New U-Pb and Sm-Nd Data and REE Distribution in Sulphides of Paleoproterozoic PGE Layered Intrusions in the Arctic Part of the Fennoscandian Shield

The U-Pb (on zircon) and Sm-Nd analysis is a popular isotope-geochronological tool for estimating the age of rocks from PGE mafic-ultramafic intrusions. Sulphides can be used to study the geochronology of ore processes as well, since they should indicate the time of ore mineralization. Gabbronorite has been sampled from the Passivaara reef at the Penikat layered intrusion (Fin-land) for U-Pb and Sm-Nd isotope single zircon analyses in order to separate sulphide minerals. The Sm-Nd isotope age of gabbronorite has been dated at 2426 ± 36 Ma; eNd(T) = −1.4 ± 0.4. The Sm-Nd isotope age on sulphides and rock-forming minerals reflects the crystallization time of the ore-bearing gabbronorite from the Passivaara reef of the Penikat layered intrusion. The mass-spectrometer analytical environment and modes of operation have been adjusted to detect REE in sulphide minerals on example of pyrite from the PGE Penikat layered intrusion (Finland) and chalcopyrite from the Talnakh deposit (Norilsk area, Russia) has been estimated. The total REE content in pyrite is ca. 3.5 ppm, which is enough to define the Sm-Nd age of pyrite. The study shows how to use the mineral/chondrite spectra to evaluate the accuracy of the REE analytical results on example of State Standard Sample 2463 (Russia).


Introduction
The current research pioneers in complex U-Pb (on zircon) and Sm-Nd (on sulphide minerals) dating of basic-ultrabasic rocks from the Penikat intrusion (Finland). The minerals have been sampled in Finland and studied in the framework of the INTERREG-TASIS international project. The paper [1] summarizes the latest U-Pb (on zircon and baddeleyite) and Sm-Nd data (obtained for the first time on rock-forming and sulphide minerals), as well as Nd-Sr and He 3 /He 4 data on Paleoproterozoic PGE layered intrusions of the NE Fennoscandian Shield (the Monchegorsk, Fedorovo-Pana and Imandra ore areas, Russia).
Basically, sulphides are ore-forming minerals in PGE deposits that can have commercially valued Pt-Pd mineralization. The study of noble metal deposits at the Kola Peninsula comprises the geochronological research and analytical treatment of its results. Data on the content and distribution of REE in minerals are required to define the age of an ore-forming event and to study the behavior of light lanthanides during the sulphide magma generation.
The REE content in sulphides is often less than 0.1 ppm. The composition of REE is not completely studied; and data on REE analyses are scarce [2] [3] [4] [5]. Palessky [6] reviews methods of REE analysis of geological samples and notes that the atomic absorption and atomic emission spectrometry is nowadays more widely adopted, compared to the X-ray fluorescence and neutron activation analyses. However, these popular methods do not allow to simultaneously reveal the whole range of elements in geological samples because of their low detection limits or constraints on the range of elements to identify. Thus, the content of such monoisotopic elements as Pr, Tb, Ho, and Tm cannot be measured by the isotope dilution method. It is possible to analyze REE by ICP MS method on ELAN-9000 (Perkin Elmer) mass-spectrometers in the amount of up to 1 ppt (instrumental detection limit is 0.1 ppt). Timofeev [7] shows a possibility of analyzing microimpurities of rare and trace elements on ELAN mass-spectrometers with no preliminary concentration. A simultaneous determination of the whole REE range with no preliminary concentration is described in [8].
Rimskaya-Korsakova et al. [4] show the REE content in the chalcopyrite of the Talnakh deposit at a ppb level that is six to thirty times lower than that in chondrite. The authors preliminarily separated REE from the matrix of the unsealed liquid samples, and concentrated REE to analyze on ICP MS, PlasmaQuad PQ2STE.

Geological Setting of the Penikat Intrusion
The age of the Penikat intrusion is 2440 Ma, according to [9]. It is ca. 23 km long  type. This megacyclicity is associated with injections of new magmas into the chamber of the intrusion during the crystallization. It is confirmed by mineral and cumulate compositional variations [9].
The thickness of the first megaunit varies from 270 to 410 m in different parts of the intrusion. The unltramafic layer of the megaunit is represented by plagioclaseand chromite-bearing bronzitites with granophyres, biotite, ilmenite and chlorine-bearing apatite. The bronzitites are overlain by plagioclase-bronzite mesocumulates and plagioclase-augite-bronzite cumulates. In the middle part of the layer, there are plagioclase-bronzite (chromite) mesocumulates with augote oicocrysts.
The second megaunit is 160 -230 m thick. It has a layer of ultramafic rocks represented by alternating websterites, lherzolites with interlayers of gabbronorites and pyroxenites. The upper part of the megaunit cross-section is mainly The third megaunit has a thickness of 75 to 330 m. The ultramafic unit is represented by websterites with dispersed chromite, sometimes websterites and lherzolites. The ultramafic unit is overlain by plagioclase-augite-bronzite cumulates, and the megaunit cross-section is completed by plagioclase-bronzite orthocumulates with poikilitic intercumulative augite and pegmatoid gabbronorites.
The fourth megaunit is 760 to 1100 m thick. The contact between the rocks of the third and fourth megaunits is uneven due to the igneous erosion [9] [10].
The cross-section of the lower zone is composed of ultramafic rocks (alternating cumulative bronzitites and augite with intercumulative augite and bronzite). The  U-Pb analyses, using the ion-exchange chromatography. First, handpicked crystals are treated in ultrasonic bath for cleaning in spirit or in acetone and then in 7N nitric acid. Next, the crystals are heated for about 15 minutes on a warm rangette and three times flushed with recurrent purification water. The chemical mineral decomposition has been performed in teflon bombs with 3 to 5 mcl of mixed 205 Pb/ 235 U tracer with T. Krogh's method [11] in concentrated nitric acid during 5 -7 days at a temperature of 210˚С. After complete decomposition, the column effluent is evaporated on a warm rangette, and then 10 drops of 6.2 N chlorohydric acid are added. The sample is placed into the thermostat for 8 -10 hours at a temperature of 140˚С -150˚С for homogenization. Pb and U are separated for isotope investigations using ion-exchange chromatography in columns with Dowex IX8 200 -400 mesh resin. Pb is eluted with 10 drops of 6.2 N chlorohydric acid. Then, one drop of 0.1 N phosphoric acid is added, and the solution is evaporated on a rangette down to 3 mcl. U is eluted separately from Pb with 20 drops of water and one drop of 0.1 N phosphoric acid added, and evaporated on a rangette down to 3 mcl. All chemical procedures are carried out in an ultraclean block with blank Pb and U contamination of ca. 1 -3 pg, and ca. 10 -15 pg, respectively. The isotope composition and concentrations of Pb and U are measured on Re bands at 7-channel mass-spectrometer Finnigan-MAT 262 (RPG), on collectors, with 204 Pb and 205 Pb measured at a temperature of 1350˚С -1450˚С in an ion counting mode, using a multiplier or quadrupole RPG accessory. Silicagel is used as an emitter. U concentrations are detected at a temperature of 1450˚С -1550˚С, using a collector and a multiplier in a mixed statically dynamic mode. When U concentrations are negligible, the multiplier or quadrupole RPQ accessory is applied in a dynamic mode. All the measured isotope ratios are adjusted for mass-discrimination (0.12% ± 0.04%) obtained during the parallel analyses of SRM-981 and SRM-982 standards. Сoordinates of points and isochron parameters are calculated using programs by K. Ludwig [12] [13]. Ages are estimated according to the accepted values of U decay constants [14], with errors indicated on a 2b level. has been exposed to water bath at a temperature of 50˚C -60˚C until fully dissolved. No HCI has been added during opening of sulphide minerals. When opening the sample, we have registered an increased pressure of acid and nitrogen oxide vapour that suppressed the volatility of the components in the sample.

Analytical Procedures
The chilled sample has been mixed with 0.1 ml H 2 O 2 , and the dissolved sample was diluted with 2% HNO 3 . The level of total REE content in the blank sample was <0.5 ppb (see the REE spectrum in Figure 1, Curve 7). This blank sample qualifies the level of analytical accuracy and the limit of element detection. Centre, Russian Academy of Sciences, Apatity. Chemical and analytical conditions of the sample decomposition are described in [8]. The fitted conditions of the sample decomposition and analysis have provided an opportunity to determine REE concentrations in samples of reference rocks. The obtained results validate certified values ( Figure 1, Figure 2).

REE analyses of sulphides from the PGE reef of the Passivaara (Finland)
Further, the Sm-Nd isotope and geochronological study has been provided for sulphide minerals and whole-rock samples from the ore-bearing gabbronorite of the Penikat layered intrusion, Finland. The research indicates considerable concentrations of REE, including Sm and Nd, in sulphides and whole rock from these gabbronorites used in the Sm-Nd isotope analysis ( Figure 3, Table 1, Table 2).
The REE spectra analysis in sulphide minerals reflects a distribution trend similar to the studied whole rock samples. Thus, it supports the conclusion that the REE distribution style in sulphides is inherited from the parental rock, and sulphides formed at a stage of rock crystallization. This is corroborated by age-related isotope and geochronological Sm-Nd data obtained for the ore-bearing gabbronorites from this intrusion.
In the current research, concentrations of REE have been estimated. Accord-        [22]. The decompositions constants are as per [12].
Values of ε Nd have been calculated according to [14] for a one-stage model and according to [23] for a two-stage model.
U-Pb and Sm-Nd isotope dating of the Sompuyarvi PGE reef of the Penikat intrusion Single zircon crystals have been sampled for further U-Pb dating. The crystals are elongated prismatic (up to 180 µm in size), colored dark brown ( Figure   5(b)). Coordinates of the two points occur on the U-Pb concordia and reflect the similar age of 2430 ± 2 Ma ( Figure 5, Table 3).
The isochron Sm-Nd age on sulphide minerals, plagioclase, clinopyroxene and   Figure 5(a), Table 3). This age complies within the error with the earlier obtained age of the Penikat intrusion to be 2410 ± 64 Ma [17]. It is especially important that figurative sulphide points lie on the isochron that corroborates the idea of the sulphide genesis and rock crystallization. U-Pb isotope data on single zircon from the studied gabbronorite show the age of 2430 ± 2 Ma, which is much coeval to the Sm-Nd age. A thin magmatic zonation has been detected in all of the zircon grains, using the cathodoluminescence technique ( Figure 5(b), Table 4).

Conclusions
The U-Pb isotope dating of zircons from gabbronorites of the PGE reef of the Penikat and Sompuyarvi intrusions (Finland) has been provided. The conducted research implies the following: • the pattern of the REE distribution in sulphide minerals from gabbronorite of the Penikat layered intrusion is inherited from the parental magma melt, while sulphides formed at the very same time with the rocks of the massif, as confirmed by the Sm-Nd isotope and geochronological data; • along with rock-forming minerals, sulphide with a high mineral genesis temperature [24] can be used for the Sm-Nd isotope dating of ore-bearing mafic-ultramafic intrusions; • the study shows a possibility to analyze REE in sulphide minerals (Talnakh deposit, Russia, and PGE Penikat layered intrusion, Finland) on ELAN 9000 DRC-e (Perkin Elmer, USA) with no preliminary separation of the mineral matrix; as a result, the REE distribution spectra have been obtained; • the concentration of Sm and Nd in sulphide minerals (Penikat, Finland) is high enough to be detected by the Sm-Nd isotope analysis; • the REE distribution spectra have been obtained for SSS 2463 to estimate concentrations of Gd, Er and Tm. Isotope Sm-Nd data on rock-forming and sulphide minerals show the age much similar to the U-Pb single zircon age. Thus, we may suggest that minerals had a narrow range of closure temperatures.
Future U-Pb isotope dating will use not only zircon. It is planned to introduce baddeleyite to compare the temperature of the U-Pb and Sm-Nd systematics *The ratios are corrected for blanks of 1 pg for Pb and 10 pg for U and for mass discrimination 0.12% ± 0.04%. **Correction for common Pb has been estimated for the age, according to Stacey and Kramers [10].