Provenance Tracing of the Green Manas Nephrite (Xinjiang, China): Insights from Folded Veins and Spheroids

The green Manas nephrite from Xinjiang (China), predominantly composed of felted tremolite fibers and typically occurring at contacts between serpentinized ultramafic rocks and enclosing basaltic–andesitic volcanics, contains diverse veins (including folded, Stage 1 straight, and Stage 2 straight varieties exhibiting ptygmatic, reticulate, and master-branching patterns, respectively) and spheroids (Type 1 and Type 2). Its provenance identification criteria have not yet been well established. This study presents geological, petrographic, and mineral chemical data from these veins and spheroids to constrain their mineral composition and texture. EPMA data indicate that the veins and spheroids are mainly composed of tremolite–actinolite fibers. The fibers in the Stage 2 branching veins are (sub-)parallel to the vein walls, whereas those in the other veins are (sub-)perpendicular to the walls. Type 1 spheroids are composed of felted fibers and commonly occur as breccia lenses within a nephrite matrix. In contrast, Type 2 spheroids are characterized by parallel-arranged fibers. The distinctive occurrence of the white (or gray) folded veins and spheroids serves as an important diagnostic criterion for provenance and is crucial for distinguishing the Manas nephrite from that of other localities.