Geochemical and mineralogical characteristics of weathered ore in the Dalucao REE deposit, Mianning-Dechang REE Belt, western Sichuan Province, southwestern China

摘要

The Dalucao deposit in western Sichuan Province, southwest China, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning-Dechang REE Belt. Moreover, this is the only deposit identified in the southern part of the belt. The deposit contains the No. 1, 2, and 3 orebodies. The No. 1 and 3 orebodies are hosted in two breccia pipes within syenite-carbonatite rocks that intrude a Proterozoic quartz-diorite pluton. Both breccia pipes have elliptical horizontal cross-sections at the surface, being 200-400 m long, 180-200 m wide, and extending to >450 m depth. The No. 1 and No. 3 orebodies have total thicknesses of 55-175 m and 14-58 m, respectively. REE mineralization is associated with four brecciation events that are recorded in both pipes. The ore grades in the No. 1 and 3 orebodies are similar, with the rocks containing 1.0-4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a mineral assemblage comprising fluorite + barite + celestite + bastnasite (i.e., Type I), whereas the No.3 orebody is characterized by an assemblage comprising fluorite + celestite + pyrite + muscovite + bastnasite + strontianite (i.e., Type II). Significant amounts of weathered high-grade REE ore (up to 60 wt.% of the rock mass) is mainly present in the No. 1 orebody. This is the main ore-type targeted for exploration within the Dalucao deposit, but is rarely present in other deposits in the Mianning-Dechang REE Belt.