Composition and formation conditions of nephrite, Nyrdvomenshor deposit, Polar Urals

   Research subject. Nephrite and related rocks from the Nyrdvomenshor deposit in the Polar Urals were studied. The Nyrdvomenshor deposit is located in the exocontact of the Rai-Iz ultramafic massif, confined to the Main Ural Fault. The deposit was developed in the process of geological exploration; a license has been issued for a part of the deposit.   Aim. To study the nephrite and related rocks from alluvial of the deposit, to formulate a model of its origin.   Methods. Qualitative characteristics were assessed visually using a binocular microscope and a special flashlight. The chemical composition was determined by the X-ray fluorescence method. The contents of trace elements were determined by ICP-MS analysis. The mineral composition was studied on a scanning electron microscope with an energy dispersive microanalysis system. Measurements of the isotopic composition of oxygen were carried out.   Results. In addition to vesuvianite rodingite, hydrogarnet rodingite was found to be common at the deposit. The studied nephrite is substandard. Tremolite predominates in nephrite, diopside forms relic grains. Uvarovite is widespread, forming both idiomorphic grains, sometimes sheath, less often elongated xenomorphic, and replacing chromite. Omphacite overgrows grains of chromite and uvarovite. Grains of the Fe-dominant mineral of the shuiskite group are noted.   Conclusions. Nephrite was formed through both metamorphic and metasomatic processes. Serpentinite was replaced by diopside, which was then replaced by nephrite. Metamorphism enhanced the metasomatism of the serpentinite melange and provided the cryptocrystalline tangled fibrous structure of the nephrite. Then metamorphism and metasomatism led to the formation of omphacite and cracking of the nephrite, which reduced its quality. As these processes progressed, the contribution of the crustal fluid increased, which is confirmed by the results of studying the oxygen isotopic composition of nephrite and other rocks of the deposit.