Formation conditions for magnetite of phoscorites of the Tomtor massif (NE, Russia)
Authors: Baranov Leonid , Tolstov Alexander 
Published: March 2022

&lt;p&gt;The Tomtor carbonatite complex with the area of 250 km&lt;sup&gt;2&lt;/sup&gt; 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&lt;sup&gt;o&lt;/sup&gt;) 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).&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;Magnetite forms massive accumulations with hypidiomorphic crystals up to a few centimeters in size. Magnetite contains (in wt%): TiO&lt;sub&gt;2&lt;/sub&gt; (1,21-4,72), MnO (0,48-1,9), MgO (0,08-0,41); Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (&amp;#1076;&amp;#1086; 0,14); BaO (&amp;#1076;&amp;#1086; 0,32); ZnO (0,06-0,53); V&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (0,25-0,52).&lt;/p&gt;&lt;p&gt;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&lt;sub&gt;2&lt;/sub&gt;-Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-FeO. Ilmenite has a significant range of Mn contents (1.34-14); it may contain MgO (up to 1.57), Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (up to 0.21), BaO (up to 1.09), ZnO (up to 0.2), V&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (up to 0.2).&lt;/p&gt;&lt;p&gt;It was established that the temperatures of magnetite formation create a continuous series from 459 to 914 &amp;#176; C; oxygen fugacity (&lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt;) varies respectively in the range from -10 to -24. These data confirm the magmatic nature of magnetite.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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).&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;ul&gt;&lt;li&gt;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&amp;#8212;106. https://doi.org/10.32454/0016-7762-2020-63-5-96-106 (in Russian).&lt;/li&gt;
&lt;li&gt;Baranov, L.N., Tolstov, A.V., Okrugin, A.V., &amp; 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).&lt;/li&gt;
&lt;li&gt;Tolstov, A.V., 1994. Mineralogy and geochemistry of apatite-magnetite ores of the Tomtor Massif (NorthwesternYakutia). Russ.Geol. Geophys.35,76&amp;#8211;84.&lt;/li&gt;
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