Petrochemistry of the Late Cretaceous-Paleogene igneous rocks in the Ikuno-Akenobe mines area, Southwest Japan

Abstract

The Ikuno and Akenobe vein deposits are known to be typical subvolcanic type being hosted in the Late Cretaceous volcanic rocks in the Ikuno mine area and in Paleozoic-Mesozoic igneous and metamorphic rocks in the Akenobe mine area. The ore veins have mostly N-S, partly NW strikes and steep dips in the Ikuno mine, and are rich in silver and base metals historically. Tin was found in later years. The ore veins as a whole show a concentric zoning of the central Sn-Cu subzone, surrounded by Sn-Cu-Zn subzone, Zn subzone, Pb-Zn subzone, Au-Ag subzone and barren subzone. The Akenobe veins have generally NW strike swinging to NE in the western part and steep dips. The ores are polymetallic but shown by two different stages of the early Pb-Zn-Cu substage and the later W-Sn-Cu substage. Late Cretaceous volcanic formation of the Ikuno Group is composed of andesitic, dacitic and rhyolitic lavas and pyroclastic rocks. By the measurement of the magnetic susceptibility and microscopic observations, these volcanic rocks are found to belong generally to the I-type magnetite-series. These rocks are plotted K-rich calc-alkaline rocks in the variation diagram. Rich ores tend to occur associated with basaltic and rhyolitic dikes in the Ikuno mine. Cu- and Sn-rich ore fluids may be originated in the basaltic and rhyolitic magmas, respectively. Abundant dikes, mostly post-ore though, in the Akenobe mine indicate that the I-type magnetite-series magmatism with wide range of compositions prevailed in this mine area and supplied the ore fluids to the early stage base metal mineralizations. But the second stage of Sn-rich ore fluids may be derived from an I-type ilmenite-series granitic body still hidden.