Potential for Porphyry Copper Deposits in Northern Tōhoku (or the Exploration Potential for Base and Precious Metal Deposits in Japan 2020)

Abstract

AbstractThere are no known porphyry copper deposits in Japan. Attempts were made in the past to explain this paradox as the Japanese islands sit above a subduction zone of the type widely accepted to give rise to porphyry‐type deposits within subvolcanic magmatic–hydrothermal systems. Is it possible that porphyry copper systems are incompatible with the volcanoplutonic setting of the Japanese islands? In other words, is the established model for the genesis of porphyry copper systems not completely correct, or is it that the elusive Japanese porphyry deposit has not yet been found? We argue that, until proven otherwise, the answer lies in the latter option.In this article, we focus on Akita Prefecture and the surrounding areas and consider the potential for the existence of porphyry copper deposits mainly from an empirical and practical point of view, in contrast with the obviously important petrogenetic and/or tectonic perspectives. We argue that this region is representative of the Tōhoku province of northern Honshu and that it serves as a proxy for Japan in general. Akita is rich in mineralization styles and deposits, including two main magmatic‐hydrothermal deposit types of Neogene age: (i) middle Miocene submarine stratiform volcanogenic massive sulfide (VMS) deposits (Kuroko) and (ii) late Miocene to Pleistocene Ag–Au‐bearing polymetallic epithermal/subepithermal veins. In particular the Au–Ag‐bearing polymetallic veins, which are comparable to intermediate‐sulfidation epithermal deposits commonly associated with porphyry copper systems, were discovered and mined for several centuries before the first Kuroko deposits were found in the late 1800s. Osarizawa, the last active vein mine, closed in 1978. Kuroko deposits, the main source of metal production in Akita in the 20th century, had peak of production in the 1970s, followed by a rapid decline during the 1980s and complete cessation of all mining by 1995. Not surprisingly, the end of mining in Akita, combined with a series of strategic decisions made by the Government and the Japanese metal mining industry, coincided with a cessation of mineral exploration, both in Akita and throughout Japan, with the exception of VMS deposits offshore and limited exploration for bonanza‐grade, low‐sulfidation epithermal gold deposits in Hokkaido. Our analysis of post‐1945 regional exploration in northern Tōhoku indicates that Kuroko deposits were by far the dominant target and that the number of drill holes in the 1990s accounted for less than 4% of the total completed from the early 1960s to 1995. These data indicate that essentially no mineral exploration has been conducted in Tōhoku for Au–Ag‐bearing polymetallic vein deposits during the past 30+ years. We conclude that exploration stopped too early and certainly before it could have benefited from the significant applicable metallogenic and technological breakthroughs of the last few decades.