Stannite from the Otoge Kaolin-Pyrophyllite Deposits, Yamagata Prefecture, NE Japan and Its Genetical Significance As Well As Ca

RESOURCE GEOLOGY, 44(6), 439•`444, 1994

Stannite from the Otoge Kaolin-Pyrophyllite Deposits, Yamagata Prefecture, NE Japan and Its Genetical Significance

Makoto WATANABE*, Ken-ichi HOSHINO*, KO KO MYINT*, Kazunori MIYAZAKI* and Hirotugu NISHIDO**

Abstract: Stannite was found in the Otoge kaolin-pyrophyllite deposits of ca. 4Ma(K-Ar), which are localized in rhyolitic

pyroclastic rocks of middle Miocene time. Stannite occurs exclusively in pyrite-rich silicified portion within the sericite zone. Under the microscope, stannite, strongly anisotropic, shows a close association with sphalerite and pyrite. Microprobe

analysis of stannite reveal that it is of Zn-rich variety with Fe/Fe+Zn atomic ratios of 0.49 to 0.71. It is noted that the atomic

contents of the (Fe+Zn) site exceed 1.000 by up to 30% for stannite which contains more than 6 wt. % of Zn, suggesting a

possibility of non-stoichiometry. The temperatures estimated based on the Fe and Zn partition are 300•‹to 340•‹C with Log fs2 of ca. -10 to -8, approximately corresponding to the corrected filling temperatures of fluid inclusions in quartz nearby stannite.

the Yatani and Otoge deposits. The resultant ages Introduction are: 3.25•}0.26 to 3.6•}0.27 Ma on adularia; As well as cassiterite, stannite, Cu2(Fe, 5.16•}1.95 Ma on sericitized rock, respectively. Zn)SnS4 , is a mineral of rather ubiquitous occur- During the course of our investigation of the rence especially in skarn deposits and plutonic kaolin-pyrophyllite deposits at the Otoge mine, veins, which are genetically related to granitic we have found stannite. As far as we know, stan- intrusives of ilmenite series(e.g., SOEDA and nite has never been known from ore deposits of. WATANABE, 1982; SHIMIZU and SHIKAZONO, this type. We will report its mineralogical charac- 1985; SOEDAet at., 1988; WATANABEet al., 1988; teristics and discuss the genesis. WATANABEand HOSHINO, 1991). Geologic Setting Stannite occurs also in polymetallic veins of subvolcanic affinity such as those of the Ikuno and The Otoge mine area (Fig. 1), located near the Akenobe deposits, Japan and the Oruro deposits, boundary between Yamagata and Fukushima Pre- Bolivia. In the paper dealing with the geochemical environments of the Neogene epithermal Au-Ag-Pb- Zn veins at the Yatani mine, located about 3 km northeast of the Otoge mine, HATTORI(1975) de- scribed only briefly an occurrence of stannite in the Kanizawa Au-Ag vein. This is likely a rare case in such epithermal environments without an associated granitic stock. SHIKAZONO(1985) measured the K-Ar ages for

Received on December 10, 1993, accepted on March 17, 1994 *Department of Earth and Planetary Sciences , Hiroshima University, Higashi-Hiroshima 724, Japan **Hiruzen Research Institute, Okayama University of Science, Okayama 682-02, Japan Keywords: Otoge kaolin-pyrophyllite deposit, 4Ma(K-Ar age), Zn-rich stannite, Genesis, NE Japan Fig. 1 Location map showing the Otoge mine.

439 440 M. WATANABE, K HOSHINO•¬ KO KO MYINT, K. MIYALAKI and H. NISHIDO RESOURCE GEOLOGY:

Fig. 2 Ore minerals from the Otoge mine. a) Microphotograph showing a mode of occurrence of stannite. Reflected light. Scale bar indicates 100 ƒÊm in length. b) Same as a), partially crossed nicols. c) Microphotograph showing a mode of occurrence of stannite. Reflected light. Scale bar indicates 100 ƒÊm in length. d) EPMA compositional image of arsenopyrite showing a zonal structure. Mineral abbreviation are: Stn=stannite; Sph=sphalerite; Py=pyrite; Cp=chalcopyrite; Apy=arsenopyrite.

fectures in northeast Japan, and forming part of prep.). Stannite occurs exclusively in the pyrite- the Green Tuff region, a major tectonic division rich silicified part of the sericite zone. and a metallogenic province of the Neogene Ter- K-Ar age determination has been made on the tiary in Inner Zone of Japan, consists mainly of sericite-rich fraction in the sericite zone, using a rhyolitic pyroclastic rocks, named the Otoge For- 30 cm radius sector-type mass spectrometer with a mation (TANIGUCHI,1969). This formation, being single collector system installed at the Okayama correlatable with the Yatani formation hosting the University of Science. The age obtained is nearby Yatani Au-Ag-Pb-Zn veins (e.g., 3.96•}0.10 Ma, which is considerably younger and HATTORI,1975; SATOet al., 1978), is divided into more precise than that reported by SHIKAZONO two, the lower and upper formations. The former (1985). Therefore, it is concluded that the kaolin- is mainly composed of acidic tuff breccias, while pyrophyllite mineralization and the nearby Pb-Zn- the latter of slightly altered welded tuffs. The ka- Au-Ag mineralization took place almost simulta- olin-pyrophyllite deposits occur selectively in the neously. uppermost part of the lower formation. Intrusive rocks such as those observed in Yatani mine area Ore Mineralogy are not recognized in the immediate vicinity of the As sulfide minerals, euhedral to subhedral py- Otoge mine. Based on mineral assemblages and rite crystals up to about 5 mm in size are predomi- their relative abundance observed at the Otoge de- nant in the silicified portion of the sericite zone. posits, a zonal distribution of ores is recognized: They occur as aggregates to form a veinlet-like pyrophyllite, sericite and kaolinite ones. Their form and as disseminations in the silicate matrix. detailed descriptions and genetical discussions Small amounts of galena and chalcopyrite are in- will be given elsewhere (MIYAZAKIet al., in cluded in the pyrite grains. Subordinate amounts 44(6), 1994 Stannite from the Otoge kaolin-pyrophyllite deposits and its genetical significance 441

Table 1 Microprobe analyses of coexisting stannite and sphalerite from the Otoge mine (wt. %).

( ): sphalerite compositions, n.d. = not determined

dicated the miscibility gap between stannite and of sphalerite with or without abundant fine- kesterite and suggested their difference in crystal grained exsolved dots of chalcopyrite are closely structure. The latter conclusion was confirmed by associated with pyrite. Sphalerite is sometimes HALL et al., (1978), who refined the structure of accompanied by a small amount of chalcopyrite stannite and kesterite in space group I42m and I4, and galena. Very fine-grained Ag-bearing min- respectively. However, in some cases, from their eral, 1 to 2 microns in size, is rarely associated chemical compositions alone, the two minerals with stannite, chalcopyrite and galena. It is noted that stannite always occurs as long and narrow are indistinguishable from each other. Recently, strips up to 0.1 mm in width and as fine-grained KISSIN and OWENS (1989) and KISSIN (1989) re- and irregularly-shaped crystals within sphalerite ported two new minerals, ferrokesterite and petru- kite, approximately the same composition as that as shown in Fig. 2(a, b, c). In reflected light, stan- of stannite, but differing crystal structures. Both nite is gray in color with an olive green tint and minerals are very similar shows a strong anisotropism (Fig. 2b). Investigat- ,to kesterite in optical ing the phase relations in the pseudobinary system properties, being weakly anisotropic, while the Cu2FeSnS4-Cu2ZnSnS4, SPRINGER (1972) dis- Otoge material is strongly anisotropic. Although, strictly speaking, its structural refinements are closed that below 680•‹C, there are two different needed, we may conclude that the tin-bearing sul- stannite-kesterite phases separated by a miscibil- fide from the Otoge mine is stannite, mainly based ity gap. Using Gandolfi X-ray camera and micro- on their optical property and chemical composi- probe analyzer, KISSIN and OWENS (1977) also in- 442 M. WATANABE, K HOSHINO•¬ KO KO MYnvT, K. MIYAZAKI and H. NisHmo RESOURCE GEOLOGY: tions, as will be stated later. Small amounts of euhedralarsenopyrite are sometimesobserved associated with pyrite. Pyrrhotite has never been observed. As gangue minerals, quartz, sericite and apatite with an euhedralhabit occur in the si- licifiedportion. EPMA analyses of stannite and associated sphaleriteare listed in Table 1. Althoughstannite from Otoge is compositionallyzoned, exsolution textures such as those describedby HARRISand OWENS(1972), KISSINand OWENS(1977) and SOEDAet al. (1988)are not recognized.Tin an Zn contents(in atomic %) of stanniteare in the range: 10.74 to 12.34 and 3.64 to 8.35, respectively. Therefore,the Otogestannite is of Zn-richvariety, with Fe/Fe+Znratios of 0.49 to 0.71. It is noted that the Zn-poor stanniteshows the atomic contents of the (Fe+Zn) site to be almost1.000, while thoseof the Zn-richone deviatefrom the value,by up to about 30 %, suggesting a possibility of non- stoichiometry as found by PETRUK (1973). This will be treated in a separate paper. On the con- Fig. 3 Fe/Zn partition diagram for coexisting stannite trary, the Yatani stannite is of Zn-poor variety and sphalerite (After NAKAMURAand SHIMA,1982). with the ratio of 0.97 (SHIMIZU and SHIKAZONO, 1985). FeS contents (in mol. % FeS) of sphalerite (NAKAMURAand SHIMA, 1982), coexisting with stannite range from 6.0 to 13.7 %. where T is temperature in kelvin. For the com- Usually, arsenopyrite contains As of 30 to 32 parison between the two geothermometers, as atom. %. KRESTCHMARand SCOTT (1976) indi- well as some problems inherent to its application, cated that in S-rich assemblage such as arsenopy- see SHIMIZUand SHIKAZONO(1985) and SOEDA et rite-pyrite, centers of arsenopyrite crystals are S- al. (1988). rich relative to rims. However, arsenopyrite from Using the partition coefficients by NEKRASOV the Otoge mine does not show such a regular zon- et al. (1975) and NAKAMURAand SHIMA (1982) ing (Fig. 2d). These analytical results will be dis- and phase relations for the Fe-Zn-S system cussed later. (BARTON and TOULMIN, 1966; SCOTT and BARNES, 1971), equilibrium temperature and sul- Stannite Genesis fur fugacity conditions can be estimated. The re- Partition of Fe and Zn between coexisting stan- sults are graphically shown in the partition dia- nite and sphalerite solid solutions can be repre- gram (Fig. 3). Compositions of coexisting stan- sented by the following exchange reaction: nite and sphalerite from some Japanese skarns and Cu2FeSnS4(Stn)+ZnS(Sph) plutonic veins (SHIMIZU and SHIKAZONO, 1985; =Cu2ZnSnS4(Stn)+FeS(Sph) WATANABE and HOSHINO, 1991) are also de- where Cu2FeSnS4(Stn) and ZnS)(Sph) indicate picted for comparison, together with those from Cu2FeSnS4 and ZnS components in stannite and the Yatani Pb-Zn-Au-Ag veins of epithermal na- sphalerite, respectively. Partition coefficients for ture (SHIMIZUand SHIKAZONO,1985). As is evi- this reaction, Kd=(Fe/Zn)sph/(Fe/Zn)stn, was ex- dent, the Otoge samples show a quite different be- perimentally determined as follows; havior, characterized by low iron contents of stan- Log Kd=-1274/T+1.174 nite and sphalerite, from both the granitoids-re- (NEKRASOVet al., 1975); lated deposits and epithermal veins. The obtained Log Kd=-2.8•~103/T+3.5 overall range of temperature and sulfur fugacity 44(6), 1994 Stannite from the Otoge kaolin-pyrophyllite deposits and its genetical significance 443

(in logarithm) is: 300•Ž to 34•Ž (210•Ž to knew that a small-scaled granite porphyry intrud- 290•Ž); -10 to -8 bar. Values in bracket are those ing the host rocks was caught by drilling in the estimated form the Kd by NEKRASOV et al. (1975). vicinity of the Otoge mine (K. IBARAKI, oral For the Kd less than 5.86 (corresponding to comm.).

383•Ž), as is clear the relationships, the Kd by References NAKAMURA and SHIMA (1982) gives much lower temperatures than that by NEKRASOV et al. (1975). BARTON,P.B.,Jr. and TOULMIN, P. III (1966): Phase relations In order to check the results, we measured the fill- involving sphalerite in the Fe-Zn-S system. Econ. Geol., 61,815•`849. ing temperatures of fluid inclusions in quartz HATTORI, K. (1975): Geochemistry of ore deposition at the nearby stannite, with the resultant temperature Yatani lead-zinc and gold-silver deposits, Japan. Econ. range from about 260•Ž to 300•Ž. These tem- Geol., 70, 677•`693. peratures are intermediate between the two tem- HALL, S. R., SZYMANSKI, J. T. and STEWART, J. M. (1978): perature ranges based on the two geothermom- Kesterite, Cu2(Zn, Fe)SnS4, and stannite, Cu2(Fe, eters. When some correction for pressure, though Zn)SnS4, structurally similar but distinct minerals. Can.

small, is made, the filling temperatures become Mineral., 16,131•`137. close to the partition temperatures based on HARRIS, D. C. and OWENS, D. R. (1972): A stannite-kesterite

NAKAMURA an SHIMA (1982). Therefore, it may exsolution from British Columbia. Can. Mineral.,11, be said that the estimated temperatures and sulfur 531•`534. fugacities are reasonable. KISSIN, S. A. (1989): A reinvestigation of the stannite Arsenopyrite compositions, ca. 30 to 36 atomic (Cu2FeSnS4)-kesterite (Cu2ZnSnS4) pseudobinary system. Can. Mineral., 27, 689•`697. % As, if they would have been buffered by the KISSIN, S. A. and OWENS, D. R. (1979): New data on stannite pyrite-native As-arsenopyrite assemblage during and related tin sulfide minerals. Can. Mineral., 17, deposition, give anomalously higher tempera- 125•`135. tures. This may be due to non-equilibrium fea- KISSIN, S. A. and OWENS, D. R. (1989): The relatives of stan-

tures reflecting the kinetics of arsenopyrite growth nite in the light of new data. Can Mineral., 27, 673•`688.

and local fluctuations in the concentration ratio KRETSCHMAR, U. and SCOTT, S. D. (1976): Phase relations in-

(S2/AS2)(e.g., KRETSCHMAR and SCOTT, 1976). volving arsenopyrite in the system Fe-As-S and their ap-

Despite some remaining uncertainties, we em- plication. Ca. Mineral., 14, 364•`386. MIYAZAKI, K., HOSHINO, K. and WATANABE, M. (in prep.): phasize that Fe and Zn partitioning between coexisting stannite and sphalerite solid solutions will Genesis of the kaolin-pyrophyllite deposits at the Otoge mine, NE Japan. be a powerful geothermometer. It is pointed out MOH, G. H. (1975): Tin-bearing mineral systems. Part II. that stannite will probably be found also in Phase relations an mineral assemblages in the Cu-Fe-Zn- epithermal environments including current geo- Sn-S system. Chem. Erde, 34, 1•`61. thermal areas with or without granitic intrusives. NAKAMURA, Y. and SHIMA, H. (1982): Fe and Zn partitioning Acknowledgements: We wish to acknowledge between sphalerite and stannite (abstr.). Joint Mtg. Soc.

with particular thanks the important contribution Mining Geol. Japan, Assoc. Miner. Petr. Econ. Geol., and

that the staff of the Otoge mine, especially Mr. M. Miner. Soc. Japan, A8 (Japanese).

MATSUKI, Mr. K. IBARAKI of the Sumitomo Metal NEKRASOV, I. J., SOROKIN, V. I. and OSADCHII, E. G. (1979): Fe

Mining Company Limited and Mr. N. SATO of and Zn Partition between stannite and sphalerite and its

Mitsubishi Materials Corporation have made to application in geothermometry. In Origin and Distribution our understanding of the Geology of Otoge mine of the Elements, (L. H. AHRENS, Ed.), Phys. Chem. Earth, 34,739•`742. and its surrounding area. Mr. A. MINAMI of PETRUK, W. (1973): Tin sulphides from the deposit of Hiroshima University kindly assisted with the Brunswick Tin Mines Limited. Can. Mineral., 12, 46•`54. electron microprobe analyses. Helpful comments SATO, T., TAKATORI, I. and YANAGISAWA, S. (1985): Geology and criticisms by Dr. N. SHIKAZONO of Keio uni- and ore deposits of the Yatani mine, Yamagata Prefecture, versity and Dr. W. PETRUK of Department of En- Japan, with special reference to some suggestions on ex- ergy, Mines and Resources, Ottawa, Canada are ploration. Mining Geol.,28, 177•`190(in Japanese with greatly appreciated. English abstract). Addendum: After submitting this report, we SCOTT, S. D. and BARNS, H. L. (1971): Sphalerite geothermom- 444 M. WATANABE, K HOSHINO•¬ KO KO MYINT, K. MIYAZAKI and H . NISHIDO RESOURCE GEOLOGY:

etry and geobarometry. Econ. Geol., 66, 653•`669. (Japanese). SHIKAZONO, N. (1985): K-Ar ages for the Yatani Pb-Zn-Au-Ag SPRINGER, G. (1972): The pseudobinary system Cu2FeSnS4-

vein-type deposits and Otoge kaolin-pyrophyllite depos- Cu2ZnSnS4 and its mineralogical significance. Can. Min-

its, Yamagata Prefecture, northeastern part of Japan. Min- eral.,11, 535•`541.

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SHINIIZU, M. and SHIKAZONO, N. (1985): Iron and zinc parti- Mining Geol., 19,113•`121.

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山形県,大峠カオリンーパイロフィライト鉱床産黄錫鉱ならびにその成因的意義

渡辺洵・星野健一・ココミント・宮崎一範・西戸裕嗣

要旨:大峠カオリンーパイロフィライト鉱床は,4m .y. 原子含有量は1.000を30%程越えるものがあり,このこと (K-Ar)前,中期中新世の流紋岩質火砕岩の一部を交代しはnon-stoichiomoetryを示唆する.共存する黄錫鉱と閃亜て生成した.黄錫鉱は ,その中のセリサイト帯の中に鉛鉱の間のFeとZnの分配に基づいて温度を推定すると約あって珪化が著しくかつ黄鉄鉱に富む部分に見いだされ 300～340°C(log fsz≒-10～-8bar)となり,これは黄錫鉱た.鏡下では,黄錫鉱は特徴的に強い異方性を示し,閃の近くの石英中の流体包有物について得られた補正され亜鉛鉱および黄鉄鉱と密接な随伴関係にある.本黄錫鉱た充填温度と調和的である.今後,大峠鉱床のような浅は,Znに富むもので,そのFe/Fe+Zn原子比は0 ,41～0.71 成環境において,黄錫鉱がみつかる可能性が大きい. である.Znを6wt.%以上含む黄錫鉱の(Fe+Zn)サイトの