Genesis of nephrite-bearing magnesian skarns in the Kavokta deposit, Vitim region, eastern Buryatia, Russia: Evidence from petrography, geochemistry, and zircon U–Pb ages and δ18O values

• Nephrite in the Kavokta nephrite deposit formed by successive replacement processes. • The Kavokta nephrite deposit formed at ca. 343.9 Ma. • The nephrite-forming fluids comprised meteoric–magmatic water and CO 2 . Nephrite consists predominantly of fine-grained tremolite–actinolite aggregates and typically occurs at the contact between granite/granodiorite and dolomitic marble. Most previous studies have focused on the mineralogy, geochemistry, age, and ore-forming fluids of nephrite deposits. However, the detailed formation processes of nephrite remain unclear, particularly in terms of the occurrence of fine-grained tremolite aggregates in nephrite and whether granitoids can be directly replaced by nephrite. The Kavokta nephrite deposit is the largest dolomite-related nephrite deposit in Russia and is an ideal target for investigating the formation of nephrite deposits because it contains a relic mineral assemblage derived from granitoids and multiple generations of minerals that crystallized during nephrite formation. In this study, we undertook a comprehensive set of observations and analyses, including back-scattered electron (BSE) imaging, TESCAN integrated mineral analysis (TIMA), X-ray fluorescence spectrometry (XRF), inductively coupled plasma–mass spectrometry (ICP–MS), electron probe microanalysis (EPMA), laser ablation inductively coupled plasma–mass spectrometry (LA–ICP–MS), and isotope ratio mass spectrometry (IRMS), to constrain the genesis of the Kavokta nephrite deposit. The igneous minerals (e.g., apatite, allanite, titanite, thorite, rutile and ilmenite) in the nephrite are relics after metasomatism of the granitoids by ore-forming fluids. The relic igneous minerals, along with the metasomatic grossular and diopside, and other multi-stage mineral assemblages (e.g., epidote, tremolite, and talc; serpentine–talc; phlogopite–chlorite; tremolite–chlorite; and tremolite–talc) in the nephrite, record magmatic, contact metamorphic, and prograde and retrograde metasomatic stages. This suggests that the nephrite with fine-grained tremolite formed by the successive replacement of coarse-grained tremolite aggregates. Both green and white nephrite samples have similar rare earth and trace element features, characterized by moderate negative Eu anomalies, U enrichment, and Nb depletion. Hydrothermal zircons occur in two samples of green nephrite collected near the host granitoids. The samples have whole-rock Zr contents of 43 and 31 ppm, and the zircons yield a concordant 206 Pb/ 238 U age of 343.9 ± 1.2 Ma (n = 10; MSWD = 2.3), similar to the age of the host granitoids. Oxygen and hydrogen isotope data indicate the ore-forming fluids in this deposit were predominantly meteoric water, very possibly with contributions from magmatic water and CO 2 derived by dolomite decarbonation. In general, the Kavokta deposit is a typical dolomite-related nephrite deposit, and the formation of nephrite at the contact between the granitoids and dolomitic marble resulted in the retention of some relic minerals from the granitoids and the crystallization of multiple generations of minerals of various sizes. We propose a model of nephrite formation in which both granitoids and dolomitic marble were metasomatized by ore-forming fluids, leading to nephrite formation and subsequent recrystallization of coarse-grained tremolite to fine-grained forms.