Kavokta Deposit, Middle Vitim Mountain Country, Russia: Composition and Genesis of Dolomite Type Nephrite

The Kavokta deposit of the dolomite type nephrite is located in the Middle Vitim mountain country, Russia (Russian Federation). The deposit area is composed of granite of the Late Paleozoic Vitimkan complex. The granite contains complex shape blocks of Lower Proterozoic rocks. They are represented by metasandstone, crystalline schist, amphibolite, and dolomite marble. The calcite–tremolite and epidote–tremolite skarns were formed on the contact of dolomite and amphibolite. Calcite–tremolite skarn contains nephrite bodies. The mineral composition of 16 core samples obtained during the geological exploration conducted by JSC “Transbaikal Mining Enterprise” within Vein 1 of Prozrachny site has been studied in thin sections using a petrographic microscope, and in polished sections using a scanning electron microscope, with an energy-dispersive microanalysis system. Twenty-five minerals have been identified. They have been attributed to relict, metasomatic associations of the pre-nephrite and nephrite stages and hydrothermal and secondary associations. The intensity of the nephrite’s green color is explained by the Fe admixture in tremolite, and the black color is explained by its transition to actinolite in the areas of contact with epidote–tremolite skarn after amphibolite. In the formation and alteration of nephrite, dolomite is replaced by diopside, diopside by tremolite, prismatic tremolite by tangled fibrous tremolite, and tremolite by chlorite. Granite provides heat for metasomatism. Participation of amphibolite in the nephrite formation determines the variety of nephrite colors. The role of metamorphism is reduced to tectonic fragmentation facilitating fluid penetration; stress provides a tangled fibrous cryptocrystalline texture.