New Mexico Mineral Symposium — Abstracts

The Naegi Pegmatite District in the Hirukawa area of Gifu Prefecture is one of the classic mineral localities of Japan. Historically, this area produced some Japan’s finest topaz crystals. Tsunashiro Wada wrote in his 1904 book Minerals of Japan (Nihon Kobutsushi) that “Among the best known minerals from Japan are stibnite, in crystals most gigantic and splendid of all metallic minerals, topaz [from the Naegi District and Tanakamiyama]…, and rock crystal which presents an uncommon form of twinning [Japan law twins].” More recently the district includes type locations for the minerals hingganite-(Ce) and proto-ferro-anthophyllite.

Mining in the area started in the latter half of the 19th century with the development of alluvial (placer) deposits that concentrated resistant minerals that had weathered from the granites and related hydrothermal veins. Later, in the 20th century, with growing interest in western building styles, quarrying of the local granite for building stone began. This exposed many pegmatites which are the main source of mineral specimens from the area. Polymetallic vein deposits that are also associated with the granites and are hosted by it and surrounding lithologies. Many of these have been mined for ores of molybdenum, tungsten, tin, bismuth, copper, lead and zinc.

During the Edo period Nakatsugawa was a post town (station) along the Nakasendo, an important travel route that stretched through the central mountains of Honshu from Edo (modern-day Tokyo) to Kyoto. Although heavily traveled, and farmed for more than a millennia, there seems to have been no mineral development in this area prior to the Meiji Restoration.

The dominant specimen producing lithologies in the Naegi District are miarolitic pegmatites of the NYF family, found throughout the Naegi-Agematsu granite, and related pneumatolytic-hydrothermal veins that crosscut the granite and the Nohi rhyolite. The granite contains many miarolitic pegmatitecassiterite, molybdenite s, ranging from a few centimeters to a meter or so in diameter.

As with most NYF family pegmatites, the dominant mineralogy of the Naegi District pegmatites is quite simple comprising beautiful crystals of smoky quartz, microcline (of a great variety of forms and twins), albite, muscovite, biotite, zinnwaldite, schorl, fluorite, beryl and most notably topaz. There are also accessory minerals, usually found in microcrystals. These include zeolites such as chabazite-Ca and stilbite-Ca, cassiterite, molybdenite and a suite of REE and actinide bearing minerals such as zircon, hingganite-(Ce), fergusonite-(Y), samarskite-(Y), monazite-(Ce), allanite-(Ce), xenotime-(Y) and gadolinite-(Y).