Enrichment of REE and HFSE during the magmatic-hydrothermal evolution of the Baerzhe alkaline granite, NE China: Implications for rare metal mineralization.

The Baerzhe Cretaceous (123.7 ± 0.9 Ma) peralkaline pluton underwent extensive fractional crystallization and extreme enrichment in incompatible elements including rare earth elements (REE) and the high field strength elements (HFSE). It is one of the largest resources of rare metals in China, containing approximately 100 million tons of ore with an average grade of 1.84 wt% ZrO 2 , 1.00 wt% REE 2 O 3 (34% heavy rare-earth oxides), and 0.26 wt% Nb 2 O 5. Zr, REE and Nb are mainly hosted by hydrothermal minerals, such as zircon, hingganite-(Y), monazite-(Ce), polycrase, pyrochlore, fergusonite and columbite. An integrated investigation of field geology, mineral textures and compositions of minerals and host granites was carried out to examine the evolution of the Baerzhe pluton and the roles of magmatic and hydrothermal processes in concentrating REE and HFSE. Most minerals in the intensively altered subsolvus granite show secondary textures or replaced pseudomorphs, such as the replacement of arfvedsonite by aegirine, zircon dissolution and reprecipitation, and the replacement of monazite-(Ce) and polycrase-(Y) by hingganite-(Y). Compositions of the key economic minerals and changes with respect to these alteration stages reveal evidence that hydrothermal alteration played a role in the mineralization of the pluton. In-situ analyses and element mappings suggest that large volume of metals were remobilized and redistributed during hydrothermal replacement, such as the replacing of monazite-(Ce) and polycrase by hingganite-(Y). It is suggested that subsolidus re-equilibration and hydrothermal alteration, in addition to magmatic fractionation, is critical for further concentrating REE and HSFE in peralkaline granitic systems. • Enrichment of REE and HFSE in the Cretaceous Baerzhe peralkaline pluton. • Rare metals concentrated in the upper subsolvus granite with intensive alteration. • Remobilization and redistribution of rare metals during hydrothermal processes. • Subsolidus re-equilibration and alteration was critical to mineralization. [ABSTRACT FROM AUTHOR]