Abstract
The Kobushi skarn deposits are near the boundary of Nagano and Yamanashi Prefectures. It has developed as a gold mine for many years. Many pits are found along the ridgeline extending northward from the Kokusigatake.
The Jurassic Kawakami Group distributes in the mining area, and consists mainly of sandstone, shale, slate, chert, lens-like limestone. It generally strikes NW-SE and inclines in the N direction, whereas near the deposits lens-like limestone strikes NE-SW and inclines steeply in the NW direction.
The Kofu granitoids intrude to the south of this area, as a result, the contact metamorphic zone developed over about 2km, and small-scale dikes of quartz porphyry are often observed at the eastern slope of the ridgeline.
The Kobushi skarn deposits are further divided into three ore deposits; Azusayama, Sanjin, and Kokushi from north, and two layers of chert are between each ore deposit. The Azusayama ore deposit is rich in magnetite, while the Kokushi ore deposit is rich in gold in the garnet skarn.
This research mainly focuses the Kokushi ore deposit consisting of 24 skarn orebodies of origin of small-scale lens-like limestone. The main consituent minerals found from the ore deposit are garnet, hedenbergite, scapolite, adularia, chlorite, epidote, vesuvianite, quartz, calcite, scheelite, apatite, magnetite, gold, bismuth, Bi-Te minerals, pyrrhotite, chalcopyrite, arsenopyrite and others.
Two big orebodies among 24 in the Kokushi ore deposit exist; one is No.2 orebody about 700m away from, another is No.9 orebody near the ridgeline of 1,000m away from the granitoids to north. No.2 orebody is in a garnet-hedenbergite skarn which mainly produced magnetite, and the size is 90m or more in the extended direction. No.9 orebody, about 30m of E-W, about 50m of N-S and about 50m of width, is lens-shaped and generally strikes NW-SE and inclines in the N direction. It is intruded by quartz porphyry with a diameter of about 10m. The skarn minerals have zonally arranged there; calcite zone (limestone), hedenbergite zone, and garnet zone. A quartz vein is observed near the boundary between the calcite and hedenbergite zones, and quartz and calcite veinlets and networks are in the garnet skarn.
Gold occurs in the garnet skarn of No.9 orebody. Its size is generally about 100μm. It occurs interstitially among fine grains of garnet in the garnet skarn, coexisting with arsenopyrite, bismuth, and Bi-Te minerals. According to the analytical results of the gold, the inside of gold is comparatively homogeneous, and Ag content is about 7 % or about 11 % in many cases. Gold with calcite is also observed.
