5:15 PM - 6:45 PM
[HRE13-P15] Granitic melt inclusions in basalt-hosted diopside megacrysts from the Sanogawa area at the Southern Fossa Magna region
Keywords:Granitic magma, Melt inclusion, Diopside, Undifferentiated basalt
Most hydrothermal metal ore deposits in island arcs are formed by hydrothermal fluids produced from granitic magma, and chemical compositions and behaviors of the fluids are closely related to original chemistries of the granitic magma. Thus, it is important to elucidate the original chemical characteristics of the granitic magma in order to examine the formation process of the ore deposits. However, many granitic bodies exposed on the earth's surface are likely to have lost their original chemical features due to crystal differentiation and separation of hydrothermal fluids, which makes it difficult to obtain primary information about the ore deposit formation from the exposed granites. To overcome this point, we have searched granitic melt inclusions in phenocrysts of undifferentiated basalts ascending from the uppermost mantle. The melt inclusions are likely to be traces of original granitic magma present in the lower crust and the upper crust. As a result, we found the presence of granitic melt inclusions, estimated to be original granitic melts, in diopside megacrysts in undifferentiated Neogene basaltic dykes around the Sanogawa River in Yamanashi Prefecture, Japan. Mineral compositions and precipitated crystals of metal sulfides and precious metal particles in the melt inclusions have been preliminarily reported (Amagai and Kurosawa 2021, 2022, Abstracts of Resources Geology Annual meeting).
In the Sanogawa River basin, undifferentiated-basalt dykes containing large amounts of chromian diopside megacrysts, several centimeters in size, were present in some parts, which corresponded to island-arc ankaramites. The diopside megacrysts contain almost completely crystallized granitic melt inclusions, which are mainly composed of precipitated crystals of quartz, plagioclase, potassium feldspar, and amphibole, with small amounts of pyrite and chalcopyrite. Small amounts of fine sulfides such as Ni, Zn, Cd, and Pb were attached to the chalcopyrite, and gold-silver compounds and platinum minerals were also observed. Other inclusions, mainly chlorite and calcite, were also present in the diopside, but no inclusions containing liquid water were observed. The granitic melt inclusions are estimated to have been captured approximately 20 to 10 km depth.
In the present study, the average chemical composition of the granitic melt was estimated based on chemical compositions and area ratios of each precipitated crystal by two-dimensional elemental mapping using SEM-EDS. As a result, the composition of granitic melts was found to correspond to that of meta-aluminous and calc-alkaline granodiorite. Based on the volume ratio of hydrous minerals, a water content of the granitic melts was low, less than 1 wt%. The average composition was well consistent with the major chemical compositions of Neogene granites of the moderate K2O series with low water content in the southern Fossa Magna region. The average concentrations of sulfur and copper, which are important for the formation of ore deposits, were comparable to or higher than those of granites with large-scale metal ore deposits in Japan. However, even if this granitic magma forms a metal ore deposit, only a small-scale metal deposit would be formed. This is because the amount of hydrothermal water generated is estimated to be small due to the low water content in the magma. Furthermore, sulfide species and their relative abundance ratios in the melt inclusions are consistent with the tendency in the sulfide species and their relative abundances in Fe-Cu-Pb-Zn and Cu-Au-Ag vein-type ore deposits associated with Neogene granitic bodies near the basaltic dykes. This suggests that melt chemistry in granitic melt inclusions trapped within undifferentiated basalts may provide clues to the characterization of metals in ore deposits associated with granitic rocks.
In the Sanogawa River basin, undifferentiated-basalt dykes containing large amounts of chromian diopside megacrysts, several centimeters in size, were present in some parts, which corresponded to island-arc ankaramites. The diopside megacrysts contain almost completely crystallized granitic melt inclusions, which are mainly composed of precipitated crystals of quartz, plagioclase, potassium feldspar, and amphibole, with small amounts of pyrite and chalcopyrite. Small amounts of fine sulfides such as Ni, Zn, Cd, and Pb were attached to the chalcopyrite, and gold-silver compounds and platinum minerals were also observed. Other inclusions, mainly chlorite and calcite, were also present in the diopside, but no inclusions containing liquid water were observed. The granitic melt inclusions are estimated to have been captured approximately 20 to 10 km depth.
In the present study, the average chemical composition of the granitic melt was estimated based on chemical compositions and area ratios of each precipitated crystal by two-dimensional elemental mapping using SEM-EDS. As a result, the composition of granitic melts was found to correspond to that of meta-aluminous and calc-alkaline granodiorite. Based on the volume ratio of hydrous minerals, a water content of the granitic melts was low, less than 1 wt%. The average composition was well consistent with the major chemical compositions of Neogene granites of the moderate K2O series with low water content in the southern Fossa Magna region. The average concentrations of sulfur and copper, which are important for the formation of ore deposits, were comparable to or higher than those of granites with large-scale metal ore deposits in Japan. However, even if this granitic magma forms a metal ore deposit, only a small-scale metal deposit would be formed. This is because the amount of hydrothermal water generated is estimated to be small due to the low water content in the magma. Furthermore, sulfide species and their relative abundance ratios in the melt inclusions are consistent with the tendency in the sulfide species and their relative abundances in Fe-Cu-Pb-Zn and Cu-Au-Ag vein-type ore deposits associated with Neogene granitic bodies near the basaltic dykes. This suggests that melt chemistry in granitic melt inclusions trapped within undifferentiated basalts may provide clues to the characterization of metals in ore deposits associated with granitic rocks.