Resource Geology, 47(4), 211-221, 1997

Petrology of and Chromitite in Wakamatsu Mine of the Tari-Misaka Ultramafic Complex, Western Japan

Kazuhiro MIYAKE*,Shoji ARAI**and Masayuki OKUNO**

Abstract: Peridotite and chromitite in Wakamatsu mine, a large mine, at the Tari-Misaka ultramafic complex are described in order to understand the genesis of podiform chromitite. Rocks in and around the Wakamatsu mine are thermally metamorphosed by granite and the main silicate mineralogy and zonal structure of relic spinel are controlled by the thermal metamorphic temperatures dependent on the distance from the granite. The primary lithologies, however, can be determined by relic textures. is relatively predominant at Wakamatsu mine area, and chromitite occurs as pods within the dunite. Primary are variable in textural and chemical characteristics of chromian spinel dependent on the distance from the chromitite pod. Harzburgite far from chromitite, typically exposed at Misaka, has low-Ti (<0.2 wt% of TiO2) and highly anhedral spinel. Harzburgite close to the boundary with dunite has less anhedral and more Ti-rich (up to 0.5 wt% of TiO2) spinel. Dunite also has relatively Ti-rich (up to 0.5 wt% of TiO2) spinel, which tends to be more abundant and euhedral towards chromitite. Chromitite has rather constant mineralogy irrespective of spinel modal amount, with up to 0.7 (mostly 0.2 to 0.4) wt% of TiO2. Cr#(=Cr/(Cr+Al) atomic ratio) of spinel in all lithologies is confined in a narrow range, from 0.4 to 0.6. These characteristics possibly indicate that a melt/harzburgite interaction controlled the genesis of the podiform chromitite and enclosing peridotites. The low-Ti harzburgite was passed by a relatively Ti-rich melt to leave dunite-chromi- tite and harzburgite envelope with relatively high-Ti mineralogy.

cussed the contact-metamorphic petrogenesis of ultra- 1. Introduction mafic rocks of the Tari-Misaka complex for the first Wakamatsu mine is located in the Tari-Misaka ultra- time. HIRANO et al. (1978) successfully discriminated mafic complex, which is distributed around the bound- the primary lithologies of contact metamorphosed peri- ary area of Tottori, Shimane, Okayama and Hiroshima dotites and recognized that dunite is relatively predomi- Prefectures, Chugoku district, western Japan (Fig. 1). It nant in the northern part of the Tari-Misaka complex, to has been the largest chromite mine in Japan, producing which Wakamatsu mine belongs. ARAI (1980) estab- Al-rich refractory-grade chrome ores (e.g., BAMBA,1963), lished the essential igneous petrological characteristics and provides us a good field for research of chromitite of the Tari-Misaka peridotite. There have been only few genesis. The Tari-Misaka ultramafic complex (ARAI, 1980) has been very famous for a large chromite field, having been paid atten- tion by many researchers, especially by ore geologists. Among them KITAHARA(1958, 1959ab, 1962ab) described various rocks and minerals from the chromite mines of this complex. BAMBA(1963) made pioneer- ing works on ultramafic rocks of the Tari- Misaka complex. IGI and ABE (1969) and Research Group of Peridotite Intrusion (1967) summarized petrographical charac- teristics of peridotites in compilation of peridotite complexes of Chugoku district and Japan, respectively. ARAI (1975) dis- Fig. 1 Index map of Wakamatsu mine.

Recived on June 3, 1997, accepted on July 17, 1997 * Stockpile Department , Metal Mining Agency of Japan, Tokyo 105, Japan ** Department of Earth Sciences, Kanazawa University, Kanazawa 920-11, Japan Keywords: Chromitite, Podiform chromite deposit, Wakamatsu mine, Melt/harzburgite interaction

211 212 K. MIYAKE, S. ARAI and M. OKUNO RESOURCE GEOLOGY:

works, however, on the petrography and petrology of the peridotite-chromitite sys- tem in the Tari-Misaka complex after ARAI (1980). MATSUMOTOet al. (1995) described petrography of ultramafic and related rocks from the central part of the Chugoku dis- trict, including the Tari-Misaka one. Fol- lowing many early descriptive works of chromitites (e.g., BAMBA,1963), ARAIand YURIMOTO(1994) discussed the origin of the chromitites of the Tari-Misaka complex based on major- and trace-element chem- istry of chromian spinel. Recently MATSU- MOTOet al. (1997) highlighted the high-Al character of the Tari-Misaka chromitite. MATSUMOTO(1997) discussed morphologi- cal and chemical variations of chromian spinel in peridotites and their bearing on the chromitite genesis. In this article we describe petrography and mineral chemistry of chromitites and associ- ated rocks in the Wakamatsu mine to make genetical relationshipbetween chromitite and associated peridotite much clearer. As in the previous works (ARAIand YuRilvIoTo,1994; MATSUMOTOet al., 1997) we focused on chromian spinel because its chemistry and texture are often preserved to be primary,the compositions of and other minerals are not reliable for the primary ones (ARAI, 1975). This article is complementary to ever published works (ARAI, 1980; ARAI and YURIMOTO,1994; MATSUMOTOet al., 1997) and also provides a background for discussion on the genesis of chromitite pods of Wakamatsu mine, the largest chromite concentration in Japan. Fig. 2 Lithological map of the north part of Tari-Misaka complex (below: after MITI, 1994) and distribution of peridotite complexes in 2. Geological background the Chugoku district (above: after ARAI, 1980). The Tari-Misaka ultramafic complex is one of the largest of many ultramafic complexes (Fig. 2) ments. The complex is intruded by two granite to gran- in the Sangun metamorphic belt, which is of intermedi- odiorite masses of Cretaceous age (ARAI, 1975). The ate high-pressure type (MIYASHIRO,1973). Most of them Tsudoko tunnel of Hirose mine, another large chromite were suffered from thermal metamorphism by Creta- mine in the complex, penetrated through the peridotite ceous granitic intrusions (ARAI, 1975). The Sangun ultra- from west to east into the granodiorite mass at its east- mafic complexes are mostly composed of massive ern end. Granodiorite phorphyry dikes are frequently harzburgite and dunite with or without small amount of observed in Wakamatsu mine. The thermal metamor- podiform chromitite (e.g., ARAI, 1980; MATSUMOTOet phism was after some serpentinization, and transformed al., 1995). One exception is the Ochiai-Hokubo com- the ultramafic rocks into deserpentinized peridotites plex which is composed of layered harzburgite-lherzo- (ARAI, 1975). Five metamorphic mineral zones are rec- lite-dunite-wehrlite (e.g., ARAI et al., 1988). ognized from the contact with granite to the center of The Tari-Misaka ultramafic complex is emplaced into the complex (ARAI, 1975; MATSUMOTOet al., 1995): (1) the Paleozoic sediments (mainly slates) and is covered chrysolite/lizardite (Zone I, thermally unmetamor- with Cretaceous rhyolitic welded tuff and Tertiary sedi- phosed), (2) antigorite (Zone II), (3) olivine-talc (Zone 47(4), 1997 Petrology of peridotite and chromitite in Wakamatsu mine 213

work a few years ago (in 1994). The chromite ore in this mine contains about 31 wt% of Cr2O3 on average, and is of refractory grade for making firebricks.

Many chromitite pods are distributed in the mining area; main deposits (pods) are, from north to south, North 7th (Kita-

Nana-go) pod, North 5th (Kita-Go-go)

pod and South 5th (Minami-Go-go) ore- body which is subdivided into six small- er pods (South 51st to South 56th) (Fig. 3). The North 7th pod, striking southeast and dipping 30•‹ south, is the largest

chromitite orebody in Japan, 20 to 30 m thick, 30 to 50 m wide and 200 m long in size. The North 5th pod has a lenticular shape, with a long axis of 50 m, a short axis of 25 m and average thickness of 15 Fig. 3 Distribution of chromitite pods around the Chubu-ko tunnel of Waka- m. The main tunnel (= Chubu-ko) con- matsu mine. (a) Plan. (b) Cross section. nects the North 7th pod to the north with the South 5th pods to the south. In addi- III), (4) olivine-anthophyllite (Zone IV), and (5) olivine- tion to the large pods, at least three small pods are orthopyroxene (Zone V). The primary lithology, dunite or found in the Chubu-ko tunnel. harzburgite, can be completely discriminated in Zones I 4. Sampling and II, where primary minerals and/or textures are often preserved. As described below in more detail chromian Samples used in this study were mainly collected at spinel is characteristically euhedral in dunite and anhe- the Wakamatsu mine and its surrounding area. Samples dral or even vermicular in harzburgite there. Dunite and from the Misaka area, where harzburgite is noticeably harzburgite can be usually distinguished even for highly predominant over dunite, were also examined for com- metamorphosed peridotites, which preserve primary tex- parison. tures, especially psudomorphs of orthopyroxene and All over the Chubu-ko tunnel, which connects main chromian spinel (HMANo et al., 1978). pods, was sampled at every 10m to check the relation- Harzburgite is generally dominant over dunite in the ship between chromitite pods and wall-rock peridotites Tari-Misaka ultramafic complex as in almost all com- (Fig. 3). Short tunnels which connect the North 7th and plexes in the Sangun zone (MATSUMOToet al., 1995). South 55th pods with the Chubu-ko tunnel were also However, dunite is relatively predominant in the north- sampled at every 10m to examine the mode of occur- ern part of the complex where some chromite mines rence of chromitite in more detail. The chromitite sam- including Wakamatsu mine are present (HIRANo et al., ples were collected at the North 7th, North 5th and 1978; MATSUMOTOet al., 1995). Gabbroic to dioritic South 5th pods as well as at small pods at the Chubu-ko rocks are frequently observed as irregular-shaped blocks tunnel. or dikes with chilled margins. Pyroxenites are character- All samples from the Misaka area were supplied from istically rare. Metal Mining Agency of Japan (MMAJ). They were collected in the investigation in the storage condition of 3. Wakamatsu Mine rare metal mineral resources by MMAJ.

Chromitite (chromite ore or chrome ore) was discov- 5. Petrography ered in 1897 in this area, and the Wakamatsu mine is the oldest mine for the chromitite. There were several 5.1 Peridotites chromite mines in the northern part of the Tari-Misaka The rocks from the Misaka area belong to Zone I or complex (BAMBA, 1963). Wakamatsu and Hirose mines Zone II in terms of metamorphic mineralogy. They usu- are the largest of them. The Hirose mine, for which the ally preserve the primary textures and often partially work of ARAI (1975) was mainly done, was closed sev- preserve primary minerals, olivine, orthopyroxene, eral years ago (in 1984) because of shortage of the clinopyroxene and chromian spinel. We can, therefore, reserve. Wakamatsu mine had been the only working clearly discriminate primary lithologies there. Harzbur- chromite mine in Japan for many years but stopped to gite, which is predominant, of the Misaka area has the 214 K. MIYAKE, S. ARAI and M. OKUNO RESOURCE GEOLOGY:

spinel and small amount of the Si-rich min- eral clots. The protoliths are harzburgite for the former and dunite for the latter, based on the comparison with the primary litholo- gies from the Misaka area. Chromian spinel is partially or wholly altered into opaque ferritchromite or . The clots of the Si-rich minerals (orthopyroxene or talc) are most probably pseudomorphs of primary orthopyroxene. Based on this classification we can esti- mate the primary lithologies even in the Wakamatsu mine area (Fig. 4). It was con- firmed that chromitites pods are always enclosed by dunite (ARAI and YuRIMOTO, 1994) as for other chromitite pods ever doc- umented (e.g., THAYER,1964). The strongly anhedral (vermicular) chromian spinel tends to be more abundant in peridotites with an increase of the distance from the pods. Petrographical characteristics of the ther- mally metamorphosed peridotites were extensively described by ARAI (1975). Sec- ondary metamorphic olivine is usually fine- grained and sometimes has a dusty appear- ance due to abundance of minute inclusions of magnetite and sulfides. Talc forms clots sometimes with trails of fine euhedral mag- netite, which may trace original cleavage of primary orthopyroxene. Metamorphic orthopyroxene, which is totally free of deformation and exsolution of clinopyrox- ene, usually forms radial aggregates. Zone IV, which is characterized by appearance of acicular anthophyllite, is characteristically very narrow and roughly coincides with the Fig. 4 Morphological characteristics of chromian spinel and distribution Chubu-ko tunnel (Fig. 10a). of primary peridotites along the Chubu-ko tunnel. 5. 2 Modal variations of chromian spinel in peridotites Modal amounts of chromian spinel are dis- protogranular texture. Orthopyroxene is usually severe- ly altered into bastitic pseudomorph. Olivine and tinctly different between harzburgite and dunite as first orthopyroxene are sometimes kinked. Chromian spinel, described by ARAI (1980) (Fig. 5a). Dunite contains 1 to which is highly anhedral (vermicular) to subhedral, is 11 volume % (most frequently around 3 %) of chromian brownish under the microscope. The vermicular spinel spinel, whereas harzburgite has less than 3 volume % is usually intergrown with . Dunite has euhe- (mostly 1 to 2 %) of spinel (Fig. 5b). It is noteworthy dral chromian spinel and rarely has pyroxenes. that the morphological change of spinel is also distinc- The rocks from the Wakamatsu mine and adjacent tive as mentioned above; eudedral spinel surpasses area, on the other hand, are thermally, metamorphosed anhedral one in dunite and vice versa in harzburgite to have mineral assemblages of Zones III to V (ARAI, (Fig. 5a). 1975). Their primary mineralogies are not evidently The variation of modal amounts of chromian spinel in determined. There are two kinds of metaperidotites; one peridotites in the northern part of the Chubu-ko tunnel is has irregular-shaped chromian spinel and clots of meta- especially noteworthy (Fig. 4). As mentioned above the morphic orthopyroxene or talc dependent on the meta- peridotites exposed at this part of the Chubu-ko tunnel are morphic grade, and the other has euhedral chromian wall of the North 7th chromitite pod to the north (Fig. 3). 47(4), 1997 Petrology of peridotite and chromitite in Wakamatsu mine 215

(a) Mode compositions of Chromian Spinel (Selected) it increases both from No. 6 to No. 1 towards the North 7th pod and from 9 to 12 towards the small pod (Fig. 4). 5. 3 Chromitites Chromitites are composed of chromian spinel and silicate matrix with various proportions. There is a gap for the chromian spinel mode, between 10 and 20 volume % (ARAI, 1980), and the rocks with more than 20 volume % of chromi- an spinel are named "chromitite" in this article. Chromian spinel in the chromitite is most frequently between 40 and 90 volume % in mode. Chromitite with high content of chromian spinel exhibits so- called foam texture, an aggrega- (b) Histogram of total Chromian Spinel tion of rounded chromian spinel. Nodular and anti-nodular textures are rarely found especially in the South 5th pods, but orbicular tex- ture is not found. Chromian spinel is sometimes fractured or even brecciated and is reddish brown in thin section. The silicate matrix is totally composed of hydrous min- erals, serpentine, chlorite, and talc in most cases. Olivine, orthopyrox- ene, cordierite and others are also found depending on the metamor- phic grade and degree of alter- ation. Ti-rich minerals, rutile and , are characteristically found in chromitites (ARAI, 1980). Rutile frequently encloses ilmenite and they altogether fill the crack of chromian spinel. Rutile also rarely occurs as a discrete grain within silicate matrix. Native cop- per, chalcopyrite, pentlandite and millelite are found in trace amount.

6. Mineral chemistry

Chemical compositions of chro- mian spinel and other minerals Fig. 5 Modal amounts morphology of chromian spinel in peridotites exposed at were determined by the SEM Wakamatsu mine. (a) Selected mode compositions of chromian spinel. (b) His- (Akashi alpha-30A)-EDAX sys- togram of total chromian spinel in two types of host rocks. tem with an energy dispersive X- ray spectrometer at Kanazawa In addition a small chromitite pod is found between University. Fe2+ and Fe3+ in chromian spinel were calcu- sampling points 11 and 12 of Fig. 4. The spinel mode dis- lated assuming spinel stoichiometry. In the calculation tinctly increases around the chromitite pods: for example, all Ti in spinel was combined with Fe2+ to form 216 K. MIYAKE,S. ARM and M. OKUNO RESOURCE GEOLOGY:

ulvospinel component. Chro- Table 1 Selected microprobe analyses of chromian spinel in rocks from Wakamatsu mian spinel was carefully ana- mine and adjacent area. lyzed at its fresh core for the primary composition in case of chromian spinel partly altered. Selected analyses are listed in Tables 1 and 2. 6. 1 Chromian spinet Trivalent cation ratios of chromian spinel in harzburgite demonstrate a systematic varia- tion dependent on mode of occurrence, especially on the distance from chromitite pods (Fig. 6). Chromian spinel in harzburgite from Misaka area, where chromitites are absent, is rather confined in composition, relatively low in Cr# (= Cr/(Cr+Al) atomic ratio), around 0.5, and Fe3+ ratio (Fig. 6). On the other hand the harzburgite close to the bound- ary with dunite from Waka- matsu mine has spinel distinc- tively higher both in Cr#, around 0.6, and Fe3+ ratio than the Misaka harzburgite (Fig. 6). Spinel in the harzburgite far from dunite, that is also far from chromitite, has an inter- mediate spinel (Fig. 6). Chro- mian spinel is also variable in dunite; spinel is higher in Cr# in the dunite directly enclosing chromitite pod (around 0.6) than in other (0.5 to 0.6) (Fig. 6). Mg# (= Mg/(Mg+Fe2+)atomic ratio) of spinel is basically variable inversely with Cr# (Fig. 7). These chemical tendencies can be also recognized through the lithologies in the Chubu-ko tunnel (Fig. 8). Chromian spinel in chromi- tite pods is not largely differ- ent in Cr# from that in peri- dotites (Figs. 6 and 7). It is noteworthy, however, that spinel from major pods is rela- tively low in Cr#, 0.4 to 0.5, whereas spinel from a small but highly spinel-concentrated Fig. 6 Ratios of trivalent cations of chromian spinel in rocks from Wakamatsu mine and adjacent area. 47(4), 1997 Petrology of peridotite and chromitite in Wakamatsu mine 217

Table 2 Selected microprobe analyses of Ti-rich minerals in chromitite from Wakamatsu (> 90 %) pod is relatively mine. high in Cr#, 0.5 to 0.6 (Fig. 7). Mg# is high in chromitite, from 0.7 to 0.8, due to high modal amount of chromian spinel (Fig. 7) (ARAI, 1980). TiO2 content in spinel is generally low (< 0.7 wt%) throughout all lithologies and almost within the range for alpine-type podiform chromi- tites (DICKEY, 1974), but is variable dependent on litholo- gy as described by ARAI (1980), ARAI and YURIMOTO (1994) and MATSUMOTOet al. (1997). Chromian spinel in the Misaka harzburgite has the lowest Ti content, distinc- tively lower than that in the harzburgite from Wakamatsu mine (Fig. 9). Dunite and harzburgite from Wakamatsu mine roughly have a similar range of TiO2 content (0 to 0.5 wt%) of spinel (Fig. 9). It is noteworthy that the dunite around chromitite pods often has relatively low-Ti spinel (Fig. 9). TiO2 content of spinel in chromitite is almost comparable with that in dunite spinel and is rather constant irrespective of the degree of spinel concentra- tion (spinel mode) (Fig. 9). Note that primary spinel in chromitite should have had slightly higher TiO2 content because Ti-rich minerals were precipitated at subsolidus as described 'above. The essen- tially similar relationship for Ti content in spinel, that is higher in chromitite-dunite than in harzburgite, is also rec- ognized in Oman and Troo- dos ophiolites (AUGE, 1987; ARAI and YURIMOTO,1994). Difference of compositional zoning in chromian spinel Fig. 7. Mg#-Cr# relationships of chromian spinel in rocks from Wakamatsu mine and dependent on metamorphic adjacent area. temperature was examined in the Chubu-ko tunnel. Chromi- 218 K. MIYAIE, S. ARM and M. OKUNO RESOURCE GEOLOGY:

Fig. 10 Difference in compositional zoning of chromian Fig. 8 Compositional variations of chromian spinel in spinel dependent on contact metamorphic temperature along the Chubu-ko tunnel of Wakamatsu mine. (a) Cr- peridotites along the Chubu-ko tunnel of Wakamatsu mine. (a) Along the Chubu-ko tunnel with small chromi- Al-Fe3+ cationic ratio of chromian spinel. (b) Plan of tite. (b) Along the tunnel from the north 7 th orebody. Chubu-ko tunnel. Metamorphic zone are also indicated.

an spinel has been altered into ferritchromite or magnetite along the rim or crack in all lithologies without an excep- tion. Observed by reflected light under the microscope the spinel core with low reflectivi- ty gradually changes into more reflective rim in peri- dotites from Zone V (olivine- enstatite zone). Chemical analysis reveals that the low- Fe3+ chromian spinel at the core gradual changes into Al- free ferritchromite or chromi- an magnetite (Fig. 10). For spinel in rocks from Zone III (olivine-talc zone), on the other hand, there is a sharp optical boundary or a compo- sitional gap between the core Fig. 9 Cr#-TiO2 relationships of chromian spinel in rocks of Wakamatsu mine and adja- and the rim (Fig. 10). cent area. 47(4), 1997 Petrology of peridotite and chromitite in Wakamatsu mine 219

6. 2 Ti-rich minerals Ti-rich minerals usually occur as fine-grained aggre- gates within spinel. Rutile rarely occurs as an discrete grain within serpentine mineral(s) as described above. In the former ilmenite encloses rutile. Rutile is almost pure TiO2 (Table 2). Ilmenite is very Mg-rich; the Mg# is around 0.68. As rutile and ilmenite usually fill cracks of spinel they are secondary subsolidus minerals. This indicates that spinel in chromitite initially had some higher contents of TiO2, although the effect of their pre- cipitation is actually very low because of their extreme- ly low modal amount (<<0.1 %).

7. Discussion The data mentioned above for chromitite of Waka- matsu mine are concordant to field observations for podiform chromitites (THAYER,1964; CASSARDet al., 1983) and also to recent interpretations for the origin of podiform chromitite (LAGOet al., 1982; ARAI,1992, 1997; ARAIand YURIMOTO,1992, 1994; ZHOUet al., 1994). They interpreted that the dunite envelope around podiform chromitite is essentially "the discordant dunite" (KELE- MEN, 1990) which is an interaction product between melt and wall harzburgite. The melt counterpart should be enriched with Si due to contribution from decompo- sition of orthopyroxene in harzburgite combined with precipitation of olivine. The details of the melt/peri- dotite interpretation were described in KELEMEN(1990) and ARAIet al. (1994). It is highly possible that the sec- ondary relatively Si-rich melt meets and is mixed with subsequently supplied relatively primitive melt within a conduit (e.g., NOLLERand CARTER,1986; PAKTUNC,1990; ARAIand YURIMOTO,1994; ZHOUet al., 1994). The mixed melt should be oversaturated with chromian spinel to solely precipitate chromian spinel (= to form spinel-rich cumulates) (IRVINE,1975, 1977). For the Tari-Misaka case, relatively Ti-rich melt was supplied from deeper parts through cracks into Ti-poor harzburgite (= Misaka-type) (Fig. 11). The melt reacted the wall harzburgite to leave discordant dunite which Fig. 11 A model for the genesis of peridotite-chromitite has relatively Ti-rich spinel (Fig. 11). The subhedral complex of Wakamatsu mine and adjacent area. character of spinel in the dunite near the boundary with (a) Invasion of a relatively Ti-rich melt into Ti-poor harzburgite is inherited from the vermicular shape of the harzburgite. Misaka-type harzburgite. This type of dunite with sub- (b) Interaction between the melt and the Ti-poor harzbur- hedral spinel is of replacive origin from harzburgite due gite to make dunite and a relatively Si-rich melt, and to decomposition of orthopyroxene (MATSUMOTO, replenishment of the Ti-rich relatively primitive melt. Note a wall of relatively Ti-rich harzburgite 1996). The harzburgite adjacent to the melt conduit was metasomatized to have also similarly Ti-rich spinel (= (c) Mixing of the two melts and precipitation of chromi- an spinel to make chromian spinel cumulate. Wakamatsu-type) (Fig. 11). The spinel in the Wakamat- (d) Formation of the zonal arrangement of podiform su-type harzburgite is less vermicular than that in the chromitite, Ti-rich dunite, Ti-rich harzburgite and Ti- Misaka-type and is thus chemically and morphological- poor harzburgite from the core outwards. ly modified equivalent to the latter one. The dunite sometimes low-Ti spinel possibly due to the dilution effect of orthopyroxene, which is very low in Ti, 220 K. MIYAKE, S. ARAI and M. OKUNO RESOURCE GEOLOGY: involved with the interaction. The almost uniform ARAI, S. (1997): Origin of podiform chromitites. Journal of chemical composition, especially for Cr# and TiO2 con- Southeast Asian Earth Sciences (in press). tent, throughout the interaction product (chromitite, ARAI, S., INOUE,T and OYAMA,T. (1988):Igneous petrology of dunite and Wakamatsu-type harzburgite) may indicate the Ochiai-Hokubo complex, the Sangun zone, western Japan: a preliminary report. Jour. Geol. Soc. Japan, 94: that fractional crystallization did not control the process 91-102 (in Japanese with English abstract). (e.g., ARAI and YURIMOTO, 1994). It is possible instead ARAI, S. and YURIMOTO, H. (1992): Origin of podiform that the minerals including chromian spinel were finally chromitites from Southwest Japan as a melt-mantle inter- re-equilibrated with each other through a melt during action. Abstr. 29th Intern. Geol. Congr., Kyoto, 1, 176. the interaction (cf., ARAI, 1980). ARAI, S. and YURIMOTO,H. (1994): Podiform chromitites of Spinel chemistry is changeable from the initial the Tari-Misaka ultramafic complex, Southwest Japan, as igneous one during the subsolidus stage, depending on mantle-melt interaction products. Econ. Geol., 89, 1279- both the modal amount and metamorphic temperature. 1288. The relatively large variation of Mg# of spinel in peri- ARAI, S., ABE, N. and NINOMIYA,A. (1994): Reaction of peri- dotites from Wakamatsu mine is possibly due the ther- dotite xenoliths with host magmas as an analogue of man- mal metamorphism; Mg# of spinel was lowered through tle-melt interaction: microscopic characteristics. Sci. Rep. Kanazawa Univ., 39, 65-99. Mg-Fe redistribution with olivine at 450 to 600•‹C dur- AUGE, T. (1987): Chromite deposits in the northern Oman ing the metamorphism. The Mg# of spinel in the Misaka ophiolite: Mineralogical constraints. Mineral. Deposita, harzburgite is uniform because the rock is basically 22,1-10. unmetamorphosed and the equilibrium temperature BAMBA, T. (1963): Genesis of the chromite deposit in Japan. should be the same in the Misaka area (ARAI, 1975). Bull. Geol. Surv. Japan., 200, 68p. The compositional gap of spinel in peridotites from the CASSARD, D., NICOLAS, A., RABINOVITCH,M., MOUTTE, J., Chubu-ko tunnel (Fig. 10) is possibly equal to a misci- LEBLANC,M. and PRINZHOFER,A. (1983): Structural clas- bility gap because the metamorphic temperature by the sification of chromite pods in southern New Caledonia. granitic intrusions is definitely lower in Zone III than Econ. Geol., 76, 805-831. Zone V (ARAI, 1975). This clearly means that the enclosed DICKEY,J. S. Jr. (1974): A hypothesis of origin for podiform chromitites have different thermal histories dependent chromite deposits. Geochim. Cosmochim. Acta, 86, 2469- 2491. on the distance from the granite. The zonal structure of HIRANO,H., HIGASHIMOTO,S. and KAMITANI,M. (1978): Geol- chromian spinel is thus a good guide to the metamor- ogy and chromite deposits of the Tari district, Tottori Pre- phic temperature or thermal history of ultramafic rocks fecture: Bull. Geol. Surv. Japan, 29, 61-71 (in Japanese in the Tari-Misaka complex. with English abstract). Acknowledgements: This paper is based on the Mas- IGI, S. and ABE, K. (1969): Ultrabasic rocks in the eastern part ter's thesis of K. M. at the Kanazawa University, which of the Chugoku Zone, Japan. Bull. Geol. Surv. Japan, 20, was supervised by Prof. T. MATSUMOTO and Prof. K. 39-50 KIRARA. We are very grateful to the stuff of Wakamatsu IRVINE,T. N. (1975): Crystallization sequences in the Muskox mine for approving us to research the mine, and espe- intrusion and other layered intrusions- II. Origin of cially to Mr. T. YAMANE (the then Chief Exploration chromitite layers and similar deposits of other magmatic Engineer) for his continuing help and hospitality in ores. Geochim. Cosmochim. Acta, 39, 991-1021. IRVINE,T. N. (1977): Origin of chromite layers in the Muskox everything we have done at the mine. We are very much intrusion and other intrusions: a new interpretation. Geol- indebted to Dr. I. MATSUMOTO for his discussion and ogy, 5, 273-277. assistance in preparation of this paper. We wish to KELEMEN,P. B. (1990): Reaction between and express our thanks to Metal Mining Agency of Japan fractionating basaltic magma I. Phase relations, the origin for encouragement and lending us the important sam- of calc-alkaline magma series and the formation of discor- ples from Misaka area. dant dunite. Jour. Petrol., 31, 51-98. KITAHARA,J. (1958): Petrology of chromite in the Tari district, References Tottori prefecture, western Japan (Part I). J. Japan Assoc. ARAI, S. (1975): Contact metamorphosed dunite-harzburgite Mineral. Petrol. Econ. Geol., 42, 1-9 complexes in the Chugoku district, western Japan. Con- KITAHARA,J. (1959a): Petrology of chromite in the Tari dis- trib. Mineral. Petrol., 52, 1-16. trict, Tottori prefecture, western Japan (Part II). J. Japan ARAI, S. (1980): Dunite-harzburgite-chromitite complexes as Assoc. Mineral. Petrol. Econ. Geol., 43, 42-54 refractory residue in the Sangun Yamaguchi zone, western KITAHARA,J. (1959b): Petrology of chromite in the Tari dis- Japan. Jour. Petrol., 21, 141-165. trict, Tottori prefecture, western Japan (Part III). J. Japan ARAI, S. (1992): Petrology of peridotites as a tool of insight Assoc. Mineral. Petrol. Econ. Geol., 43, 90-100 into mantle processes: a review. Jour. Mineral. Petrol. KITAHARA,J. (1962a): Mineralogy of chromite in the Tari dis- Econ. Geol., 87, 351-363 (in Japanese with English trict, Tottori prefecture, western Japan (Part I). J. Japan abstract). Assoc. Mineral. Petrol. Econ. Geol., 47, 167-174 47(4), 1997 Petrology of peridotite and chromitite in Wakamatsu mine 221

KITAHARA,J. (1962b): Mineralogy of chromite in the Tari dis- 1992 fiscal year. 140pp (in Japanese) trict, Tottori prefecture, western Japan (Part II). J. Japan MITI (1994): Report on the the investigation in the storage Assoc. Mineral. Petrol. Econ. Geol., 47, 223-221 condition of rare metal mineral; Dogoyama area, 1993 fis- LAGO, B. L., RABINOWICH,M. and NICOLAS, A. (1982): Podi- cal year. 204pp (in Japanese) form chromite ore bodies: a genetic model. Jour. Petrol., NOLLER, J. S. and CARTER, B. (1986): The origin of various 23,103-125. types of chromite schlieren in the Trinity Peridotite, Kla- MATSUMOTO,I. (1995): Degree of mantle-melt interaction and math Mountains, California. In Metallogeny of Basic and genesis of podiform chromite in the dunite-harzburgite- Ultrabasic Rocks (regional presentations). (CARTER, B., chromite complexes of the Sangun zone, Southwest Japan. CHOWDHURY,M.K.R., JANKOVIC,S., MARAKUSHEV,A. A., Ph D. thesis, Kanazawa University, 99pp MORTEN,L., ONIKHIMOVSKY,V. V., RAADE, G., ROCCI, G. MATSUMOTO,I., ARAI, S., MURAOKA,H. and YAMAUCHI,H. and AUGUSTITHIS, S. S., eds.), Theophrastus, Athens, (1995): Petorological characteristics of the dunite-harzbur- Greece, 151-178. gite-chromite complexes of the Sangun zone, Southwest PAKTUNC,A. D. (1990): Origin of podiform chromite deposits Japan. J. Japan Assoc. Mineral. Petrol. Econ. Geol. (in by multistage melting, melt segregation and magma mix- Japanese with English abstract). ing in the upper mantle. Ore Geol. Rev., 5, 211-222. MATSUMOTO,I., ARAI, S. and YAMAUCHI,H., (1997.): High-Al Research Group of Peridotite Intrusion (1967): Ultrabasic podiform chromitites in dunite-harzburgite complexes of rocks in Japan. J. Geol.Soc. Japan, 73, 543-553 the Sangun zone, central Chugoku district, Southwest THAYER, T. P. (1964): Principal features and origin of podi- Japan. Journal of Southeast Asian Earth Sciences (in press) form chromite deposits and some observation on the Gule- MIYASHIRO,A. (1973):Metamorphism and metamorphic belts: man-Soridag district, Turkey: Econ. Geol., 59, 1497- George Allen and Unwin, London, 492pp 1524. MITI (Japanese Ministry of International Trade and Industry) ZHOU, M.-F., ROBINSON,P. T. and BAI, W.-J. (1994): Forma- (1993): Report on the the investigation in the storage con- tion for podiform chromitites by melt / rock interaction in dition of rare metal mineral resources; Dogoyama area, the upper mantle. Mineral. Deposita, 29, 98-101.

若松 鉱 山のか ん らん岩お よび クロム鉄鉱岩 の岩石学 的研 究

三宅 一弘 ・荒 井章 司 ・奥野 正幸

要 旨:多 里-三 坂 超 苦 鉄 質 岩 体 中 の 若 松 鉱 山 の か ん ら ん ネ ル の モ ー ド組 成 に 関 わ ら ず む し ろ 均 質 な 鉱 物 組 成 を 持 岩 と ク ロ ミ タ イ トを 調 べ,ボ デ ィ フ ォ ー ム 型 ク ロ ム 鉱 床 ち,0.7wt%に 達 す るTi含 有 量(多 く は0.2～0.4)を 示 す. の 成 因 に つ い て 考 察 し た.若 松 鉱 山 の 坑 内 お よ び 周 辺 の す べ て の 岩 相 に お い て,Cr#(=Cr/Cr+Al:原 子 比)は0.4～ 岩 石 は 花 崗 岩 に よ り熱 変 成 を 受 け て い る が,岩 石 の 初 生 0.6の 狭 い 範 囲 に 収 ま る. 構 造 は 岩 石 組 織 の 遺 骸 に よ り判 別 で き る.ダ ナ イ ト は 鉱 こ の よ う な 特 性 は,ポ デ ィ フ ォ ー ム ・ク ロ ミ タ イ ト と 山 周 辺 で 多 く見 ら れ,ク ロ ミ タ イ トは ダ ナ イ ト中 に ポ ッ そ れ を 取 り巻 く橄 欖 岩 の 成 因 を 支 配 す る,メ ル ト/ハ ル ド状 に 出 現 す る.こ れ ら の か ん ら ん 岩 は ク ロ マ イ ト ・ポ ッ バ ー ジ ャ イ ト相 互 反 応 の 存 在 を 示 唆 す る.つ ま り,低 ッ ドか ら の 距 離 に よ り,ク ロ ミ ア ン ・ス ピ ネ ル の 組 成 や いTi含 有 量 を 持 つ ハ ル ッ バ ー ジ ャ イ トか ら,Tiに 富 む 化 学 組 成 が 変 化 し て い る.ク ロ マ イ ト鉱 床 か ら 十 分 離 れ メ ル トの 通 過 に よ っ て,Tiに 富 む ダ ナ イ トー ク ロ ミ タ イ て い る 三 坂 地 域 の ハ ル ツ バ ー ジ ャ イ ト中 の ス ピ ネ ル は, トお よ び そ れ を 取 り巻 くハ ル ッ バ ー ジ ャ イ トが 形 成 さ れ Ti含 有 量 が 低 く(<0.2wt%),複 雑 な 他 形 の 形 態 を 呈 し て た と 考 え ら れ る. い る.ダ ナ イ ト と の 境 界 に 近 い ハ ル ツ バ ー ジ ャ イ ト中 の ス ピ ネ ル は,他 形 の も の が 少 な く な り や やTiに 富 む(0.5 日本語表記 wt%).ダ ナ イ ト中 の ス ピ ネ ル もTiに 富 み(0.5wt%),ク ロ Wakamatsu 若 松, Tari 多 里, Misaka 三 坂, ミ タ イ トに 向 か っ てTi含 有 量 は 増 加 し,自 形 度 の 高 い ス Ochiai 落 合, Hokubo 北 房, Hirose 広 瀬, ピ ネ ル が 多 く見 ら れ る.一 方,ク ロ ミ タ イ ト は そ の ス ピ Chubu-ko 中 部 坑.