THE GEOCHEMICAL CHARACTERISTICS OF THE CARBONATITE BRECCIAS OF THE SALLANLATVA COMPLEX: A KEY TO UNDERSTANDING THE NATURE OF THE ORE-BEARING FLUIDS
E.N. Kozlov1, E.N. Fomina1, M.Yu. Sidorov1
1Geological Institute of the Kola Science Centre of the Russian Academy of Sciences, Apatity, Russia
Keywords: Carbonatites, breccias, rare earth elements, apatite, fluids, Sallanlatva, Kola Alkaline Province
Abstract
This paper presents the results of a geochemical study of carbonatite breccias from the Sallanlatva massif (Devonian Kola Alkaline Province). It has been established that, despite the three-stage history of deformation, only the second and third stages of the associated fluid influx significantly influenced the petrochemical characteristics of the rocks. The second stage involved the contribution of Ca, Mg, Na, Si, Al, P, S, high-field strength elements (excluding Th), and heavy rare-earth elements (REE). The third stage featured the enrichment in Ba, Sr, S, light REE, and Th. The results revealed no significant fluid-mediated transport of Fe and Mn: the main source of these elements was the siderite in the breccia clasts, and their redistribution occurred via dissolution–reprecipitation reactions. Our research has established that the redistribution of REE was controlled by two processes occurring at different stages: the enrichment in heavy REE is associated with hydrothermal apatite mineralization (the second stage), whereas the concentration of light REE is related to the superimposed ancylite mineralization (the third stage). A consistent geochemical trend for breccias and carbonatites in the Ba–Sr–REE coordinate system indicates that the enrichment processes affecting the massif to a depth of the first few hundred meters were of a common pattern. The crystallization of the main ore phase—ancylite—occurred at temperatures of ~250–300 °C, and the rate of its precipitation decreased upon cooling of the system and the depletion of rare earth elements, giving way to the precipitation of strontianite and baryte. The joint transfer of Ba and S was facilitated by the reducing properties of the fluid, whereas the extensive deposition of baryte and pyrite was driven by the disproportionation of dissolved sulfur compounds (S⁴⁺) during cooling. The proposed model of fluid system evolution, involving deep fractionation of the alkaline-carbonate melt, the generation of phosphorus-enriched residual fluid, and the subsequent vertical transport of Ba, Sr, and light REE by late aqueous carbonic‑sulfurous solutions, explains the rare-metal metallogenic specialization not only of the Sallanlatva massif but also of other carbonatite massifs in the Kola Alkaline Province.
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