In Situ U–Th–Pb Dating of Parisite: Implication for the Age of Mineralization of Colombian Emeralds

Abstract

Parisite-Ce (Ca(Ce,La)2(CO3)3F2) is a rare-earth (REE) fluorocarbonate mineral first described from the world-famous emerald mines of the Muzo district, Boyacá Province, Colombia. Four samples of parisite-Ce collected from outcrops near Muzo have been geochemically studied and dated using the in situ laser ablation U–Th–Pb method. Our study shows that the REE abundance of parisite is controlled by the leaching of the wall rocks (black schist). Furthermore, we show that the parisite-Ce crystals formed in textural equilibrium with the emeralds, suggesting a similar time of crystallization. Our analysis demonstrates the capability of parisite as a geochronometer and shows that precise and accurate U–Th–Pb ages can be obtained from parisite after common 207Pb correction. A higher precision date was obtained with the Th–Pb ratio rather than with the U–Pb ratio because of the relatively higher content of Th than U in the samples. The samples yielded 208Th–232Pb ages ranging from ~47 to 51 Ma. The new ages are ~10 Ma older than previously reported Ar–Ar ages and ~10 Ma younger than previously reported Rb/Sr ages. These results will have significant implications for understanding the timing of mineralization and crystallization of emerald deposits in Colombia. Furthermore, this study opens new avenues for dating similar deposits worldwide.

Keywords

• geochronology
• U–Th–Pb dating
• parisite
• calcite
• emeralds
• Muzo