兵庫県明延鉱床およびその周辺地域における多金属鉱化作用と金銀鉱化作用

抄録

Polymetallic and gold-silver mineralizations in and around the Akenobe ore deposits are related to the acidic to intermediate magmatism during the Cretaceous to Paleogene. The polymetallic mineralization occurs in galena-sphalerite, chalcopyrite-sphalerite, chalcopyrite-cassiterite and cassiterite-ferberite veins formed by subvolcanic and poly-ascending hydrothermal activities. These are hosted by basic volcanic rocks of the lower member of the Permian Maizuru Group and located at Akenobe. The gold-silver mineralization, associated with minor base metal sulfides, is spatially distributed on the southeast (Mikohata) to southwest (Ohmidani) of the polymetallic zone of Akenobe. It occurs in quartz veins in the Yakuno Igneous Complex and in slate of the middle member of the Maizuru Group. The transitional mineralization occurs in sphalerite-chalcopyrite-gold-silver-quartz vein located between the polymetallic and gold-silver zones. Pearceite-polybasite, tetrahedrite-tennantite, argentite (acanthite) and electrum-native silver are common minerals found in all of the polymetallic, gold-silver and transitional zones. Canfieldite occurs in both the gold-silver and transitional zones, indicating that the hydrothermal solution contained a minor amount of tin during the gold-silver ore deposition. The ore mineral assemblages suggest that the gold-silver ore was formed at a late stage ((Cu-) Sn-W stage) of the polymetallic ore deposition. Ag content in tetrahedrite-tennantite, ranging from 0 to 24 wt.%, tends to increase towards the gold-silver zone from the polymetallic zone. The temperatures estimated based on electrum compositions for the gold-silver ores are 260-310°C which are similar to homogenization fluid inclusion temperature of the ores formed at the late stage of the polymetallic ore deposition.