Composition and Genesis of Serpentinite-Type Nephrite, Mount Bikilyar, South Urals, Russia

The object of this study is nephrite found on Mount Bikilyar, South Urals, Russia. The purpose of the study is to determine the mineral and chemical composition of the nephrite and host rocks and to develop a model of the nephrite’s formation. The mineral composition was studied using a scanning electron microscope with an energy-dispersive quantitative microanalysis system. We studied the chemical, trace element, and oxygen isotopic composition. The nephrite’s composition is dominated by tangled fibrous and downy aggregates, with large lamellar grains of actinolite. Magnesian hornblende and resorbed diopside grains, as well as chromite, titanite, apatite, chlorite, sulfides, and secondary minerals, are less common. The host rocks are metagabbro of diverse mineral compositions. The nephrite is characterized by a subchondritic trough-like distribution spectrum of REE with a negative Eu anomaly. The chondrite-normalized spectrum of the metagabbro is characterized by significantly higher REE content values. The δ18O isotopic composition of the nephrite is +5.53 to +5.80‰, indicating the deep origins of oxygen inherited from ultramafic rocks. The extremely heterogeneous and heavier composition of the metagabbro is +4.42 to +11.27‰, reflecting different contributions of the mantle and crustal sources to its formation. The nephrite was formed as a result of ultramafic xenoblock transformation under the influence of gabbro. Initially, serpentine was replaced by diopside, after which actinolite developed to form the nephrite. The Bikilyar nephrite differs in its mineral composition from the nephrite of other deposits. Despite the unique mineral composition, Bikilyar nephrite is a typical serpentinite in terms of paragenesis and chemical composition. The nephrite-specific mineral and chemical composition is explained by the small size of the ultramafic block that is in a significant volume of gabbro.