Platinum Group Element Geochemistry And Metallogenic Potential Of Mafic-ultramafic Intrusions From The Eastern Area Of Jilin Province

The eastern part of Jilin province (east of Siping–Changchun–Yushu-line) is locatedin the eastern Central Asian Orogenic Belt, and it crosses two distinct geotectonic unitsincluding North China Craton and Xing–Meng Orogenic Belt. This region is thought tohave experienced many large-scale tectonic movements, which give rise to a series of deepfault zones. These large faults can provide conduits for mantle-derived magmas. Thespecial tectonic conditions lead to the formation of numerous mafic–ultramafic intrusionsand generally the exposed area of each of the intrusions is less than1km2. Theseintrusions frequently occurred as dykes, clustered repeatedly in corridors generallyoriented east–west. They follow the subsidiary northwestern (NW) or northeastern (NE)faults of the major Huifahe–Gudonghe and Benxi–Tonghua crustal fractures which seemto control the localization of the mafic–ultramafic complexes.Variable lithofacies are commonly developed in the mafic–ultramafic intrusions,which is the result of magma differentiation. The intrusions have overall compositionsequivalent to Fe-rich basic–ultrabasic rocks. This is proven by chemical analysis that mostof the analyzed mafic–ultramafic samples have m/f and Mg#values greater than2.0and70,respectively. Analysis of rare earth elements (REE) shows that the mafic–ultramaficsamples have a similar chondrite normalized REE pattern with light REE enrichment andheavy REE depletion. Moreover, the mafic–ultramafic samples also have comparableprimitive mantle normalized trace element patterns, both of which can be regarded asindicators for similar genesis of the mafic–ultramafic intrusions. Furthermore, they are allmarkedly depleted in high field strength elements, such as Nb, Ta and Ti. Then, weconclude that the mantle-derived differentiated magmas, to some extent, have beencontaminated by continental crust during ascent and emplacement, which is also demonstrated by Re–Os isotope study. The addition of crustal components from thecontamination may play an important role in triggering segregation and concentration ofdroplets of liquid sulfide from the mafic–ultramafic magmas.Most of the mafic–ultramafic complexes are ore-bearing intrusions of considerableeconomic value. They compose a number of magmatic Cu–Ni sulfide deposits, such as theHongqiling, the Piaohechuan, the Changren and the Chibaisong deposits. Among thesedeposits, the Hongqiling deposit is the second largest resource of Ni in China after thegiant Jinchuan magmatic Cu–Ni–(PGE) sulfide deposit. The dominant ores at thesedeposits are of the disseminated Cu–Ni sulfide variety, which contains pyrrhotite,pentlandite, chalcopyrite and pyrite. The Cu–Ni mineralizations most often occur in themore basic lithofacies, and element concentrations of ores and their host rocks areanalyzed using canonical correlations analysis. The result shows that there is a significantcorrelation between the ore elements and the common rock-forming oxides, which mayindicate that sulfide melts generated from sulfide segregation are not capable of migratingthrough partially solidified silicate rocks, or sulfide droplets may be carried in suspensionby flowing mafic–ultramafic magmas.Mafic–ultramafic intrusions in the study area are characterized by strong depletion ofplatinum group elements (PGEs). No economically viable PGE ore body has been foundso far. Most of the mafic–ultramafic rocks have relative high Ni/Cu values (1to10) andrelative low Pd/Ir values (<40), which suggest that the intrusions are generated by highdegree of partial melting of mantle. Moreover, Pt/(Pt+Pd) values (0.4to0.5) and(Pt+Pd)/(Os+Ir+Ru) values (5to25) of the ore samples fall between those of the oresassociated with komatiites and Sudbury deposit, which indicate that degree ofdifferentiation of the mafic–ultramafic magmas is greater than that of the komatiite, butlower than that of the Sudbury intrusion.We discuss the factors that may influence the PGE content of the mafic–ultramaficintrusions, such as the presence of PGE in the mantle, the degree of partial melting and theevolution of the mantle on comparison of temporal and spatial distribution of some typical Cu–Ni–PGE sulfide deposits at home and abroad. Then we expand on the relationshipbetween the PGE content of mafic–ultramafic intrusions and the age of mafic–ultramaficintrusions and the nature of the mantle. Finally, we draw a conclusion that PGEmineralization preferentially occurs in older mafic–ultramafic intrusions, especially thosederived from a fertile mantle.The mafic–ultramafic intrusions in the eastern part of Jilin province are small in size,and mainly formed from the late Paleozoic to early Mesozoic. They are quite small andyoung compared to the world-class PGE deposits which generally formed during the lateArchaean and early Proterozoic. Moreover, the intrusions in the study area arepredominantly located at an active craton margin, which is also really different from theworld-class PGE deposits emplaced into stabilized cratonic lithosphere. Furthermore,primary magmas of the mafic–ultramafic intrusions from the study area are derived fromdepleted mantle sources, which have been demonstrated by analysis of existingneodymium (Nd) isotopic data. All of the above factors may be the main causes of thePGE depletion in the mafic–ultramafic intrusions. Finally, we conclude that the probabilityof success in future search for sulfide deposits enriched in PGE would consequently bevery low in the mafic–ultramafic intrusions from the eastern part of Jilin province.