Tungsten skarn deposit of the Fujigatani Mine, Southwest Japan

The Fujigatani tungsten deposit occurs in Triassic sediments composed mainly of pelitic rocks and chert within the contact aureole of an Upper Cretaceous granodiorite to monzogranite batholith. The orebodies were formed in nearly pure limestone lenses in pelitic hornfels. The Akemidani orebodies occur at an estimated 100 to 300 m above the contact. The mineralizing solutions may have been supplied through original fissures which were later filled with quartz veins.Scheelite precipitated in the alteration zones formed along the fissures in and the margins of the limestone lenses and also in quartz veins near the orebodies. The alteration consists mainly of clinopyroxene-garnet skarn ( approximately 10 m) and, locally, plagioclase-muscovite-chlorite rock ( approximately 10 m), in which euhedral or subhedral scheelite occurs widely together with small amounts of interstitial sulfide minerals such as pyrrhotite. The plagioclase-muscovite-chlorite rock, characterized by a higher content of scheelite as well as a lack of skarn minerals, occurs only in thick skarn, along quartz veins. This rock may have been formed from the skarn due to intense metasomatism. A thin zone of wollastonite skarn (2-4 cm) formed along the contact with limestone, but the skarn and adjacent limestone were locally replaced by quartz-calcite rock ( approximately 50 cm) in the late stage, when the pore space of the ore would have been filled with quartz and calcite together with sulfide minerals. The hornfels contact with the ore was altered to barren epidote-clinopyroxene skarn (2-50 cm).Chemical data for the principal minerals were obtained by wet chemical and electron microprobe analysis. The ore of the Fujigatani deposit is unique in its Fe (super +3) -poor chemistry. Major gangue minerals, clinopyroxene, and garnet are close to hedenbergite (about 90 mole %) and poor in andradite component (< 20%), respectively. Scheelite is practically Mo free.Physicochemical conditions can be estimated from the mineral stabilities and sulfur isotope geothermometry. The deposit was formed around or above 400 degrees C but below 600 degrees C, at about 1 kb. Oxygen fugacity was lower than the pyrrhotite-pyrite-magnetite buffer, and sulfur fugacity was at or below the pyrrhotite-pyrite boundary during mineralization. The low f (sub O ₂ ) and f (sub S ₂ ) conditions caused the Fe (super +3) -poor nature of the ore. The formation of extremely pure scheelite may have been related to the reducing conditions as well as to an Mo-poor environment.Tungsten-rich skarn deposits in Japan are divided into two types: a scheelite type, including the Fujigatani deposit, and a molybdoscheelite type, characterized by Fe (super +3) -rich chemistry and Mo-bearing scheelite. These two types are related to the ilmenite and magnetite series granitic rocks, respectively. The reduced nature of the Fujigatani deposit was presumably genetically connected with reducing ilmenite series granitic magma.

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