Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

Abstract. Determinations of SO 3 and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO 3 and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined. Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain >0.1 wt% S as SO 3 . Such high SO 3 contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (>2 wt%). The SO 3 contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally <0.1 wt%. On the other hand, the SO 3 contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO 3 contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high. In terms of the Cl and SO 3 contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO 3 contents of apatite is a feature of early stage magmatic differentiation. The SO 3 contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.