Reconfirmation of jadeite in the Sanbagawa belt of the Shibukawa region, central Japan: Occurrence within a veinlet cutting dunite

Abstract

The Shibukawa region of the Sanbagawa belt, central Japan, is one of the historic jadeite localities in Japan. Although its petrological and mineralogical characteristics of the locality were briefly reported over 35 years ago, no further studies have been conducted since the disappearance of the jadeite-bearing outcrop. Recently, jadeite was discovered within a veinlet in a host dunite boulder, a member of an ultrabasic body that is surrounded by the Sanbagawa crystalline schists in the Shibukawa region. The veinlet is composed mainly of diopside, OH-bearing grossular, jadeite, chlorite, pargasite and serpentine. Relict olivine and spinel, which were probably members of the host dunite, rarely occur in the veinlet. The jadeite grains vary from <1 to 50 µm in size, and occur as patchy aggregates with amoeba-shaped grossular grains. The jadeite grains have Na₂O and Al₂O₃ contents of 14.7-15.4 wt.% and 24.3-25.5 wt.%, respectively, and are 91-98% jadeite contents [(Na-Fe³⁺)×100].