Hydrothermal fluids of apatite-magnetite ores of the Tomtor carbonatite massif (NE, Russia)
Authors: Baranov Leonid , Tolstov Alexander , Prokopyev Ilya 
Published: March 2020

&lt;p&gt;The Tomtor carbonatite complex, with an area of 250 km&lt;sup&gt;2&lt;/sup&gt;, 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&lt;sup&gt;o&lt;/sup&gt;) 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.&lt;/p&gt;&lt;p&gt;Apatite contains a multiphase (crystal-fluid) inclusions with gas, liquid and 1-5 visible crystalline phases. The gas phase is represented by CO&lt;sub&gt;2&lt;/sub&gt;, contains subordinate amounts of H&lt;sub&gt;2&lt;/sub&gt;O, H&lt;sub&gt;2&lt;/sub&gt;S and SO&lt;sub&gt;2&lt;/sub&gt;. The liquid phase is represented by H&lt;sub&gt;2&lt;/sub&gt;O with SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;, HSO&lt;sup&gt;4-&lt;/sup&gt; and HCO&lt;sup&gt;3-&lt;/sup&gt; ions. The solid phases in the inclusions are represented by mainly halite (NaCl) and sylvite (KCl), with strontianite (SrCO&lt;sub&gt;3&lt;/sub&gt;), barite (BaSO&lt;sub&gt;4&lt;/sub&gt;) and Ca-Sr-REE F-carbonate crystals. Complete homogenization occurs in the temperature range from 290 to 350 &amp;#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&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;), strontianite (SrCO&lt;sub&gt;3&lt;/sub&gt;), REE phosphates and sulfates of Sr and Ba. Complete homogenization occurs at 250-300 &amp;#176;C, the concentration is 35-55 wt. % of NaCl-eq. The gas phase of the fluid inclusions in K-feldspar is predominantly CO&lt;sub&gt;2&lt;/sub&gt;; the liquid phase is H&lt;sub&gt;2&lt;/sub&gt;O. The solid phases are represented by witherite (BaCO&lt;sub&gt;3&lt;/sub&gt;) and calcite (CaCO&lt;sub&gt;3&lt;/sub&gt;). The homogenization temperature of fluid inclusions occurs at 350-375 &amp;#176;C.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;The work was supported by the Russian Science Foundation (RSF), project # 19-17-00013.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;ol&gt;&lt;li&gt;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&amp;#8211;693.&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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