Comparison On Petrologic Geochemistry Of Three Mafic-ultramafic Intrusions Associated With Ni-Cu Sulfide Deposits In Northern Xinjiang

Recently, the increased demand of Cu, Ni, Pt and Pd, etc., greatly promotes the research onthese mafic-ultramafic rocks associated with magmatic Ni-Cu-(PGE) sulfide deposits.Furthermore, the mafic-ultramafic rocks can be an important object to understand crust-mantlemagmatism and the process of continental geodynamics.In the past twenty years about 20 magmatic Ni-Cu-(PGE) sulfide deposits and occurrenceshave been discovered in northern Xinjiang, China. These Ni-Cu-(PGE) sulfide deposits, relatedto mafic-ultramafic intrusions, are one component of the large scale metallogenesis duringpost-collisional tectonism in Late Carboniferous–Early Permian within the Central AsianOrogenic belt. To understand the origin of these Ni-Cu-(PGE)-bearing mafic-ultramaficintrusions is potentially important not only to the guidence exploration on Ni-Cu-(PGE) sulfidedeposits in orogenic belt but also to the study on the evolution of the CAOB. However, theprevious studies were focused on these Ni-Cu-(PGE) deposits, with little attention to the originand evolution of these mafic-ultramafic intrusions. Thus, three intrusions, including Baishiquanintrusions in Central-Tianshan Block, Huangshandong intrusions in Jueluotage orogenic beltand Kalatongke intrusions in Altay orogenic belts, are chosen for objects in this dissertation.The characteristics of petrology, mineralogy and geochemistry on these intrusions areinvestigated for the purpose as follow: (1) to determine the petrogenesis and evolution of theserocks;(2) to provide some clues for Ni-Cu-(PGE) sulfide ore genesis;(3) to discuss the mantlesource regions to understand the geodynamic processes;(4) to compare these characteristicsand find the reasons of similarities and differences, to discuss the regularities of formation forthe magmatic Ni-Cu-(PGE) sulfide deposits in orogenic belt. (5) to provide some constraints forthe process of post-collisional tectonism.On the basis of the systematically research on three mafic-ultramafic intrusions hostingNi-Cu-(PGE) sulfide deposits in the CAOB, some conclusion has been drawn:(1) The Baishiquan, Huangshandong and Kalatongke mafic-ultramafic intrusions occurredin the postcollisional setting and were related to lithospheric delamination and magmaticunderplating.(2) These intrusions have different rock assemblages and host rocks for Ni-Cu deposits,although the petrogenetic minerals are same. The Baishiquan and Huangshandong intrusionsare composed of peridotite, pyroxenite, hornblendite, troctolite, norite, gabbro and diorite, andthe Kalatongke intrusions consist of norite, gabbro-norite and biotite diorite. The peridotite andpyroxenite are the main host rocks for the Cu-Ni ores in the Baishiquan and Huangshandongintrusion, while norite in Kalatongke intrusion.(3) All of the intrusions are characterized by enrichment in large ion lithophile elements(LILE, e.g., Rb, Sr, K, U, Pb and Th) and light rare earth elements (LREE), depletion in highfield strength elements (HFSE, e.g., Nb, Ta, Ti and P) in relative to primitive mantle andMORB. Their Sr, Nd, Pb and O isotope data suggest a mixture of depleted asthenosphericmantle with MORB-like isotopic signature and subducted slab with EMⅡisotopic signature.The absolute PGE abundances are low, and the PGE is mainly affected by crystallizationdifferentiation in the Baishiquan and huangshandong intrusions.(4) The parental magmas are high-magnesium tholeiite, but have different contents in MgO.(5) The primitive magmas are derived from the asthenospheric mantle contaminated bypreviously subduction material. The primary magma with high-magnesium componentsexperienced two stages evolution. Olivine is the main fractionated phase in first stage of magmaevolution, and sulfides segregated from the silicate magma. The parental magma, which isolivine-and sulfide-laden crystal mush, entered the high-level magma chamber anddifferentiated in situ to form these intrusions. The sulfides segregation can be ascribed to thecrust contamination and the olivine crystallization.