Occurrence of Nb, Ta, and Cs in Precambrian low-grade metamorphic basement rocks in South China: Mineralogical characterization and petrogenetic significance

Abstract

This paper describes the occurrence of Nb, Ta, and Cs in Precambrian low-grade metamorphic basement rocks in South China. The occurrence of Nb, Ta, and Cs in slates was investigated using a combination of chemical and structural characterization techniques (i.e., trace element geochemistry, electron probe microanalysis, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy). Cs⁺ is hosted in either Cs-bearing sericite. Nb⁵⁺ and Ta⁵⁺ occur mainly in the octahedral sites of sericite and Fe-chlorite. During prograde metamorphism, the Cs⁺, Nb⁵⁺, and Ta⁵⁺ ions that were originally adsorbed on the surfaces of kaolinite and montmorillonite were incorporated into the lattice of phyllosilicate minerals such as sericite and Fe-chlorite. Crystal-chemical calculations reveal a clear compositional trend from Al- to Fe-chlorite with increasing temperature, along with an increase in Nb contents. Our results provide direct mineralogical evidence for the pre-enrichment of ore-forming elements in the basement rocks of South China, as well as in similar settings worldwide; e.g., the Northern Cape and southern Ghana Li-Cs-Ta deposits in Paleoproterozoic shales of the Wa-Lawra Belt. The Nb- and Ta-bearing Fe-chlorite and Cs-bearing sericite are examples of new occurrences of critical metals in low-grade metamorphic rocks.