Mineralogical Characteristics And Significance Of Spinels In The Kimberlite And Petrographic Characteristics Of Kimberlite In Wafangdian

The investigated region is in Wafangdian,Liaoning Province,which is an important diamond-producing area located in the southern margin of North China Craton.The study of diamondiferous kimberlite in this area is of great significance to understand the characteristics of mineralization of diamond deposits in China and to guide the exploration of ore deposits in this area.Spinels occur as common accessory minerals in kimberlite.The petrographic characteristics and chemical composition of spinels reflect the environment of mineral formation,which is important to evaluate the potential of diamond in the kimberlite.There are three Kimberlite metallogenic belts in Wafangdian area.This paper chose four kimberlite pipes(NO.50,NO.30,NO.42,NO.1)as research objects.The first three pipes have high ore grade,and the last one pipe has low ore grade.In this paper,we did petrographic observation of kimberlites and mineral identification in 4 pipes.We also did electronic probe and Raman spectrum analysis on spinels from NO.30 pipe.In addition,we collected the composition data of the spinels in kimberlite from NO.50 pipe and Changmazhuang to make a comparative analysis with the data of NO.30 pipe.Hand specimens and petrographic observations suggest the kimberlite from Wafangdian has porphyritic texture and brecciated structure.Phenocryst minerals are mainly olivine and a small amount of phlogopite.The matrix is composed of olivine and phlogopite.The olivine has all been altered to serpentine Phlogopitization,serpentinization and carbonation commonly developed in Kimberlite The kimberlite contains two types of mantle xenocrysts(olivine xenocryst and spinel xenocryst)and three types of xenoliths(homologous xenoliths,non-homologous xenoliths and wall rock xenoliths).Homologous xenolith refers to former kimberlite which are broken up and trapped by latter kimberlitic magma,whose texture,structure and mineral composition is consistent with latter kimberlite.Non-homologous xenolith refers to ultrabasic rocks brought by kimberlitic magma from deep mantle.Wall rock xenolith refers to shallow wall rock breccia.Homologous and non-homologous xenoliths are dark green,spherical or elliptical,whose are named rock-ball.The kimberlites from 4 pipes in Wafangdian can be furthered divided into several types according to the characteristics of phlogopite and xenoliths in kimberlite The type of kimberlite in No.50 pipe is mainly porphyritic kimberlite,which has a little phlogopite and xenolite.The kimberlite in NO.30 mainly includes two types of kimberlites,one type is kimberlite with a large amount of rock-balls and wall rock xenoliths,and another type is kimberlitic breccia.The type of kimberlite in NO.42 is mainly kimberlite with a large amount of rock-balls and wall rock xenoliths.The type of kimberlite in No.1 is mainly phlogopite-rich kimberlite.NO.50,NO.30,NO.42 and NO.1 pipes are in a descending order of ore-bearing.The Ore-rich kimberlite has more olivine xenocryst which is in larger particle size,lower phlogopite content and more non-homologous xenoliths.Spinel xenocrysts in No.30 pipe are dark,large particle diameter(500-1000?m),mainly 700 ?m,partially embayed as well as corroded.The shape of spinel xenocrysts are mainly elliptical and little sub-angular.With high Cr2O3 content(up to 66.56wt%)?MgO content(8.88-16.68wt%)and low TiO2 content(mostly<1wt%),spinel xenocrysts are classfied as magnesium chromite.The morphology and chemical composition characteristics show that the spinel xenocrysts are mantle xenocrysts.Partial spinel xenocrysts in NO.30 pipe are located in the area of the diamond,which indicates that these spinel xenocrysts grow with diamonds and the NO.30 kimberlite has the ability to carry the diamond.Compared with kimberlite in No.50 pipe and Changmazhuang,the potential of diamond in the kimberlite of NO.30 is weak.