The age of rare-earth mineralisation in carbonatites of the Vuoriyarvi massif (NW Russia): Implications from in situ apatite Sm&#8211;Nd geochronology
Authors: Kozlov Evgeniy , Fomina Ekaterina 
Published: March 2022

&lt;p&gt;Our previous Rb&amp;#8211;Sr&amp;#8211;O&amp;#8211;C isotope data on the rare-earth carbonatites of the Petyayan-Vara area of the Vuoriyarvi massif (NW Russia) showed that these rocks were formed in two stages. The earliest burbankite-bearing carbonatites are sourced from Ba-Sr-REE-rich magnesiocarbonatite melt. The interaction of this melt with the host silicate rocks yielded high-Ti contact rocks, locally rich in apatite, HFSEs, and HREEs. An orthomagmatic S-rich fluid remobilized Ba, Sr, and REEs from burbankite-bearing carbonatites into fractures and breccia zones and deposited them as ancylite ores with strontianite and baryte. Sm-Nd isotopic data on bulk samples showed that very little time passed (by geological standards) between the intrusion of carbonatite melt and this early postmagmatic fluid REE process. The age estimates for ancylite ores (365 Ma) coincide with the age of igneous rocks. These findings are essential for understanding the reasons for the accumulation of such ores, selecting practical prospecting tools for exploration in the Vuoriyarvi massif, and searching for similar deposits.&lt;/p&gt;&lt;p&gt;We studied the Sm-Nd system and the distribution of REEs in apatite using local methods to test this hypothesis. High-Ti carbonatites were formed simultaneously with burbankite-bearing carbonatites and underwent considered fluid processing. They contain two apatite generations: early Ap-1 (almost free of admixtures) and late Ap-2 (rich in REEs, Sr, and S). The chemical specificity of Ap-2 suggests that it was formed under the action of the fluid that remobilized the REEs. Chondrite-normalized REE spectra in Ap-2 are similar to the spectra of ancylite ores with LREE&lt;sub&gt;CN&lt;/sub&gt;&gt;&gt;HREE&lt;sub&gt;CN&lt;/sub&gt; (Nd&lt;sub&gt;CN&lt;/sub&gt;&gt;Sm&lt;sub&gt;CN&lt;/sub&gt;), which confirms the above conclusion. The spectra of Ap-1 and Ap-2 are very different. The point is that Ap-2 is rich in HREEs, which results in LREE&lt;sub&gt;CN&lt;/sub&gt;&amp;#8804;HREE&lt;sub&gt;CN&lt;/sub&gt; (Nd&lt;sub&gt;CN&lt;/sub&gt;&lt;Sm&lt;sub&gt;CN&lt;/sub&gt;). Recent experimental works have shown that the interaction of carbonatite magma and Si-undersaturated silicate rocks occurring in the presence of alkalis leads to enrichment in HREEs. Thus, Ap-1 inherited the chemical components involved in the contact interaction of carbonatite melt and host rocks. Due to the contrasting differences in the Nd&lt;sub&gt;CN&lt;/sub&gt;/Sm&lt;sub&gt;CN&lt;/sub&gt; ratios for Ap-1 and Ap-2, the points of these phases in the &lt;sup&gt;147&lt;/sup&gt;Sm/&lt;sup&gt;144&lt;/sup&gt;Nd&amp;#8211;&lt;sup&gt;143&lt;/sup&gt;Nd/&lt;sup&gt;144&lt;/sup&gt;Nd diagram form two clusters, plotted far apart from each other along the &lt;sup&gt;147&lt;/sup&gt;Sm/&lt;sup&gt;144&lt;/sup&gt;Nd axis. The errorchron drawn through these clusters gives an age of 350.7&amp;#177;16.7 Ma (3&amp;#963;) at MSWD=2.3, which, within the error limits, coincides with the age estimates for both magmatism and ancylite ores. This estimate confirms the earlier assumption about the formation of ancylite ores during the late magmatic stage shortly after the magmatic event. Our results show that the in situ Sm-Nd analysis of apatite is an effective geochronological tool for assessing the age of REE ores for the case of interaction of carbonatite melt with Si-undersaturated silicate rocks and subsequent fluid processing.&lt;/p&gt;&lt;p&gt;&lt;em&gt;This research was funded by the Russian Science Foundation, grant &amp;#8470; 19-77-10039&lt;/em&gt;.&lt;/p&gt;