Mineralogy and geochronology of pegmatites in the Kontum Massif, Central Vietnam: Implications for evolution of rare-metal mineralization

Abstract

The Lavi pegmatite located within the Kontum Massif of central Vietnam, represents a newly discovered rare-element pegmatites with significant economic potential. However, the emplacement age of the Lavi pegmatite and processes governing rare-element mineralization remain unclear. This study endeavors to elucidate these aspects through an examination of mineral chemical compositions and U-Pb isotopes of columbite-tantalite (coltan) group minerals and monazite sourced from distinct types of Lavi pegmatite. Four distinct pegmatite types were identified based on mineral compositions: muscovite-albite (type I), muscovite-albite-beryl-montebrasite (type II), muscovite-cassiterite (type III), and lepidolite- topaz-albite (type IV). The detailed EPMA analysis of minerals found in different types indicates that the Lavi pegmatite can be generally classified as the rare-element pegmatite, with a notable lepidolite subtype. Rare elements are primarily hosted within minerals such as montebrasite, lepidolite, beryl, coltan, and cassiterite. Geochronological data reveal a weighted mean ²⁰⁶Pb/²³⁸U age of 254.7 ± 2.8 Ma for monazite (type I) and 255 ± 5.7 Ma for coltan (type IV), suggesting the close temporal association of barren pegmatite and Li- mineralized pegmatites. Additionally, the linear correlation of K/Rb vs. Rb and K/Cs vs. Cs in mica compositions indicates continuous fractional crystallization from a common magma source. These findings provide insights into the pegmatitic evolution and underscore the economic potential of the Lavi pegmatite for rare metal extraction.