Tin, Arsenic, Zinc and Silver Vein Mineralization Many Vein

MINING GEOLOGY, 38(5), 407•`418, 1988

Tin, Arsenic, Zinc and Silver Vein Mineralization in the Besshi Mine, Central Shikoku, Japan

Katsuo KASE*

Abstract: Intense Sn-As-Zn-Ag vein mineralization was found at the 26th Level of the Besshi mine, of which deposit is a conformable massive sulfidetype (Besshi-type).The mineralization resulted in formation of such rare Sn minerals as rhodostannite, hocartite and a franckeite-like mineral, as well as common stannite and cassiterite. Pyrite, arsenopyrite, sphalerite, tetrahedrite, manganese carbonates, quartz, tourmaline and so on occur in association with these Sn minerals. The prominently polymetallic ores are composed of minerals that were formed at several stages during the mineralization sequence. The present microprobe analyses, combined with previous chemicaldata, indicate that the substitution of Ag for Cu is extensivein rhodostannite, whereas that of Zn for Fe is very limited. The substitution relationship of these elementsin stannite and hocartite is just the opposite to that found in rhodostannite. Divalent Sn may substitute for Pb in the franckeite-like mineral, which is considered to be mainly responsible for the extensive solid solution observed in this mineral. The mineralization may have taken place nearly simultaneously with contact metamorphism, which converted the massive pyrite ores and surrounding pelitic schiststo massivepyrrhotite ores and biotite hornfels, respectively. A granitic intrusion, which is supposed to be hidden in the deeper part of the Besshimining district, probably caused the vein mineralization and contact metamorphism. The geological situation and mineralogical characteristics of the vein mineralization are quite similar to those observedin the Sn deposits adjacent to Miocene granitic intrusives in the Outer Zone of Southwest Japan at Kyushu. The Miocene Sn metallogenicprovince in the Outer Zone of Southwest Japan at Kyushu should be extended eastward to the Besshimining district in Shikoku.

currence of high temperature Sn deposits has 1. Introduction been found so far (MIYAHISA,1973). Many vein-and skarn-type Sn deposits oc- Thin hydrothermal stibnite veins sometimes cur in the aureoles around the Miocene penetrate the conformable pyrite-chalcopyrite granitic intrusives in the Outer Zone of ores of the Besshi-type deposits occurring in Southwest Japan at Kyushu (described as in Shikoku (e.g., the Yuryo and Choshidaki the outer zone of Kyushu hereafter). They in- deposits, TAKEDAet al., 1973). Chalcostibite clude such deposits as Obira, Ho-ei, Mitate, and tetrahedrite occur occasionally in the reac- Matsuo, Osuzu and Suzuyama mines (Fig.1), tion zones formed between the stibnite veins and constitute a major Sn metallogenic pro- and massive ores. In the Besshi mine, stibnite, vince in Japan. In Shikoku which is located to chalcostibite and tetrahedrite are also known the east of the province, however, only quartz- to occur (YUI, 1971; UCHIDA et al., 1981). stibnite and quartz-cinnabar veins are known SHIMADAand TSUNORI(1962) found small as Miocene mineralizations (e.g., the. Ichino- amounts of stannite and arsenopyrite in kawa stibnite deposit), and no remarkable oc- specimens from the fault zones of the massive ores in the Besshi mine. Received on December 8, 1987, accepted on June 15, Intense Sn-As-Zn-Ag vein mineralization 1988 * Department of Earth Sciences is found at the 26th Level of the Besshi mine. , Faculty of Science, This vein mineralization resulted in formation Okayama University, Tsushima-Naka 3-1-1, Okayama of such rare Sn minerals as rhodostannite, 700, Japan. hocartite and a franckeite-like mineral, as well Keywords: Rhodostannite, Hocartite, Franckeite, Besshi mine, Sn mineralization, Metallogenic province, Mio- as common stannite and cassiterite. This is the cene granite. first remarkable occurrence of Sn mineraliza-

407 408 K. KASE MINING GEOLOGY:

Fig. 2 Geologic cross section of the massive sulfide deposit of the Besshi mine (partially modified from UCHIDAet al., 1981). 1: basic schist, 2: pelitic schist, 3: siliceous schist, 4: mineral deposit, 5: epidote amphibolite, and 6: fault.

alternation of sulfide-rich and quartz-chlorite- rich layers. The total thickness of the ore-body is usually 2 to 3 m. Fig. 1 Locations of the Besshi mine, the major Sn The Besshi deposit was subjected to the mines in Kyushu (1 to 6), and the mines in Shikoku glaucophanitic regional metamorphism in the of which the deposits are related to the Sb mine- late Mesozoic. The metamorphic conditions ralization (7 to 9). observed in the Sambagawa metamorphic M.T.L.: Median Tectonic Line, 1: Obira , 2: Ho-ei, 3: Mitate, 4: Matsuo, 5: Osuzu, 6: Suzuyama, 7: rocks of the Besshi mining district correspond Ichinokawa, 8: Yuryo, and 9: Choshidaki. to a metamorphic facies transitional from the glaucophane schist facies to epidote-amphi- tion in Shikoku. This paper describes the bolite facies (BANNO,1961; KASE, 1972). The mineral characteristics of the Sn-As-Zn-Ag reagionally metamorphosed massive ores con- vein found at the Besshi mine and discusses its sist of pyrite with subordinate chalcopyrite, genetic relation to the Miocene Sn deposits oc- sphalerite and silicate gangues. Bornite some- curring in the outer zone of Kyushu. times occurs, constituting a pyrite-chalcopy- 2. Geology and Mineral Deposit rite-bornite assemblage. of the Besshi Mine Contact metamorphism was superimposed on the regional one at levels deeper than the The Besshi mine is situated about 10 km 18th L of the Besshi mine, which is probably south of Niihama, central Shikoku (Fig . 1). due to the thermal effect derived from a The conformable massive sulfide deposit of plutonic intrusion hidden in the deeper part of the mine occurs in the piles of pelitic and basic the mine (KASE, 1977). By this thermal effect, schists of the Sambagawa metamorphic belt , pyrite is converted to pyrrhotite. Polytypes with thin beds of siliceous schists at its hang- and Fe contents of the pyrrhotite change pro- ing-wall and footwall (Fig. 2) . The deposit ex- gressively with increasing depth from the tends about 1800 m along the strike (N40•KW- monoclinic type (18th L-25th L) to the Fe-rich N70•KW) and more than 2500 m down the dip hexagonal type with exsolved troilite (below (45•K-70•KN). It consists of two layers of 30th L) through the mixture of monoclinic massive sulfide ores and a layer of banded ores and hexagonal types. Pelitic schists are con- between them. The banded ores comprise thin verted to the biotite hornfels. Diopside, scap- 38(5), 1988 Tin, arsenic, zinc and silver vein mineralization in the Besshi mine 409 olite, wollastonite and cordierite appear in strongly contact-metamorphosed rocks with optimum composition at the deepest levels of the adits (IMAI, 1978).

3. Vein Mineralization Thin sulfide veins less than 1 cm wide develop densely in the eastern end of the ore- body at the 26th L of the Besshi mine. The vein mineralization is commonly accompanied with strong carbonatization and brecciation, hence making it difficult to define the original host rocks. Such carbonate-rich, sulfide- disseminated ores are called brecciated ores in this paper (Fig. 3-A). Sulfide veins develop also in siliceous schists at the hanging-wall and footwall sides of the massive ores, without displaying a remarkable wall rock alteration (Fig. 3-B). The veins frequently intersect massive pyrrhotite ores. The full extent of the vein mineralization is not known, but it appears to extend further at depth. Sphalerite with abundant chalcopyrite dots, pyrite, arsenopyrite and stannite are the main constituents of the brecciated ores. They are followed by cassiterite, rutile, rhodostannite, Fig. 3 Photographs of ore specimens. A: brecciated ore (manganese carbonate-rich, sulfide- hocartite, tetrahedrite, chalcopyrite, pyr- disseminated ore). White parts are dominantly made rhotite, galena and a franckeite-like acicular of kutnahorite and rhodochrosite, B: sulfide veins in mineral. Transparent minerals are dominantly siliceous schist. manganese carbonates such as kutnahorite and rhodochrosite, and quartz, with lesser tween 0.5 mm and 0.2 mm is relatively rare. amounts of tourmaline, fluorite and musco- The chalcopyrite dots and small inclusions of vite. All of these minerals occur together in a stannite are more abundant in the coarse-grain- small ore specimen. ed sphalerite. Discrete grains of stannite occur Arsenopyrite and pyrite in the brecciated with sizes ranging from 0.1 mm to 0.3 mm in ores are usually euhedral with grain sizes rang- diameter. Also, stannite occurs overgrowing ing from less than 0.1 mm to 1 mm across. on, or replacing, sphalerite and cassiterite. No Arsenopyrite commonly shows the cataclastic exsolution mineral can be observed in stan- texture, the cracks of which are usually filled nite. with stannite. Pyrrhotite is occasionally found Hocartite and rhodostannite occur in close in the cores of arsenopyrite grains. association with stannite in the brecciated In the brecciated ores, sphalerite always oc- ores. Hocartite involves frequently euhedral curs intimately associated with stannite and stannite (Fig. 4-B). The weak but distinct contains small inclusions of stannite as well as reflection pleochroism and anisotropism of abundant chalcopyrite dots, with an apparent hocartite observed under the microscope exsolution texture (Fig. 4-A). The grain sizes reveal that polysynthetic twinning is devel- of sphalerite are usually larger than 0.5 mm in oped in this mineral. The reflection color is diameter but grains smaller than 0.2 mm are evidently more brownish grey with a violet tint not uncommon. Sphalerite with grain sizes be- compared to that of stannite. Polishing hard- 410 K. KASE MINING GEOLOGY:

Fig. 4 Photomicrograph (A) and back-scattered electron images (B, C and D), showing the mode of occurrence of sphalerite and rare Sn minerals. A: sphalerite with abundant chalcopyrite dots (cp), and inclusions of stannite (st) and arsenopyrite (asp), B: hocartite (ho) involving arsenopyrite and euhedral stannite, C: porous rhodostannite (rs) and closely associated stannite, and D: fibrous franckeite-like mineral (fr) replacing galena (gn).

ness is only slightly softer than stannite. franckeite group (e.g., franckeite, potosiite or Rhodostannite shows a porous texture with incaite). No structural determination could, grain sizes from 0.1 mm to 0.3 mm in diameter however, be carried out on this mineral be- (Fig. 4-C). The reflection color of rhodostan- cause the amount of sample is not adequate nite is prominently pinkish compared with for such determination. Thus, this mineral is that of stannite. The mineral is weakly called franckeite-like mineral in this paper. pleochroic and anisotropic, with a polishing Tetrahedrite is rarely observed as discrete hardness similar to that of stannite. grains in the brecciated ores. Chalcopyrite and A franckeite-like mineral occurs as aggre- pyrrhotite occur as dots in sphalerite and as in- gates of fibrous or feathery crystals with the clusions in arsenopyrite, respectively and no largest size of 0.1 mm long and 0.05 mm wide. discrete grains of them could be observed in It appears to have replaced galena (Fig. 4-D). the brecciated ores. This franckeite-like mineral is weakly pleo- Chalcopyrite, pyrrhotite, stannite and jame- chroic and distinctly anisotropic from light sonite are the main constituents of the sulfide grey with a brownish tint to dark grey, which veins developed in siliceous schists, followed together with its characteristic crystal habit in order of abundance by arsenopyrite, tetra- indicates that this mineral may belong to the hedrite, sphalerite, marcasite, cassiterite and 38(5), 1988 Tin, arsenic, zinc and silver vein mineralization in the Besshi mine 411 galena. The mineral assemblages and the mod- Table 1 Representative microprobe analyses of al proportion of the constituent minerals in sphalerite from brecciated ores (B) and a sulfide the sulfide veins are markedly different from vein in siliceous schist (V) those of the brecciated ores. Vaguely-defined symmetrical mineral zoning is observed in these veins; i.e., the chalcopyrite-cassiterite- tetrahedrite (or arsenopyrite) zone in the wall sides and the stannite-jamesonite-sphalerite zone in the central parts. Pyrrhotite and marcasite derived from pyrrhotite through alteration occupy the intermediate zone. Tourmaline and quartz are major transparent minerals dispersed ubiquitously in the sulfide veins.

4. Analytical Procedure

Microprobe analyses of sphalerite, Sn minerals, tetrahedrite and chalcopyrite were carried out using a JEOL Model 733, operating at an accelerating voltage of 25 kV and with a specimen current of 0.02ƒÊA on copper metal. The standards used for analyses are chalcopy- * calculated after Cu and Fe needed to form CuFeS2 rite, synthetic ZnS, synthetic MnS, synthetic are subtracted from the obtained compositions. CdS, galena and pure metals (Sn, Ag, Sb and 1(B): coarse-grained sphalerite with the maximum Fe As). The measured X-ray intensities were content, 2(B): coarse-grained sphalerite with the max- corrected for background and deadtime, and imum Cd and Mn contents, 3(B): coarse-grained then converted into concentration according sphalerite, and 4(B): fine-grained sphalerite. to the method described by SWEATMAN and LONG (1969). Homogeneous areas were se- mole% (Table 1). lected for analyses after detailed microprobe The MnS and CdS contents are plotted line scanning. The chemical homogeneity of against the FeS contents of sphalerite from the the franckeite-like mineral could not be, how- brecciated ores and sulfide veins in siliceous ever, confirmed because of its small grain size. schists in Fig. 4, together with those of spha- The chemistries of the analysed minerals are lerite from the regionally and contact meta- described in some details in the next chapter, morphosed ores in the same deposit for com- with their representative compositions shown parison. The overall variation of FeS for 21 in Tables 1 to 5. sphalerites from the brecciated ores and sul fide veins ranges from 6.5 to 21.2 mole%, in 5. Results which 6 analyses with FeS less than 10 mole% 5.1 Sphalerite are obtained from the fine-grained sphalerite Small amounts of Cu, usually less than 0.5 in the brecciated ores. The coarse-grained wt.% (Table 1), are almost always detected in sphalerite has, on the contrary, FeS ranging sphalerite. It is, however, difficult to discrimi- from 14.5 to 21.2 mole%. The FeS contents nate the amounts of Cu dissolved in sphalerite are between 17.1 and 20.0 mole% for from those in chalcopyrite included as an sphalerite accompanied with pyrrhotite from impurity. Copper and iron, needed to form the sulfide veins in siliceous schists. CuFeS2, were therefore subtracted here from The CdS contents of sphalerite from the the obtained composition, and then the Zn, brecciated ores show only a little variation, remaining Fe, Mn and Cd contents were cal- ranging from 0.3 to 0.6 mole%. The contents culated in terms of ZnS, FeS, MnS and CdS are markedly higher than those of sphalerite 412 K. KASE MINING GEOLOGY: Ag2(Fe,Zn)SnS4 for stannite and hocartite based on 8 total atoms per formula unit, with very small numbers of Ag and Cu up to 0.06 and 0.09, respectively. The Fe/(Fe+Zn) atomic ratios in stannite range from 0.74 to 0.92 (Table 2), and no Zn-rich variety could be obtained. The ratios are relatively high for stannite coexisting with pyrrhotite from the sulfide veins in siliceous schists (e.g., st-5 in Table 2), whereas they are relatively low for stannite coexisting with pyrite from the brec ciated ores (e.g., st-1 in Table 2). Stannite has the intermediate values between above two groups when it coexists with pyrite and arsenopyrite in the brecciated ores (st-2 and st- 3 in Table 2). The Ag contents seem to be relatively low for stannite with high Fe/(Fe+Zn) atomic ratios from the sulfide veins in siliceous schists. The Fe/(Fe+Zn) atomic ratios of hocartite are quite uniform, ranging from 0.45 to 0.50. Fig. 5 Relations between MnS and FeS and between CdS and FeS of sphalerite from brecciated ores The composition corresponds to just the in- (solid circles), sulfide veins in siliceous schists termediate one between Ag2FeSnS4 (hocartite) (open circles), contact-metamorphosed massive and Ag2ZnSnS4 (pirquitasite). ores (solid triangles) and regionally metamorphos- The chemical composition of rhodostannite ed massive ores (open triangles) in the Besshi mine. yields the chemical formula close to the stoichiometric (Cu, Ag)2FeSn3S8,based on 14 from the regionally and contact metamorphos- total atoms per formula unit (Table 2). The ed ores (Fig. 5). The sphalerite from veins in analysed rhodostannite contains Ag from 3.5 siliceous schists appears to have intermediate to 4.4 wt. % (from 0.13 to 0.16 in terms of CdS contents relative to the above two groups. Ag / (Cu + Ag) atomic ratios). The MnS contents of sphalerite from the The franckeite-like mineral may consist brecciated ores appear to increase with increas- essentially of Fe, Pb, Sn, Sb and S (Table 3). ing FeS contents. The MnS contents of FeS- Variable amounts of Bi may substitute for Sb. rich sphalerite from the brecciated ores are Copper is commonly and zinc rarely detected markedly higher than those from the contact in trace amounts. The structural formula was metamorphosed ores. The MnS contents of calculated from the composition on the basis the FeS-poor sphalerite are, however, not so of 25 total atoms per formula unit, which different between those from the regionally yields a metal: sulfur ratio of approximately metamorphosed ores and brecciated ores. The 11:14, consistent with the ratios of such fran- MnS contents of sphalerite from veins in ckeite group minerals as franckeite, potosiite siliceous schists seem to be intermediate be and incaite (MAKOVICKY,1974; WOLF et al., tween those from the brecciated ores and con 1981; SUGAKIet al., 1983; KISSINand OWENS, tact metamorphosed ores as also found for 1986). Compared with the ideal chemical for- CdS contents. mula of franckeite, FePb6Sb2Sn2S14,proposed 5.2 Stannite, Hocartite, Rhodostannite by SUGAKIet al. (1983), the Besshi franckeite- and Franckeite-like Mineral like mineral has the following chemical The microprobe analyses yield the struc characteristics; (1) numbers of Sn (Nsn) and tural formulae close to Cu2(Fe,Zn)SnS4 and Nsb+Bi+Snare greater than 2 and 4, respec- 38(5), 1988 Tin, arsenic, zinc and silver vein mineralization in the Besshi mine 413

Table 2 Representative microprobe analyses of stannite (st), hocartite (ho), chalcopyrite (cp) and rhodostannite (rs)

*from Vila Apacheta, Bolivia (SPRINGER,1968). **rhodostannite with the maximum Ag content from the Pirquitas deposit, Argentina (JOHANand PICOT,1982). ***based on total atoms per formula unit of 8 for stannite and hocartite, 4 for chalcopyrite and 14 for rhodostannite, respectively. st-1: associated with pyrite and sphalerite of 3(B) in Table 1, st-2: occurring as an inclusion in hocartite shown in Fig. 4-B, st-3: associated with pyrite, arsenopyrite and coarse-grained sphalerite, st-4: associated with fine-grained sphalerite of 4(B) in Table 1 and containing the maximum Ag of the analysed stannites, st-5: from sulfidevein in siliceous schist, ho: shown in Fig. 4-B, cp: from sulfide vein in siliceous schist, and rs-1 and rs-2: containing the minimum and maximum Ag, respectively. tively, whereas Nsb+Bi are smaller than 2, (2) NPb+Fedecrease with increasing Nsn (Fig. 6). 5.3 Tetrahedrite and Chalcopyrite The analysed tetrahedrite is essentially free of As and contains significant amounts of Ag ranging from 9.4 to 20.9 wt.% (Table 4). The Ag contents are usually higher in tetrahedrite from the brecciatd ores than those from sulfide veins in siliceous schists, although chemical heterogeneity due to the substitution of Ag for Cu is intense in a single grain. The Fe/(Fe+Zn) atomic ratios are, on the con- Fig. 6 Relationship between the numbers of Sn and trary, rather uniform around 0.8. of (Pb+Fe) in the franckeite-like mineral, based Chalcopyrite from the sulfide veins in on the formula calculated on the basis of 25 total atoms per formula unit. The solid line is the least- siliceous schists was analysed, with the results squares fit expressed as Nsn=-0.71NPb+Fe+8.5, that trace but significant levels of Sn are with a correlation coefficient of -0.93. detected, and that Fe/Cu atomic ratios deviate slightly from unity up to 1.06 (Table 2). Back-scattered electron images reveal that no analysed chalcopyrite. Tin may therefore Sn minerals are included as impurities in enter into chalcopyrite as a solid solution. 414 K. KASE MINING GEOLOGY:

Table 3 Representative microprobe analyses of the franckeite-like mineral 6. Discussion 6.1 Mineral Chemistry The Fe/(Fe+Zn) atomic ratios from 0.45 to 0.50 for the present hocartite indicate that hocartite may form an extensive solid solution toward Ag2ZnSnS4 (pirquitasite) as suggested by JOHANand PICOT(1982). The mineral contains, however, only about 1 wt.% of Cu. Previous chemical data also show that dissolu- tion of Cu is very limited (CAPE et al., 1968; JOHANand PICOT, 1982). The analysed rhodostannite contains Ag from 0.13 to 0.16 in terms of Ag/(Cu+Ag) atomic ratios. The rhodostannite described originally from the Vila Apacheta, Bolivia is free of Ag (SPRINGER,1968). JOHANand PICOT (1982) described, however, rhodostannite with the ratios ranging from 0.16 to 0.56 from the Pirquitas deposit, Argentina (rs-4 in Table 2). BERNHARDTet al. (1984) also reported high Ag Listed in order of increasing Sn content. *based on 25 rhodostannite. The substitution of Ag for Cu total atoms per formula unit. **number of calculated may be extensive in rhodostannite series in con- divalent Sn (see text). ***meaning NPb+Fe+1/2Cu-. trast with the fact that the substitution is very limited between the stannite-kesterite series Table 4 Representative microprobe analyses of and the hocartite-pirquitasite series (JOHAN tetrahedrite from sulfide veins in siliceous schists and PICOT, 1982). The present and previous (V) and a brecciated ore (B) data indicate, however, that Zn contents are very low in rhodostannite series (SPRINGER, 1968; JOHANand PICOT, 1982; BERNHARDTet al., 1984), showing a marked contrast with the minerals of stannite group in which the substitution of Zn for Fe is extensive (SPR- INGER,1972; JOHANand PICOT, 1982). The chemical characteristics of the franckeite-like mineral is examined, based on the ideal chemical formula of franckeite, i.e., FePb6Sb2Sn2S14.An electric charge balance is assumed to be maintained in the mineral. Total numbers of Sb, Bi, and Sn are always greater than 4 in the formulae of the Besshi franckeite-like mineral. After filling the sites of Sb and Sn, the mineral has thus further excess Sn. The excess Sn may take a divalent valence state with a radius similar to that of Pb, and occupy the sites of Pb. WOLFet al. (1981) has already suggested that some proportions

Listed in order of increasing Ag content . *based on of Sn enter into the structure of potosiite with 29 total atoms per formula unit . a divalent valence state. Actually, the NPb+Fe 38(5), 1988 Tin, arsenic, zinc and silver vein mineralization in the Besshi mine 415

stage. Finally, the franckeite-like mineral

replaced galena. Rhodostannite appears to be simultaneous with the stannite of major stage. SPRINGER (1968) considered, however, that rhodostannite from the Vila Apacheta, Bolivia, is an alteration product of stannite, and its

porous nature resulted from the shrinkage of volume due to the alteration. The temperatures of mineral deposition

were estimated from the distribution of Fe/Zn between the coexisting sphalerite and stannite. The temperature dependency of Fe and Zn partitioning between sphalerite and stannite was

determined by NEKRASOV et al. (1979) and Fig. 7 Relationship between the numbers of NAKAMURA and SHIMA (1982). The equations calculated divalent Sn and of (Pb+Fe+Zn+ obtained by them are as follows; 1/2Cu) in the franckeite-like mineral. The solid log Kd=(1.274 •~ 103/T)-1.174 line is the least-squares fit expressed as NSu2+= (NEKRASOV et al., 1979), -0 .77NPb+Fe+Zn+1/2Cu+6.8, with a correlation coefficient of -0.86. log Kd=(2.8 •~ 103/T)-3.5 (NAKAMURA and SHIMA, 1982). of the Besshi franckeite-like mineral decreases In these equations, T is the absolute tempera- with increasing numbers of the calculated di- ture and Kd means (XCu2FeSnS,/XCu2ZnSnS4)st/ valent Sn. In Fig. 7, Npb+Fe+Zn+1/2Cuis used (XFeS/XZnS)sp, where X is the mole fraction of instead of Npb+Fe, taking into account that the subscript component in stannite (st) and small amounts of Zn and monovalent Cu may sphalerite (sp). substitute for Fe in the Besshi franckeite-like SHIMIZU and SHIKAZONO(1985) compared the mineral. The slope (ca. -0.8) of the linear equilibrium temperatures estimated from the least-squares regression deviates slightly from sphalerite-stannite geothermometer with those - 1 , which is believed to be due to analytical estimated from the homogenization of fluid in- differences. The substitution of divalent Sn for clusions and from the sulfur isotope fractiona- Pb could be responsible for the chemistries of tion between galena and sphalerite in Japanese further Sn-rich varieties in franckeite group vein-type and skarn-type deposits. According minerals such as incaite (FePb4Sb2Sn4S14),but to them, the equation proposed by NAKAMURA accompanied with slight changes in structural and SHIMA (1982) gives the temperatures con- configurations (KISSINand OWENS,1986). sistent with those derived from studies on the 6.2 Sequence and Environment of Mineral fluid inclusions and sulfur isotopes. The NAKA- Deposition MURA-SHIMA's equation is, therefore, applied The paragenetic sequence observed in polish- here to the coexisting sphalerite and stannite ed sections of the brecciated ores is as follows. pairs from the Besshi mine (Fig. 8). Pyrite, arsenopyrite and cassiterite appear to It is evident in Fig. 8 that the temperatures have formed at an earlier stage, with small of precipitation (ca. 320•K-345•Ž) for coarse-

amounts of pyrrhotite as a forerunner. The grained sphalerite are significantly higher than coarse-grained sphalerite and stannite closely those (ca. 260•K-270•Ž) for fine-grained sphal- associated with it may have precipitated also erite in the brecciated ores. The temperatures at this stage. The majority of stannite (i.e., of mineralization appear to become lower to- that occurring as discrete grains, and over- ward the later stages. The replacement of the growing on, or replacing, sphalerite and cassit- franckeite-like mineral for galena may have erite), hocartite, fine-grained sphalerite, tetra- thus occurred at further low temperatures. hedrite and galena may have formed at a later The brecciated ores are therefore considered 416 K. KASE MINING GEOLOGY:

sphalerite and stannite occurring in the sulfide veins are 278•K, 284•K and 302•Ž (Fig. 8). The narrow temperature range may be due to the

limited occurrence of sphalerite in the central

parts of the veins. A temperature range wider than that estimated is inferred from the common occurrence of marcasite derived from the

alteration of pyrrhotite. 6.3 Genetic Relation with the Sn Deposits in the Outer Zone of Kyushu Granitic rocks related to the Sn deposits in the outer zone of Kyushu have narrow range of K-Ar ages (14•}1 Ma), according to the data compiled by NOZAWA (1968) and SHIBATA

(1978). For the Sn deposits, MIYAHISA (1958) Fig. 8 Distribution of Fe and Zn between coexisting pointed out the following characteristics; (1)

sphalerite and stannite. Broken lines indicate the the deposits are prominently polymetallic, (2) isotherms of 250•K, 300•K and 350•Ž, obtained by telescoped ores are commonly found, and (3) NAKAMURA and SHIMA (1982). the mineral zoning is sometimes conspicuous open circles: coarse-grained sphalerite and stannite around the granitic intrusive bodies with Sn- in brecciated ores, solid circles: fine-grained As deposits occurring close to the intrusives sphalerite and stannite in brecciated ores, and open and Ag-Sb deposits away from the intrusives. triangles: from sulfide veins in siliceous schists. The Sn deposits occur in general within the to be composed of mixtures of minerals form- zones of biotite hornfels adjacent to the ed at different stages and different tempera- granitic intrusive bodies (MIYAHISA, 1969). tures during the mineralization sequence. Miocene granitic intrusives with K-Ar ages They could be called telescoped ores. of 14•}1 Ma, which are associated with the Sn The interaction of the hydrothermal solu- deposits in the outer zone of Kyushu, are also tions with the contact-metamorphosed mas- sporadically distributed in the outer zone of sive pyrrhotite ores could be responsible for Shikoku(SHIBATA, 1978). Although no granitic the mineral assemblages and the chemical rocks are found in the Besshi mining district, a characteristics of minerals of the sulfide veins granodiorite with similar K-Ar age crops out in siliceous schists, such as the dominant occur- at Omogo, 30 km southwest of the Besshi mine rence of pyrrhotite and the CdS and MnS con- (NOZAWA, 1968). It is thus highly probable tents of sphalerite intermediate between those that the Sn mineralization occurring in the from the brecciated ores and from the contact- zone of biotite hornfels at the Besshi mine is metamorphosed massive ores. The vein mine- genetically related with a Miocene granitic in- ralization may have occurred immediately trusive rock, now hidden in the deeper part of after the contact metamorphism, because the the mine. hornfels are completely replaced by hydro- Stibnite veins occur at the shallow and in- thermal minerals in the brecciated ores, but termediate levels of the Besshi mine (UCHIDA temperatures of the massive ores were proba- et al., 1981). It appears that there is a regional bly still elevated high enough for the ex- zoning in distribution of vein sulfide minerals change of such metals as Mn and Cd with the in the Besshi mining district, with a mineraliza- hydrothermal solutions. The sulfur fugacity in tion center in the deeper part of the mine. The the sulfide veins was also strongly affected by Sn vein mineralization at the Besshi mine is massive ores composed dominantly of pyr- characterised by the polymetallic nature and rhotite. telescoping of ores. The geological and mine- Temperatures estimated from three pairs of ralogical features of the Sn mineralization in 38(5), 1988 Tin, arsenic, zinc and silver vein mineralization in the Besshi mine 417 the Besshi mine are thus quite similar to those and Tsugio SHIBATAof Okayama University of the Sn deposits in the outer zone of Kyushu. for their critical reading of the manuscript and The Sn metallogenic province in the outer invaluable suggestions. Thanks are also due to zone of Kyushu should be extended eastward Dr. Kenichiro HAYASHIof Tohoku University to Shikoku. who kindly gave the important references. Technical assistances are indebted to Mrs. 7. Concluding Remarks Hiroshi FUJIWARAand Toshiaki SAITO of 1. Intense Sn-As-Zn-Ag vein mineraliza- Okayama University. The present study was tion was found at the 26th L of the Besshi supported in part by Grant-in-Aid for Fun- mine, which resulted in formation of such rare damental Scientific Research (No. 60303013) Sn minerals as rhodostannite, hocartite and a from the Ministry of Education of Japan franckeite-like mineral as well as common given to Prof. Tadashi MARIKOof Waseda Uni- stannite and cassiterite. versity. 2. The present microprobe analyses, combined with previous chemical data, indicate References that the substitution of Ag for Cu is extensive BANNO, S. (1964): Petrological studies on Sambagawa in rhodostannite, whereas that of Zn for Fe is crystalline schists in the Besshi-Ino district, central very limited. On the contrary, the substitution Shikoku, Japan. Jour. Fac. Sci., Univ. Tokyo, Sec. of Ag for Cu is very limited and that of Zn for 2, 15, 203•`319. Fe is extensive in stannite and hocartite. Tin- BERNHARDT, H. J., COIRA, B. and de BRODTKORB, M. K. rich compositions of the franckeite-like (1984): A new occurrence of silver-rhodostannite. In mineral are probably due to the substitution Sulfosalt: Observations and descriptions, ex- of divalent Sn for Pb. periments and applications (G. MOH, ed.), Neues Jahrb. Mineral. Abhandl., 150, 25-64.

3. The polymetallic Sn ores are composed of CAYE, R., LAURENT, Y., PICOT, P. and PIERROT, R. (1968):

minerals formed at different stages and temper- La hocartite, Ag2FeSnS4, une nouvelle espece miner-

atures. The ores could be called telescoped ale. Bull. Soc. Franc. Mineral. Cryst., 91, 383•`387.

ores. IMAI, H. (1978): Besshi mine, Ehime Prefecture. In 4. If appears that there is a regional zoning Geological studies of the mineral deposits in Japan in distribution of the vein sulfide minerals in and East Asia (H. IMAI, ed.), Univ. Tokyo Press, the Besshi mining district, with Sn and As 256•`258. minerals forming in the mineralization center JOHAN, Z. and PICOT, P. (1982): La pirquitasite, and Sb minerals away from the center. Ag2ZnSnS4, un nouveau membre du groupe de la 5. The vein mineralization may have occur- stannite. Bull. Mineral., 105, 229•`235. KASE, K. (1972): Metamorphism and mineral assemblages red nearly simultaneously with the contact of ores from cupriferous iron sulfide deposit of the metamorphism. Besshi mine, central Shikoku, Japan. Jour. Fac. Sci.,

6. The geological and mineralogical features Univ. Tokyo, Sec. 2, 18, 301•`323.

of the Sn mineralization are quite similar to KASE, K. (1977): Sulfide minerals and their assemblages of

those of the Sn deposits in the outer zone of the Besshi deposit-Studies on sulfide minerals in Kyushu, which are related genetically with metamorphosed ores of the Besshi and Hitachi cop-

Miocene granitic intrusives. A Miocene gra- per deposits (1)•\. Mining Geol., 27, 355•`365. nitic intrusive rock, now hidden in the deeper KISSIN, S. A. and OWENS, D. R. (1986): The properties and part of the Besshi mining district, probably modulated structure of potosiite from the Cassiar di- caused the mineralization and the contact strict, British Columbia. Can. Mineral., 24, 45•`50. metamorphism. MAKOVICKY, E. (1974): Mineralogical data on cylindrite 7. The Sn metallogenic province in the outer and incaite. Neues Jahrb. Mineral. Monatsh., H. 6, 235•`256. zone of Kyushu should be extended eastward MIYAHISA, M. (1958): Tertiary acid intrusives and related

to Shikoku. metallogenetic provinces in the outer zone of Kyushu

Acknowledgements: The author expresses his and Shikoku, Japan. Mem. Ehime Univ., Sec. 2, 3, sincere gratitude to Drs. Masahiro YAMAMOTO 145•`155 (in Japanese with English abstract). 418 K. KASE MINING GEOLOGY:

MIYAHISA, M. (1969): Geological studies on the ore de- possible indicator of temperature and sulfur fugacity.

posits of Obira-type in Kyushu (2) some geochemical Mineral. Deposita, 20, 314•`320. features of granitic rocks. Mem. Ehime Univ., Sci., SPRINGER, G. (1968): Electronprobe analyses of stannite

Ser. D, 11, 9•`20 (in Japanese with English abstract). and related tin minerals. Mineral. Mag., 36,

MIYAHISA, M. (1973): Tin ores and tin deposits. In Mineral 1045•`1051.

deposits of Japan: Shikoku district (T. WATANABE, T. SPRINGER, G. (1972): The pseudobinary system Cu2FeSnS4-

SAWAMURA and M. MIYAHISA, eds.), Asakura, p. 191 Cu2ZnSnS4 and its mineralogical significance. Can.

(in Japanese). Mineral., 11, 535•`541. NAKAMURA, Y. and SHIMA, H. (1982): Distribution of SUGAKI, A., UENO, H., KITAKAZE, A., HAYASHI, K.,

Fe/Zn between coexisting sphalerite and stannite. SHIMADA, N., SANJINES, O., VELARDE, O., SANCHEZ, A.

Abst. Joint Meet. Soc. Mining Geol. Japan, Mineral. and VILLENA, H. (1983): Geological and

Soc. Japan, and Japan Assoc. Mineral. Petrol. Econ. mineralogical studies on the polymetallic hydrother-

Geol., C-15 (in Japanese). mal ore deposits in Andes area of Bolivia. Report of

NEKRASOV, I. J., SOLOKIN, V. I. and OSADCHII, E. G. (1979): Overseas Scientific Survey, Sendai, pp. 191.

Fe and Zn partitioning between stannite and SWEATMAN, T. R. and LONG, J. V. P. (1969): Quantitative

sphalerite and its application in geothermometry. In electron-probe microanalysis of rock-forming

Origin and distribution of the elements (L. H. minerals. J. Petrology, 10, 332•`379.

AHRENS, ed.), Phys. Chem. Earth, 34, 739•`742. TAKEDA, H., SUZUKI, T. and MIYAHISA, M. (1973):

NOZAWA, T. (1968): Radiometric ages of granitic rocks in Kieslager-type deposits in Sambagawa and Mikabu

outer zone of southwest Japan and its extension: metamorphic belts. In Mineral deposits of Japan:

1968 summary and northshift hypothesis of igneous Shikoku district (T. WATANABE, T. SAWAMURA and M.

activity. Jour. Geol. Soc. Japan, 74, 485•`489 (in MIYAHISA, eds.), Asakura, 66•`140 (in Japanese).

Japanese with English abstract). UCHIDA, K., ASAMI, N., OHTANI, K., SUZUKI, T. and MAT-

SHIBATA, K. (1978): Contemporaneity of Tertiary granites SUKI, M. (1981): Prospecting of kieslager-type

in the outer zone of southwest Japan. Bull. Geol. mineral deposits in Besshi-Sazare district, Shikoku,

Surv. Japan, 28, 551•`554 (in Japanese with English Japan. In Prospecting of mineral deposits in Japan

abstract). (1). Soc. Mining Geol. Japan, 221•`277 (in Japanese).

SHIMADA, S. and TSUNORI, T. (1962): Discovery of stannite WOLF, M., HUNGER, H. J. and BEWILOGUA, K. (1981):

in a fault zone in the Besshi mine, Ehime Prefecture. Potosiite, a new mineral of the cylindrite-franckeite

Mining Geol., 12, 223•`224 (in Japanese with group. Freiberger Forsch., 364, 113•`133.

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別子鉱山における錫,砒素,亜鉛および銀の鉱脈型鉱化作用

加瀬克雄

要旨:別子鉱山26番抗において,黄錫鉱,錫石の他にる.フランケ鉱様鉱物では2価の錫が鉛を置換し,その rhodostannite,黄錫銀鉱(hocartite)およびフラソケ鉱様結果フランケ鉱様鉱物の錫含有量に多様性が生じ得るも鉱物などの稀産錫鉱物を含む,多金属性鉱脈型鉱化作用のと考えられる. が見いだされた.これらの錫鉱物には,黄鉄鉱,硫砒鉄鉱脈型鉱化作用は,別子鉱床の塊状黄鉄鉱鉱体を塊状鉱,閃亜鉛鉱,四面銅鉱,マンガン炭酸塩鉱物,石英お磁硫鉄鉱鉱体に,また周辺の泥質片岩を黒雲母ホルンフよび電気石などが伴う.これらの鉱物の生成時期は2～ェルスに変化させた接触変成作用とほぼ同時期に生じた 3のステージに区分される. ものと考えられる.鉱脈の性質およびその産状は,九州 X線マイクロアナライザーによる今回の分析結果と外帯の中新世花崗岩質岩に伴う錫鉱床のそれらに類似し既存の分析データは, rhodostannite中では,著量の銀ている.別子鉱山の錫鉱物を伴う鉱化作用も,鉱床深部が銅を置換して固溶し得るが,鉄─ 亜鉛間の置換は微量に潜在していると予想される中新世花崗岩質岩の貫入にである事を示す.これらの元素の黄錫鉱,黄錫銀鉱中に関連して生成されたものと考えられる.九州外帯の中新おける置換関係は, rhodostanniteの場合と全く逆であ世錫鉱床区は,四国まで延長して考えるべきであろう.