Tin.Bearing Chalcopyrite from the Izumo Vein. Toyoha Mine. Hokkaido

Canadian Mineralogist Vol. 25, pp. 9-13 (1987)

TIN.BEARINGCHALCOPYRITE FROM THE IZUMO VEIN. TOYOHAMINE. HOKKAIDO. JAPAN

KATSUO KASE Depafttnent of Earth Sciences,Faculty of Science, Okayoma University, Tswhina Naka 3-I-1, Okayama 7@, fapan

ABSTRAcT Ag, Zn, Sn and so on, only at levels up to several thousandppm (Harris et ol. l9M, Cabi et al. 1985), Chalcopyrite containing rp to 2.34 wt.9o Sn occurs in One of the reasons for these low concentratious is - sphalerite chalcopyrite - tetrahedrites - stannite ores that the range of high-temperature solid-solution from mine, the Izumo vein of the Toyoha Japan. shrinks quickly with decreasing temperature, as Microprobe analysis confirms tlat Sn enters into chal- expectedfrom the rapid rate of equilibration of this copirite as a solid solution toward stannite by the substi- mineral (Barton tution of Sna+Fd+ for Fe3+Fe3+.The Sn contents are & Skinner 199). strongly d€p€nd€nton the orientation ofa chalcopyritecrys- Chalcopyrite with up to 2.34 wt. 9o Sn in solid tal, and they are especiallyhigt in directions of fast growth. solution occttrs, however, in the Izumo vein of the The original composition of the tin-bearing chalcopyrite Toyoha mine, Hokkaido, Japan.Chalcopyrite con- has been preserved witiout significant change, owing to faining such high Sn has not been reported previ- quenching of the mineral assemblagefrom a temperature ously. This paper describesthe mode of occurrence of approximately 3@oC. and chemistry of the chalcopyrite. Kelwords: t'm-bearingChalcopyrite, stannite, Toyoha mine, Japan. MTNERALDeposrrs oF TrrE Toyona MNe

The mineral depositsof the Toyoha mine, situated SoMMerne about 30 km southwestof Sapporo (Fig. l), are now the most productive Pb-Zn veins in Japan. More De la chalcopyrite dont la teneur en €tain s'€lbvejusqu'i than 40 veinsoccur in alteredvolcanic rocks and clas- 2.34r/o (en poids) se pr6ente dans le minerai i sphaldrite 1is 5sdimsafary rocks of Miocene age; theseare cor- : chalcopyrite - tdtraddrite (solution solide) - stannite du related with the so-called "Green tuff" formations, filon Izumo de la rnine Toyoha (Japou). L'analyse d la the host rocks of the Kuroko-type massive-sulfide mioosonde confirme que l'6tain setrouve d:ns la chalcopy- deposits. The geology of the Toyoha m ining district rite i f&at de solution solide en direction de la stannite, has been described by mauy authors (e.g., par zubstitution de Sna+Fd+ e Fe3+Fe3+.La teneur en Sn Hashimoto et al. 1977, Kuwahara et al. 1983). ddpend fortement de I'orientation du cristal de chalcopy- Most of the veins are composed of sphalerite, rite; elle est particulibrement 6lev€edans les directions de galena, pyrite aud quartz, with small amounts croissancerapide du cristal. La compositioh originelle de of la chalcopyrite d teneur d'6tain a €t6 prdservdesans chan- chalcopyrite, ma9netite, hematite and tetrahedrite- gementnotable, grdcei la trempe de l'assemblageminft3t tennantite (describedas tetrahedritd* hereafter). In i une temp6rature d'environ 3@"C.

(Iraduit par la R&action)

Mots-clds: chalcopyrite i teneur d'6tain, smnnite, Toyoha (mine), Japon.

INTRoDUcTIoN

Small amounts of sphalerite; stannite and tetrahedrite may occur in chalcopyrite as lamellae and star- and dot-shaped inclusions (Ramdohr 1969). They have been ascribed to exsolution, which sug- gests tle incorporation of Zn, Sn, Sb and As in chalcopyrite solid-solution at elevated temperatures. Nondestructive in situ analyses of natural chalcopyrite, however, show that this mineral sontains Ftc. l. Location of tle Toyoha mine, Hokkaido, Japan. 9 10 TI{E CANADIAN MINERALOGIST addition to quartz, rhodochrosite and manganoan 3CI'C (Yajima & Ohta 1979).Mineral deposition at calcite are the dominant gangue minerals in some a sliebtly higher temperature is one of tle factors veins. Silver minerals, such as acanthite' electrum for the unique mineralogicralnature of the Izumo and pyrargyrite, occlu in small amounts Kojima el vein. al. 1979).The Ag content of the ores averages120 g/t (Kuwahara et al. L983).The veins of the Toyoha SAMPLEDescRrPrroN mine have been considered to be epithermal (e.9., Shikazono 1975).Tin and W minerals are generally The ore samplesin which tin-bearing chalcopyrite absentin most of the Pb-Zn veins. Yajima (1977) occurswere taken from drill coresof the Izumo vein. and Yajima & Ohta (1979) have, however, found Sphalerite, tetrahedrites, arsenopyrite, pyrite, chal- stannite, cassiteriteand wolframite in the Izumo vein copyrite, stannite and quartz are the main sonsti- of this mine. They pointed out its xenotlermal tuents. Galena,wolframite, cassiterite,acanthite and nature, basedon the mineralogical characteristicsof loellingite are present in small or trace amounts. the Izumo vein, in contrastto other veinsof the mine. Finely banded struclures imply the successivedepo- Filling temperaturesfor fluid inclusions in guartz sition of theseminerals. Chalcopyrite, tetrahedriteo, and sphalerite from several Pb-Zn veins of the sphalerite and stannite are locally intimately inter- Toyoha mine are in the range from 150 to 250'C gxown, but no distinct exsolution-intergrowths are (Tokunaga 1970, Shikazono 1975, Enjoji & observed.Minute grains of tetrahedrite* occur only Takenouchi lW 6, Y ajima &Ohta 1979).Filling tem- along and within chalcopyrite grains, suggesting peratures of the Izumo vein are somewhat higher exsolution from tlat mineral. than those of other veins, and range from 2([ to A marked compositional 2sning is revealed by

Frc.2. a, Photomicrograph of tin-bearing chalcopyrite. crossed nicols. Reflected tigbt. weak anisotropism revealsthe banded structure. b and c. Back-scatteredelectron images of the samearea, displaying the distinctly banded structure seenunder.reflected light. Note that brightest bands coincide with the poorly developedplanes. d. Snln charac- teristic X-ray scanning photograph taken in the same field as in c. TIN-BEARING CHALCOPYRITE FROM HOKKAIDO, JAPAN l1

microprobe analysisiu sphalerite and tetrahedrite*; TABLEI. CHEI,IICALCO{POSITION OF TII{-BEARII{GCHALCOPYRITE compositions range from I to 6 mole 9o FeS, and from l0 to 98 mole 9o tetrahedrite component, Cu 34.33 34.31 34.39 U.29 34.@ 33.87 Fe 30.52 30.32 30.25 30.40 30.12 29.37 respectively. The tetrahedriteo commonly contains zn 0.09 0.18 0.14 0.10 0.18 0,29 considerable Ag (up to about 13 wt. 9o). The tin- Sn 0.86 l.l2 1.27 1.35 1.50 ?.U s 34.52 34.18 34.03 U.21 34.62 34.40 bearing chalcopyrite displays weak reflection- Total 100.32 l00.ll 100.08 100.35 100.51 100.27 pleochroism and anisotropism, which permit obser- tlunber of atomsDer foflil'la unltr vation of its internal structure (Fig. 2a). Cu t.990 1.998. 2.007 1.995 1.977 "t.977 Fe 2.013 2.010 2.009 ?.013 1.987 1.952 Zn 0.005 0.010 0.008 0.005 0.010 0.016 Felzn 2.018 2.020 2;017 2.018 1.998 1.968 Anar,ytcal Dana Sn 0.027 0.035 0.040 0.v2 0.M7 0.074 s 3.965 3.947 3.936 3,945 3.979 3.981 Back-sgatteredelectron imagesobtained by using Stannlte(wt.t) a JEOL electron microprobe reveal a distinct zonal 3.ll 4.05 4.60 4.89 5.43 8.48

structure in the chalcopyrite @igs.2b, c). The lighter r calculated on the basls of I total atons. Composition domains of the imagescorrepond to higher Sn con- detennlhed by eiectrcn nicroprob€. tents (Figs. 2c, d). Figure 3 shows line profiles for CuKo, FeKcu,Sntra andZnKa acrossthe light and tron images become brighter, with a maximum Sn dark chalcopyrite bands. An increasein intensity of content measuredof 2.34 W. 90. This Sn value is, Sntrtr corresponds to a decreasein that of FeKcu. however, exceptionally high; Sn contentsare usually Quantitative analysesmainly were made on the lesstlan I .5 wt. 90. The analyseswith unusuallyhigh pale domains of the back-scatteredelectron images. Sn contents were obtained on the poorly developed The standardsused are chalcopyritefrom the Kosaka crystal planes (Figs. 2b, c). In Figure 4, numbers of mine, synthetic ZnS and Sn metal. The measured Fe + Zn (Nr"+zJ and Cu (NqJ atoms are plotted ratios ofintensity werecorrected for background and against Nro, the number of Sn atoms, calculated on deadtime and then converted into concentrations the bdsis of 8 total atoms per formula unit. NF"+zn according to the method of Sweatman & Long decreases monotonously with ioslsasing .l/s' (1969). The results are gven in Table l. although Zn is restricted to domains with high Sn The Sn contentsincrease as'the back-scattered elec- contents. The relationships betweenNr"121 rnd Nso

Ftc.3. FeKa, SnLa, ZnKa and CtrKa scanning profiles across the tin-bearing chalcopyrite, taken along the solid line shorqn on the back-scattered electron image (inset) (cps: counts per second). t2 TI{E CANADIAN MINERALOGIST

exceeded3CI'C (Yajima & Ohta 1979).It is thus highly probable that chalcopyritewitl unusually high Sn contentshas precipitated metastablyin the Izumo vein. The observation that the Sn contents are anomalously high on planes characterizedby rapid c gowth supports this idea. In this regard, several N grains of chalcopyrite containing symplectitic stan- f nite from the polymetallic veins of the Ikuno mine z were Sn were found to be !t analyzed; their contents J level of the microprobe (ca. 300 (.t below the detection z ppm). This result indicates that the composition of chalcopyrite changes rapidly during the sseling process.The Toyoha vein systemis believedto have formed at very shallow depth (Yajirna & Ohta 199), and therefore quenching of the tin-bearing chalcopyrite can reasonably be inferred.

AcK}.IowLEDGEN{ENTS Nsn The author expresseshis sinceregratitude to Drs. Fto.4. Relationshipbetween a) numberof Fe + Zn atoms Takashi Fujii, Hidehiko Tsugio (NF"+zo,solid circles)and b) N6" (opencircles), and Shimazaki and Nsoin chalcopyrite.Number of atomsis calculatedon Shibata for their sritical reading of the manuscript the basisof 8 total atomsper formula unit. and invaluable suggestions.Thanks are also due to Drs. Robert F. Martin, Alan H. Clark and Louis J. Cabri, who kindly reviewedthe manuscript and sug- obtained by least-squares regression is: gestedimprovements. The ore samplesof the Izumo - Nre+zn= l.24Nso*2.06. No systematicrelation- vein were provided by the staff of the exploration ship is noticed between Nq, and Nso. branch of the Toyoha [\dining Co. Ltd., through the good offices of Mr. Hiroshi Ogasawara,President DncussroN of the company. Technical assistancegiven by Mr. Toshiaki Saito is acknowledged. The valence state of Cu in chalcopyrite and stannite photo- is monovalent, accorrling to the X-ray REFERENCES elestron spectroscopicdata of Nakai et al. (1976). Iron in chalcopyrite and Sn in stannite are trivalent BeRroN,P.8., Jn. & Srnwrn, B.J. (1979):Sulfide and tetravalent, respwtively, according to tle Miiss- mineral stabilities..In Geochemistryof Hydrother- bauer spectraof Greenwood& Whitfield (1968).The mal ore Deposits(2nd edition, H.L. Barnes,ed.). valence state of Fe in stannite may be divalent to John Wiley & Sons, New York. maintain a balance of electric charges, thus differ- ing from that in chalcopyrite. Mixing of chalcopyrite Cannr, L.J., Cetvrpsst-L,J.L., Lananaun, J.H.G., and stannite therefore involvs coupled substitution Luon, R.G., MexweLL,J.A. & Scorr, J.D. (1985): of Sna+Fdr for Fd+Fd+ . Znc may replaceFd+ in Proton-miooprobeanalysis of traceelements in sul- Sn-rich compositions. In the Toyoha chalcopyrite, fides from some massive-sulfidedeposits, Can. Nre+zn decreaseslinearly with increasing Nso, and Mineral. 23, t33-t48. the slope for the relationship doesnot depart much ENrorr,M. & Tennxoucm,S. (1970: Presentand from unity (Fig. 4). Tin enters into chalcopyrite as future researchesof fluid inclusionsfrom vein-type a solid solution toward stannitebytle aboveschemes deposits.Minins Geol, (Iapun),Spec. Isme?r85-l0o of atomic substitution. (in Japanese). Moh (1975), however, showed that only 3 wt. 9o stannite is soluble in low-temperature (tetragonal) GnsENwooo,N.N. & Wurrr'rcr,p,H.J. (1968):Mdss- chalcopyrite, which is stable below about 550oC in bauer effect studies on cubanite (CuFqft) and Chem.Soc. (A), Inorg. Phys. the system Cu-Fe-S (Yund & Kullerud 196Q. For relatediron sulfides. J. Theor.,1697-1699. high-temperature(rsometric) chalcopyrite, tle extent of solid solution is greater (Mob 1975). The maxi- Har.pus,D.C,, Cannr,L.J. & NosrlrNc, R. (1984): mum Sn content determined in tle preseut study Silver-bearingchalcopyrite, a principal sourceof sil- Q.34 wt.9o) correspondsto about 8.5 wt. % stan- ver in the Izok Lake massive-sulfidedeposit: con- nite (Table l), but the temperature of mineral depo- firmation by elegffon-and proton-microprobeana- sitiou in the Izumo vein is estimated not to have lyses.Can. Minerol. X2,493498. TIN.BEARING CHALCOPYRITE FROM HOKKAIDO, JAPAN r3

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