Bournonite from Daikoku, Chichibu Mine, Saitama, Japan*

MINERALOGICAL JOURNAL, VOL. 6, No. 3, pp. 186-188, SEPT., 1970

SHORT COMMUNICATIONS

BOURNONITE FROM DAIKOKU, CHICHIBU MINE, SAITAMA, JAPAN*

KAZUO HARADA Central Research Laboratory, Sumitomo Metal Mining Company, Ichikawa, Chiba, Japan

OSAMU SAKAMOTO Section of Geology, Chichibu Museum of National History, Nogami-machi, Saitama, Japan

KAZUSO NAKAO and Kozo NAGASHIMA Department of Chemistry, Tokyo University of Education, Otsuka, Tokyo, Japan

A thick sequence of late Carboniferous to early Permian limestones, cherts and slates widely develops in the Chichibu Mining Territory where some limestones are metamorphosed as well as metasomatized by the intrusion of a diorite body of unknown geo logic age, forming various pyrometasomatic ore bodies (Miyazawa, Hashimoto & Harada, 1970). The Daikoku ore-body has been worked for Fe, Zn, Cu and Pb. This ore-body consists of a huge lump of magnetite with pyrite and pyrrhotite in the lower part. The irre- gular chimney shaped upper part is about 200m high, bears sulfide minerals but not magnetite and produces, as the result of the reaction of ore solutions containing Sb, Bi, As, Ag and Cu, many interesting minerals such as chalcopyrite, marmatite, jamesonite (Seki , 1960), arsenopyrite, freibergite (Harada & Kodama, 1963), semseyite (Uetani , et al., 1966) and bournonite. The wet chemical analysis of freibergite

* Mineralogical Contribution No . 1. Central Research Laboratory of the Sumitomo Metal Mining Company. Bournonite from Daikoku, Chichibu Mine, Saitama , Japan 187

by Nakao and X-ray study by Harada confirmed the chemical com

position as Ag, 9.22%; As, 0.85%; Sb, 19.78%, and a=10.41A•}0.0lA. Spectrochemical analysis by Harada revealed the presence of 0.01- 1.00% of Bi in the material. However, the properties of this freibergite are left for further studies because of a limited amounts of the specimen. This note presents the mineralogical properties of bournonite found generously in the upper part of the Daikoku ore body (3rd ore-body) intimately associated with freibergite mentioned above, galena and pyrite.

Mineralogy of bournonite

Form: Simple prismatic, sometimes showing wheel-like twining up to 2cm long and 1cm wide with faces, c {001}, b {010}, a {100}, m {110} and n {011}.

Physical properties: Brilliant lead grey in colour, with Streak in

steel grey, and purplish white in reflected light from smooth surfaces.

Conspicuous twin lamellation on (110) is sometimes found under the

microscope. Hardness, 2.5. Specific gravity, 5.732 (determined by a

pycnometer (5 cc) using 12g of the sample at 20•Ž) and 5.778 (calcu-

lated value from the chemical and X-ray data).

X-ray powder data: X-ray powder data obtained with an X-ray

diffractometer using Ni-filtered CuKal (ƒÁ=1.5405A) radiation and

silicon as internal standard gave the following unit-cell edges:

a=8.149, b=8.716, c=7.798 (all•}0.003A) and V=554.3A3.

Those values are a little smaller than those given by Hellner and Leineweber (1956), and Takeuchi and Haga (1969). This fact will be accounted for by the minor substitution of As (seligmanite molecule) for Sb (bournonite molecule) in the present specimen as referred to below.

Chemical composition: Using 15g of the pure sample separatedd by careful hand-picking was analysed by Nakao according to the wet method: Zn, 0.46; Cu, 13.15; Pb, 40.18; Fe, 1.44; As, 1.13; Sb, 188 K. HARADA, O. SAKAMOTO, K. NAKAO and K. NAGASHIMA 23.73; S, 19.68; insol., 0.41; total, 100.18. When S is set to be 3.000, the following structural formula (Z=4) will be obtained: (Pb0.947,Zn0.034, Fe0.013)0.994 Cu1.011(Sb0.952, AS0.074)1.026 S3.000. This corresponds well to the ideal formula of bournonite 4 (PbCuSbS3), and minor substitutions of As for Sb has thus been confirmed.

Discussion The lump-shaped ore-body in the lower part of the Daikoku ore deposit gradually changes its form into chimney-like cylinder towards the upper part. This chimney clearly shows the telescopic arrange ment of ore minerals within 200m in vertical height as: 1) heden bergite- andradite-bustamite-ilvite-magnetite-chalcopyrite-pyrite- pyrohotite-siderite, followed by 2) pyrite-marmatite-chalcopyrite- arsenopyrite-native gold-dolomite-ankerite-quartz, followed by 3) pyrite-jamesonite-marmatite-galena-arsenopyrite-dolemite-manga- noan ankerite-quartz, followed by 4) pyrite-jamesonite-sphalerite (amber color)-freibergite-bournonite-galena-semsyite-rhodochrosite- calcite-quartz. The concentration of Sb, As, Bi, Ag, Pb and Mn in the uppermost part of the Daikoku ore-body will indicate the nature of the ore solutions differentiated gradually from the host ore-solution rich in Fe. REFERENCES Harada, K. & Kodama, H. (1963). J. Japan. Ass. Miner. Petrol. & Econ. Geol., 49, 115. Hellner, E. & Leineweber, G. (1956). Zeits. Krist., 107, 150. Miyazawa, T., Hashimoto, M. & Harada, K. (1970). "Nagatoro-Chichibu Mine" Excursion Guidebook,IMA-IAGOD MEETINGS'70. Seki, T. (1960). J. Japan. Ass. Miner. Petrol. & Econ. Geol., 44, 14. Takeuchi, Y. & Haga, N. (1969). Zeits. Krist., 130,254. Uetani, K., Kato, A. & Okada, Y. (1966). Bull. Nat'l. Sci. Mus., Tokyo, 9, 431. Manuscript received 10 February 1970 and in revised form 8 June 1970.