Hf-isotope constraints on contrasting magma sources in economic and non-economic ultramafic-mafic intrusions of the Noril’sk area (Russia): Evidence from zircon

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Malich, Kreshimir۱; Belousova, Elena۲; Griffin, William۲; Pearson, Norman۲; Khalenev, Vladimir۱
Holding Date 08 September 2008

World-class platinum-group-element (PGE)-Cu-Ni deposits closely linked to intracontinental paleorift-related ultramafic-mafic intrusions are located in the northwestern corner of the Siberian craton, Russia. Recent U-Pb SHRIMP studies (Petrov et al., 2006; Malich et al., 2007) identified distinct age groups of zircon from the main types of ultramafic-mafic intrusions of the Noril’sk area. Most of concordant U-Pb ages lie in the range 230-270 Ma. Minor zircon populations show older U-Pb ages, which cluster around 300 and 340 Ma.
In situ Hf-isotope data (350 analyses) were collected on the dated spots within single zircon grains from the main lithological units of economic (Noril’sk-1, Talnakh and Kharaelakh), subeconomic (Chernogorsk and Vologochan), prospective Mikchangda and non-economic (Nizhny Talnakh and Zelyonaya Griva) intrusions of the Noril’sk area. The analysis used a New Wave LUV213 laser-ablation microprobe attached to a Nu plasma MC-ICP-MS at GEMOC (Griffin et al., 2002).
Zircons from economic intrusions with U-Pb ages between ca. 230-340 Ma yielded mean epsilon Hf(T) (parts in 104 difference between the zircon sample and the chondritic reservoir) values of + 9.4 (n=45) at Kharaelakh, + 11.3 (n=96) at Talnakh and + 12.2 (n=83) at Noril’sk-1, close to the mean value of the depleted mantle reservoir at that time. Zircons from subeconomic and prospective intrusions (n=38) with U-Pb ages between ca. 230-290 Ma show similar ranges in Hf-isotope values (mean epsilon Hf(T) +9.1 at Chernogorsk, +12.5 at Vologochan and +10.9 at Mikchangda, respectively). In contrast, zircons from non-economic Nizhny Talnakh and Zelyonaya Griva intrusions with U-Pb ages between ca. 215-305 Ma yielded epsilon Hf(T) values +1.0 (n=80) and +1.7 (n=11), respectively.
The Hf isotope data suggest that zircons from economic and subeconomic intrusions are characterised by the signature of a juvenile mantle-derived magma. The less radiogenic Hf isotope values of zircons from non-economic intrusions indicate mixing between mantle and crustal magma sources. Our new findings suggest the interaction of distinct magmas, indicating that ultramafic-mafic intrusions of the Noril’sk area have a more complex geological history than is commonly assumed. The "radiogenic" Hf isotope composition of zircon is an effective fingerprint for identifying prospective intrusions and consequently is useful in exploration for PGE-Cu-Ni sulphide ores associated with Noril’sk-type intrusions.