ISSN 1028334X, Doklady Earth Sciences, 2010, Vol. 434, Part 1, pp. 1272–1274. © Pleiades Publishing, Ltd., 2010. Original Russian Text © O.V. Petrov, S.A. Sergeev, E.M. Prasolov, V.O. Khalenev, K.I. Lokhov, 2010, published in Doklady Akademii Nauk, 2010, Vol. 434, No. 3, pp. 388–390.
GEOCHEMISTRY
Geochronological and Isotope Geochemical Characteristics of Mafic Intrusions of the Norilsk Region O. V. Petrov, S. A. Sergeev, E. M. Prasolov, V. O. Khalenev, and K. I. Lokhov Presented by Academician N.S. Bortnikov May 12, 2010 Received May 17, 2010
DOI: 10.1134/S1028334X1009028X
Layered intrusions of the Norilsk group contain unique deposits of copper–nickel, cobalt and plati noid ores, and platinoids. The origin of silicate matrix was traditionally considered to be mantle. However, the first study of the isotopic composition of fluid components (He and Ar [1] and sulfur [2]) revealed largescale mantle–crust interaction. The complex isotopic investigations of rocks and ores of mafic intrusions of the Norilsk region performed at the Karpinskii Geological Research Institute in 2003– 2008 demonstrated the quantitative genetic indicators allowing us to refine the models of the formation of unique Cu–Ni–PGE deposits. We performed several hundred isotopic analyses: isotopic composition of helium and argon from fluid microinclusions in rocks and ores; sulfur, copper, and nickel from sulfides. The absolute (isotopic) age of sul fide ores and zircons from rocks in intrusions with var ious ore potential was determined. To obtain an inte grated pattern, we also used the latest data of Russian and foreign researchers in this region [3–5 and refer ences therein]. The helium isotopic composition (3Не/4Не ratio) is considered to be the only reliable and strong criterion of the link between mineralforming fluids and the mantle. It is revealed that the ratio of helium isotopes in the Earth’s upper mantle (1.2 × 10–5) is ~1000 times higher than in helium formed in rocks of the Earth’s crust (~ 2 × 10–8). This allows us to reveal and calculate the portion of mantle helium, if it exceeds 1% of the total helium in the sample. Measurements demon strated that crustal helium dominated in paleofluids from the Norilsk intrusions and the contribution of mantle helium ranged from 1 to 22%. The contribu tion of helium of such origin is especially low (1–4%)
Karpinskii Geological Research Institute, St. Petersburg, Russia email:
[email protected]
in rich and intermediate intrusions. Poor intrusions significantly differ by this parameter (4–22%). The argon isotopic composition (40Ar/36Ar) pro vides a possibility for precise calculation of the atmo spheric argon portion in mineralforming fluids. Migration together with infiltration and sedimenta tion waters is the only way of air argon penetration to a depth. Because of this, the ratio of isotopes charac terizes the degree of participation of nearsurface waters in the mineral formation. The results of mea surements provide evidence for a high portion of air argon in fluid inclusions of the studied objects (60– 100%). It is especially high in rich intrusions (from 88 to 100%). Airsaturated waters of host sedimentary rocks participated in the formation of rocks and ores in intrusions of the region. Intermediate intrusions sig nificantly differ from the rich ones: they contain only 60–85% of atmospheric argon. All intrusions differ among each other by the com plex of data on helium and argon isotopes. Intrusions with rich, intermediate, and poor ore potential are dis tinguished into definite criterion zones on the diagram with 3Не/4Не and 40Ar/36Ar coordinates. Thus, the helium–argon isotopic criterion provides the possibil ity to estimate the ore potential of mafic intrusions in the Norilsk region at the initial stages of the geological prospecting investigations by an effective and eco nomically profitable method. The isotopic composition of sulfur in sulfides (34S/32S ratio) studied in sulfide ores is most likely ade quate to that in fluid sulfur. The measurements dem onstrate that rich intrusions contain isotopically heavy sulfur (the high concentration of 34S, δ34S = 7–15‰) related to the crustal source by this parameter. Sulfur of intermediate and poor intrusions is not distinguish able by the isotopic composition (0–9‰); the contri bution of crustal sulfur is lower in them. Thus, the sul fur isotopic composition is involved as an additional criterion for the estimation of the ore potential of the studied intrusions.
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GEOCHRONOLOGICAL AND ISOTOPE GEOCHEMICAL CHARACTERISTICS
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U–Pb age of zircons of various genesis and Re–Os isochrone age of sulfide ores in mafic intrusions of the Norilsk region (Ma) Zircons magmatic
postmagmatic
inherited
Massive and impreg nated ores
Norilsk1
251 ± 2
228 ± 2
1900
–
Kharaelakh
265 ± 11
230–235
290, 347 ± 16
250 ± 2
Talnakh
256 ± 1
221–231
No
251 ± 13
Zub Marksheider
249 ± 4
225
No
248 ± 14
Imangda
243 ± 4
223 ± 3
320–330
–
Chernogorskaya
244 ± 3
227
290
248 ± 14
PyasinoVologochan
242 ± 10
225
330, 2730
248 ± 14
Mikchangda
256 ± 2
230
293
–
Yuzh. Pyasinskaya
242 ± 3
210
No
248 ± 14
Kruglogorskaya
241 ± 4
225
303–306
–
Zel. Griva
241 ± 6
No
No
250 ± 14
Nizh. Talnakh
254 ± 4
220–230
270, 300
247 ± 45
Nizh. Norilsk
247 ± 6
230
1900
251 ± 20
Binyuda (Taimyr)
249 ± 11
No
No
251 ± 13
Intrusion
The data on the isotopic composition of copper and nickel in sulfides may be used for this purpose as well. It is established that industrial orebearing intru sions contain isotopically fractionated copper with a clear negative correlation between the δ34S and δ65Cu values. Isotopic and geochronological investigations by the local U–Pb method SIMS SHRIMP by accessory zir cons of intrusions and Re–Os method by sulfide ores (table) revealed the chronology of the main geological events in the Norilsk region. (1) The formation of magmatic zircon in each of 14 studied intrusions reflects the time of their silicate material crystallization. (2) Two successive episodes (pulses) of intrusion and crystallization of silicate melt are distinguished: early, 254 ± 4, and late, 244 ± 4 Ma. (3) The similarity of the ore age (250 Ma, Re–Os method) provides evidence for the link between the ore formation and intrusion and crystallization of the silicate melt. Early intrusions are usually orebearing. They also contain a high concentration of xenogenous zircons of Permian and Carboniferous age. Such zir cons reflect the age and the composition of host rocks and provide evidence for the degree of their assimila tion. Single zircon grains of Proterozoic and Archean age (1.9 and 2.7 Ma) were found only in the Norilsk and PyasinoVologochan intrusions, which points to the presence of an ancient basement in the Norilsk region. (4) Secondary metasomatic processes resulting in the formation of metasomatic generation of zircons occurred in the period of 220–230 Ma. Intrusion of DOKLADY EARTH SCIENCES
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Bolgotokh granite at 229 ± 0.4 Ma [6] and throughout formation of biotite with an age of 225–230 Ma (40Ar/39Ar) occurred at that time in the Norilsk region as well. Comparison of the isotopic systems (Lu–Hf, zir cons, and Sm–Nd rocks) demonstrate that almost all intrusions of the Norilsk region are characterized by excessive radiogenic hafnium. Figurative points on the graph εNd(T)–εHf(T) plot above the correlation area for magmatic rocks of the Earth [7]. The value of excessive radiogenic hafnium: δHf(T) = εHf(T) – {1.5εNd(T) + 3} > 5. Such an exotic component with δHf(T) > 2 could be inherited only from a relatively old source with an anomalously high Lu/Hf ratio, most likely from sedimentary rocks with a significant por tion of clay material. Consequently, the formation of ores is directly con trolled by assimilation of matter of host sedimentary rocks (including fluids) by mantle magmas. This is evi dent from isotopic characteristics of noble gases, sul fur, copper, neodymium, and hafnium. Age bound aries are established for intrusions with different ore potential and sulfide ores on their own. The complex isotopic criterion of the ore potential of the Norilsk type intrusions was established. The most promising are intrusions with an age of 254 ± 4 Ma, the presence of excessive radiogenic hafnium (δHf(T) > 5), the prevalence of atmospheric–crustal argon and helium, isotopically heavy sulfur, and frac tionated copper from sedimentary rocks. The results of our investigations have important theoretical and applicable importance and may be effectively used
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during predicting and estimative investigations in the Norilsk region. ACKNOWLEDGMENTS The authors are grateful to K.N. Malich and I.Yu. Badanina for their help in obtaining unique materials for this investigation. REFERENCES 1. S. S. Neruchev and E. M. Prasolov, Platinum of Russia (Geoinformmark, Moscow, 1995), pp. 94–101 [in Rus sian].
2. V. A. Grinenko and L. N. Grinenko, Geochemistry of Sulfur Isotopes (Nauka, Moscow, 1974) [in Russian]. 3. N. T. Arndt, G. K. Czamanske, R. J. Walker, et al., Econ. Geol. 98, 495–515 (2003). 4. S. L. Kamo, G. K. Czamanske, Y. Amelin, et al., Earth Planet. Sci. Lett. 214, 75–91 (2003). 5. G. K. Czamanske, J. L. Wooden, R. J. Walker, et al., Intern. Geol. Rev 42, 895–927 (2000). 6. S. L. Kamo, G. K. Czamanske, Y. Amelin, et al., J. Conf. Abs 5 (2), 569 (2000). 7. K. I. Lokhov, I. N. Kapitonov, E. M. Prasolov, and S. A. Sergeev, Dokl. Akad. Nauk 425 (5), 660–663 (2009) [Dokl. Earth Sci. 425A, 463 (2009)].
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