Structural Control of the Kuroko Deposits of the Hokuroku District

Clusters of Kuroko deposits of Miocene age in the Hokuroku district of northern Honshu lie along one of a conjugate set of lines with NNW and NE strike and most of the larger deposits lie at or close to the inter sections of these lines. A wide variety of structural features of northern Honshu have NNW and NE strikes cross cutting the predominant N-S structural trends of Miocene and younger age and are thought to be of basement (pre-Tertiary) origin. It is proposed that fractures in the basement of the Hokuroku district and particularily their intersections were responsible for the distribution of the Kuroko deposits. Presumably, such sites facilitated the diapiric rise of dacite domes and provided a suitable plumbing system for metal-bearing hydrothermal fluids. Several intersections of the NNW and NE lines do not have known Kuroko deposits and may be good exploration targets.  The Kuroko deposits of Japan are regarded world-wide as the "type" volcanogenic CuPb-Zn-Ag massive sulfides against which all others are compared. Decades of geological research as revealed to the English-speaking world in two notable compilations, Volcanism and Ore Genesis (TATSUMI, 1970) and Geology of Kuroko Deposits (ISHIHARA, 1974), and in recent reviews (e.g. HASHIMOTO, 1977; HORIKOSHI, 1969; LAMBERT and SATO, 1974; SATO, 1977; URABE and SATO, 1978) have provided a good understanding of the geology, petrology and mineralogy of these deposits. Comparative studies, such as SPENCE (1975) and those summarized in SANGSTER and SCOTT (1976), have shown that there are significant similarities between the young (middle Mi ocene, 13 m.y.) Japanese Kuroko deposits and much older deposits elsewhere (e.g. Noranda, Quebec, Canada; Archean age). Consequently, an understanding of the factors controlling the genesis and localization of the Kuroko ores is important for interpreting the origins of other volcanogenic massive sulfides and might be an aid in their exploration. The formation of the Kuroko deposits by hydrothermal emanations onto the sea floor ac companying explosive felsic volcanism and dacite domes, their regional distribution near the eastern margin of the Green Tuff volcanic belt of the Japanese island arc and their short time interval of deposition are reasonably well known. However, less certain is the control, if any, on the distribution of deposits within a particular ore district. The most important and extensively studied Kuroko deposits in Japan are those of the Hokuroku district of northern Honshu (Fig.1). The currently popular explanation for their distribution is that they were formed around the margins of a sedimentary basin (SATO et al., 1974) and exploration over past decades based on this hypothesis has been successful in locating new deposits (K. HASHIMOTO, personal communication, 1977). However, recent discoveries by Dowa Mining Company of the Fukazawa deposit in the center of the basin and of the Etsuri deposit well removed from the margin have demonstrated that this working hypothesis is not correct and has opened large areas of the Hokuroku basin to more intensive exploration. In this paper I * Received April 17, 1978; in revised form May 18, 1978.  Department of Geology, University of Toronto, Toronto, Ontario, Canada, M5S 1A1.

Field of study: Geology