Vein skarn with ferrobustamite in the Mukai-hi Seibu ore body of the Fujigatani mine, Yamaguchi Prefecture

Abstract

The Fujigatani mine is located about 15 km west of Iwakuni City, Yamaguchi Prefecture. The Mukai-hi Seibu ore body of the mine is a skarn type scheelite deposit and is emplaced at the boundary between Triassic lenticular limestone and slate. The skarns developed in the deposit are classified as Vein skarn and Shelllike skarn on the basis of the mode of occurrences. Scheelite ore, which is accompanied by small amount of pyrrhotite, chalcopyrite and sphalerite, developed in the vicinity of the intersection of both skarns. The vein skarn is about 1 meter in width trending N15°E in the ore body. It shows clear zonal arrangement from marble to its center in the following sequence; (1) marble with carbonaceous material-(2) white marble-(3) brown ferrobustamite zone-(4) white ferrobustamite zone-(5) fine-grained clinopyroxene zone-(6) garnet-clinopyroxene zone-(7) coarse-grained clinopyroxene zone.
Scheelite is deposited within limited zones such as in (5) and (6) of the vein.
Microprobe study reveals the following sudden compositional change of coexisting clinopyroxenes and garnets in the ferrobustamite zones [(3) & (4)]. That is, towards the marble side, ferrous iron content decreases while manganese increases in clinopyroxenes, whereas in garnet ferric iron increases while ferrous iron and manganese decrease. In the same zones, manganese content of ferrobustamite also decreases steeply towards the marble side. Considering this compositional variation in these minerals, it is assumable that environment in the vicinity of marble is in a relatively oxidized condition.
Superiority of reduced skarn is assumed to be caused by carbon in marble and slate. High manganese contents in these minerals of Vein skarn might have been derived from underlying manganiferrous sedimentary rocks.
Taking into account these phenomena, scheelite precipitation in the ore body is presumed to have taken place under environment of relatively low fo ₂ and high fCO₂ conditions.