Kuroko mineralization

The most important geologic events at the time of Kuroko mineralizations are rapid subsidence just before the mineralization and bimodal volcanism (contemporaneous basic and felsic volcanism) (Konda, 1974). 

The summary of the bulk chemical compositions (major elements, minor elements, rare earth elements), Sr/ Sr (Farrell et al., 1978 Farrell and Holland, 1983), microscopic observation, and chemistry of spinel of unaltered basalt clarifies the tectonic setting of Kuroko deposits. Based on the geochemical data on the selected basalt samples which suffered very weak alteration, it can be pointed out that the basalt that erupted almost contemporaneously with the Kuroko mineralization was BABB (back-arc basin basalt) with geochemical features of which are intermediate between Island arc tholeiite and N-type MORE. This clearly supports the theory that Kuroko deposits formed at back-arc basin at middle Miocene age. 

The age of Kuroko mineralization can be estimated from (1) K-Ar ages of igneous rocks associated with Kuroko deposits and (2) foraminiferal assemblages in mudstone directly overlying Kuroko deposits. 

Pyrophyllite and diaspore alterations were reported from several Kuroko deposits, although they are not common (Urabe, 1974a). This type of hydrothermal alteration is thought to have occurred at a later stage than the hydrothermal alterations associated with Kuroko mineralization (sericite, chlorite, and zeolites) (Utada, personal communication, 1995). 

Figure 1.37. Estimation of minimum depth of seawater at the time of Kuroko mineralization. P-T diagram of NaCl-H20-C02 is from Drummond (1981) (Ohmoto et al., 1983).

Positive Eu anomaly is observed for hydrothermal solution issuing from the hydrothermal vent on the seawater at East Pacific Rise (Bence, 1983 Michard et al., 1983 Michard and AlbarMe, 1986). Guichard et al. (1979) have shown that the continental hydrothermal barites have a positive Eu anomaly, indicating a relatively reduced environment. Graf (1977) has shown that massive sulfide deposits and associated rocks from the Bathurst-Newcastle district. New Brunswick have positive Eu anomalies. These data are compatible with positive Eu anomaly of altered basaltic rocks, ferruginous chert and Kuroko ores in Kuroko mine area having positive Eu anomaly and strongly support that Eu is present as divalent state in hydrothermal solution responsible for the hydrothermal alteration and Kuroko mineralization. 

The REE data, combined with alteration minerals and concentration of major elements in hydrothermally altered rocks, could be used to reconstruct the structure and evolution of a submarine geothermal system accompanied by Kuroko mineralization (Shikazono, 1999a). 

At the stage of Kuroko mineralization, evolved reacted seawater enriched in Eu, Ca, and Sr formed at low seawater/rock ratio (ca. 1 by mass) and at relatively reduced condition (Eu +/Eu + greater than 1). Selective leaching of Eu, Ca and Sr occurred from the dacitic rocks underlying the Kuroko ores. The hydrothermal solution enriched... 

Evolution of tectonics and hydrothermal system associated with epithermal and Kuroko mineralizations... 

As noted already, intense submarine hydrothermal activity took place in the Japan Sea in 15-12 Ma, associated with Kuroko mineralization. However, it is uncertain that submarine hydrothermal activities associated with the Kuroko mineralization took place in the other periods from middle Miocene to present in the Japan Sea. Therefore, the geochemical features of sedimentary rocks which formed from the Japan Sea at these ages have been studied by the author because they are better indicator of age of hydrothermal activities than those of hydrothermally altered igneous rocks because the samples of continuous age of sedimentation are able to be collected and the ages are precisely determined based on microfossil data (foraminiferal, radioralian and diatom assemblages). 

Inoue, A. and Utada, M. (1991) Hydrothermal alteration in the Kamikita Kuroko mineralization. Mining Geology, 41, 203-218. 

Ishikawa, Y. (1988) Geochemical characteristics of hydrothermal alteration related to Kuroko mineralization around the Fukazawa deposits, Akita Prefecture. Mining Geology Special Issue, 12, 57-66. 

Matsuhisa, Y. and Utada, M. (1993) Hydrothermal activity responsible for the Kuroko mineralization inferred from oxygen isotopic ratios of altered rocks from the western area of the Hokuroku district, northern Japan. Bull. Geol. Surv. Japan, 44 (213/4), 155-168 (in Japanese). 

Sato, Kazuo, Delevaux, M.H. and Doe, B.R. (1981) Lead isotope measurements on ores, igneous, and sedimentary rocks from the Kuroko mineralization area. Geochem. J., 15, 135-140. 

Sato, Takeo (1973) A chloride complex model for Kuroko mineralization. Geochem. J., 7, 245-270. 

Shikazono, N., Aoki, M., Yamada, R., Singer, D., Kouda, R. and Imai, A. (1992) Epithermal gold and kuroko mineralization in northeast Honshu (A07). In Mineral Deposits of Japan and the Philippines, 29th IGC field trip guide book, Vol. 6. Tokyo Soc. Resource Geol., pp. 61-100. 

Singer, D. and Kouda, R. (1992) Regional view in the search for Kuroko deposits. In 29th IGC Field Trip A07 (Epithermal Gold and Kuroko Mineralizations in Northeast Honshu), pp. 83-91. 

Yoshida, T. and Yamada, R. (2001) Relationships between the evolution of the Neogene volcanism and the Kuroko mineralization in the vicinity, of the Hokuroku district. NE Japan. Abst. Min. Soc. Japan and Assoc. Mineral. Petrol. Econ. Geol, 272 (in Japanese). 

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