Isotope Geology of the Norilsk Deposits

The results of helium and argon isotope composition measurements (³He/⁴He, ⁴⁰Ar/³⁶Ar ratios and others) are presented in the chapter. In paleofluids from the Norilsk intrusions, the crustal helium is dominant, and the fraction of mantle helium is in the range from 0.1 to 22%. The contribution of crustal helium (0.1–4%) in rich and medium intrusions is especially low. In this parameter, poor intrusions (4–22%) are significantly different, with much more mantle helium. In fluid inclusions of the studied targets, the share of air argon is high, 60–100%. It is especially high in rich intrusions, from 88 to 100%. Consequently, air-saturated waters from enclosing sedimentary rocks actively participated in the formation of rocks and ores of the intrusions in the region. Average (by reserves) intrusions differ significantly from the rich ones; they contain only 60–85% of atmospheric argon.

The chapter presents the results of sulphur isotope composition measurements (δ³⁴S, in ‰), which showed that in rich intrusions sulphur is in most cases isotopically heavy (δ³⁴S = 9–13, average about 11‰), and in this parameter corresponds to the crustal source, apparently, in most cases, sedimentary anhydrite with δ³⁴S = 16.5 ± 1.5. The sulphur from medium and poor intrusions is indistinguishable by the isotopic composition and is characterized by a much lower δ³⁴S with a wide range of variations (0–9‰). Clear regularities in the distribution of sulphur isotope composition along the section of intrusions have not been revealed. The criterion should be considered as additional because of the possible fractionation of sulphur isotopes in the formation of ores. Thus, in the richest intrusions, sulphur, like helium, is of predominantly crustal origin; it appears to have been borrowed from anhydrites of the enclosing strata as a result of intensive migration (circulation) of water caused by injection. The origin of sulphur in poor and medium intrusions remains unclear. It is possible that some of the sulphur has a mantle genesis.

The chapter describes a technique of analysing copper and nickel isotopes in ores and magmatic rocks of deposits. Average δ⁶⁵Cu value strongly diverges in rich intrusions: Kharaelakh (−1.55‰), Talnakh (−0.7‰), and Norilsk-1 (+0.25‰). Perhaps this is due to the mixing of the crustal and mantle sources of matter in the formation of these massifs. The Kharaelakh intrusion by δ⁶⁵Cu has a large share of crustal matter. In the remaining massifs, the average value of δ⁶⁵Cu is almost the same (−0.8 to −0.3‰) and does not correlate with the isotope composition of sulphur. The average values of nickel ⁶¹Ni/⁶⁰Ni isotope composition vary within narrow limits from −0.6 to 0‰ in all intrusions of the region and point to a single source of this metal. Data on copper and nickel isotopes in the products of mining and metallurgical companies in the world are given. Variations in the isotopic composition of copper are more associated with raw materials sources of companies, and the isotope composition of nickel, with different technological processes. A possible change in the isotopic composition of nickel during carbonylation (a mond process) is considered.

The chapter presents data on Rb–Sr and Sm–Nd TIMS analysis of pyroxenes, plagioclases, gross samples of igneous rocks and sulphide ores. In most Sm–Nd analyses of minerals, the isotope system (pyroxene–plagioclase) does not reflect the age of intrusions and shows a rejuvenated or aged isochronous age. Only for the Kharaelakh intrusion, one 241 ± 32 Ma isochron was obtained, close to the U–Pb age of zircons. The primary isotope ratio ⁸⁷Sr/⁸⁸Sr and the ε_Nd parameter of samples are calculated for the age of 250 Ma based on the average U–Pb age of zircons. The primary ratio of plagioclases strontium from magmatic rocks of intrusions (0.7032–0.7090) is lower than that of ore sulphides (0.7081–0.7116). The heterogeneity of Sr and Nd isotope composition of minerals and rocks within a single intrusion is caused by different degrees of impact of ore and host-rock contamination. In the gross samples of igneous rocks from the studied massifs, the primary ratio of strontium varies from 0.7049 to 0.7128, and ε_Nd varies from −7.7 to +7.0. These data indicate a significant effect of the crustal component in the formation of igneous rocks during the injection of mantle material with the parameters ⁸⁷Sr/⁸⁸Sr = 0.7037 and ε_Nd = +9.5. Based on Nd and Sr isotopic data, it is impossible to clearly distinguish groups of intrusions with different ore content.

The discussed below results on Pb isotopic compositions have been obtained by both TIMS and LA-ICP-MS-MC techniques on sulphide and plagioclase mg-weight specimens and single grains correspondingly. General Pb isotopic compositions imply derivation from both, the mantle and crustal sources with the latter being dominant. The obtained results from the Talnakh intrusion notably discrepant from that of the Norilsk and Kharaelakh being less radiogenic, while the latter two cluster together, demonstrating a minor divergence between massive and disseminated ores. Analysis of ²⁰⁷Pb/²⁰⁴Pb versus δ³⁴S along with Th/U assumes three sources of mantle, lower curst and upper crust, while Pb isotopes alone do not provide distinguishing of the massive ores from poor one. Disparity of Pb isotopes in sulphides and plagioclases assumes their chemical and isotopic disequilibrium, precluding their coexistence in a single batch of melt.

Lu–Hf isotope is an informative geochemical tool. Comprehensive in situ study of U–Pb and Lu–Hf isotope systems in zirconium minerals (zircon, baddeleyite) by LA-ICP-MS allows obtaining characteristics of the initial substance and evolution of the earth’s crust. The data bear the crucial important information about the parent rocks and ore source in the Norilsk-Taymyr district. This chapter presents data obtained in VSEGEI isotope laboratory and in Australia. In Norilsk ores, significant variations in Hf isotopic composition have been revealed. Zircons of commercially ore-bearing intrusions have an increased value of εHf (to +10, weighted average εHf(T) = 8.2 ± 1.8). Compared with them, the Lower Talnakh low ore-bearing intrusion has practically zero εHf values. Thus, according to the results of the studied zircon sample, we can say that high εHf values are a necessary isotope criterion of ore content.

He, Ar, S, and Cu isotope characteristics of the Norilsk-1, Talnakh, and Kharaelakh ore-rich intrusions have been found to diverge. The intrusions’ isotope characteristics form regression lines with the Kharaelakh intrusion having the highest atmospheric contribution of Ar and the Norilsk-1—the least one: this correspond to a degree of interaction with the host-rock, believed to be a source of atmospheric Ar, though may relate to the intrusions occurrence depth. δ³⁴S has been found to decrease at growth of m (³He/⁴He) with the highest proportion of mantle S and He revealed in the Norilsk-1 intrusion. δ³⁴S versus δ⁶⁵Cu demonstrates negative correlation suggesting probable Cu income from different sources rather than its isotope fractionation: in this instance the Kharaelakh ores are dominated by the crustal Cu, Ni isotopic compositions vary narrowly, displaying no pronounced correlation with He, Ar, S, and Cu isotopes, implying: (1) a single mantle source of Ni and its minute fractionation; (2) contamination of ore system and its fluids by S, Cu and noble gases from the host-rocks; (3) the revealed isotopic variations may be accounted for two or three sources of matter.

The chapter presents new geochronologic results of various isotope techniques (U–Pb SIMS and Re–Os TIMS) along with their comparison with already published data. The main intrusions of the Norilsk district are demonstrated to be emplaced almost simultaneously with two possible magma intrusion pulses at 254 ± 4 and 244 ± 4 Ma (U–Pb SIMS), assuming c. 10 Ma duration of igneous activity. This is corroborated by sulfides Re–Os dating (245–250 Ma), suggestion synchroneity of intrusion and the ore formation. Some Permian and Carboniferous zircon xenocrysts have been found along with Precambrian grains (c. 1.9 and 2.7 Ga), while no Devonian xenocrysts has been revealed. A group of 145–150 Ma old mafic rocks has also been discovered: those nature and relation to the ore-bearing Norilsk intrusions yet to be studied. The geochronologic study suggests, that: (1) ore-bearing massifs belong to the early emplacement phase (250–255 Ma); (2) ore-bearing massifs contain xenogenic Palaeozoic zircons, pointing to important role of the host-rocks; (3) Late Triassic igneous activity (225–230 Ma) has not affected ore systems.