, in sections perpendicular to the foliation, the flakes are ragged and locally lamellar. The medium crystal grain-size is between 1 and 3 mm. The subtype G2 is related to quartzofeldspathic intercalations generated by in situ partial melting of the gneiss itself. The flakes are characterized by being immersed in a matrix of felsic composition and igneous texture. On the basal cleavage plane, the crystal morphology tends to hexagonal forms, in almost all cases, displaying rounded margins. Grain-size varies, on average, between 3 and 5 mm. The subtype G3 is typified by graphite schist intercalations between 0.5 and 1 m thick and normally associated with the G2 subtype. Such rocks are melanocratic, essentialy composed of graphite flakes. On the basal cleavage plane, crystals have a lamellar form. Grain-size reaches up to 2 cm.
Faria, LF, Ms Thesis, Universidade Federal de Minas Gerais, Departamento de Geologia, Belo Horizonte, Brazil., 154 p., (1997).
Morais, EN, Principais Depósitos Minerais do Brasil, Departamento Nacional da Produção Mineral, Brasília, IV-C, 179-183, (1997).
Pedrosa-Soares AC, Vidal Ph, Leonardos OH & Brito-Neves BB, Geology, 26, 519-522, (1998).
Reis, LB & Pedrosa-Soares, AC, 40 Cong. Brasileiro Geol, Anais, 312, (1998).
The mineral composition of sulfur ore in the Mesopotamia basin is relatively simple. The main minerals parageneti-cally related to sulfur ore are calcite, gypsum and bitumens. The oxidation zone abounds in minerals of the alum group and, to a lesser extent, in jarosite, melanterite, alunite, epsomite and quartz. All these minerals constitute what seem to be dispersion halos in oxidization zone, containing epigenetic accumulations of sulfur. The presence of sulfur-rich oxidized oil and bitumens in sulfur-bearing rocks serves as an indicator of past sulfate-reducing processes. Sulfur deposits are surrounded by a zone of re-crystallized gypsum and calcite, the frontal zone of rock metasomatism. This is confirmed, too, by the data of isotopic analysis of sulfate and native sulfur. They show that native sulfur was formed by reduction of sedimentary sulfates under favorable conditions created upon percolation of hydrocar-bon-saturated water through beds of gypsum or anhydrite.
The high concentration of 34S in native sulfur, which is similar to that in primary gypsum, indicates that sulfate reduction was relatively thorough, i. e., that sulfur-rich ore should occur here. Bacterial reduction thereby proceeds at a different rate and to a different extent because of differences in the concentration of organic matter, the pH of the medium and other factors. In our case this was responsible for the considerable variations in the isotopic composition, of native sulfur and sulfates. On the other hand, the carbon isotopic composition of the carbonate depends on the sulfur content of the ore: high sulfur samples contain lighter carbon.
Sulfur-free limestones with carbon of the same isotopic composition as in the sulfur ore occur at other sulfur depo-sits. This is explained by the fact that sulfur deposits are formed in two stages: first the parent sulfate is reduced to hydrogen sulfide with formation of calcium carbonate, inheriting isotopicly light carbon of organic origin, and then hydrogen sulfide is oxidized to elemental sulfur. Under certain conditions oxidation of hydrogen sulfide does not occur and only calcite with light carbon, formed during reduction of sulfate, remains in situ.
In Northeastern Sahara, where similar processes of sulfate reduction by hydrocarbons are taking place, recrystallized carbonates show a light 13C in fractured zones, which served as hydrocarbon's migration pathways. Higher quantities of recrystallized calcite with light carbon are always associated to folds with "dead oil" occurrences. This means that, carbon isotope criteria are decisive in distinction between recrystallized carbonate owing to hydrocarbon migration and carbonate formed in pure mineral environment. In this respect, no other mineralogical criteria are available. What's more is that, when prospecting, the probability of finding a recrystallized carbonate (with light carbon) in epigenic or presurface zones, is much more that to find remains or traces of oil (or native sulfur). This is due to much more stability of recrystallized carbonates in such erosion environment.
In the final analysis, sulfur isotope composition (34S) characterize native sulfur abundances, whereas carbon iso-tope composition (13C) characterize, first and foremost, the carbonate ion origin (organic or mineral) in the carbonates. The abundance of recrystallized carbonate (with light carbon) characterize, in turn, the quantity of sulfate reduced and the volume of hydrocarbons involved, as well. Values of 13C = - 10‰, and more negative, in carbonate carbon may be recommended as prospecting clues in the search for sulfur-bearing limestones and especially for petroleum prospecting in the Mesopotamian, North Saharan and other basins. This approach already helped in discovering new oil pools in the Mesopotamia basin.
The mineralogy of the matrix of the Akrotiri tuffs and lapilli tuffs of the Santorini island (in the centre of the South Aegean Sea Volcanic Arc) is dominated by clinoptilolite and smectite, occasionally illite/smectite. The clinoptilolites have been characterised by means of thermal tests and EMPA. After firing at 460°C and at 560°C, in both cases for 12 h, the total reduction of the 020 diffraction peak of the clinoptilolites was found to range from 6 to 45%. The Si/Al ratio ranges from 4.37 to 5.18 (mean 4.72). Opal-CT, mordenite, feldspar, amphibole, Fe-Ti oxides, quartz and halite complete the mineralogy. The CEC of the zeolitized materials goes up to 210-220 meq/100g. In contrast, mordenite dominates the mineralogy of the western-northwestern exposures of the pyroclastics from Polyegos island (on the west flank of the South Aegean Sea Volcanic Arc). Some typical Na,Ca-mordenite as well as exceptionally K-rich mordenite were found. Illite, more rarely illite/smectite, occasionally clinoptilolite, opal-CT, feldspar, quartz, kaolinite, biotite and halite are also found. The CEC of the zeolitzed materials goes up to 170-180 meq/100g.
The geochemistry of the zeolitized pyroclastics was examined by using XRF with more than one hundred samples. Major elements were determined on fused glass beads and trace elements on pressed powder pellets. The XRF analyses were plotted on the SiO2 vs. K2O, SiO2 vs. Na2O+K2O and Zr/TiO2 vs. Nb/Y diagrams. It was obvious that the geochemical diagrams using SiO2, Na2O and K2O are not suitable at all for use with zeolitized materials. The spread of the plots was clearly much wider than it should be expected for volcanics of a distinct calc-alkaline affinity from the South Aegean Sea Volcanic Arc. The results were obviously strongly affected by the mobilization and re-distribution of elements during zeolitization, especially of the alkalis. Potassium has clearly been affected by more intensive mobilization and re-distribution processes. By using less mobile elements, such as Zr, Ti, Nb and Y much more reliable conclusions can be drawn. It was revealed that the parent precursors were dacites to rhyodacites in Santorini and dacites-rhyodacites to trachyandesites in Polyegos. Some degree of magma differentiation, but no significant change in the alkaline affinity were recorded for the Santorini pyroclastics. In contrast, some change in alkalinity, but no significant magma differentiation were recorded for the Polyegos pyroclastics. It seems that even when so-called immobile elements are used, the reliability of the relevant diagrams is somehow limited under high water:rock ratio conditions in intense hydrothermal alteration regime. Therefore, it is suggested that these diagrams should also be cautiously used.
Mineral exploration of clay raw materials has increased significantly in the past decade, because of an improved situation on the market (Maccioni et al., 1995; Uras, 1994). In order to assess the economic potential of a clay ore deposit, it is important to estimate the useful mineral types and contents and the geometric features as well as the strike and dip of the mineralised bodies. As regards the mineral types and contents, several methods (XRF, XRD and optical microscope) are commonly used to investigate the quality of the material and evaluate its useful mineral contents. Moreover, the physical-chemical properties of materials can be determined by technological tests. As regards the geometric features of the mineralised bodies, it is not always possible to estimate the effective reserves of clay deposits from outcropping geological-structural features. This study is intended as a contribution to this problem by integrating geological-structural surveys of clay deposits with the application of quick, accurate and low-cost geophysical techniques. The results obtained in the exploration of the «Piscina Collusco» bentonite deposit in Southern Sardinia are presented as an example. The deposit is hosted in Miocenic limestones and Paleozoic shales (Ligas, 1990) and originated from the alteration of a single ash-lens of Oligocene-Miocene calcalkaline volcanism. It outcrops over an area of 50.000 m2 and is exposed for a thickness of about 5 m. The main mineral is montmorillonite, while cristobalite, sanidine, quartz and illite are minor. Smectite is of the Ca-Mg type (2% < CaO < 9%; 0.4% < Na2O < 0.8%; 0.4% < K2O < 0.9%; 3.3% < MgO < 4%; 7.0% < Fe2O3 < 9%). Technological tests (U.N.I.,1970) show a sand and a carbonate contents of 6% and 7% respectively. CEC ranges from 35 to 55 meq/100 g, the WL (Water Limit) is 270%, and Swelling 14 ml/2 g.The VLF (Very Low Frequency) electromagnetic method was found to have a high resolution power in detecting lateral variations in this geological context. In fact, the method showed spatial significant variations in the electrical properties (i.e. conductivity) of the surveyed formations. The interpretation of VLF-em data was assisted by the application of a well-known filtering technique (Karous and Hjelt, 1983) to VLF in-phase component data in order to obtain the current density pseudosections along profiles crossing the bentonite deposit. These pseudosections allowed the acquisition of useful information on the lateral continuity and depth of the target buried deposit, and led to an improvement of the quality of the general information at a lower cost than drilling. The successful application of integrated geological and geophysical methodologies demonstrates that it can strongly improve the estimation of reserves and consequently allows a better evaluation of the economic potential of deposits.
Karous M & Hjelt SE, Geophysical Prospecting, 31, 782-794, (1983).
Ligas P, Studio geogiacimentologico e geofisico delle bentoniti di Piscina Collusco (Siurgus-Donigala, Sardinia, Italy). Unpubl. Thesis, Dep. of Geol. Science, Univ. of Cagliari (Italy), 109, (1990).
Maccioni L, Marchi M, Padalino G, Palomba M & Sistu G, Expl. and Min. Geol, 4 (1), 74-79, (1995).
UNIStandard Normative 6716-70, Bentoniti per fonderia. Caratteristiche e prove. U. N. I. (Ente Naz. It. di Unif.), Milano, 16 p, (1970).
Uras I, Proceed. of "Giornata di Studio in ricordo del Prof. S. Zucchetti", Torino, 12/5, 3-12, (1994).
In Tresnuraghes district (NW Sardinia, Italy) hydrothermally altered volcanic rocks occur forming kaolin deposits and consistent silicified bodies. From the geological point of view, this area is characterized by the occurrence of Tertiary rhyolitic ignimbrites and Quaternary basaltic lava flows (Garbarino et al., 1994). The paleo-hydrothermal activity was related to the Tertiary calcalkaline magmatic activity. Principal fluid conduits were invariably steeply inclined faults and/or fractures whereas lithological control was minor. Intensity and rank of alteration decrease moving horizontally away from the fluid conduits. Different hydrothermal styles were recognized based on different hydrothermal mineral occurrences. The kaolin deposits show a consistent zonation of alteration minerals ranging from opal-kaolinite-alunite at the center of alteration through kaolinite-montmorillonite-cristobalite and finally to the fresh rock. The alteration mineral assemblage, related to the open filling veins, consists of quartz-K-feldspar-illite while the alteration assemblage related to the deposition of siliceous sinter consists of quartz-montmorillonite-illite. Each alteration assemblage reflects typical physical-chemical conditions of the mineralizing fluids (Browne, 1978). Sinter and quartz veins were generated by neutral-pH upflowing hydrothermal fluids. The sinter was deposited where the paleo-water table intersected the paleo-surface whereas data are insufficient to evaluate the paleo-depth of the quartz veins. The kaolin deposits were formed by low-pH and low temperature fluids, draining downward from the paleo-surface, as suggested by occurrence of opaline horizon resulting from the silica remobilization during supra-water table acid leaching (Schoen et al., 1974; Sillitoe, 1993). As the kaolin origin was from descensum and the fluids were neutralized by the host rock during their passing through, these deposits do not seem to extend in depth; therefore, they have low potentiality from an economic point of view. As these features have been commonly observed in the shallow part of precious metal epithermal systems, these kaolin deposits may overlie a precious metal mineralization.
Garbarino C, Masi U, Padalino G & Palomba M, Chem. Erde, 54, 213-233, (1994).
Browne PRL, Ann. Rev. Earth Planet Sci., 6, 229-250, (1978).
Schoen R, White DE & Hemley JJ, Clays and Clay Min., 22, 1-22, (1974).
Sillitoe RH 1993 40: 403-417., In Kirkham RV, Sinclair WD, Thorpe RI & Duke JM eds., Mineral Deposit Modeling: Geological Association of Canada, Special Paper, 40, 403-417, (1993).
Mineral exploration of zeolites increased significantly in the past years, because of an improved market situation. Recent researches evidenced important volcaniclastic zeolitized (clinoptilolite and/or mordenite) deposits in Central Sardinia. A wide area of about 200 km2 and including the cities of Laconi to SE and Allai to NW, has been investigated to give a contribution in the evaluation of its economic potential. This area is characterized by Oligocene-Miocene volcanoclastites. Zeolites mainly occur in the altered ignimbritic facies (de' Gennaro et al., 1998). From bottom to the top, five volcanic Units were identified (Assorgia et al., 1995): Luzzana, Allai, Samugheo, Ruinas, Monte Ironi. Results of regional investigation showed considerable contents of zeolites. On the basis of a preliminary survey, a rock sampling was carried out, followed by a mineralogical investigation carried out by analytical techniques such as XRPD and optical microscopy. Further detailed investigation were performed on selected Units. Quantitative evaluation of zeolite content was carried out both by RIR and Rietveld methods (Larson and Von Dreele, 1994; Chipera and Bish, 1995).
Allai: This Unit generally shows a marked explosive character and zeolites are mainly concentrated in the pumice layers. The main mineralogical association consists of prevailing clinoptilolite, mordenite, feldspars (10-41%) and subordinate quartz and opal-CT. Quantitative analyses of zeolites show that mordenite content ranges between 0-34% while clinoptilolite ranges between 0 and 56%. Smectite is also present, ranging from 5 to 25%.
Samugheo: This Units consists of a thick strongly welded pyroclastic formation; the main minerals are feldspars (12-38%); clinoptilolite content is variable (24-63%) as well as mordenite (0-9%); quartz (6-8%) and hematite (0-1%) are minor. Smectite is always less than 10%.
Ruinas: The pyroclastic formation of Ruinas, consisting of volcanic ash with white pumice, is frequently strongly altered and very poorly welded. It mainly consists of clinoptilolite (63%) and feldspars (16%); quartz (6%) and smectite (5%) are subordinated.
Monte Ironi: Volcanic products are constituted by pumice in a welded reddish ash matrix, somewhere strongly altered. The main mineralogical association consists of feldspars (32%) and mordenite (52%), clinoptilolite (4%) and quartz (6%) are minor minerals.
Detailed mineral exploration in Central Sardinia for zeolites assessed the zeolite deposits with reserves amounting to over several Mt with a mean mordenite-clinoptilolite content of about 30-40%, preferentially concentrated in the Allai Unit. To evaluate the proper industrial employment, studies about technical properties of materials are now in progress.
Assorgia A, Balogh K, Lecca L, Ibba A, Porcu A, Secchi F & Tilocca G., Proceed. of Conv. Rapporti Alpi-Appennino, Peveragno (CN), 31 mag/1 giu, 397-424, (1995).
Chipera SJ & Bish DL, Powder Diffraction, 10, 1, 47-55, (1995).
de' Gennaro M, Langella A, Padalino G & Palomba, M, Proceed. of IV Conv. Naz. Scienza e Tecnologia delle Zeoliti, Cernobbio (Como, Italy), Sept. 7-10, Univ. dell'Insubria "A. Volta", Ed. by E. Fois and A. Gamba, 57-59, (1998).
Larson AC & Von Dreele RB, Laur, 86-748, Los Alamos National Laboratory, 1-185, (1994).
Sardinian fluorites are well-known minerals both for their several industrial employments and mineralogical association with other minerals (galena, sphalerite, argentite, barite, calcite). Fluorite from several Sardinia deposits were analyzed for REE (Rare Earth Elements) content in order to find a correlation between geochemical characteristics, genetic type, geological age and, moreover, to evaluate their potential for development of REE-bearing deposits. In fact, cations commonly found to substitute for Ca include REE and Y (Eppinger and Closs, 1990). The trivalent REE ions complex preferentially with hard ligands, containing highly electronegative donor atoms (oxygen and fluorine) (Wood, 1990).
The Paleozoic fluorite deposits originated from the Ordovician volcanism (Mazzella et al., 1979). During the Hercynian orogenesis the fluorite was mobilized and concentrated in veins. The filling of karst cavities and/or fractures began in the Permian age. The Tertiary fluorite deposits originated from the Oligocene-Miocene volcanism (Pani and Valera, 1996).
Eighteen Sardinia deposits were sampled, representative of all the genetic types belonging to mineralization of different geological age, and analyzed for REE.The solutions, prepared by selected pure fluorites crystals, were analyzed by ICP-MS. Analytical results show that:
- whole samples show variable REE (5 < (sum)REE < 222) and Y contents (4 < Y < 354). LREE are more enriched than HREE. - karst fluorite from Cambrian limestones has low REE (5 < (sum)REE < 70 ppm) and Y contents (4 < Y < 200), low REE-fractionation (0.5 < (La/Lu)n < 4.2), variable Eu anomaly (0.98 < (Eu/Eu*)n < 1.14);
- fluorite veins in the Ordovician formations show moderate REE (42 < (sum)REE < 222 ppm), high Y contents (94 < Y < 354 ppm), slight REE-fractionation (1 < (La/Lu)n < 2.9), variable Eu anomaly (0.35 < (Eu/Eu*)n < 1.38);
- fluorite from Silurian shales show moderate REE (123 < (sum)REE < 145 ppm) and Y contents (91 < Y < 126 ppm), variable REE-fractionation (2.7 < La/Lu)n < 21.9), slight positive Eu anomaly (1.17 < (Eu/Eu*)n < 1.14);
- fluorite from vein of Monte Genis, in the Hercynian granitoids, has low REE (82 ppm), high Y contents (244 ppm), low REE-fractionation ((La/Lu)n = 4.2), negative Eu anomaly ((Eu/Eu*)n = 0.45);
- Tertiary fluorites have variable REE (37 < (sum)REE < 127 ppm) and Y contents (14 < Y < 122 ppm), low REE-fractionation (1.5 < (La/Lu)n < 5.8) and variable Eu anomaly (0.69 < (Eu/Eu*) n < 2.36).
The (La/Lu)n ratios show small variations in all types of fluorites. This may suggest similar stages of fluorite precipitation. The positive and insignificant Eu anomalies may be respectively attributed to more or less intense alteration of feldspar in wall rocks (Constantopoulos,1988). The REE concentration in the Sardinia fluorites is too low towards an economic exploitation.
Constantopoulos, J., Econ. Geol., 83, 626-636, (1988).
Eppinger RG & Closs LG, Econ. Geol, 85, 1896-1907, (1990).
Mazzella A, Melis MF, Porcu R, Rivoldini A & Violo M., Res. Ass. Min. Sarda, LXXXIV (2), 39-62, (1979).
Pani D., Valera R. ()., Proceed. of "Congr. Intern. per il Centenario dell'Associazione Mineraria Sarda: 1896-1996". Sardegna Mineraria dal II al III millennio. Miniere - Cave - Ambiente. Sessione III, 323-331, (1996).
Wood SA, Chem. Geol, 82, 159-186, (1990).
Anorthositic rocks are common in several major geological provinces in Norway. Most occur at scattered localities in different parts of the country, but the two largest anorthosite complexes in Western Europe are situated in the western part of Norway. These two Precambrian massifs, the Inner Sogn - Voss province (~ 1700 Ma.), and the Egersund-Rogaland Massif (~ 1000 Ma.) have, in recent years, been investigated for use as a raw material for various industrial uses, both now and as a potential resource for the future.
For some industrial applications, the aluminium (and often also the calcium) content of the rock is of interest. A leaching process is applied to achieve liberation of the desired elements. Laboratory investigations have shown that plagioclase has a solubility which is dependent upon the An-content of plagioclase. Only anorthosites with more than 70% An (bytownite) are properly soluble in mineral acids. Such basic anorthosites are relatively rare, but the large massifs in the Sogn-Voss region contain immense quantities of easily soluble anorthosites with an Al2O3 content of about 31%. These bytownite-anorthosites have been evaluated as a potential raw material at various times since the beginning of the century, because they represent an alternative to imported bauxite as a source of aluminium 'ore' for the large Norwegian aluminium production industry. A new process is at present being studied which incorporates consumption of CO2 emissions from gas power plants. The acid soluble anorthosite will thereby give products like ammonium nitrate and calcium carbonate in addition to aluminium oxide. Other industrial products such as a polymer for cleaning of drinking and waste water and as a refractory material used in aluminium smelters are also currently under development using high An-plagioclase anorthosite from Sogn.
The 500 km2 Egersund -Rogaland Massif is dominated by an andesine - plagioclase (An40-50) and is thus not soluble in acids. The anorthosite is perhaps best known for the large ilmenite deposit at Tellnes, but the anorthosite itself is also of economic interest for several end uses. The altered white variety is being exploited for aggregate and filler purposes. Unaltered anorthosite is attractive both as dimension stone and aggregate. Finally, the rather high-silica plagioclase of the Egersund area has been established as being suitable for a new innovative industrial process where both aluminium and silicon are produced in a refined continuous electrical melting operation.
Anorthosite has a varied spectrum of industrial applications and the large Norwegian anorthosite massifs situated close to the coast will continue to be of commercial interest.
A geological and technological study was performed with the aim of apply different silicatic rocks to portland cement manufacture, near Apiaí city, south of São Paulo State, Brazil.
A medium capacity cement plant, running since 1970 decade, uses a calcitic limestone, that occurs as layers in a Proterozoic stratigraphic unit (Açungui Group), and needs blending with aluminum-silicate materials to achieve proper raw mix.
Two deposits of silicatic metassedimentary rocks were investigated, named Vieira deposit and Pirizal phyllite, the first around 2 km far from the plant and the last very close to it. Coga although with good characteristics for the desirable raw mix composition, have a bad location.
The study included field sampling, technological characterization of the materials and experimental tests in a cement laboratory. Several techniques were used to characterize the materials: XRF, to chemical composition determination; XRD, for mineralogy determination; optical microscopy and SEM, to measure the fineness of the raw materials, and EDS, for microanalysis.
Vieira deposits consists of intercalated layers of metassediments, where can be distinguished a portion richer in quartzite and another in mica-schist. The quartzite presents an inadequate amount of coarse quartz grains for cement manufacture.
Pirizal phyllite is over a dolomitic limestone deposit neighboring the cement plant. The rock is very weathered showing high porosity and low density. The quantity of coarse quartz is much lower than in the other deposit. Rich in quartz and phyllosilicates, this material shows low presence of alkalis components in its chemical composition.
Coga clay also have inadequate amount of coarse quartz. Otherwise the situation of the deposit is problematic, because of factory distance and city vicinity.
Experimental tests were carried out with the materials from Pirizal phyllite and Vieira deposit. The raw mix with small amounts of phyllite showed an easier burnability, probably due to its fineness and mineralogy.
Geology department, Ibn Zohr University, B.P. 28/S, Agadir, Morocco
The Ougnat inlier (Oriental Anti-Atlas Morroco) contains upper Neoproterozoic sediments which consist of turbiditic sequences. That was deformed by the pan-african event and later intruded by granitoids. The whole is unconformably overlain by the terminal Neoproterozoic rocks composed by three sub-tabular superposed volcano-clastic formations and surmounted by the Palaeozoic cover. The Ougnat tectonic history demonstrates that emplacement of vein mineralizations took place at three geologic events : 1) an early event of Bou-Madine massif polymetallic mineralizations occured in PIII as N160°E tension fissure associated to N30°E senestral shear zone, 2) the Hercynian event characterised by Pb-Zn-Cu mineralizations with a quartz and a carbonate gangue, controlled by an E-W Reidel system, 3) the Mesozoic event consists in an emplacement of Zn-Pb-Cu mineralizations in barytic gangue related to the N45°E extensional faults.The mineralizations indicate variable galena isotopic composition (Pb/Pb) and show difference in parageneses and structural contexts.
Baricite has been revealed during the investigation of postcarbonatite hydrothermal assemblages of the phoscorite-carbonatite complex of the Kovdor alkaline-ultrabasic complex. Barichite-bearing samples were gathered by O.M.Rimskaja-Korsakova about 40 years ago and are stored in the Museum of the Geological Institute KSC RAS, Apatity. It is a first occurrence found in Kovdor, and the only one known in the Russia and former USSR.Baricite occurs in cavities of dolomite carbonatite as 1-2 cm aggregates of platted transparent blue crystals whith greyish tint. The minerals associated with baricite are late dolomite, magnesite, collinsite and pyrite. Optical and physical properties of studied baricite are close to the described ones for the holotype specimen from fracture fillings in low-temperature assemblages (Rapid-Creek area, Yukon territory, Canada). The chemical composition was determined at a microprobe "MS-46 Cameca" (wt.%): MgO 15.7, MnO 0.1, FeO 18.9, P2O5 31.9, H2O (calc.) 33.4. The valence of iron was confirmed by wet chemical analises (permanganathometry). Empirical formulas of baricite are: (Mg1.74Fe1.17Mn0.01)2.92P2O7.92*8.23H2O,and (Mg1.72Fe1.17Mn0.01)2.90P2O7.90*7.25H2O. An X-ray diffraction pattern of baricite in the space group C2/m gives the following parameters: a=10.07(3) Å, b=13.42(2) Å, c=4.68(3) Å, ß=104.9°(2), V=610.9 Å3, Z = 2, dcalc.=2.33 ±3 g/cm3. The origin of baricite in Kovdor massif is related to by low-temperature hydrothermal activity in cataclased dolomitic carbonatites. This rare mineral occurs in lower levels of the crust of weathering enriched by Fe (besides carbonatites, phoscorittes were also weathered). Upper levels of the crust containing occurrences of vivianite. Now, only bobierrite, a practically pure Mg phosphate, occurs in non-weathered dolomitic carbonatites. This study was funded by the Russian Foundation For Fundamental Investigations (RFFI) through grant 98-05-64365.
Crandallite group minerals from hydrothermal assemblages occurring in the carbonatite-phoscorite complex of the Kovdor alkaline-ultrabasic massif are studied. Goyazite, gorceixite and crandallite occur in cavities of dolomitic carbonatites veins; crandallite was identified also in crust of weathering and in carbonate-fluorapatite bearing breccia. A natural evolution of specie-forming cations in the order Sr-Ba-Ca is defined on the basis of an analysis of space relationships of the minerals in studied assemblages. Crystalls of the studied phosphates are characterized by a similar morphology of faces. Two generations of goyazite are distinguished. Zoned fine bladed crystalls of goyazite-II have an inner zone composed of (Sr0.69Ba0.25Ca0.01)0.95 Al2.83[PO3CO0.5(OH)0.5]2 (OH)5.39 and a 5 m external zone of (Ba0.62Sr0.23Ca0.14)0.99Al3.26[PO4] [PO3OH] (OH)6.76. Chemically all the described minerals are unique due to the lack of admixture of REE in first cationic position (all analyses yielded REE abundance lower than the sensitivity of electron microprobe). X-ray diffraction patterns indicated the absence of mechanic impurities. These facts made it possible to obtain X-ray diffraction patterns and unit-cell parameters that approach JCPDS: for crandallite (space group R3 m) a=b=7.009(9) Å, c=16.15(2) Å, for goyazite (R3 m) a=b=7.021(6) Å, c=16.64(2) , for gorceixite (Cm) a=12.20(4) Å, b=7.048(8) Å, c=7.049(9) Å, ß=125.20 (1)grad. The chemical composition and cell-parameters of gorceixite are practically identical with those defined for gorceixite from Rapid-Creek formation, Yukon territory, Canada. For crandallite a significant discrepancy between dmeas.=2.91 g/cm3 and dcalc.=3.27 g/cm3, is calculated in terms of its trigonal symmetry. However, after calculating X-ray data of crandallite in space group Cm, peculiar to gorceixite values dcalc. and dmeas. are identical. This fact permits us to suppose a monoclinic distortion in the real structure of crandallite, which is probable also in in structure of goyazite.In nature there is a distinct correlation between low-temperature hydrothermal occurrences of crandallite group minerals and scandium phosphates (juonniite, Kovdor; kolbeckite, Mrima Hill; pretulite, Styria et al.). On this basis we make a conclusion about the importance of crandallite group minerals as a circumstantial evidence of secondary concentration of scandium and the possibility of formation of scandium phosphates.This study was funded by the Russian Foundation For fundamental investigations (RFFI) through grant 98-05-64365.
Scandium mineralization of a new genetic type has been discovered in the Kovdor alkaline-ultrabasic massif, Kola Peninsula, Russia. It is represented by a new mineral, juonniite, simplified formula CaMgSc(PO4)2(OH)*4H2O. This mineral is from overite group minerals according to X-ray diffraction data, physical properties and stoichiometry. The new scandium phosphate occurs in low-temperature hydrothermal assemblages in the phoscorite-carbonatite complex of the Kovdor massif. The mineral forms spherulites up to 0.8 mm in diameter and is associated with late carbonates and hydrous phosphates. Juonniite occurrences are confined to a NE-trending linear long-living fault, which intersects phoscorites and carbonatites. Metasomatism of phoscorites during late phase of formation of calcite carbonatite caused almost a two-fold Sc enrichment in clinohumite-bearing units. Late hydrothermal solutions affected cataclased rocks along the fault zone and have caused subsequent dissolution of sulfides, primary fluorapatite and Sc-bearing minerals (e.g., forsterite and Zr-Nb accessories: zircon, baddeleyte, pyrochlore), which was accompanied by mobilization of traces of Sc, most likely, in the form of carbonate and sulfate. Fluorapatite-bearing carbonatites with abundant cavities provided the necessary conditions for the precipitation of scandium in the form of a hydrous phosphate, juonniite. Isotopic data, defined on hydrothermal phosphates paragenetic to juonniite, show that hydrothermal solutions were derived from calcite carbonatites, but not from dolomitic carbonatites hosting this kind of mineralization and had were not mixed with crustal sources, e.g. meteoritic water. The recently discovered occurrences of two new phosphates Sc (the described juonnite and the pretulite from lazulite-bearing quartz vein, Stiria) have a hydrothermal genesis. This fact illustrates mobility of Sc on a postmagmatic stage in hydrothermal conditions and a possibility of formation of high Sc concentrations in late mineral assemblages.
This study was funded by the Russian Foundation For Fundamental Investigations (RFFI) through grant 98-05-64365.
Owing to use our invention the Sluice System «Goverla» (patent # 2095147, Russia) we discovered a new type of gold flour (size 0.15-0.05 mm.) placers connected with Quaternary and Cretaceous silts, slimes, sands, shelly detritus and twice only - with a gravels. All this palasomes are allocated within delta facies of the northern part of the Azove-Black Sea depression (Ukraine). Side by side the ordinary clactogenic type (namely-izometric, rounded, irregular, prolate, oblate and so on) we observed unusual morphosculpture, viz-cubic, octahedronic, prismatic, globular forms down to correct sphere, ovoid, bizarre grains with a long lugs, accretions of globules like a vine clusters, spired forms like a gastropod, thin plate with an oval bulge to periphery, etc.
Sometimes this unusual grains was shown oversize than rock-making grains of their palasomes. We suppose that these forms had pullulated due to chemogenic and biogenic processes of gold precipitation from the river-sea water surface. Industrial native gold flour deposits take place when the integration of clactogenic (terrigenic), chemogenic and biogenic processes of precipitation start up and then authigeneous gold flour localization have been realizing inside loose bottom sediments. It is poligenic natural placers.
Due to use our Sluice System 'Goverla' we finded that dredge's tailings of the Amur Region goldfields (Russia) content a big amount of finest and flour native gold (size 0.15-0.05 mm. mainly). We observed outgrowths of gold globules on rounded gold particles and on irony hobnails from old-time abandoned mines. Mean content of gold is 439 mg. per cu.m. Resources of Djalinda goldfield is 59.9 ton of pure gold (weight of extracted gold by dredges is 44.8 ton). It is technogenic-authigenic placer.
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