The Petrogenesis And Mineralization Potential Of Two Types Of Mafic-ultramafic Intrusions, Gansu Beishan, China

Beishan area is located in the southern margin of the Central Asian Orogenic Belt，and inthe northeast of the Tarim plate. The tectonic structural is complicated. For a long time, it isone of the hot area of geological research. Gansu Beishan is paid more attention in recentyears because in discovering Heishan Ni-Cu deposit，and have been found Guaishishan Cu-Nimineralized intrusion, Dashantou-Sangejing mineralized clues, and also distributed NorthHeishan, Yingmaotuo and Xilinkebo mafic-ultramafic intrusion. Studies have proved thatHeishan deposit and other mineralized intrusions were formed in the Late Devonian, activecontinental margin or continental margin rift. Diagenesis and mineralization background isdifferent from the East Tianshan and Beishan in Xinjiang. There are other mafic-ultramaficintrusions which formed in Permian and related with Tarim Large Igneous Province inBeishan Gansu. This paper selects Sangejing and Luotuoshan mafic–ultramafic intrusion,different tectonic setting and time, to research geological characteristics, petrology,mineralogy and geochemistry, investigate magma source, primary magma and magmaticevolution, evaluate mineralization potential. Conclusions are as following:（1）The zircon SHRIMP U-Pb dating for gabbro of Luotuoshan yielded282.6±2.5Ma,combined with Pobei intrusion. Geochemical studies indicate it was formed in Early Permianby joint action of extending after the collision and the plume. The Sangejing intrusion wasformed in Devonian, consistent with Heishan, Dashantou, Guaishishan intrusions which arelocated in the same tectonic belt. Tectonic setting is active continental margin.（2）The Sangejing intrusion is belonged to duplex small intrusion. and ironbasic-ultrabasic rocks. The rock alterations is strong, and geochemical characteristics of rocksare poor alkaline, alkaline and poor quality aluminum; The scale of Luotuoshan intrusion isbigger relatively. rocks are iron mafic rocks and alteration is medium. Geochemicalcharacteristics of rock are weak alkaline and low aluminum. The Sangejing intrusion is moreconducive to mineralization from the perspective of mineralization.（3）Comparative study of REE characteristic of ore-bearing and no mineralizationintrusions in East Tianshan-Beishan area, indicate that the total content of ore-bearingintrusions REE is bigger than no mineralization intrusions, fractionation of LREE,HREE andtheir internal elements are relatively strong. Characteristics of REE shows Sangejing andLuotuoshan intrusion are similar with ore-bearing intrusion in East Tianshan-Beishan area.Ratio of Cu/Zr implies that Sangejing and Luotuoshan intrusion occurred sulfide liquation. （4）Characteristics of rock-forming minerals show that olivines of Sangejing andLuotuoshan intrusion have a rich Si, Mg, Ni and Fe-poor characteristics. Olivine，scharacteristics are similar with the olivines of magmatic sulfide deposits. Sangejing intrusionhas rich Opx and Amp, but Luotuoshan intrusion has limited content of these two typesmineral. It is more conducive to mineralize for Sangejing intrusion.（5）Lower limit content of primary magma MgO in Sangejing intrusion is9.42%, andLuotuoshan intrusion is9.55%. Primary magma are all high magnesium tholeiitic basalts,similar to Ni-Cu-PGE deposit intrusions.（6） Sangejing intrusion was derived from subducting slab breakoff causingasthenosphere upwelling, asthenosphere and the mantle wedge occurred partial melting,which is similar to Heishan. Luotuoshan magma originated from depleted mantle, and hadobviously been transformed by subducted fluid.（7）Combined with mineralization indicators, Sangejing intrusion have potential to find"Heishan style" copper-nickel sulfide deposits. The tectonic setting of Luotuoshan intrusion issimilar with mineralization intrusions in Pobei Xinjiang, more research are needed on itsmineralization potential.