Mineralogical Characteristics And Indicative Significance Of Chromium Spinel In Kimberlite,Wafangdian, Liaoning Province

Wafangdian, Liaoning province is an important diamond-producing region inChina. Study on kimberlite from this area help understanding the metallogenic modelof diamond deposit and have practical significance for prospecting. The Cr-spineloccurs in kimberlite as a ubiquitous accessory mineral, it is more resistant tometasormatism and corrosion than other mantle or magma genesis minerals, thechemical composition reflects the physical and chemical condition of its crystalizationprocess, which provides the information of mantle condition and the genesis ofkimberlite.Hand-specimens observation associated with rock slices analyzing suggest thatthe kimberlites from Wafangdian are mostly porphyritic kimberlite, porphyritic micakimberlite and breccia kimberlite. The major mineral composition of Wafangdiankimberlite includes primary minerals like olivine and phlogopite, and altered mineralssuch as andadite, sphene, barite, magnetite and metal sulfide. Cr-spinel in Wafangdiankimberlite can be divided into Cr-spinel phenocryst and Cr-spinel in matrix base onthe occurence, the Cr-spinel phenocrysts display rounded crystal edges and surfacecorrosion features, while the Cr-spinels in matrix are small idiomorphic octahedralcrystals with alteration girdle zones. Furthermore, kimberlites in this area haveexperienced intense alteration, serpentinization, carbonatization and siliciication arecommonly seen.Electron microprobe analyzing indicates that the Cr-spinel phenocrysts arechromites, characterized by high Cr, Mg and low Ti, while the Cr-spinels in matrix areferro-chromites or aluminum chromites.It is inferred that the Cr-spinel phenocryst is xenolith from deep mantle thatclosely linked with diamond in genesis, its chemistry composition matches well withthe Cr-spinel inclusions in diamond from Wafangdian, thus can be used as animportant indicator for diamond-bearing kimberlite. On the basis of olivine-spinelgeothermometry, the crystallization temperature of Cr-spinel phenocryst is estimatedto be1261.26℃. The chromium spinel in matrix is formed in late magatic stage, the zoning feature reflet it is crystalized in low oxygen fugcity environment and subjectedmetamorphism after the magma consolidated.