Canadian MineralogistVol. 25, pp. 9-13 (1987)

TIN.BEARING CHALCOPYRITE FROM THE IZUMO VEIN.TOYOHA MINE. HOKKAIDO. JAPAN

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

ABSTRAcT

Chalcopyrite containing rp to 2.34 wt.9o Sn occurs insphalerite - chalcopyrite - tetrahedrites - stannite oresfrom the Izumo vein of the Toyoha mine, Japan.Microprobe analysis confirms tlat Sn enters into chal-copirite as a solid solution toward stannite by the substi-tution of Sna+Fd+ for Fe3+Fe3+. The Sn contents arestrongly d€p€nd€nt on the orientation ofa chalcopyrite crys-tal, and they are especially higt in directions of fast growth.The original composition of the tin-bearing chalcopyritehas been preserved witiout significant change, owing toquenching of the mineral assemblage from a temperatureof approximately 3@oC.

Kelwords: t'm-bearing Chalcopyrite, stannite, Toyoha mine,Japan.

SoMMerne

De la chalcopyrite dont la teneur en €tain s'€lbve jusqu'i2.34r/o (en poids) se pr6ente dans le minerai i sphaldrite: chalcopyrite - tdtraddrite (solution solide) - stannite dufilon Izumo de la rnine Toyoha (Japou). L'analyse d lamioosonde confirme que l'6tain se trouve d:ns la chalcopy-rite i f&at de solution solide en direction de la stannite,par zubstitution de Sna+Fd+ e Fe3+Fe3+. La teneur en Snddpend fortement de I'orientation du cristal de chalcopy-rite; elle est particulibrement 6lev€e dans les directions decroissance rapide du cristal. La compositioh originelle dela chalcopyrite d teneur d'6tain a €t6 prdservde sans chan-gement notable, grdce i la trempe de l'assemblage minft3ti 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 tetra-hedrite may occur in chalcopyrite as lamellae andstar- and dot-shaped inclusions (Ramdohr 1969).They have been ascribed to exsolution, which sug-gests tle incorporation of Zn, Sn, Sb and As in chal-copyrite solid-solution at elevated temperatures.Nondestructive in situ analyses of natural chal-copyrite, however, show that this mineral sontains

9

Ag, Zn, Sn and so on, only at levels up to severalthousand ppm (Harris et ol. l9M, Cabi et al. 1985),One of the reasons for these low concentratious isthat the range of high-temperature solid-solutionshrinks quickly with decreasing temperature, asexpected from the rapid rate of equilibration of thismineral (Barton & Skinner 199).

Chalcopyrite with up to 2.34 wt. 9o Sn in solidsolution occttrs, however, in the Izumo vein of theToyoha mine, Hokkaido, Japan. Chalcopyrite con-faining such high Sn has not been reported previ-ously. This paper describes the mode of occurrenceand chemistry of the chalcopyrite.

MTNERAL Deposrrs oF TrrE Toyona MNe

The mineral deposits of the Toyoha mine, situatedabout 30 km southwest of Sapporo (Fig. l), are nowthe most productive Pb-Zn veins in Japan. Morethan 40 veins occur in altered volcanic rocks and clas-1is 5sdimsafary rocks of Miocene age; these are cor-related with the so-called "Green tuff" formations,the host rocks of the Kuroko-type massive-sulfidedeposits. The geology of the Toyoha m ining districthas been described by mauy authors (e.g.,Hashimoto et al. 1977, Kuwahara et al. 1983).

Most of the veins are composed of sphalerite,galena, pyrite aud quartz, with small amounts ofchalcopyrite, ma9netite, hematite and tetrahedrite-tennantite (described as tetrahedritd* hereafter). In

Ftc. l. Location of tle Toyoha mine, Hokkaido, Japan.

10 TI{E CANADIAN MINERALOGIST

addition to quartz, rhodochrosite and manganoancalcite are the dominant gangue minerals in someveins. Silver minerals, such as acanthite' electrumand pyrargyrite, occlu in small amounts Kojima elal. 1979). The Ag content of the ores averages 120g/t (Kuwahara et al. L983). The veins of the Toyohamine have been considered to be epithermal (e.9.,Shikazono 1975). Tin and W minerals are generallyabsent in most of the Pb-Zn veins. Yajima (1977)and Yajima & Ohta (1979) have, however, foundstannite, cassiterite and wolframite in the Izumo veinof this mine. They pointed out its xenotlermalnature, based on the mineralogical characteristics ofthe Izumo vein, in contrast to other veins of the mine.

Filling temperatures for fluid inclusions in guartzand sphalerite from several Pb-Zn veins of theToyoha mine are in the range from 150 to 250'C(Tokunaga 1970, Shikazono 1975, Enjoji &Takenouchi lW 6, Y ajima &Ohta 1979). Filling tem-peratures of the Izumo vein are somewhat higherthan those of other veins, and range from 2([ to

3CI'C (Yajima & Ohta 1979). Mineral deposition ata sliebtly higher temperature is one of tle factorsfor the unique mineralogicral nature of the Izumovein.

SAMPLE DescRrPrroN

The ore samples in which tin-bearing chalcopyriteoccurs were taken from drill cores of the Izumo vein.Sphalerite, tetrahedrites, arsenopyrite, pyrite, chal-copyrite, stannite and quartz are the main sonsti-tuents. Galena, wolframite, cassiterite, acanthite andloellingite are present in small or trace amounts.Finely banded struclures imply the successive depo-sition of these minerals. Chalcopyrite, tetrahedriteo,sphalerite and stannite are locally intimately inter-gxown, but no distinct exsolution-intergrowths areobserved. Minute grains of tetrahedrite* occur onlyalong and within chalcopyrite grains, suggestingexsolution from tlat mineral.

A marked compositional 2sning is revealed by

Frc.2. a, Photomicrograph of tin-bearing chalcopyrite. crossed nicols. Reflected tigbt. weak anisotropism reveals thebanded structure. b and c. Back-scattered electron images of the same area, displaying the distinctly banded struc-ture seen under.reflected light. Note that brightest bands coincide with the poorly developed planes. d. Snln charac-teristic X-ray scanning photograph taken in the same field as in c.

TIN-BEARING CHALCOPYRITE FROM HOKKAIDO, JAPAN l 1

microprobe analysis iu sphalerite and tetrahedrite*;compositions range from I to 6 mole 9o FeS, andfrom l0 to 98 mole 9o tetrahedrite component,respectively. The tetrahedriteo commonly containsconsiderable Ag (up to about 13 wt. 9o). The tin-bearing chalcopyrite displays weak reflection-pleochroism and anisotropism, which permit obser-vation of its internal structure (Fig. 2a).

Anar,ytcal Dana

Back-sgattered electron images obtained by usinga JEOL electron microprobe reveal a distinct zonalstructure in the chalcopyrite @igs .2b, c). The lighterdomains of the images correpond to higher Sn con-tents (Figs. 2c, d). Figure 3 shows line profiles forCuKo, FeKcu, Sntra andZnKa across the light anddark chalcopyrite bands. An increase in intensity ofSntrtr corresponds to a decrease in that of FeKcu.

Quantitative analyses mainly were made on thepale domains of the back-scattered electron images.The standards used are chalcopyrite from the Kosakamine, synthetic ZnS and Sn metal. The measuredratios ofintensity were corrected for background anddeadtime and then converted into concentrationsaccording to the method of Sweatman & Long(1969). The results are gven in Table l.

The Sn contents increase as'the back-scattered elec-

TABLE I. CHEI,IICAL CO{POSITION OF TII{-BEARII{G CHALCOPYRITE

Cu 34.33 34.31 34.39 U.29 34.@ 33.87Fe 30.52 30.32 30.25 30.40 30.12 29.37zn 0.09 0.18 0.14 0.10 0.18 0,29Sn 0 .86 l . l 2 1 .27 1 .35 1 .50 ? .Us 34.52 34.18 34.03 U.21 34.62 34.40

Total 100.32 l00.ll 100.08 100.35 100.51 100.27tlunber of atoms Der foflil'la unltr

Cu t .990 1.998. 2.007Fe 2.013 2.010 2.009Zn 0.005 0.010 0.008

Felzn 2.018 2.020 2;017Sn 0.027 0.035 0.040s 3.965 3.947 3.936

1.995 1.977 " t .977?.013 1.987 1.9520.005 0.010 0.0162.018 1.998 1.9680.v2 0.M7 0.0743,945 3.979 3.981

Stannlte (wt.t)3. l l 4.05 4.60 5.434.89 8.48

r calculated on the basls of I total atons. Compositiondetennlhed by eiectrcn nicroprob€.

tron images become brighter, with a maximum Sncontent measured of 2.34 W. 90. This Sn value is,however, exceptionally high; Sn contents are usuallyless tlan I .5 wt. 90. The analyses with unusually highSn contents were obtained on the poorly developedcrystal planes (Figs. 2b, c). In Figure 4, numbers ofFe + Zn (Nr"+zJ and Cu (NqJ atoms are plottedagainst Nro, the number of Sn atoms, calculated onthe bdsis of 8 total atoms per formula unit. NF"+zndecreases monotonously with ioslsasing .l/s'although Zn is restricted to domains with high Sncontents. The relationships between Nr"121 rnd Nso

Ftc.3. FeKa, SnLa, ZnKa and CtrKa scanning profiles across the tin-bearing chalcopyrite, taken along the solid lineshorqn on the back-scattered electron image (inset) (cps: counts per second).

t2 TI{E CANADIAN MINERALOGIST

Nsn

Fto.4. Relationship between a) number of Fe + Zn atoms(NF"+zo, solid circles) and b) N6" (open circles), andNso in chalcopyrite. Number of atoms is calculated onthe basis of 8 total atoms per formula unit.

obtained by least-squares regression is:Nre+zn = - l.24Nso*2.06. No systematic relation-ship is noticed between Nq, and Nso.

DncussroN

The valence state of Cu in chalcopyrite and stan-nite is monovalent, accorrling to the X-ray photo-elestron spectroscopic data of Nakai et al. (1976).Iron in chalcopyrite and Sn in stannite are trivalentand tetravalent, respwtively, according to tle Miiss-bauer spectra of Greenwood & Whitfield (1968). Thevalence state of Fe in stannite may be divalent tomaintain a balance of electric charges, thus differ-ing from that in chalcopyrite. Mixing of chalcopyriteand stannite therefore involvs coupled substitutionof Sna+Fdr for Fd+Fd+ . Znc may replace Fd+ inSn-rich compositions. In the Toyoha chalcopyrite,Nre+zn decreases linearly with increasing Nso, andthe slope for the relationship does not depart muchfrom unity (Fig. 4). Tin enters into chalcopyrite asa solid solution toward stannite bytle above schemesof atomic substitution.

Moh (1975), however, showed that only 3 wt. 9ostannite is soluble in low-temperature (tetragonal)chalcopyrite, which is stable below about 550oC inthe system Cu-Fe-S (Yund & Kullerud 196Q. Forhigh-temperature (rsometric) chalcopyrite, tle extentof solid solution is greater (Mob 1975). The maxi-mum Sn content determined in tle preseut studyQ.34 wt.9o) corresponds to about 8.5 wt. % stan-nite (Table l), but the temperature of mineral depo-sitiou in the Izumo vein is estimated not to have

exceeded 3CI'C (Yajima & Ohta 1979).It is thushighly probable that chalcopyrite witl unusually highSn contents has precipitated metastably in the Izumovein. The observation that the Sn contents areanomalously high on planes characterized by rapidgowth supports this idea. In this regard, severalgrains of chalcopyrite containing symplectitic stan-nite from the polymetallic veins of the Ikuno minewere analyzed; their Sn contents were found to bebelow the detection level of the microprobe (ca. 300ppm). This result indicates that the composition ofchalcopyrite changes rapidly during the sselingprocess. The Toyoha vein system is believed to haveformed at very shallow depth (Yajirna & Ohta 199),and therefore quenching of the tin-bearing chal-copyrite can reasonably be inferred.

AcK}.IowLEDGEN{ENTS

The author expresses his sincere gratitude to Drs.Takashi Fujii, Hidehiko Shimazaki and TsugioShibata for their sritical reading of the manuscriptand invaluable suggestions. Thanks are also due toDrs. Robert F. Martin, Alan H. Clark and Louis J.Cabri, who kindly reviewed the manuscript and sug-gested improvements. The ore samples of the Izumovein were provided by the staff of the explorationbranch of the Toyoha [\dining Co. Ltd., through thegood offices of Mr. Hiroshi Ogasawara, Presidentof the company. Technical assistance given by Mr.Toshiaki Saito is acknowledged.

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Rreived October 18, 1985, revised manuscript acceptedMay 28, 1986.