MINING GEOLOGY, 27, 355365, 1977

Sulfide Minerals and Their Assemblages of the Besshi Deposit Studies on Sulfide Minerals in Metamorphosed Ores of the Besshi and Hitachi Copper Deposits (1)*

Katsuo KASE**

Abstract: The contact metamorphism was superimposed on the regional glaucophanitic one in the deeper levels of the Besshi deposit. The mineral assemblages change progressively with increasing depth by this event. Sulfide mineral assemblages were studied on regional and contact metamorphosed ores of the deposit. Pyrite, chalcopyrite and sphalerite assemblage is the majority of ores suffered the regional metamorphism. Pyrite-chalcopyrite-bornite and pyrite-hematite-magnetite assemblages,which constitute univariant equilibria in Cu-Fe-S and Fe-S-O systems, are sometimesobserved in these ores. T and fS2of the metamorphism were discussed from the mutual relation of these univariant equilibria. T and fS2 were also obtained at high pressures from FeS contents of sphalerite (0.48-0.90 mole %) associated with pyrite-chalcopyrite-bornite assemblage. They are 280°-350°C and 10-8.71-10-6.4 atm, supposing 5 Kb lithostatic pressure. By contact metamorphism, pyrrhotite is formed from pyrite in the levels deeper than 18 L. Fe contents of pyrrhotite increase with increasing depth, from monoclinic modification containing 46.7 atomic % Fe and hex agonal one with 47.3 atomic % Fe in 18-25 L to 47.5-48.0 atomic % Fe in 26-33 L. FeS contents of sphalerite associated with pyrrhotite are in the range of 16 to 20 mole %, which does not correspond to pyrrhotite with 47.5- 48.0 atomic % Fe. Hexagonal pyrrhotite coexists commonly with pyrite, which shows sometimes euhedral shape. This association is incompatible in the light of the low temperature phase relation of Fe-S system.hitherto estab lished. Pyrrhotite alone may have changed its composition during the retrogressive metamorphism, reflecting the T-fS2 environments. Sphalerite and pyrite, on the contrary, may preserve the high temperature states. It is there fore difficult to obtain metamorphic T and fS2 from the compositions of coexisting sphalerite and pyrrhotite, unless detailed study is made on the compositional change of these minerals in the retrogressive metamorphism. The association of hexagonal pyrrhotite and euhedral pyrite may be showing that pyrrhotite of high temperature type and pyrite were once in equilibrium at the climax of the metamorphism. Sphalerite has no exsolution dots in the contact metamorphosed ores. The absence of exsolution dots will be due to the low fS2 environments in the retrogressive metamorphism, corresponding to pyrrhotite with 47.5- 48.0 atomic % Fe.

latter in the southern Abukuma metamor 1. Introduction phic terrain of andalusite-sillimanite type The Besshi and Hitachi copper deposits (Fig. 1.). are the representative strata-bound massive Both deposits suffered the intense regional sulfide deposits in Japan. The former is metamorphisms. The Besshi deposit suffered situated in the Sambagawa glaucophanitic the contact metamorphism by hidden terrain of the central Shikoku island and the granitic intrusion in the deeper levels of the deposit (KASE,1972; MWYAZAIUet al., 1974). * Received June 30, 1977; in revised form September The granodiorite intruded in the northern 20, 1977. area of the Hitachi district, and gave the ** Department of Earth Sciences, Faculty of Science, contact metamorphism to surrounding rocks Okayama University, Tsushima Naka 3-1-1, and ores. Okayama Mineralogical studies have been carried Key words: Besshi deposit, Strata-bound massive sulfide deposit, Sulfide mineral assemblage, Regional metamo out on ores of the Besshi deposit by KASE rphism, Contact metamorphism. (1972, 1974) and MIYAZAKIet al. (1974),

355 356 K. KASE MINING GEOLOGY:

vations are practically quite similar to those of MIYAZAKIet al. (1974), but an inter pretation different from MIYAZAKIand his coworkers will be given to the observed mineralogy in this paper. The Besshi deposit occurs conformably in pelitic schists between the thick piles of basic schists of Paleozoic age (KASE, 1972). The deposit extends about 1800 m along the strike and more than 2500 m along the dip. It consists of two layers of massive sulfide ores and another layer of banded ores between them. The banded ores comprise repeated fine bandings of sulfide-rich and silicate-rich layers. The total thickness of ores is usually 1-2 m but reaches 10 m in some places. Main sulfide minerals observed in the Besshi deposit belong to Cu-Fe-Zn- S system. The primary main sulfides are supposed to be pyrite, chalcopyrite and sphalerite. Pyrrhotite was formed by the desulfurization of pyrite in the process of Fig. 1 Location map showing the Besshi and the contact metamorphism. The variation Hitachi deposits. of sulfur fugacity (fS2) in the metamorphisms and the variation of mineral assemblages is therefore stressed in this paper. A special with increasing depth (equals to the in attention is paid to the change of the sulfide creasing grade of the contact metamorphism) mineral assemblages during the retrogressive was clarified. IZAWAand MUKAIYAMA(1972) metamorphisms. summarized briefly the mineralogical char- acteristics of ores from the contact metamor- 2. Metamorphisms and Classifica tion of Ores phosed strata-bound massive sulfide deposits in Japan, including the Besshi and Hitachi The mineral deposit suffered the regional deposits. They concluded that contact glaucophanitic metamorphism (high pres metamorphism is characterized by the sure, low temperature) at late Mesozoic irreversible desulfurization reactions, for time. The metamorphic grade of ores example, the formation of pyrrhotite from corresponds to the transitional facies between pyrite. the glaucophane schist and epidote- Our knowledge on the progressive miner amphibolite facies (BANNO, 1964; KASE, alogical changes of sulfide minerals in 1972). The assemblages of epidote-chlorite- regional and contact metamorphisms, how hornblende and epidote-chlorite-hornblende- ever, is still very incomplete. It is therefore glaucophane with albite, quartz, muscovite, interesting to compare the behaviors of calcite, pyrite and hematite characterize. sulfide minerals in regional and contact the basic schists of this facies. Biotite does metamorphosed ores of the Besshi and not occur in basic schists as well as pelitic Hitachi deposits. As a part of this compara schists. Ilmenite occurs only as exsolution tive study, sulfide mineral assemblages of intergrowth in hematite. Pyrrhotite is very the Besshi deposit are discussed in relation rarely observed in basic schists. In ores, to the regional and contact metamorphisms pyrite, chalcopyrite and sphalerite are the in this paper. The present author's obser- main constituent sulfides. 27(6), 1977 Sulfide Minerals and Their Assemblages of the Besshi Deposit 357

The contact metamorphism caused by chalcopyrite, bornite and sphalerite. hidden granitic intrusion was superimposed Pyrite is the predominant sulfide mineral on the regional one in the deeper levels of but pyrrhotite becomes predominant locally the deposit (KASE, 1972; MIYAZAKIet al., in this zone. In ores free from pyrrhotite, 1974). The silicate mineral assemblages as the sulfide mineral assemblages and their well as sulfide and oxide ones change pro- textures are similar to those in ores of the gressively with increasing depth by this regional metamorphosed zone. event (KASE, 1972). In ores and basic schists Pyrrhotite is observed in 18 and 22 L suffered the contact metamorphism, pyr along the fault zones. It occurs in the rhotite, ilmenite, bigtite and diopside ap ordinary sulfide ores in levels deeper than pear. Pyrrhotite becomes a predominant 23 L. In pyrrhotite-poor ores, it is usually sulfide mineral instead of pyrite in the levels formed along cracks of pyrite grains. In deeper than 26 L. Glaucophane and hem pyrrhotite-rich ores found locally, this atite disappear. Chlorite in banded ores mineral is generally granular in shape and becomes rich in MgO with increasing the grain size is about 0.1-0.2 mm. Pyr amount of pyrrhotite, from 0.5-0.6 of rhotite of this zone shows stronger reflection atomic ratio Fe/Fe + Mg in 6-14 L to 0.3- pleochroism and anisotropism than those 0.4 in 26-32 L. observed in the contact metamorphosed The deposit was classified from these zone. variation of mineral assemblages into the Chalcopyrite and sphalerite are mixed regional metamorphosed (1-17 L), transi with pyrrhotite in pyrrhotite-rich ores. tional (18-25 L) and contact metamor- Minute dots of chalcopyrite in sphalerite phosed zones (26-33 L). The boundaries of are commonly observed in this zone (Fig. each zone were defined by the appearance 5b). of pyrrhotite, and predominance of pyr Bornite included in pyrite grains is also rhotite relative to pyrite. often observed, coexisting with pyrrhotite. The occurrence of interstitial bornite is 3. Sulfide Minerals of Cu-Fe-Zn-S very rare and restricted to pyrrhotite-free System and Their Assemblages ores. 3.1 Regional Metamorphosed Zone Mackinawite is sometimes found in chal The observed sulfide minerals of Cu-Fe- copyrite and along boundaries between Zn-S system are pyrite, chalcopyrite, chalcopyrite and other minerals such as bornite and sphalerite. sphalerite and silicates,. having a needle- Pyrite is a predominant sulfide mineral. like shape. The length is less than 0.1 mm. It is always euhedral to subhedral in The mineral always coexists with pyrrhotite. shape, and the grain size is usually 0.1 to 3.3 Contact Metamorphosed Zone 0.5 mm. Pyrrhotite becomes predominant instead Chalcopyrite and sphalerite occur filling of pyrite in the levels deeper than 26 L. The the interstices of pyrite grains. Very minute observed sulfide minerals of this system are dots of chalcopyrite are always observed in pyrrhotite, pyrite, mackinawite, chalco sphalerite under magnified ore-microscope pyrite and sphalerite. (Fig. 5a). Lamellar pyrrhotite is commonly observed Bornite occurs sometimes filling the inter in this zone. The grain size is generally stices of pyrite grains. This mineral appears very small, and from 0.01 to 0.2 mm in more frequently as minute inclusions in length. In copper-rich ores, fine banding pyrite grains. structure composed of pyrrhotite-rich and 3.2 Transitional Zone chalcopyrite-rich layers is conspicuous. Re The observed sulfide minerals of this flection pleochroism and anisotropism of the system are pyrite, pyrrhotite, mackinawite, pyrrhotite are weak, compared with those 358 K. KASE MINING GEOLOGY: of pyrrhotite in the transitional zone. 3.5 Polytypes and Fe Contents of pyr Euhedral to subhedral pyrite of 0.1 to rhotite 0.5 mm in size coexists sometimes with Polytypes and Fe contents of pyrrhotite pyrrhotite in this zone (Fig. 5d). Anhedral from the transitional and contact metamor- and granular pyrite is also found occasion phosed zones were determined by X-ray ally in pyrrhotite. The grain size is very diffraction with Fe radiation and internal small, usually smaller than 0.2 mm. The standard of silicon (a•¬= 5.4307 A). amount is also very small. The euhedral Fe contents were calculated from d(102)- pyrite seems to have been in equilibrium composition curve given by TOULMIN and with pyrrhotite in certain stage of the met BARTON (1964). Frequency of polytypes and amorphism. On the other hand, anhedral Fe contents of pyrrhotite is shown in Fig. 2. pyrite is probably a relict mineral. Details were already described by KASE Chalcopyrite and sphalerite occur elon- (1974) and not repeated here. Features on

gated to the same direction of the lamellar polytypes and Fe contents of pyrrhotite are pyrrhotite in pyrrhotite-rich ores. Chal briefly summarized. copyrite dots in sphalerite were not observed Monoclinic pyrrhotite with or without

in this zone (Fig. 5c). pyrite and monoclinic pyrrhotite + Fe-poor The mode of occurrence of mackinawite hexagonal one (47.3-47.4 atomic % Fe) with is similar to that in the transitional zone. or without pyrite assemblages are predomi

3.4 Sulfide Mineral Assemblages nant in the transitional zone (Fig. 2). In the regional metamorphosed zone, pyrite + chalcopyrite + sphalerite is the common sulfide assemblage. Bornite coexists with chalcopyrite and pyrite which corre sponds to a univariant equilibrium in Cu- Fe-S system (supposing that the assem blage was formed in the presence of S vapor, and that the effect of pressure is small). The equilibrium is expressed by the following equation. Cu6FeS4+4FeS2=5CuFeS2+S2 (1) Oxide minerals are also found sometimes in massive ores with an assemblage pyrite- hematite-magnetite with chalcopyrite and sphalerite (Fig. 5e). It represents also a univariant equilibrium in Fe-S-O system, because T and fO2 are not independent in this assemblage. The equilibrium is expressed by the following equation. 4Fe2O3 + FeS2 = 3Fe3O4 + S2 (2) In the transitional zone, bornite included in pyrite + pyrrhotite with chalcopyrite and sphalerite assemblage is characteristic. In the contact metamorphosed zone, pyrrohitite + chalcopyrite + sphalerite as semblage is commonest. Euhedral pyrite + Fig. 2 Frequency of polytypes and Fe contents of pyrrhotite and anhedral pyrite + pyrrhotite pyrrhotites. with chalcopyrite and sphalerite assemblages The diagram was constructed with relative are sometimes observed. precision of 0.05 atomic % Fe for obtained values. 27(6), 1977 Sulfide Minerals and Their Assemblages of the Besshi Deposit 35

9In 26 L, monoclinic pyrrhotite is also Table 1. FeS contents of sphalerites and thei detected. Hexagonal one of 47.3-47.4 atomic r opaque mineral assemblages. % Fe with or without pyrite, however, is common. More Fe-rich hexagonal pyrrhotite (47.5-47.6 atomic % Fe) with or without pyrite assemblage is sometimes observed (Fig. 2). Beneath the 26 L, monoclinic pyrrhotite was not detected by X-ray powder diffrac tion. Fe contents of hexagonal pyrrhotite increase progressively with increasing depth (Fig. 2). They are mostly 47.4-47.6 atomic % Fe in 27-28 L, and 47.5-47.8 in 30-33 L. Fe contents of a few pyrrhotite reach about 48.0 atomic % Fe. They exceed the limit Fe content of low temperature type and terrestrial pyrrhotites investigated by DESBOROUGHand CARPENTER(1965) and ARNOLD(1967). Fe contents of pyrrhotite are not decided by the presence or absence of pyrite in the levels deeper than 26 L (KASE, 1974). The remarkable feature is the association of euhedral pyrite and Fe rich hexagonal pyrrhotite (Fig. 5d). Grain size of pyrrhotite in ores of the * determined by electron probe microanalyses, Besshi deposit is too small to be separated ** always coexistedwith chalcopyrite, *** taken for detailed X-ray identification. Further frompaper by MIYAZAKIet al. (1974),**** deter information on this mineral could not be mined by X-ray diffraction,using equation given obtained in this study. by BARTONand TOULMIN(1966). 3.6 FeS Contents of Sphalerite Abbreviations.py: pyrite, (py): pyrite in small FeS contents of sphalerite from the amount, bn: interstitialbornite, [bn]: bornite in various sulfide assemblages were measured cluded in pyrite, hm: hematite, mt: magnetite, by electron probe microanalyzer. In Table 1, po: pyrrhotite,(po) : pyrrhotitein smallamount. FeS contents of sphalerite associated with analyzed due to the poor quality of samples. characteristic mineral assemblages are shown MIYAZAKIet al. (1974) reported that the together with their opaque mineral assem- FeS contents of sphalerite of this assemblage blages. Table 2 shows FeS contents of are 0.48 to 0.90 mole % FeS (Table 1). sphalerite and Fe contents of associated FeS contents of sphalerite associated with pyrrhotite. All sphalerites investigated are pyrite, hematite and magnetite in the homogeneous in FeS contents. Minute regional metamorphosed zone are in the chalcopyrite dots in sphalerite are always range from 3.5 to 6.9 mole % (Table 1). observed in ores of the regional metamor- Sphalerite associated with bornite in phosed zone. They are common but not cluded in pyrite and, chalcopyrite but not always in ores of the transitional zone. No with pyrrhotite in the regional metamor- dots are found in sphalerite of the contact phosed and transitional zones has 3.5 to metamorphosed zone. mole % FeS (Table 1). Sphalerite from the assemblage of inter The range of FeS contents in sphalerite stitial bornite + pyrite + chalcopyrite in is between 11.4 to 20.2, mostly between 16 the regional metamorphosed zone was not to 20 mole % FeS, in both assemblages of 360 K. KASE MINING GEOLOGY:

Table 2. FeS contents of sphalerites and Fe con tents of associated hexagonal pyrrhotites.

Fig. 3 Fugacity of S2 temperature diagram for

Cu-Fe-Zn-S system, and for pyrite-hematite-

magnetite equilibrium at low pressure. * determined by X-ray diffractionwith Fe radia B: the intersected point of two equilibrium

tion and internal standard of silicon(ao = 5.4307 curves; pyrite-chalcopyrite-bornite and A), by using d(102)-compositioncurve given by pyrite-hematite-magnetite. TOULMINand BARTON(1964), ** determined by T: T-fS2 corresponding to the pyrrhotite with

electronprobe microanalyses 47.5-48.0 atomic % Fe observed in levels

Abbreviations.Hx-po: hexagonalpyrrhotite, M: deeper than 26 L.

monoclinicpyrrhotite. Other abbreviationsare the Data source: py + bn-cp; BARTON and TOULMIN same as thosein Table 1. (1964), py + hm-mt; calculated from the ther pyrite + pyrrhotite + sphalerite and pyr - mochemical data compiled by ROBIE and rhotite + sphalerite. FeS contents do not WALDBAUM (1968), py-po; SCOTT and BARNES increase with increasing Fe contents of (1971), po-iron; calculated from the thermo- associated pyrrhotite, that is, with increasing chemical data compiled by ROBIE and WALDBAUM depth (Table 2). They seem to be related (1968), FeS contents of sphalerite; calculated neither to polytypes and Fe contents of from the equation by SCOTT and BARNES (1971) and thermochemical data by ROBIE and

pyrrhotite nor to the association with WALDBAUM (1968), and Fe contents of pyrrhotite; pyrite in the transitional and contact TOULMIN and BARTON (1964). metamorphosed zones (Tables 1 and 2). Abbreviations. cp :chalcopyrite. Other abbreviations 4. Discussion on the Sulfide Mineral are the same as those in table Assemblages s ected point gives therefore the upper limit temperature of mineral formation in this

4.1 Regional Metamorphosed Zone zone. Chalcopyrite is a predominant copper

Two equilibrium curves (1) and (2) mineral and bornite + pyrite assemblage intersect on l/T-log fS2 plane (Fig. 3). At without chalcopyrite was not observed. Thus, temperatures higher than this intersected this intersected point gives also the upper point, pyrite-hematite-chalcopyrite assem limit value of fS2. They are about 500•Ž blage is not stable. This assemblage, how and 10-2 atm, as indicated by a point B in ever, is commonly found in ores of this Fig. 3. zone, Pyrite-interstitial bornite-magnetite The Sambagawa metamorphism is known assemblage was not observed. This inter- as a high pressure type one (MIYASHIRO, 27(6), 1977 Sulfide Minerals and Their Assemblages of the Besshi Deposit 361

corporation of 15 mole % FeTiO3 in

hematite gives 320•‹-230•Ž at 4-5 Kb as temperatures of the intersected point

(dotted curve in Fig. 4). The metamorphic environments (T, fS2) are also evaluated from pyrite-chalcopyrite- bornite-sphalerite equilibrium. Sphalerite

with 0.90 mole % FeS associated with

pyrite-bornite-chalcopyrite, however, can not be explained by Fig. 3. The effect of pressure on FeS contents of

sphalerite along the pyrite-bornite- chalcopyrite univariant curve was taken in account along the line given by CZAMANSKE Fig. 4 Temperature-pressure relation of the inter sected point of two equilibrium curves; pyrite- (1974). The equation (1) is rewritten as below, substituting 2 FeS2 = 2[FeS]Sp + S2. chalcopyrite-bornite and pyrite-hematite- magnetite. Cu5FeS4+2FeS2+2[FeS]SP=5CuFeS2 (3) solid line: calculated by using the thermo- Here, [FeS]Sp means FeS molecule in chemical data of end member sphalerite. The pressure effect on equation

minerals, compiled by ROBIE and (3) can be shown that WALDBAUM(1968), •¬Vr, dp= 2RT dln (aFes) (4) dotted curve: calculated taken in account the •¬ Vr, is the difference of molar volume of incorporation of 15 mole % FeTiO3 equation (3) and aFes is the activity of FeS in hematite as ideal solid-solution. in sphalerite. Using the partial molar volume

1965). The effect of lithostatic pressure on data of BARTON and TOULMIN (1966) for equations (1) and (2) was calculated, based FeS in sphalerite, molar volume data of on the relation; log K = (•¬Vs•~ P)! RUBLE et al. (1966) for bornite and pyrite, 2.303RT + log fS2. Here, K is the equilib and HALL and STEWART (1973) for chal rium constant and •¬Vs, is the difference of copyrite, •¬Vr is calculated to be about 0.5 molar volume of solid phases between prod cal/bar for reaction (3). It can be regarded ucts and reactants in equations (1) and (2). as independent to temperature (CZAMANSKE, P is the lithostatic pressure. Data of molar 1974). Integration of (4) gives, volume were taken from papers by HALL •¬Vr (P2-P1) = 4.606 RT (log aFeS.P2-log and STEWART (1973) for chalcopyrite, and aFeS .P1) (5) by ROME et al. (1966) for other minerals Here, aFeS = (YspFes) (NspFes). Where, YspFeS rep- concerned. resents the activity coefficient of FeS in Fig. 4 shows the pressure dependency of sphalerite and NspFes the mole fraction of FeS this intersected point. From the figure, an in sphalerite. YspFes can be regarded as con increasing lithostatic pressure shifts the stant, and equals to about 2.5 (BARTON TOULMIN, 1966). point to lower temperature side. The temperature of this point (220-150•Ž) FeS contents of sphalerite along the corresponding to 4-5 Kb pressure, generally pyrite-chalcopyrite-bornite univariant curve accepted as realistic pressure for the Samba were obtained experimentally at 1 Kb, and 271•‹and 395•‹by CZAMANSKE (1974), gawa metamorphism, seems to be too low. Hematite, however may contain 10-20 as about 0.19 and 0.55 mole % FeS, respec mole % FeTiO3 in general in this district tively. Thus, FeS contents of sphalerite (BANNO and KANEIHRA, 1961). Supposing along the univariant curve at high pressures

the ideal solid-solution of ilmenite-hematite can be isothermally calculated. At 5 Kb, series in the FeTiO3-poor portion, the in- they are 0.47 (at 271•‹) and 1.13 (at 395•Ž) 362 K. KASE MINING GEOLOGY:

mole % FeS. The temperatures correspond temperature type pyrrhotite were once in

ing to 0.5-0.9 mole % FeS of sphalerite equilibrium. No informations can be ob measured by MIYAZAKI et al. (1974) in the tained as to metamorphic temperature Besshi deposit are 280•‹-350•‹ at 5 Kb. from the sulfide mineral assemblages in

The corresponding fS2 are 10-8.7-10-6.4 this zone. atm. This temperature range may be more 4.3 Contact Metamorphosed Zone reliable than that obtained from thee Fe-poor hexagonal pyrrhotite coexists

intersected point of pyrite-chalcopyrite- with pyrite in 26 L. In levels deeper than bornite and pyrite-hematite-magnetite equi 26 L, hexagonal pyrrhotite of 47.6-47.7 librium curves. The range appears to be atomic % Fe associates sometimes with

quite realistic as temperature of the Samba euhedral to subhedral pyrite (KASE, 1974). gawa metamorphism. These assemblages can not be explained 4.2 Transitional Zone by the phase relations hitherto established. A non-equilibrium mixture of minerals Polytypes and Fe contents of pyrrhotite formed by regional and contact metamor may be correlated to the prevailing en

phisms constitutes an apparently incom vironments in the cooling process of the patible assemblage of bornite included in contact metamorphism, and pyrite may be

pyrite + pyrrhotite in this zone (see Fig. 3). a relict mineral as in the case of the tran Pyrite + monoclinic and Fe-poor hexagonal sitional zone. The fS2 was very low, corre

pyrrhotites constitute another incompatible sponding to 47.5-48.0 atomic % Fe in assemblage in Fe-S system (DESBOROUGH levels deeper than 26 L (shown by T in and CARPENTER, 1965; MUKAIYAMA and Fig. 3).

IZAWA, 1966; NAKAZAWA and MORIMOTO, The association of euhedral pyrite and

1971). pyrrhotite will be showing that pyrite and It is well known that pyrrhotite does not pyrrhotite of high temperature type were preserve its composition and varies according once in equilibrium at temperatures higher to the change of environments until very than ƒÀ transformation of pyrrhotite. The low temperature. This situation is supported temperature of ƒÀ transformation is about by the fact that terrestrial pyrrhotite is 300•Ž after DESBOROUGH and CARPENTER always a low temperature type, and the (1965). But more complicated phase rela compositional range is very narrow (ARNOLD, tions are given by NAKAZAWA and

1967). On the contrary, pyrite is known MORIMOTO (1971) and SCOTT and KISSIN as refractory mineral. It is therefore reason (1973). The low temperature limit of able to consider that pyrrhotite alone has coexistence of pyrite and hexagonal pyr changed its composition in the course of the rhotite can not be precisely obtained. retrogressive metamorphism. Pyrite is FeS contents of sphalerite from the thought a relict mineral in these two contact metamorphosed zone as well as incompatible assemblages just described . from the transitional zone seem to be related Polytypes and Fe contents of pyrrhotite neither to Fe contents and polytypes of may reflect the environments of the retro associated pyrrhotite nor to the presence of gressive metamorphism. Sulfur fugacity (fS,) pyrite. The observed FeS contents (mainly in the retrogressive metamorphism of this 16-20 mole % FeS) are markedly'. zone may not differ significantly from that from those obtained experimentally at fS2 corresponding to the monoclinic and Fe T conditions corresponding to 47.5-48.0 poor hexagonal pyrrhotite association. atomic % Fe of pyrrhotite (BARTON and As described previously, pyrrhotite is TOULMIN, 1966). A pressure effect is one of usually formed along cracks of pyrite in this the possible explanations for this discrepancy. zone. This texture does not necessarily in The pressure at the time of contact metamor dicate that pyrite and hexagonal high phism is estimated from the thickness of rock 21(6). 1977 Sulfide Minerals and Their Assemblages of the Besshi Deposit 363

b): Sphalerite with minute chalcopyrite exsolu tion dots in the transitional zone. The associated opaque minerals are pyrite (py), chalcopyrite (cp), magnetite (mt) and ilmenite. 20 L, No. 66081602. (C): Sphalerite without exsolution dots of chalcopyrite in the contact metamor- phosed zone. The associated opaque minerals are Fe-rich pyrrhotite (po. 47.75 atomic % Fe) and chalcopyrite. 30 L, No. BS72-07. ( d): The association of euhedral-subhedral pyrite (py) and Fe-rich pyrrhotite (po, 47.58 atomic % Fe) in the contact meta- (a): Sphalerite with very minute chalcopyrite morphosed zone. 28 L, No. 67072208. exsolution dots in the regional metamor- (e): Pyrite (py), hematite (hm), magnetite phosed zone. (mt) and chalcopyrite assemblage in the The associated opaque minerals are pyrite massive ore of the regional metamorphosed ( py). chalcopyrite (cp), magnetite and zone. 14 L, No. 66081617. hematite. 14 L, No. 66081617.

Fig.5 Photomicrographs showing the modes of occurrence of minerals and mineral assemblages.

pile underlaying the acidic effusive rocks in retrogressive process of the contact metamor the Mt. Ishizuchi district, about 30 km phism, showing contrast with pyrrhotite. southwest of the Besshi mine. They are To estimate the environments at the cli supposed to be related to the contact max of the metamorphism from FeS con metamorphism in the Besshi district. The tents of sphalerite (not from Fe contents pressure is calculated to be 1.0-1.5 Kb at of pyrrhotite) seems therefore to be more most. The pressure is too low to explain reasonable. The observed FeS contents the discrepancy (SCOTT, 1973). FeS con suggest that the metamorphic environments tents of sphalerite may be preserved in the are closely expressed by pyrite-pyrrhotite- 364 K. KASE MINING GEOLOGY:

sphalerite equilibrium at the climax of the Japan. The 24th Intern. Geol. Congress, Mont contact metamorphism. Supposing 1.0-1.5 real, Sec. 4, 455•`462.

Kb pressure, the observed FeS contents fit KASE, K. (1972): Metamorphism and mineral as semblages of ores from cupriferous iron sulfide more closely to those on pyrite-pyrrhotite- deposit of the Besshi mine, Central Shikoku, sphalerite solvus (SCOTT, 1973). This situa Japan. Jour. Fac. Sci., Univ. Tokyo, Sec. 2, tion is supported by the association of 18, 301•`323. euhedral pyrite and pyrrhotite in levels KASE, K. (1974): Pyrrhotite from the Besshi mine. deeper than 26 L. Pyrrhotite has changed Jour. Miner. Soc. Japan, 11, Spec. Issue, No. its composition to Fe-rich (47.5-48.0 atomic % 2, 97•`106. (In Japanese.) Fe) at levels deeper than 26 L in the MIYASHIRO, A. (1965): Metamorphic rocks an retrogressive metamorphism, corresponding d metamorphic belts. Iwanami, Tokyo. (In to the prevailing environments. The absence Japanese.) of exsolution dots in sphalerite at these MIYAZAKI, K., MUKAIYAMA, H. and IZAWA, E. levels may be due to these low fSa environ- (1974): Thermal metamorphism of the bedded ments in the retrogressive metamorphism. cupriferous iron sulfide deposit at the Besshi mine, Ehime prefecture, Japan. Mining Ge References ology (Japan), 24, 1•`12. (In Japanese.)

ARNOLD, R. G. (1967) : Range in composition and MUKAIYAMA, H. and IZAWA, E. (1966): Phase

structure of 82 natural terrestrial pyrrhotites. relations of pyrrhotite. Jour. Mining Inst.

Can. Miner., 9, 31•`50. Kyushu, 34, 194•`213. (In Japanese.)

BANNO, S. (1964) : Petrological studies on Samba NAKAZAWA, H. and MORIMOTO, N. (1971): Phase

gawa crystalline schists in the Besshi-Ino relations and superstructures of pyrrhotite, district, Central Shikoku, Japan. Jour. Fac. Fe1-xS. Mat. Res. Bull.,6,345•`358.

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別子 鉱床 の硫 化鉱物 について 別子 お よび 日立鉱床 の硫 化鉱物 の比較研 究(1)

加 瀬 克 雄

要 旨

別 子 鉱床 は藍 閃石 型 の 広 域 変 成 作 用 を受 け た層 状 含 銅 条 件 で実 験 的 に求 め られ たFeS量 よ り少 ない.ま た鉄 に 硫 化 鉄 鉱 々床 で あ る.さ ら に深 部 にお い て接 触 変 成 作 用 富 む 磁 硫鉄 鉱 に は黄 鉄 鉱 が しば しば共 存 して い る.黄 鉄 を受 け,鉱 石 鉱物,脈 石 鉱 物 お よび そ れ ら の鉱 物 組 み 合 鉱 は 時 に 自形 ～半 自形 を示 す 事 が あ る.こ の共 生 は低 温 わせ を著 しく変 化 させ て い る. でのFe-S系 の 相 図 に合 わ な い.磁 硫 鉄 鉱 は 後退 変 成作

藍 閃石 変 成作 用 を 受 け た鉱 石 中 では,黄 鉄 鉱,黄 銅 鉱 用時 に,そ の組 成 を変 化 させ た もの と思 わ れ る.他 方 閃 閃亜 鉛 鉱 が主 要 な鉱 石 鉱 物 で あ る.と き に黄 鉄 鉱一 黄 銅 亜 鉛 鉱,黄 鉄 鉱 は 高 温 の状 態 を保 存 してい る の で あ ろ 鉱 一 斑 銅 鉱 一 閃亜 鉛 鉱 の 共 生 が 認 め られ る.そ の平 衡 か う.し た が っ て共 存 す る閃 亜 鉛 鉱 と磁 硫 鉄 鉱 の組 成 をそ ら,5kbの 圧 力 を仮 定 して,280°～350℃ の温 度 が求 め ら の ま ま,地 質 温 度 計,硫 黄 蒸 気 圧 の地 質圧 力 計 と して用 れ た.こ の温 度範 囲 は 別 子 鉱 床 の広 域 変 成 温 度 と して は い る事 は 因 難 で あ る.鉄 に富 む磁 硫 鉄 鉱 と 自形 ～半 自形 妥 当 な もの で あ ろ う. の黄 鉄 鉱 の共 生 は,高 温 型 の磁 硫 鉄 鉱 と黄鉄 鉱 が か っ て 接 触 変 成 作 用 に よ って,18L以 深 で は 黄 鉄 鉱 の磁 硫 鉄 平 衡 に共 生 して い た こ とを示 す 組 織 で あろ う.26L以 深 鉱化 が著 しい.磁 硫 鉄 鉱 の 鉄 含 有 量 は深 部 に向 か って, の鉄 に富 む 磁 硫 鉄 鉱 と共 生 す る閃 亜 鉛 鉱 に は,黄 銅 鉱 あ 累 進 的 に増 加 し,26L以 深 では47.5～48.0の 原 子%の も るい は磁 硫 鉄 鉱 の 離 溶 を生 じて い な い.こ れ は後 退 変 成 の が普 通 に認 め られ る.共 存 す る閃亜 鉛 鉱 のFeS含 有 量 作 用 時 の 硫 黄 蒸 気 圧 が低 か っ た こ とに よる の で あろ う. は16～20モ ル%で あ り,そ の磁 硫鉄 鉱 の組 成 に対 応 す る