3:45 PM - 4:00 PM
[SCG44-08] Copper mobilization systems associated with the Neogene magmatism in the Northeast Japan arc
Keywords:copper mobilization system, magma mixing / mingling, Northeast Japan arc
A large number of Neogene vein-type copper deposits are distributed in the Northeast Japan arc, which is a volcanic arc associated with subduction of the oceanic plate but lacks porphyry-type copper deposits. Those vein-type copper deposits associated with neighboring Miocene and Pliocene intrusive rocks. The purpose of this study is to clarify how copper mobilized and concentrated as an ore deposit in the magma supply system. Felsic intrusive rocks and included enclaves (MME; mafic magmatic enclave / mafic microgranular enclave) in the Arakawa area, which have been clarified to be temporally and spatially related to the Arakawa copper deposit. In order to estimate the amount of copper released with the fluid during magma solidification, the copper content of the melt estimated using the partition coefficient was compared with the whole rock copper content.
The three felsic intrusive rocks in the Arakawa area consist of plagiorhyolite and amphibole rhyodacite stocks. The MME contained in felsic intrusive rocks is basaltic to dacite and is mainly classified into two types: clinopyroxene-bearing hornblende gabbro and hornblende dacite. All plagioclase in basaltic MME is lath-shape with An 36-89 % and Cu 2.8-35.7 ppm. Felsic intrusive rocks mainly contain tabular-shaped plagioclase with An 11-38 % and Cu 0.5-5.1 ppm. The lath-shaped plagioclase contained in basaltic MME and dacitic MME shows a negative correlation between the An value and Cu content. In the An value vs. Cu diagram, the plot is linearly continuous regardless of the type of MME. Tabular-shaped plagioclase has a small An value and Cu content as a whole, and shows a clear compositional difference from lath-shaped plagioclase. The Cu content of the whole rock is 1 ppm for felsic intrusive rocks, 2 ppm for dacitic MME, and 65-106 ppm for basaltic MME.
The copper content in the melt estimated using the partition coefficient between plagioclase and melt is about 9-24 ppm for felsic intrusive rocks and about 120-150 ppm for basaltic MME. The melts that crystallize lath-shaped plagioclase and tabular-shaped plagioclase in dacitic MME are 74-138 ppm and 8-36 ppm, respectively.
Thus, these analytical results suggest that the dacitic MME with an intermediate composition was generated by the magma mixing of the differentiated mafic magma that formed the basaltic MME and felsic magma. It is suggested that the difference between the estimated copper content of the magma that formed the felsic intrusive rocks and dacitic MME and the whole rock copper content was distributed to the fluid phase during degassing process to form the ore forming fluid.
The three felsic intrusive rocks in the Arakawa area consist of plagiorhyolite and amphibole rhyodacite stocks. The MME contained in felsic intrusive rocks is basaltic to dacite and is mainly classified into two types: clinopyroxene-bearing hornblende gabbro and hornblende dacite. All plagioclase in basaltic MME is lath-shape with An 36-89 % and Cu 2.8-35.7 ppm. Felsic intrusive rocks mainly contain tabular-shaped plagioclase with An 11-38 % and Cu 0.5-5.1 ppm. The lath-shaped plagioclase contained in basaltic MME and dacitic MME shows a negative correlation between the An value and Cu content. In the An value vs. Cu diagram, the plot is linearly continuous regardless of the type of MME. Tabular-shaped plagioclase has a small An value and Cu content as a whole, and shows a clear compositional difference from lath-shaped plagioclase. The Cu content of the whole rock is 1 ppm for felsic intrusive rocks, 2 ppm for dacitic MME, and 65-106 ppm for basaltic MME.
The copper content in the melt estimated using the partition coefficient between plagioclase and melt is about 9-24 ppm for felsic intrusive rocks and about 120-150 ppm for basaltic MME. The melts that crystallize lath-shaped plagioclase and tabular-shaped plagioclase in dacitic MME are 74-138 ppm and 8-36 ppm, respectively.
Thus, these analytical results suggest that the dacitic MME with an intermediate composition was generated by the magma mixing of the differentiated mafic magma that formed the basaltic MME and felsic magma. It is suggested that the difference between the estimated copper content of the magma that formed the felsic intrusive rocks and dacitic MME and the whole rock copper content was distributed to the fluid phase during degassing process to form the ore forming fluid.