Abstract

The Chuncheon nephrite deposit in Korea is among the largest nephrite jade deposits worldwide. The deposit is hosted in Precambrian dolomitic marble and amphibole schist that were intruded by the posttectonic (Late Triassic) Chuncheon granite. Carbonates in the dolomitic marble and nephrite deposit have oxygen and carbon isotope compositions in the range of –0.1 to +18.2 per mil and –4.3 to +0.9 per mil, and –0.4 to +3.5 per mil and –9.9 to –4.7 per mil, respectively. These data are in agreement with decarbonation processes driven by fluid infiltration forming the nephrite deposit from dolomitic marble in the temperature range of 330° to 430°C. Stable isotope compositions of silicates from the nephrite deposit are homogeneous and extremely depleted in 18O and D; tremolite has δ18O = –9.9 to –7.9 per mil and δD = –118 to –105 per mil; diopside has δ18O = –13.6 to –11.5 per mil; three samples of grossular have δ18O = –4.4, +10.4, and +11.5 per mil; clinochlore has δ18O = –9.5 to –9.0 per mil, and δD = –103 to –94 per mil; and two talc veins have δ18O = –7.7 and –7.3 per mil, and δD = –86 and –90 per mil. Quartz and amphibole in nearby amphibole schists also have variable oxygen isotope compositions of –4.7 to +0.5 per mil and –8.4 to –0.6 per mil, respectively, and are not in isotopic equilibrium. It is suggested that the formation of this nephrite deposit postdated the metamorphism of the country rocks. The distinctly negative δ18O values also indicate that the infiltrated fluid was mainly of meteoric origin and its circulation was most likely related to the intrusion of the nearby posttectonic Chuncheon granite. Model calculations demonstrate that the fluid/rock ratios were high, that both oxygen and hydrogen isotope compositions of nephrite were mainly buffered by the fluid phase, and that XCO2of the fluid phase was very low during the nephrite formation.

These conditions contrast with the formation of nephrite deposits that are spatially associated with serpentinites (e.g., Fengtien nephrite deposit, Taiwan). Such deposits are formed by fluid infiltration during regional metamorphism and their oxygen isotope compositions are mainly buffered by the host serpentinite.

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