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[SCG47-P19] Geology and Epithermal gold-silver mineralization of Bousawa deposit in Akita, Japan
Keywords:Low sulphidation epithermal deposits, Early Miocene, Vermiculite
The Bosawa deposit, located in Senboku City, Akita Prefecture, is hosted in the early Miocene Katsurabuchi Formation, which consists of felsic volcanic rocks of about 20 Ma. The mineralization age of the Bosawa deposit is similar to that of the Katsurabuchi Formation, and that of the Sado deposit, the second largest gold-producing deposit in Japan. The purposes of this study are to clarify the geological background and occurrence of gold-silver mineralization of the Bosawa deposit, and discuss gold-silver potential of the study area.
In this study, we conducted surface geological mapping of the Bosawa deposit and conducted petrological and mineralogical study for the samples collected.
The lithology of the Katsurabuchi Formation mainly consists of pumiceous lapilli tuff and tuff breccia. The formation rarely contains carbonized wood fragments, tuff with accretionary lapilli and tuffaceous mudstone. The formation is intruded by sub-parallel felsic dikes , which are similar to the strikes of the quartz veins. t The alteration zone of the surveyed area is divided into kaolinite zone, sericite - potassic feldspar zone, and chlorite zone. The kaolinite zone and the sericite - potassic feldspar zone are found around the mineralized quartz veins and a western part of the surveyed area. The kaolinite zone overlaps the sericite - potassic feldspar zone. Almost all of the lithologies (116 sites), except for the tuffaceous mudstone, showed the presence of vermiculite.
The quartz vein samples found in the mine waste show several stages based on mineral combination and microstructure. The quartz veins consist mainly of quartz and include potassic feldspar and silver-black banded crusitform veins. Hydrothermal breccia and bladed quartz textures are also observed. The ore minerals in the silver-black band are electrum and argentite, and are found in symbiosis with potassic feldspar, which exhibits pseudo-acicular texture.
The presence of vermiculite indicates a terrestrial weathering environment after the deposition of the Bosawa deposit. The presence of sericite-potassic feldspar zones around the deposit and the presence of potassic feldspar in symbiosis with silver-black bands suggest that the gold-silver mineralization was caused by alkaline-neutral fluids above 200℃. Kaolinite is of steam-heated alteration origin, and the symbiotic potassic feldspar and silver-black bands suggest that the gold-silver mineralization was caused by boiling. The kaolinite and sericite - potassic feldspar zones distributed in the western part of the Bosawa area, may suggest another hydrothermal alteration center that may host an undiscovered vein beneath it.
In this study, we conducted surface geological mapping of the Bosawa deposit and conducted petrological and mineralogical study for the samples collected.
The lithology of the Katsurabuchi Formation mainly consists of pumiceous lapilli tuff and tuff breccia. The formation rarely contains carbonized wood fragments, tuff with accretionary lapilli and tuffaceous mudstone. The formation is intruded by sub-parallel felsic dikes , which are similar to the strikes of the quartz veins. t The alteration zone of the surveyed area is divided into kaolinite zone, sericite - potassic feldspar zone, and chlorite zone. The kaolinite zone and the sericite - potassic feldspar zone are found around the mineralized quartz veins and a western part of the surveyed area. The kaolinite zone overlaps the sericite - potassic feldspar zone. Almost all of the lithologies (116 sites), except for the tuffaceous mudstone, showed the presence of vermiculite.
The quartz vein samples found in the mine waste show several stages based on mineral combination and microstructure. The quartz veins consist mainly of quartz and include potassic feldspar and silver-black banded crusitform veins. Hydrothermal breccia and bladed quartz textures are also observed. The ore minerals in the silver-black band are electrum and argentite, and are found in symbiosis with potassic feldspar, which exhibits pseudo-acicular texture.
The presence of vermiculite indicates a terrestrial weathering environment after the deposition of the Bosawa deposit. The presence of sericite-potassic feldspar zones around the deposit and the presence of potassic feldspar in symbiosis with silver-black bands suggest that the gold-silver mineralization was caused by alkaline-neutral fluids above 200℃. Kaolinite is of steam-heated alteration origin, and the symbiotic potassic feldspar and silver-black bands suggest that the gold-silver mineralization was caused by boiling. The kaolinite and sericite - potassic feldspar zones distributed in the western part of the Bosawa area, may suggest another hydrothermal alteration center that may host an undiscovered vein beneath it.