Origin of the Fangmayu Chromite Deposit, Miyun, Beijing: Constraints from Electron Microprobe Analyses of Cr-Spinel

Stratiform chromite deposits in China have long been considered to form by fractional crystallization of mafic magma. However, some researchers recently proposed that some chromite deposits are also hosted in ophiolite. In this study, Cr-spinels of the dunites, wehrlites and clinopyroxenites from the Fangmayu chromite deposit in Beijing are analyzed by EPMA. Results show that Cr-spinels of dunites and wehrlites at the early stage of magma differentiation are rich in Cr with an average Cr₂O₃ content of 43.32%. At the late stage of magma differentiation, the crystallization of clinopyroxenites consumed large amounts of Cr³⁺ and elevated the oxygen fugacity, leading to the Cr-rich magnetite and Al-rich spinel with average Cr₂O₃ contents of 10.32% and 15.77%, respectively. By comparing with the chemical composition of different types of Cr-spinels from the world, the suggestion is that Fangmayu chromite deposit was a magmatic deposit hosted in Alaskan-type intrusion, and was not related to ophiolite. The constraints on the genesis and metallogenic specialization provide evidence for mineral prospecting of Cr, Cu-Ni and PGE deposits hosted in mafic-ultramafic intrusions.