Zircon from the economic ultramafic-mafic Kharaelakh intrusion (Russia): first U-Pb and Hf-isotope constraints on timing and source composition

Malitch K.N.*, Griffin W.L.**, Badanina I.Yu.*, Petrov O.V.*, Tuganova E.V.*, Belousova E.A.**, Pearson N.J.**, Knauf V.V.***, Presnyakov S.L.*

* A.P. Karpinsky Russian Geological Research Institute (VSEGEI), St. Petersburg, Russia; ** GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia; *** NATI Research JSC, St. Petersburg, Russia

Introduction. World-class PGE-Cu-Ni sulphide deposits occur in the Noril’sk-1, Talnakh and Kharaelakh areas in the northwestern part of the Eastern Siberia, Russia. They are associated with sill-like ultramafic-mafic intrusions, which are controlled by a long-lived intracontinental paleorift (Tuganova, 2000). These ‘Noril’sk-type’ intrusions range in thickness up to 360 m and in length up to 25 km. In this study, we present new U-Pb and Hf-isotope data for 23 zircon grains from the Kharaelakh intrusion in order to: (1) provide age constraints on the magmatic evolution and (2) specify the sources of materials involved in the generation of the host rocks.

Samples and analytical methods. A distinctive feature of the Kharaelakh intrusion is the presence of two horizons of ultramafic rocks, distinguishing this intrusion from the Talnakh and Noril’sk-1 intrusive bodies. The rocks investigated comprise (from top to bottom) olivine gabbro (samples 844-1, 844-6), plagiowehrlite (844-7) and melanotroctolite (844-10,11). Forty grains of zircon were extracted using a ppm-mineralogy technique at NATI Research JSC (Russia). Grains of zircons were hand picked from each concentrate, imaged by SEM and subsequently mounted in epoxy blocks together with grains of the TEMORA and 91500 reference zircons. Transmitted and reflected light photomicrographs and cathodeluminescence images were made in order to select grains and choose sites for analyses, avoiding cracks and inclusions. The Sensitive High-Resolution Ion Microprobe (SHRIMP-II) at the VSEGEI was used to perform 26 in-situ U-Pb analyses by applying a secondary electron multiplier in a peak-jumping mode following the procedure described by Williams (1998). Age calculations use the routines of Ludwig (2003) and follow the decay constant recommendations of Steiger and Jager (1977). In-situ Hf-isotope data were collected on the dated spots within zircon grains. Twenty-six Hf-isotope analyses were carried out with a New Wave LUV213 laser-ablation microprobe attached to a Nu plasma MC-ICP-MS at GEMOC, following the analytical procedures reported by Griffin et al. (2002). The measured ¹⁷⁶Lu/¹⁷⁷Hf ratios and the ¹⁷⁶Lu decay constant of 1.865x10^-11 yr^-1 reported by Scherer et al. (2001) were used to calculate initial ¹⁷⁶Hf/¹⁷⁷Hf ratios. The present-day chondritic values of ¹⁷⁶Lu/¹⁷⁷Hf=0.0332 and ¹⁷⁶Hf/¹⁷⁷Hf=0.282772 reported by Blichert-Toft and Albarede (1997) were used for the calculation of epsilon Hf values (parts in 10⁴ difference of initial Hf isotope ratios between the zircon sample and the chondritic uniform reservoir).

Morphology and internal structure. Zircon has been observed in thin sections as single grains or aggregates enclosed in clinopyroxene, amphibole or mica, frequently intergrown with apatite. Separated zircon grains are commonly idiomorphic to subidiomorphic prismatic, transparent to semi-transparent, light beige crystals that may contain fissures; the elongation of zircon grains varies from one to three (sometimes up to 6-7). In the Pupin diagram (Pupin, 1980), type D crystals predominate (76 %) among the investigated grains, indicative of alkaline-high temperature conditions of formation. Observations under the petrographic microscope revealed distinct zircon populations with different internal structure. However, zircons are characterized by similar fuzzy cathodeluminescence, commonly with a total absence of the growth zoning typical of zircons from magmatic rocks.

U-Pb ages. Twenty-three zircon grains used for the U-Pb determinations yielded four groups of concordant U-Pb ages. Colorless cores from complex grains 844-1_10 and 844-1_7, defined as population 1 (ZR 1), produced a ²⁰⁶Pb/²³⁸U age of 347 ± 16 Ma (mean square of weighted deviates (MSWD) = 1.2), which is significantly older than ages found in the other types. Indeed, rims of zircons from these complex grains (population 2, ZR 2) gave a ²⁰⁶Pb/²³⁸U age of 265 ± 11 Ma (MSWD=2.3), whereas dark cores from two other complex grains (i.e., 844-1_19 and 844-6_28) and the main set of homogeneous grains (n=15) yielded slightly younger concordant ²⁰⁶Pb/²³⁸U age of 253.8 ± 1.7 Ma, MSWD=0.064). A younger concordant ²⁰⁶Pb/²³⁸U age of 235.7 ± 6.1 Ma (MSWD=2.0) is typical of overgrowths over dark cores and a minor group of homogeneous grains. For consistency, we refer to the groups of zircon with ²⁰⁶Pb/²³⁸U ages of 253.8 ± 1.7 and 235.7 ± 6.1 as population 3 (ZR 3) and population 4 (ZR 4), respectively.

Hf-isotope data. The studied zircons show a wide range in Hf-isotope composition. Zircons of population 1, represented by colorless cores in complex grains with ²⁰⁶Pb/²³⁸U ages between ca 338-355 Ma, yielded e_Hf (parts in 10⁴ deviation of initial Hf isotope ratios between the zircon sample and the chondritic uniform reservoir) values from + 8.1 to + 8.7 (mean e_Hf + 8.4). Zircons of population 2 with ²⁰⁶Pb/²³⁸U ages between ca. 264.9-270 Ma (exemplified by rims from the same complex grains) have Hf isotope compositions that are less radiogenic than the other zircon populations (e.g. ZR1, ZR 3 and ZR 4). Population 3 zircons with ²⁰⁶Pb/²³⁸U ages between ca. 241.8-267.2 Ma and population 4 zircons with ²⁰⁶Pb/²³⁸U ages between ca. 229.4-239.6 Ma have similar mean e_Hf values (10.5 and 9.95, respectively), which are only slightly higher than that of type 1. However, populations 3 and 4 zircons show a significant range in e_Hf (from +5.1 to +16.7 and + 4.9 to +13.1, respectively) with some points plotting close to the Depleted Mantle reference line, while the least radiogenic points fall close to the values observed in ZR 2.

Genetic implications and concluding remarks. The U-Pb ages of the four zircon populations cover a significant time span (347 ± 16, 265 ± 11, 253.8 ± 1.7 and 235.7 ± 6.1 Ma) suggesting a prolonged evolution of the magmatic system. This magmatic activity is consistent with two recognised stages of active tectonism in the development of the Siberian Craton (Malitch, 1975). The crystallisation of zircon population 1 corresponds to the final stages of the Middle Paleozoic (D₃-C₁) tectonic cycle, whereas that of populations 2 to 4 matches the known duration of the Early Mesozoic (P₂-T₂) tectonic event.

On the basis of results reported here we propose that mafic-ultramafic magmas were emplaced in the lithospheric mantle or deep crust ca 90 Ma before the flood basalts, and that the magmas generated in Early Mesozoic (around 230-260 Ma) picked up zircons from these earlier intrusions. We also suggest that this might represent a single long-lived magma chamber. The age of zircon population 3 (253.8±1.7 Ma) slightly precedes that suggested for the basalts (248.7±0.6 – 250.3.2±1.1 Ma, Reichow et al., 2009). Therefore, the timing of the magmatic episode exemplified by ZR 3 might be closely linked and possibly interpreted as coeval with flood-basalt volcanism. ZR 4 can be attributed to thermal recrystallisation during or after the emplacement of the intrusion, clearly postdating the eruption of the basalts.

Hf isotope data show that majority of studied zircons have a clear juvenile component, and there is good evidence of mixing with an older, possibly lithospheric component. The Hf-isotope data for zircons of population 3 suggest that the Kharaelakh magmas represent mixing between a juvenile source equivalent to the Depleted Mantle, and a lithospheric source with Hf-isotope composition similar to the zircons of population 2. The smaller range of initial ¹⁷⁶Hf/¹⁷⁷Hf values in population 4 might reflect progressive homogenization of these magmas during crystallisation, but the present dataset is too small to say this with confidence.

The depleted mantle component was not previously recognised by whole-rock Nd- and Sr-isotope studies (Arndt et al., 2003, among others). All rocks at Kharaelakh from which zircons were separated are characterised by e_Nd values from + 0.8 to + 1.1. However, the existence of a highly depleted mantle source (e_Nd in the range of +3.7 and +5.1) has been confirmed (Kogarko et al., 1988; Horan et al., 1995) in picritic lavas, both at Noril’sk (Gudchihinsky suite) and Maimecha-Kotui (Maimechian suite) areas. Therefore, the slightly lower uniform Nd isotopic values typical of ore-bearing ‘Noril’sk-type’ ultramafic-mafic intrusions were attributed to certain levels of crustal contamination, presumably in deep staging magma chambers. Our data confirm that a lithospheric component is present. The Sm-Nd whole rock data reflect this (and the homogenising effect of whole-rock analysis) more strongly, while the zircon data allow us to put better constraints on the end-members of the mixing (DM vs an old lithospheric component with e_Hf ≈ 0 at 250 Ma).

In conclusion, our new findings suggest a prolonged period of mafic-ultramafic magmatic activity, from Middle Paleozoic to Early Mesozoic, and interaction between juvenile magmas and an older lithospheric component, at least during the ca 253 Ma magmatism. Consequently, these processes could lead to high degrees of separation and concentration of ore elements and formation of specific ore magmas of unique scales and concentrations. We further propose that ‘radiogenic’ Hf-isotope composition of zircon can be employed as an effective fingerprint for identifying prospective intrusive host rocks and consequently is useful in exploration for sulphide-rich ores associated with ‘Noril’sk-type’ intrusions. Finally, our new data suggest that the Hf-isotope composition of defined zircon types can provide a unique set of previously unknown constraints on petrologic history of the Kharaelakh intrusion.

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