北海道の新生代鉱床区における熱水活動履歴(<特集>堆積盆の熱履歴を探る(その1))  [in Japanese] Cenozoic hydrothermal activity in the metallogenic province, Hokkaido, Japan(<Special issue>Thermal histories of sedimentary basin: technique and case studies(Part1))  [in Japanese]

東北日本弧に属する西部北海道鉱床区と千島弧に属する東北北海道鉱床区には後期新生代の熱水性鉱床および熱水変質帯が分布する.これらの鉱床区における鉱化作用・熱水活動のステージは,M-Iステージ(19-15.5Ma),M-IIステージ(15.5-10Ma),M-IIIステージ(10-5Ma),M-IVステージ(5-0Ma)の4つに区分される.西部北海道鉱床区と東北北海道鉱床区東部ではM-IIステージに黒鉱鉱床などの海底熱水鉱床が,M-III〜IVステージに浅熱水性の鉱脈鉱床や陸上火山噴気熱水性鉱床が形成された.これに対し,東北北海道鉱床区の西部ではM-II〜IVステージに浅熱水性の鉱脈鉱床や陸上火山噴気熱水性鉱床が形成され,このうち鉱脈鉱床の形成年代(14.4〜0.3Ma)は北から南へと順次若くなる.熱水性鉱床や熱水変質帯の偏在性は,古環境,基盤岩の物性や構造,火成活動,広域テクトニクスに規定される.西部北海道鉱床区と東北北海道鉱床区東部では中期中新世の引張応力,後期中新世以降の圧縮応力をもたらした太平洋プレートの沈み込みに起因する火成活動が重要な役割を果たした.また,東北北海道鉱床区内部では中期中新世以降のユーラシアプレートのオホーツクプレートへの潜り込みに起因する圧縮応力とマントルダイアピルの上昇による火山活動によって熱水性鉱床や熱水変質帯が形成された.

Many hydrothermal ore deposits and hydrothermal alteration zones related to Late Cenozoic magmatism of the Northeast Japan Arc and the Kuril Arc are distributed in the West Hokkaido Metallogenic Province (WHMP) and the Northeast Hokkaido Metallogenic Province (NHMP) in Japan. In this article, I discuss the spatial and temporal process of hydrothermal activity and ore mineralization, and the factors controlling the heterogeneous distribution of the hydrothermal alteration zones and hydrothermal ore deposits in Hokkaido. Based on K-Ar ages of hydrothermally altered mineral, the modes of occurence of ore deposit and stratigraphic evidence of host rocks, the metallogenic period is divided into following four stages and six substages; M-I stage 19-15.5Ma, M-II stage 15.5-10Ma (M-IIa: 15.5-13.5Ma, M-IIb stage: 13.5-10Ma), M-III stage 10-5Ma (M-IIIa stage: 10-8Ma, M-IIIb stage: 8-5Ma), M-IV stage 5-0Ma (M-IVa stage: 5-2.5Ma, M-IVb stage: 2.5-0Ma). The metallogeny in the WHMP, is characterized by 1) skarn Fe deposits during M-I stage, continental-margin terranes, 2) volcanogenic massive sulfide Cu-Pb-Zn, Ba deposits and bedded Mn deposits during M-II stage, associated with submarine volcanism, 3) epithermal vein-type Cu-Pb-Zn, Mn, Au-Ag deposits during M-III to IV stage, and Land-volcanic exhalative hydrothermal S, FeS deposits during M-IV stage, related to terrestrial volcanism of the Northeast Japan inner arc. The metallogeny in the eastern part of the NHMP, similarly as one in the WHMP, is related to arc volcanism of the Kuril inner arc. The metallogeny in the western part of the NHMP is characterized by 1) epithermal vein-type Au-Ag and Cu-Pb-Zn deposits, and Land-volcanic exhalative hydrothermal S, FeS deposits during M-II to IV stage, associated with terrestrial volcanism, 2) mineralization ages (14.4 to 0.3Ma) tend to young southward, resulted from Miocene-Pleistocene terrestrial volcanism, which was initially widespread, but gradually became spatially restricted towards the south in the NHMP. The western part of the NHMP was subjected to volcanism due to uplifting mantle diapirs related the collision tectonics of Eurasian, Okhotsk and Pacific plates, resulting in regional ore mineralization and hydrothermal activity.