Genesis and multistage crystallization of zircon in Zr-Y-REE deposit of Sakharjok, Kola Peninsula

Lyalina L.M., Zozulya D.R., Bayanova T.B.

Geological Institute Kola Science Centre RAS, Apatity, Russia

lialina@geoksc.apatity.ru

 

Complex Zr-Y-REE deposit is confined to the Sakharjok alkaline rock massif on the Kola Peninsula. The massif represents the dyke-type intrusion of 7 km length and of 4-5 km² outcrops. It intrudes the Late Archean Keivy peralkaline granite and gneiss-diorites of TTG basement. The massif is composed of alkaline syenite and nepheline syenite of 2680 and 2610 Ma age, correspondingly. The rocks were affected by regional metamorphism of 1750-1800 Ma age. The ores are confined to the nepheline syenite body and represents the linear zones of 200-1350 m length and of 3-30 m thickness. The main ore-forming minerals of the Sakharjok deposit are zircon, britholite group and pyrochlore. The genetic model of zircon presented in this paper is based on the comprehensive mineralogical studies.

Zircon from the ore block of the Sakharjok massif is highly variable on the morphology, inner structure (anatomy), physical properties and chemical composition.

Five morphological types were established from the study of zircon habits,: prismatic zirconic ({110}+{111}), prismatic zirconic-hyacinthic ({110}+{100}+{111}), sharply angular dipyramidal ({331}±{221}+{111}), dipyramidal-prismatic ({110}+{111}+{221}), and dipyramidal ({111}+{221}±{110}). The quantitative proportion of the morphotypes varies in different lithologies of nepheline syenites. As a whole it could be estimated as: ~75 vol. % of dipyramidal crystals, ~20-25 vol. % of dipyramidal-prismatic crystals, and other morphotypes do not exceed 5vol. %.

The inner structure of the most of crystals shows the rough phase heterogeneity expressed in the presence of the inner zones and outer rims. The inner zones are represented by the small segments and disintegrated fragments. They reveal the weak thin rhythmical zonality and are mostly light in BSE and have the complicated form with partial preserving of linear boundaries (Fig. 1a). The character of the inner zones’ phase heterogeneity depends strongly from the habits of the crystals. Prismatic and dipyramidal-prismatic morphotypes do not contain inclusions (Fig. 1a), and the dipyramidal ones have the numerous mineral inclusions, among which albite (Ab) dominates and rare pyroxene (Px) is found (Fig. 1b). The growth of the outer rims starts from healing of cavities in the early stage zircon from the inner zones which is followed by euhedral crystallization with inheritance of the primary morphology of the crystals. The rims may reach of 80% of crystals volume and contain the rare large mineral inclusions (Ab, Px). The rims reveal the thin rhythmical zonality which is good visible in CL-images (Fig. 1a, e).

The widely distributed zircon from the pegmatoid nepheline syenite has the inner zones of the porous structure and is impregnated with the numerous mineral and gas-and-fluid inclusions. The similar porous structure as the sharp zones, overgrowing the early zircon, is established in the mineral from the schlieren pegmatite in nepheline syenite (Fig. 1c, d).

The least distributed crystals are characterized only by the thin rhythmical zonality without inner zones and outer rims (Fig. 1f).

Our data show that zircon at the Sakharjok deposit was formed in several stages of rock formation: magmatic, postmagmatic/hydrothermal, and metamorphic. Zircon of magmatic stage is represented by the inner zones of the heterogeneous crystals. The change of crystal habits indicates on the broad temperature interval of crystallization (900-500 ºC, according to J.P. Pupin (1980)), and two substages could be distinguished: the early magmatic (prismatic crystals) and late magmatic (dipyramidal crystals). The growth of the low temperature crystals of dipyramidal habit occurred in sufficiently consolidated environment that has caused the trapping of numerous inclusions. The formation of the porous zircon marks the postmagmatic/hydrothermal stage of the rock formation. During this stage the rapid growth of zircon has happened in the high-fluid conditions, and the mineral absorbed other phases like a “sponge”. The trapping of the numerous inclusions at rapid crystallization is confirmed by the experimental data (Krasnova, Petrov, 1997). Magmatic and postmagmatic zircons were subjected to intensive dissolution (irregular boundaries, disintegrated fragments) and recrystallization, forming the outer rims during the metamorphic stage. Despite of outer rims in heterogeneous crystals, the metamorphic zircon forms also the individual rhythmically zoned crystals (Fig. 1f).

The conclusion on multistage formation of zircon based on studying morphology and anatomy of its crystals is confirmed by chemical composition of zircon (Fig. 2). Magmatic stage is subdivided into two substages: early magmatic characterized with elevated ZrO₂/HfO₂ ratio (average 90) and late magmatic with moderate ZrO₂/HfO₂ ratio (average 63). Zircon of the postmagmatic/hydrothermal stage has the very low ZrO₂/HfO₂ ratio (average 29) and high admixtures of CaO (0.1-0.3 wt. %), Y₂O₃ (0.5-1.5 wt. %) and ThO₂ (up to 1.5 wt. %). The successive decreasing of the ZrO₂/HfO₂ ratio at these stages is explained by evolution of the melt and of the postmagmatic fluids. The zircon of the metamorphic stage is characterized with averaged ZrO₂/HfO₂ ratio (45), similar to the same one from the nepheline syenite. The metamorphic zircon has very negligible U content, differing from the higher one in magmatic species, which is explained by selective removal of U by the metamorphic fluids. At a whole, the zircon from Sakharjok deposit is characterized by low contents of “dangerous” admixtures (f.e., U: 10-90 ppm, Th: 10-80 ppm) that is favorable for its wider industrial use.

It is concluded that zircon of the Sakharjok Zr-Y-REE deposit has been formed at a broad temperature interval for a long time (ca. 800 Ma) during the all stages of the deposit formation (magmatic, postmagmatic/hydrothermal, metamorphic).

Fig. 1. REM photos of zircon (a, b, d, e – BSE; c, f – CL). Arrow indicates the porous zone in zircon from pegmatite in nepheline syenite.

  

Fig. 2. ZrO2-HfO2 diagram for zircon from the nepheline syenites of the Sakharjok deposit: 1а – inner zones of early magmatic prismatic crystals (squares), 1b - inner zones of late magmatic dipyramidal crystals (diamonds), 2 – inner “porous” parts of postmagmatic/hydrothermal crystals, 3 – outer rims of heterogeneous zircons of different morphogenetic types (transparent symbols) and individual rhythmically zoned crystals (crosses) of metamorphic stage. Triangles – inner zones of zircons from pegmatites.

 

The study was fulfilled under the frame of project 66 of RAS and Bulgarian Academy of Sciences scientific cooperation. The work was partly supported by RFBR, project 08-05-00324.

 

References:

Krasnova N.I., Petrov T.G. Genesis of individual and aggregated minerals. St-Petersburg: Nevskiy Kur’er, 1997. 228p.

Pupin J.P. Zircon and granite petrology // Contribs. Miner. and Petrol. 1980. V. 73. № 3. P. 207-220.