Mineral Chemistry and Genetic Constraints on Yiyuan Jade, Eastern Qinling Orogen, China

Abstract

Yiyuan jade, a specific gem-quality jade variety in the East Qinling Orogen, occurs as lenticular and vein-shaped bodies hosted within the dolomite and biotite-quartz marble of the Neoproterozoic Meiyaogou Formation. Based on detailed petrological and mineralogical characterization, three subtypes are defined: tremolite jade (nephrite), serpentine jade (a green variety of serpentine group minerals that resembles true jade), and their transitional varieties. Among these, tremolite jade is predominantly white, moon-white, and light green, whereas serpentine jade (gem-quality serpentine) exhibits dark green, yellow-green, and yellow coloration. Comprehensive mineral composition analyses are presented, encompassing amphibole, serpentine, carbonate minerals, and titanite in metagabbro, aiming to constrain the physicochemical conditions and genetic model of these jades. We propose that tremolite jade and serpentine jade are probably formed simultaneously via metasomatic processes, primarily governed by variations in fluid volume and the activities of SiO2 and CO2. The serpentine jade is interpreted as serpentinized marble, within which local serpentine aggregation forms serpentine jade akin to Lantian jade in the western East Qinling region. The tremolite jade closely resembles Hetian tremolite jade within geochemical and mineralogical characteristics but differs significantly from serpentine-type tremolite jade associated with ultramafic rocks, thereby confirming its classification as dolomite-type jade. Application of the Ti-in-tremolite geothermometer yields a formation temperature exceeding 400 °C. Integrated with regional geochronological constraints, our findings link the formation of Yiyuan tremolite jade and serpentine jade to tectonic activity driven by the Late Devonian subduction of the Erlangping back-arc basin. This study not only enhances the current understanding of jade mineralization within collisional orogenic settings but also establishes a robust foundation for future exploration in the East Qinling Orogen.