Voimakan Deposit of Dolomite-Type Nephrite, Middle-Vitim Mountain Country: Formation Conditions

The Voimakan deposit of dolomite-type nephrite has been investigated to elucidate the features of its formation. Twelve nephrite samples and five host rock samples have been studied using a binocular stereomicroscope, gemological flashlight, and polarizing petrographic microscope. The contents of macro- and microcomponents and the oxygen isotope composition have been determined. Nephrite is light-salad, salad, gray-salad, and brown (honey). It forms separations in calcite‒tremolite skarn bodies at the contact of dolomite marble and amphibolite transformed into epidote‒tremolite skarn. The δ18O value is from ‒18.5 to ‒18.8‰ for nephrite, ‒17.4‰ for calcite–tremolite skarn, from ‒4.4 to +2.6‰ for epidote–tremolite scarn, and 26.1‰ for dolomite. Nephrite meets the requirements for gemstone raw materials. Diopsidite with nephrite lenses and interlayers can be used for carving multicolored products or inlays. The dolomite type of nephrite is confirmed by the Mg to Fe ratio; low Cr, Ni, and Co contents; high F content and Sr to Ba ratio; and shape of the rare-earth element distribution. The rare-earth element spectrum of nephrite is mainly inherent of initial dolomites, but has signs of the influence of skarning of the main rocks. The source of anomalously isotopically light oxygen in nephrite is a pore fluid, probably of meteoric origin, depleted in 18O due to decarbonation of dolomite. Granite only provides regional heating, which activates the pore fluid. Both the metasomatic and metamorphic processes have been involved in the nephrite formation and transformation. The formation of nephrite is associated with the formation of calcite–tremolite and epidote–tremolite skarns. The tectonic stresses caused crushing of the rocks, which facilitated penetration of the fluid and ensured the formation of a cryptocrystalline tangled fibrous structure of nephrite. Further regressive metamorphism has led to the development of chlorite and talc, which have worsened the quality of nephrite.