Kutnahorite from carbonatites of the Bol’shetagninskiy massif (East Sayan, Russia)

All-Russian Scientific-Research Institute of Mineral Resources (VIMS); vims-sokol@mail.ru

The paper reports newly obtained data on kutnahorite CaMn(CO₃)₂ from fluorite carbonatite of the Bol’shetagninskiy massif. Optical microscopy, X-ray phase analysis, and microprobe analysis were employed to the distinguish mineralogical type of kutnahorite-bearing carbonatites and identify minerals accompanying kutnahorite, including such Mn-rich carbonates as calcite, siderite, dolomite, and rhodochrosite. According to its chemical composition, the kutnahorite was classified with its ferrous variety (7.70-11.71 wt % FeO).

Kutnahorite is a Ca and Mn carbonate of the dolomite group, whose idealized formula is written as CaMn(CO₃)₂and which was named after its type locality of the Kutná Hora deposit of Mn ores in Bohemia (Bukowsky, 1901). The mineral was then found in several regions worldwide, including the United States, Japan, Brazil, and Russia (Khibina massif in the Kola Peninsula and the Urals), at which it was produced by various geological processes: hydrothermal, carbonatite, metamorphic, and sedimentary.

In the carbonatite of the Bol’shetagninskiy massif, which belongs to the formation of alkaline-ultrabasic rocks, kutnahorite was first described by M.Ya. Somina (1975). In studying fluorite mineralization related to a carbonatite stock in this massif, the author obtained data on kutnahorite of carbonatite genesis (Sokolov, 2007, 2008).

The kutnahorite-bearing carbonatites can be classified in a number of types according to their mineralogical composition. The most widely spread type is bicarbonate carbonatite with albite or microcline, whose quantitative proportions of calcite and kutnahorite broadly vary. The feldspar-free manganoan siderite-kutnahorite (± manganiferous calcite) and manganiferous calcite-kutnahorite (± manganiferous siderite) varieties occur not as widely, and the practically monocarbonate kutnahorite carbonatite is even more rare. Kutnahorite with fluorite, calcite, feldspars (albite and microcline), and apatite are major rock-forming minerals and account for 15-20 to 40% of the rocks by mass; the rocks may locally contain as much as 60-70% kutnahorite. All of the aforementioned carbonatite types contain minor amounts of hematite and pyrite.

The carbonatites usually contain kutnahorite as fine-grained aggregates, most commonly in association with calcite. These rocks can be readily distinguished thanks to their color: calcite is white, whereas kutnahorite is pale beige to dark brown. Under an optical microscope, kutnahorite is brownish-gray, in contrast to colorless calcite, and has higher birefringence and well-defined pseudo-absorption.

The calcite-dominated carbonatites contain kutnahorite in the form of subhedral crystals of subrhombohedral habit of size 0.2-0.3 mm across. It is worth mentioning that fluorite is concentrated mainly in calcite-rich domains and is notably less abundant in association with kutnahorite.

The locally fine-grained fabric of the carbonatites (whose mineral grains are rather common 10-20 µm) led us to use X-ray phase analysis (analyst I.S. Naumova, VIMS) and JXA-8100 Superprobe microprobe (analyst N.I. Chistyakova, VIMS) for their investigation. This allowed us to more accurately evaluate the mineralogical composition of the rocks and reveal that kutnahorite is contained in them in association with various micrometer-sized minerals. It was determined that all of the carbonatite types contain pyrochlore, Ti-columbite was identified only in the microcline type; zircon was found only in the calcite-kutnahorite type; and Nb-rutile, V-ilmenorutile, and REE fluorcarbonates (bastnaesite and parisite) are typical of feldspar-free varieties.

Our analyses indicate that the carbonatites contain a widespread group of Mn-rich carbonates. Along with kutnahorite, these are manganiferous calcite (1.96-2.69% MnO) and manganocalcite (6.43-7.04%), manganiferous siderite (4.66-11.89%) and manganoan siderite (22.79-28.06%), Fe-Mn dolomite (8.89-10.06%), and rhodohrosite (50.79-52.88%). This association suggests that the fluorite carbonatites of the massif were produced in a Mn-rich environment. It is pertinent to recall that carbonatites of other massifs in the Eastern Sayan (Beloziminskiy and Sredneziminskiy) and in most alkaline-ultrabasic massifs in the Kola Peninsula do not contain any widespread Mn-rich carbonates.

According to X-ray diffraction data (analyst G.K. Krivokoneva, VIMS), the kutnahorite belongs to the series between dolomite-ankerite, on the one hand, and kutnahorite, on the other, being notably closer to the latter. Microprobe analyses (Table 1) indicate that the mineral is a Fe-rich and Mg-poor variety of kutnahorite. Its composition is comparable with that of ferroan kutnahorite from ankerite-rhodochrosite carbonatites of the Khibina massif (Zaitsev, 1996), which contains roughly the same FeO concentration (8.11-11.38%) as the mineral from the Bol’shetagninskiy massif but less MgO (0.97-2.68%) and more MnO (17.47-21.52%). X-ray and chemical data confirm the viewpoint (Essene, 1987) that a broad interval of solid solutions is stable between the dolomite-ankerite series and kutnahorite.

The textural relations revealed in hand-specimens and petrographic thin sections suggest that the mineralogical composition of the carbonatites evolved, and this pertains, first and foremost, to the crystallization succession of the carbonates. From the geochemical standpoint, this provides evidence of the evolution of the proportions of the mineral-forming cations from calcite (Ca) through Fe-Mn dolomite and kutnahorite (Ca, Mn, Fe, and Mg) to siderite (Mn, Fe), and rhodochrosite (Mn) and corresponds to gradual replacement of strength bases by weak ones. The earlier crystallization of abundant calcite suggests that the mineral-forming medium was relatively highly alkaline. According to D.S. Korzhinskiy, a decrease in the activity of basic elements at decreasing temperature provides grounds to believe that the acidity gradually increased. A decrease in the alkalinity of the solutions also follows from the increase in the number of sulfide species (sphalerite and galena crystallized simultaneously with pyrite) and their total amount and from the crystallization of REE fluorcarbonates. The latest low-temperature crystallization of the kutnahorite carbonatites was associated with an increase in the alkalinity and the oxidation potential, with the Mn-calcite and kutnahorite replaced by pyrolusite and the pyrite + hematite association replaced by pyrite + barite (Sorokin et al., 1997).

Table 1. Chemical composition (wt. %) and end members (mol. %) of Fe-kutnahorite and Fe-Mn-dolomite

Mineral	Fe-kutnahorite		Fe-Mn-dolomite
Mineral	Range	Average (n=8)	Range	Average (n=3)
CaO	26.16-29.80	28.49	27.75-30.41	29.00
MgO	3.45-5.69	4.38	7.06-7.92	7.59
FeO	7.70-11.51	9.48	7.28-11.99	9.94
MnO	13.86-15.63	14.65	8.89-10.06	9.62
CO₂	41.57-42.51	42.03	42.94-43.27	43.09
Total		99.03		99.24
CaCO₃		53.2		52.8
MgCO₃		11.4		19.2
FeCO₃		13.8		14.15
MnCO₃		21.6		13.85
Total		100.0		100.0

Contents of SrO, BaO, REE₂O₃, and Y₂O₃ in all analyses are below the detection limits.

CO₂concentrations, end member proportions, and cation proportions were calculated by the author.

Bukowsky A. Kuttenberger manganmineralien. Anzeiger Der III Congr. Böhm. Naturforsch. und Ärzte. Prague. 1901. 293 s.

Essene E.J. Solid solutions and solvi among metamorphic carbonates with applications to geologic thermobarometry. // Carbonates: mineralogy and chemistry. Reviews in Mineralogy. 1983. Vol. 11. P. 77-96.

Sokolov S.V. New data on mineralogy of fluorite ores of the Bol’shetagninskiy deposit. // The function of mineralogy in knowledge of ore-forming processes. M.: IGEM RAS, 2007. P. 306-311 [in Rus.].

Sokolov S.V. Fluorite carbonatites of the Bol’shetagninskiy massif. // Geochemistry of magmatic rocks. Proceedings of the XXV All-Russian Seminar. School « Alkaline magmatism of the Earth». Abstract Volume. SPb: SPbSU. 2008. P. 144-145 [in Rus.].

Somina M.Ya. Dolomite and ankerite carbonatites of the Eastern Siberia. M.: Nedra. 1975. 191 p. [in Rus.].

Sorokin V.I., Dadze T.P., Kashirtseva G.A. Sulfide-Sulfate Relationships in the Hydrothermal Process // Petrology. 1997. No 1. P. 63-72 [in Rus.].

Zaitsev A.N. Rhombohedral carbonates from carbonatites of the Khibina massif, Kola Peninsula, Russia. // Canadian Mineralogist. 1996. Vol. 34. Pt. 2. P. 453-468.