<p>The Tomtor carbonatite complex, with an area of 250 km<sup>2</sup>, is confined to the eastern framing of the Anabar Anteclise; it is located withtin the Ujinsky province of ultrabasic alkaline rocks and carbonatites (Northeast of Siberian Platform) (Erlich, 1964). The complex has a concentric zonal structure: the outer ring is composed of alkaline and nepheline syenites, the inner incomplete ring is nepheline-pyroxene rocks of the foidolite family, the core is represented by carbonatites. All rocks of the massif are intersected by dikes and explosion tubes of picrites and alneites. Onkuchakh apatite-magnetite deposit is located on the northeastern border of the carbonatite core. Apatite-magnetite ores (camaforites, phoscorites, nelsonites) form a series of ore steeply dipping (75-80<sup>o</sup>) lenticular bodies of north-western strike. The resources of the apatite-magnetite ores of the Tomtor massif are about 1 billion tons of iron (Tolstov, 1994). Primary and pseudo-secondary fluid inclusions were studied in apatite, calcite and potassium feldspar of camaforites. Inclusions have isometric or elongated shapes up to 50 microns. Most of the studied inclusions have a negative crystal form located in the central parts and zones of apatite growth.</p><p>Apatite contains a multiphase (crystal-fluid) inclusions with gas, liquid and 1-5 visible crystalline phases. The gas phase is represented by CO<sub>2</sub>, contains subordinate amounts of H<sub>2</sub>O, H<sub>2</sub>S and SO<sub>2</sub>. The liquid phase is represented by H<sub>2</sub>O with SO<sub>4</sub><sup>2-</sup>, HSO<sup>4-</sup> and HCO<sup>3-</sup> ions. The solid phases in the inclusions are represented by mainly halite (NaCl) and sylvite (KCl), with strontianite (SrCO<sub>3</sub>), barite (BaSO<sub>4</sub>) and Ca-Sr-REE F-carbonate crystals. Complete homogenization occurs in the temperature range from 290 to 350 &#176;C, the concentration is 30-45 wt. % of NaCl-eq. Calcite has the similar in composition fluid inclusions. The solid phases are mainly represented by halite (NaCl) and sylvite (KCl), as well as the dolomite (CaMg(CO<sub>3</sub>)<sub>2</sub>), strontianite (SrCO<sub>3</sub>), REE phosphates and sulfates of Sr and Ba. Complete homogenization occurs at 250-300 &#176;C, the concentration is 35-55 wt. % of NaCl-eq. The gas phase of the fluid inclusions in K-feldspar is predominantly CO<sub>2</sub>; the liquid phase is H<sub>2</sub>O. The solid phases are represented by witherite (BaCO<sub>3</sub>) and calcite (CaCO<sub>3</sub>). The homogenization temperature of fluid inclusions occurs at 350-375 &#176;C.</p><p>The results show that the hydrothermal fluids of camaforites of the Tomtor massif are represented by the concentrated high-medium temperature sulfate-carbonate-chloride solutions of complex composition . The fluid composition is explained by the evolution of the carbonatite melt.</p><p>The work was supported by the Russian Science Foundation (RSF), project # 19-17-00013.</p><p>References</p><ol><li>Erlich, E.N., 1964. The new province of alkali rocks on the north of Siberian platform and its geological aspects. Proc. All-Soviet Mineral.Soc.93,682&#8211;693.</li> <li>Tolstov, A.V., 1994.Mineralogy and geochemistry of apatite-magnetite ores of the Tomtor Massif (NorthwesternYakutia). Russ.Geol. Geophys.35,76&#8211;84.</li> </ol>
The northeastern Siberian platform (Republic of Sakha, Yakutia) hosts the Udzhinskaya province of alkaline ultrabasic massifs with carbonatites as final phases of magmatic system evolution; they form i ts central carbonatite core and are characterized by elevated Fe, Al and P concentrations. They also contain a complex of rare and rare-earth elements. Crust of laterite weathering of up to 400 m thick is present within the massifs. Phosphate, Nb, Y, Sc and TR content in crust of carbonatite weathering is much higher compared to unaltered rock differences. Their maximum values are in the sequence of specific sedimentary deposits f ormed a s denudation products of ore-bearing carbonatite crust precipitated in minor lake depressions and due to their intensive chemogenic transformation in hot humid climate. They are unique high-grade ores, with no world analogs in terms of mineral potential. Sometimes, these rocks are their natural concentrates averaging (in weight %) 7,21 Nb2O5, 0,578 Y2O3, 0,045 Sc2O3 and 10,16 TR2O3. The rocks composing the ore-bearing sequence show distinct evidence of sedimentary genesis: well-pronounced layered texture and facial zoning, presence of carbonized vegetable detrite and bacteriomorphic aggregates. Therefore, it is reasonable to regard a set of these formations as an independent stratigraphic unit, Tomtor sequence. Geological data suggest that it formed 340-280 Ma. Tomtor sequence can be an important prospecting criteria in prospecting for rare and rare-earth elements.
<p>The Tomtor carbonatite complex with the area of 250 km<sup>2</sup> is confined to the Eastern framing of the Anabar Anteclise. It is located in the Udja province of ultrabasic alkaline rocks and carbonatites (Northeast of Siberian Platform). The Tomtor apatite-magnetite deposit is located on the Northeastern border of the carbonatite core. Apatite-magnetite ores (camaforites, phoscorites, nelsonites) form a series of ore steeply dipping (75-80<sup>o</sup>) lenticular bodies of the Northwestern strike. The resources of the apatite-magnetite ores of the Tomtor massif are about 1 billion tons of iron (Tolstov, 1994).</p><p>The subject of research is magnetite with ilmenite decomposition structures, which composes up to 70% of phoscorite. The microprobe analysis established the compositions of 34 grains of magnetite isolated from the core of well No. 801; and ilmenite, which forms decomposition structures in these grains. Based on the compositions, the temperatures of their formation and oxygen fugacity were calculated.</p><p>Magnetite forms massive accumulations with hypidiomorphic crystals up to a few centimeters in size. Magnetite contains (in wt%): TiO<sub>2</sub> (1,21-4,72), MnO (0,48-1,9), MgO (0,08-0,41); Cr<sub>2</sub>O<sub>3</sub> (&#1076;&#1086; 0,14); BaO (&#1076;&#1086; 0,32); ZnO (0,06-0,53); V<sub>2</sub>O<sub>3</sub> (0,25-0,52).</p><p>Ilmenite varies within a wide range in the content of hematite minal (2.15 - 62.19%), corresponding to the ilmenite-hematite trend on the diagram in the coordinates TiO<sub>2</sub>-Fe<sub>2</sub>O<sub>3</sub>-FeO. Ilmenite has a significant range of Mn contents (1.34-14); it may contain MgO (up to 1.57), Cr<sub>2</sub>O<sub>3</sub> (up to 0.21), BaO (up to 1.09), ZnO (up to 0.2), V<sub>2</sub>O<sub>3</sub> (up to 0.2).</p><p>It was established that the temperatures of magnetite formation create a continuous series from 459 to 914 &#176; C; oxygen fugacity (<em>f</em>O<sub>2</sub>) varies respectively in the range from -10 to -24. These data confirm the magmatic nature of magnetite.</p><p>Magnetite is the main and one of the highest-temperature minerals of the Tomtor phoscorites. Accordingly, the upper limit of the obtained temperatures is the minimum for fractionation of the P-Fe melt and characterizes the onset of crystallization of phoscorites.</p><p>The obtained results confirm the magmatic nature of the phoscorites of the Tomtor massif from the initial P-Fe melt with the participation of the crystallization differentiation mechanism and reaffirm the conclusions of previous studies based on the results of studies of the mineralogical-geochemical (thermo-barometric) and structural and textural features of apatite-magnetite ores (Baranov, 2018; Baranov, 2020).</p><p>References</p><ul><li>Baranov L.N., Tolstov A.V. Typomorphic features of magnetite from tomtor massif camaphorites. Proceedings of higher educational establishments. Geology and Exploration. 2020;63(5):96&#8212;106. https://doi.org/10.32454/0016-7762-2020-63-5-96-106 (in Russian).</li> <li>Baranov, L.N., Tolstov, A.V., Okrugin, A.V., & Sleptsov, A.P. (2018). New in mineralogy and geochemistry of apatite-magnetite ores of the Tomtor massif, northeast of the Siberian platform. Ores and metals, (2) (in Russian).</li> <li>Tolstov, A.V., 1994. Mineralogy and geochemistry of apatite-magnetite ores of the Tomtor Massif (NorthwesternYakutia). Russ.Geol. Geophys.35,76&#8211;84.</li> </ul>
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