Petrology, Geochemistry, Ages And Ore-bearing Property Evaluation Of The Mafic-ultramafic Intrusions, The Middle Segment Of Inner Mongolia

Located in western part of the northern margin of the North China Craton, central Inner Mongolia exposes a mafic-ultramafic belt, EW extending about300km long and30km wide. This mafic-ultramafic belt, located on both sides of the fracture of the northern margin of the North China Craton, shows good facies differentiation and it is a favorable metallogenic belt for Cu-Ni (PGE) sulfide deposits. A large amount of Cu-Ni (PGE) deposits (ore occurrences) have been discovered here in the past years. Four ore-bearing intrusions, including Xiaonanshan, Huanghuatan, Kebu and Erbutu in this belt, are the subjects of this dissertation. The features of mineralogy, petrology, geochemistry and ages on these ore-bearing intrusions are studied for the purpose as follows:(1) to discuss the ages, tectonic setting and magmatic evolution of the four ore-bearing intrusions;(2) to set up a regional metallogenic model for the Cu-Ni (PGE) deposits in this area based on the features of ore-bearing intrusions;(3) to compare the geochemincal characteristics of mineral and petrology with other Cu-Ni ore-bearing intrusions as to find the similarities and differences, and to evaluate the prospecting potential for the studied intrusions preliminarily;(4) to provide some clues for regional magmatic evolution. Main results are as follows:The Xiaonanshan gabbros are mafic rocks (m/f=0.96-1.15) and belong to the tholetiitic series in petrochemical composition. Most of rocks are relatively enriched in large ion lithophile elements (Rb, Ba, U, Pb) and depleted in high field strength elements (Nb, Ta, Ti), and exhibit right-oriented linear REE patterns with LREE enrichment relative to HREE (LaN/YbN=3.03to31.67); The Huanghuatan gabbros and gabbro-diorites are mafic rocks (m/f=0.96-1.21), and belong to the calc-alkaline series in petrochemical composition. Most of rocks are relatively enriched in large ion lithophile elements (Rb, Ba, Sr, K, U, Pb) and depleted in high field strength elements (Nb, Ta, Zr, Hf, Ti), and exhibit right-oriented linear REE patterns with LREE enrichment relative to HREE (LaN/YbN=4.97to8.82). The Kebu intrusions are mafic-ultramafic (m/f=2.92-3.54) and belong to the tholetiitic series in petrochemical composition. Most of rocks are relatively enriched in large ion lithophile elements (Rb, Sr, Ba, Pb) and depleted in high field strength elements (Nb, Ta, Zr, Hf, Ti) and exhibit LREE-enriched ((La/Yb)N=2.47to9.64), right-inclined REE distribution patterns. The primary magma of Kebu intrusions should be high Mg basult with10.2%MgO and14.6%FeO by using olivine-melt equilibrium. The Erbutu intrusions are mafic-ultramafic (3.57-4.44) and belong to the tholetiitic series in petrochemical composition. Most of rocks are relatively enriched in large ion lithophile elements (Rb, Th, K, U, Pb) and depleted in high field strength elements (Nb, Ta, P) and exhibit LREE-enriched ((La/Yb)N=3.43to8.31), right-inclined REE distribution patterns. The primary magma of Erbutu intrusions should be high Mg picrite with16.0%MgO and10.1%FeO. LA-ICP-MS zircon U-Pb dating of Xiaonanshan, Huanghuatan and Kebu ore-bearing intrusions, yield mean ages of272.7±2.9Ma (MSWD=2.2),262.4±1.1Ma (MSWD=0.51) and258.1±1.8Ma (MSWD=2.3), respectively, indicating that three plutons emplaced at the mid-late Permain. The mineralogy, petrology, geochemistry and ages of the intrusion, combined with previous studies, suggest that there are two mafic-ultramafic magmatic activities in this area:one, associated with arc setting, occurred in Early Permain (294-285Ma); the other, associated with intraplate tectonic setting occurred in Mid-late Permain (272-258Ma). The first one formed Erbutu Ni deposit, whereas the other formed Xiaonanshan Cu-Ni (PGE) deposit, Huanghuatan Cu-Ni (PGE) deposit and Kebu Cu-Co-Ni deposit.Based on the study of geochemistry features of mineral, pertrology and the magmatic differentiation and comparison with typical Cu-Ni ore-bearing intrusions in the world, it is suggested that Wengeng, Kebu and Erbutu ore-bearing intruions have good conditions to form large deposits, whereas Xiaonanshan and Huanghuatan do not.