1982 Volume 32 Issue 176 Pages 443-456
Two types of zoned skarns occur at the Akemidani No. 5 ore body of the Fujigatani mine in the Kuga district, the most important tungusten province in Japan. One is "incrustation skarn", up to 15 m wide, formed around the boundary between a lenticular limestone and its surrounding slate and chert. The other is"vein skarn", up to 2 m wide, developed in the limestone host.
As a basis for the future discussion of metasomatic process, we have made an effort to clarity the characteristics of zoning observed in the vein skarns.
The zonal arrangements of the vein skarn, generally symmetrical, are as follows: from limestone side towards the center, coarse-grained calcite zone, wollastonite zone, garnet-clinopyroxene zone, clinopyroxene-garnet zone, and central zone. As is evidenced by field observations, the central zone was originally a fracture, into which solutions invaded to yield the vein skarns. Preservations of original texture indicate that the inner zone replaced the outer zone towards the limestone side.
Modal and chemical analyses and calculation of components derived from both data indicate the following characteristic tendencies towards the zoning of the vein skarns.
(1) Whilst modal ratios of early skarn minerals, wollastonite, clinopyroxene, garnet, and vesuvianite, are relatively constant in each zone, they change abruptly at the boundaries.
(2) Clinopyroxene do not show a significant variation in composition from the garnet-clinopyroxene zone to the central zone, that is, containing 80 to 90 mol. % hedenbergite, while those in the wollastonite zone are rich in Fe near the garnet-clinopyroxene zone and poor in Fe near the limestone side. The fact that the Fe/Mg ratios of clinopyroxenes in the wollastonite zone is not continuous is due to the partial substitution of the latter by the former, Fe-rich one.
(3) CaO contents decrease from the limestone side towards the central zone, while SiO2 contents reveal the reverse change. On the other hand, A/2 (AlO 3/2+FeO 3/2) and F (FeO+MnO+MgO) components exhibit the inverse correlation, because they are dependent on the modal composition of garnet and clinopyroxene.
