Coloration and Genesis of Calcite-Dominated Jade from Xinjiang, China: Evidence from Spectroscopy, U-Pb Dating, and C-O Isotope

Carbonate jade has emerged as a recently recognized commercial variety in the Chinese gemstone market. Systematic gemological and mineralogical investigations on carbonate jade, however, remain very scarce. Three Xinjiang calcite-dominated jade samples were investigated by using Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, ultraviolet-visible (UV-Vis) absorption spectroscopy, microbeam X-ray fluorescence (Micro-XRF) spectrometry, trace element analysis, in situ U-Pb dating, and C-O isotope analysis. The orange-red color originates from staining by hematite and magnetite inclusions, while the green color is produced by d-d electronic transitions of lattice-bound Fe3+ and Mn2+. The provenance comparison reveals systematic differences in trace element compositions between the Xinjiang carbonate jade and Pakistani Lvwen stone: the Xinjiang samples are characterized by extremely low Cu and Sr contents, whereas the Pakistani Lvwen stone has high Cu, Mn and Sr contents, and low Fe content. The U-Pb age obtained for the Xinjiang carbonate jade sample coincides with a Late Cretaceous rapid cooling event. Enriched light rare earth element (LREE) and C-O isotope (δ13CV-PDB = −1.19–2.21‰, δ18OV-SMOW = 15.00–20.01‰) signatures indicate that the carbonate-precipitating fluids were derived from marine carbonate wall rocks. These findings provide new mineralogical and geochemical constraints on the coloration mechanism, provenance, and fluid evolution of carbonate jade from Xinjiang.