This paper is the result of mineral exploration engineering.

Objective Dazhuang deposit is a newly discovered medium-sized Nb-REE deposit (Nb₂O₃=16245t; Associated TRE₂O₃=30147 t)in Fangcheng, Henan Province and its ore body occurs within alkaline syenite.

Methods In this paper, the authors compared the differences between ore-bearing and barren alkaline syenites in terms of petrology, petrography and geochemistry based on detailed field observation.

Results The results show that both ore-bearing and barren alkaline syenites belong to typical A-type granite which is rich in alkaline and aluminium and there is no significant differences in alkali metal contents between them, whereas the ratios of (Na₂O+K₂O) /CaO, FeO*/MgO, K₂O/MgO are obviously different. Moreover, the ore-bearing alkaline syenite has high contents of high field strength elements such as Nb, Ta, Ce, U, Th, Zr and Y, but is depleted in Ba, Sr, P and Eu. The barren alkaline syenite displays high content of large ion lithophile elements of Rb and some high field strength elements such as Nb, Ta, Y and is depleted in Ba, U, Sr, P, Ti and heavy rare earth elements however, its depleted degree is lower than that of ore-bearing alkaline syenite.

Conclusions The comprehensive analysis implies that the ore-bearing alkaline syenite in Dazhuang deposit was not the product generated by hydrothermal alteration (albitization) of barren alkaline syenite, but formed from the same magmatic system at different evolution stages. The fractionation degree of the ore-bearing alkaline syenite is obviously higher than that of barren syenite and the enrichment of rare elements and rare earth elements in Fangcheng Dazhuang deposit is closely related to the high evolution process of A-type magma. These results may provide some petrological evidences for the REE prospecting in Fangcheng district.