MINING GEOLOGY, 28, 267•`276, 1978 Sub-types and Their Characteristics of Kuroko-type Deposits* Ei HORIKOSHI** and Naotatsu SHIKAZONO*** Abstract: The Kuroko-type deposits in the Hanaoka-Kosaka district are divided into three sub-types based on the ratio of major base metals in a single unit deposit. The ratio of copper to lead and zinc increases in order of the "B", "C" and "Y" sub-types. There is the north-southerly lateral zoning of these sub-type deposits in the Hanaoka and Kosaka districts. •¬ D values are distinctly high in the "B" sub-type deposits. •¬34S values are distinctly high in the "C" sub-type deposits and iron contents of sphalerite are low in the "Y" sub-type deposits. The higher •¬D values of "B" sub-type deposits occurring in the central part of basins may indicate sea-water as the predominant source for ore fluids. Using iron contents of sphalerite and •¬34S values of pyrite the possible depositional environments of three sub-type deposits were estimated on fo2-pH diagram. In conclusion, each ascending ore fluid responsible for different sub-type deposits was different in geo- chemical nature. This suggests that the chemical and physical changes of ore fluid circulating in a hydrothermal system are important for the cause of differences among three sub-types of the Kuroko-type deposits. ores, and describe briefly their occurrence and Introduction geochemical characteristics, and discuss the In terms of a hydrothermal sedimentary possible cause. This paper concerns mainly theory the schematic section of a single unit with three sub-types in the Hanaoka-Kosaka of Kuroko-type deposits was first proposed by district, the northern part of Akita Prefecture. Horikoshi (1960). Thereafter, many schematic Black Ore Sub-type sections were proposed and modified slightly with the progress of investigations. There is, HIRABAYASHI(1908) defined first the however, no fundamental difference among Kuroko-type deposits as follows: Black ore the schematic sections appearing in 1970's. (Kuroko) is a compact mixed ore composed For example, the section published by of sphalerite, galena and barite. Furthermore, HORIKOSHI(1976) is essentially identical with he mentioned that Kuroko is usually accom- SATO'S (1970). It seems that the general panied with chalcopyrite and pyrite in addi- feature of a single unit of Kuroko-type deposits tion with the above minerals, and contains gold has been established among geologists, espe- and silver. According to HIRABAYASHI'Sdef- cially in Japan. inition, the Kuroko-type deposits are mineral It is possible, however, to classify the deposits composed mainly of the black ore. Kuroko-type deposits into more sub-types on It seems likely that his definition was affected the basis of some factors. Papers with this line by the economic value of ore in the Kuroko appeared already (HORIKOSHI,1965; URABE, deposits. The ore at that time was mined only 1974a. HORIKOSHIand OHMOTO,1978). The for silver contents of black ore, especially of the authors will classify the Kuroko-type deposits weathered ore. Therefore, the present yellow into three sub-types on the basis of constituent and siliceous ores, if any, were not regarded * Received May 20, 1978. as the economic ore. There are, however, ** Department of Earth Sciences, Toyama University, the Kuroko-type deposits composed exclusively Gofuku, 930 Toyama. of black ore. These mineral deposits are *** Geological Institute, University of Tokyo, Hongo, designated the Black Ore sub-type in the 113 Tokyo classification of this paper. Keywords: Kuroko, Classification, •¬D, Sphalerite, Ore fluid The Shakanai No. 1 deposit in the Shakanai 267 268 E. HORIKOSHI and N. SHIKAZONO MINING GEOLOGY: siliceous ores are not exactly known, because the Doyashiki deposit is accompanied by many satellite deposits. It may be more than 10 million tons. The quantity of black ore is roughly equal to yellow or siliceous ore. The second largest deposit of Kuroko-type is probably the Motoyama deposit of this sub- type in the Kosaka mine. A part of the deposit was already eroded away, when the deposit was discovered in 1861. However, about 7 million tons of ore were mined until 1946, when the mining was ended. The ore contains 2.3 million tons of black, 1.1 million tons of yellow and 3.7 million tons of siliceous ores. That is to say, this mineral deposit consists of roughly equal quantities of black, yellow Fig. 1 Available data on the Cu, Pb and Zn ratio of and siliceous ores (Fig. 1). total ore in a single unit deposit in the Hanaoka- A part of geology of the Composite Ore Kosaka district, marked with three sub-types. sub-type is exhibited strikingly in the abandoned open-pit of the Motoyama deposit. mine is a good example for this type and was The Uchinotai-nishi deposit of the same excellently described by KAJIWARA(1970a). sub-type was, however, better described by There are more mineral deposits of this sub- HORIKOSHI(1969) and HORIKOSHIand SATO type in the Hanaoka-Kosaka district. The ore (1970). The hydrothermal activity responsible of Black Ore sub-type deposits consists pre- for the mineralization of the Composite Ore dominantly of galena and sphalerite compared sub-type deposits was mostly preceded by the to chalcopyrite (Fig. 1). Ore deposits of uplift of lava dome and the subsequent steam this sub-type are usually not directly associated explosion. with lava dome. It is known, however, that dome-shaped dacite occurs below some of Yellow Ore Sub-type this sub-type deposit (KAJIWARA, 1970a; Some of the Kuroko-type deposits consist URABE, 1974b; TANIMURA et al., 1974). predominantly of pyrite containing a little Total ore quantity of a single unit deposit is quantity of chalcopyrite. In this paper the generally small, about one million tons. mineral deposits composed predominantly of Composite Ore Sub-type pyrite, either with an economical value of chalcopyrite or not, are called the Yellow Ore Mineral deposits of this sub-type are sub-type, though the yellow ore means usually often called typical Kuroko-type deposits. SATO copper ore (Fig. 1). (1970), HORIKOSHI(1976) and others published Generally speaking, mineral deposits of their schematic sections of Kuroko-type copper-rich Yellow Ore sub-type occur above deposits referring to the general geology of lava dome or lava flow. While copper-poor this sub-type. The most characteristics are deposits occur mostly in pyroclastic rocks and that, from the economic point of view, the are associated with a lot of gypsum. The major Kuroko-type deposits belong to this Matsumine deposit in the Hanaoka mine is sub-tune. typical of the Yellow Ore sub-type, whose It seems that the largest Kuroko-type deposit geology was described by ITO et al. (1974). is the Doyashiki deposit of the Composite Ore The Matsuki and Takadate deposits in the sub-type in the Hanaoka mine. The total ore Matsuki mine also belong to this sub-type quantity and ratio of black, yellow and (KURODA,1978). Many pyrite-rich ore bodies 28(4), 1978 Sub-types and Their Characteristics of Kuroko-type Deposits 269 associated with a large quantity of gypsum ern part of the Hanaoka district were mined ore were mined in the Hanaoka mine many years ago. Hence, their forms and (TAKAHASHIand SUGA, 1974). It seems likely characteristics are slightly obscure. However, that these ore bodies are composed of several mineral deposits in the other area are mostly unit deposits of this sub-type. well documented. Figure 2 shows the distribu- tion of different sub-types of the Kuroko Areal Distribution deposits in the Hanaoka district. Roughly Hanaoka and Kosaka are the most produc- speaking, the "Y" sub-type deposits are tive Kuroko mining districts. Probably more distributed in the westernmost zone and the than 30 single units of Kuroko-type deposits "B" sub -type deposits are in the easternmost were already discovered and exploited. These zone of the district. The "C" sub-type de- deposits include all sub-type deposits men- posits occur between both zones. No mineral tioned in the preceding chapters. Hence, both deposit has been found in the western area districts are suitable to investigate the areal beyond the Tsutsumizawa fault. distribution of different sub-types of the In Figure 2, Kuroko-type deposits are Kuroko deposits. The authors will use the projected on the upper surface contours of abbreviations in the following sentences: "B", M1 mudstone occurring below mineral deposits sub-type for the Black Ore, "C" for the (TAKAHASHIand SUGA, 1974). M1 mudstone Composite Ore and "Y" for the Yellow Ore. shows weakly subsided trough-like structure. Mineral deposits located at the northwest- The axis is located between the "C" sub-type and "B" sub-type zones, and dips slightly to the southeast. This structure is slightly disturbed in the northern part, because large lava domes associated with the "C" sub-type deposits intruded in the area. The Hanaoka formation, in which most mineral deposits occur, becomes thicker to the east in the district. Furthermore, some pyroclastics in this formation moved down to the east as far as their flow directions were confirmed (HORIKOSHI, 1966; TAKAHASHIand SUGA, 1974). HORIKOSHI(1966) described, however, the field evidence that a tuff breccia moved to the east and was dammed up along the eastern zone over the Shakanai No. 1 deposit. It seems likely that the trough-like form of the sea floor existed already at the stage, when mineral deposits were formed. Mineral deposits from the Shakanai No. 1 to the Matsuki occur in crater-like depressions. This occurrence is indicated by mudstone distributed in the restricted areas on or above the mineral deposits (HORIKOSHI,1965; 1966; KURODA, personal communication).