2011

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Evolution of the residual liquid in syenites of Azov ore-bearing stock (fluid inclusion data)

Kulchytska G., Chernysh D.

Semenenko Institute of Geochemistry, Mineralogy and Ore Formation, Kyiv, Ukraine; kulchec@igmof.gov.ua

Anorthoclase-micropertite from syenites preserve two types of primary inclusions and four types of diagenetic inclusions. The last ones were formed in diagenetic stage as the result of segregation of admixture atoms inside host crystal. Inclusions evidence the increase of oxygen potential and water content in residual liquid while the temperature decreases.

Introduction. Azov stock in the Eastern Azov area is known by its rich deposits of zirconium-rare earth ores (Меlnikоv еt аl., 2000a). The stock consists of two varieties of monofeldspathic (hypersolvus) pegmatoidal syenites: melanocratic in the external zone and leucocratic in the interior. Common rock minerals of the external zone are highly ferriferous amphibole (hastingsite) and alkali feldspar, the minor are fayalite, hedenbergite, annite, fluorite, and quartz. Alkali feldspar predominates in the interior zone (95%), the other minerals are annite, fluorite, quartz, magnetite, siderite. Alkali feldspar is represented by anorthoclase-micropertite (AmP) that contains at about 70% of albite phase. Such composition corresponds to the temperature minimum composition in the Ab-Or system (700 °C). Ore minerals, such as zircon, britholite, allanite, bastnaesite are concentrated in melanocratic variety of syenites, the so-called “taxites”, in the contact with the leucocratic. There is no consensus on the genesis of the deposit. Mineralogical finds of the inclusions with crystallized silicate and salt melts in zircon, fluorite and anorthoclase (Melnikov et al, 2000b) settled the dispute of the adherents of the metasomatic (Strekozov et. al., 1998) and magmatic hypotheses of ore formation in favour of the last.

Primary syngenetic inclusions. Two types of primary crystallized melt inclusions were found in hastingsite and AmP of "taxites": (1) the inclusions in the form of host mineral negative crystals filled with polyphase aggregates of minerals that crystallized after the host mineral. Neither liquid nor gas were found in these inclusions. Effect of crystallization differentiation of melt, which resulted in zonal arrangement of minerals, is clearly visible in one of the large inclusions (> 1 mm) in AmP. Iron oxides (magnetite, hematite) replace highly ferriferuos mica from the top to the middle of the negative crystal, and the residual portions of the liquid were crystallized like the fluorite-albite eutectic; (2) globular inclusions of fluorite (± albite) that are very common in AmP from melano- and leucocratic syenites, and are rare in the amphibole and annite. Most likely, they are drops of immiscible fluoride or fluoride-silicate melt which separated from the silicate magma, as the permissible concentration of halogens in it was exceeding.

Secondary diagenetic inclusions. In addition to halogen globules and rare inclusions in the form of negative crystals, AmP contains a lot of other inclusions. These are simpler in composition and more diverse in form. All inclusions consist of solid phases only with no visible traces of liquid or gas. Four morphological types of inclusions were distinguished (Kulchitska, Melnikov, 2008): inclusions-laths, inclusions-boxes, inclusions-disks and inclusions-polyhedrons. The size of the inclusions in this line reduces sequentially. The length of laths reaches the first millimetres while the size of polyhedrons does not exceed 20 microns. Despite the diversity of inclusions, their forms are confined to crystallographic elements of the host mineral. Every inclusion is more or less flattened in accordance with anorthoclase perfect cleavage. However, none of these types of inclusions can be classified as secondary epigenetic inclusions “healing” cracks along the cleavage. The location and size of some of them indicate more in favor of primary inclusions. Accordingly to their peculiarities all four types of inclusions were classified as secondary diagenetic inclusions (Kulchitskaya, Chernysh, 2009). Anorthoclase during rapid crystallization absorbed large quantity of admixture atoms, which segregated in the diagenetic stage in the consolidated host mineral and formed the secondary inclusions with residual liquid. Crystallization of residual liquid occurred before the formation of perthites. Following this perthitization dissolved the external boundaries of diagenetic inclusions. They remained intact in rare cases, when residual liquid crystallized after feldspar disintegration.

Green silicate predominates in the inclusions-laths. It is displaced by albite (± fluorite, rarely ± calcite) at the end or in the middle of the laths. In spite of visual similarity the optical and x-ray characteristics of lath green silicates vary considerably. They are often x-ray amorphous while the others correspond to alkali amphiboles (catophorite, arfvedsonite, riebeckite), annite, stilpnomelane, i.e. silicates with high Fe²⁺ proportion. These and other facts enable to suggest that the inclusions-laths in AmP of melanosyenites are not captured amphibole crystals, but rather crystallization product of highly ferriferous fluor-bearing alkali silicate melt in the channels. The last ones usually appear in the front of crystal face during its rapid movement in supersaturated conditions. This is the earliest type of diagenetic inclusions, similar to primary inclusions of residual liquid.

Inclusions-boxes are represented by annite flakes (200×200×10 microns) with irregular external contours and leucocratic core filled with KAlSi₃O₈, NaAlSi₃O₈ and CaF₂ in different proportions. Admixture of other minerals is negligible. Inclusions-boxes are located exclusively on the (001) anorthoclase plane. They were formed a little later than inclusions-laths. Intermediate types of the inclusions are known between them. Boxes have preserved F-Fe-K-Na-silicate melt extruded into a perfect cleavage plane of the host mineral. After annite crystallized along the perimeter of the inclusion, remaining in the centre residual liquid of approximately the same volume gave rise to the leucocratic core.

Inclusions-disks are flattened along [010] axis of anorthoclase crystal. Sometimes their form transforms to rhomboid plates. Disks and rhomboid plates consist of fluorite (25–50 %), Fe silicate (0–5 %) and feldspar. Fluorite forms rounded crystals dendrite in the feldspar matrix, the boundaries of which were “dissolved” during perthitization. Iron silicate flakes located at the inclusion periphery enable to define the true shape and size of inclusions-disks (10 to 100 microns). They have preserved halogen-silicate liquid with a small Fe percentage. The formation of inclusions-disks is synchronized with formation of inclusions-boxes, but the inclusions-disks do not occur with them on the plane (001). The inclusions-disks fixed the process of separation of immiscible fluoride-silicate melt, which was observed in the magma, but in isochemical conditions.

Inclusions-polyhedrons consist of two phases only. Unlike the previous types of inclusions their ratio is constant: 1/3 of magnetite, 2/3 of albite. These inclusions are the latest. They characterize the last stage of anorthoclase diagenetic transformation, following the perthite formation in conditions of constant temperature and isotropic strain. A distinctive feature of this stage is Fe³⁺ increasing in the alkaline-iron-silicate residual liquid. Calculations reveal that the ratio of Fe³⁺, Na and Si in the relict liquid corresponds to the stoichiometric composition of aegirine. However, other minerals have crystallized instead of pyroxene. The most likely reason for this is the excess of Al₂O₃, as well as cooling down below the temperature aegirine stability.

Inclusions in minerals of the interior zone of stock also prove the considerable temperature decrease (<400 °C). Four similar types of diagenetic inclusions were found in AmP of leucosyenites, but the mineral composition of formed phases is different. With this respect composition of the inclusions-laths is especially significant. Channel cavities in the AmP leucosyenites are filled with albite. Crystals of slightly ferriferous mica (amphibole is absent), magnetite, fluorite, hydromica aggregates, sequentially displacing each other along the cavity, are immersed into albite matrix. In other cases crystals of iron oxides and hydroxides, fluorite, and siderite are randomly dispersed in the amorphous transparent matrix. Both liquid and gas phases are absent. Ferrous composition of the residual silicate liquid remained in the AmP of leucosyenites, but due to PT-conditions change, oxides, hydroxides and carbonate of iron crystallized instead of the Fe-silicates.

Diagenetic inclusions in fluorite also show the evolution of the residual fluid, but its composition is specific (Kulchitska, 2007). Silicate component in it is of subordinate importance. The inclusions of salt (chloride) melt prevail. With cooling down from 800 to 60 °C it becomes enriched with water and formed brine-melt, supersaturated solutions, and finally slightly salted chloride solutions.

Conclusions. The composition of inclusions in AmP of melanosyenites proves that the residual silicate liquid even after crystallization of amphibole was strongly enriched with Fe²⁺. Alkalinity of the residual melt was locally increasing, in comparison with the initial magma; alkali amphiboles, absent in syenites, were crystallized in diagenetic inclusions. One more distinguishing feature of the residual liquid is accumulation of fluoride in it. Increasing concentrations of fluoride led to the separation of immiscible halide or halide-silicate melt. While the temperature deacreased the residual liquid, segregated in diagenetic inclusions, evolved toward oxygen and volatiles enrichment. Residual liquid of the leucosyenite evolved in the same way. In the residual silicate fluid, which is similar to the silica gel, H₂O and CO₂ were completely fixed by the low-temperature minerals. Liquid H₂O inclusions were found in fluorite only. The evolution of residual liquid in the Azov stock is not entirely compatible with the experimental data, which is probably due to the initially strongly reducing conditions of syenite crystallization.

References:

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Kulchytskaya A.A..,Chernysh D.S. Diagenetic inclusions in minerals as a products of the transformation of nanodefects to microdefects / Mineralogical intervention in micro- and nanoworld: materials of Int. mineralogical. seminar (Komi Republic, Russia, 9–11 June 2009). Syktyvkar: Geoprint. 2009. P. 209–211 (in Russian).

Kulchytska G.O., Melnykov V.S. Inclusions of Crystalline Melts in Anortoclase-Microperthite from Syenites of the Azov Stock (Ukrainian Shield) // Mineral. Journ. (Ukraine). 2008. Vol. 30, № 4. P. 21–40 (in Ukrainian).

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Melnikov V.S., Kulchitska A.A., Kryvdik S.G. et al. The Azov deposit – a new type of rare-metal objects of Ukraine // Mineral. Journ. (Ukraine). 2000b. Vоl. 22, № 5/6. P. 39–49.

Strekozov S.N., Vasilchenko V.V.,.Gurskiy D.S. et al. Geological structures and ore features characteristic of the Azov deposit // Mineral resources of Ukraine. 1998. № 3. P. 6–9 (in Russian).