Kuroko deposits

Precious metal vein-type deposits include enrichments in Au, Ag, Hg, Te, Se, Sb, As, S, and Bi. Base metal vein-type deposits contain Pb, Zn, Mn, Ag and Cu, whereas Kuroko deposits are enriched in Cu, Pb, Au, Ag and Ba. 

Kuroko deposits in Northeast Japan originated from these materials. Antimony, mercury and sulfur in the Hg-Sb deposits in Southwest Honshu may have been derived from the shallow level of the crust under the Shimanto Group. 

Main hydrothermal ore deposit types of Neogene age that formed in and around the Japanese Islands are Kuroko deposits and epithermal vein-type deposits. This classification is based on the form of the deposits. 

Kuroko deposits are strata-bound and massive in form (Fig. 1.7) and syngenetically formed on the seafloor and/or sub-seafloor environment. Vein-type deposits are fissure-filling and epigenetically formed (Fig. 1.8). 

Elemental association can be used to sub-classify these deposits. Major metal elements produced from Kuroko deposits are Cu, Pb, Zn, Ba, Ca, Fe, Au, and Ag. Average ore grade and tonnage are summarized in Table 1.1. Horikoshi and Shikazono (1978) classified Kuroko deposits into three sub-types C sub-type (composite ore type). 

Y sub-type (yellow ore type), and B sub-type (black ore type), according to Cu, Pb and Zn ratios (Fig. 1.9). However, the variation in the ratio is not wide, compared with epithermal vein-type deposits. Therefore, characteristic differences in each sub-type of Kuroko deposits are not discussed here. 

Figure 1.9. Available data on the Cu, Pb and Zn ratio of total ore in a single unit deposit in the Hanaoka-Kosaka district, marked with three sub-types of Kuroko deposits (Horikoshi and Shikazono, 1978).

Hirabayashi (1907) defined Kuroko as an ore which is a fine compact mixture of sphalerite, galena, and barite. This definition can be applied to black ore , but not to yellow ore or siliceous ore because these minerals are not abundant in these ores. Kinoshita (1944) defined Kuroko deposit as a deposit genetically related to the Tertiary volcanic rocks, consisting of a combination of Kuroko (black ore), Oko (yellow ore), Keiko (siliceous ore), and/or Sekkoko (gypsum ore) (Matsukuma and Horikoshi, 1970). The deposit is generally defined as a strata-bound polymetallic sulfide-sulfate deposit genetically related to Miocene bimodal (felsic-basaltic) volcanism (T. Sato, 1974). 

Kuroko deposits occur in the Green tuff region which is characterized by thick altered volcanic and sedimentary piles of Miocene age. 

Distributions of Kuroko deposits and names of the representative mines are given in Fig. 1.10 and Table 1.1. Metals produced during the past are summarized in Table 1.1. 

Large Kuroko deposits occur in the Hokuroku district in Northeast Honshu (Fig. 1.11). Small numbers of Kuroko deposits are found in other districts such as Southwest Hokkaido, the northern part of Honshu (Shimokita Peninsula district), the middle of Northeast Honshu (Wagaomono and Aizu districts) and western Honshu (San-in district) (Fig. 1.10). 

It is clear in Fig. 1.10 that the distribution of Kuroko deposits is restricted in a narrow zone in the Green tuff region which was called a Kuroko belt by Inoue (1969). This belt was formed by rapid subsidence under the extensional stress regime and is thought to have been a back-arc depression zone at middle Miocene age. The relationship between tectonic setting and formation of Kuroko deposits is discussed in section 1.5. 

Many Kuroko deposits are distributed in the Hokuroku di.strict. Northeast Honshu (Fig. 1.9). Therefore, general geology and stratigraphy of the Hokuroku district is briefly described below mainly following T. Sato (1974, 1977), Tanimura et al. (1983) and Ishikawa (1991) (Table 1.3). 

Kuroko-type Gypsum or Barite Green Tuff Beit of Japan Clusters of Kuroko Deposits... 

The formation which is mostly composed of dacite lavas, tuff breccia and mudstone (Hanaoka, Yukisawa, Uwamuki formations) conformably overlies the Hotakizawa and Sasahata formations. The thickness of these formations is 300-400 m. Kuroko ore deposits occur at the upper part of this formation. White rhyolite lava domes characterized by intense sericite alteration have a close spatial relationship with Kuroko deposits. 

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

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. 

Ma. Considering uncertainty of foraminiferal and K-Ar ages it seems reasonable that the Kuroko deposits in Hokuroku district formed in 14-12 Ma (more likely 13.6-... 

However, several small Kuroko deposits (e.g., Yunosawa in Hokuroku district, Kuroko deposits in Hokkaido) occur in the formation younger than middle Miocene age (16-14 Ma), suggesting younger ages (12-13 Ma). 

Figure 1.9 shows the proportion of Cu, Zn and Pb contents of Kuroko ore (Tatsumi and Ohshima, 1966 Horikoshi and Shikazono, 1978). Horikoshi and Shikazono (1978) divided Kuroko deposits in the Hanaoka-Kosaka area of Hokuroku district into three sub-types based on the ratio of Cu to Pb and Zn which increases in order of the B (black ore), C (composite ore), and Y (yellow ore) sub-types (Fig. 1.9). Characteristic features of these three sub-types were summarized by Horikoshi and Shikazono (1978) and are briefly decribed below.

C sub-type deposits are often called typical Kuroko deposits. Sato (1970) and Horikoshi (1976) published the schematic sections of Kuroko deposits referring to the general geology of this sub-type. The major Kuroko deposits belong to this sub-type. The largest Kuroko deposit is the Doyashiki deposit in the Hanaoka mine which belongs to C sub-type. The total ore quantity may be more than 10 million tons. The second largest deposit of Kuroko deposits is the Motoyama deposit of this sub-type. About 7 million tons of... 

Some of the Kuroko deposits consist predominantly of pyrite containing a small amount of chalcopyrite. The ore deposits consisting predominantly of pyrite, either with an economical value of chalcopyrite or not, are called the Y sub-type deposits, which occur above dacite lava dome or lava flow, while copper-poor deposits occur mostly in pyroclastic rocks and are associated with a large amount of gypsum. The Matsumine deposit in the Hanaoka mine is typical of the Y sub-type. The Matsuki and Takadate deposits in the Matsuki mine are also classed as this sub-type (Kuroda, 1978). Many pyrite-rich ore bodies... 

Figure 1.12. Distribution of two different sub-types of Kuroko deposits in the Kosaka district, Akita Prefecture. Y sub-type deposits have not yet been discovered in the area. The top pre-Tertiary basement is contoured showing some depressed structures (Horikoshi and Shikazono, 1978).

Figure 1.13. Distribution of three different sub-types of the Kuroko deposits in the Hanaoka district. The top of M mudstone is also shown to visualize the structure of country rocks (Horikoshi and Shikazono, 1978).

The Y, C and B sub-types roughly correspond to types 1, 2 and 3 as defined by Urabe (1974a), who classified Kuroko deposits based on hydrothermal alteration and ore mineral assemblages type 1, kaolinite-pyrophyllite-diaspore-type type 2, sericite-chlorite-type type 3, sericite—chlorite-carbonate-type. Hydrothermal alterations in the Kuroko mine area are described in section 1.3.2. 

Most large Kuroko deposits belong to type 2 (or C-subtype). Type 1 occurs mostly in Northeast Honshu and Hokkaido. Type 3 deposits are distributed in middle Honshu and Southwest Honshu (San-in district). Most of the previous studies have been carried out on the deposits in the Hokuroku district. A summary of the mineralogical and geochemical characteristic features of Kuroko deposits in this district is given below (sections 1.3.2 and 1.3.3). 

Figure 1.14 shows the distribution of minerals in each ore zone (Matsukuma and Horikoshi, 1970). The occurrence of ore minerals in Kuroko deposits was described in Shimazaki (1974), Matsukuma et al. (1974) and Urabe (1974a). 

Iron contents of sphalerite are different in layered ores in different ore deposit and different sub-types of Kuroko deposits. Iron contents of sphalerite from the B sub-type deposits (Uwamuki No. 4, Shakanai No. I, Ezuri, and Fukazawa deposits) show wide range but generally less than 0.2 wt% (Ono and Sato, 1995). The average value, however, is probably lower than the C sub-type deposits (e.g., Uehinotai deposits). This may... 

Figure 1.14. Schematic diagram showing mineralogical changes in various kinds of ores of Kuroko deposits (Matsukuma and Horikoshi, 1970).

Tetrahedrite-tennantite composition varies widely in Kuroko deposits (Yamaoka, 1969 Yamaoka and Nedachi, 1978a Yui, 1971 Horii, 1971 Shimazaki, 1974 Kouda, 1977 Shikazono and Kouda, 1979 Ono and Sato, 1995 Ishizuka and Imai, 1998). 

Generally, tetrahedrite-tennantite composition from Kuroko deposits is characterized by high Zn content, low Fe content, high Cu content, and low Ag content compared with those from vein-tyjre deposits in Japan (Fig. 1.16). Rarely, it contains Hg up to 1 wt% (Ishizuka and Imai, 1998). 

Figure 1.16. Chemical composition of tetrahedrite-tennantite (Shikazono and Kouda, 1979). A Au-Ag vein-type deposits, B Kuroko deposits, C Taishu-Shigekuma Pb-Zn vein-type deposits, D Skam deposits (Kamioka).

The mode of occurrence of electrum was described by Matsukuma (1985). Chemical compositions of electrum from Kuroko deposits were summarized by Shikazono (1981) and Shikazono and Shimizu (1988a). The Ag content of electrum from Kuroko deposits varies widely from 4.7 to 89.4 atomic% (Fig. 1.17). Electrum with low Ag... 

Figure 1.17. Frequency histogram for the Ag content of electrum from Kuroko deposits in Japan (Shikazono...

Figure 1.18. Variation of Fe " /(Fe + Mg) and tetrahedral AI of chlorite from hydrothermal ore deposits Japanese Neogene Cu-Pb-Zn vein-type (open circle) and Kuroko deposits (solid circle). Localities 1 Ashio, 2 Yatani, 3 Toyoha, 4 Kishu, 5 Sayama, 6 Mikawa, 7 Furutobe, 8 Hanaoka, 9 Wanibuchi, 10 western Bergslagen (Shikazono and Kawahata, 1987).

Dominant gangue minerals in Kuroko deposits are quartz, barite, anhydrite, gypsum, chlorite, sericite, and sericite/smectite. Morphology of quartz changes from euhedral in the centre to the irregular in the margin of the deposits (Urabe, 1978). No amorphous silica and cristobalite have been found. 

Kuroko deposits are characterized by large amounts of sulfate minerals (barite, anhydrite, and gypsum). Estimated total amount of barite and sekko (gypsum + anhydrite) from individual deposit is shown in Table 1.4. Sr contents of gypsum, anhydrite and barite... 

Figure 1.25. Map of probabilities of centres of Kuroko deposits based on sodium depletion, sericite, and gypsum plus anhydrite (Singer and Kouda, 1988).

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