Voimakan deposit of dolomite type nephrite, Middle-vitim mountain country: formation conditions

Abstract

The Voimakan deposit of dolomite type nephrite has been investigated in order to clarify the features of its formation. 12 samples of nephrite and 5 samples of host rocks were studied. A binocular stereomicroscope, a gemological flashlight and a polarizing petrographic microscope were used. The contents of macro- and micro-components, the isotopic composition of oxygen were 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 value of δ 18 O of nephrite is –18.5 ÷ –18.8%; calcite-tremolite skarn –17.4%; epidote-tremolite scarn –4.4, 2.6%; dolomite 26.1%. Nephrite meets the requirements for gemstone raw materials. Diopsidite with nephrite lenses and interlayers can be used for carving multicolored products or inlays. The green shade of nephrite increases with an increase in the Fe 2+ content. The brown color of nephrite is determined by Fe 3+ in the tremolite structure. The dolomite type of nephrite is confirmed by the ratio of Mg and Fe, a reduced content of Cr, Ni, Co, an increased content of F and the ratio of Sr to Ba, and the nature of the REE distribution. The distribution of REE in nephrite is determined by the composition of the initial dolomite under the influence of epidote-tremolite scarn. The source of abnormally isotopically light oxygen of nephrite is a meteoric fluid depleted in 18 O as a result of dolomite decarbonation. Granite only provides regional heating, activating the fluid. Both metasomatic and metamorphic processes were involved in the nephrite formation and transformation. The formation of nephrite is associated with the formation of calcite-tremolite and epidote-tremolite skarns. Tectonic stresses caused the crushing of rocks, facilitating the penetration of fluid, provided the formation of a nephrite cryptocrystalline tangled fibrous structure. But further regressive metamorphism led to the development of chlorite and talc, which worsened the nephrite quality.