THE OCCURRENCE OF STANNOIDITES FROM THE XENOTHERMAL ORE DEPOSITS OF THE AKENOBE, IKUNO, AND TADA MINES, HYOGO PREFECTURE, AND

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1 MINERALOGICAL JOURNAL, VOL. 5, No. 6, pp , MAR., 1969 THE OCCURRENCE OF STANNOIDITES FROM THE XENOTHERMAL ORE DEPOSITS OF THE AKENOBE, IKUNO, AND TADA MINES, HYOGO PREFECTURE, AND THE FUKOKU PREFECTURE, MINE, KYOTO JAPAN AKIRA KATO Department of Geology, National Science Museum, Ueno Park, Tokyo and YOSHINORI FUJIKI Department of Mineral Development Engineering, Faculty of Engineering, University of Tokyo ABSTRACT Stannoidite was found in copper-tin-sulphide ores from xenothermal ore deposits of the four mines mentioned in the title. The mineral assemblages include chalcopyrite, mawsonite, bornite, sphalerite, galena, tennantite-tetrahedrite, cassiterite, and quartz as the common associates. Electron micro probe analyses verified the ideal formula Cu5 (Fe, Zn)2SnS8 with minor substitution for of copper by silver, and possible deficiency of sulphur leading to Cu5 (Fe, Zn)2SnS7. The occurrence of this mineral is rather common in copper-tin sulphide ores from Japanese xenothermal ore deposits. Introduction Stannoidite, Cu5 (Fe, Zn)2SnS8, is a new mineral first described from the Konjo mine, Okayama prefecture by Kato (1969). It is an ore-forming mineral of copper-tin-sulphide ores bearing cassiterite,

2 418 The Occurrence of Stannoidites from the Xenothermal Ore stannite, chalcopyrite, galena, tetrahedrite, quartz, and siderite. This mineral is very similar to hexastannite (Ramdohr (1944) (1960)) in many respects including optical properties, X-ray powder data, and the mode of occurrence. But the substantital identity has not de finitely been established yet, mainly because of the unsettled chemical composition and crystallography of hexastannite. Recently, Levy (1967) described a copper-iron-tin-sulphide mineral with the name ' stannite jaune' (interpreted as yellow stannite in this paper) and with the general formula Cu2+xFeSn1-xS4, where 0<x<0.44. As stated later, some hexastannites and yellow stannites may be iden tical with stannoidite. The ore deposits of these four mines are of xenothermal vein type and characterized by the development of complex polymetallic ore mineral assemblages including copper, iron, zinc, lead, tin, silver, wolfram, arsenic, and antimony minerals. These ore deposits form a metallogenic province in the western Kinki District (Imai, Fujiki, & Tsukagoshi (1967)), in which the Konjo mine is situated (Fig. 1). The first report dealing with the occurrence of this mineral was made by Yamaguchi (1939) together with a description of the ores from the Ikuno mine, Hyogo prefecture. In this report, he used the name brown stannite, recognizing its difference from ordinary stan nite in optical properties, which no doubt indicates the identity of his brown stannite with stannoidite. In 1966, Shimizu, Kato, and Matsuo found hexastannite in the ores from the Fukoku mine, Kyoto prefecture, based on the identification by optical and X-ray powder methods. In the next year, Imai, Fujiki, and Tsukagoshi recognized the presence of at least two different copper-iron-tin sulphides in copper-tin-sulphide ores from the Ikuno and Tada mines. One of them was identified as mawsonite and the other as hexastannite from the optical and chemical studies, and they designated the ideal formula of hexastannite to be Cu5Fe2SnS8 from the electron micro probe analyses. Just after this finding, the occurrence of hexastannite

3 A. KATO and Y. FUJIKI 419 Fig. 1. Index map; 1. Akenobe mine, Hyogo prefecture. 2. Ikuno mine, Hyogo prefecture. 3. Tada mine, Hyogo prefecture. 4. Fukoku mine, Kyoto prefecture. 5. Konjo mine, Okayama prefecture. was found from the Akenobe mine by the authors chemical, and X-ray powder studies. of stannoidite from their optical, During the course of the study from the Konjo mine by the first author, the identity of the original stannoidite with these four hexastannites was proved by optical, chemical, and X-ray powder methods. The present paper describes the modes of occurrence, optical properties, X-ray powder data, and chemical composition together with the discussion on the possible range of the chemical composi tion of this mineral.

4 420 The Occurrence of Stannoidites from the Xenothermal Ore The authors express their sincere thanks to Professor Hideki Imai, Faculty of Engineering, University of Tokyo, and to Professor Takeo Watanabe, Faculty of Science, Nagoya University, for their kind advice. Part of this study was made while the first author was in the Smithsonian Institution, U. S. National Museum, as a visiting research associate sponsored by the National Research Coun cil, U.S.A., to which he is greatly indebted. The specimens used in the present study were kindly placed at the authors' disposal by Mitsubishi Metal Mining Company, Nihon Mining Company, and Japan Earth Science Company. Modes of occurrence a) Stannoidite from the Akenobe mine The xenothermal ore deposits of the Akenobe mine are composed of veins cutting Paleozoic sedimentary and pyroclastic rocks and later diorite and quartz porphyry. They include chalcopyrite, spha lerite, galena, cassiterite, wolframite, scheelite, arsenopyrite, and bornite as the principal ore minerals (Saigusa (1958)). Very recently the occurrence of roquesite and some bismuth minerals was reported (Kato and Shinohara (1968)). The studied specimen coming from the Eisei vein consists of quartz with dark brown gray vaguely concave patches composed of stannoidite, cassiterite, and quartz. They reach 1cm across and grade into stringers connecting the other patches. In some specimens are light brown masses of siderite. Quartz is somewhat drusy apart from the patches. Under the ore microscope, the patches are composed of subround masses of stannoidite less than 2mm across with or without cassi terite. Quartz is present in their interstices. The stannoidite masses consist of very fine-grained crystals, with occasional coarser-grained ones reaching 0.2mm across which are exclusively found around some cassiterite and quartz grains and especially when the quartz crystals are idiomorphic. Cassiterite grains exhibit simple twinning

5 A. KATO and Y. FUJIKI 421 Fig. 2. Stannoidite (Std) including partly replaced inclusions of cassiterite (Cas). Gangue mineral (black) is quartz (Q). Akenobe mine, Hyogo Prefecture. very often and light yellow brown internal reflections. They are idiomorphic in quartz, and replaced by stannoidite when surrounded by it (Fig. 2). This means the presence of cassiterite and idiomorphic quartz prior to the formation of stannoidite. Chalcopyrite is found as very small bodies in stannoidite or very thin veinlets cutting stannoidite. There is a remarkable difference in the texture of quartz between the patch and the other parts. In the former, very fine-grained quartz from some hundredths to some tenths of a milimeter across forms mosaic aggregate with a small amount of sericite, whereas the latter is occupied by quartz grains exceeding a few milimeters across with occasional development of zoning due to dusty inclusions. b) Stannoidite from the Ikuno mine The ore deposits of the Ikuno mine are in many respects similar

6 422 The Occurrence of Stannoidites from the Xenothermal Ore to those of the Akenobe mine, but the wallrocks are late Cretaceous (?) sedimentary and pyroclastic rocks and later andesite dikes. The principal ore minerals are chalcopyrite, sphalerite, galena, cassiterite, ferberite, scheelite, arsenopyrite, - bismuth, and some silver minerals. The occurence of such rare minerals as ikunolite (Kato (1959)), pa vonite (Kato (1964)), and sakuraiite (Kato (1965)) is known. The studied specimen was collected at No. 18 stope, 6th level of the Senju mae vein, where the vein contains an unusually large number of mineral species, such as chalcopyrite, sphalerite, galena, stannite, cobaltiferous arsenopyrite, bismuth, bismuthinite, pyrite, tennantite, ferberite, scheelite, cassiterite, quartz, calcite, and gypsum. Stannoi dite was found in small masses reaching 5mm across in quartz, which also includes chalcopyrite with or without tennantite. Stan noidite masses are absent around drusy parts of the vein in which chalcopyrite and tennantite crystals are present. Under the ore microscope, the masses include stannoidite grains with interstitial graphic intergrowths of chalcopyrite and zincian stannite (Fig. 3), the latter being less anisotropic and more bluish than ordinary stannite, with a chemical composition; Cu 31.2, Zn 6.4, Fe 5.9, Sn 27.5, and S 26.6 (total 97.6%) according to electron micro probe analysis. The stannoidite grains are generally less than 0.5mm across and some of them are polysynthetically twinned. Tennantite occurs in veinlets cutting stannoidite and chalcopyrite very often and replaces cobaltiferous arsenopyrite contained as skeleton-like inclu sions therein. Other ore minerals observed in the polished sections are galena, ferberite partly replaced by scheelite, cassiterite, and pyrite. The gangue minerals are quartz and minor calcite. Stan noidite showing the above-mentioned texture was studied by the X-ray powder method, but not chemically analysed. Very probably, it is identical with that studied by Imai, Fujiki, and Tsukagoshi (1967). In this stope is massive bornite-rich copper ore apart from

7 A. KATO and Y. FUJIKI 423 Fig. 3. Stannoidite (Std) with graphic intergrowths of zincian stannite (St, light grey) and chalcopyrite (Cp, white) together with tennantite (Ten). Gangue mineral is quartz (Q). Ikuno mine, Hyogo Prefecture. the above ore. Under the ore microscope, it consists of bornite con taining granular mawsonite and stannoidite less than 0.2mm across and chalcopyrite with quartz. The chemical composition of this stannoidite is given in Table 2. The microprobe analysis of the associated mawsonite gave Cu 43.0, Fe 14.3, Sn. 11.6, and S 29.6 (total 98.5%). c) Stannoidite from the Tada mine The ore deposits of the Tada mine consist of some parallel veins in acidic pyroclastic and volcanic rocks correlated with those of the Ikuno area (Ikebe, et al. (1961)), and one of them named 'Hyotan vein' has been worked mainly for copper and silver. The principal ore minerals therefrom include bornite, chalcopyrite, sphalerite, galena, stannoidite, mawsonite, cassiterite, tetrahedrite, silver, and argentite, and the gangue minerals are quartz, calcite, and fluorite. Among

8 424 The Occurrence of Stannoidites from the Xenothermal Ore them, bornite is the predominant ore mineral, and stannoidite as well as mawsonite are exclusively found in the ores containing bornite and quartz. The common associates therewith are cassiterite, chal copyrite, tetrahedrite, and sphalerite. The ore microscopic observa tion of this ore reveals the presence of remnant inclusions of cas siterite in stannoidite, which is in turn surrounded by mawsonite (Fig. 4). This texture may be interpreted that the stannoidite was a reaction product of pre-existent cassiterite and ascending materials containing copper, iron, and some zinc together with sulphur, and the mawsonite was that of this stannoidite and the latter. Since the direct contact of cassiterite and mawsonite has not been found in any polished section, this association may be unstable under such conditions as these ore minerals are formed, leading to the possible formation of stannoidite as a reaction product of cassiterite and mawsonite. However, the authors rejected this possibility due to Fig. 4. Stannoidite (Std) surrounding partly replaced inclusions of cassiterite (Cas). They are in aggregate of mawsonite (Mw) grains. Tada mine, Hyogo Prefecture.

9 A. KATO and Y. FUJIKI 425 the presence of zinc in stannoidite. For, if stannoidite was formed by the reaction of cassiterite and surrounding mawsonite, which is virtually void of zinc, the stannoidite should be poor in zinc. However, this is not the case with the present material. The analysed stan noidite is in association with mawsonite, which has Cu 43.0, Ag 0.2, Fe 13.6, Zn tr., Sn 12.6 and S 30.2 (total 99.6) according to electron microprobe analysis. d) Stannoidite from the Fukoku mine The ore deposits of this mine are fissure-filling veins cutting a Triassic sandstone and shale alternation, which is intruded by a granodiorite body in the northern part of the deposits. They were once worked for copper, zinc, lead, gold, and silver, but has since been closed. The major ore minerals are chalcopyrite, pyrrhotite, sphalerite, pyrite, arsenopyrite, galena, and bismuthinite (?), and the gangue minerals are quartz and calcite. The occurrence of cosalite, boulangerite, and mawsonite has been reported by Shimizu, Kato, and Matsuo (1966), who also found there hexastannite now to be called stannoidite. The stannoidite bearing specimen coming from a dump is a part of a chalcopyrite bearing quartz vein, in which the grains of stannoidite and chalcopyrite reaching 3mm across are present. Ore microscopic observations showed the grain to be an aggregate of core chalcopyrite rimmed by mawsonite and then by stannoidite (Fig. 5). The mawsonite has the composition: Cu 45.8, Fe 12.6, Zn tr., Ag. 0.1, Sn 15.0, and S 26.9 (total 100.4%), according to electron microprobe analysis. Along the outside of some mawsonite rims, very minute intergrowths of chalcopyrite and a stannite-like mineral are observed. Some core chalcopyrite grains contain very small wit tichenite droplets. As far as the available polished sections indicate, stannoidite makes direct contact with chalcopyrite, stannite, sphalerite, bornite,

10 426 The Occurrence of Stannoidites from the Xenothermal Ore Fig. 5. Stannoidite (Std) including zoned aggregate of mawsonite (Mw) and chalcopyrite (Cp). Note the presence of minute grain aggregates between mawsonite and stannoidite. Fukoku mine, Kyoto Prefecture. galena, mawsonite, tetrahedrite-tennantite, bismuthinite, cassiterite, ferberite, scheelite, quartz, and siderite. When mawsonite exists close to cassiterite, there is always found stannoidite between them. The optical properties of these stannoidites are essentially the same as those of stannoidite from the Konjo mine (Kato (1969)), except the Fukoku material, which has slightly purplish reflection colour. Stannoidite from the Tada mine is less sensitive to HNO 3 (1:1) than the other. X-ray powder study The X-ray powder data for the analysed Akenobe material and unanalysed Ikuno material are given in Table 1, in which those for the analysed Konjo material and yellow stannite from Vaulry, France are also tabulated. The chemical composition of the same stannoidite

11 A. KATO and Y. Fu.tnKi 427 Table 1. X-ray powder data for stannoidites and yellow stannite. 1. Stannoidite. Akenobe mine, Hyogo prefecture. Cu/Ni radiation. Diffractometer method. Analysed material. 2. Stannoidite. Ikuno mine, Hyogo prefecture. Cu/Ni radiation. Diffractometer method. Unanalysed material, but probably identical with that studied by Imai, Fujiki, and Tsukagoshi (1967). 3. Stannoidite. Konjo mine, Okayama prefecture. Cu/Ni radiation. Diffractometer method. Analysed material. After Kato (1969). 4. Yellow stannite. Vaulry, France. After Levy (1967).

12 428 The Occurence of Stannoidites from the Xenothermal Ore Table 2. X-ray powder data for sphalerite, chalcopyrite, stannite and stannoidites. a0=5.411a Z=4 [ZnS] a0=5.280a c0=10.409a Z=4 [CuFeSZ] a0=5.47a c0=10.746a Z=2[Cu2FeSnS4] a0=10.76a b0=5.40a c0=16.09a Z=3[Cu5(Fe, Zn)2SnS8]

13 A. KATO and Y. FUJIKI 429 from the stope of the Ikuno mine as the one studied here has been reported as (Cu 4.93Ag0.07)5.00 (Fe1.60Zn0.36)1.96Sn1.03 S8.26 (taking total metal=8) by Imai, Fujiki, and Tsukagoshi (1967). The line with d=11a of yellow stannite indexed as (100) by the unit cell constants of the Konjo stannoidite is lacking in the present stannoidites and this reflection should be absent according to the possible space group of the original stan noidite, I222, I212121, Immm or Imm2. Except for this line and d=2.80a and 1.69A (indexed as (312) and (611) respectively), the other lines agree very well with those of the original one. For the Tada material, the available amount of the pure material was too small to obtain good powder data. However, some strong lines including 5.40(m), 3.11(vs), 2.69(m), 1.90(s), and 1.62(m) are present to prove its substantial iden tity, and those of the Fukoku material here analysed are 5.43(10), 4.82(10), 3.11(100), 2.70(20), 1.906(40), 1.627(20b), and 1.616(5), according to Shimizu, Kato, and Matsuo (1966). X-ray powder data for stannoidite are very similar to those for stannite, suggesting a stru ctural similarity between them. In Table 2, those for sphalerite, chalcopyrite, stannite, and stannoidite are given for comparison. Chemical composition The chemical analyses of the Akenobe, Ikuno, Tada, and Fukoku materials were made

14 430 The Occurrence of Stannoidites from the Xenothermal Ore Table 3. Chemical analyses of stannoidites, yellow stannite and hexastannites. 1. Stannoidite. Akenobe mine, Hyogo prefecture. X-ray studied. 2. Stannoidite. Ikuno mine, Hyogo prefecture. Not X-ray studied. 3. Stannoidite. Tada mine, Hyogo prefecture. 4. Stannoidite. Fukoku mine, Kyoto prefecture. 5. Stannoidite. Konjo mine, Okayama prefecture. After Kato (1969). 6. Yellow stannite. Montmins, France. After Levy (1967). 7. Hexastannite. Tingha, New South Wales. After Markham and Lawrence (1965). 8. Hexastannite. Mt. Pleasant, New Brunswick, Canada. After Boorman and Abbott (1967). * including In 0.04.

15 A. KATO and Y. FUJIKI 431 by the electron microprobe method. In Table 3, they are compared with those of the Konjo material (Kato (1969)), yellow stannite from Montmins (Levy (1967)), and hexastannites from Tingha, New South Wales (Markham and Lawrence (1965)), and Mt. Pleasant, New Brun swick (Boorman and Abbott (1967)). The calculations tentatively assuming the number of metal atoms to be eight give the following empirical formulae: Akenobe; Ikuno; Tada; Fukoku; Konjo; Montmins; Tingha; Mt. Pleasant; (Cu4.90Ag0.01)4.91(Fe1.60Zn0.39)1.99Sn1.11S7.93 (Cu4.99Ag0.01)5.0(Fe1.39Zn0.63)1.92Sn1.09s8.21 (CU4.82Ag0.05)4.87(Fe1.69Zn0.53)2.12Sn1.00S6.93 (Cu5.04Ag0.01)5.06(Fe1.34Zn0.42)1.76Sn1.19S6.89 (Cu4.83Ag0.01)4.84(Fe1.86Zn0.15)2.01Sn1.15S8.03 Cu4.96(Fe1.90Zn0.05)1.95Sn1.10S7.95 Cu4.83(Fe1.61Zno.51)2.12Sn1.0.S7.35 Cu4.64(Fe1.70Zn0.44)2.14Sn1.21S7.02. Those of the Akenobe, Ikuno, and Konjo materials are very close to Cu5(Fe, Zn)2SnS8, whereas that of the Tada material is to Cu5(Fe, Zn)2SnS7. Because the X-ray powder data for stannoidite are very similar to those for stannite (Table 3) in which metal/s=1 in mole ratio, the former formula is believed to be the ideal one. The present calculations based on the assumption of the total metal=8 reasonably explains the composition of the Tada material, as a sul phur deficient derivative. Since the substitution for iron by zinc is known in stannite (Koucky (1959)), and copper by silver is also in tetrahedrite, metal atoms are considered to form three groups: (Cu +Ag), (Fe+Zn), and Sn. From the above empirical formulae, it may be concluded that (Cu+Ag) tends to be less than 5, whereas Sn is correspondingly more than 1. Thus, the actual formula of stannoidite may be expressed as (Cu, Ag)5-x(Fe, Zn)2Sn1+xS8-y, where x?? 0.2, y?? l, as far as these Japanese stannoidites are concerned. It is not certain whether zinc is essential for stannoidite or not, though

16 432 The Occurrence of Stannoidites from the Xenothermal Ore the low zinc content in yellow stannite from Montmins (Levy (1967)) seems to suggest that it is not provided that this yellow stannite is identical with stannoidite. For the Fukoku material showing the largest deviation from the ideal formula, its identity to stannoidite is supported by its X-ray powder data. However, the reflection colour is, as already stated, discernibly purplish compared with that of the other stannoidites. Problem of iron-zinc substitution As shown by the chemical analyses of stannoidites (Table 3), there is a considerably wide range of iron-zinc substitution in this mineral as well as in stannite (Koucky (1959)) and in briartite (Fran cotte, Levy, Moreau, and Ottenburgs (1965)). However, those maw sonites associated with stannoidites from the Ikuno, Tada, and Fukoku mines virtually lack zinc in spite of their high iron contents. This situation is the same as in orange bornites identified as mawsonite (Levy (1967)) and in the original mawsonite (Markham and Lawrence (1965)). All of the above tin- or germanium-bearing sulphide minerals produce X-ray diffraction patterns similar to those of sphalerite and chalcopyrite, suggesting themselves to be the materials with a modi fied sphalerite structure. Thus, it has become evident that these iron-bearing sulphides exhibit at least two ways of taking the modified sphalerite structure, according to the capability of substitution for iron by zinc. One prohibits the substitution for iron by zinc and the other does not. The former includes chalcopyrite, mawsonite, and possibly talna khite, and the latter stannite, briartite, sakuraiite, and stannoidite. It is a well-known fact that neither copper nor iron in chalcopyrite can be substituted by zinc irrespective of its structural similarity to sphalerite. The reasonable interpretation of this fact has not been presented. However, when the interpretation is made possible,

17 A. KATO and Y. FUJIKI 433 it will be applicable to iron in mawsonite as well, and cubanite may then well become to be looked upon as a member of the chalcopyrite group, or the group excluding the substitution for iron by zinc. From the structural similarity between chalcopyrite and stannite, it is surmised that the next neighbours to the iron atom, that is, copper atoms in chalcopyrite and copper and tin atoms in stannite may play a significant role in the substitution. For, the nearest neighbours are sulphur atoms, and are situated in the lattices of chalcopyrite and stannite nearly in the same manner. REFERENCES BERRY, L. G. & THOMPSON, R. M. (1962). Geol. Soc. Amer. Memoir, 85. BOORMAN, R. S. & ABBOTT, D. (1967). Canad. Miner., 9, 166. FRANCOTTE, J., LEVY, C., MOREAU, P. & OTTENBURGS, R. (1965). Bull. soc. franc. miner. crist., 88, 432. IKEBE, N. et al. (1961). Geological map of Hyogo prefecture and the ex planatory text: Hyogo Prefecture Office. IMAI, H., FUJIIKI, Y. & TsUKAGOSHI, S, (1967). Kozan Chishitsu (Mining Geology), 17, 50. (Abstr.) (in Japanese) KATO, A. (1959). Miner. J. 2, 397. KATO, A. (1964). J. Miner. Soc. Japan, 7, 77. (Abstr.) (in Japanese) KATO, A. (1965). Chigaku Kenkyu (Earth Science Studies), Sakurai Volume, 1-7 (in Japanese) (Amer. Miner., 53, 1421 (1968)). KATO, A. (1969). Bull. Nat. Sci. Museum, 12, 165. KATO, A. & SHINOHARA, K. (1968). Miner. J., 5, 276. KOUCKY, F. L., Jr. (1959). Bull. Geol. Soc. Amer., 70, (Abstr.). LEVY, C. (1967). Bur. Rech. Geol. Min., Mem. No. 54, 1. MARKHAM, N. L. & LAWRENCE, L. J. (1965). Amer. Min., 50, 900. RAMDOHR, P. (1944). Abh. Preuss Akad. Wiss. Math. -naturw. KI. Nr. 4, 1. RAMDOHR, P. (1960). Die Erzmineralien and ihre Verwachsungen: Akademie Verlag, Berlin. SAIGUSA, M. (1958). Kozan Chishitsu (Mining Geology), 8, 218. (in Japanese) SHIMIZU, T., KATO, A. & MATSUO, G. (1966). Chigaku Kenkyu (Earth Science Studies), Masutomi Volume, 201. (in Japanese) YAMAGUCHI, K. (1939). Jour. Jap. Assoc. Miner. Petro. Econ. Geol. 21,189. (in Japanese) Manuscript received 20 November 1968.