2011

News	Registration	Abstract submission	Deadlines	Excursions	Accommodation	Organizing committee
First circular	Second circular	Abstracts	Seminar History	Program	Travel	Contact us

Features of fluid evolution under formation of the PGE-bearing layered West Pansky intrusion (the Kola Peninsula, Russia) according to the gas isotope (He, Ar) evidence

V.A. Nivin, T.V. Rundkvist

Geological Institute, Kola Science Centre RAS, Apatity, Russia; nivin@geoksc.apatity.ru

The He and Ar isotope composition was studied in the rocks and minerals of the West Pansky ultramafic-mafic massif. Hypabyssal conditions of the massif formation provided losses of primordial mantle volatile components and a dilution of the magmatic fluid with the subsurface paleometeoric water comprising the dissolved air. These processes are suggested to start at the pre-crystallization stage. The relationship between distribution of noble gas isotopes and ore components does not indicate the derivation of the latter due to the assimilation of the Earth crust material and testifies to their mantle origin.

The West Pansky massif (WPM) is one (central) of the three large structural blocks in the Early Proterozoic Fedorov-Pansky layered ultramafic-mafic complex located in the north-eastern part of the Baltic Shield. Though the massif has been investigated in much detail, a lot of problems relating to the sources, mechanism and formation conditions of the ore horizons, low sulfide complex PGE mineralization (in particular the fluid regime) have not been taken into consideration. As an approach of their comprehending, new data on the He and Ar isotope compositions of the WPM rocks and minerals accumulated during the last years have been compiled and systematized. WPM mainly consists of fine-grained and medium-grained gabbronorites forming the same name zone (GNZ). The Cu-Ni and PGE mineralization is mostly located in two layered horizons to follow: the Lower (LLH) and Upper (ULH) ones, which thickness is 40-80 and 100-150 meters respectively. LLH and ULH are composed of layers and lenses of gabbronorites, norites, leucocratic gabbro and anorthosites. The rhythmical layered horizon of the olivine-bearing rocks (OH) overlays ULH. Moreover, many layers and lenses of magnetite-bearing gabbro often occur in GNZ. GNZ consists of the Lower (beneath LLH), Middle (between LLH and ULH) and Upper (above ULH) parts (L-, M- and UGNZ, respectively). The isotope compositions of He and Ar have been studied in 44 samples. The isotope-gas measurement technique has been repeatedly represented earlier, e.g., in (Tolstikhin et al., 1999). The two methods to follow have been used for gas extraction from the samples: melting in the high-vacuum electric furnace and mechanical milling in vacuumed glass ampoules. The former yields gas from the whole volume of the sample, the latter evokes the gas mainly from fluid microinclusions (FI) in minerals. The isotope analyses of He and Ar have been carried out using the single-cascade magnetic mass spectrometers MI-1201 and MI-1201IG. The interpretation of gas-geochemistry data have been based on the well-known evaluations of the isotope composition for the noble gases in the meteoric water saturated with the atmosphere air, in the Earth crust and in the subcontinental lithosphere mantle. The ratios used are as follows: ³He/⁴He, ⁴⁰Ar/³⁶Ar, ³He/³⁶Ar and ⁴He/⁴⁰Ar respectively: 1.4×10^-6 ; 295.6; (5 - 30)×10^-8 ; 1×10^-4 (meteoric water); (1 - 7) ×10^-8 ; >40 000; (1.7 - 20) ×10^-4; 4 - 6.4 (Earth crust) and (0.8 - 1.0) ×10^-5 ; < 40 000; 0.1 - 1; 0.3 - 3 (lithosphere mantle).

Some significant variations for He (0.8 - 19.2 cm³/g) and, in particular, for Ar concentrations (9.1 - 280 cm³/g), for ³He/⁴He ratio (1.8×10^-8 - 3.2×10^-7), and for ⁴⁰Ar/³⁶Ar ratio (800 - 19 634) have been revealed in the gases extracted via melting of samples. The estimated part of the mantle-derived He in the total gas volume does not exceed 1 %, and the amount of the atmosphere-derived Ar varies from 6 to 12 % in different parts of the WPM geological section. The data obtained for the radiogenic gas isotopes and their parental U, Th, Li and K allows estimating the initial ³He/⁴He (from 20×10^-8 to 70×10^-8) and ⁴⁰Ar/³⁶Ar (~ 1150) ratios in the fluid captured by the rocks. The ULH ore differs from its barren rocks by lower He and Ar isotope concentrations and a slightly less amount of mantle-derived He and atmosphere-derived Ar. To the contrary, features of the LLH ore are relatively high concentrations of both ³He and ⁴He and ⁴⁰Ar, and relatively high ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios. In the fluid inclusions the scatter in the ⁴⁰Ar/³⁶Ar and ³He/⁴He isotope ratios is slightly wider, values of the latter are higher in general (up to 56.4×10^-8). In average, about 4 % of ⁴He, 16 % of ³He and ⁴⁰Ar and 31 % of ³⁶Ar relative to their total amount are extracted from the fluid inclusions. When compared to the crystal matrix of the samples, the ⁴⁰Ar/³⁶Ar ratio in the fluid inclusions is usually four times lower, and ³He/⁴He ratio is 5-10 times higher. The lowest values for ³He/⁴He ratio are found in the rocks of the layered horizons. The noble gas isotope distribution in the mono-mineral fractions shows that the variations of the gas isotope characteristics for the rocks in different parts of the geological section are dictated mainly by He and Ar isotope composition in the cumulate minerals. Meanwhile, for the rocks of the layered horizons these are determined by the inter-cumulate and post-magmatic minerals. In sulfides, the fraction of the mantle-derived He is twice as lower than in associated enstatite and plagioclase, while the amount atmosphere-derived Ar in the sulfides is 5-7 times larger. The statistic analysis has revealed a considerable positive correlation between the total ³He abundance, ³He/⁴He ratio and Pd and Pt concentrations. Also positive but more weak relations between Cu, Ni, S and ³He, Cu and ³He/⁴He have been defined. A weak negative correlation is noted for the following pairs Pd - ³⁶Ar, Pd - ⁴⁰Ar, Pt - ⁴⁰Ar, Au - ⁴⁰Ar/³⁶Ar, Cu - ⁴⁰Ar/³⁶Ar, Ni - ⁴⁰Ar/³⁶Ar and S - ⁴⁰Ar/³⁶Ar. Concentrations of both ³He and ⁴He in the gas from the fluid inclusions are directly related to the content of nearly all ore components in the rocks, with the most severe connection being observed for PGEs. A positive correlations between PGEs and ⁴⁰Ar/³⁶Ar ratios have been determined here along with a negative correlation of ⁴⁰Ar with S and all the ore elements.

The factors to follow indicate losses of the primary mantle-derived fluid and its dilution by the crustal fluid before or during the magma crystallization, probably, by the trans-vaporization mechanism: (i) features of He and Ar isotopes distribution and interrelation in the WPM rocks and minerals, (ii) comparison of measured gas isotope characteristics and the ones calculated from the age of the massif and concentration of the radioactive elements parental for the radiogenic isotopes of He and Ar, (iii) deduced evaluation of initial isotope ratios in the captured gases. The trans-vaporization-like mechanism of fluid mixing has been assumed, e.g., during the formation of the Monchegorsk Pluton (Tolstikhin et.al., 1992) and the Norilsk trappean intrusions (Pokrovsky et al., 2005). The most part of radiogenic isotopes produced in situ during ~2.5 Ga has proved lost. The observed noble gas isotope compositions could be derived, if the added crustal fluid were represented by the young (at that time) meteoric water with the dissolved atmosphere air, demonstrating a low ⁴⁰Ar/³⁶Ar ratio and small He concentration. Probable contribution of the subsurface meteoric water to the magmatic system is particularly confirmed with an increase of the atmosphere-derived component fraction in the noble gas compositions up from the bottom on the geological section. The larger portion of the atmosphere-derived component in the fluid inclusions (some being of secondary genesis) as compared to gases extracted by sample melting and the better preservation of the mantle-derived component in the weakly metamorphosed rocks suggest the subsurface hydrothermal water circulation in the magmatic chamber also during the later stages of the massif formation. The pattern of variations in the He and Ar isotope compositions confirms the previous suggestion that during the WPM formation the melt repeatedly came in the magmatic chamber. These variations also testify to the following: (i) in comparison with the gabbronorite melts, the ones forming the ore horizons were degassed more intensively, (ii) the horizons consolidated in a longer time period and wider temperature interval (iii) the ULH rocks are more saturated with the fluid in comparison with the LLH rocks. Features of interrelation of gas geochemical characteristics and ore genetic elements suggest a mantle, not the crustal origin of the latter, particularly of PGEs and S. It corresponds with the sulfur isotope composition (Shissel et al., 2002). Hence, at least for WPM, the hypotheses that the ore-bearing magma generated the Early Proterozoic layered intrusions in the Baltic Shield is related to the assimilation of the Archaean supracrustal rocks, in particular, sulfide-bearing metasediments enriched by the ore components and fluids with Cl and S (Voloshina et al., 2008; Sharkov, Duyzhikov, 2008, etc.) has not been proved. At the same time, the conclusions based on the petrography and mineralogical data testifying to the post-magmatic processes partaking in the ore generation (Rundkvist, 1999; Voloshina et al., 2008) have been confirmed. It is necessary to combine investigations of fluid inclusions (thermo-barometry), abundance and composition of the main fluid components in rocks and minerals as well as He and Ar isotope compositions for better understanding of the role of volatile compounds and the mechanism of ore generation in WPM.

The investigation has been financially supported by RFFI grant 03-05-64257.

References:

Voloshina Z.M., Karzhavin V.K., Petrov V.P. Metamorphism and ore genesis in the platinum-bearing Pansky intrusive massif (Kola Peninsula). Apatity: Kola Science Center of RAS Publishers, 2008. 140 p. (in Russian).

Pokrovsky B.G., Sluzhenikin S.F., Krivolutskaya N.A. Conditions of the Norilsk trappean intrusions and host rocks interaction based on isotope data (O, H, C) // Petrologia, 2005. Vol. 13. No. 1. P. 56-80 (in Russian).

Rundkvist T.V. Late- and post-magmatic mineral generations in the Pansky massif (Kola Peninsula). Apatity: Polygraph, 1999. 66 p. (in Russian).

Tolstikhin I.N., Kamensky I.L., Marty B. et al. Low mantle plume component in Devonian Kola ultrabasic-alkaline-carbonatite complexes: Evidences from rare gas isotopes and related parent elements. Reprint. Apatity-Nancy-Bruxelles, 1999. 97 p. (in English and Russian).

Sharkov E.V., Dyuzhikov O.A. Generation of large and unique PGE-Cu-Ni deposits in the large igneous provinces (e. g. North Siberia and Baltic Shield) // Problems of the ore genesis in the Precambrian Shields. Materials of the all-Russian Scientific Conference. Apatity, November 17-18, 2008. Apatity: Kola Science Center of RAS Publishers, 2008. P 158-161(in Russian).

Schissel D., Tsvetkov A.A., Mitrofanov F.P., Korchagin A.U. Basal platinum-group elment mineralization in the Fedorov Pansky Layered Mafic Intrusion, Kola Peninsula, Russia // Economic geology, 2002. Vol. 97. P. 1657-1677.

Tolstikhin I.N., Dokuchaeva V.S., Kamensky I.L., Amelin Yu.V. Juvenile he-kium in ancient rocks: II. U-He, K-Ar, Sm-Nd, and Rb-Sr systematics in the Monche Pluton. 3He/4He ratios frozen in uranium-free ultramafic rocks // Geochimica et Cosmochimica Acta, 1992. Vol. 56. P. 987-999.