3:30 PM - 5:00 PM
[SCG48-P07] Epithermal gold-silver mineralization in the Miyata region, Nishiki Town, Akita, Japan
Keywords:epithermal deposit, fluid inclusion, hydrothermal alteration
In the central part of Akita Prefecture, Pliocene-Pleistocene hydrothermal metal deposits such as the Miyata, Shirakomori, and Nibetsu deposits are distributed in an east-west direction around the Taiheizan area. In the central part of the Taheiyama area, Cretaceous Taheizan granitoids are widely distributed, and most of the related igneous rocks in the Miyata, Shirakomori, and Nibetsu deposits are not exposed on the ground surface. The Miyata Deposit has not been explored extensively, and the geology and characteristics of the gold-silver mineralization have not been clarified. In this study, the following research methods were used to clarify the mineralization process of the Miyata Deposit.
The following research methods were used in this study -Geological survey in the study area -Fabrication of polished thin sections of samples and microscopic observation -Whole rock chemical composition analysis of volcanic rocks using X-ray fluorescence analysis -Identification of altered minerals by X-ray powder diffraction analysis -Estimation of salinity and homogenization temperature by fluid inclusion microthermometry
-Determination of FeS content in sphalerite using SEM-EDS Analysis.
Around the deposit, biotite andesite lava of Quaternary and Neogene age, basaltic clinopyroxene andesite lava, olivine basaltic intrusive rock, andesitic intrusive rock, tuff, lapilli tuff, sandstone and siltstone of Neogene age are distributed, and all rocks are altered and illite/smectite mixed-layer clay zone is widely distributed. Smectite, chlorite, illite, and pyrophyllite zones are also present. In the outcrops near the veins, a progressive sequence of siliciclastic, illite-chlorite, and chlorite zones is observed from the vein center.
In this deposit, quartz and calcite boulders were used as research samples. The quartz boulders with relatively low gold content are designated as Type I, the quartz boulders with relatively high gold content as Type II, and the boulders composed of calcite as Type III. In the Type I, argentite, natural silver, pyrargyrite, and sphalerite, in the Type II, argentite, natural silver, galena, and pyrite, and in the Type III, silver and sulfide minerals occur in the mosaic-like quartz, and argentite, natural silver, and pyrite are recognized as the main ore minerals. The mineral name of the electrum produced in this deposit is natural silver because the Atomic% value of gold is 42.61% in Type I, 36.86% in Type II, and 37.43% in Type III, which is less than 50%.
The results of homogenization temperature measurements of fluid inclusions in quartz and calcite, which are gas-liquid two-phase and liquid-dominant, indicate that fluid inclusions could not be measured at about 140-220 degree, 2.0-4.0 Wt.% in Type I and Type II, respectively. In Type III, measurements were made at three locations in calcite, and the temperatures were about 180-201 degree, 2.4-3.8 Wt.%, 171-187 degree, 2.3-7.0 Wt.%, and 170-196 degree, 2.2-3.1 Wt.%, respectively.
The results of fluid inclusions and quartz microstructures suggest that the hydrothermal fluid that caused the gold-silver mineralization in Type I was boiling. The gold-silver mineralization in Type III occurred with quartz after the CO2-rich hydrothermal fluid crystallized calcite. The deposit is characterized as intermediate sulfidation based on the homogenization temperature and iron content in sphalerite, and the presence of calcite and the wide distribution of neutral alteration zones suggest that the deposit is located in a CO2-rich steam-heated zone.
The following research methods were used in this study -Geological survey in the study area -Fabrication of polished thin sections of samples and microscopic observation -Whole rock chemical composition analysis of volcanic rocks using X-ray fluorescence analysis -Identification of altered minerals by X-ray powder diffraction analysis -Estimation of salinity and homogenization temperature by fluid inclusion microthermometry
-Determination of FeS content in sphalerite using SEM-EDS Analysis.
Around the deposit, biotite andesite lava of Quaternary and Neogene age, basaltic clinopyroxene andesite lava, olivine basaltic intrusive rock, andesitic intrusive rock, tuff, lapilli tuff, sandstone and siltstone of Neogene age are distributed, and all rocks are altered and illite/smectite mixed-layer clay zone is widely distributed. Smectite, chlorite, illite, and pyrophyllite zones are also present. In the outcrops near the veins, a progressive sequence of siliciclastic, illite-chlorite, and chlorite zones is observed from the vein center.
In this deposit, quartz and calcite boulders were used as research samples. The quartz boulders with relatively low gold content are designated as Type I, the quartz boulders with relatively high gold content as Type II, and the boulders composed of calcite as Type III. In the Type I, argentite, natural silver, pyrargyrite, and sphalerite, in the Type II, argentite, natural silver, galena, and pyrite, and in the Type III, silver and sulfide minerals occur in the mosaic-like quartz, and argentite, natural silver, and pyrite are recognized as the main ore minerals. The mineral name of the electrum produced in this deposit is natural silver because the Atomic% value of gold is 42.61% in Type I, 36.86% in Type II, and 37.43% in Type III, which is less than 50%.
The results of homogenization temperature measurements of fluid inclusions in quartz and calcite, which are gas-liquid two-phase and liquid-dominant, indicate that fluid inclusions could not be measured at about 140-220 degree, 2.0-4.0 Wt.% in Type I and Type II, respectively. In Type III, measurements were made at three locations in calcite, and the temperatures were about 180-201 degree, 2.4-3.8 Wt.%, 171-187 degree, 2.3-7.0 Wt.%, and 170-196 degree, 2.2-3.1 Wt.%, respectively.
The results of fluid inclusions and quartz microstructures suggest that the hydrothermal fluid that caused the gold-silver mineralization in Type I was boiling. The gold-silver mineralization in Type III occurred with quartz after the CO2-rich hydrothermal fluid crystallized calcite. The deposit is characterized as intermediate sulfidation based on the homogenization temperature and iron content in sphalerite, and the presence of calcite and the wide distribution of neutral alteration zones suggest that the deposit is located in a CO2-rich steam-heated zone.