Mineral composition and formation model of dolomite type nephrite, Voimakan deposit, Middle-Vitim mountain country

Research subject . The Voimakan deposit of dolomite type nephrite. Aim. Determination of the mineral composition and development of a model for the formation of nephrite. Materials and Methods. 12 samples of nephrite and 5 samples of host rocks were studied. A binocular, a gemological lantern, a magnifying glass, and a petrographic microscope were used. The mineral composition was studied using a scanning electron microscope with an energy dispersive quantitative microanalysis system. Results. Nephrite is light salad, salad, gray-salad and brown (honey). It forms segregations in calcite-tremolite skarn bodies at the contact of dolomite marble and amphibolite transformed into epidote-tremolite skarn. Nephrite minerals are classified as parageneses: relict (minerals of dolomite, amphibolite and skarns): dolomite, magnetite, uraninite, fluorapatite, zircon, epidote I; metasomatic before nephrite: diopside, quartz I, okermanite, olivine; metasomatic nephrite: calcite I, tremolite; regressive metasomatic: quartz II, serpentine, talc, chlorite, epidote II; secondary: anglesite, vanadinite, wulfenite, hollandite (?), sylvin, uranophane, native bronze, copper, silver. Conclusion . Nephrite meets the requirements for gemstone raw materials. Intensive replacement with chlorite and, especially, talc has been developed, which significantly worsens the quality of raw materials. Diopsidite with nephrite lenses, interlayers can be used for carving multicolored products or inlays. A model of nethrite formation is presented with the initial formation of diopside on dolomite, its replacement by tremolite or calcite-tremolite aggregate; the early prismatic tremolite is replaced by a tangled fibrous cryptocrystalline tremolite. Skarn calcite can also be replaced by tremolite to form nephrite. With the continuation of the regressive process, tremolite is replaced by chlorite or talc in association with calcite.