Contribution of magma mixing to hydrothermal Cu mineralization: Evidence from the Arakawa area, Akita, Japan

• Ore-related intrusions in the Arakawa area contain dacitic to basaltic enclaves. • Dacitic enclaves contain two types of plagioclases with different Cu contents. • Cu-rich, oxidized magma was formed by mixing of Cu-rich basaltic magma with oxidized rhyolitic magma. • Magma mixing may have caused Cu contained in the basaltic magma to migrate into the related hydrothermal system. A large number of Neogene vein-type Cu deposits are distributed in the Northeast Japan Arc, a volcanic arc formed by the subduction of the Pacific Plate. These deposits, which are associated with neighboring Miocene and Pliocene felsic intrusions, constitute the largest Cu metallogenic province in the Japan Arc. Ore-related intrusions near the Arakawa Cu deposit, which is a typical vein-type Cu deposit in the area, are composed of microgranite and rhyolite, which include dark-gray dacitic to andesitic enclaves and dark-green basaltic enclaves. Except for the sulfide-bearing basaltic enclaves, these igneous rocks formed under oxidizing conditions above or near the SSO buffer and contain very small amounts (1–3 ppm) of Cu. Plagioclase crystals in these igneous rocks contain ≤17.9 ppm Cu and show two distinct clusters in Cu–An% space: one with ≤5.1 ppm Cu at An <60 %, and another showing an increasing trend from 2.8 (at An ≈ 89 %) to 17.9 ppm Cu (at An ≈ 14 %). The former assemblage occurs in rhyolitic and microgranitic intrusive rocks and in andesitic enclaves, while the latter is found in basaltic enclaves; dacitic enclaves contain both assemblages. The dacitic enclaves formed by mixing a Cu-poor rhyolitic magma with a Cu-rich differentiated basaltic magma. The initial Cu contents of these two magmas were calculated using the Cu contents of early-crystallizing plagioclase crystals and reported plagioclase–melt partition coefficients. Comparison with the observed whole-rock Cu contents allowed estimation of the amounts of Cu released from the magmas during crystallization. The characteristics of the magmatic enclaves indicate the presence of intermediate-composition magmas formed after differentiation by crystallization and by mixing with a Cu-rich basaltic magma. These magmas are inferred to have supplied Cu to the magmatic–hydrothermal system that formed the Arakawa Cu deposit.