Abstract:  The Chachaxiangka deposit in Qinghai Province is located in the eastern segment of the QaidamAltun UHPM complex in the northeastern Qaidam basin. It is the first albite aplite vein related U-Th-Nb-REE deposit recognized in China. Mineralization can be divided into two types: vein and disseminated veinlet types. Vein type mineralization is entirely hosted in albite aplite, while disseminated veinlet type refers to disseminated veinlets produced in greenschist wall rock. Three mineralization stages have been delineated according to detailed mineralogical analyses, i.e. magmatic, hydrothermal main mineralization and post-ore stages. The magmatic stage is characterized by the “droplet” U-Th-Nb mineral (uraninite, uranothorite, thorite, viggezzite, etc.) inclusions hosted in the albite grains. The main hydrothermal mineralization stage is mainly the REE mineralization stage, accompanied by Nb and Th mineralization. The major ore minerals include allanite, bastnaesite, thorite, Nb-titanite, zircon, etc., closely intergrown with low-temperature gangue minerals such as apatite, calcite, chlorite and quartz. The post-ore stage marks the termination of the mineralization system and is represented by the widespread calcite quartz stockworks. Based on our comprehensive analyses of mineralogical, major and trace elemental compositions, we suggest that the albitite vein is magmatic-hydrothermal in origin and derived from the partial melting of the REE- and Nb-rich metasomatized mantle source. Although phase separation between silicate and carbonate/phosphate melts could take place in the magmatic stage, immiscibility between silicate melt and chloride-dominated fluids is still the most important mechanism for REE mineralization and cause of Nb-Th re-mobilization and enrichment. The red color of albitite aplite vein is a useful prospecting mark in the field, and more mineralization can be expected in the deeply buried or peripheral areas of the deposit according to the metallogenic model of similar U-Th-Nb-REE mineralization systems.

Key words: Chachaxiangka deposit, U-Th-Nb-REE mineralization system, albitite, partial melting of metasomatized mantle, magmatic-hydrothermal mineralization