Some dolomitic skarns in the Kamineichi area, Iwate Prefecture, were formed by contact metamorphism associated with emplacement of the Miyako granodiorite in Early Cretaceous age. The characteristic features of the fluid forming the dolomitic skarns were investigated on the basis of mineral assemblages of the dolomitic skarns, homogenization temperatures and salinities of fluid inclusions in calcite and quartz, and carbon and oxygen isotopic ratios of calcite and dolomite.
The Dainikadokami, Neichi, Nejo, Kebaraichi and Kadokami dolomitic skarns occur in recrystallized limestone intercalated with metamorphosed mudstone and chert. The apparent distances from the Miyako granodiorite to the dolomitic skarns are 0 km (almost contact) for the Dainikadokami and Neichi, 0.1 km for Nejo, 0.9 km for Kebaraichi and 1.3 km for Kadokami dolomitic skarns. The major mineral assemblages of the dolomitic skarns are calcite-dolomite-forsterite±spinel, and calcite-tremolite-diopside. The formation temperature of dolomitic skarns was estimated to be around 530°C from the mineral assemblage and chemical composition of calcite coexisting with dolomite. The homogenization temperatures of fluid inclusions in calcite range from 300 to 500°C with a peak around 460°C and from 100 to 325°C with a peak around 220°C. The salinities of the inclusion fluids are divided into 30 wt% equivalent NaCl and from 8.4 to 3.7 wt% equivalent NaCl. Based on the for-maion temperature of dolomitic skarns around 530°C and homogenization temperature of fluid inclusions in calcite around 460°C, the formation pressure was estimated to be from 1.0 to 0.7 kb. The fluid of 30 wt% equivalent NaCl was formed by phase separation of fluid having salinity from 8.4 to 3.7 wt% equivalent NaCl around 0.7 kb and 550°C. The trap temperature of fluid having the homogenization tempterature around 220°C is estimated to be around 280°C at 0.7 kb. The fluid was predominant in hydrothermal stage, in which ore minerals such as pyrrhotite and chalcopyrite were precipitated in the dolomitic skarns of the area.
The ranges of carbon and oxygen isotopic ratios of calcite from the dolomitic skarns in the area are 0.6 to 0.1 ‰ and 24.6 to 23.5 ‰ for the Dainikadokami, 0.2 to -1.7 ‰ and 23.8 to 20.4 ‰ for the Neichi, -1.5 and 17.8 ‰ for the Nejo, -2.3 to -2.9 ‰ and 16.5 to 13.6 ‰ for Kebaraichi and 0.1 to -1.0 ‰ and 23.3 ‰ for Kadokami dolomitic skarns. The oxygen isotopic ratios of calcite in the area were divided into higher oxygen isotopic ratios (23.3 to 24.6 ‰) and lower oxygen isotopic ratios (13.6 to 17.8 ‰). These carbon and oxygen isotopic ratios are lower than those of unaltered dolomite rocks (4.5 to 3.9 ‰ for carbon and 26.0 to 25.7 ‰ for oxygen), are higher than those of calcite from Japanese skarn deposits.