MINERALOGICALJOURNAL, VOL. 7, No. 3, pp. 282-288, OCT., 1973

Carrollite from the Shirataki mine, Kochi Prefecture, Japan

SHIRO ITOH, YUJI ENDO, TAKEO SATO and YOSHIHIKOSHIMAZAKI Geological Survey of Japan, 135 Hisamoto, Takatsu, Kawasaki 213

ABSTRACT Carrollite was identified from the ores of the cupriferous pyrite deposit of the Shirataki mine. It occurs closely associated with chalcopyrite and pyrite. Electron microprobe analysis showed the composition to be very close to CO2CUS4.The mineral is believed to be formed by reaction between chalcopyrite and cobalt which was expelled from pyrite during intense metamorphism. This is the second occurrence of carrollite in Japan.

Introduction Carrollite, a copper-bearing member of the linnaeite series (Co, Ni)2(Co, Ni, Fe, Cu)S4, usually occurs in copper deposits associated with copper, nickel, and iron sulphides. It is a very rare mineral reported from several localities such as Carroll County of Maryland, USA (Shannon, 1926), Raipas mine of Norway (Vokes, 1956), and some deposits of Central African Copper-belt (e. g., Chibuluma mine by Darnly and Killingworth, 1962, and Camoto mine by Bartholome et al., 1971). In Japan, the only confirmed occurrence of the mineral was found by F. Nakayama from the ores of the Sazare mine (in Kane- hira and Tatsumi, 1970). The mineralogical data were later reported by Tatsumi and Nakayama (1973). During the course of the study of the geochemical behavior of cobalt in cupriferous pyrite deposits by one of the writers (S. I.), S. ITOH, Y. ENDO, T. SATO and Y. SHIMAZAKI 283 microscopic observation of the ores of the Shirataki mine brought to the attention of the writers an unusual mineral which was identified to be carrollite by subsequent studies (Itoh and Endo, 1972). In this paper, the mode of occurrence, chemistry and optical properties of the mineral are presented together with discussion of the genesis.

Occurrence

The ore deposit of the Shirataki mine, Kochi Prefecture is geologically located in the Sambagawa metamorphic belt and is one of the largest bedded cupriferous pyrite deposits in Japan. The area has undergone intensive metamorphism and the deposit is one of the most strongly metamorphosed deposits in Japan.

Fig. 1. Sketch showing the occurrence of carrollite (black) associated with pyrite (blank), chalcopyrite (dotted) and quartz (lined) from the Shirataki mine. 284 Carrollite from the Shirataki mine, Kochi Prefecture, Japan

The ores are massive in texture and their mineral composition

is relatively simple consisting mainly of subhedral pyrite with

chalcopyrite filling the interstices of the coarse pyrite grains. A

small amount of bornite is also present. Carrollite is embedded in

chalcopyrite and is in direct contact with pyrite. It is subhedral

to unhedral and is less than 0.02mm in diameter (Fig. 1). Although

the distribution of the mineral within the ore deposit is not traced

thoroughly, it seems to occur mostly in the lower part of the orebody.

Optical and Chemical Studies

The optical properties of the mineral determined under reflection

microscope are as follows:

Color: Creamy white with slight pinkish tint.

Bireflection: None.

Anisotropy: None.

Internal reflection: None.

Polishing hardness: •âchalcopyrite, •ƒpyrite.

Reflectivity: •„chalcopyrite, •ƒpyrite.

The chemical composition of the mineral was determined by

JXA-3 electron microprobe. The measured values were corrected for background, dead time, the atomic number effect (Poole-Thomas' method) and absorption (Philbert's method). Chalcopyrite with known composition was used as standard for Cu, Fe, and S, and pure metals for other elements. The results is shown in Table 1 together with previous data. The composition of the Shirataki specimen is very close to CO2CuS4.

Though X-ray work could not be made because of the difficulty in separation of the mineral, it seems logical to conclude from the occurrence, optical and chemical properties that the mineral in question is carrollite. S. ITOH, Y. ENDO, T. SATO and Y. SHIMAZAKI 285 Table 1. Analyses of carrollite.

1. CO2CUS4. 2. Shannon, E. V. (1926). 3. de Jong, W. F. and Hoog, A (1927). 4. Permingeat, F. and Weinryb, E. (1960). 5. Darnley, A. G. and Killingworth, P. J. (1962). 6. Vokes, F. M. (1967). 7. Tatsumi, T., Nakayama, F. and Urabe, T. (in preparation). 8. The present study CO2.00(Cu0.75Ni0.07 Fe0.06CO0.06)0.95S4.05. 286 Carrollite from the Shirataki mine, Kochi Prefecture, Japan

Discussion

The cobalt content of the ores from the cupriferous pyrite deposits of the Sambagawa metamorphic belt is in the range of 0.03 to 0.5 per cent. The element is mostly contained in pyrite replacing iron. In the case of the highly metamorphosed ores, the cobalt is concentrated along the rim of the pyrite grains, while it is very heterogeneously distributed within the grains in the weakly metamorphosed and non-metamorphosed ores (Itoh, 1973). In spite of the relatively high cobalt content in the ores of the region, the occurrence of cobalt minerals was not reported until very recently. The first description of such mineral was cobaltite from the Besshi mine by Kase (1972), and the following two reports of carrollite are the only known occurrence of these minerals to date. Cobaltpentlandite has been reported to occur in the pyrrhotite predominant ores of the Shimokawa mine, Hokkaido (Kato and Sato, 1963), possibly as a primary mineral. In this case it is inferred that cobalt in ore solution formed its own sulphide mineral, cobalt- pentlandite, because the content of the element in pyrrhotite, chalco- pyrite, and other sulphides is extremely small, whereas pyrite which usualy contains significant amount of cobalt was a minor constituent in the vicinity. In the pyrite predominant ores, however, cobalt was contained in the pyrite crystals and formed cobaltiferous pyrite (Itoh, 1973). The cobaltite from the Besshi mine was also found in the pyrrhotite ores of the lower parts of the orebody where pyrrhotite could have been formed from pyrite by thermal metamorphism (Kase, 1972). In this case, it is considered that the cobalt mineral was formed secondarily by the decomposition of cobaltiferous pyrites. The pyrites were metamorphosed to pyrrhotite and the cobalt formed an independent mineral as it was expelled from pyrite. In the case of the deposits of both high nickel and cobalt S. ITOH, Y. ENDO,T. SATOand Y. SHIMAZAKI 287

contents, different cobalt minerals could be formed . Vahatalo (1953) reported the occurrence of linnaeite minerals in the ores of the Outokumpu mine, Finland. The mineral species have not been es- tablished, but they are most likely to be linnaeite proper (Co, Ni)3S4 and siegenite (Ni, Co)3S4. The significant difference between carrollite and these minerals is the low nickel and high copper contents of the former. On the other hand, the nickel content of the sulphide ores from the cupriferous pyrite deposits in Japan is usually much lower than the cobalt content. For example, the cobalt/nickel ratio of pyrites from the Shirataki mine ranges from 4 to 53 with average of 21 (Itoh, 1971). Furthermore, nickel is likely to enter many ore minerals, such as magnetite, chalcopyrite, and pyrrhotite, which is very dif- ferent from cobalt. From these reasons, it is very unlikely to find nickel-rich members of the linnaeite series in the cupriferous pyrite deposits in Japan. The rarity and the restriction of the occurrence of carrollite to the highly metamorphosed ores of the cupriferous pyrite deposits suggests that the mineral was formed by reaction between chalco- pyrite and cobalt which was expelled from pyrite during intense metamorphism.

Acknowledgement-The writers wish to express their gratitude to Professor Tatsuo Tatsumi of the Tokyo University for providing his unpublished analytical data.

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Received July 2, 1973