The role of fluids in trace elements mobility: a case study of the contact zones of the Khibina and Lovozero alkaline massifs, Russia

Arzamastsev A.A.*, Arzamastseva L.V.*, Shanina S.N.**, Zaraisky G.P.***

*Geological Institute of the Kola Science Centre RAS, Apatity, Russia

**Institute of Geology, Komi Science Centre RAS, Syktyvkar, Russia

***Institute of Experimental Mineralogy RAS, Chernogolovka, Moscow, Russia

The Paleozoic Khibina and Lovozero massifs are mostly composed of agpaitic syenites with minor foidolites, ultrabasic alkaline rocks and carbonatites, which occupy the area of ca. 2000 km². The host rocks exposed in the contact zones are represented by the Late Archean tonalite-trondhjemite-granodiorite gneisses. Metabasalts and green schists of the Early Proterozoic Pechenga-Imandra-Var­zu­ga paleoriftogenic complex occur only in the southern contact of the Khibina massif. Geological observations indicate that the thickness of the fenites varies from few meters up to 100 meters and even more. Common are alkaline syenite veins ("umptekite" in the Khibina) and albite-microcline pegmatoids (the Lovozero) in fenitized gneisses.šš š

The hallmark of the agpaitic syenites is their immense abundances of REE, Y, Sr, Zr, Hf, Nb, Ta, and Th. These are either concentrated in apatite, titanite, perovskite, and other accessories or, given their concentrations in melts were high enough, form their own discrete minerals, such as loparite, pyrochlore, and eudialyte, which economic deposits provide the basis for the region’s mining industry.

In order to decipher the behavior of gases and trace elements in the fenitization process, two profiles across the contacts of the Khibina and Lovozero massifs have been investigated. 78 samples from AR gneisses, fenites, alkaline syenite veins, zircon and ilmenite bearing albite-microcline veins have been analysed for major and trace elements (ICP-MS for V, Cr, Co, Ni, Ga, Rb, Sr, Y, Zr, Nb, Ba, Hf, Ta, Pb, Th, U). The microprobe data have been used for the mineralogical characteristics of the fenites. All samples have been analysed for gases (F, Cl, CO₂, H₂O, H₂S).

The endocontact zones. The data obtained so far unequivocally give evidence for different agpaitic magma - gneiss interaction in the Khibina and Lovozero endocontacts. In the Khibina, in the proximal vicinity of the contact the alkaline rocks are represented by syenites, which are suggested to be formed as a result of contamination of agpaitic syenites by gneisses. The endocontact extends up to 100 m. Within this zone the volatile content (H₂O, F, Cl, CO₂) in the agpaitic syenites ("khibinites") is at least twice as higher when compared with the average for the rock complex, therefore, pectolite, natrolite, and lamprophyllite are ubiquitous. The endocontact agpaitic syenites ("khibinites") are significantly depleted in Zr, Hf, Nb, Ta, REE, whereas concentrations of Rb, Sr and Ba are close to average in the whole complex. In the Lovozero massif the endocontact zone is only 5 - 15 m wide. In contrast to the Khibina, the Lovozero contact rocks exhibit no evidence of volatile concentration. However, the endocontact lujavrite is significantly enriched in Zr, Hf, Nb, Ta, REE, Y, which enter eudialite, loparite, apatite and other rare minerals.

The exocontact zones. Two types of fenite can be distinguished in the exocontacts of the Khibina and Lovozero massifs.

The first type fenite (fenite I) form the narrow (< 5 m) zone in the immediate contact of the nepheline syenites. Concentrations of Zr, Hf, Nb, Ta, REE, Y in the Khibina apogneiss fenite are notably higher when compared with neighbouring nepheline syenite. The peak values of the above mentioned trace elements occur 3 m off the contact of the massif. In the 10 m profile across the contact (Ce/Yb)_N changes from 32.8 in agpaite syenite to 17.2 in apogneiss fenite, whereas average (Ce/Yb)_N in gneiss is 25±2. These data are in good agreement with those obtained previously in our experiments (Zaraisky et al., 2008), which show preferable migration of HREE from agpaitic syenite into gneiss during the metasomatic interaction process. Similarly, accumulation of Nb in apogneiss fenite in respect of the near-contact syenite fits well with our experimental data.

The fenites II are closely related to the veins of alkaline rocks and pegmatoids that spread within the 500 m range off the intrusions. Fenites II form 3-20 m wide halos, in which gneiss gradually transforms into a totally recrystallized fenite. The newly formed alkaline phases, such as nepheline, and the protolith phases, e.g. quartz, are common even in a single specimen. In the northern contact of the Lovozero massif fenites II form a 500 m wide zone surrounding a net of albite-microcline veins with zircon and ilmenite. HFSE and REE concentrations in fenites II positively correlate with intensity of metasomatic alteration. Furthermore, in the Lovozero and Khibina totally altered fenites F content is higher than that in adjacent alkaline veins.

Since the volatiles and most of the trace elements exhibit distinct signs of mobility in the metasomatic process, we have attempted to assess the role of the H₂O, F, CO₂ in the migration of LILE, REE and HFSE. Additionally, we have performed a stepwise heating for extraction of H₂, N₂, CH₄, C₂H₄, C₂H₆ within the range of 20-400, 400-500, 500-600, 600-700 and 700-800^oC in the set of the Khibina samples. From the correlation analysis between the above volatiles and trace elements, the following conclusions can be drawn.

(1) The strong positive correlation with H₂O appears to suggest that hydrothermal fluids, which followed alkaline pegmatoid veins were responsible for the transport of the REE, Zr, Hf, Nb, Ta. Rb and Sr into the host gneisses. š

(2) As the temperature of the fluid increases to >400^oC, the positive correlation between REE and CH₄ decreases. For example, the K_La gradually changes from 0.74 to 0.56. By contrast, correlation between REE and C₂H₆ during heating increases. Zr, Hf, Nb, Ta, Rb show the same behavior during the step-wise heating.š

In general, the now available information on the contact zone of the Khibina and Lovozero intrusions appears to suggest that the alkaline magma-host rock interaction has occurred via porous media flow (in the immediate contact) or by the discrete channels formed during marginal fracturing of the calderas. The obtained U-Pb age of single zircons (359 ±5 Ma, SHRIMP II data) shows that formation of albite-microcline veins followed the main Lovozero and Khibina intrusions, which occurred at ca 370 Ma (Kramm and Kogarko, 1994). Numerous concentric fissures were filled by post-magmatic hydrothermal fluids, which are suggested to be the main agent of fenitization of Archean gneisses.

This study was financially supported by the RFBR grant 09-05-00224.

References:

Kramm U., Kogarko L.N. Nd and Sr isotope signatures of the Khibina and Lo­vozero agpaitic centres, Kola Alkaline Province, Russia. Lithos. 1994. Vol.32. P.225-242.

Zaraisky G.P., Shamatrina, A.M., Arzamastsev, A.A. Experimental modelling of alkaline metasomatic reactions in the contact zone of the Lovozero massif. // Geochemistry of igneous rocks. Proceedings of the XXV International seminar. School: "Alkaline magmatism of the Earth". (St. Petersburg, 23-26.05.2008). St.Petersburg State University Publ., 2008. P.59-60.š