Late Cenozoic Metallogeny of Southwest Hokkaido, Japan

Abstract: This paper reviews the Miocene to Pleistocene tectonic framework, geology, magmatic style and stress field of southwest Hokkaido, Japan, and compiles deposit form, type, ore and alteration minerals, strike and length of mineralized veins, and associated igneous activity. The late Cenozoic tectonic regime of the Sapporo‐Iwanai ore district is divided into five periods on the basis of the subduction mode of the Pacific plate: Period 1 (15.0–12.1 Ma) oblique‐subduction setting with an orthogonal convergence rate (OCR) of 51–81 mm/y; Period 2 (12.1–6.2 Ma) normal subduction with an OCR of 81–94 mm/y; Period 3 (6.2–3.6 Ma) oblique subduction setting with an OCR of 73–99 mm/y; Period 4 (3.6–1.5 Ma) normal‐subduction setting with an OCR of 99–103 mm/y and Period 5 (1.5–0 Ma) oblique‐subduction setting with an OCR from 99 to 57 mm/y.The hydrothermal deposits in the district include Kuroko deposits of Period 1 and epithermal vein‐type deposits of Periods 2 to 5. The Kuroko deposits were accompanied by submarine monogenetic rhyolite volcanism associated with tholeiitic basalt in the backarc region. In contrast, Late Miocene to Pliocene epithermal vein‐type deposits were associated mainly with polygenetic andesite and monogenetic rhyodacite volcanism of calc‐alkaline series. These different styles of magmatism occurred in an extensional tectonic regime (Period 1), and weakened extensional (Periods 2–3) or neutral tectonic regimes (Periods 4). Periods 2–5 epithermal vein‐type deposits are divided into base‐metal and precious‐metal deposits. The base‐metal deposits are associated mainly with large (>5 km in diameter) polygenetic andesitic volcanoes and subvol‐canic intrusions. The precious‐metal deposits are associated with small (<5 km in diameter) polygenetic or monogenetic volcanoes and/or subvolcanic intrusions of andesite, dacite and rhyolite near the volcanic arc front. This difference in distribution is ascribed to different states of horizontal differential stress.Productive vein‐type deposits, such as Toyoha, Inakuraishi, Ohe and Chitose, formed in the neutral regime with a large horizontal differential stress during Period 4, which may have promoted strike‐slip faulting and non–extrusive, subvol‐canic intrusion. This tectonic regime and stress field resulted from the normal subduction of the Pacific plate with elevated velocity. This observation leads to the conclusion that large metallic deposits in southwest Hokkaido are expected to have formed primarily during Pliocene magmatic‐hydrothermal activity, when the orthogonal convergence rate was highest and strike‐slip faulting was active.