16:15 〜 16:30
[HRE12-09] Prolonged evolution of VHMS-forming magmatic systems: Insights from silicic products of the Hokuroku District, Japan
キーワード：Kuroko deposits, Magma, VHMS mineralization, Ore, Melt inclusions
The Hokuroku District, Akita, Japan, hosts significant volcanic-hosted massive sulfide (VHMS) deposits, known as Kuroko, which are associated with rhyolitic lavas and tuffs deposited by middle Miocene submarine volcanism. The volcanic rocks in the Hokuroku District can be grouped into pre-syn-ore and post-ore units, relative to VHMS mineralization. The pre-syn-ore units consist of quartz-free rhyolitic lava (R3-R4) and tuff (T3-T4), and the post-ore units consist of quartz-rich rhyolitic lava (R2) and tuff (T2). The physiochemical properties of pre-syn-ore magma are yet to be fully understood. Previous studies suggested increasing seawater depth with increasing magmatic heat flux to explain the episodic nature of ore deposition. Here, we comprehensively analyzed crystal textures and size distributions (CSD) of plagioclase, chemical compositions of the melt inclusions, and U-Pb zircon ages of pre-syn- and post-ore volcanic rocks to assess the lifespan and conditions of ore-forming and barren magmas.
The representative studied samples of the pre-syn- and post-ore felsic volcanic rocks from the Hokuroku District have rhyolite compositions, with SiO2 contents higher than 70 wt% in whole rock analyses and over 75 wt% in the melt inclusions. These rocks exhibit a range of plagioclase crystal sizes and textures and display a kinked CSD pattern. Smaller plagioclase crystals are typically unzoned, while larger crystals exhibit zoning, along with dissolution and sieve textures. Zircon crystals commonly show oscillatory zoning in cathodoluminescence, with some exhibiting resorption textures in both the core and rim. U-Pb zircon dating indicates long magma residence times (>1 m.y.) for both the pre-syn- and post-ore magmas. Despite these similarities, major element compositions of melt inclusions of pre-syn-ore rocks display higher contents of MgO, FeO, CaO, TiO2, and Al2O3 compared to those of the post-ore rocks. Furthermore, trace element compositions of melt inclusions in the pre-syn-ore rocks show higher concentrations of Cu (0.8-14.7 ppm, average of 5.5 ppm) and Zn (35.8-82.6 ppm, average of 55.8 ppm) but similar Pb (5.9-14.8 ppm, average of 11.5 ppm) compared to those of the post-ore samples. The dissolution texture and kinked CSD signatures of plagioclase reflect multiple recharges that kept the magma in a partially molten state over a prolonged period. Our earlier study revealed that the temperature of pre-syn-ore magma was higher than that of the post-ore magma. A high temperature long-lived magma chamber, initially rich in Cu and Zn, may have facilitated the protracted circulation of ore-bearing hydrothermal fluids and the deposition of Kuroko ores.
The representative studied samples of the pre-syn- and post-ore felsic volcanic rocks from the Hokuroku District have rhyolite compositions, with SiO2 contents higher than 70 wt% in whole rock analyses and over 75 wt% in the melt inclusions. These rocks exhibit a range of plagioclase crystal sizes and textures and display a kinked CSD pattern. Smaller plagioclase crystals are typically unzoned, while larger crystals exhibit zoning, along with dissolution and sieve textures. Zircon crystals commonly show oscillatory zoning in cathodoluminescence, with some exhibiting resorption textures in both the core and rim. U-Pb zircon dating indicates long magma residence times (>1 m.y.) for both the pre-syn- and post-ore magmas. Despite these similarities, major element compositions of melt inclusions of pre-syn-ore rocks display higher contents of MgO, FeO, CaO, TiO2, and Al2O3 compared to those of the post-ore rocks. Furthermore, trace element compositions of melt inclusions in the pre-syn-ore rocks show higher concentrations of Cu (0.8-14.7 ppm, average of 5.5 ppm) and Zn (35.8-82.6 ppm, average of 55.8 ppm) but similar Pb (5.9-14.8 ppm, average of 11.5 ppm) compared to those of the post-ore samples. The dissolution texture and kinked CSD signatures of plagioclase reflect multiple recharges that kept the magma in a partially molten state over a prolonged period. Our earlier study revealed that the temperature of pre-syn-ore magma was higher than that of the post-ore magma. A high temperature long-lived magma chamber, initially rich in Cu and Zn, may have facilitated the protracted circulation of ore-bearing hydrothermal fluids and the deposition of Kuroko ores.