A new type U-Th-REE-Nb mineralization related to albitite: A case study from the Chachaxiangka deposit in the northeastern Qaidam Basin of China

Abstract

The U-Th-REE-Nb (Ta)-polymetallic mineralization is generally related to either the silica-undersaturated syenites, the silica-oversaturated alkaline/peralkaline granites or igneous carbonatites. In this study, the authors report a new mineralization type, which is related to the magmatic-hydrothermal albitite (with mineral assemblage predominated by albite with volume content > 90%), as exemplified by the Chachaxiangka deposit in Qinghai Province of China. The Chachaxiangka deposit is the first albitite-related U-Th-REE-Nb deposit recognized in China and the mineralization can be divided into 3 types: the vein-type, the disseminated veinlet type and breccia type, of which the former 2 are predominant. Three mineralization stages can be identified according to the detailed mineralogical analyses, including the magmatic stage, main hydrothermal mineralization stage and post-ore stage. By comprehensive analyses of the mineralogical, major and trace element compositions, the authors suggest that the albitite vein is magmatic-hydrothermal in origin and both the magmatic evolution and overprint of the hydrothermal fluids play important roles in the formation of the albitite and related polymetallic mineralization. Phase separation between the silicate melt and carbonate/phosphate melt might take place in the magmatic stage, yet the immiscibility between the silicate melt and chloride-dominated fluids is the most important mechanism for the REE mineralization and also causes the Nb-Th re-mobilization and enrichment. The red color of the albitite aplite vein is an eye-catching prospecting mark in the field and more mineralization can be expected at depth and in the surrounding areas. The discovery of the new albitite type U-Th-REE-Nb mineralization give rise to new ideas during future U-Th-REE-Nb exploration, not only in the Qaidam-Altun belt, but also other areas across China.