Magmatic evolution and metal systematics of back-arc volcanic rocks of north–east Japan and implications for deposition of massive sulphide Kuroko ores

Abstract

The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatures of 880–940 °C, and Fe–Ti oxide equilibrium indicates oxygen fugacity of ca. FMQ + 1.5. Melt inclusions have high-SiO2 rhyolite compositions (> 75 wt%, on an anhydrous basis), and the plot of normative mineral compositions in the granitic triplot indicates low pressure of magma stalling and crystallisation (< 1 kbar) at cotectic compositions. Melt inclusion metal contents plotted vs incompatible element Y suggest contrasting behaviour of different metals during fractionation and degassing. Zinc was mostly retained in the melt during crystallisation, whereas other metals, such as Pb, Cu, Sn and Mo, were released to an exsolving fluid phase. The latter may have thus been transferred to the hydrothermal system from a degassing magma. Shallow storage of relatively hot magma would have induced vigorous hydrothermal circulation on the seafloor, a precondition for ore deposition.