Origin of Light-Colored Nephrite Relative to Green Nephrite in Ospa, Russia

In serpentinite-related green nephrite, light-colored nephrite commonly occurs as veins or interlayers at nearly every nephrite locality. However, its characteristics have been poorly understood. In this study, samples containing both green and light-colored nephrites from the Ospa deposit in East Sayan, Russia, were selected for integrated petrographic and geochemical analyses. Petrographic observations identified three domains transitioning from green to light-colored nephrite: the green domain, the transition band, and the light-colored domain. Although both the green domain and the transition band consist of oriented tremolite fibers, the fiber orientations differ between these two domains. In contrast, the light-colored domain typically exhibits non-oriented fibers and fine grains with flow textures. Geochemical data reveal a decrease in Cr and Ni contents from the green domain to the light-colored domain, while Fe and Mn contents remain relatively stable. Additionally, significant enrichments of Hf and Zr were observed in the light-colored domain. These observations suggest that the light-colored domain precipitated from a later-stage fluid distinct from the fluid sources responsible for the green nephrite, with the transition band resulting from the late fluid reacting with syn-deformational green nephrite. Thus, a model for light-colored nephrite formation can be simplified as a process in which a late fluid, low in Cr and Ni, precipitates predominantly within the stretched structures of green nephrite. UV-Vis spectroscopy on the studied samples, combined with a comparative analysis of chromogenic complexes in nephrites of various colors, further indicates that the green hue of the specific grayish-green in light-colored nephrite is dominantly controlled by Cr, whereas its grayish hue is modified by the ratio of (Fe + Mn) to Cr. These findings elucidate the formation sequence and genetic mechanisms behind light-colored nephrite, offering a thorough comprehension of nephritization in serpentinite-related deposits.