2011

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Genesis of apatite ores from the Magan alkali-ultrabasic carbonatite massif (northern Siberia): data of melt inclusion study

L.I. Panina, A.T. Nikolaeva, E.Yu. Rokosova

Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, Novosibirsk, Russia panina@igm.nsc.ru

Melt inclusions in apatite from ijolite veins (ijolite type) and exocontact aegirinites (fenite type) from the Magan massif have been studied. Both types of apatite mineralization were found to contain alkaline sulfate-carbonate inclusions, and fenite type also contained alkaline aluminosilicate inclusions. A conclusion is made that apatite in ijolite type was the result of autometasomatic influence of alkaline sulfate-carbonatite melts on ijolites, and the formation of fenite type is related to the contamination of host quartzites by primary magma and further effect of carbonatite melts.

The Magan massif is part of the Maimecha-Kotui alkali-ultrabasic carbonatite province (northern Siberia) and is one of the three most promising apatite deposits of the region. The shape of the massif is an elongated oval stretching in circummeridium direction for 9 km, widening toward the north (6.5 km) and narrowing toward the south (4 km).

The central part is occupied by the rocks of ijolite-jacupirangite series that make up to 80 – 85 % of the massif area. Here one can also observe a small (about 1 km in diameter), nearly isometric, stock of dolomite carbonatites which borders on the field of foskorites and magnetitites. In many places ijolites are cut by submeridian veins of phlogopitized ijolite pegmatites and later dykes of microijolites. In the northern peripheral part, cancrinite-nepheline syenites occur in ring fractures. Among ijolites there are small, isometric fragments (xenoliths) of altered olivinites. Around the body of ijolite-jacupirangites, at the contact with host quartz sandstones of the Riphean one can observe a thick (70 – 600 m) zone of apatite-bearing aegirinites (fenite type of apatite ores). The zone is the most promising for apatite mineralization, whose resources, according to different estimations, are about 50 – 150 million tons P₂O₅. The zones consist of several subzones, which are replaced from the exocontact of intrusion in the following sequence: nepheline aegirinites, K-feldspar-nepheline aegirinites, aegirine-apatite ores, tveitasites, syenite- porphyries, quartzites. The rocks are cut by the veins of apatite-bearing ijolites. Moreover, the body of ijolite-melteigites itself in the massif is apatite-bearing (ijolite type of ores).

Some researchers (Egorov, 1976, 1991) think that the fenite-type apatite ores were formed under the influence of pneumotolite-hydrothermal solutions of ijolite intrusion on host rocks, and the ijolite-type ores resulted from autometasomatic transformation of ijolites.

Using the methods of thermobarogeochemistry, we studied inclusions in apatite from ijolite veins of the exocontact zone in the Magan massif and in apatite from aegirinites hosting these veins.

Apatite from ijolite veins was found to contain salt alkaline sulfate-carbonate inclusions of prismatic and irregular shape. Their sizes range from 10х12 to 25х50 μm. Homogenization of inclusions proceeded at 980 – 1080 ^оС. The average chemical composition of inclusions, according to microprobe analysis, (wt.%): 44.8 CaO, 8.6 Na₂O, 0.9 K₂O, 3.3 P₂O₅, 5.4 SO₃, 0.9 SrO, 0.4 BaO and hundredth fractions % TiO₂, MgO, Cl. In terms of normative composition they contained (%) 72.3 calcite, 8.5 gregorite (Na₂CO₃), 8.3 thenardite (Na₂SO₄), 1.7 arcanite (K₂SO₄), 1.3 strontianite (SrCO₃), and 0.5 witherite (BaCO₃). In other words, their composition is very similar to alkaline carbonatite lavas of Oldoinyo Lengai (Tanzania).

Apatite of exocontact aegirinites also contained the alkali sulfate-carbonate inclusions and inclusions of alkali aluminosilicate composition. The chemical composition of alkali salt inclusions and their homogenization temperatures were similar to those in apatite of ijolite veins. The inclusions of alkali aluminosilicate compositions homogenize at temperatures conisderably higher than 1000 ^оС. Their chemical composition varies (wt.%): 46.6 – 54.2 SiO₂, 27.9 – 33.1 Al₂O₃, 0.98 – 1.47 CaO, 4.6 – 10.3 Na₂O, 0.6 – 1.04 K₂O, 0.2 – 0.3 P₂O₅. The normative composition of these inclusions contains (%) from 3 to 30 orthoclase, 40 – 70 albite, 5 – 7 anorthite up to 1 apatite, and up to 20 corundum.

The existence of inclusions of alkali aluminosilicate and alkali sulfate-carbonate composition, which are enriched in phosphorus, in apatite from exocontact aegirine-apatite rocks suggests that the formation of aegirine-apatite mineralization resulted from the contamination of host quartzites by primary alkali-ultrabasic magma. The magma was immiscible with the alkali-sulfate-carbonatite melt which was not spatially isolated from it yet. During the interaction of alkali-ultrabasic magma with quartzites, the magma underwent loss of alkalies, Mg, Fe, Ca and enrichment in Si at inert behavior of Al. As a result, at first aegirinite rocks formed from contaminated magma, after which the melt had an alkali aluminosilicate composition, from which further tveitasites and syenite-fenites formed. It is interesting that in terms of the normative composition, the alkali-aluminosilicate melts could be the source of crystallization not only of feldspars and alkali carbonates but also of a rather great amount of corundum. It is worth noting that we found the corundum crystallites in apatite grains from aegirinites. More detailed studies of exocontact rocks will, probably, reveal the occurrence of corundum as a rock-forming mineral in these rocks. Notice that corundum was earlier found in the Vishnevye mountains of the Urals in the exocontact zones of alkali pegmatites occurring in fenites and gneisses. These pegmatites are surrounded by a zone of feldspar and feldspar-corundum composition, which is accompanied by a change in the primary mineral composition of host rocks (Es’kova, Zhabin, Mukhitdinov, 1964).

The enrichment of aegirinites in apatite at the Magan massif, most likely, resulted from the drastic increase in acidity in contaminated melts, which was accompanied by a drastic decrease in temperature. The interaction of contaminated melt with alkali sulfate-carbonatite melt also contributed significantly to the formation of apatite mineralization of this type. The occurrence of alkali sulfate-carbonate inclusions in apatite of ijolite veins in aegirinites explicitly evidences the direct genetic relationship between their apatite content and autometasomatic influence of salt carbonatite melt on ijolites.

This study was financially supported by RFBR grant 11 – 05 – 00283.

Литература:

Egorov L.S. Apatite resources of northern Siberia. Leningrad: NIIGA, 1976. 187 p.

Egorov L.S. Ijolite-carbonatite plutonism. Leningrad: Nedra, 1991. 260 p.

Es’kova E. M., Zhabin A. G., Mukhitdinov G. N. Mineralogy and geochemistry of trace elements from the Vishnevye mountains. Moscow: Nauka, 1964. 318 p.